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PHYSIOLOGY AND BIOCHEMISTRY
IN MODERN MEDICINE
BY
J. J. RR. MACLEOD, M.B.
PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF TORONTO, TORONTO, CANADA; FORMERLY
PROFESSOR OF PILYSIOLOGY IN THE WESTERN RESERVE UNIVERSITY,
CLEVELAND, OHIO
ASSISTED BY Roy G. Prarce, B.A., M.D.
Director of the Cardiorespiratory Laboratory of Lakeside Hospital,
Cleveland, Ohio
AND BY OTHERS
WITH 233 ILLUSTRATIONS, INCLUDING
11 PLATES IN COLORS
ST. LOUIS
C. V. MOSBY COMPANY
1918
Ts
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Copyricit, 1918, By C. V. Mossy Company
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C. V. Mosby Company
St. Louis
TO
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PREFACE
The necessity of allotting the various subjects of the medical curric-
ulum to different periods, so that the more strictly scientific subjects
are completed in the earlier years, has the great disadvantage that the
student, being no longer in touch with laboratory work, fails to employ
the scientific knowledge with full advantage in the solution of his clin-
ical problems. He is apt to regard the first two or three years in the
laboratory departments as inconsequential in comparison with the sup-
posedly more practical instruction offered during the subsequent clinical
years. He is taught by his laboratory instructors to observe accurately,
and to correlate the observed facts, so that he may be enabled to draw
conclusions as to the manner of working of the various functions of the
animal body in health, and before proceeding to his clinical studies, he
is required to show a proficiency in scientific knowledge, because it is
recognized that this must serve as the basis upon which his knowledge
of disease is to be built. When the clinic is reached, however, the meth-
ods of the scientist are not infrequently cast aside and an understanding
of disease is sought for largely by the empirical method; namely, by the
endeavor to see and examine innumerable patients, to diagnose the case
according to the grouping of the signs and symptoms, and to treat it by
the prescribed methods of experience. So much has to be learned and so
much has to be seen during the clinical years, that the student gives little
thought to the nature of the functional disturbance which is responsible
for the symptoms; he fails to realize that after all, there is no essen- '
tial difference between the condition brought about in his patient by
some pathologic lesion, and that which may be produced in the labora-
tory by experimental procedures, by drugs or by toxins. It must of
course be recognized that just as the science of medicine originated by
the grouping of symptoms into more or less characteristic diseases for
which the most favorable method of treatment had to be discovered by
experience, so must a certain part of the medical training be more or
less empirical but it should at the same time be realized that such a
method is only a means to an end, and that the real understanding of
disease can be acquired only when every abnormal condition is inter-
preted as a primary or secondary consequence of some perverted bodily
function, and when the training in observation and the inductive method
is carried from the laboratory into the clinic.
Vv
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vi PREFACE
It is a constant experience of clinical instructors who would employ
scientific methods of instruction, that they find the students not only
indifferent to an analysis of their cases from the functional standpoint,
but also that they are too inadequately prepared in fundamental phys-
iologic knowledge, to make the analysis possible. The student may
have a superficial acquaintance with the main facts of physiologic science
but have failed to acquire the enquiring habit of mind which will en-
able him, through reflection, comparison, and personal research, to ap-
ply the knowledge in practical medicine and surgery.
For this lack of correlation between the laboratory and clinical stud-
ies, the clinical instructors are not alone responsible. The laboratory
courses are frequently given without any attempt being made to show
the student the bearing of the subject in the interpretation of disease,
or to train him so that in his later years he may be able to adapt the
methods of investigation which he learned in the laboratory, to the study
of morbid conditions. It is self-evident that (without any knowledge
of disease) the-extent to which the student in the earlier years of the
course could be expected to appreciate the clinical significance of what
he learns in the laboratory is limited, but this should not deter the in-
structor from indicating whenever he can, the general application of
scientific knowledge in the interpretation of diseased conditions. But
the chief remedy of the evil undoubtedly lies partly in the continuance
of certain of the laboratory courses into the clinical years, and partly
in the study of medical literature in which the application of physiology
and biochemistry in the practice of medicine is emphasized.
Notwithstanding the sufficient number of excellent textbooks in phys-
iology available to the medical student, there is none in which partie-
ular emphasis is laid upon the application of the subject in the routine
practice of medicine. In the present volume the attempt is made to
meet such a want, by reviewing those portions of physiology and bio-
chemistry which experience has shown to be of especial value to the
clinical investigator. The work is not intended to be a substitute,
either for the regular textbooks in physiology, or for those in functional
pathology. It is supplementary to such volumes. It does not start like
the modern test in functional pathology, with a ‘consideration of the
diseased condition, and then proceed to analyze the possible causes and
consequences of the disturbances of function which this exhibits; but
it deals with the present-day knowledge of human physiology in so far
as this can be used in a general way to advance the understanding of
disease. In a sense it is therefore an advanced text in physiology for
those about to enter upon their clinical instruction, and at the same
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PREFACE Vil
time, a review for those of a maturer clinical experience who may desire
to seek the physiological interpretation of diseased conditions.
In attempting to fulfil these requirements, it has been deemed essen-
tial to go back to the fundamentals of the subject, and to explain as
simply as possible the physical and physicochemical principles upon
which so large a part of physiological knowledge depends. Physiology
may be considered as an application of the known laws and facts of
physics and chemistry to explain the functions of living matter, and it is
only after the extent to which this application can be made has been
‘ appreciated, that the knowledge may be used to serve as the foundation
upon which a superstructure of clinical knowledge can be built.
In order that the volume might be maintained of reasonable size, it
has been necessary to select certain parts of the subject for particular
emphasis, the basis of selection being the degree to which our knowledge
clearly shows the value of the application of physiological methods both
of observation and of thought in the study of diseased conditions. This
has not been done to the extent of omitting the apparently less essential
parts, for these have been treated in sufficient detail to link the others
together so as to preserve a logical continuity, and show the bearing of
one field of knowledge on another. There are however certain parts
of the science, particularly the physiology of nerve and muscle, of the
special senses, and of reproduction, for which application in the general
fields of medicine and surgery is limited, and these parts have been
omitted entirely. It has been judged that this perhaps somewhat arbi-
trary selection is justified on the ground that the ordinary text in
physiology covers these subjects sufficiently, except for the specialist,
for whom on the other hand, no adequate review would have been pos-
sible within the limits of such a volume as this. With reference to bio-
chemistry, no attempt is made to review the properties or describe the
characteristic tests of the various chemical ingredients of the body tis-
sues and fluids. This is already sufficiently done in the textbooks on
biochemistry, and in the numerous manuals on clinical methods. Bio-
chemical knowledge is treated rather from the physiologist’s stand-
point, as an integral part of his subject, particular attention, neverthe-
less, being paid to the far-reaching applications of this latest depart-
ment of medical science, in the elucidation of many obscure problems
of clinical medicine, such as those of diabetes, nephritis, acidosis, goiter
and myxedema. To make the volume of value to those who may not
have had time or opportunity to familiarize themselves with the techni-
cal methods of the physiologist and biochemist as used in the modern
clinie, a certain amount of space is devoted to a brief description of the
methods that appear at present to be receiving most attention, and to
he of greatest value.
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vili PREFACE
Finally, it should be mentioned that the principles of serum diagnosis
and therapy are omitted, since these belong to a highly specialized science
requiring an intensive training of its own.
In the hope that the volume may be instrumental in arousing sufficient
interest to stimulate a more intensive study of the various subjects
which it introduces, a brief bibliography is given at the end of each
section. The references selected are to papers that are more partic-
ularly known to the author; they are not necessarily the most impor-
tant publications on the subject, but are often chosen because of the
useful reviews of previous work contained in them, rather than because
of their own originality. Some of the papers, however, are referred to
as authority for statements of fact which may arouse in the reader a
desire to ponder for himself the evidence upon which these are based.
The references are usually divided into two groups, ‘‘monographs’’ and
‘original papers,’’ and it is only occasionally that specific reference is
made to the former in the context. The original papers, on the other
hand, are referred to by numbers. With the general field of the subject
so well covered by such excellent textbooks as Bayliss’ ‘‘Principles of
General Physiology,’’ Stewart’s, Howell’s, Starling’s, and Halliburton’s
“Human Physiologies,’? and Leonard Hill’s ‘‘Recent and Further Ad-
vances in Physiology,’’ the author has felt free to pick and choose from
the monographs and original papers, topics that are ordinarily passed
over cursorily in the textbook, and when this has been done, the refer-
ences are somewhat more extensive. Such is the case for example in
the chapters relating to the chemistry of respiration, to the metabolism
of carbohydrates and fats, to the problems of dietetics and growth, to the
physicochemical basis of neutrality regulation in the animal body, and to
the action of enzymes.
‘Acknowledgment is gratefully made for the assistance and advice
in the preparation of the book, particularly to Doctor R. G. Pearce, for
the contribution of several chapters, to which his name is attached, and
for which he is entirely responsible; and to Doctor HE. P. Carter, whose
criticisms, after patient perusal of the unfinished manuscript, were of
inestimable value in its final revision. Acknowledgment is also made
to Doctor R. W. Scott and Professor F. E. Lloyd, for valuable criticism
and advice, and to the former for a chapter on the ‘‘Clinical Applica-
tion of Electrocardiographs.’’ To Miss Achsa Parker, M.A., the author
owes a great debt of gratitude for the thorough and painstaking way in
which she prepared the manuscript for the press, and for her never-
tiring endeavors to have the spelling and punctuation in conformity
with Webster’s Dictionary. For assistance in the preparation of the
index thanks are due to Miss Marian Armour and Mrs. MacFarlane,
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PREFACE ix
and for permission to use certain of the figures and illustrations, to the
various authors and publishers who granted it. For the excellent man-
agement and careful execution of the presswork, the author wishes to
thank the publishers, whose courteous and friendly dealings have always
made the work' easier.
J. J. R. Macueop.
University of Toronto,
Toronto, Canada.
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CONTENTS
‘PART I
‘THE PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL
PROCESSES
Seige I PAGE
GENERAL CONSIDERATIONS . . . * oe a. Has TL
The Laws of Solution, 3; Gas Laws, 3; Gamotte Press, 4; " Biological Methods
for Measuring Osmotic Pressure, 6; Hemolysis, 7; Plasmolysis, 8.
CHAPTER II
Osmotic PRESSURE (CONT’D) . . . oo . 10
Measurement by Depression of icant Point, 10; The Role of Oamasts Dif-
fusion, and Allied Processes in Physiologic Mechiniamss 11.
CHAPTER IIT
ELEcTRICc CONDUCTIVITY, DISSOCIATION, AND IONIZATION . . . . . . . « . « 16
Biological Applications, 19.
CHAPTER IV
THE PRINCIPLES INVOLVED IN THE DETERMINATION OF HYDROGEN-ION CONCENTRATION 22
Titrable Acidity and Alkalinity, 22; Actual Degree of Acidity or Alkalinity,
23; Mass Action, 23; Application to the Measurement of H-ion Concentration,
26; Application in Determining the Real Strength of Acids or Alkalies, 28.
CHAPTER V
THE PRINCIPLES INVOLVED IN THE MEASUREMENT OF HYDROGEN-ION CONCENTRATION
(ConT’D) . . iy EG ee eo aS OR aD
The Electric Method, 29; The Auiiteatos Method, 32.
CHAPTER VI
REGULATION OF NEUTRALITY IN THE ANIMAL BoDY AND ACIDOSIS . . . 386
Buffer Substances, 36; Theory of Acidosis, 38; Measurement of the Hessie
Alkalinity, 41; Titration Methods, 41; CO,combining Power, 42; Indirect
Methods, 46.
CHAPTER VII
CoLLoms . . . bn MY oe ech Oi eee Le eye 0
Characteristic Prapertieg, 50; Characteristics of True Colloidal Solutions,
51; Tyndall Phenomenon, 51; Relative Indiffusibility, 51; Electrie Proper-
ties, 55; Brownian Movement, 57; Osmotic Pressure, 57.
Xi
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CHAPTER VIII PAGE
CoLLoIDs (ContT’D) . . . 60
Suspensoids and muleoiae, 60; ‘Galatinleation, 61; Imbibiion, 62; Aviton, of
Electrolytes on Colloids, 63; Proteins as Colloids, 63; Surface Tension, 64;
Adsorption, 65; Everyday Reactions Depending on Adsorption, 66; Conditions
Influencing or Influenced by Adsorption, 67; Physiologic Processes Depending
on Adsorption, 69.
CHAPTER IX
FERMENTS, OR ENZYMES .. . . 71
The Nature of Enzyme Action, 72; ‘Properties ‘of Busymes, 733 Reversibility
of Enzyme Action, 77; Specificity of Enzyme Action, 79; Peculiarities of
Enzymes, 80; Types of Enzyme, 81; Enzyme Preparations, 82; Conditions for
Enzymic Activity, 82
PART II
THE CIRCULATING FLUIDS
CHAPTER X
Buioop: Irs GENERAL “PROPERTIES (By R. G. PEARCE) .. . - 85
Quantity of Blood in the Body, 85; Water Content, 86; Broteing, 87; Fer
ments and Antiferments, 89.
CHAPTER XI
Tie Boop CeLts (By R. G. Pearce) . . . 91
Red Blood Corpuscles, or Erythrocytes, 91; Orielt, 92; ates, of Heactorted
93; Hemolysis, 95; Leucocytes, 96; Blood Platelets, 97.
“CHAPTER XII
Buioop CLOTTING . . . 98
Visible Changes in Sikes Blood Durine Clotting, ‘98; Methoda of Helasiting
Clotting, 99; Nature of the Clotting Process, 101; Tafivence of Calcium Salts,
103; Tafinence of Tissues, 104.
CHAPTER XIII
Bioop CLorring (ConT’D) . . . 106
Theories of Blood Clotting, 106; rk aactiay Clotting, 107; Neaaeaineay of
the Clotting Time, 108; Blood Clotting in Various Physiologic Conditions, 110;
Blood Clotting in Disease, 111; Hemorrhagic Diseases, 112; Thrombus Forma-
tion, 113. I
J
CHAPTER XIV
LYMPH FORMATION AND CIRCULATION . . . 7 er es - 115
General Considerations, 115; Experimental Tiestigatiome, 118; Edema, 120.
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CONTENTS xii
PART III
CIRCULATION OF THE BLOOD
CHAPTER XV PAGE
BuLoop PRESSURE . .. a ae . 122
The Mean Arterial Blood Peas 123; Mercury Manomeiee arpasiiee; 193;
Spring Manometer Tracings, 126; Clinical Measurements, 128.
CHAPTER XVI
TIE FAcToRS CONCERNED IN MAINTAINING THE BLOOD PRESSURE . . . . 134
Pumping Action of the Heart, 134; Peripheral Resistance, 134; Aarisunt ‘of
Blood in the Body, 135; Effects of Hewoihage and Transfusion, 139; Viscos-
ity of the Blood, 140; Elasticity of Vessel Walls, 142.
CHAPTER XVII
Tur ACTION OF THE HEART . . : . 144
The Pumping Action of the Heart, “144; Tatisionedins “Preasave aera, 146;
Comparison of the Curves, 148.
CHAPTER XVIII
Tue PuMpPING ACTION or TIE Hearr (Cont’p) *. . . 151
Contour of the Intracardial Pressure Curves, 151; ‘Westientir Cuea: 151;
Auricular Curve, 153; The Mechanism of Opening and Closing of the Valves,
154; The Heart Sounds, 157; Causes of Sounds, 157; Records of Sounds
(Electrophonograms), 158.
CHAPTER XIX
THe NUTRITION OF THE HEART .. . 7 ¢ g eae @ LOL
Blood: Supply, 161; Perfusion of the Heart Outside fe: Holly, 161; Resuscita-
tion of the Heart in Situ, 164; Relationship of the Chemical Composition of the
Perfusion Fluid in Cold-blooded and Warm-blooded Hearts, 165.
CHAPTER XX
PIIYSIOLOGY OF THE HEARTBEAT . . . - 2 ve ooehtO
Origin and Propagation of the Beat, 170; ifyoqetiie Hypothesis, 171; Neuro-
genic Hypothesis, 172; The Pacemaker of the Heart and Heart- block, 174;
Physiologic Chuiackeristies of Cardiac Muscle, 176.
CHAPTER XXI
PHYSIOLOGY OF TIE HEARTBEAT (Cont’D) . eo = TSB
Origin and Propagation of the Beat in the Manimalian Heat, ‘182 5 @ondiuete
ing Tissue in the Mammalian Heart, 182; Site of Origin of Beat, ‘187.
CHAPTER XXII
PHYSIOLOGY OF THE HEARTBEAT (CONT’D) . . . F . 191
Mode of Propagation of the Beat in the ‘hieieles ara ie ithe deetdies te ‘the
Ventricles, 191; Spread of Beat in the Ventricle, 193; Fibrillation of the Ven-
tricles and Auricles, 195.
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CHAPTER XXTITI PAGE
THE BLOODFLOW IN THE ARTFRIES . . ..... . . . 198
The Pulses, 198; General Characteristics, 198; Rate of eityeenaniitaetnat of ‘Pie
Waves, 198; Contour of the Pulse Curve, 200; Velocity Pulse, 200; Palpable
Pulse, 202; Analysis of the Curve, 202; The Dicrotic Wave, 203; Causes of
Disappearance of the Pulse in the Veins, 205.
CHAPTER XXIV
RATE OF MOVEMENT OF THE BLOOD IN THE BLOOD VESSELS .. . . 206
Velocity of Flow, 206; Mass Movement of the Blood, 208; The Viseetal Blood-
flow in Man, 212; Work of the Heart, 212; Circulation Time, 213; Movement
of Blood in the Veins, 214.
CHAPTER XXV
THE CONTROL OF THE CIRCULATION ... ioe, eS
Nerve Control, 217; Vagus Control in dire Cola: plavded sat the ‘Mamuindiltan
Heart, 217; Tonie Vagus Action, 221; Afferent Vagus Impulses, 222; Mechan-
ism of Vagus, 224; Termination of the Vagus Fibers in the Heart, 995; Sym-
pathetic Control, 227.
CHAPTER XXVI
THE CONTROL OF THE CIRCULATION (CONT’D) . . . 229
Nerve Control of Peripheral Resistance, 2294 Dateatlon of Vaasmetes Fibers
in Nerves, 231; Origin of Vasomotor Nerve Fibers, 232; Vasomotor Nerve
Centers, 235; Independent Tonicity of Blood Vessels, 236.
CHAPTER XXVII
THE CoNTROL OF THE CIRCULATION (CONT’D) . . . . 237
Control of the Vasomotor Center, 237; Hormone Control, 237; Merve ential,
238; Pressor and Depressor Impulses, 239; Reciprocal Innervation of Vascular
Areas, 243; Influence of Gravity on the Circulation, 244.
CHAPTER XXVIII
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA . . . . 247
Circulation in the Brain, 247; Anatomical Peculiarities, 247; Physical Condi-
tions of Circulation, 249; Vasomotor Nerves, 252; Intracranial Pressure, 253;
Circulation through the Lungs, 253; Cireulation through the Liver, 255; The
Coronary Circulation, 257. : :
CHAPTER XXIX
CLINICAL APPLICATIONS OF CERTAIN PuysIOLoGiG METHODS ...... . . 259
Electrocardiograms, 259; The Ventricular Complex, 262.
CHAPTER XXX
CLINICAL APPLICATIONS OF CERTAIN PItySIOLOGIC METHODS (CONT’D) . . . 266
Electrocardiograms of the More Usual Forms of Cardiac Teapulacitiss, 266;
Sinus Arrhythmia, 266; Sinus Bradyeardia, 266; The Extrasystole, 266; Parox-
ysmal Tachycardia, 269; Auricular Fibrillation, 269; Auricular Flutter, 269;
Heart-block, 270.
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CHAPTER XXNI PAGE
CLINICAL APPLICATIONS OF CERTAIN PirysioLocic MrTHopsS (CONT’D) . . + ai 273
Polysphygmograms, 273; Venous Pulse Tracings, 273; Simultaneous Arterial
Pulse Tracings, 276; Abnormal Pulses, 276.
CHAPTER XXXII
CLINICAL APPLICATIONS OF CERTAIN PirysIoLocic MeTHops (ConT’D) . . . 281
Measurement of the Mass Movement of the Blood, 281; The Normal ‘How,
282; Clinical Conditions Which Affect the Bloodflow, 283.
CHAPTER XXXIII
SuHock ... bo c8, Geog oo WG - 287
Gravity Bice 287; Pietieahngs Shock, 288; Ananthatté Shock, 288; Spinal
Shock, 288; Nervous Shock, 289; Surgical Shock, 289; Bxpertnents In-
vestigation of Shock, 289; Treatment, 295; Cause of Shusadany Symptoms,
295.
PART IV
RESPIRATION
CHAPTER XXXIV
RESPIRATION . . . . 299
The Mechanics ae Respivation, 209; Piesuce and senwait of ‘Abe in é the Ties
299; Respiratory Tracings, 303; The Intrapleural Pressure, 304; Influence
on Blood Pressure, 306.
CHAPTER XXXV
THE MECIIANICS oF RESPIRATION (CoNT’D) (By R. G. Pearce) . . ‘ . 310
Variations in Dead Space, Residual Air and the Mid- and Vital Gapadties in
Various Physiologie and Pathologic Conditions, 310.
CHAPTER XXXVI
THE MECHANICS OF RESPIRATION (ContT’D) (By R. G. PEaRCE) . . » 815
The Mechanism of the Changes in Capacity of the Thorax and thes, 315;
The Movements of the Ribs, 315; The Action of the Musculature of the Ribs,
319; The Action of the Diaphragm, 320; The Effects of the Respiratory Move-
ments on the Lungs, 325.
CHAPTER XXXVII
Ture CONTROL OF RESPIRATION . . . . . . 327
The Respiratory Centers, 527; Reflex Contiel of thie, Reaptiatory Genter, 331.
CHAPTER XXXVIII
Tur CONTROL OF RESPIRATION (CONT’D) . . . . ww. tee ee oe we OOO
Hormone Control of the Respiratory Center, 335; Tension of CO, and O, in
Arterial Blood, 337; Tension cf CO, and O, in Alveolar Air, 339; Tension of
CO, in Venous Blood, 342.
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CHAPTER NXNXIX PAGE
TIE CONTROL OF RESPIRATION (ConT’p) (By R. G. Prarcr) 3 -
Estimation of the Alveolar Gases, 344; Method for Normal uly sees, 345;
Clinical Method, 347.
CHAPTER XL
Tire CONTROL OF RESPIRATION (CONT’D)
The Nature of the Respiratory Hormone, 349; Helatioaghip petuees co, “ee
Inspired Air and Pulmonary Ventilation, 350; Possibility that CO, Specifically
Stimulates the Center, 352; Relationship among Acidosis, Alveolar CO, and
Respiratory Activity, 354.
CHAPTER XLI
Tue ConTROL or RESPIRATION (CoNT’D)
The Constancy of the Alveolar CO, Tension ener Noriaal ‘Conditions, 256;
Sensitivity of the Center to Changes in the CO, Tension of the Alveolar Air,
357; Alveolar CO, Tension during Breathing in a Confined Space, 357, in
Rarefied Air, 360, and in Apnea, 362.
CHAPTER XLII
THE CONTROL oF RESPIRATION (CONT’D)
The Effect of Muscular Exercise on the Reapiation, 356.
CHAPTER rt
Tur CONTROL OF RESPIRATION (CONT’D) . . . ws
Periodic Breathing, 371; Types of Periodic Breathing, 371; Gisines of ‘Poriodie
Breathing, 372.
CHAPTER XLIV
RESPIRATION BEYOND TIE LUNGS
Transportation of Gases by the Blood, "379; “Dednapovtatien of Os gen, " 379;
Dissociation Curve, 383; Difference beeen Curves of Blood and Pane sonia
Solution, 383; Bate of Dissociation, 386; Dissociation Constant, 388.
CHAPTER XLV
RESPIRATION BEYOND TIE LUNGS (Coxv’D)
Means by Which the Blood Carries the Glands: 390; Onyeen Requirement of
the Tissues, 393; Mechanism by Which the Demands of the Tissues for Oxy-
gen Are Met, 397.
CHAPTER XLVI
THE PrysIoLOGy OF BREATIIING IN COMPRESSED AIR AND IN RAREFIED AIR
Mountain Sickness, 399; Compressed Air Sickness (Caisson Discase), 402;
Practical Application in Treatment, 406. ;
CHAPTER XLVII
THE CIRCULATORY AND RESPIRATORY CHANGES ACCOMPANYING MUSCULAR EXERCISE
Mechanical Factor, 410; Nervous Factor, 412; Hormone Factor, 415.
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. 349
. 856
. 366
871
. 378
. 890
. 399
410
CONTENTS xvii
PART V
DIGESTION
CHAPTER XLVIII PAGE
GENERAL PHYSIOLOGY OF THE DIGESTIVE GLANDS . . . . 418
Microscopic Changes during Activity, 418; Mechanism of Becidtion “420; ‘Other
Changes during Activity, 421; Gantrel of Glandular Activity, 422; Nervous
Control, 423.
CHAPTER XLIX
“PHYSIOLOGY oF THE DIGESTIVE GLANDS (CONT’D) . . . ia we oe ee ees
Hormone Control, 425; Nervous Control of the Pancreas, 427.
CHAPTER L
PITYSIOLOGY OF THE DIGESTIVE GLANDS (CONT’D) . . . & Ae dee eo
Normal Conditions of Secretion, 430; Normal Secretion ‘gf Saliv a, 431; Secre-
tion of Gastric Juice, 432; The Intestinal Secretions, 441.
CHAPTER LI
THE MECHANISMS OF DIGESTION . . . . 444
Mastication, 444; Deglutition, 445; The ‘Candide, Sihintton. 448; vontting,
449,
CHAPTER LIT
THE MECIIANISMS OF DIGESTION (CONT’D) . . . 451
Movements of the Stomach, 451; Character of the figsveriénte; 451; Effect
of the Stomach Movements on the Food, 454; Emptying of the Stonigch,
456; Control of the Pyloric Sphincter, 456; Rate of Emptying of the Stomach,
458; Influence of Pathologie Conditions on the Emptying, 450; Gastroenter-
ostomy, 461.
CHAPTER LIII
THE MECHANISMS OF DIGESTION (CONT’D) . . . ate ‘ - 463
Movements of the Intestines, 463; Movements of fhe Small Teteadine, 463;
Movements of the Large Tndextine, 468; Effect of Clinical Conditions on the
Movements, 470.
CHAPTER LIV
HUNGER AND APPETITE .. . - 471
Hunger Contractions of Stowach, 471; Remoto Bftects of Hiner ‘Gaibide,
tions, 474; Hunger during Starvation, 475; Control of the Hunger Mechanism,
476.
CHAPTER LV
BIOCHEMICAL PROCESSES OF DIGESTION . . . . 481
Digestion in the Stomach, 481; Functions: of fis Uydenehoite hae. 482;
Amount and Source of the Acid, 482; Action of Pepsin, 485; Clotting of
Milk in the Stomach, 488.
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CHAPTER LVI PAGE
BIOCHEMICAL PROCESSES OF DIGESTION (CONT’D) . . . 489
Digestion in the Intestines, 489; Pancreatic Digestion, 489 ; The Bile, 492;
Chemistry of Bile, 494.
CHAPTER LVII
BACTERIAL DIGESTION IN THE INTESTINE . .. as Wat ee es ee ee Ae Cae 9)
Bacterial Digestion of Protein, 501; Botulism, 503.
PART VI
THE EXCRETION OF URINE
CHAPTER LVIII
TuE EXCRETION OF URINE (By R. G. PEARCE) . ... . - 507
Structure of Kidney, 507; Mechanism of the Excretion of Trina, 510; "phisonies
of Renal Function, 511; Diuretics, 518; Albuminuria, 519; Influence of the
Nervous System on the Secretion of Urine, 519.
CHAPTER LIX
TuE AMOUNT, COMPOSITION AND CHARACTER OF THE URINE (By R. G. Pearce) . 521
Amount, 522; Specific Gravity, 522; Depression of Freezing Point, 523; Re-
action, 524; Solid Constituents, 525.
PART VII
METABOLISM
CHAPTER LX
METABOLISM .. . . 554
Energy Balance, 535; ‘Methods nor ‘Meaaring oe ‘Gait, 536; - Nowmal
Values, 538; Influence of Age and Sex, 541; Influence of Diseases, 542
The Material Balance of the Body, 543; Mathes for Measuring Output, nia,
Caleulation of the Results, 544.
CHAPTER LXI
TIE CARBON BALANCE . . . . 547
Respiratory Quotient, 547; Tafluense of Diet, 547; Tidinange of AMstabéliwin,
549; Magnitude of the Respiratory Exchange, 550; Influence of Body Tem-
perature, 551.
CHAPTER LXII
A CLINICAL METHOD FOR DETERMINING THE RESPIRATORY EXCHANGE IN MAN (By
R. G. PEARCE) . . oe Age DOE
The Valves, 555; Tissot Atkrornstars, 556; Douglas Bag, ‘558; Haldane Gas-
analysis Apparatus, 559; Calculations, 562.
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CONTENTS X1x
CHAPTER LNIII PAGE
STARVATION... . . . . 566
Excretion of Ritronen, 566; Bnergy Output, 563; Sirosenie Mctabolites, 568 ;
Excretion of Purines, 569; Excretion of Sulphur, 569; Normal Metabolism,
570; Nitrogenous Equilibrium, 571; Protein Sparers, 571.
CHAPTER LXIV
NUTRITION AND GROWTH . . Osage 0 BR ee ee WAS DES
The Food Factor of Grevth, 87 74; Relationship of Proteins to Growth and
Maintenance of Life, 574.
CHAPTER LXV
NUTRITION AND GROWTH (CONT’D) . . foe ce ee 2 SBS
Relationship of Carbohydrates’ and Fats to Guna, "583; eewegsons Food
Factors, or Vitamines, 584; Relationship of Inorganic Salts, 586.
CHAPTER LXVI
DieteTics ... . ioe snd . 588
Calorie eciuliecists, 588 ; The Protein Requirement, "590; ‘ eceaeey: oad
Factors, 593; Digestibility and Palatability, 593.
CHAPTER LXVII
Tite METABOLISM OF PROTEIN . . . . 595
Introductory, 595; Chemistry of Protein sii of the janine cide, 507.
CHAPTER LXVIII
Tue METABOLISM OF PROTEIN (CONT’D) . . Bee oy OR SRLS Be 7 O06
Amino Acids in the Blood and Tissues, 606; Fate of the Amino Acids, 610.
CHAPTER LXIX
Tir METABOLISM OF PROTEIN (CONT’D) . . . bo oR a OE Ge Ge we ae we, OTS
End Products of Protein Metabolism, 613; Urea and Ammonia, 615; In-
fluence of Acidosis on Ammonia-urea Ratio, 616; Influence of Liver on Am-
monia-urea Ratio, 617; Perfusion of Organs, 618; Clinical Observations, 620.
CHAPTER LXX
Tire METABOLISM OF PROTEIN (CONT’D) . . . . . . ey . 622
Creatine and Creatinine, 622; Essential Chemical Pacts, 622; Aesabolism,
624; Influence of Food, Age, and Sex, 621; Origin of Creatine and Creatinine,
626. :
CHAPTER LXXI
Tun METABOLISM OF PROTEIN (Conr’D) . . . 629
Undetermined Nitrogen and Detoxication Compounds, 629; Ethoveal Bul plates
and Glycuronates, 632.
CHAPTER LXXII
Uric ACID AND THE PuRINE BopIES . . . se oe = 634
Chemical Nature of the Purines, 634; Chentieal Taine. of ine Substances
Containing Purine and Pyrimidine Bases, 637; History of Nucleic Acid in the
Animal Body, 638; Balance between Intake and Output of Purine Substances
under Various Physiologic and Pathologic Conditions, 641.
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XX CONTENTS
CHAPTER LXXIII PAGE
Uric ACID AND THE PURINE Bopies (ContT’D) . . . . ee . 643
Source of Endogenous Purines, 643; Influence of asin Phiyatologie Ga:
ditions, of Drugs, and of Disease on the Endogenous Uric-acid Excretion,
647; Urie Acid of Blood, 648.
CHAPTER LXXIV
METABOLISM OF TIIE CARBOIIYDRATES . . - 652
Capacity of the Body to Assimilate Carbohydrates, 652; asein(lation Limite,
652; Saturation Limits, 654; Digestion and Absorption, 656; Sugar Level in
the Blood, 657; Value of Blood Examinations in Diagnosis of Diabetes, 659;
Relationship Between Blood Sugar and the Occurrence of Glycosuria, 660.
CHAPTER LXXV
METABOLISM OF THe CARBOHYDRATES (CONT’D) . . . 1 6 6 ee ee + + 662
Fate of Absorbed Glucose, Gluconeogenesis, 662; Storage of Sugar, 662;
Sourees of Glyeogen, 662; Gluconcogencsis in Normal Animals, 667.
CHAPTER LXXVI
METABOLISM OF THE CARBOHYDRATES (CONT’D) . . . . 669
Fate of Glycogen, 669; Regulation of the Blood Suse Level, 671; ecye
Control and Experimental Diabetes, 672; Nervous Diabetes in Man, 674;
Hormone Control and Permanent Diabetes, 676; Utilization of Glucose in
Tissues, 677; Relation of the Pancreas to Sugar Metabolism, 678; Diabetes
and the Ductless Glands, 678; Diabctic Acidosis or Ketosis, 683; Starvation
Treatment, 684.
CHAPTER LXXVII
Fat METABOLISM... é . 686
Chemistry of Fatty BiGhatenocs: 686; Dipestion of Fats, 690; ‘Kbsouption of
Fats, 691.
CHAPTER LXXVIII
Fat METABOLISM (CONT’D) . . . 696
Fat of Blood, 696; Methods of Detarnintion; ‘696; ‘Variations in “Blood Fat,
697; Depot Fat, 700; Fat in the Liver, 701.
CHAPTER LXXIX
Fat METABOLISM (CONT’D) . . . « 107
Production of Fatty Acid Out of Guitahadeate: 707; Method by Which the
Fatty Acid is Broken Down, 709.
CHAPTER LXXX
CoNTROL OF BoDy TEMPERATURE AND FEVER .. . . 714
Variations in Body Temperature, 714; Factors in Maintaining hs Boily- Tem-
perature, 715; Control of Temperature, 719; Fever, 721; Causes, 721; Changes
in the Body during Fever, 723; Heat-regulating Center, 725; Significance of
Fever, 726.
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CONTENTS Xxi
PART VIII
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
CHAPTER LXXXI PAGE
THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS .. . aed a ed
Methods of Investigation; 730; Adrenal Gland, 731; Cortex, 731; Medulla,
732; Adrenalectomy, 733; Suprarenal Extracts, 734; Physiologic Action, 734.
CHAPTER LXXXII
ADRENAL GLAND (CONT’D) . . . ay Sloe . 738
Variations in Physiologic ‘detigtty "738; Assaying tg Rpinepliring. Content
of the Gland, 738; Epinephrine Content of the Blood, 739; Autoinjection
Method, 743; Adrenalemia, 745; Association of the Adrenal with Other Iin-
docrine Organs, 746.
CHAPTER LXXXIII
THYROID AND PARATHYROID GLANDS . . . oe ew » 749
Structural Relationship, 749; Thyroid Ghani 750; Gondiden of Gland, 750;
Experimental Phy-rotdectouty, 752; Disease of the Thyroid, 753; Relation
with Other Endocrine Organs, 757; Parathyroids, 758; Experimental Parathy-
roidectomy, 758; Relationship with Other Endocrine Organs, 761.
CHAPTER LXXXIV
Piruirary Bopy .. . i sls ee ae ae She ee sy ee Sa Mg GS
Structural Halikiaashine: "762 ‘ uaaiions 764; Clinical Characteristics, 771;
Relationship with Other Bindoerine Organs, 773.
CHAPTER LXXXV
THE PINEAL GLAND AND THE GONADS .. . . 776
Pineal Gland, 776; Gonads or the Gevertize Or gans, 776; Generating Gina
of the Male, 776; Ganarative Organs of the Female, 778.
PART IX
THE CENTRAL NERVOUS SYSTEM
CHAPTER LXXXVI
THE EVOLUTION OF THE NERVOUS SYSTEM .......... 2... . 781
CHAPTER LXXXVII
PROPERTIES OF EACH PaRT OF THE REFLEX ARC... { . . 788
Receptor, 788; Epicritic and Protopathic Receptors, 790; Peculiniitien of the
Separate Sensations, 791; Temperature, 791; Touch, 793; Pain, 795.
CHAPTER LXXXVIII
THE PROPERTIES Or EActt Part OF THE REFLEX ARC (CONT’D) . . . - 796
The Nerve Network, 796; Network on Skin’ Nerves, 796; The Siniaipala, 797;
The Nerve Cell, 799; The Intermediate or Internuncial Neuron, 802.
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Xxii CONTENTS
CHAPTER LXXXIX
REFLEXES OF THE SPINAL ANIMAL AND SPINAL SHOCK .. . : - 803
Spinal Shock in Laboratory Animals, 803; Spinal Shock in Mew, 806; ‘Gunse
of Spinal Shock, 807.
CHAPTER XC
PHYSIOLOGIC PROPERTIES OF THE SIMPLE REFLEX ARC ... . - 809
Latent Period, 809; Grading of Intensity, 809; After-effect, 810; Summation,
810; Irreversibility of the Direction of Conduction, 810; Refractory Period,
gil; Successive Degeneration, 813.
CHAPTER XCI
ReciprocaL INNERVATION . . ot see dal Seine a ae ote . . 814
Reciprocal Inhibition, 814; Action of Strychnine aia Betis Toxin, 819.
CHAPTER XCII
INTERACTION AMONG REFLEXES .. . 1G fel or one . 821
Integration of Allied Reflexes, 822; Tategeation of Antagonistic Randa:
824; Other Factors Which Determine Occupancy of Final Common Path, 824;
Irradiation, 826.
CHAPTER XCTIITI
Tue TENDON JERKS; SENSORY PATHWAYS IN SPINAL CorD . . . . . . . . . 828
The Teudon Jerks, 828; Afferent Spinal Pathways, 830.
CHAPTER XCIV
EFFECTS OF EXPERIMENTAL LESIONS OF VARIOUS Parts or TIE NERVOUS SYSTEM . 835
Anterior Roots, 835; Posterior Roots, 836; Spinal Cord, and Brain Stem, 839;
Medulla, 839; Corpora Quadrigemina, 840; Removal of the Cerebral Hemi-
spheres, 840.
CHAPTER XCV
CEREBRAL LOCALIZATION. . . « 843
Ablation of the Motor Cexkers, 843; Stinilation ‘of Bid ‘Motor. Catitors, S44;
Clinical Observations, 849.
CHAPTER XCVI
CEREBRAL LOCALIZATION (CONT’D) . .. . : ve = & 5 800
Sensory Centers, 850; Sense Centers, 851; ‘Angociatinn ‘Accas, 852.
CHAPTER XCVII
CONDITIONAL AND UNCONDITIONAL REFLEXES . . . «. . « «© « « « we + + 856
CHAPTER XCVIII
HIGHER FUNCTIONS OF TIIE CEREBRUM IN Man; APHASIA . . . . . . . . . 860
Psychopathological Applications, 862.
CHAPTER XCIX
FUNCTIONS OF THE CEREBELLUM . . : ts . 865
Localization of Function, 867; Gircumpertbed Hixtivpalion, 869; Clinical Ob-
servations, 870.
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CONTENTS xxiii
CHAPTER C
THE CEREBELLUM AND THE SEMICIRCULAR CANALS; FUNCTIONAL TESTS . » 873
Association between the Eye Movements and the Semicircular Canals, 875.
CHAPTER CI
Ture AuTONOMIc NERvouS SYSTEM ...... 7 ee BOTT
General Plan of Construction, 877; Thoracicolumbar Outflow, or Sympathetic
System Proper, 880; Bulbosacral Outflow, or the Parasympathetic System, 882;
Axon Reflexes, 883; Functions of Autonomic Nerves, 884; Afferent Fibers of
the Autonomic System, 885.
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ILLUSTRATIONS
FIG. PAGE
1. Diagram of osmometer . 2 2. 1 eee ee ee ee ee eee CS
2, Hematocrite . . 2... 1. wwe ee Bie SB a a ied
3. Plasmolysis in cells from Tradescantia discotot Bh ee OE 8 a ee ap a 29.
4. Apparatus for measurement of the depression of freezing point “ot solution . 11
5. Diagram of conductivity cells . ... ol oe Gee oe AS
6. Wheatstone Bridge for the measurement of alaateie reutstaniee gfe: ee So cae AS
7. Diagram to show type of electrodes used in studying electromotive force . . 30
9. Chart of tints as used in colorimetric measurement of H-ion concentration.
(Color Plate.) . . . . . 34
8. Diagram of apparatus for ceca measur srernent ae the i -ion ‘guiesntraion ey 28d
10. Diagram of apparatus for saturating blood and plasma with expired air . 43
11. Van Slyke’s apparatus for measuring the CO,-combining power of blood in
blood plasma .. . ety ashi teio atey aries esas a se Man a ae ay AA
12. Ultramicroscope (slit type) “for the examination of olloidel solutions . . 52
13. To show diffusion into gelatin of a crystalloid stain, and the nondiffusion
of a colloid stain . . . ‘ Sy a - . 53
14, Diagram from W. Ostwald showing the blative size of various panhieles and
colloidal dispersoids compared with a red blood corpusele and an
anthrax bacillus . ¢ 6 6 © a we 6 8 ee 8 ee we ww OB
15. Capillary analysis of colloids . ......... 2.4 464+. + 56
16. Diagram to show structure of gels . . ©. 2. 2. ee ee ee ee ee 6261
17. Diagram to illustrate surface tension . . ~~... . 1... ee ss 64
18. Traube’s stalagmometer . . . . 2. 1 6 ee ee ew ee ee 65
19. Diagram of the graphic coagulometer iis (any Soren See ite ee Re ae Se ise 109
20. Coagulometer . . . mPa s. is « » 110
21. Mercury manometer oF sina figenel, arianied for racondliris the mean ar-
terial blood pressure in a laboratory experiment ... . . 124
22. The arterial blood pressure recorded with a mercury manometer Ciomer tial
ing) along with a tracing of the respiratory movement of the thorax . 125
23. Hiirthle’s spring manometer . .... . B) Stoo Ao Goud oe #126
24, Arterial pressure recorded by a spring en eo . 126
25. Diagram based on experiments ou dogs to show the setalie, diastonte and
mean blood pressures at different parts of the circulatory system . . 127
26. Apparatus for measuring the arterial blood pressure in man . .. . . . 129
27. Effect of cutting the vagus nerve on the arterial blood pressure . . . . . 135
28. Effect of stimulating the peripheral end of the right vagus on the arterial
blood pressure... ea eee - 136
29, ffect of stimulation of the lett leashes nerve on the anterial ‘load pres-
sure. . PNW ES BAe eee. cee. eS a> Go od BT
30. The effect of — and slow hemorrhage on ‘ie neue blood pressure . . 138
31. Diagram of experiment to show that the diastolic pressure depends on the
elasticity of the vessel wall . . 2. 1 ee ee ee ee ew + 148
32. Diagram of Wiggers’ optical manometer . . cm ag Eh ee 2d
XXV
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XXV1 ILLUSTRATIONS
FIG.
33.
34
35.
36.
37.
38.
39.
40.
41,
42.
43,
44.
45.
46.
47.
48,
49.
50.
51,
52.
53.
54.
55.
56.
57.
58.
60.
61,
62.
63.
64.
65.
66.
PAGE
Optical records of intraventricular pressure . . ..... +... 47
Superimposed pressure curves after being graduated . . . . be epi ED
Von Frank’s maximal and minimal valve, which is placed in tie course of
the tube between heart and mercury manometer . ..... . + 152
Diagram to show the positions of the cardiac valves . 155
Diagram showing the position of the cardiac chambers and valves during
presystole and during the sphymic period . . . . 1 1 1 ws e . 156
Electrophonograms along with intraventricular pressure curves from three
different experiments ........ ...+4868 4. . 159
One form of apparatus for recording tracings from an excised heart . . 163
Volume curve of ventricles of cat (lower curve) in a heart-lung. perfusion
preparation . . ey aw Se ae a ee OO
Heart and cardiac nerves a Titenals olgbanias alg ost a See Se Sa oe DNS:
Heart-block produced by applying clamp .. . » . 2175
Tracing of contraction of ventricle, showing the effect of the ‘baal appli-
cation of heat to the auricle . . . i oa we Oe AID
Frog heart showing the position of the first ‘ait | ealares of Stannius 176
Effects of stimuli of increasing strength on skeletal and cardiac muscle to
illustrate the ‘‘all or nothing’’ principle in the latter . . .. . 177
The effects of successive stimuli on skeletal and cardiac muscle to show the
prominence of the staircase phenomenon, or treppe, in the latter . 178
The effects of successive stimuli and of tetanizing stimuli on skeletal muscle
and cardiac muscle ........ aw 0 E79.
Myograms of frog’s ventricle, showing effect of ‘exeliation by breale indue-
tion shocks at various moments of the cardiac cycle . . . . . « . . 180
Heart of tortoise as suspended a4 . 183
Dissection of heart to show sateuloventriodiay bundle aoe SS » . « 184
Photograph of model of the auriculoventricular bundle and its Sinidlbatiore,
constructed from dissections of the heart ... . see 6 184
Diagram of an auricle showing the arrangement of the iamatile andes the
concentration point; and the outline of the node ..... . . . 186
Diagram to show the general ramifications of the Sear tissue in the
heart of the mammal ...... ° ~ + 2 » 186
Diagram to illustrate the development and — of tthe wave of negativity
in a strip of muscle (curarized sartorius) when stimulatéd at the end . 188
Simultaneous electrocardiograms to show the cause for extrinsic deflections 190
Diagram of experiment by Lewis showing the times at which the excitation
wave appeared on the front of the heart : . 194
Diagram of Chauveau’s dromograph . 200
Diagram to show principle of Pitot’s tubes for measuring gelehe: aise . 201
Dudgeon’s sphygmograph ar . 201
Pulse tracing (sphygmogram) taken iy eigenen . 202
Forms of apparatus for measurement of blood velocities . . 207
Plethysmograph for recording volume changes in the hand and Poirenert . 210
Simultaneous tracings from auricle and ventricle of turtle’s heart . 218
Effect of vagus stimulation on heart of turtle ee ee ee . 218
Tracing to show that vagus stimulation may diminish transmission from
auricles to ventricles . 2. 1 1. 6 1 ew ee ee ew ew . 219
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ILLUSTRATIONS XXvii
FIG. PAGE
67. Tracing to show that vagus stimulation may facilitate transmission from
auricles to ventricles . . . ee. Bo Tebviet, “ae sb rae SMe eee
68. Diagram to show the innervation of ths iene in the frog or turtle. (Color
Plate.) . oe % . 3 Bs BS Gy CUNT se CORY We es. Te ae
69. Frog heart paatug sliowtiae fy ‘eso of nicotine . . . . .. . . . 226
70. Schematic representation of the innervation of the heart of the mammal.
(Color Plate.) . . . . 226
71. Tracings showing the effects. on the haairthest of ‘the tee vemulting froin
stimulation of the sympathetic nerves prior to their union with the
Vagus NERVE: 6 gow zo a ak Rw WO a A ae a ee a DB
72. Roy’s kidney oncometer . . . . i! dee aI an a oe ee. 230
73. Fall of blood pressure from deatiattinn of ‘thie depressor nerve . . . 289
74, The effect of strong stimulation (heat) of the skin of the foot on ithe ar-
terial blood pressure and respiratory movements ...... . . 241
75. Diagram showing the probable arrangements of the vasomotor reflexes . 242
76. Aortic blood pressure, showing the effect of posture .. . « . «245
77. Tracing to show the effect of gravity on the arterial blood pressure . . 245
78. The effect of gravity on the aortic pressure after division of the spinal
cord in the upper dorsal region . . . . . » . » 246
79. Schema to show the relations of the Pacchionian hotles i ‘tie sinuses . . 248
80. Tracing showing simultaneous records of the arterial blood pressure, the
venous pressure, the intracranial pressure, the pressure in the venous
sinuses . . i at tar a anaes ee ee oe ae we SET
81. Hicotroeasdlopeaplie Sy aaee as ‘tintie iy the Gate tae Scientific Ma-
terials COs: a. Gs RR BO es RS a Ge Se ce Sw BED
82. Normal electrocardiogram . . . Z . 261
83. Electrocardiogram (dog) taken staruttarradnsty with curves droit auisisle andl
ventricle . 6 1. ee ke es re Be . Raph gs a . « 262
84, Records of electrocardiogram and radoinend of sadicinls of frog shes
that when the apex is warmed a typical T-wave appears in place of a
wave in the opposite direction appearing when the apex is cooled . . 264
85. Sinus bradycardia . . 2. 2. 2. 1 ee ee we ee we ee ew we 267
86. Auricular extrasystole . . . eve oe we ale Gh 2EF
87. Ventricular extrasystoles arising in tite right ventniola ge Gh: er ih Re ee
88. Ventricular extrasystole arising in the left ventricle . . ... . . . 267
89. Paroxysmal tachycardia . . . 2. 2. 1 1 1 ee ee we ew we we 268
90. Auricular fibrillation . 2. 2. 1. 1. 1 ee ee ee we we ew ws 268
91. Auricular flutter 2. 1 ww ek ke ee ee ee ee ee 270
92. Delayed conduction . . . . 1... ee ee ee ee ee et we 270
93. Partial dissociation . 2. 2. 1. 1 1 we ee eee ee ee ee B71
94. Complete dissociation . . 2. 2. 1 1 ee ee ee ee eee we OI
95. Polysphygmograph . . 1. 1... we ee ee ee ee ee ee we Bh
96. Normal jugular tracing . . ars . . 274
97. Reduced tracings from carotid, suite: genkeaie: avid and jiilan: ie stow
the general relationships of the various waves . . . . +. +. +. + 275
98. Polysphygmograms including jugular, apex and radial tracings . . . . 275
99. Delayed conduction time . . . . 2 1. 2. ee eee wee ee BIT
100. Dropped beats . . . oan Fags 9. tr al a oi ae etd RE
101. Premature beats uibracpatoles) vanttiontan i in origin . . ..... . 278
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XXVili ILLUSTRATIONS
FIG.
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103.
104,
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106.
107.
108.
109.
110.
111.
112.
113,
114,
115.
116.
117.
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119.
120.
121.
122.
123.
12-4,
125.
126.
127.
128.
129.
150.
131.
132.
PAGE
Paroxysmal tachycardia SG ity ceetetecnas) ee Shi. tBeecie: oS She ety a ee canes RON
Auricular ‘flutter 6 8 ow ee eR ea we ew TD
Auricular flutter a iS Nee wep tees ane a een gh 3 . oe . 279
Auricular fibrillation . . . . . ee . 280
Showing the appearance of the bios veaséls in tthe ears oe a ee in
a state of deep shock. (Color Plate.) . . . . . 290
Diagram showing amounts of air contained by tis enue in various piesen
of ordinary and of forced respiration . . ....... .-. . 301
Pneumograph . .. . oR . 303
Body plethysmograph for “veenrting séuphiation . 304
Effect of abdominal and chest breathing on the pulse and ‘toed pressure
of man . . . eae
First dorsal wettabes, sixth Wout ride aid it: Axis of sdbatton ieee
- 308
in each case . 316
Lower half of the fiona “From, the ‘6th dorsal to the “4th vartolvi, seen
from the front . . . . ae gO OO Jt ah, Te BO Be ew ess SES
Intercostal muscles of 5th and 6th spaces... Ree 8 . 319
Hamberger’s schema to demonstrate the functional smnlamentent of internal
and external intercostals . . . . .... owe oe we eo OLD
Schema to demonstrate that the function of the internal intercar-
‘tilaginous intercostals is identical with that of the external in-
terosseous intercostals . . . . . 320
Diagram to show the effect of high a law pasion of the Gaphragia
on the costal angle-. . . ae MS alkaline earths > alkalies;
and the anions:
SO, > Cl > NO, > Br > I > CNS.
The influence of a given electrolyte varies extraordinarily with the reac-
tion of the colloid, a fact which must be carefully regarded in all work
in this field.
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CHAPTER VIII
COLLOIDS (Cont’d)
SUSPENSOIDS AND EMULSOIDS
According to whether colloids form solutions that are more or less
viscid than the suspension medium, they are divided into emulsoids and
suspensoids. Examples of the former class are silicates and gelatin, and
of the latter, dialyzed iron and arsenious sulphide. The following char-
acteristics are used to distinguish between suspensoids and emulsoids:
1. Measuring the time it takes, at a standard temperature, for a given
volume of the fluid to flow out of a standard pipette (10 c.c.) shows the
viscosity to be, roughly, inversely proportional to the time of outflow. In
the case of suspensoids the viscosity is no different from that of the
dispersion medium alone, and does not vary much when the solution is
cooled. The viscosity of emulsoids even in very dilute solutions is, on
the other hand, considerably greater than that of the dispersion medium
itself, and it becomes greatly increased by cooling.
2. Suspensoids are much more readily coagulated by the addition of
electrolytes than emulsoids. This is particularly true when water is
the dispersion medium (so-called hydrosols), and when electrolytes hav-
ing a polyvalent ion (such as Al or Mg.) are employed. Thus, practically
all suspensoids are coagulated in the presence of 1 per cent of alum,
which has no influence on emulsoids. We shall return to this phase of
our subject later on.
The division of colloids into emulsoids and suspensoids is more or less
arbitrary, since one class may be changed into the other, the determining
factor being the water content of the dispersoid. The water content of
suspensoids is low (lyophobe), while that of emulsoids is high. By
changing the relative amounts of water and solid of which a colloidal
solution is composed, the nature of the dispersoid may be changed. If
the water is diminished,. the dispersoid behaves as a suspensoid and be-
comes readily precipitated. The practical importance of this fact is
that it explains the salting out of proteins—a process extensively used
in their separation. Ordinarily these behave as emulsoids, but the addi-
tion of salt raises the osmotic pressure of the dispersion medium, and
thus attracts water from the dispersoids, with the result that they come
60
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COLLOIDS 61
to behave as suspensoids, and are accordingly precipitated by the elec-
trolytes.
Another property of emulsoids of biological importance is the pro-
tection which they can afford against the precipitating influence of
electrolytes on suspensoids. If a colloidal solution of gold is mixed with
a trace of gelatin, the subsequent addition of salts will be found to
produce no precipitation. The explanation of this is that the emulsoid
bécomes distributed as a film on the suspensoid particles, thus practically
converting them into emulsoids.
Gelatinization
One of the best known properties of emulsoids is that of gelatiniza-
tion, which has an interesting bearing on many problems of biology.
After the gel has set, an enormous pressure is required to squeeze out
any water from it, indicating that the water no longer forms the con-
tinuous phase but must be enclosed in vesicles formed of more solid
material.
A
eses,
Ca%e
Oa%
B
Fig. 16.
As a gelatin solution cools, the gel at first forms a polarized cone of
light, but the very fine particles which are responsible for this effect
soon increase in number and size so that they obstruct one another in
their Brownian movements and adhere, giving an appearance of fine
felt-like threads throughout the solution. A sort of impervious sponge
work of the more solid phase is therefore formed, the more fluid phase
being inclosed in the meshes.
If, as in the accompanying diagram, the dispersion medium is repre-
sented by white and the dispersoid in black, the relationship between
the two in a suspensoid is as in A, and that in a gel as in B. To express
any of the dispersion medium in B, it will require a pressure sufficient to
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62 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
cause the more fluid phase to penetrate the more solid. If the gel is
treated with reagents like formaldehyde, the liquid can be readily pressed
out. This occurs during fixation for histological purposes.
Imbibition
Closely related to gel formation is the process of imbibition—the
power of taking up large quantities of water without actually forming
liquid solutions. Besides gelatin the dried tissues of plants and animals
exhibit the phenomenon, and it is undoubtedly of importance in many
physiologic processes such as growth and the passage of water into
cells, ete. The materials present as vacuoles in plant cells attract water
from the environment of the cell by imbibition, and thus exert on the
cell wall a pressure which, acting along with the osmotic pressure,
maintains the turgor of the cell. The initial growth of pollen is also
dependent upon imbibition, and important observations on this process,
under varying conditions, are likely to furnish us with useful informa-
tion concerning the significance of imbibition in connection with growth
of cells in general.
By measuring the rate of increase in length of long, narrow strips of
gelatin placed in Petri dishes containing solutions of varying composi-
tion, the factors that influence the imbibition process can be quantita-
tively investigated. Working in this way, F. H. Lloyd’? has found that
for all acids there is a certain concentration (about N/320 H,SO,) which
induces a maximum rate of swelling, and another, much weaker
(N/2800 H,SO,), in which the rate of swelling is even less than in pure
water. In higher concentrations of acid than N/320, the gelatin at first
swells very quickly, but the rate slows off so that it soon comes to be
less than that with intermediate concentrations. Regarding ‘alkalies,
at high concentrations the effect is similar to that of‘acids. Salts alone
seem to repress the swelling below that of water. It should be pointed
out that the concentrations of acid and alkali in the above observations
are much greater than those that could occur in the animal body. The
experiments recall the attempts made some years ago by Martin Fischer
to explain edema as due to excessive imbibition of water by. the pro-
teins of the tissues because of increased acidity of the blood and tis-
sue fluids. That imbibition might possibly play some role in such
processes is not denied, but Fischer disregards entirely the now well-estab-
lished facts that hydrogen-ion concentration is one of the most constant
properties of the blood, that very low concentrations of acid may dimin-
ish rather than increase imbibition, and that it is manifested only in
the absence of inorganic salts.* Moreover, the fluid in edema can often
*Determinations of the hydrogen-ion concentration of the blood recently published from Fischer's
laboratory do not inspire confidence.
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COLLOIDS 63
be drained off by hollow needles, and it passes by gravity from one part -
of the blood to another, neither of which processes would be possible
if imbibition were the essential factor concerned. If further evidence
against this hypothesis should be demanded, it might be found in the
utter failure of the therapeutic measures—alkali administration—that
are. recommended to combat the edema.
Action of Electrolytes on Colloids (apart from their effect on osmotic
pressure).—It has been stated above that the charge which a colloidal
particle assumes may be neutralized by a charge of opposite sign car-
ried by an ion present in the dispersion medium. The neutralization
of the electric charge causes coagulation of the suspensoids but not of
the emulsoids. Of the positive and negative ions into which the elec-
trolytes dissociate, the one producing the coagulation is that which is
opposite in sign to the electric charge of the colloidal particle.
A quantity of electrolyte which is capable of producing complete pre-
cipitation when added all at once to suspensoids will be ineffective when
added in small quantities at a time. This phenomenon, which is also
known to be exhibited when toxins and antitoxins are mixed together, is
probably owing to the fact that precipitation depends on inequality and
irregular distribution of electric charges, a condition which becomes
established when the electrolyte is suddenly added, but not so when it
is gradually added. The particles in the latter case become, as it were,
acclimated to the electric charges introduced by the addition of the
electrolyte.
Proteins as Colloids.—The most prominent colloids in the field of bio-
chemistry are the proteins. On account of complexity of structure,
however, certain factors intervene which render the investigation of
their behavior very difficult. As we shall see later, proteins are made
up of combinations of amino acids, each of which contains basie (NH,)
and acid groups (COOH). .The various amino acids are linked together
in protein by the COOH of one uniting with the NH, of another, with
the elimination of water—thus, CO ;OH+H; HN—but some NH, and
COOH groups are left uncombined. According to the relative number
of these uncombined radicles, the protein (or polypeptid, page 601)
will exhibit faintly acid or basic or neutral properties. With acids, for
example, a salt will be formed by union with the NH, groups, which will
dissociate into the anion of the acid and a large organic cation; whereas
with alkalies union will oceur with the COOH group, and the salt on
dissociating will form a small cation of the metal of the salt and a large
complex anion. We may therefore obtain the protein with either a
positive or a negative electric charge by altering the chemical nature of
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64 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
the fluid in which it is dissolved, so that the reaction towards other
colloids and towards electrolytes will vary.
One feature of proteins of importance in this connection is that known
as the isoelectric point, at which the protein exists with a maximum of
electrically neutral molecules. This point is reached by adding acid to
a protein solution. -The acid represses the dissociation of the protein
acting as an acid, and therefore diminishes the number of free hydrogen
ions; and at the same time it combines with the NH, groups and neutral-
izes the basic characteristics. The alteration in electric charge thus in-
duced alters the water-absorbing powers of the protein and therefore
all of the properties which we have seen to be associated therewith
(page 63).
SURFACE TENSION
Before we consider a very important property of colloids known as
adsorption, by means of which they are able to perform many reactions
that do not conform with the laws of mass action, it will be well to
Fig. 17.—Diagram to illustrate surface tension. The rings A and B inclose soap films in
which a very fine loop of silk is suspended. In A it is loose but in B, where the film inclosed
in the loop has been broken, it is drawn into a circle by the tension of the soap film. (From
Bayliss.) :
say a few words concerning the physical phenomenon upon which this
depends—namely, surface tension. The creation of this force is due
to the fact that, whereas the molecules within a liquid are subjected to
equal forces of attraction on all sides, at the surface these forces act on
one side of the molecules only, and therefore tend to pull them inwards.
This causes the surface to pull itself together so as to occupy the least
possible area, and it is this force which constitutes surface tension.
The surface behaves as if stretched. There are various simple experi-
ments that reveal the presence of surface tension. If a film is made on
a loop of wire by dipping it in soap solution, a fine silk thread ean be
floated in the film, so that it forms a loop that is quite loose. If the
portion of the film inside the loop is destroyed by touching it with filter
paper, the film will break in the loop, which will now be pulled into a
circular shape by the tension of the film around it (Fig. 17).
For the measurement of surface tension, various methods are used.
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COLLOIDS 65
The size of drops of liquid falling from an orifice is dependent on sur-
face tension; the larger the drops, the greater the surface tension. If
the number of drops obtained by allowing a liquid to-drop from a stand-
ard orifice in a given time is counted, we have a measure of the surface
tension. Account must of course also be taken of the specific gravity
of the liquid. The instrument used for this purpose is called a
stalagmometer (Fig. 18). Another method depends on the fact that
the height to which a fluid rises in a capillary tube is dependent on
surface tension (and inversely on the diameter of the capillary). The
difference in the heights to which two liquids rise in capillary tubes of
known bore permits us to compare their surface tensions, and if this
is known for one of the solutions, it can be determined for the other.
Besides existing between liquid and air, surface tension also exists at
the interface between two immiscible liquids, and at that between sus-
Fib. 18—Traube’s stalagmometer. The surface tension is proportional to the number of
drops formed in a given time. ‘he right-angled tubes are for thin liquids, and the straight
one for blood and other viscous fluids.
pended solid particles and liquid, as in colloidal solutions. Since, as
we have seen, the surface area (interface) is enormously increased in
these solutions, a very great surface energy is present, for this is equal
to the surface tension multiplied by the surface area.
ADSORPTION
The surface tension between liquid and air is lowered when organic
substances are dissolved in the liquid, but is slightly raised when inor-
ganic salts are dissolved. The degree of lowering varies markedly ac-
cording to the organic substance dissolved, being very pronounced with
bile salts, upon which fact the well-known (Hay) test for the presence
of bile in urine is based. Between liquid and liquid, as well as between
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66 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
solid and liquid, the surface tension is always lowered by dissolving sub-
stances in the liquid. Now, at the interfaces between solid particles and
liquid there must be a local accumulation of free surface energy, which
will be equal to the surface tension multiplied by the surface (inter-
face) area. A constant tendency exists for such free’ energy to be de-
creased and, since dissolved substances have this effect; they will become
concentrated at the interface. This is known as the principle of Willard
Gibbs, and it is of fundamental importance to the biochemist, because
on it depends the phenomenon known as adsorption, which in the case
of colloidal solutions may therefore be defined as the local concentra-
tion or condensation of dissolved substances at the interface between
the two phases. The amount of substance concentrated at the interface
can be calculated by a formula which takes into account the concentra-
tion of the dissolved substance, the temperature, and the surface tension
at the interface (the'Gibbs formula). After absorption has occurred, vari-
ous reactions of a chemical, electrical or purely physical nature (e. g., dif-
fusion) may follow at a rate which depends on the amount of the
condensation. -
Every-day Reactions Which Depend on Adsorption
1. Decolorization of liquids by charcoal. That no chemical reaction oc-
curs in such a case is readily shown by the ease with which the pigment
can be extracted from the charcoal.
2. Adsorption of gases by such solids as charcoal and spongy platinum.
In these cases there must be great condensation, even a liquefaction of the
gas, during which heat must be evolved. By adsorbing oxygen and hydro-
gen, spongy platinum causes these gases to combine and form water. The
hemoglobin of blood may take up oxygen by a similar process.
3. Formation of solid surface films on solutions of protein, saponin, ete.
The condensation may lead to coagulation, which explains why,:if the
froth produced by beating the white of an egg is allowed to stand, it can
not be again beaten into a froth, the albumin having gone out of solution
by surface coagulation.
An interesting phenomenon depending on the surface tension occurs
when the protoplasmic contents of a ciliated infusorian is pressed out in
water. A new membrane forms on the protoplasm. because of surface con-
centration of all constituents which lower surface energy. By application
of the principle of Willard Gibbs, A. B. Macallum** concludes that not only
adsorption, as exhibited in a colloidal solution, but also the local accumula-
tions of material often seen in cells, are associated with changes in sur-
face energy. His conclusions are based largely on microscopic studies
of various forms of cell exhibiting different degrees and types of activity,
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COLLOIDS ; 67
and ingeniously stained for potassium by cobalt hexanitrite. By such
a means the potassium stains intense black. In vegetable cells, local
accumulations of potassium occur either near the interface between the
clear and the chlorophyl-containing parts of the cell (spirogyra) or
under a portion of the cell wall from which later a protrusion grows out
to form the first stage in conjugation. The outgrowth from the cell,
as well as the accumulation of potassium, may be the result of a low
surface tension. In unicellular animal organisms, such as Vorticella,
much less potassium is present, being confined to the base of the cilia,
which Macallum believes indicates that the structures are produced as
an outcome of low surface tension.
In the cells of higher animals, deposits of potassium are also localized ;
in striated muscle, for example, they occur in a zone at each end of the
doubly refractive band and immediately adjacent to the singly refrac-
tive band. Changes in surface tension, associated with changes in the
distribution of potassium, are believed by many te be responsible for
muscular contraction. In nerves and nerve cells, potassium is concen-
trated at the axon and at the surfaces of the cells. Interesting sugges-
tions are offered to explain the relationship among changes in surface
tension at the terminations of axons (synapses, terminations in gland and
muscle cells) brought about by the nerve impulse acting as a change in
electric potential. Surface condensation of potassium has also been
observed at the lumen border of gland cells (pancreas), and on the lu-
men surface of the cells of the renal tubules. Such observations indicate
in what way surface tension may be called into play to control cellular
activities. The field is new and almost unexplored, but there is already
much to indicate that surface energy plays a most important role in the
performance of many cellular activities.
Conditions That Influence or Are Influenced by Adsorption
Electric Changes.—Besides mere concentration, other forces come
into play to assist or retard adsorption. One of the most important of
these is electrical. Most solids when present as particles in a fluid carry
a negative charge of electricity, some a positive one. In conformity with
the Willard Gibbs law, a constant tendency will exist for this free energy
to be diminished by the neutralization of the electric charge. This can
oceur by deposition on the interface of other particles carrying an
electric charge of opposite sign or by the action of that present on ions.
Charcoal in suspension in water, for instance, has a negative charge.
If colloidal iron, which has a positive charge, is added to the solution, it
will become deposited on the charcoal, as will also the cations of an
inorganic salt. On account of electric adsorption, dyestuffs and bile
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68 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
salts are adsorbed much more freely than they would be if the process
depended solely on surface condensation; that is, if the Gibbs formula is
used to calculate the adsorption, it will give values that are much below
those actually found.
‘If the dissolved substance and the particles both have the same electric
sign, adsorption will not occur. Filter paper, for example, has a nega-
tive charge and can not therefore adsorb a negative dye such as congo
red (as shown by the depth to which it becomes stained) ; whereas it
readily adsorbs night blue, which is positively charged. If the negative
charge of the paper is lowered, it becomes capable of adsorbing some of
the negative congo red. This can be effected either by placing the paper
in alcohol or by adding inorganic salts (NaCl) to the water with which
it is in contact. The positive-charged ions of Na, produced by dissocia-
tion, neutralize some of the negative charge on the paper, and allow a
certain amount of adsorption of the negative-charged congo red to oc-
eur. As would be expected, acids and alkalies are capable of greatly
altering the electric charges by the H and OH ions which they contribute.
Chemical Forces.—If the nature of the phase at the surface of which
adsorption occurs is such that it can enter into chemical combination
with the substance adsorbed, reactions will occur that do not obey the
laws of mass action. By adsorption, reactions of a certain type may be
encouraged over other reactions, even although the necessary reacting
substances may be present in the solution (specific adsorption). The
adsorbing substance itself is not, however, usually susceptible of chem-
ical change even when it exists as very minute particles, as in the case of
colloidal solutions. Nevertheless, adsorption may accelerate chemical
reactions by bringing together in concentrated form substances of high
chemical reactivity. In such cases the adsorbing substance itself does
not enter into the chemical reaction, and can be recovered at the end
in an unchanged condition. It acts as a catalyst (page 72). As we
shall see later, enzymes act in this way—i.e., their rate of reaction. is
controlled by adsorption.*
The distinguishing feature of such adsorption phenomena is that a
eurve of the reaction (drawn by plotting amount of chemical change
e *Another instance of the influence of surface energy on the course of chemical reactions is seen
in the accelerative influence of charcoal on such reactions as the oxidation of formic acid, glycerol,
etc. Surface tension may also cause retardation of chemical reactions, as’ a seen in the turbidity
(due to the separation of chloroform) which gradually develops when a 7 NasCOs solution is
; ‘ M ¢ ‘ i
mixed with a—~ chloral hydrate solution. The surface remains clear, because surface energy, has
prevented the reaction. ; ;
An important effect of surface tension on chemical reactions is also seen in the relationship
between it and the absorption coefficient of gases (volume of gas dissolved by unit volume of
liquid). The lower the surface tension, the greater the solubility of the gas. Oxygen and nitrogen
are, for example, much more soluble in alcohol, hydrocarbons or oil than in water. This shows
the futility of attempting to prevent the loss of gases from fluids such as blood by covering them
with oils or hydrocarbons.
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.
COLLOIDS 69
against concentration of reacting substances) is a parabola, indicating
that the laws of mass action (page 23) are no longer followed. In
order to be able to determine whether some particular process—as, for
example, a fermentation process, or the absorption of oxygen by blood—
is caused by adsorption, we must compare its curves, constructed ac-
cording to the same principles, with the typical adsorption curve. A
formula may be used in constructing the curves. In arriving at this
formula, two facts have to be remembered: (1) As adsorption proceeds
and less and less of the free energy on the adsorbing surface remains
to be neutralized, the reaction slows off, until equilibrium is reached.
The more dilute the solution, the greater is the proportion of its con-
tents to be adsorbed, which means that if a is the amount of substance
adsorbed from a certain solution, then, from a solution of twice that
strength, somewhat less than 2 u will be adsorbed—i.e., u multiplied
by some root of 2. Although the formula is one belonging to the class
known as parabolic, it must not be assumed that every reaction which
happens to give such a parabolic curve (such as the combination of O,
with hemoglobin under certain conditions) (see page 383) must be one
dependent on adsorption.
It must be understood that although the substance that is removed
from a solution by adsorption is no longer capable of contributing to the
conductivity or the osmotic pressure of the solution, it is nevertheless
not so firmly fixed that it can not be set free again by purely mechanical
means, as by constant dilution of the fluid. If charcoal which has ad-
sorbed sugar is placed in a dialyzer made of membrane the pores of
which allow sugar but not charcoal to pass through, the sugar will
gradually-be removed if the dialyzer is immersed in running water. A
certain equilibrium exists between the substance adsorbed and the same
substance still remaining in solution. If the latter is constantly dimin-
ishing by dialysis, the adsorption compound must break down to main-
tain the equilibrium. It is clear, however, that the process of removal
will be extremely slow. The ability of adsorbed substances to withstand
removal by washing is taken advantage of by nature in holding back
foodstuffs in the soil.
Physiologic Processes Depending on Adsorption
Instances in which adsorption undoubtedly plays a most important
part in physiologic processes are as follows:
1. The action of enzymes (see page 71).
2. The combination of toxin with antitoxin occurs according to the laws
of adsorption rather than those of mass action. In this case it is im-
portant to note that when the toxin of diphtheria is added in small suc-
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70 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
cessive quantities to diphtheria antitoxin, more toxin is neutralized than
when the toxin is all added at once. A similar phenomenon ean also be-
observed by adding filter paper to congo red, more of the pigment being
adsorbed when the paper is added in small quantities than when added
all at once. The explanation is that relatively more adsorption of a
given substance occurs from a dilute than from a strong solution (cf.
page 69).
3. The sensitizing of leucocytes by opsonins, as well as the subsequent
ingestion of bacilli by the sensitized leucocytes, both of which follow the
course of an adsorption reaction.
4. The formation of adsorption compounds, such as the inorganic salts
and proteins and the complex lecithin compounds that can be extracted
from egg yolk or brain tissue. In such compounds the laws of chemical
proportion no longer hold, and properties may be exhibited that are quite
different from those of either one of its components. When yolk of egg
is extracted with ether, for example, a compound of lecithin with vitellin
goes into solution, although vitellin itself is quite insoluble in ether.*
There can be no doubt that adsorption compounds of this character are
very abundant in living cells, and that they are constantly being formed
and broken down.
*By mixing solutions of egg albumin, congo red and a dye called fustic in the presence of
alum, the colloidal particles of which each is composed run together to form larger colloidal ag-
gregates, which by ultramicroscopic examination can be seen, to be composed of a red, a yeliow
ane a green colloidal particle. The attractive force holding the particles together is electric in
this case.
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CHAPTER IX
FERMENTS, OR ENZYMES
One of the most striking developments of modern research in biochem-
istry concerns the nature of enzyme action. So remarkable are many of
the facts that have been brought to light that it can not fail to interest
every one engaged in the study of life phenomena—whatever the nature
of that study may be—to know something of the main questions at
‘present occupying the attention of investigators in this field. In this
chapter a brief survey will be given of some of these questions; no at-
tempt will be made at completeness, and only where necessary for the
sake of example will reference be made to individual types of enzyme
action. ‘
The discovery by Buchner that an enzyme can be expressed from yeast
cells which is capable of instantly bringing about the alcoholic fermen-
tation of dextrose solutions has been responsible for a great deal of the
modern advance. Formerly, yeast cells were believed to bring about
alcoholic fermentation as a result of their growth: it was believed to be
a life phenomenon, or ‘‘vital process.’? Now we know that yeast cells
produce an intracellular ferment or endo-enzyme* to which its sucroclastic
properties are due and which can act apart from the cells that produce it.
It is no great stretch of imagination to think of all chemical reactions
mediated by cellular activity as due to a similar mechanism, and this thought
has led to the hypothesis that all processes of intermediary metabolism in
the animal and plant are caused by enzyme action. Before Buchner’s
day we knew only of the extracellular enzymes (such, for example, as
the digestive ferments), that is to say, of enzymes, produced indeed by
cells, but secreted from them and acting outside their protoplasm; now
we must recognize intracellular enzymes acting where they are produced,
in the protoplasm of the cell. But we must not permit this conception to
carry us too far. Without further investigation we must not imagine
that the riddle of life is thus solved.
As an example of the réle which extra- and intracellular enzymes are
supposed to play in the animal economy may be cited the metabolism of
protein. Proteolytic enzymes are very widely distributed in the active
tissues of the animal and plant. By their agency in animal life, the com-
*The terms “ferment” and “‘erizyme’” are synonymous, but the latter is preferable as the noun,
leaving the former to be used as the verb.
71
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72 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
plex protein molecule is split up to render it absorbable from the intes-
tine, and the tissues appropriate from the blood those of the degradation
products that they require for the construction of protoplasm, which,
later, they decompose so as to utilize the energy which the organism
demands. All these processes are believed to be the work of enzymes.
The Nature of Enzyme Action
The changes brought about by enzymes can also be accomplished by
ordinary chemical means, but these have often to be of a very energetic
nature to accomplish what the enzyme can so quickly and quietly
perform.
It is the custom to regard enzymes as catalysts. A catalyst is a sub-
stance which accelerates (or retards) a chemical reaction which in its
absence could proceed at a different (usually slower) pace. The action
of catalysts has been aptly likened to that of a lubricant. A weight
placed at the top of an inclined plane, so held that the weight only slowly
slips down, has its velocity greatly increased if its under surface be
oiled. The oil accelerates the action but does not affect the ultimate
result. Catalysts do not combine with the final products of the reaction,
these being, as a rule, the same as they would have been had no catalyst
been added. Another characteristic is the tremendous amount of chem-
ical change which even a trace of catalyst can induce. There are many
examples of catalysts in the inorganic world, among which may be cited
the action of spongy platinum on hydrogen peroxide. This substance
normally tends to decompose into water and .oxygen, but if a small
amount of spongy platinum is added to it, the decomposition is greatly
accelerated: H,O, = H,O+ 0.
A very good example of the action of an inorganic catalyst is that of
‘the hydrogen ion on cane sugar, or other disaccharides, in the presence
of water. It accelerates the hydrolysis. Cane sugar solution at room
temperature does not indeed, in sterile solution, undergo any appreciable
hydrolysis, but at 100° C. it does, which leads us to believe that, though
inappreciable, the action also occurs at room temperature. By adding
a little hydrochlorie acid, or other acid not having an oxidizing effect
‘on sugar, we greatly accelerate the hydrolysis because of the hydrogen
ions present in the acid solution. Within certain limits the rate of hy-
drolysis is proportional to the amount of catalyst present.
Enzymes, like other catalysts, produce their action when present in
very small amounts (e. g., sucrase can hydrolyze 200,000 times its weight
of cane sugar; diastase can convert starch to sugar in a dilution of
1-1,000,000) and there is a distinct relationship between the amount of
enzyme present and the rate of the reaction. The final product of the
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FERMENTS, OR ENZYMES 73
reaction is, however, the same at whatever. rate it proceeds, and the
enzyme does not appear in the final products. Many enzymes such as
diastase can be found unaltered in amount after they have completed
their action. This is determined by adding a fresh supply of substrate
(that is, of material to be acted on), when the enzymic action proceeds
again in the usual way. The same is no doubt true for all enzymes,
though as yet it can actually be proved for only a few of them. Enzymes,
therefore, may be defined as catalysts produced by living organisms.
The Properties of Enzymes
Although enzymes are examples of catalysts, they exhibit many proper-
ties that appear to differ from those of inorganic catalysts. It will,
therefore, be advisable in considering each quality to compare it in
catalysts and enzymes, for by this method a much clearer conception of
the nature of enzyme action can be gained (Bayliss'*). Those properties
that are strictly peculiar to enzymes we shall consider later.
1. Most enzymes are remarkably specific in their action, whereas inor-
ganic catalysts are very much less so. Thus, in the case of the enzymes
which bring about inversion of disaccharides, this specificity is clearly
shown. There is a special enzyme for each of the three disaccharides—
maltose, lactose and cane sugar—and one of these can not replace
another.
Still more strikingly is this specificity of enzyme action demonstrated
in the fact that certain enzymes, such as zymase (expressed from yeast),
will act only on bodies having a certain configuration, that is, having
their side chains arranged in a certain way. Thus, there are two varie-
ties of dextrose (a and 8), which differ from each other solely in the
fact that the side chains are arranged in different positions with rela-
tion to the central chain of carbon atoms. This form of isomerism is
called stereoisomerism because the two bodies rotate the plane of polar-
ized light to an equal degree in opposite directions. Zymase acts on one
of these but not on the other, and there are innumerable examples of the
same kind. Indeed, of all bodies that exist in two stereoisomers only
one is found in living cells and it is on this variety alone that the enzymes
in animals can act. A similar specificity exists between certain drugs and
their pharmacologic action.
Specificity of action is explained by supposing that a union occurs
between the substrate and the enzyme, and for this union to take
place the enzyme must possess a configuration which corresponds accu-
rately with that of the substrate. The process has been compared to a
lock and key; the key must be shaped to fit the lock, or it ean not
operate. The specificity does not, however, in itself disprove the close
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74 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
relationship between enzymes and inorganic catalysts, for on the one
hand there are several enzymes which do not exhibit this property, and
on the other, there are inorganic catalysts which do. For example,
lipase, the fat-splitting enzyme of pancreatic juice, decomposes not only
fats but to a greater or less degree a number of bodies of the same gen-
eral build (esters), and tyrosinase can decompose, not tyrosin alone,
but all phenol compounds. Conversely, the hydrogen ion—to the pres-
ence of which acids owe their catalytic powers—can decompose the ordi-
nary esters (that is, of acids containing the carboxyl or COOH group)
but it has no action on the sulphonic esters. However, enzymes are cer-
tainly much more specific in their action than inorganic catalysts.
2. Temperature does not influence catalysis and enzyme action in the
same way. As the temperature is raised in the ease of inorganic catalysts,
the reaction becomes about doubled in rapidity for each rise of 10° C.,
whereas in the case of enzymes it becomes increased up to a certain opti-
mum. temperature, beyond which, as the temperature rises, the reaction is
first slowed and then disappears altogether.
This peculiarity of enzymes as compared with inorganic catalysts need
not in itself disprove the analogy between the two, because enzymes do
not form true, but colloidal solutions. Colloidal solutions, as we have
seen, are really fine suspensions of ultramicroscopic particles; there is no
splitting into ions of the dissolved substance, as is the case with true
(molecular) solutions, but the colloid is suspended in the water or other
solvent to form a heterogeneous system (page 51), on which account
the surface area of the menstruum is enormously increased. Rise in
temperature alters the extent of the surface area, and thereby intro-
duces an influence which progressively opposes catalysis.
Although inorganic catalysts in molecular solution show no optimum
temperature but increase in activity in proportion as the temperature is
raised, inorganic colloidal catalysts may show an optimum temperature.
Thus, spongy platinum shows an optimum temperature in its action on a
mixture of hydrogen and oxygen. It has therefore been suggested that
it is because they are colloids that enzymes exhibit this property.
3. Inorganic catalysts frequently carry the reaction to a further stage
than that attained by the action of enzymes. For example, acid breaks
down the protein molecule much more completely than do the proteolytic
enzymes. This difference is perhaps explained by the fact that enzymes
are retarded in their activities when there comes to be a certain accumu-
lation of the products of the reaction present. The final stages in the
reaction may become so slow as to be almost inappreciable. This de-
crease in activity is partly due to a union between the enzyme and the
products of its activity.
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FERMENTS, OR ENZYMES 75
4. The velocity constant in the case of inorganic catalysts remains un-
changed throughout the reaction, whereas in the case of enzymes it be-
comes either less or greater as the process proceeds. When a substance is
changed by eatalytic action, it is, of course, constantly being diminished
in concentration so that less and less of it remains to be acted on. This
implies that there are fewer molecules present for the same amount of
catalyst to act on and consequently that the amount changed in a unit
of time is progressively less. At any moment, therefore, the rate of
catalysis will be proportional to the amount of substance (substrate)
left. To understand this we must refer back to what we have learned about
mass action. If we suppose that two substances A and B react to form
two other substances C and D, then, by the law of mass action, the reac-
tion will not go on.to completion but will stop when a certain equilibrium
is reached. The reaction can be represented by the equation
A+B2C+D, which means that it proceeds at a rate proportional to
the reacting molecules. In some cases this reaction goes on until either
A or B has practically disappeared (that is, the equilibrium point is very
near the right of the equation), as is the case in the inversion of cane
sugar: i
Cy, Hi, 01; 7 H,0 — C, Hi, 0, + C. Hip 0,
Taking place as it does in an excess of water, and there being very
little tendency for this reaction to go in the opposite direction (cf. re-
versible action) (page 25), the only thing which will influence its
velocity is the concentration of cane sugar; in other words, the velocity
of the reaction at any moment will depend solely on the concentration,
C, of the material still left undecomposed. This can be determined by
means of an equation.*
The value of such an equation is that it gives us a figure K, represent-
ing the amount of inversion that would occur in each unit of time if the
cane sugar were kept in constant concentration. When, for example,
it is stated that K for a particular strength of acid acting on cane sugar
solution is 0.002, this means that when volume, concentration of acid and
*If x be the amount of sugar inverted if time ¢, and if we use a figure called a constant (K) to
express the fundamental rate of the reaction (which will therefore be differcnt for different reac-
tions), then — = KC. But C can not be the same at any two consecutive periods of time, because
the reaction is going on continuously. This renders it necessary to use the notation of the differential
calculus, and we have = = KC. The sign § indicates that the reaction is a constantly changing
one so that 5x and §t represent such infinitely small amounts that they can not be measured. By
methods of integration, however, it can be shown that the above equation may be written:
Qi
— a log. nat. ——
TT Ca
thus RSF aIE us to find the value “of K (Cy Cz being the concentrations of cane sugar at the
times T, Te).
Any two determinations during the course of the reaction can be used for calculating K. These
equations apply only to cases in which but one substance is changing (monomolecular reaction).
hen two substances are involved, the equation is more complicated.
K=
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76 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
temperature are constant in a gram-molecular solution of sugar, 0.002
gram-molecule of sugar would be inverted the first minute and 0.002
gram each succeeding minute, provided we could keep the solution con-
stantly a gram-molecular one, that is, provided we could add sugar just
as quickly as it becomes inverted.
At first sight it may appear of little practical importance to determine
K. In our present discussion concerning the nature of enzyme action,
it is however of great value for, whereas with inorganic catalysis K is
really of constant value, with enzyme action it is not so. Thus, when
cane sugar is inverted by sucrase—an enzyme present in the intestine
and in yeast—the constant gradually rises; for most other unimolecular
reactions mediated by enzymes it gradually falls; for example, the action
of trypsin on proteins.
Where there is a great excess of substance to be acted on, in compari-
son with the amount of enzyme present, it will be found that a more
constant value than K is obtained when we compute the absolute amount
of substance decomposed in a given time. In such a case, too, the
amount of change in a given time will be proportional to the amount of
enzyme present, indicating that some sort of combination between en-
zyme and substrate must be the first step in the fermentative process.
This fact has been noticed by us in connection with the hydrolysis of
glycogen in the liver. When there is an excess of glycogen present, the
amounts which disappear in equal intervals of time after death are the
same; when, on the contrary, there is not much glycogen, the amount
which disappears gradually declines, but, if K be computed by the above
equation, it is constant.
To make these facts clear it may be well to pause for a moment to
consider an illustration. The conditions obtaining when there is a large
excess of substrate over. enzyme may be compared to those governing
the removal of a pile of bricks from one place to another by a number of
men. The pile of bricks represents the substrate; the men, the enzyme.
If each man works up to his capacity, it is plain that the number of
bricks transferred in a given time will not depend at all on the size of
the pile to be transferred. When, however, the pile of bricks gets small,
though the same number of men continue to work the number of bricks
transferred in a given time falls off, because the men interfere with one
another’s activities in securing their loads from the pile. When a similar
stage is arrived at in enzyme processes, we have to use the velocity con-
stant to show how much work could be done by the enzyme if the amount
of substrate were maintained of constant amount.
In the large volume of recent work which has been done with the
object of discovering the cause of these variations in the velocity con
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FERMENTS, OR ENZYMES 17
stant in the case of enzymes, four important conditions have been recog-
nized: (1) reversibility; (2) gradual destruction of the enzyme; (3) com-
bination of the enzyme with products of the reaction; (4) autocatalysis.
Of these four influences the only one which could be held accountablé
for an increase in the activity of the enzyme is autocatalysis; in this
process the enzyme by its action produces substances which intensify
its own activity. In some eases at least—for example, the action of
invertase on cane sugar—these are acid bodies, a moderate increase in
acidity favoring the action of this enzyme.
The other influences all tend to retard the reaction and progressively
lower the value of K. Negative autocatalysis occurs when the enzyme
produces products which interfere with its activity. Gradual destruc-
tion of the enzyme and its union with the products of its activity will
manifestly also decrease its power. There is plenty of evidence that
both of these processes may occur.
Reversibility of Enzyme Action
But the most important of all the causes of retardation of enzyme
activity is undoubtedly reversibility of action, which is an application of
the law of mass action (page 25). If we take the saponification of an
ester, the equation is:
CH,CH,CH,COOCG,H, + H,O S CH,CH,CH,COOH + C,H,OH.
(ethyl butyrate) (butyric acid) (ethyl alcohol)
The equilibrium point is not so near the position of complete hydrol-
ysis as in the case pf the inversion of saccharose; in other words, the
tendency for the bodies produced by the hydrolysis to reunite and: form
the original substances is quite marked, so that the reaction comes to an
end before all the ethyl butyrate has been decomposed. For some time
before the equilibrium point is reached, there will have existed a progres-
sively increasing opposition to the breakdown of the ester, as a conse-
quence of which, when enzymes are used to accelerate the reaction, the
velocity constant, as determined by the above equation, will gradually
fall as the reaction proceeds. Conversely, in a mixture of ethyl alcohol
and butyric acid there is very slow synthesis to ethyl butyrate, and here
again lipase accelerates the process; it induces a recognizable synthesis
within a short time. Ethyl butyrate is usually employed for these ex-
periments because, on account of its odor, the ester is readily recognized.
Thus, if the alcohol and acid be mixed alone, no ester will be detectable,
but if some lipase be added, it will soon become so. Similar synthetic
action of lipase has also been demonstrated for mono- and tri-olein.
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78 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
It should be clearly understood that pure catalysts, such as the hydro-
gen ion, in accelerating a reaction like the above, do so equally in both
directions, so that the position of equilibrium remains unchanged. En-
zymes may, however, cause this position to change because of their form-
ing intermediate combinations.
The reverse phase of certain reactions is probably the cause of at least
some of the synthetic processes which occur in the animal body. A great
difficulty in accepting such a view, however, is the fact that the equilib-
rium point of all hydrolytic reactions, in the presence of an excess of
water, is so near complete hydrolysis that very little synthesis can be
possible. That is true so long as the substance synthesized is soluble,
but if it is nearly insoluble in water, or if it is immediately removed
from the site of the reaction by diffusion, or in any other way, then it is
obvious that it will go on being synthesized by the reaction. Thus, in the
intestine neutral fat is hydrolyzed by pancreatic lipase into fatty acid
and glycerin, which are absorbed into the epithelium, where they again
come under the influence of intracellular lipase. This latter will tend to
accelerate the synthesis of neutral fat from the fatty acid and glycerin
until the equilibrium point of the system (fat acid + glycerin = neutral
fat + H,O) is again reached; but this point, although it is near the right
hand of the equation, will really never be reached for the reason that the
neutral fat, as quickly as it is formed, will become deposited in insoluble
globules in the protoplasm and thus be removed from the equation. In
support of this view it has been found that lipase is present in intestinal
mucosa after all traces of adherent pancreatic juice have been washed
away. By similar reactions the fat of the tissues becomes decomposed to
fatty acid and glycerin and passes out of the blood when the concentra-
tion of fat in this fluid falls below a certain level. Provided one of the
substances synthesized is insoluble or can in some other way be removed
from the reaction, it is plain that, even though the equilibrium point is
very near to that of complete hydrolysis, yet the reversion will be suf-
ficient to do all that is required of it.
Results such as the above have prompted many to conclude that it is
by such reversible action that all synthetic processes occur in the living
organism. But the demonstrable synthesis of an ester must not be taken
as evidence that all other syntheses are explainable on the same basis.
For example, we have seen above that in the case of cane sugar the equi-
librium point in the equation is so-near that of complete hydrolysis, that no
measurable amount of cane sugar is formed when dextrose and levulose are
allowed to act on each other, and that cane sugar does not appear
when sucrase is added to the mixture. If instead of sucrase we take
another of the sugar enzymes—namely, maltase, which accelerates the
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FERMENTS, OR ENZYMES 79
decomposition of maltose into two molecules of glucose—there is, how-
ever, evidence of synthesis as a result of the acceleration of a reversible
reaction. To understand these results we must remember that ordinary
dextrose is a mixture of two stereoisomers designated a and 8. When
two molecules of a dextrose condense (that is, fuse togther with the
loss of a molecule of water) maltose is formed, but when two molecules
of B dextrose condense isomaltose results. There is some controversy
as to whether maltose is really responsible for the synthesis of a dextrose
molecules to maltose, it being claimed by some that this is accomplished
by another enzyme, emulsine. If this were true it would materially
minimize the importance of reversible action as a factor in cellular syn-
thesis. The latest evidence goes to show, however, that it is maltase
and_ not emulsine that is responsible in the above case (cf. Bayliss).
Evidence, both direct and indirect, is also steadily accumulating to
show that enzymes may accelerate the synthesis of proteins. As pieces
of direct evidence we have: (1) the retardation of the digestive action
of trypsin, ete., which sets in after the process has gone on for a time,
and (2) the recommencement of a digestive process apparently at an
end, if the products of the digestion are removed by dialysis or other
means. As direct evidence may be cited the formation of synthetic
products when pepsin is added to concentrated solutions of peptone,
and the diminution in the number of small molecules, as judged by meas-
urements of electrical conductivity, when trypsin is added to the prod-
ucts of tryptic digestion of caseinogen. Protamine—a simple form of pro-
' tein—has also been found to be produced when trypsin—obtained from
a molluse—was added to a tryptic digest of the same protamine. The
significance of these facts in connection with the metabolism of the
amino aids will be evident when we come to study this subject (page
598) .*
Specificity of Enzyme Action
Although in all of the above features of enzyme action there is nothing
to contradict the view that they are catalytic agents, there remains one
peculiarity which at first sight seems uninterpretable on such a basis.
This is with regard to their often remarkable specificity of action. Thus,
as we have seen, maltase can hydrolyze maltose alone (which is com-
posed of two a-dextrose molecules), but not isomaltose (composed of
B-dextrose). This means that mere difference in the configuration of
molecules is sufficient to alter the influence of enzymes on them. Since
such differences could not influence that of inorganic catalysts we must
*We have been unable in this laboratory to demonstrate any synthesis of glycogen when gly-
cogenase is added to a hydrolysis mixture of dextrine, maltose and glucose produced by the prolonged
action of glycogenase on pure glycogen. ;
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80. PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
explain the cause of the difference. This has been done on the basis
either that enzymes are colloids or that the active (catalytic) group of
the enzyme is attached to a colloid molecule. Before a substance can
be acted on, it must combine with the colloid, which it does by the proc-
ess of adsorption (see page 65). This can occur, however, only when
there is a harmony between the adsorbing substance and the substance
adsorbed. Instances of the specificity of adsorption have already been
given.
In support of this view it has been found that of the two proteases,
a and 8, in the spleen, one is adsorbed but not the other when a solu-
tion containing them is shaken with Kieselguhr. Furthermore, when
solutions of invertase are shaken with certain inert powders, the in-
vertase is adsorbed by some of them but not by others. In strong sup-
port of the adsorption. hypothesis is also the fact that the same mathe-
matical laws as apply in the process of adsorption are obeyed in the
ratio which exists between the activity of an enzyme and its concen-
tration in the solution.
To sum up, then, catalysis as exhibited by enzymes involves three
processes: (1) contact between the enzyme and the substrate, which will be
dependent on their rates of diffusion; (2) adsorption between them, which
will depend on their configurations (ef. the lock and key simile); and
(3) the chemical change which itself probably takes place in two stages.
In connection with the third process, it is probable that an initial ecom-
pound of a definite chemical nature is first formed, followed by the
hydrolytic or other chemical change, after which the enzyme group
becomes free.
It is very significant in this connection to note that in their solubil-
ities there exists a distinct relationship between the ferments and the
substrates on which they react. Thus, trypsin is very soluble in water
and acts on water-soluble proteins; lipase is soluble in fat solvents.
Certain Peculiarities of Enzymes
Notwithstanding the very strong case that is made out for the ecata-
lytic hypothesis, there are certain facts which many find it difficult to
make conform with such a view. One of these is that dextrose. ean
undergo three distinct and separate types of decomposition according
to the enzyme allowed to act on it. These are alcoholic fermentation,
butyric acid fermentation and lactic acid fermentation. It is difficult
to see how simple catalytic action ean be responsible for all three results.
The enzyme must not only initiate the changes but also direct their
course.
Another peculiarity is that when certain enzymes—e. g., rennin, pep-
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FERMENTS, OR ENZYMES 81
sin, ete.—are inoculated in animals, they cause specific antienzymes to
appear in the blood of the inoculated animal. Thus, when antirennin
serum is added to milk it greatly hinders clotting on the subsequent
addition of rennin. It is probable that powerful antienzymes are pro-
duced in the animal body for the purpose of protecting the tissues from
attack by enzymes. It is on account of the presence of antienzymes
that intestinal parasites can exist in the intestine, and the immunity
from digestion which the mucosa of the gastrointestinal tract enjoys,
is believed to be due to the same cause. But there is considerable doubt
regarding this claim. Fresh pancreatic juice when injected into the
empty intestine digests its walls. When food is present in the intes-
tine it evidently prevents digestion of the walls by diverting the enzyme
to itself.
Types of Enzyme
Having learned something about the general nature of enzyme action,
we may now turn our attention to certain details that have a practical
importance. In the first place, with regard to nomenclature, in the
earlier work each newly discovered enzyme received a name which was
often quite inappropriate. Many of these names are retained, such as
pepsin, trypsin, ptyalin, ete. but it is now customary to name the
enzyme according to the substance on which it acts. This is done either
by replacing the last part of the name of the substance acted on by the
termination -ase (for example, the enzyme which inverts maltose is called
maltase), or by merely adding -ase to the name of the substance acted
upon (thus, the enzyme which hydrolyzes glycogen is called glycogenase).
Most of the enzymes in the animal body accelerate hydrolytic proc-
esses and are classified according to the chemical nature of the sub-
strate on which they work. Thus, we have:
1. The amylases—accelerating the hydrolysis of polysaccharides, e. g.,
ptyalin (in saliva), amylopsin (in pancreatic juice), glycogenase (in
liver), diastase (in malt).
2. The invertases—accelerating hydrolysis of disaccharides, e. g., malt-
ase, lactase and suerase (in succus entericus).
3. The proteinases—accelerating hydrolysis of proteins, e.g., pepsin
(in gastrie juice), trypsin (in pancreatic juice), erepsin, intracellular
proteinases.
4. The lhpases—accelerating disruption of neutral fats, e.g., steapsin
(in pancreatic juice), intracellular lipases.
5. Arginase—accelerating hydrolysis of- arginin into. urea and or-
nithin, (intracellular).
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82 PHYSICOCHEMICAL ‘BASIS OF PHYSIOLOGICAL PROCESSES
6. Urease—acecelerating hydrolysis of urea to ammonium carbonate
(in many microorganisms and in the soy bean).
7. Glyorylase—econverting glyoxals into lactic acid (page 666).
Other enzymes accelerate oxidative processes and are called oxidases
and peroxidases. Others bring about the displacement of an amino
group by hydroxyl (desamidases). Others cause coagulation (coagula-
tive ferments), e.g., thrombin, rennin. One of the enzymes present in
succus entericus acts by converting the zymogen (trypsinogen) into the
enzyme (trypsin).
Enzyme Preparations
So far it has been impossible to prepare enzymes in a pure state al-
though, being colloidal in nature, they are readily precipitated or ad-
sorbed along with other colloids.
Since most enzymes exist in cells, it is necessary to break up the cells
in order to isolate the enzyme. This is done in various ways. By one
method the cells are ground in a mortar with fine sand, then made into
a paste with infusorial earth (Kieselguhr), the paste enclosed in stout
canvas and placed under an hydraulic press at about 300 atmospheres
pressure; a clear fluid separates and this contains the enzymes. An-
other way is to freeze the tissue with liquid air and grind it in a steel
mortar by.means of a machine. Still another and less expensive method,
and one which we have found most useful for organs and tissues, con-
sists in reducing the tissue to a pulp and, after sieving it to get rid of
connective tissue, etc., spreading the pulp on glass plates and drying
in a slightly warmed, dry air current. The scales of dried material are
then ground in a paint mill with toluene, and the resulting suspension
filtered ; the powder which remains on the filter, after thorough washing
with toluene, is dried and kept for future use. The toluene removes all
the fatty substances, so that when shaken with water, ete., the enzymes
dissolve.
Conditions for Enzymic Activity
Reactions brought about by intracellular enzymes are very readily
inhibited when there comes to be a certain accumulation of their prod-
ucts of action. Thus, yeast ceases to ferment sugar when the alcohol
has accumulated to a certain percentage. This action is partially due
to a toxic action of the alcohol on the cell, which paralyzes its power of
absorbing the substance to be acted on by the intracellular enzyme. If
these products be not in some way removed, they will ultimately kill
the cell and stop the fermentation. We have seen above how the ac-
cumulation of products may interfere with the activities of enzymes in
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FERMENTS, OR ENZYMES 83
other ways in which the enzyme does not suffer destruction, as is shown
by the fact that it resumes its original activities on removal of the
products.
Enzymes, both intracellular and extracellular, are very sensitive to-
wards the inorganic composition of the medium in which they are act-
ing. For the intracellular enzymes this is what we should expect when
we bear in mind the profound influence of inorganic salts on the heart
beat and on cell growth and division. This influence of salts and of
reaction (acidity, ete.) on the life of the cell is so pronounced as to lead
some observers to believe that abnormal cell multiplication in the body,
as in the case of tumor formation, is due to changes in the inorganic
composition of the tissue fluids. Extracellular enzymes are also very
susceptible to the influence. of inorganic salts but more especially so
towards the reaction of the solution. In terms of modern chemistry
we may say that the concentration of H- and OH’ ions has a profound
influence on the activities of enzymes. Most of the enzymes of the an-
imal body perform their action normally in the presence of a slight ex-
cess of OH’ ions, that is, in faintly alkaline reaction. Indeed the only
exception of importance to this is the pepsin of gastrie juice, which nor-
mally acts in an acid medium. An excess of either OH’ or H- ions
inhibits the activity of the enzyme and usually destroys it permanently.
The activities of enzymes are also influenced by light, many -of them
being destroyed by sunlight; cells such as microorganisms are similarly
affected.
Before being secreted the digestive enzymes exist in the cells which
produce them as inactive precursors called zymogens. The granules seen
in resting gland cells are of this nature. The activation of the zymogen,
or its conversion into the enzyme, occurs after it has left the cell, and
this has been considered as another safeguard to digestion of the cell.
Sometimes the activation does not occur until the zymogen has travelled
some distance along the gland duct, as in the case of the proteolytic
enzyme of pancreatic juice. Till it reaches the intestine, this exists as
trypsinogen (the zymogen), but it is here acted on by another enzyme-
like body produced by the intestinal epithelium and called enterokinase.
PHYSICOCHEMICAL REFERENCES
(Monographs and Original Papers)
1Bayliss, W. M.: Principles of General Physiology, Longmans, Green & Co., 1915.
ns = ite Physical Chemistry, Its Bearing on Biology and Medicine, Arnold,
ed. 2,
3McClendon, J. S.: Physical Chemistry of Vital Phenomena, Princeton University
Press, 1917.
4Starling, E. H.: Principles of Human Physiology, ed. 2, 1915, Lea and Febiger.
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84. PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES
“Kahlenberg, L.: Jour. Physical Chem., 1906, x, 141.
‘Reid, E., Weymouth: Jour. Physiol., 1898, xxii, lvi.
7Wilson, T. M.: Am. Jour. Physiol, 1905, xiii, 150.
8Haldane, J. S., and Priestley, J. G.: Jour. Physiol., 1916, 1, 296; Priestley, J. G:
Ibid., p. 304.
°Clark, W. M., and Lubs, H. A.: Jour. Bacteriology, 1917, ii, 1 and 109.
1oHenderson, L. J.: The Excretion of Acid in Health and Disease, Harvey Lectures,
J. B. Lippincott Co., 1915, x, 132.
11Henderson, L. J.: The Fitness of the Environment, Macmillan, N. Y., 1913.
12Van Slyke, D. D.: Jour. Biol. Chem., 1917, xxx, 289, 347.
13Levy, R. L., and Rowntree, L. G.: Arch. Int. Med., 1916, xvii, 525.
14Cullen, G. E.: Jour. Biol. Chem., 1917, xxx, 369.
15Palmer, W. W., and Henderson, L. J.: Arch, Int. Med., 1913, xii, 153.
iSellards, A. W.: The Principles of Acidosis and Clinical Methods for Its Study,
Harvard University Press, Cambridge, 1917.
ivLloyd, F. H.: Private communication.
1sMacallum, A. B: Surface Tension and Vital Phenomena. University of Toronto
Studies, No. 8, 1912; also Ergebnisse der Physiologie, 1911, ii, 598.
19sBayliss, W. M.: Enzymic Action, ed. 2. Monographs in Biochemistry, Longmans,
Green & Co.
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PART II
THE BLOOD AND THE LYMPH
CHAPTER X
BLOOD: ITS GENERAL PROPERTIES
By R. G. Prearcr, B.A., M.D.
The blood, being the carrier of the nutritive and waste substances of
the body’s metabolism, must at one time or another contain all the ma-
terials which compose the tissues in addition to those which are peculiar
to the blood itself. It is a very complex fluid, and all of its constituents
are not fully known. Structurally it is composed of water in which are
dissolved various gases and organic and inorganic bodies, the corpuscles
and platelets.
THE QUANTITY OF BLOOD IN THE BODY
The most accurate method of determining the volume of blood in
the body is by bleeding and subsequently washing out the blood from
the vessels and then estimating the amount of hemoglobin in the total
fluid (Welcher’s method). This method employed in the case of two
criminals who had been decapitated gave the weight of the blood as
7.7 and 7.2 per cent of the body weight. Bloodless methods for deter-
mining the total volume of blood are based upon the principle of add-
ing a definite quantity of a known substance to the circulation and then
estimating its concentration in a sample of blood withdrawn from the
body shortly afterward. If the substance can not leave the blood vessels
and does not cause fluid to be withdrawn from the tissues, the total quantity
of blood in the body ean be calculated from the concentration of the
injected substance in the blood. The most accurate methods based on
this principle are Haldane and Smith’s, in which carbon monoxide gas
is inhaled in a given amount and the carbon monoxide hemoglobin sub-
sequently determined colorimetrically; and Keith, Rowntree and Ger-
aghty’s, which employs vital red, a dye of low diffusibility. The dye
remains long enough in the body to be thoroughly mixed with the
blood, and its concentration in the plasma is determined colorimetrically
85
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86 THE BLOOD AND THE. LYMPH
by comparing with a suitable standard mixture of dye and serum. These
methods give the total amount of blood in the body as from 5 to 8.8 per
cent of its weight. Meek has recently developed a method in which gum
acacia is used. After mixing with the blood, the concentration of this
substance is determined from the calcium content. Being colloid, none
of the gum leaves the blood vessels.
The newer methods have shown that the volume of the circulating
fluid is maintained fairly constant in spite of influences tending to alter
it. The body accomplishes this by drawing upon the reserve fluid in
the tissues and by varying the rate of water excretion, particularly
through the kidneys. Years ago the doctrine of an increased amount of
blood in the body (plethora) gave rise to the therapeutic use of bleeding.
Especially was this thought to be useful in conditions which we now
recognize as chronic hypertension, and which show no increase in blood
volume. Indeed variation in blood volume is not common, although
plethora may occur in polycythemia, chlorosis, and anemias, and there
may be a temporary reduction in the amount of blood in diseases in
which there is a great depletion of water, as in Asiatic cholera, and fol-
lowing very severe hemorrhage.
While the total quantity of the blood in the body does not vary greatly,
the concentration of its various constituents is subject to distinct change.
The volume percentages of the corpuscles and the plasma can be approx-
imately determined by allowing oxalated blood to sediment or by cen-
trifuging in a graduated cylinder by the use of the hematocrit. Such
methods are not very reliable, but may yield some important information.
Normally 45 to 50 per cent of the volume of blood is composed of cor-
puscles. It varies more or less directly with the number of red blood
cells.
THE WATER CONTENT OF THE BLOOD
Since the blood plasma is essentially a watery solution, some idea of
its water content can be obtained by a determination of the specific
gravity. The most accurate method for accomplishing this is to deter-
mine directly the weight of a given volume of blood and compare it
with the weight of the same volume of water. Since this method re-
quires a rather large amount of blood, indirect methods using smaller
amounts have been devised. One of these (Hammerschlag’s) uses a
solution of chloroform and benzol of a specific gravity of about 1.050,
in which a drop of blood is suspended by delivering it cautiously from
a pipette bent at right angles near its tip. If the drop sinks, chloroform
is added; if it rises, benzol is added until the drop remains suspended.
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BLOOD: ITS GENERAL PROPERTIES 87
The specific gravity of the benzol-chloroform mixture is then determined,
and this-value is supposed to give the specifie gravity of the blood.
The specific gravity of the blood determined in this way varies be-
tween 1.040 and 1.065. It is somewhat less after eating and increases
after exercise; it is slightly lower during the day than at night, and
the variation in individuals is considerable. The changes which occur
in the specifie gravity of the blood in disease are chiefly due to variation
in the percentage of protein, since the salt content of the blood is rela-
tively fixed. It is only when great changes occur in the concentration
of the noncolloidal salts that they markedly affect the specific gravity.
From 90 to 92 per cent of the plasma and from 59.2 to 68.7 per cent of
the corpuscles consist of water. Of the whole blood, from 60 to 70 per cent
by volume or about 55 per cent by weight consists of plasma; and from
40 to 30 per cent by volume or 45 per cent by weight consists of cor-
puscles.
THE PROTEINS OF THE BLOOD
The plasma obtained by centrifuging the blood rendered noncoagula-
ble by oxalates, hirudin or other means (see page 99), contains 5 to 8
per cent of coagulable proteins. These proteins are serum albumin,
serum globulin, and fibrinogen. They can be separated from each other
by the use of acids and neutral salts. Their proportion varies under dif-
ferent conditions, but is approximately as follows:
Fibrinogen ......ceec seca ncrccuaee 0.15-0.6%
Serum globulin ..............0.005 3.8%
Serum albumin ................ fee e. 2.5%
The amount of fibrinogen is subject to the greatest variation (Mathews).
Fibrinogen
The least soluble of the blood proteins is fibrinogen. The plasma is
almost freed of it by half-saturation with sodium chloride, or with a
small amount of acetic acid. It is precipitated as fibrin in the process
of blood coagulation (see page 99), and is estimated by weighing the
amount of fibrin which it produces.
Serum Globulin and Serum Albumin
Globulins are ordinarily defined as being insoluble in distilled water,
and albumins as being soluble. It is, however, impossible to separate
serum globulin and albumin satisfactorily in this manner. The globu-
lin obtained by dialysis can be returned to solution by the addition of
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88 THE BLOOD AND THE LYMPH
a suitable amount of water, which makes the salt adherent to the pre-
cipitate a weak saline solution. In neutral or acid solutions it is coag-
ulated by heat at about 75° C. But it does not act as an individual pro-
tein, since a portion of it is precipitated by dialysis or by carbon diox-
ide. Probably serum globulin really consists of two or more proteins.
The serum albumin remaining in solution after saturation with am-
monium sulphate likewise does not represent a chemical entity. It is
possible by carefully heating the solution of serum albumin to distin-
guish three separate coagulation temperatures. This fact has been in-
terpreted as meaning that the serum albumin consists of at least three
closely related proteins.
Since the refractive index of the blood depends primarily upon the
amount of protein present, it has been taken as a means of determining
variations in the concentration of the proteins. It has been found that
the concentration of the blood proteins varies somewhat; during ex-
ercise it is increased probably because of the taking up of water by
the tissues, and during profuse bleeding it is diminished because
large amounts of fluid are being added to the blood from the lymph,
which is relatively poor in proteins. The ingestion of considerable
amounts of salts has been found to reduce the concentration of the blood
proteins for a short time. In pathologic conditions, as in diabetes, when
rapid changes in the body weight due to alterations in the diet are oc-
curring, changes in the fluid content of the blood are often observed.
Likewise in edema caused by faulty renal function, there may be a re-
tention of fluid in the blood before there is any indication of edema. The
hydremic condition of the blood can therefore be considered as a useful
diagnostic aid in determining the water metabolism.
The relative concentration of the proteins of the blood is also of some
interest, especially since in some diseases a considerable amount of
blood protein is lost. By refractrometric methods it is possible to sep-
arate the globulin and albumin fractions. Normally the total proteins
range between 6.7 and 8.7 per cent, of which the albumins lie between
4.95 and 7.7 per cent, and the globulins between 1 and 2.54 per cent. In
some diseases, as in chronic nephritis, pneumonia, and syphilis, the
total proteins of the blood are decreased and the relative amount of
serum globulin is increased On the other hand, in many mild infections
and chronic septic conditions the globulin fraction may be increased
with no change occurring in the total protein content.®
Our knowledge of the origin and the function of the blood proteins is
quite unsatisfactory. Previous to the discovery of amino acids, the
building stones of the proteins, in the blood it was thought that the
nitrogenous nutrients were converted somehow into blood proteins dur-
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BLOOD: ITS GENERAL PROPERTIES 89
ing or immediately following their absorption from the alimentary
canal, and that the tissue cells were nourished from this common pro-
tein. It is now known that the amino acids are not immediately syn-
thetized into blood proteins after their absorption from the digestive
system. The blood proteins are radically different from the tissue pro-
teins. Substances which retard or accelerate nitrogen metabolism do
not alter the relationship existing between the protein bodies of the
blood. This fact indicates that the serum proteins have a function quite
independent of the nitrogenous metabolism of the body. They un-
doubtedly maintain the viscosity of the blood and assist in preserving
its neutrality. Attempts to localize the site of formation of the blood
proteins have not been successful. There is some evidence that fibrin-
ogen is formed for the most part in the tissues of the splanchnic area
(liver). It is quite possible that the blood forms its own proteins, just
as do other tissues, from the amino acids it contains.
THE FERMENTS AND ANTIFERMENTS OF THE BLOOD
The blood plasma contains many of the ferments present in the tissues.
The nature of these ferments has been the subject of many investiga-
tions in recent years, primarily because it has been found that they are
intimately connected with the problems of immunity.
Among the ferments the following have been demonstrated in the
blood:
Proteases are probably present normally in the human blood serum
in small amounts, but they are found in large amounts in the white
blood corpuscles. A protein foreign to the body if injected into the
blood ordinarily produces no untoward symptoms, but a second injec-
tion following the first by some days will produce symptoms of poison-
ing known as anaphylaxis. This fact has led to the assumption that
the injection of any foreign protein into the blood promptly leads to
the appearance therein of specific proteolytic enzymes which will digest
the strange protein into its derivatives, which are poisonous. This
power of the body to produce specific proteases has been the subject
of much research and debate, and Aberhalden proposed a test for preg-
nancy, for cancer, and for other conditions in which he made use of this
phenomenon. He believes the presence of placenta or tumor tissue to
cause the presence of proteins that bring about the production of specific
ferments whose duty it is to rid the system of these substances. Other
investigators fail to find the specificity in proteolytic action claimed by
Abderhalden, and believe that proteolytic ferments which are capable
of digesting foreign proteins are absorbed from the alimentary canal
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90 THE BLOOD AND THE LYMPH
from the digestive juices (Boldyreff). Some investigators fail to confirm
the claim that the proteolytic activity of the blood serum is increased under
the above conditions.
Blood contains an antiferment known as antitrypsin. This can be
removed from the blood serum by several substances, among which are
kaolin, colloidal iron and starch. Serum thus treated shows strong pro-
teolytic activity and autodigestion will occur. In this case there can be
no question of the specific origin of proteases. Abderhalden believes
that the ferments of the blood of the pregnant woman are able to digest
the placental tissue. Human placental tissue has the ability of absorb-
ing antitrypsin and it is very questionable as to whether the test pro-
posed by Abderhalden is due to the new formation of ferments or to
the removal of the antitrypsin and the action of the protease normally
present in the blood.
Nuclein ferments are capable of decomposing nucleic acid and purins
into the simpler bodies.
Lipases have been demonstrated in the blood.
Amylase—The presence of starch-splitting ferments in the blood was
first shown by Magendie in 1841, and later Bernard showed that gly-
eogen or starch injected into a vein produced glycosuria. Since then
it has been proved conclusively that diastatic enzymes are normally
present in the blood and lymph. The source of these enzymes has given
rise to much speculation. Some observers believe that they are derived
from the amylopsin of the pancreatic secretion, while others believe. that
they are manufactured by the liver. Ligature of the pancreatic ducts
is said to increase the amount of amylase, while removal of the pan-
creas may (Carlson and Luckhart) or may not (Schlesinger) increase
the amylase of the blood. In some forms of experimental diabetes the
amylase of the blood has been found increased, and this is the case in
human diabetes (Myers and Killian). If this is true, a cause for the
inability of the diabetic to store up glycogen is found. In impairment
of renal function, there is usually an increase in the blood amylase and
a decrease in the urine amylase. This has been suggested as being of
diagnostic value.
The blood contains a feeble glycolytic enzyme capable of destroying
glucose. It is claimed that this power is reduced in diabetics (Lepine).
Catalase is found in the blood and tissues generally. It has the power
of liberating oxygen from hydrogen peroxide without any accompany-
ing oxidation process. Its physiologic significance is not known. It
is said that the amount of catalase is increased during excitement and
exercise, and is decreased in conditions where the body’s activity is
lowered. Its determination is clinically unimportant at present.
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CHAPTER XI
BLOOD: THE BLOOD CELL
By R. G. Pearce, B.A., M.D.
THE RED BLOOD CORPUSCLES, OR ERYTHROCYTES
The most prominent function of the blood is to carry oxygen to the
tissues. It owes this property chiefly to the red blood cells which are
present in large numbers (5,000,000 per ¢.mm. of blood). These cells
are biconcave discs, having a diameter of about 7.7 ». They are con-
structed out of a framework composed largely of lipoidal material, in
the meshes of which is deposited a substance called hemoglobin, to
which the remarkable oxygen-carrying power of the blood is due. Nei-
ther the manner by which the red cell carries its hemoglobin nor the
intimate structure of the cell itself is accurately known. It is com-
monly believed that the hemoglobin is held enmeshed in a framework
or stroma, or encased in the cell membrane. One thing is certain, how-
ever, that the union of hemoglobin with the stroma of the red cell is
a fairly strong one, since mere fragmentation of the corpuscle fails to
liberate the hemoglobin. The fact that the framework contains a large
amount of lipoidal substances enables the corpuscles to maintain their
shape and is responsible for their characteristic permeability.
Hemoglobin is a very complex substance belonging to the group of
conjugated proteins. By chemical means it can be broken up into a
simple globulin and a pigment hematin, containing iron. When com-
pletely saturated, oxygen is present in hemoglobin in the proportion
of two atoms of oxygen to one atom of iron (Peters); or 401 ec.c. of
oxygen can be carried by hemoglobin containing one gram of iron, the
molecular weight of the molecule being about 16.669, or some multiple
thereof (Barcroft and Peters) (see also p. 397). At this figure the
iron in the molecule would represent 0.34 per cent of the total weight
of the molecule. The corpuscular surface area has been estimated to
be 3200 square meters. There is therefore a very large surface avail-
able for the absorption of oxygen from the alveolar air, as the blood
corpuscles pass in single file through the capillaries of the lungs.
Since the amount of oxygen which the blood can carry depends upon
its hemoglobin content, it is of some importance clinically to have
91
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92 THE BLOOD AND THE LYMPH
methods of determining the approximate amount present. The amount
of hemoglobin present in a quantity of blood ‘is usually determined
colorimetrically by comparing the color of the blood with standard col-
-ors which correspond to known strengths of hemoglobin. In normal
persons the amount of hemoglobin varies greatly at different ages, and
in order to determine whether or not a given blood contains more or
less hemoglobin than normal, it is imperative to consider the age. The
greatest variations occur between birth and the sixteenth year. After °
the sixteenth year the blood in males usually contains a larger amount
than that in females (Williamson‘). Instruments used in determining
the amount of hemoglobin should be standardized to give the value in
grams hemoglobin per 100 c.c. of fluid.
The amount of hemoglobin which is present in each corpuscle in
terms of normal is therefore of some clinical interest. This relation of
the number of red cells to the amount of hemoglobin is known as the
color index and is computed as follows: The average red count in man
is 5,000,000 to the cmm., and the average minimal amount of hemo-
globin is taken as 13.88 grams in 100 cc. of blood (= 80,:Sahli; = 90,
Miescher; = 86, Plesch; and 110, Tallquist methods). These relative
values give a color index of one. The percentage of normal red cells
divided by the percentage of normal hemoglobin present gives the
color index.
The Origin of the Red Blood Cells
In fetal life the spleen and the liver are generally believed to be re-
sponsible for the formation of the red blood cells. In extrauterine life
this function is taken over by the red bone marrow. In the primitive
condition all red blood cells are supposed to be nucleated. In extra-
uterine life the nuclei of the red cells are lost, and nonnucleated forms
are alone present in the blood stream. In fetal life and in certain path-
ologic conditions, the rate of blood formation is so rapid that some
nucleated cells appear in the blood. The normal response of the body
to a loss of red blood corpuscles consists in an increased activity of the
blood-forming cells of the red bone marrow. It is not easy to follow
the course of the regeneration of the red corpuscles or to discover the
mechanism of their formation in the bone marrow, since this tissue pre-
sents a mixture of cells which are precursors of the varied corpuscles
found in the blood and the identity of which can ‘not be determined.
Recently new methods of staining blood for microscopic examina-.
tion have allowed more detailed study to be made on the site and
method of blood cell formation. When fresh unfixed blood is treated
with solutions of various dyes, such as brilliant cresyl blue, polychrome
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THE BLOOD CELL 93
methylene blue or neutral red, an otherwise invisible structure appears
in some cells in the form of coarse granular particles or threads, which
give a reticulated appearance to the corpuscles. These reticulated cells
are more abundant in infants’ blood and in patients suffering with se-
vere anemia or hemolytic jaundice than in normal blood, and may be
taken as evidence of the youth of the red cell and not as a degenera-
tive process. Since the number of the reticulated cells that are present
in the blood is more or less-directly proportional to the hemopoietic
activities of the bone marrow, enumeration of the reticulated cells is
of clinical importance in anemias. In conditions in which animals have
been made plethorice by the transfusion of blood, it has been found that
the number of reticulated cells is decreased; the bone marrow of these
animals also shows a marked reduction in reticulated erythroblasts.
The diminished rate of blood cell formation sometimes noted after blood
transfusions may be explained by assuming that the stimulus. which
awakens the formation of red cells in the bone marrow is absent or
made subnormal on the injection of red cells into the blood, and thus
the formation of red cells is depressed. Small transfusions are there-
fore preferable to large ones in cases in which the rate of blood forma-
tion is greatly impaired. By means of living cultures of red bone mar-
row the different stages of the development of the normoblasts into
true red corpuscles may be studied (Tower and Herm®). Some evidence
has been gathered from such studies which points to the conclusion that
in place of the red cells being cells which have lost their nucleus, as is
the current teaching, they are rather cells which develop as a nuclear
bud and eseape into the circulation as true red cells. The nucleated
red cell and the red nucleated corpuscle of the bird are the product of
intranuclear activity and are morphologically identical.
Rates of Regeneration of Erythrocytes
Microscopic examination of the blood during rapid regeneration of
red cells shows the presence of nucleated forms. Nucleated red cells
in the blood have therefore been taken as an inevitable feature of rapid
blood regeneration. The evidence upon which this belief depends,
however, is hardly complete, since changes in the manner of red blood
cell formation may be responsible for the nucleated forms. The red
bone marrow is considered the seat of red cell formation, and it is true
that an abnormal increase in the red bone marrow usually accompanies
increased red cell formation. The nature of the stimulus which brings
about the new formation of red cells is not understood. Oxygen want
may be an important factor, since we find the presence of an abnormally
large number of red cells in conditions where there is a scarcity of
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94 THE BLOOD AND THE LYMPH
oxygen in the inspired air, as in life at high altitudes, or a difficulty in
its absorption through the lungs, as in congenital heart disease.
The red cells produced following hemorrhage and in simple anemia
contain less than the normal amount of hemoglobin, but their shape and
size are approximately normal, and few nucleated cells are present. In
the regeneration of red cells which is found in pernicious anemia, we
find the cells containing an unusually large amount of hemoglobin.
The red cells in this disease have abnormal forms, many being large,
with or without a nucleus, and containing basic staining granules.
This type of blood cell formation is due to degenerative changes.
The Fate of the Erythrocytes
The length of life of the red blood cell is unknown. Estimates based
upon the daily excretion of bile pigments are not reliable, since Hooper
and Whipple have shown that the pigments, in part at least, arise from
pigments which the liver has made in excess of its needs for the manu-
facture of hemoglobin, and which, not being needed, are excreted.’
There is no question however that every erythrocyte sooner or later
undergoes disintegration, a process formerly thought to be ushered in
by the ingestion of the red blood cell by a phagocyte in the spleen or
in a hemolymph gland, the hemoglobin of the disintegrated cell being set
free and carried to the liver, where it is broken up into hematin, which
the body stores for future use, and into bile pigments, which are ex-
ereted. Rous and Robertson® fail to find evidence that this process
occurs in man to an extent sufficient to account for the normal destruc-
tion of the blood cells. However they have recently found another and
unsuspected method for blood destruction in all animals thus far
studied—namely, the disintegration of the blood cells by fragmentation
while they are circulating, without loss of their hemoglobin. These
fragmented cells are found most frequently in the spleen. They believe
that the small ill-formed cells, known as microcytes and poikilocytes,
observed in severe experimental anemias, are due not to the fact that
they are produced by the bone marrow, but rather to the fact that the
marrow in its anemic condition is not able to produce a resistant ery-
throcyte, and fragmentation therefore takes place too readily. [ Opening from
== Sinus to auric
Auriculo-ventricular
valves Hook from
‘Heart lever
Ventricle
von Bezolds Ganglion
la Auricular septum
Bidders Ganglion
in auriculo-ventricular junction
timulating electrodes
in Sino-auricular junction [Crescent]
Sympathetic fibres= dotted lines
Fig. 68.—Diagram to show the innervation cf the heart in the frog or turtle. The electrodes
are represented as applied to the white crescentic line where they will stimulate some postganglionic
fibers. (From Jackson.)
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THE CONTROL OF THE CIRCULATION 225
(page 814). It is not the nerve fibers themselves that are responsible
for the inhibitory effect, for it has been found that if the peripheral
end of a eut vagus nerve is connected with the central end of one of
the anterior roots of the cervical portion of the spinal cord, the axons
of the latter when they grow down into the vagus trunk during the
regeneration which follows, stimulation of the regenerated fibers will
still produce inhibition of the heart. The nature of the fibers can not
therefore be the factor upon which the inhibiting influence of the vagus.
is dependent. This leaves the terminal apparatus of the vagus fibers in
the heart as the structures in which the stimulus conveyed to them is
rendered inhibitory in nature.
There has been considerable speculation as to what kind of change
must be occurring in the heart in order to cause the inhibition, but
practically nothing that is definite is known. One significant fact, how-
ever, is that the electrical current led off through nonpolarizable elec-
trodes from two portions of the auricle one of which is injured, does not
take the same direction when the vagus nerve is stimulated as that which
it takes when the motor nerve of a similarly observed muscle is stimu-
lated. A positive instead of a negative variation is observed. Now,
since a negative variation is always accompanied by active chemical
breakdown changes occurring in the muscle to supply its energy of
contraction, it is assumed that the positive variation accompanying stim-
ulation of the vagus must indicate that, instead of a katabolic process,
a building up, or anabolic process, is being excited. This conclusion
would fit in perfectly with the well-known fact that, after the heart has
been held in standstill for some time by vagus stimulation, the beats are
stronger after the inhibition has passed off than they were before. The
vagus seems to have a conserving influence on the heart. During the
inhibition produced by it energy material is apparently stored up in the
heart, so that when the beat is reestablished it is stronger than before.
The Manner of Termination of the Vagus Fibers in the Heart.—This
subject is of considerable pharmacologic and therefore therapeutic in-
terest. In approaching the problem it must be remembered that the
vagus fibers belong to the so-called cerebral autonomic system of nerves
(see page 882). They are therefore fibers which have cell stations situ-
ated near their peripheral termination—cell stations, that is to say, in
which ganglionic medullated fibers, by forming synapses around nerve
cells, become connected with. postganglionic nonmedullated fibers. The
existence of ganglia in the heart, particularly of the frog, has been
known for a long time. These ganglia are located at the sinoauricular
junction, at the interauricular septum, and in the ventricle near the
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226 THE CIRCULATION OF THE BLOOD
auriculoventricular junction. The function of the ganglia is to serve as
cell stations on the course of the vagus nerves. (Fig. 68.)
Nicotine is a drug which in certain concentrations, if applied locally
to sympathetic ganglia, specifically paralyzes the synapses between the
ends of the preganglionic fibers and the cells from which the post-
ganglionic fibers arise. If this drug is applied in a 1 per cent solution
to the heart, stimulation of the vagus trunk no longer produces inhibi-
tion, but if the stimulus is applied to a portion of the heart known as
OPIS ewer!
CUT
Fig. 69.—Frog Sioa tracing showing the action of nicotine. The vagus trunk was stimulated
as indicated. In the normal (lower) tracing inhibition occurs but after nicotine (second tracing) °
no inhibition follows. Stimulation of the crescent in the next two lines still is followed by inhibi-
tion. The final effects of the drug are shown in the last two (upper) tracings. (From. Jackson.)
the white crescentic line, inhibition still occurs, because at this point the
postganglionic nerve fibers come near to the surface and therefore are
stimulated (Fig. 69). On the other hand, atropine is a drug which
paralyzes the postganglionic fibers, so that after its application to the
heart inhibition can not be produced by stimulating either the vagus
trunk or the white crescentic line. Pilocarpine and muscarine are drugs
which have aii‘action exactly opposite or antagonistic to that of atro-
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NE
Postganglionic fibers
are dotted thus ----
- —,
WN
q
dulla . BESS
“oblongata P \
N. p
N.X
Jugular ganglion (Gang. of the root)
Depressor (Fall in pressure or slowing of heart)
(Sensory) Separate nerve in rabbit and opossum.
‘adosum ganglion (Gang. of the trunk) Yarrington)
it Inhibitory cranial autonomic fibers
‘Superior cervical ganglion
Descending sympathetic fibers in cord
Cervical vago-sympathetic trunk
| — ee
7 _ Electrodes (slowing or stoppage of
\Subclavian heart. Augmentation in some
i art ; animals.)
2
Accessory n.~{,
to trapezius
Spinal
medulla
(cord)
Rami Inferior
; cervical
communican-
tes going to gang. oF %
symp. gang., X. fe Aortic arch
(preganglionic) ( “
Ansa, SX
subclavia
(Annulus of
ae
ral
+
t
ThoracieA3\ ||
nerves~ NK.
“ tS [|
? | ae!
slept |S
Electrodes WT Ks
Acceleration, or
augmentation of heart)
First thoracic ganglion
(Stellate)
Fig. 70.—Schematic representaticn of the innervation of the heart of the mammal. The red
continuous lines represent the sympathetic (accelerator) preganglionic fibers, and the broken red
lines, their postganglionic fibers. The cell stations are in the inferior cervical and stellate ganglia,
some extending up to the superior cervical ganglion. The green continuous lines represent the
vagus preganglionic fibers, and the broken green lines, their postganglionic fibers. The cell stations
in this case are located in the heart itself. It will be observed that electrodes applied to the so-
called vagus low down in the neck may stimulate some sympathetic fibers. (From Jackson.)
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THE CONTROL OF THE CIRCULATION 227
pine ; that is, they stimulate the postganglionic fibers and produce a
slowing and possibly an enfeebling of the beat.
In the mammalian heart a large number of the fibers in the right
vagus nerve proceed directly to the sinoauricular node, where it can
be shown histologically that considerable masses of nervous tissue exist.
On the other hand, the great majority of the fibers in the left vagus
proceed to the auriculoventricular bundle, in which also nervous strue-
tures are abundant (page 184). As already indicated, the experimental
results which follow stimulation of either nerve can be explained by the
influence which the nerve exerts on the particular structure to which
the majority of its fibers proceed. In brief, stimulation of the right
vagus is likely to produce slowing and weakening of the beat, whereas
stimulation of the left vagus is more likely to institute a condition of
partial heart-block.
On account of the different results which may be obtained by stimu-
lating the vagus, some authorities have assumed that the heart must
contain four kinds of fiber, more strictly, of vagus nerve endings, one for
each kind of influence which the vagus can develop. These four influ-
ences are, it will be remembered, on the strength, the rate and the
propagation of the heartbeat, and the excitability of the cardiac muscle.
It is, however, almost certainly unnecessary to make such an assump-
tion, for the results can be explained as merely dependent upon dif-
ferent degrees of stimulation of the same kind of fiber and upon the
exact part of the heart to which the fiber runs. Sometimes, for ex-
ample, when the right vagus nerve is stimulated very feebly, there may
be only a diminution in the force of the beats without any change in
their rate, indicating that the effect.-has been upon the musculature of
the auricular walls and not on the sinoauricular node. If the stimulus
is increased a little, then both an enfeebling and a slowing of beat occur,
indicating that the stimulus has now passed both to the auricular mus-
culature directly and to the sinoauricular node.
The Sympathetic Control
The effect of the sympathetic nerve on the heart may be described as
being exactly opposite to that of the vagus. The pathway along which
the fibers of this nerve travel to the heart is more or less a devious one.
They arise in the mammal from nerve cells in the gray matter in the
upper thoracic portion of the spinal cord. The fibers leave by the cor-
responding spinal roots and pass by the white rami communicantes into
the sympathetic chain, up which they travel to the stellate and inferior
cervical ganglia. Around the nerve cells of the stellate ganglion the
fibers end by synapsis, and the axons of the cells are then continued on
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228 THE CIRCULATION OF THE BLOOD
as postganglionic fibers, proceeding to the heart through branches com-
ing off from the stellate ganglion itself, or from the ansa subclavii or
inferior cervical ganglion. (Fig. 70). In cold-blooded animals, such as
the frog, the sympathetic fibers run up to the upper end of. the cervical
sympathetic and join the vagus immediately after it leaves the cranial
cavity. They then proceed along with this nerve—forming the vago-
sympathetic—to the heart. The effect of stimulation is shown in Fig. 71.
The sympathetic nerve differs from the vagus in that a much longer la-
tent period elapses before its influence becomes effective, and this persists
for a much longer period after the stimulus is withdrawn. If the vagus
Fig. 71.—Tracings showing the effects on the heartbeat of the frog resulting from stimulation of
the sympathetic nerves prior to their union with the vagus nerve. (From Brodie.)
and sympathetic are stimulated at the same time, as by exciting the vago-
sympathetic in the frog, the first effect observed is that of the vagus
usually followed, after removal of the stimulus, by the sympathetic ef-
fect. If the stimulus is maintained for a long time, so that the vagus
becomes fatigued, escapement will occur earlier than with pure vagus
stimulation, and augmentation may become apparent. The sympathetic
influence is, however, never so strong as that of the vagus. The two
nerves are therefore not antagonistic in the sense that the one neutralizes
the effect of the other; but when both are stimulated, the heart responds
first to the vagus and later to the sympathetic.
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CHAPTER XXVI
THE CONTROL OF THE CIRCULATION (Cont’d)
THE NERVE CONTROL OF THE PERIPHERAL RESISTANCE
As already explained, the nerve control of the peripheral resistance
takes place through the action of vasoconstrictor and vasodilator nerve
fibers on the musculature of the arteriole walls. The vasoconstrictor
impulses like those in the vagus of the heart are tonic, so that when a
nerve containing such fibers is cut, the corresponding blood vessels un-
dergo dilatation (see page 135), and when their peripheral ends are stim-
ulated artificially, constriction occurs. On the other hand, the vasodi-
lator impulses do not appear, at least under ordinary circumstances, to
be tonic, so that the cutting of such fibers does not cause vasoconstriction ;
their stimulation, however, causes marked dilatation. Vasomotor fibers
are contained in most of the efferent (motor) nerve trunks, and to
detect their presence the nerve must be either cut or stimulated and-the
condition of the blood vessels of the innervated area observed.
Methods for the Detection of Constriction or Dilatation
Several methods, varying with the exact area under observation, can
be used for the detection of vasoconstriction or dilatation. In many cases
visual inspection is sufficient, as in the well-known experiment of Claude
Bernard on the blood vessels in the ear of the rabbit (see Fig. 106). When
this is held with a light behind it, and the cervical sympathetic of the
corresponding side is cut, marked dilatation will become evident and
vessels will spring into view where previously there were none visible.
Visual inspection is usually also a satisfactory method of demonstrat-
ing vasodilatation or constriction in exposed glands, in mucous pas-
sages and in the vessels of the skin.
Another comparatively simple method is the observation of the tem-
perature of the part, this being particularly useful when the vascular
area is one situated in the peripheral part of the body, such as the hand
or foot (see page 209). When dilatation occurs the temperature of the
part rises, because the warmer blood from the viscera flows with greater
freedom through the peripheral regions, where it is cooled off by radia-
tion. When a thermometer is placed between the toes of a dog or cat, a
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230 THE CIRCULATION OF THE BLOOD
distinct rise in temperature will be observed when the sciatic nerve of the
corresponding limb is cut. The application of this principle in deter-
mining the mass movement of blood by the amount of heat given off from
the hands or feet has already been explained.
Other methods depend upon observation of the outflow of blood from
the veins of the part. A simple application of this method can be used in
the case of the ear of the rabbit. If the tip of the ear is cut off, bleeding
under ordinary circumstances is only very slight, but if the cervical
sympathetic is cut, it becomes quite marked, slowing down again or
even stopping entirely when the peripheral end of the nerve is stimu-
lated. By making measurements of the volume of the outflow of blood
from a vein by this method, the extent of constriction or dilatation can
y tube to recorder
oil enclosed
by membrane
Fig. 72.—Roy’s kidney oncometer. (From Jackson.)
be followed quantitatively. Vasodilatation also causes changes in the
character of the venous flow, the usually continuous flow becoming pul-
satile and the color of the blood brightening. Comparison of the pressures
in the arteries and the veins of a part is also often of value in the detec-
tion of changes in the caliber of the blood vessels, for, of course, the
greater the difference in pressure between the two manometers, the
greater must be the resistance offered to the flow.
. For experimental purposes, however, the standard method is that
known as the plethysmographic. For this purpose the organ or tissue is
enclosed in a so-called plethysmograph or volume recorder, the prin-
ciple of which will be clearly seen by consultation of the accompanying
diagram of one adapted for the kidney (Fig. 72). Any increase de-
tected by this means in the volume of the part must be due either to
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THE CONTROL OF THE CIRCULATION 231
an increase in blood flowing into the vessels because of increased heart
action or to a local vasodilatation; and vice versa, when shrinkage oc-
curs. We can not tell from the volume tracing itself which of these
changes is really responsible for the observed alteration, but we can do
so by simultaneously observing the mean arterial blood pressure. If this
falls when the volume decreases, it means that the volume of blood flow-
ing to the part must have become diminished. If, on the other hand, the
blood pressure remains constant or rises while the volume decreases, it
means that the blood vessels have locally constricted.
Methods for the Detection of Vasomotcr Fibers in Nerve Trunks
If we wish to find out through which nerve trunks a given vascular
area is supplied with vasoconstrictor or vasodilator impulses, we should
proceed by the use of one of the above described methods to observe the
effect produced on the vessels by cutting the nerve and then by stimu-
lating the peripheral end of the cut nerve. As a result of such observa-
tions it has been found that the vasomotor fibers are frequently dis-
tributed so that those having a vasoconstricting action are collected
mainly in one nerve trunk and those having a dilating action in another;
in some nerve trunks, however, the relative numbers of the opposing
fibers are about equal. Nerves containing a great preponderance of vaso-
constrictor fibers are the great splanchnic and the cervical sympathetic;
and those containing a great preponderance of vasodilator are the chorda
tympani nerve to the submaxillary gland and the nervi erigentes to the
external genitalia. t
It must be clearly understood that, although one kind of vasomotor
fiber may preponderate in one of these nerves, yet the opposite kind is
also present. In the cervical sympathetic, for example, some vasodila-
tor fibers extending to the blood vessels of the mucous membrane of the
nose and cheeks can readily be demonstrated, as shown by the flushing
of these parts when the peripheral end of the nerve is stimulated; and
similarly, even in the great splanchnic nerve itself, vasodilator fibers
supplying the suprarenal capsule can quite readily be made out. When
the vasoconstrictor fibers greatly preponderate over the vasodilator, the
effect of the latter may be demonstrated by taking advantage of the fact
that ergotoxine paralyzes the vasoconstrictor but not the vasodilator
fibers, so that after its administration stimulation of the great splanch-
nic nerve gives rise to a vasodilatation instead of a vasoconstriction.
The presence of vasoconstrictor fibers in the so-called vasodilator nerves
(chorda tympani and nervi erigentes) has not however, been demon-
strated.
A good example of a nerve trunk containing about an equal admix-
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232 THE CIRCULATION OF THE BLOOD
ture of both kinds of vasomotor fibers is the sciatic. If the hind limb of
a dog is placed in a plethysmograph and simultaneously a record of the
mean arterial blood pressure taken, it will be found on cutting the sciatic
nerve that the volume of the limb increases, whereas the blood pressure
remains practically constant. Before placing the limb in the plethysmo-
graph, the muscles must of course be paralyzed by means of curare;
otherwise muscular contractions would confuse the result. If the
peripheral end of the cut nerve is now stimulated, vasoconstriction will
readily be observed. So far, then, the results demonstrate the presence
of vasoconstrictor nerve fibers alone. :
To demonstrate the presence of vasodilators a different procedure is
necessary. This is based on the following facts: (1) The vasodilator
nerve fibers degenerate more slowly than the vasoconstrictor; (2) they
are less depressed in their excitability by cooling the nerve; and (3) they
are more sensitive to weak slow faradic stimulation than the vasocon-
strictor fibers. Accordingly, if we cut the sciatic nerve two or three
days before the actual experiment, and then, while observing the volume
of the limb, proceed to stimulate the half-degenerated nerve with feeble
electric stimuli of slow frequency we shall usually observe a dilatation
of the limb instead of constriction; and even if we cool a stretch of a
freshly cut nerve before applying the stimulus, the same result will
often be obtained. :
The Origin of Vasomotor Nerve Fibers
Having seen how the presence of vasomotor fibers may be detected in
peripheral nerves, we must now proceed to trace them back to their
origin from the central nervous system. The method for doing this con-
sists, in general, in observing the effect on the blood vessels produced by
cutting or stimulating the various nerve roots through which the fibers
might pass on their way to the nerve trunks. As a result of such obser-
vations it has been found that all of the vasoconstrictor fibers. emanate
from the spinal cord in the region between the level of the second thoracie
and that of the second or third lumbar spinal roots, but from nowhere
else in the cerebrospinal axis. Section of the spinal cord below the level
of the second lumbar spinal roots produces no change in the volume of
the hind limb, provided the muscles be thoroughly curarized, nor does
stimulation of the lower end of the cut spinal cord have any effect. If
the last two thoracic or the first two lumbar spinal roots are stimulated,
however, evidence of vasoconstriction will be obtained.
The restriction of the origin of vasoconstrictor fibers to the above-
mentioned regions of the spinal cord indicates that in proceeding to
the mixed nerve trunks they must travel along special nerve paths.
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THE CONTROL OF THE CIRCULATION 233
These are provided by the sympathetic chain and its branches (Fig. 228).
The vasoconstrictor fibers in the anterior spinal roots leave the latter
by way of the corresponding white rami communicantes, and pass into
the neighboring sympathetic chain, along which they either ascend or
descend, according to their ultimate destination. In their course they
come into contact with the sympathetic ganglia, through one or two of
which they may pass without any change, but ultimately each fiber ar-
rives at some ganglion, in which it terminates by forming a synapsis
around one of the ganglionic nerve cells. The axon of this nerve cell
then continues the course by the nearest gray ramus communicans back
to the spinal nerve beyond the union of its anterior and posterior roots.
Up to the point where the fiber forms a synapsis with a ganglionic nerve
cell, it is medullated and is known as the preganglionic fiber. Beyond
the nerve cell, it is nonmedullated and is known as postganglionic
(page 877).
The exact ganglion in which a.given vasoconstrictor fiber becomes connected with a
nerve cell can be determined by the nicotine method of Langley. Local application to
the ganglion of a weak solution of this drug (1 per cent) paralyzes the synaptic con-
nection, so that a stimulus applied to the preganglionic fiber no longer produces its
effect. Suppose, for example, that a vasoconstrictor fiber has been found by the stimula-
tion method to travel through several ganglia, and we wish to determine in which of
these the synapsis occurs: we can do so by applying the stimulus at a point central to
the ganglia after painting each of them in turn with the nicotine solution. If the
application of the drug to a given ganglion is found 1o cause no alteration in the
effect produced by stimulation, then we know that there can not be any synaptic
connection in that ganglion, and we proceed in the same way till we have located
the ganglion in which synapsis occurs. It is important to remember that the post-
ganglionic vasoconstrictor fibers in a gray ramus communicans do not come from the
preganglionic fibers of the corresponding spinal rcot, but from fibers coming through
white rami at a highcr or a lower level.
The above description applies to the vasoconstrictor fibers proceeding to the vessels of
the anterior and posterior extremities. those for the former arising (in the dog) from
about the fourth thoracic to the tenth; and those for the latter, from the lowest thoracic
and the first three lumbar nerve roots. The cell station for the fibers to the fore limbs
is in the stellate ganglion, and for the hind limbs in the last two lumbar and first two
sacral ganglia of the abdominal sympathetic chain.
The vasoconstrictor fibers to the vessels of the head and neck run a somewhat dif-
ferent course, there being no convenient cerebrospinal nerve along which the post-
ganglionic fibers may run. The fibers to the blood vessels of the head leave the cord
by the second to the fourth or fifth thoracie roots and pass by the corresponding white
rami communicantes into the sympathetic chain, up which they run, passing through the
stellate ganglion, the ansa subclavii, and the inferior cervical ganglion, then ascending
in the cervical sympathetic to the superior cervical ganglion, where their cell station
exists. The postganglionic fibers on leaving this ganglion travel to their destination
mainly along the outer walls of the blood vessels.
The vasoconstrictors to the abdominal viscera are carried by the splanchnic nerves,
the fibers.of which come off from the lower seven thoracic and the uppermost lumbar
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234 THE CIRCULATION OF THE BLOOD
roots, The thoracic fibers pass down the sympathetic chain, which they leave by the
great splanchnic nerves. The lumbar fibers form the lesser or abdominal splanchnic
nerves. As preganglionic fibers, therefore, these fibers are carried by the greater and
lesser splanchnic nerves into the abdomen, where the former comes into close relation-
ship with the suprarenal glands, giving off a branch to the suprarenal ganglion. The
main course of the nerve is continued on to the solar plexus, in the various ganglia of
which most of the preganglionic fibers end by synapsis, the postganglionic fibers then
proceeding along the blood vessels to the vessels of the abdominal viscera. (See also
page 879).
Vasodilator fibers have a more varied origin than vasoconstrictor, and
they run an entirely different course. Vasodilator impulses may be
transmitted by fibers arising from practically any level of the cerebro-
spinal axis, not only by the motor roots, but by the sensory as well.
Thus, they pass out of the spinal cord in the posterior sacral roots to
enter the nerves of the hind limbs, as has been demonstrated by observ-
ing an increase in the volume of the curarized limb during electrical
stimulation of the exposed rootlets. The apparent inconsistency of these
observations with the well-known law concerning the direction of the
impulses contained in the posterior spinal roots is explained by assum-
ing that the dilator impulses are transmitted along the ordinary sensory
fibers, since there are no efferent fibers in these roots. They are impul-
ses which go against the ordinary stream (antidromic). In support of
this explanation it is of importance to note that at their termination
near the skin many sensory fibers split into several branches, some of
which run to blood vessels, and others to receptor organs (page 797).
Stimulation of the latter branches may cause dilatation of the local blood
vessels nearby, indicating that impulses must be transmitted up to the
point at which the branching occurs and then down the vascular branch,
this result being obtained even after the main trunk of the nerve has
been cut above the division.
For the blood vessels of the anterior extremity, the vasodilator impulses are similarly
transmitted through the posterior spinal roots of the lower cervical region of the spinal
cord. The vasodilator fibers to the abdominal viscera are transmitted with the splanchnic
nerves, but they may also be derived from the posterior spinal roots, for it has been
found that stimulation of posterior rcots in the splanchnic area causes dilatation in the
intestine (Bayliss). Vasodilator fibers are also contained in the cranial nerves, par-
ticularly the seventh and the ninth, being distributed in the former nerve to the an-
terior portion of the tongue and the salivary glands, and in the latter to the posterior
portion of the tongue and the mucous membrane of the flocr of the mouth. The vaso-
dilator fibers for the mucous membrane of the inside of the cheeks and nares have their
course in the cervical sympathetic, being distributed to the Deeper region inthe
branches of the fifth cranial nerve.
There is evidence to show that the vasodilator fibers, like the vasoconstrictor, become
connected by synapsis with nerve cells somewhere in their course. In the case of the
vasodilator fibers in the chorda tympani and nervi erigentes, such cell stations have
been clearly demonstrated in the hilus of the submaxillary gland in the former nerve
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THE CONTROL OF THE CIRCULATION 235
and in the hypogastrie plexus situated on the neck of the bladder in the latter.
It is therefore commonly assumed that, although not recognizable by histologic methods,
such terminal cell stations must also exist in close association with all blood vessels
to which the vasodilator fibers run. Whether or not such peripheral cell stations exist,
there is a marked difference between the course of vasodilator and of vasoconstrictor
fibers.
The Vasomotor Nerve Centers
Our next problem is to trace these fibers farther into the central
nervous system, and find the location and study the characteristics of
the nerve centers from which they are derived. We must postulate the
existence of both vasoconstrictor and vasodilator centers, but since there
is no adequate evidence at the present time which enables us to locate
the latter, we must confine our attention to the vasoconstrictor centers.
These exist at two levels in the cerebrospinal axis: (1) in the gray mat-
ter of the spinal cord, and (2) in the gray matter of the medulla
oblongata.
The spinal, or as they are often called, the subsidiary vasoconstrictor
centers, are represented by certain cells of the lateral horn of gray mat-
ter in the thoracic portion of the spinal cord, from which the pregan-
glioniec vasoconstrictor fibers above described are derived. The exact
location of the nerve cells composing the chief centers in the medulla has
not as yet been definitely made out; they undoubtedly lie near those of
the vagus center (see Ranson). The axons of the medullary cells de-
scend in the lateral columns of the spinal cord to end by synapses
around the cells of the subsidiary vasoconstrictor center in the lateral
horns.
The experimental evidence which indicates the existence of chief and
subsidiary centers is quite definite. Thus, if the spinal cord is cut at the
lower cervical region (below the phrenic nuclei, so as not to interfere
with the movements of the diaphragm), the arterial blood pressure falls
profoundly, because the pathway connecting the two centers is broken.
After several days, however, the blood pressure will gradually rise again.
If after this has occurred, the spinal cord is destroyed by pushing a wire
down the vertebral canal, the arterial blood pressure will again fall,
indicating that the vascular tone which had been reacquired after sec-
tion of the pathway between the main and the subsidiary centers must
have been brought about by the development in the subsidiary centers
of an independent power of reflex tonic action. This experiment there-
fore demonstrates that in the intact animal the subsidiary centers do not
by themselves discharge tonic impulses. In other words, the subsidiary
centers ordinarily serve merely as transfer stations for the tonic im-
pulses coming from the chief center, but when these impulses no longer
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236 THE CIRCULATION OF THE BLOOD
arrive, then a hitherto dormant power e tonic activity becomes devel-
oped in the subsidiary centers.
Independent Tonicity of Blood Vessels
Even after complete disconnection of the spinal cord from the blood
vessels, as by cutting of the splanchnic nerve to the abdomen or abla-
tion of that portion of the lower spinal cord from which the fibers to
the hind limb arise, the disconnected blood vessels, although at first
completely dilated, may later reacquire an independent tone of their
own, indicating therefore, that they must possess some neuromuscular
mechanism which can act independently of the nerve centers, and which
may be stimulated to activity by the presence of hormones in the blood.
The hormone was at. one time thought to be epinephrine (sec page 745).
Epinephrine control is indicated in the effect produced upon arterial
blood pressure by stimulation of the great splanchnic nerve. Careful
analysis of the curve, shown in Fig. 29, shows that the rise is both im-
mediate and delayed; that is, the curve mounts immediately, then flat-
tens out a little, and then assumes a further rise. This delayed response
seems to depend upon the exeretion of epinephrine into the blood, for it
does not occur when the suprarenal veins are occluded, and is much de-
layed by temporarily clamping the suprarenal veins on the same side
as that on which the splanchnic nerve is stimulated. It has been stated
by certain observers that, after occlusion of the adrenal veins, there is
a downward tendency of the blood pressure, which however develops
with extreme slowness; and that a distinct elevation of blood pressure
follows the removal of a clamp temporarily placed on the adrenal veins.
This rise is pronounced if the splanchnic nerve is stimulated during the
occlusion of the veins. It must of course be understood that the imme-
diate rise in blood pressure following splanchnic stimulation is caused by
vasoconstriction in the splanchnic area itself, as is evidenced by the
fact that it does not occur, or is only very faint, when the abdominal
blood vessels are ligated prior to the stimulation of the splanchnic nerve.
Even after ligation of the adrenal veins and of the blood vessels of the
splanchnic area, stimulation of the splanchnic nerve may still cause a
slight rise in arterial blood pressure, possibly because some fibers may
run from the splanchnic to vascular areas not situated within the realm
of the splanchnic nerve—for example, the blood vessels of the lumbar
muscles.
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CHAPTER XXVII
THE CONTROL OF THE CIRCULATION (Cont’d)
CONTROL OF THE VASOMOTOR CENTER
The activities of the vasomotor center are controlled partly "by hor-
mones and partly by afferent impulses.
The Hormone Control
As with the respiratory center, the chief hormone is the hydrogen-ion
concentration of the blood. When this is inercased, as in asphyxia, the
vasoconstrictor part of the vasomotor center becomes stimulated, so
that the blood vessels are constricted and the blood pressure rises. Tak-
ing, as our criterion of hydrogen-ion concentration, the tension of the
carbon dioxide in the blood (see page 354), we may proceed to investi-
gate the relationship by observing the blood pressure during changes
in the carbon-dioxide tension brought about by causing the animal to
breathe atmospheres containing known percentages of the gas (Mathi-
son’), Thus, if a decerebrate cat is made to respire an atmosphere
containing 5 per cent or more of carbon dioxide, an immediate rise
occurs in the arterial blood pressure. That the inhaled carbon dioxide
acts by raising the hydrogen-ion concentration of the blood is indicated
by the fact that a similar rise in blood pressure can be obtained by intra-
venous injection of a weak solution of lactic acid (2 ¢.c. N/15) in a de-
cerebrate cat:
Instead of injecting the lactic acid, we may cause it to be produced
in the muscles of the animal itself by greatly diminishing their oxygen
supply. When a decerebrate cat, for example, is made to breathe an
atmosphere of almost pure nitrogen, there is, after a latent period of
about 30 seconds, a sudden rise in arterial pressure. The existence of
this latent period in the latter case, as compared with its absence when
carbon dioxide is inspired, is owing to the time taken for lactic acid to
be produced in the muscles on. account of the oxygen deprivation. It
is important to note in the above experiment that decerebrate animals
are employed so as to avoid the necessity of using anesthesia, under
which the results are much less definite. The faet that oxygen depriva-
tion causes excitation of the vasoconstrictor center has been known for
237
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238 THE CIRCULATION OF THE BLOOD
some time, but the explanation that has usually been given has been that
it is due to a direct effect of oxygen want on the center.
The sensitivity of the medullary center towards the hydrogen-ion is
many times greater than that of the subsidiary centers in the spinal
cord. If an animal is kept alive by artificial respiration for some time
after cutting the cervical spinal cord, the subsidiary vasomotor centers
will, as we have seen, gradually acquire a tonie action, and the lowered
blood pressure will gradually rise again. If, when this has been attained,
the animal is made to breathe an atmosphere rich in carbon dioxide, a sud-
den rise in blood pressure will occur, but to produce it a very much
greater percentage of this gas must be inspired than when the pathway
between the chief and subsidiary centers is intact. Whereas 5 per cent:
carbon dioxide is sufficient to cause a rise of pressure in an animal hav-
ing its chief vasomotor center, it takes 25 per cent and upward to pro-
- duce a like effect on a spinal animal; and similarly, although 2 «.c. of
N/15 lactic acid will stimulate the chief vasomotor center, it takes 5 e.c.
of N/2 to excite the spinal-cord centers.
The Nerve Control
However important hormones may be in maintaining a tonic stimula-
tion of the center, the more sudden changes in activity are mainly
brought about by afferent nerve impulses. The afferent impulses are
of two classes: (1) those causing a rise in blood pressure, called
pressor, and (2) those causing a fall in blood pressure, called depressor.
The effect produced on the arterial blood pressure by stimulation of
either pressor or depressor fibers is usually more or less evanescent,
especially in the case of the depressor fibers; and when the change fol-
lowing stimulation of the nerve passes off, the blood pressure always
returns to its former level. This indicates that the afferent impulses do
not affect the tonic control which the vasomotor center exercises on the
blood vessels. It has, therefore, been assumed by Porter™ that there are
really two kinds of vasomotor centers: one concerned merely in the
bringing about of temporary reflex changes, the other concerned in the
maintenance of the vascular tone. It may be that the activities of the
former are primarily dependent upon afferent impulses, and the latter,
upon hormones. Justification for this view has been found in observa-
tions made on the effects of stimulation of pressor and depressor fibers
in animals under the influence of curare or alcohol. With the former
drug, stimulation of a nerve containing a preponderance of pressor or
depressor fibers produces double its usual effect, but the mean level of
the blood pressure apart from this effect remains unchanged. With the
latter drug (alcohol), on the other hand, the reflex response entirely
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THE CONTROI OF THE CIRCULATION 239
disappears, although it immediately reappears when the alcohol effect
has passed off, and there is no evidence of a change in tone. The tonic
and the reflex mechanisms of the vasomotor center can not therefore be
identical.
At the present stage of our knowledge, it is only possible for. us to
study the effect of stimulation of pressor and depressor fibers on the
vasoreflex center. Such fibers are contained in practically every sen-
sory nerve of the body, and it would appear that a fairly equal mixture
of both kinds of fiber exists in most of these nerves.
Pressor and Depressor Impulses.—Depressor impulses are alone present
in the cardiac depressor nerve. Sometimes as in the rabbit, this exists
as an independent nerve trunk, originating by two branches, one from
the superior laryngeal, the other from the vagus, and descending close to
OTH
Ra A ee
STMT TTT er
NSU
Tig. 73.—Fall of blood pressure from excitation of the depressor nerve. The drum was
stopped in the middle of the curve and the exvitation maintained for seventeen minutes. The line
of zero pressure should be 30 mm. lower than here shown. (From Bayliss.)
the vagus trunk, to end around the arch of the aorta. In other animals
the depressor is bound up with the vagus trunk from which it can some-
times be separated by careful dissection. The first prerequisite in inves-
tigating the cause of the changes produced by stimulation of these nerves
is the elimination of any chance of an alteration in heartbeat as a result
of simultaneous stimulation of afferent vagus fibers. This may be done
either by cutting both vagi or by administering atropine.
Stimulation of the central end of the cardiac depressor nerve in such
an animal causes an immediate fall in blood pressure, accompanied by an
increase in volume which can be demonstrated either in the hind limb or in
one of the abdominal viscera—evidence of general vasodilatation (Fig. 73).
When the central end of a sensory nerve, such as the sciatic, is acted
on by a stimulus of moderate strength, it will usually be found that the
arterial blood pressure rises and that the volume of the limb or of some
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240 THE CIRCULATION OF THE BLOOD
abdominal viscus becomes diminished—evidence of general vasoconstric-
tion. But when the sensory nerve is stimulated with extremely weak
faradic shocks, an entirely different result is likely to be obtained;
namely, a fall of blood pressure and an increase in volume of the limb
or viscus, indicating that in this manner we have stimulated depressor
fibers. By careful experimentation with quantitatively graduated elec-
trical stimuli, it has been found by Martin and others’ that on stimu-
lating an afferent nerve with weak shocks, a fall in blood pressure is
the first effect to be observed, and that this becomes more and more
marked as the strength of the stimuli is increased, until a certain opti-
mum is reached, after which the fall in blood pressure becomes less evi-
dent. When a certain strength of stimulation is exceeded, a rise instead
of a fall occurs. After this point additional increase in stimulation causes
more and more marked elevation of blood pressure through a very long
range of stimuli.
Stimulation of two afferent nerves at the same time usually produces
a greater reflex vasomotor change than the stimulation with an equiva-
lent strength of current of either nerve alone. That is to say, the effect
produced by stimulating the central end of both sciaties simultaneously
will be greater than that produced by stimulating either alone with double
the strength of stimulus.
As has been stated above, the reflex change in blood pressure is often
quite transitory in nature, although the stimulation of the pressor nerve is
maintained. When this decline has occurred, the pressor reaction can
often be renewed by shifting the stimulation to a second nerve. These
faets concerning the greater efficacy of combined stimulation of several
nerves are of considerable importance in connection with the general
question of reflex changes in blood pressure. For instance, many of the
pressor fibers found in the sciatic nerve are connected with the receptors
that mediate the sensations of the skin. "When these receptors are
stimulated, as by heat or cold, reflex changes in blood pressure occur
(pressor reaction), (Fig. 74), and it is important to remember that
localized stimulation of the skin is less efficient in bringing about such
vascular changes than stimulation applied over large areas, even when
the local stimulus is intense and the general stimulus mild in character.
Jumping into a moderately cold bath will cause a much greater rise in
arterial blood pressure than plunging the hand into ice cold water.
Mechanism of Action of Pressor and Depressor Impulses.—When we
consider the exact mechanism by which these afferent impulses operate,
we have to bear in mind four possibilities: the reflex fall produced by
‘stimulation of a depressor afferent fiber may be due either to a stimula-
tion of the vasodilator part of the center or to an inhibition of the tone
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THE CONTROL OF THE CIRCULATION 241
cf the vasoconstrictor part; and, conversely, a rise in arterial pressure
caused by vasoconstriction may be dependent either on a stimulation of
the vasoconstrictor part of the center or on an inhibition of the tone of
the vasodilator part. All of these changes have, as a matter of fact, been
shown to occur, at least under certain conditions, although the evidence
Heat ieee
Fig. 74.—The effect of strong stimulation (heat) of the skin of the foot on the arterial blood
pressure and respiratory movements. Upper tracing, thoracic movement; lower tracing, arterial
bléod pressure.
for the inhibition of dilator tone is as yet a little uncertain (see Fig. 75).
Without going into the subject in detail, we may nevertheless take
as an example of the methods by which the information has been ob-
tained, the experiment performed by Bayliss,* showing that the vasodi-
lation which results from stimulation of the depressor nerve is owing
partly to removal of vasoconstrictor tone and partly to vasodilator
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242 THE CIRCULATION OF THE BLOOD
stimulation. The volume of the hind limb of a curarized and vagotomized
rabbit increases when the central end of the cardiae depressor nerve is
stimulated. In order to determine whether this dilatation is due solely
to the removal of vasoconstrictor tone, the above experiment was repeated
on a rabbit in which the sympathetic chain had been cut below the level
of the second lumbar spinal roots. By such an operation all the vaso-
constrictor fibers to the vessels of the hind limb are severed, but the
vasodilator fibers, since they emanate through the sacral sensory roots,
are left intact. It was nevertheless found on stimulating the depressor
nerve that dilatation of the hind limb still occurred, thus indicating
C.C,
Fig. 75.—Diagram showing the probable arrangements of the vasomotor reflexes.
A. Muscle of artefole.
_ D. Vasodilator nerve fiber terminating on A and inhibiting its natural tonus, as indicated by
sign.
D. Vasoconstrictor’ fiber also ending in A, but exciting it (+). These two kinds of fiber arise
from the dilator center (DC) and the constrictor center (CC) respectively.
F. Afferent depressor fiber, dividing into two branches, one of which (—) inhibits the con-
strictor center, while the other (+) excites the dilator center causing dilatation of the arteriole and
fall of blood pressure. 2
R. Pressor fiber exciting CC and inhibiting DC, and therefore causing vasoconstriction and rise
of blood pressure.
a, b, c, and d represent the synapses of the pressor and depressor branches with the efferent
neurons. (From Bayliss.)
that stimulation through vasodilator fibers must have taken place. Con-
versely, in another experiment, instead of the sympathetic chain, the
spinal cord was cut below the level of the second lumbar segment, thus
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THE CONTROL OF THE CIRCULATION 243
severing the dilator but not the constrictor path, and again depressor
stimulation caused the volume of the limb to increase, indicating that
an inhibition of constrictor tone must have occurred.
Reciprocal Innervation of Vascular Areas
It must not be imagined that changes in the caliber of the blood ves-
sels occurring in one vascular area are necessarily occurring all over
the body. On the contrary, a most important reciprocal relationship
exists in the blood supply to different parts. After food is taken, for
example, more blood is required by the digestive organs than when they
are at rest, and this is insured by dilatation of their own vessels along
with reciprocal constriction of those of other parts of the body. On
account of the relatively great capacity of the abdominal vessels, their
dilatation during digestive activity is usually greater than the reciprocal
constriction of the other vessels, so that the diastolic blood pressure falls,
necessitating a more powerful cardiac discharge in order to maintain
the mean pressure. After taking food, the systolic pressure does not
as a rule fall so much as the diastolic, if it falls at all; and the pres-
sure pulse therefore becomes greater and causes a greater live load: to
be applied to the vessels with each heartbeat. During the sudden strain
that is thrown on them, weakened arteries may give way,-especially in
the brain.
Another example of reciprocal action of the vascular system is seen
in muscular exercise. The vessels of the active muscles dilate, while
those elsewhere constrict. The local dilatation in this case is, however,
not entirely at least a nervous phenomenon, being caused in fact, as we
shall see, by hormone action on account of the local increase in hydro-
gen-ion concentration (see page 414). There can be little doubt that
local irritants to the surface of the body, such as hot applications, lini-
ments, etc., act.in the same way; they cause local dilatation of the super-
ficial and perhaps of the immediately underlying vesgels and constric-
tion of those elsewhere in the body. Application of cold to local areas
of skin similarly causes local constriction accompanied by reciprocal
dilatation elsewhere. This action of cold is very marked in some parts of
the body, such as the hands, where by Stewart’s method (page 283) it
can be shown, not only that the bloodflow of the hand to which the cold
is applied is greatly curtailed, but also that of the opposite side.
Experimental demonstration of reciprocal vascular innervation is fur-
nished by numerous experiments. If the central end of the great auric-
ular nerve of the ear is stimulated in a rabbit, a blanching of the ves.
sels of the ear occurs at the same time as a rise in arterial blood pres-
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244 THE CIRCULATION OF THE BLOOD
sure (Lovén reflex). Similarly when the central end of one of the sen-
sory roots of the leg of a dog is stimulated, there is a rise in arterial
‘blood pressure and an increase in the volume of the limb.
THE INFLUENCE OF GRAVITY ON THE CIRCULATION
If the arterial blood pressure is measured in the arm and leg in a man
standing erect, a difference corresponding to the hydrostatic effect of
gravity will be found between the two readings. In comparison with
the high pressure normally existing in the arteries, this difference is,
however, of little significance. On the other hand, in the veins, where
the average pressure is low, gravity would cause serious embarrassment
to the circulation of blood were it not for the valves and the forces
which move the blood beyond them (page 214).
In erect animals the part of the circulation in which blood might stag-
nate as a result of gravity is the splanchnic area. Were such stagna-
tion to occur, the blood would not be returned to the right heart, so
that the arteries would not receive sufficient blood to maintain an ade-
quate circulation, particularly in the vessels of the brain.
Simple experiments devised by Leonard Hill’ ?* illustrate these prin-
ciples. When a snake, for example, is pinned out on a long piece of
wood and an opening made opposite the heart, this organ can be seen
‘to fill adequately with blood as long as the animal is maintained in the
horizontal position. When placed vertically, however, the heart be-
comes bloodless. If now the tail end of the animal is placed in a cylinder
of water so as to overcome the effect of gravity, the heart will be seen
to fill again with blood. Evidently in such an animal there is no mechan-
ism to compensate for gravity.
If a domestic rabbit with a large pendulous abdomen is held in the
vertical tail-down position, stagnation of blood in tlie splanchnic ves-
sels occurs to such an extent that in from fifteen to twenty minutes the
animal dies from cerebral anemia. If an abdominal binder is first of all
applied, the vertical position will not have the same consequences. This
experiment illustrates clearly the possible evil effects that gravity may
produce in animals in which no mechanism exists to compensate for it.
Placing an animal such as a dog under light ether anesthesia in the
vertical tail-down position produces an immediate fall in arterial blood
pressure, as shown in the tracing (Fig. 76), followed by a certain de-
gree of compensation even while the animal is still in the erect position.
The extent to which this compensation occurs varies with the depth of
the anesthesia. If the experiment is repeated after administering a large
dose of chloroform, not only will the initial fall be much greater, but
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THE CONTROL OF THE CIRCULATION 245
subsequent compensation will be practically absent. The application of
these facts in the operating room will be self-evident.
Leonard Hill has shown that three factors are involved in the com-
pensating mechanism: (1) the tonicity of the abdominal musculature;
oF 1 Ws ene
r f | r My iho ii Hi
V 5 oe ry i
Se
Fig. 76.—Aortic blood pressure, showing the effect of posture: A, vertical, head-up; B, hori
zontal; C, vertical, head-down; D, horizontal. (L.H.)
(2) the tone of the splanchnic blood vessels; (3) the pumping action of
the respiratory movements. The importance of the first-mentioned fac-
tor can be readily shown by making a crucial incision of the abdom-
inal walls in an animal in the erect position (Fig. 77), and that of
Hi |
7 |
Ay
| ay
Fig. 77.—Tracing to show the effect of gravity on the arterial blood pressure. At A, the
animal was placed in the vertical position; at B, the abdomen was compressed; at C, a crucial
incision was made in the abdomen; at D, the pleural cavity was opened; at F, the animal was
returned to the horizontal position. (From Leonard Hill.)
the second factor by cutting the great splanchnic nerves, or the spinal
cord. After such an operation, even while in the horizontal position, as
we have seen, the blood pressure falls to a considerable extent. If the
animal is now placed in the vertical tail-down position, however, it falls
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246 THE CIRCULATION OF THE BLOOD
to the zero line and the animal soon dies (Fig. 78). The influence of the
third factor is not so great as of the other two, but can be shown by the
increased respiratory activity which is likely to develop in the vertical
Ee LCT Ta
oe RS nine
Fig. 78.—The effect_of gravity on the aortic pressure after division of the spinal cord in the
upper dorsal region. By placing the animal in the vertical feet-down posture, the pressure fell
almost to zero, but on returning it to the horizontal posture, the circulation was restored. (From
Leonard Hill.)
tail-down position, the anemic condition of the respiratory center being
no doubt the cause of the increased respiration.
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CHAPTER XXVIII
PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA
Up to the present we have been considering the circulation of the blood
from a general point of view. There are certain organs and tissues, how-
ever, in which the general mechanism is altered in order to meet pecu-
liar requirements of blood supply. Thus, it is evident that the brain,
ineased as it is in the rigid cranium, will be unable to contract and
expand as a result of vasoconstriction or vasodilation. On the other
hand, we know that the blood supply to this organ does vary con-
siderably from time to time. What is the nature of the mechanism by
which such changes are brought about? In the ease of the liver the cir-
culation is peculiar on account of the fact that blood is carried to the
organ by two vessels, in one of which it is supplied under high pressure
and in the other, under low pressure. We must investigate the rela-
tionship of these two sources of blood supply. The circulation through
the coronary and pulmonary vessels must likewise receive special atten-
tion on account of the highly specialized functions of these organs.
THE CIRCULATION IN THE BRAIN
Anatomic Peculiarities
Serious curtailment of the blood supply to the brain is guarded against
by the existence of the circle of Willis. Besides the four main arteries—
the vertebrals and the two carotids—the spinal arteries contribute to
the blood supply of the circle, and consequently in certain animals, such
as the dog, the four main arteries may be ligated without causing death.
In man, however, ligation of both carotids is usually fatal. The free
anastomosis displayed in the circle of Willis is not maintained in the
case of the arteries which run from it to supply the brain structure. On
the contrary, these vessels are more or less terminal in character; that
is to say, the capillary system produced by the different vessels does not
freely anastomose, so that the obstruction of one vessel, or an important
branch, is followed by death of the supplied area. The vessels which go
to the pia mater, however, break up into numerous smaller branches,
which freely anastomose before entering the brain tissue.
sae 247
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248 THE CIRCULATION OF THE BLOOD
The venous blood is collected by the small, very thin-walled and. valve-
less cerebral veins. These run together to form larger veins dis-
charging into the sinuses, the openings into which are kept patent by
the arrangement of dura mater around the orifices. The sinuses exist
between the dura and skull and are so constructed that they can not
be compressed, particularly those at the base of the brain. From them
the blood is conveyed mainly to the internal jugular vein, some of it
however escaping by the anastomoses existing between the cavernous
sinus and the opththalmic veins, and by the venous plexus of the spinal
cord. The most striking peculiarities of the veins are their patulous con-
dition and the absence of valves, so that any change in the blood pres-
sure in the internal jugular vein must be immediately reflected in that of
the venous sinuses. This explains why compression of the abdomen
Fig. 79.—Schema to show the relations of the Pacchionian bodies to the sinuses: d, d, Folds
of the dura mater, inclosing a sinus between them; v.b., the blood in the sinus; @, the arachnoidal
membrane; p, the pia mater; Pa., the Pacchionian body as a projection of the arachnoid into the
blood sinus. (Irom Howell’s Physiology.)
causes venous blood to flow from an opening made in the longitudinal
sinus.
In considering the cerebral circulation, another factor that must be
borne in mind is the presence of cerebrospinal fluid. This is contained
in the subarachnoid spaces of the brain and spinal cord, these spaces, in
the case of the brain, being often considerably enlarged to form the
cisterne. The cerebrospinal fluid is also present in the ventricles of the
brain, which it will be remembered communicate with the subarachnoid
spaces through the foramen of Magendie, etc. It is unlikely that the
cerebrospinal fluid is of much importance in connection with the control
of the blood supply to the brain tissue. It may be merely a lubricating
fluid; at least it is so small in amount (60 to 80 ¢.c. in man) as to be
apparently of little value in bringing about an alteration in brain volume.
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PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 249
Although normally so scanty, its secretion can become remarkably stim-
ulated under certain conditions as in-fractures of the base of the skull.
Under these conditions in man, it may drain away at the rate of about
200 ¢.e. a day or more.
The fluid is apparently secreted from the choroid plexus, for when
the pathways by which the ventricles communicate with the subarach-
noid space are obstructed, it collects in the ventricles, producing internal
hydrocephalus. Under certain conditions its absorption is also very
rapid, as shown experimentally by the rapidity with which physiologic
saline is absorbed when it is injected into the subarachnoid space. This
absorption is believed to occur through the Pacchionian bodies, which
are minute sac-like protrusions of the arachnoid into the interior of a
venous sinus. The membrane that separates blood and cerebrospinal
fluid is extremely thin at these places (Fig. 79).
Physical Conditions of Circulation
On account of these anatomic peculiarities, the physical factors con-
trolling the circulation of blood to the brain are considerably different
from those obtaining in any other part of the body, with the possible
exception of the bones. In other vascular areas, we have seen that, when
dilatation or constriction of the vessels occurs, a marked increase or
diminution of the volume of the part becomes evident. Such a change
in volume is evidently impossible in the case of the brain because of
the rigid cranium in which it is contained. In fact, from a physical
point of view we must consider the blood vessels of the brain as pro-
jecting into a rigid case filled with incompressible material. Under
these conditions it is obvious that the vessels as a whole could neither
contract nor dilate without some increase or decrease in the volume of
the contents of the cranial cavity (Leonard Hill’).
Some have thought that the cerebrospinal fluid as it flows into or out
of the spinal cord might accomplish this alteration in the cranial con-
tents, but the relatively small amount of available cerebrospinal fluid,
the smallness of the openings between the brain and the spinal cord, and
the lack of experimental evidence that such changes in volume of cere-
brospinal fluid in the spinal cord do actually occur, all stand in contra--
diction to such a view. However, although the vessels as a whole might
not contract or expand, yet some vessels, like the arteries, might con-
tract simultaneously with a corresponding dilatation of other vessels,
such as the smaller cerebral veins. In admitting the possibility of some
reciprocal relationship between arteries and veins, we must remember
that it is only before the well-protected sinuses are reached that a
change in the caliber of the veins would be possible. But it is difficult
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250 THE CIRCULATION OF THE BLOOD
to see how such reciprocal dilatation and constriction could be of any
advantage except perhaps in causing certain areas to receive more
blood than others. A reciprocal relationship might also exist between
adjacent arterioles as well: as between arterioles and veins; when, for
example, the arm center becomes active, it is conceivable that its
arterioles might dilate at the same moment that those of a neighboring,
less active center become constricted. Alterations obviously might oc-
cur without causing any perceptible change either in the volume of the
brain as a whole or in the condition of venous flow.
In consideration of these factors, most observers are agreed that the
total volume of blood in the brain must be constant at all times (Monro
and Kellie doctrine). Alteration of blood supply ean, however, still be
brought about by changes in the velocity with which the blood traverses
the vessels. When more blood is required in the brain to supply the
increased metabolism which we must presume accompanies heightened
mental activity, it is not accomplished as in other parts of the body by
an increase in the capacity of the vessels as compared with those of
other vascular areas, but by a hurrying up of the circulation through
vessels whose caliber remains unaltered.
The main factors determining the velocity of bloodflow through the
brain must, therefore, be dependent upon changes occurring elsewhere
in the vascular system, a conclusion for which there is abundant experi-
mental evidence. Of the many ingenious methods that have been de-:
vised to secure this evidence, we will cite but one in this place. Records
are taken of changes in: (1) the venous blood pressure of the brain by
connecting a cannula either with the vein immediately after leaving the
skull or, better still, with the torcular Herophili; (2) the brain volume,
by connecting a very sensitive receiving tambour with a trephine. hole
in the cranium so that its open end lies against the pia mater.*’ Al-
though, as we have seen, while incased in the rigid cranium the brain
volume can not change to any degree, yet this will oceur when a
portion of the cranium is removed, so that pulsations correspond-
ing to those in the blood vessels will be observed; (3) the circula-
tory conditions elsewhere in the body, by taking arterial and
venous pressures and plethysmograms. The results in a normal an-
imal show the following points (see Fig. 80): (1) The tracings of
the arterial blood pressure (A), the brain volume (C) and the intra-
cranial venous pressure (C) have exactly the same contour—that is,
the respiratory and the cardiac waves in all three of them are identical.
The venous blood as it flows into the jugular veins also pulsates in
*This receiving tambour really consists of a brass tube of the same diameter as the trephine
hole, into which it is tightly fitted. The brass tube is closed at its inner end by thin rubber membrane,
and its outer end is connected with the receiving tambour.
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PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 251
unison with the artery. (2) Any change in the blood pressure of the
systemic venous system is immediately reflected in the blood pressure
of the sinuses of the brain and in the brain volume (not well shown in
accompanying tracing). (3) A change never occurs in the vessels of
the brain which can not be accounted for by some change occurring
Fig. 80.—To show simultaneous records of the arterial blood pressure (4), the venous pres-
sure (B), the intracranial pressure (C), the pressure in the venous sinuses (dD). The fall in ar-
terial pressure produced by stimulation of the cerebral end of the vagus will be found to cause
a fall of intracranial and cerebral venous pressure, accompanying that in the arteries, but a rise
in that of the venous system. (From Leonard Hill.)
elsewhere in the vascular system outside the cranial cavity. This re-
sult is important because it shows that there can not be vasomotor
nerve control of the brain vessels.
Taking into consideration not only the results of such experiments,
but also the peculiar physical conditions existing in the eranial cavity,
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252 THE CIRCULATION OF THE BLOOD
we must conclude that changes in blood supply depend on changes in
the velocity of the bloodflow, and that such alterations in velocity are
dependent upon changes occurring in the aortic and more especially
in the vena-cava pressure. When the aortic pressure rises, more blood
will flow into the cerebral arteries and move along them at an increased
velocity, the increased pressure probably causing a moderate degree
of passive dilatation, to allow extra room for which the numerous
small cerebral veins become compressed. This compression of the veins
probably does not obstruct the greater flow of blood through them, be-
cause, taken as a whole, they are ordinarily much more ecapacious than
need be. On the other hand, if the aortic pressure should remain con-
stant, but that in the vena cava increase, then there would be obstruc-
tion to the passage of blood in the intracranial arteries, and conse-
quently a diminished velocity of flow.
Vasomotor Nerves
It might be inferred that, since the bloodflow through the cerebral
vessels is mainly dependent on vascular conditions elsewhere in the
body, there would be no need, as in the vessels of other vascular areas,
for vasomotor fibers. Histologists have, however, discovered the pres-
ence of such fibers, and it has become necessary for the physiologist to
find out if they are really of importance in connection with the regula-
tion of the blood supply to the brain. Even if it is admitted that the
arterioles could not contract or expand as a whole without producing
local changes in venous pressure or cranial volume, it is yet of course
always possible, as has already been pointed out, that one set of arte-
rioles might contract at the same moment that another set expanded.
That the vessels can undergo a process of constriction has been shown
by experiments in which the volume of outflow from the vessels of
the brain was measured in perfused preparations of brain. "When
epinephrine was added to the perfusion fluid, curtailment of outflow
was observed to occur (Wiggers). Since this drug causes constriction of
vessels only when these are supplied with constrictor fibers (see page
736), the conclusion may be drawn that the cerebral blood vessels do
contain such nerve fibers. Nevertheless, the local vasomotor control of
the cerebral blood vessels can not have the significance in connection
with changes in blood supply that it has for other vascular areas (Hill
and Macleod?*). No doubt nerve fibers are present in the cerebral
blood vessels, and presumably under certain conditions they are capable
of causing the blood vessels to undergo alterations in caliber, but it is
impossible to see of what real value this can be under normal conditions.
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PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 253
Intracranial Pressure
One word more with regard to what is known as intracranial pressure,
that is, the pressure in the space between the skull and the brain.
Under ordinary conditions it must be equal to that in the cerebral capil-
laries, and may be measured by connecting a sensitive manometer with
a tube screwed into the cranium as described above. It has been found
to vary from 0 mm. Hg in a man standing erect to 50-60 mm. Hg in a
dog poisoned by strychnine. It becomes increased, not only by com-
pression of the veins of the neck and by an increase in general arterial
pressure, but also in pathologic conditions, such as hydrocephalus. A
new growth in the brain, if it occupies more space than the tissue which
is destroyed, exerts pressure on all parts of that region of the cranial
cavity, but this pressure may not be transmitted equally throughout
the cranial contents, for the falciform ligaments and the tentorium sup-
port a part of it, thus directing the spread of pressure along certain
pathways. The structures at the base of the brain, the optic nerves,
the veins of Galen and the Sylvian aqueduct are most: affected in this
way. If the pressure is rapidly applied, however, it may rise through-
out the cranial contents. In such cases the pressure is, of course, cir-
culatory in origin, since immediately after death from cerebral tumor
the intracranial pressure is not found to be raised.
The major symptoms of cerebral compression are no doubt due to
anemia of the medulla oblongata, which may be the result either of
pressure applied locally in the bulbar region, where the presence of a
very small foreign body or only trivial tumor formation is sufficient to
destroy life, or of pressure transmitted from the cerebral cavity, in
which case, on account of the support offered by the tentorium, a much
larger growth is required to affect the medulla. Internal hydrocephalus
produced by blocking of the aqueduct of Sylvius and the veins of Galen
causes the greatest rise in intracranial tension, and may affect the me-
dulla, because the brain is driven downwards so as to pinch the bulb
against the occipital bone. It must be emphasized that it is not the
pressure per se that causes the symptoms, but the attendant anemia,
the symptoms of acute cerebral anemia and of compression being iden-
tical (Leonard Hill’®). To relieve the compression, trephining is the
common practice. The trephine hole should be as large and as near |
to the source of compression (tumor, ete.) as possible.
CIRCULATION THROUGH THE LUNGS
The pulmonary or lesser circulation, as it is called, is quite different
from the systemic circulation. In the first place, because the pressure
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254 THE CIRCULATION OF THE BLOOD
in the pulmonary arteries does not amount to more than about 20 mm.
Hg, or about one-sixth of that of the systemic arteries, the peripheral
resistance in the blood vessels of the lungs is much less than that of
the body in general. This lower resistance is owing partly to the large
diameter of the arterioles and the small amount of muscular fibers in
their walls, and partly to the fact that the capillaries are held con-
stantly in a somewhat dilated condition on account of the subatmos-
pherie pressure in the thorax (see page 306).
Another peculiarity of the pulmonary circulation is that ‘the caliber
of the vessels is to a very large extent dependent upon the changes
that occur in the intrathoracic pressure with each inspiration and ex-
piration. They become dilated on inspiration and contracted on ex-
piration. The extent to which these respiratory changes affect the
amount of blood contained in the lungs, is very considerable. At the
height of inspiration it is computed that a little more than eight per
cent of the whole blood in the body is contained in the lungs, whereas
on expiration it diminishes to between five and seven per cent.
A third peculiarity is that the pulmonic blood vessels are not sup-
plied with vasomotor nerve fibers—at least with such as can readily be
demonstrated. It is said that, when the pulmonary vessels are per-
fused and the outflow measured, a diminution in the latter is found to
occur when epinephrine is added to the injection fluid—a result which
is, however, denied by certain investigators. Changes in the bloodflow
have not been observed to occur when the vagus or sympathetic nerve
fibers running to the lungs are stimulated. In short, the conelusion
which we must draw is. much the same as that for the blood vessels
of the brain—namely, that although, as a result of the epinephrine ex-
periment, we must admit that a vasomotor supply may possibly be
present, yet it is one which can be of no significance under normal
conditions.
When there is obstruction to the outflow of blood from the left ven-
tricle, as, for example, in cases of high aortic pressure, the blood is not
entirely discharged with each beat of the left ventricle, and therefore
dams back through the left auricle into the lungs. On account of the
marked distensibility of the pulmonary capillaries, a large amount of
this blood may collect there and thus make the lungs serve as a kind of
reservoir of the heart. When the capacity of this reservoir has, how-
ever, been overstepped, an increased peripheral resistanee will come to
be offered to the movement of blood in the pulmonary arteries, the
pressure in which will consequently rise and sooner or later interfere
with the discharge from the right ventricle, causing as a result a stag-
nation of blood in the systemic veins, and a consequent increase in vol-
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PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 255
ume of such viscera as the liver and kidneys. The same changes will
obviously also supervene when there is regurgitation of blood from the
left ventricle to the left auricle, as in cases of mitral insufficiency.
CIRCULATION THROUGH THE LIVER
The liver is the only gland in the body receiving both venous and
arterial blood, the former being supplied to it at a very low pressure
by way of the capacious portal vein, and the latter at very high pressure
by the strikingly narrow hepatic artery. Except for the relatively
small amount of blood which is supplied to the walls of the blood vessels
and the piliary ducts, none of the hepatic artery blood mixes with that of
the portal vein until the vessels enter the hepatic lobules. Beyond this
point the two blood streams mix and the combined stream is. drained
away by the sublobular and hepatic veins.
Methods of Investigation
To study the relative importance of these two sources of blood sup-
ply, and also to investigate the manner in which the latter is controlled,
the most satisfactory method has consisted in measurements of changes
in volume flow rather than in those of changes in pressure. The vol-
ume-flow measurement has been made either by connecting stromuhrs
(page 207) to the hepatic artery or portal vein, or by measuring the out-
flow of blood from the hepatic vein into the vena cava, first with both ;
inflow vessels intact, and then with one of them ligated. An objec-
tion to the first (the stromuhr) method is the possible interference with
bloodflow or blood pressure produced by inserting the stromuhr into
the entering vessels, and also the fact that simultaneous measurement
of the flow in both vessels can not be made satisfactorily.
To measure the outflow from the hepatic veins, the aorta is ligated
below the celiac axis and a wide cannula is inserted into the central
end of the vena cava below the level of the liver, a loose thread being
placed around this vessel just above the diaphragm. By pulling on this
thread the vena cava becomes obliterated, and the blood from the
hepatic veins is therefore diverted into the cannula, through which it
flows into one end of a vessel shaped somewhat like a sputum cup (the
receiver), the other end being connected by tubing with a piston re-
corder, from the movement of which the volume of blood flowing into
the receiver can readily be computed. To measure the flow of blood,
a clip on the tube of the receiver is removed at the same moment that
the thread around the vena cava above the diaphragm is tightened,
and when the receiver has filled with blood, this thread is again loosened
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256 THE CIRCULATION OF THE BLOOD
and the receiver tilted up so that the blood flows at low pressure back
into the circulation. The receiver being of known capacity, the length
of time it takes the blood to fill it as determined by the piston recorder,
furnishes us with the necessary data from which to calculate the rate
of flow. The receiver is chosen of such a size that it takes only a few
seconds to fill, the diversion of blood into it not causing any material
fall in arterial pressure. The observations are repeated frequently.
Results.—By the use of these methods it has been found that the total
mass movement of blood to the liver of the dog varies between 1.46 and
2.40 e.c. per second for 100 grams of liver. Considerable changes may
occur in the arterial pressure without affecting the liver flow. When
the hepatic artery is occluded, the flow diminishes by about 30 per
cent, or conversely, when the portal vein is obstructed but the hepatic
artery left intact, by about 60 per cent, indicating that about one-third
of the total bloodfiow through the liver is contributed by the hepatic
artery and two-thirds by the portal vein. Some blood, however, gains
the liver through anastomotic channels between it and the diaphrag-
matic veins.
The relative supply by the two vessels is subject to various condi-
tions. That through the hepatic artery, for example, may be very con-
siderably altered on account of vasoconstriction in this vessel, for its
walls can easily be shown to be liberally supplied with vasoconstrictor
fibers carried by the hepatic plexus. This can be demonstrated by
_ the rise in blood pressure which occurs in a branch of the hepatic artery
during stimulation of the plexus. On the other hand, alterations in the
bloodflow in the portal vein can not be brought about by active con-
striction or dilatation of the intrahepatic branches of this vessel, no
active vasomotor fibers having been demonstrated by stimulation of
the hepatic nerves, although, as in the case of the brain and lung blood
vessels, a certain amount of constriction may occur under the influence
of epinephrine.
The bloodflow through the portal vein is dependent on changes oc-
curring at either end of the distribution of the vessel, that is, changes
occurring in the liver itself or in the intestine. Of these factors the lat-
ter is no doubt the more important, an increase not only in portal blood
pressure but also in portal bloodflow being readily produced by dila-
tation of the splanchnic blood vessels; for example, as the result of sec-
tion of the splanchnic nerve. Alterations in portal bloodflow brought
about by changes in the caliber of the vessels in the liver itself are
partly dependent upon changes in the branches of the hepatic artery.
Let us consider briefly how this may be brought about. At the point
where the portal and hepatic arteries come together—that is, at the in-
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PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 257
trahepatic capillaries—the pressure of the blood in them must become
equal, which means that in its course through the interlobular connec-
tive tissue, the branches of the hepatic artery must offer much resistance
to the blood flowing through them. This frictional resistance resides in
the hepatic arterioles, and since these are richly supplied with constric-
tor nerves, great variation in hepatic inflow becomes possible. These
changes will affect the degree of tension of the interlobular connective
tissue in which the arterioles lie. In this tissue, however, also lie the
thin-walled branches of the portal vein. When therefore the tension
of this tissue becomes greater, as a result, for example, of vasodilatation
in the hepatic artery, the portal vein radicles will become compressed
and the bloodflow along them impeded. Conversely, when vasocon-
striction occurs in the hepatie arteries, the congestion of the connective
tissue becomes diminished, the veins dilate, and the blood flows through
them more readily (Macleod and R. G. Pearce”!). Experimental evi-
dence in support of the above view is furnished by observing the out-
flow of blood from the liver before and during stimulation of the he-
patic plexus. The first effect is an increase in the outflow, which very
soon returns to its original amount, even though the stimulation of the
plexus is kept up during the experiment. This return to the normal
flow must indicate either that the constriction of the hepatic artery has
not been maintained, or that it has been maintained but is accompanied
by a compensatory increase in the flow through the portal vein. As
a matter of fact, we know that the hepatic artery remains constricted
as long as the hepatic plexus is stimulated, indicating that the conges-
tion of the connective tissue in which the venules lie has become reduced
to such an extent, as a result of the constriction, that these open up and
permit the blood to flow through them more readily. The initial in-
crease in outflow immediately following upon stimulation of the hepatic
plexus, is no doubt caused by the squeezing out of the blood already in
the hepatic vessels, and it is a result which is often observed in other
organs during stimulation of vasoconstrictor nerve fibers.
THE CORONARY CIRCULATION
We have already studied the effect produced on the heartbeat by in-
terfering with the flow of blood in the coronary vessels, and it remains
for us to study: (1) peculiarities in the bloodflow through them, and
(2) whether this bloodflow can be altered by dilatation or constriction
of the vessels brought about through nerves. With regard to the pecu-
harities of bloodflow, it may be stated that there is said to be two periods
in each cardiac cycle during which an increase takes place in the mass
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258 THE CIRCULATION OF THE BLOOD
movement of blood in the coronary vessels—namely, at the beginning
of systole, and again at the beginning of diastole. Nevertheless the
pressure pulse has the same contour in the coronary as in the systemic
circulation. (W. T. Porter.??) During systole the intramural branches
of the coronary artery are compressed and the blood pressed out of
them. This emptying of the vessels favors the flow of blood through
the heart walls.
Regarding the presence of coronary vasomotor nerves, there is at pres-
ent a certain amount of doubt. When strips of the coronary artery are
suspended in.a solution of epinephrine, they undergo relaxation instead
of contraction. On the assumption that the action of epinephrine on
blood vessels is the same as that of stimulation of the vasoconstrictor
fibers, this result has been taken as evidence of the absence of such
fibers and the possible presence of vasodilator fibers. A somewhat
similar type of experiment has been performed by injecting epineph-
rine into the fluid used to perfuse the excised mammalian heart,
with the result that, when such injections are made into a heart that
is not beating, evidence of vasoconstriction is obtained, whereas when
injected into a beating heart, dilatation occurs. This latter result
. may, however, bé owing to the action of the epinephrine in stimulating
the cardiac contraetions. Other observers, however, deny that the in-
jection of epinephrine into the coronary circulation has any influence
upon the outflow of the perfusion fluid. Taking the result of these
observations as a whole, we may. at least conclude that epinephrine
does not produce the same marked vasoconstriction that it produces in
other blood vessels—a fact, which, as already stated, may be taken
advantage of in bringing about the rise in coronary pressure that is
necessary for successful resuscitation of the heart.
Attempts to demonstrate the presence of vasomotor fibers by electrical
stimulation of the vagus or sympathetic nerve have yielded results which
are quite inconclusive, although some observers assert that the vagus
nerve carries vasoconstrictor fibers to the coronary vessels, and that
the sympathetic carries vasodilator.
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CHAPTER XXIX
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
METHODS*
In the following chapters a brief account will be offered of the clinical
use of the electrocardiogram, of polysphygmograms, and of bloodflow
measurements. This is done to show how physiologic technic is being
employed for the accurate investigation of cardiovascular disease.
ELECTROCARDIOGRAMS
To observe the electrical change produced by the spread of the excita-
tion wave over the heart from auricles to ventricles, it is not necessary
to place the electrodes directly on the heart, but, as already hinted, we
may follow the electrical change by leading off from electrodes applied
to the surface of the body. From such electrocardiographic tracings
extremely important facts concerning the propagation of the heartbeat
may be ascertained. In order to make an observation the hands and the
left foot are each placed in a solution of sodium chloride contained in
porous jars, immersed in larger vessels containing a saturated solution of
ZnSO, and zine terminals.| An arrangement like that in Fig. 81 may also
be used. By manipulation of suitable keys the extremities may then be
connected with the electrocardiograph in the following manner: Lead 1,
right arm and left arm; lead 2, right arm and left leg; lead 3, left arm
and left leg. Through lead 1, the current acting on the galvanometer will
be that produced more especially at the base of the heart. Through lead
2, the current will pass through the long axis of the heart, and through
lead 3, it will pass mainly along its left border.
‘When any pair of leads is-connected with the galvanometer, it is ob-
served that the string is deflected to one side owing to electrical cur-
rents arising from the skin. Before taking a record of the cardiac
movements of the string, it is necessary to compensate for this skin cur-
rent by introducing into the circuit in the opposite direction the re-
*A certain amount of repetition of matter previously discussed has been found advisable in these
chapters for which the indulgence of the reader is requested.
tIt is really unnecessary to use the so-called nonpolarizable electrodes. Glass vessels containing
20 per cent NaCl solution with the zine plates dipping into them are quite satisfactory.
259
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260 THE CIRCULATION OF THE BLOOD
quired amount of current, called the compensating current, to bring the
strong shadow back to the zero or midposition. In order that the rec-
ord obtained may be quantitative in character, it is further necessary
that the movement of the string be standardized. This is done by as-
certaining to what extent the string moves when a current of known
voltage is sent through it and by altering the tension of the string so that
one millivolt of current causes an excursion of one centimeter of the
string shadow on the photographie plate. It would take us beyond the
Klectrocardiographic apparatus as made by the Cambridge Scientific Materials Co. Con-
tact electrodes are shown, but the immersion electrodes described in the context are preferable.
Fig. 81.
confines of this volume to go in any greater detail into the technic in-
volved in taking electrocardiograms, but it may be said that this is by
no means difficult, provided the instructions which are supplied with
the instrument are carefully followed. In practice the taking of elec-
trocardiograms is indeed quite a simple matter, and the extremely im-
portant information which they give us concerning the mechanism of
the heartbeat and the evidence of myocardial disease should make their
employment a universal practice in all cardiac clinics. Some of these
clinical applications are described elsewhere (page 266).
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ELECTROCARDIOGRAMS 261
What particularly interests us here is the contour of the electrocardio-
gram in a normal person (Fig. 82). It will be observed that there are
three waves above the line of zero potential and two waves below it.
They have been lettered from before backward, P, Q, R, 8, and T,
and in all such records when correctly obtained, the waves above the
line of zero potential indicate that the base of the heart is negative to
the apex. The exact cause of each wave has been ascertained by taking
simultaneously with the electrocardiogram a record of the mechanical
changes occurring in the heart during each cardiac cycle. Such records
Fig. 82.—Normal electrocardiogram. Leads 1, 2, 3. Note that the height of the R deflection in
lead 3 equals the difference between the height of R, and Ro.
have been secured by taking intracardiac pressure curves with the results
as shown in Fig. 83. The top curve represents auricular and the second
one ventricular pressure, whereas the lowest is an electrocardiogram.
It will be observed: (1) that the P-wave occurs just antecedent to con-
traction of the auricles; (2) that the small positive wave, Q, which is ab-
sent in these tracings, must occur just before the beginning of the con-
traction of the ventricles; (3) that the negative wave, R, occurs just be-
fore and during the early part of ventricular systole—that is, during
the presphygmic period; and (4) that the long upward wave, T, culmi-
nates at the moment the ventricle begins relaxing.
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262 THE CIRCULATION OF THE BLOOD
Although such comparisons give us considerable insight into the cause
of several of the waves, there yet remain certain peculiarities of the
electrocardiogram to be considered. These are: (1) the cause of the
slight positive wave, Q; (2) the cause of the positive wave, S; (3) the
cause for the period of equal potential at the base and apex during ven-
tricular systole indicated by the portion of the curve between S and T;
(4) the cause for the negative wave, T. To solve these problems it is
necessary to compare electrocardiograms taken from the surface of the
body with those from electrodes placed directly on the base or apex of
the ventricle of the exposed heart.
_ Fig. 83.—lilectrocardiogram (dog) taken simultaneously with curves from auricle and ven-
tricle. It will be observed that wave P slightly precedes auricular systole and that wave R occurs
just before the presphygmic period starts in the ventricle. (From Tewis.)
The Ventricular Complex
In view of the nature of the electric change which occurs in a strip
of denervated muscle when a wave of contraction passes along it (page
188), the simplest interpretation of the ventricular part of the above
eurve is that the contraction must pass into the ventricle at a little dis-
tance from the base, thus causing the latter, for a moment of time, to be
positive to the rest of the ventricle, and accounting for the slight down-
ward wave, Q. Immediately after this the base of the ventricle becomes
negative to the apex, giving us the marked upward wave, R, which
however lasts for but a short period of time, being followed by an inter-
val during which the base and apex are of the same electrical potential
(horizontal part of wave between R and T). Finally the base again be-
comes negative to the apex, thus accounting for the smaller upward
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ELECTROCARDIOGRAMS 263
wave, T. ~The cause of the occasionally observed downward wave, 8,
following R, is obscure.
The most significant fact in the electrocardiogram is therefore that
the base is' negative to the apex at the beginning (R-wave) and again at
the end (T-wave) of the ventricular contraction. How may this be ex-
plained? When electrocardiograms are taken through electrodes placed
directly on the basé and apex of the ventricle of the exposed heart, it
has been found that the contour of the electrocardiogram is like that
which is obtained from a strip of muscle when a wave of contraction
passes along it: it is diphasie in character. (page 188), a result which
may be interpreted as indicating that the wave of contraction starts at
the base and ends at the apex. This rules out the explanation, at one
time suggested for the T-wave, that the wave starts at the base, then
proceeds to the apex, and finally ends at the base, following the disposi-
tion of the muscular fibers of the ventricle in a folded or loop form,
with the bend of the loop at the apex and the free ends at the base. Al-
though the explanation seemed at first to conform with the embryo-
logic fact that the heart is developed from a folded tube, it can not hold,
as has been shown by observing the course of the excitation wave se-
cured through electrodes placed at various points on the surface of the
exposed ventricle (page 194).
The explanation which is accepted by the majority of observers at the
present time is to the effect that the T-wave is caused by the longer con-
tinuance of the electric change at the base of the ventricle than at the
apex. To test this hypothesis the crucial experiment would evidently
be to see whether a T-wave could be induced in an electrocardiogram,
such as that of the frog ventricle, in which no T-wave exists, by hurry-
ing up the contraction process at the apex without affecting it at the
base. This can be done by local warming of the apex, or by applying
the ventricular electrode at varying parts of the ventricle in an excised
heart beating in Ringer’s solution of relatively high H-ion concentra-
tion. Mines showed that under these conditions a typical T-wave ap-
pears in the electrocardiogram, as shown in Fig. 84.*
The existence of the small Q-wave, indicating that the ‘contraction
does not really start from the base, conforms with the observation that
the Purkinje system of fibers ends about the papillary muscles, which
therefore would be the first to contract, and with the observations of
Lewis, already alluded to above, on the appearance of the negative vari-
ation on the surface of the exposed heart.
The most important clinical application of the electrocardiogram is
*This tracing was found among those left by Professor Mines of McGill University, and for
permission to use it the author is indebted to the authorities of that institution.
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264 THE CIRCULATION OF THE BLOOD
A.—Normal
B.—Apex cooled
C.—Apex warmed
Fig. 84.—Records of electrocardiogram and movement of ventricle of frog showing that when
the apex is warmed a typical ‘I-wave appears in place of a wave in the opposite direction appear-
ing when the apex is cooled. (From Mines.)
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ELECTROCARDIOGRAMS 265
undoubtedly in connection with the determination of the rate of trans-
mission of the excitation wave from auricle to ventricle; thus, the P-R
interval, as it is called, indicates the time taken for the impulse to
travel from the sinoauricular to the auriculoventricular node and bundle.
In delayed transmission this interval becomes abnormally long. Obvi-
ously also conditions of heart-block, of auricular fibrillation, or of auric-
ular flutter will be immediately revealed by the electrocardiogram. The
interpretation of abnormalities in the contour of the ventricular portion
of the curve is, however, not so easy a matter, and should never be
undertaken unless curves from the three leads have been secured, for it
will be found that the corresponding electrocardiograms differ from
one another in detail; for example, the R-wave is usually most prominent
in lead 2, although sometimes it is more prominent in lead 3. T is always
upright in normal individuals in curves taken from lead 2, but it is not
infrequently inverted in those of lead 3, and may show partial inversion
in those from lead 1. The Q-R-S group is often of peculiar contour in
curves from lead 3. These variations are possibly dependent upon the
relative preponderance of the musculature in the left and right ven-
tricles, for it is evident that the amount of muscle included in the path-
way between the two leads will vary.
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CHAPTER XXX
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
- METHODS (Cont'd)
CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY
The Electrocardiogram in the More Usual Forms of Cardiac
Irregularities
By R. W. Scorr
The principle of the application of the string galvanometer to the
study of cardiac irregularities has been indicated. It is our object here
to outline some of the more common forms of irregular heart action,
with a brief description of the abnormalities in the electrocardiogram
resulting therefrom. For the sake of comparison a normal electrocar-
diogram is shown in Fig. 82. The cause and relationship of the various
deflections have been explained (see page 262).
Sinus Arrhythmia—This irregularity is seen commonly in children
and young adults, and is without pathologic significance. The electro-
cardiogram presents the normal deflections and shows by the varying
spaces between the P deflections that the cardiac impulse has been gen-
erated at slightly irregular intervals.
Sinus Bradycardia.—The electrocardiogram in a simple case of sinus
bradycardia is usually normal, except that the deflections occur at an
unusually slow rate (Fig. 85). This indicates that the cardiac impulse
is built up at a slow rate, but when generated it evokes a normal auric-
ular and ventricular contraction.
The Extrasystole—The extrasystole may be either auricular or ven--
tricular in origin. Occasionally a rare type is seen in which the im-
pulse arises in the junctional tissues between the auricle and ventricle.
When the focus of impulse production is at or near the sinoauricular
node, the resulting electrocardiogram complexes are practically normal.
If, however, the seat of impulse formation is removed from the S-A
node, the P deflection may be distorted or actually inverted, followed
by a normal Q-R-S-T complex (Fig. 86).
In the ease of ventricular extrasystole, the cardiac impulse originates
in either the right or the left ventricle. This abnormal site, together
266
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CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY 267
Fig. 85.—Sinus bradycardia. Rate 32 per minute. Note the normal appearance of the electro-
cardiogram. P-R interval = .17 seconds.
}
Fig. 86.—Auricular extrasystole. Two auricular extrasystoles following two normal complexes.
Note the ectopic origin of the extrasystoles indicated by the inversion of P.
Fig. 87.—Ventricular extrasystoles arising in the right ventricle.
Fig. 88.—Ventricular extrasystole arising in the left ventricle.
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268 THE CIRCULATION OF THE BLOOD
with the path which the impulse takes, produces a much greater differ-
ence of electric potential than is seen in the normal electrocardiogram.
‘When the impulse arises in the right ventricle near the base, the prin-
Fig. 89.—Paroxysmal tachycardia. Auricular origin. Note that the P deflection falls back on T.
Rate 200 per minute.
cipal R deflection is upwards in both leads 1 and 2. Arising near the
apex, the principal R deflection is up in lead 1 and down in lead 2. Two
extrasystoles both arising in the right ventricle are shown in Fig. 87.
Fig. 90.—Auricular fibrillation. Leads 1, 2, 3. Note the coarse fibrillation waves between the
R peaks, and the absence of any B deflections in relation to R. Also the unequal spacing of the R
deflections.
In the case of the left ventricle, a basal impulse gives a downward
principal deflection in lead 1 and up in lead 2. When the aberrant fo-
cus is located near the apex of the left ventricle, the principal deflec-
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CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY 269
tion is down in both leads 1 and 2. Any one or several of the general
types of extrasystole may occur in the same patient. Fig. 88 shows
an extrasystole originating from the left ventricle.
Paroxysmal Tachycardia.—Electrocardiographie records taken in the
interval between the paroxysms may appear normal. During the tachy-
cardia the records normally show only two deflections, R and a combina-
tion of T and the succeeding P (Fig. 89). If the paroxysm is of auric-
ular origin, the P deflection may be inverted, indicating that the new
focus of impulse production is located at some other site than the sino-
auricular node. Rarely the new focus may be in the ventricles. Records
taken during the paroxysm may show a rapid succession of deflections,
simulating isolated ventricular extrasystoles. —
Auricular Fibrillation—The electrocardiogram in auricular fibrilla-
tion shows three distinctive features:
1. Absence of the P deflections typical of auricular contractions.
2. The ventricular complexes (Q-R-S-T waves) occur in irregular se-
quence and may vary in height.
8. The presence of small irregular oscillations best seen between the
ventricular complexes. A typical tracing of this condition is shown in
Fig. 90.
The dependence of the P-wave upon auricular contraction has been
indicated (page 261). Its absence in auricular fibrillation is accounted
for by the fact that the individual muscle fibers of the auricles contract
independently of one another, so that some fibers are in a state of con-
traction while others are relaxed. This renders impossible a coordinate
contraction of the auricle as a whole.
The multiple impulses from the fibrillating auricles reach the ventri-
cles and evoke a contraction provided the ventricle is not already in a
state of contraction (refractory period, page 178). These irregular
ventricular responses will of course produce unequal spacing of the
ventricular complexes in the electrocardiogram. The variations in the
height of the R deflections is thought to be due to the distortion causéd
by the superimposition of the small waves representing auricular ac-
tivity. These small waves must occur throughout the whole cardiac
cyele, but are more or less masked by the ventricular complexes, appear-
ing as separate oscillations only during diastole.
Auricular Flutter—Auricular flutter was discovered by the electro-
cardiograph, and it is practically impossible to make a diagnosis of this
condition without the use of the string galvanometer. The auricular
deflections are usually rhythmic and in the average case vary in rate
from 200 to 350 per minute. The initial deflection of P may be base
negative or apex negative—up or down—depending on the site of the
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270 THE CIRCULATION OF THE BLOOD
origin of the auricular impulse (when arising from some other source
than the S-A node the impulse is said to be ectopic). Usually a regular
succession of P deflections can be traced throughout the record (Fig.
91).
Since it is impossible for the ventricle to respond to all the impulses
coming from the auricles, a condition of partial heart-block obtains
(2:1—3:1—4:1, ete.). The ventricular complexes will occur regularly
except when a 3:2 rhythm exists.
Fig. 91.—Auricular flutter. Auricular rate 300. Ventricular rate 80. Note the inversion of the P
deflections.
Usually the ventricular complexes are such as to indicate that the
stimulus arose in the auricle (supraventricular). The height of the
individual deflections Q-R-S-T may vary, depending on the predominance
of a right or left ventricular hypertrophy.
Fig. 92.—Delayed conduction. Note the normal appearance of the electrocardiogram except for
the prolongation of the P-R interval, which measures .23 seconds.
Heart-block—There are three degrees of severity in heart-block: (1)
delayed conduction, (2) partial dissociation, and (3) complete dissocia-
‘tion.
Any one of these conditions may be present in the same patient at
successive intervals.
DELAYED ConpucTion.—When the conducting tissues of the heart are
so affected as to cause an abnormal prolongation of the P-R interval,
the condition is called delayed conduction. The ventricles respond to
each stimulus originating at the sinus node, but the time required for the
impulse to pass through the conducting tissues is longer than normal,
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CLINICAL APPLICATIONS OF ELECTROCARDIOGRAPHY 271
In a simple case the electrocardiogram may appear perfectly normal,
but when the P-R interval is measured accurately, it will be found to be
lengthened beyond the extreme limits of the normal (0.20 seconds) (Fig.
92),
PartiaL Dissoctation.—In the typical case of partial dissociation the
Fig. 93.—Partial dissociation. Note the failure of ventricular response following the second P,
which has been preceded by two extrasystoles (x) of ventricular origin.
ventricles respond to the impulse coming from the auricle most of the
time, but occasionally fail to do so, when the condition is called ‘‘dropped
beat.’’ The electrocardiogram records a P deflection but no ventricular
complex, showing that the auricles have contracted at their usual rate
but that the ventricles failed to respond to the stimulus coming from
the sinoauricular node (Fig. 93).
Fig. 94.—Complete dissociation. Note that the P wave spaces regularly and bears no definite re-
lation to the R wave of the ventricular complex. Auricular rate 72. Ventricular rate 40.
CompLete Dissociation.—In a simple case of complete dissociation
the auricles beat independently of the ventricles; hence the P deflection
of the electrocardiograms bears no relation to the ventricular complex
(Q-R-S-T) (Fig. 94). The P deflections space regularly and are easily
made out when they fall during diastole of the ventricle. Occasionally
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272 THE CIRCULATION OF THE BLOOD
the auricle will happen to contract during ventricular systole, causing a
distortion of the ventricular complex by the superimposition of a P
deflection. Except when this occurs the Q-R-S-T complex is the normal
supraventricular type. The P deflections occur more frequently than
the Q-R-S-T complex, showing that the auricles are beating more often
ban the ventricles. The auricular rate in the average case of complete
heart-block is about 72, while the ventricular rate is much slower (35
to 40).
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CHAPTER XXXI
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
: METHODS (Cont’d)
POLYSPHYGMOGRAMS
In taking polysphygmograms, the following technic is usually followed:
Venous Pulse Tracings.—Direct the observed person to lie down with
his head slightly raised by a cushion and bent to the right side. Place
the receiver (thistle funnel) over the jugular bulb on the right side of
the neck. This lies immediately above the inner end of the clavicle.
Bring the style of the recording tambour to write with a minimal
amount of friction on the paper or drum. Since a venous pulse tracing
can not be interpreted without a simultaneous tracing from an artery,
now adjust the button of a receiving tambour over the radial artery and
adjust the style of its recording tambour so as to write on the drum it in
the same perpendicular as the style of the venous tambour.
Tracings should be taken with the recording surface at a moderate
speed. Before disturbing the relative positions of the writing points,
allow them to make vertical marks (with recording surface stationary)
at various parts of the tracings. These alignment marks permit of ac-
curate comparisons between the curves. Repeat the above, using the
carotid instead of the radial. A time tracing (4% sec.) should always be
taken simultaneously. The polysphygmograph is shown in Fig. 95.
To interpret the venous curve, make a vertical mark on the arterial
pulse tracing corresponding to the beginning of the pulse upstroke. If
this is done on the radial pulse tracing, measure one-tenth of a second in
front of it, and make a vertical mark to allow for the time lost in propa-
gation of the pulse from the heart to the radial artery.
This line 3 (corrected in case of radial pulse) corresponds to the be-
ginning of the sphygmie period of ventricular systole—i. e., to the open-
ing of the semilunar valves: Measure the distance from it to the near-
est vertical line that was made to indicate the relative position of the
writing points. Then measure off the same distance in the venous trac-
ing from the corresponding indicator line. This will fall at the begin-
ning of the small wave (¢), which is due to the bulging into the auricles
of the closed auriculoventricular valves. (Fig. 96.)
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274 THE CIRCULATION OF THE BLOOD
The auricular wave (a) occurs one-fifth of a second in front of c, and
may now be ascertained by measuring off this distance in front of c.
This is line 1.
Now measure the distance on the radial pulse tracing from the begin-
ning of the upstroke to the dicrotic notch. The distance between these is
the sphygmie period (E).
Fig. 95.—Polysphygmograph. This instrument records in ink on glazed paper two simul-
taneous tracings, i. e., radial pulse and one other, such as carotid, jugular, apex beat, etc., in addi-
tion to the time tracing. The ink tracings are both more convenient and permanent than smoked
paper tracings. The clockwork operates at variable speeds, permitting the taking of protracted
records at different speeds.
Measure off the same distance on the venous tracing from c. Line
5 will be found to fall just before a small wave (wv), which is due to the
sudden opening of the tricuspid valves. This practically coincides with
the dicrotic notch on the radial pulse tracing. Sometimes a little wave
Fig. 96.—Normal jugular tracing. The spacing below shows the duration of the a-c interval.
(From E. P. Carter.)
occurs on the upstroke of wave v just where line 5 falls. This co-
incides with the closure of the semilunar valves. The distance between it
and wave v corresponds to the postsphygmiec period.
The cause for the depression (marked x) following ec will readily be
understood by referring to the intraauricular curve (Fig. 97), to which,
as already explained, the venous pulse tracing is qualitatively similar.
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POLYSPHYGMOGRAMS 275
The rise in the curve following depression x is caused by the filling of
the auricle with blood. This goes on until v, when the tricuspid valves
open, allowing the blood to fall into the ventricle.
lim
E
Lo!
C.
ut
Ae
c
AS.
sf
{
eI
Fig. 97.--Reduced tracings from carotid, aorta, ventricle, auricle and jugular, to show the
general relationships of the various waves. An electrocardiogram is also shown. Note that the
jugular and auricular curves have the same contour, and that the depression (x) in them occurs
during systole of the ventricles. (After Lewis.)
To interpret the cardiogram, adjust receiving tambours to the radial and
open beat with both writing styles in the same perpendicular, and following
the other directions described under ‘‘venous pulse’’ mark on the
. cardiogram: (See Fig. 98.)
Jugatur 4 a
&
0,2 Second
MARAAAAAAAAARAAAA AAPA AAA AA A RR A A
Fig. 98.—Polysphygmograms including jugular, apex and radial tracings. Line 4 on the radial
tracing is first of all located. It is then transferred (by measurement from the alignment mark on
the right edge of the tracing) to the jugular and 1/10 second subtracted from it, giving line 3.
When this is similarly transferred to the apex tracing, it falls somewhere on the upstroke the be-
ginning of which is line 2,
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276 THE CIRCULATION OF THE BLOOD
1. The beginning of the sphygmie period (E) (line 5).
2. The end of the sphygmic period (E) (line 5).
3. The auricular wave.
4. The beginning of ventricular systole (difference between 1 and 4
equals presphygmic interval).
5. The opening of auriculoventricular valves (lowest point in tracing).
Mark also on the tracing the exact moment at which the heart sounds
are heard.
It is important to make certain that the button of the tambour is ac-
curately over the apex beat, since otherwise a depressed or negative
wave may be inscribed at ventricular systole.
Simultaneous Arterial Pulse Tracings.—The velocity of the transmis-
sion of the pulse wave is calculated by measuring the time between the
systolic rise in the carotid and in the radial arteries, tracings of which
are taken by applying one receiving tambour to the carotid artery and
another to the radial artery.
Abnormal Pulses
The following is a brief description of the main character of abnormal
pulses:
The Ventricular Pulse—In this no ‘‘a’’ waves are present in the
jugular tracing, the heart action being either regular or irregular. In
the former case, the absence of the ‘‘a’’ waves may depend on: (1) over-
filling of the right auricle, (2) increase in the heart rate, or (3) complete
heart-block associated with auricular fibrillation. When the heart is
irregular, the absencé of the ‘‘a’’ waves signifies auricular fibrillation.
Delayed Conduction and Heart-block——This causes a change in the
time relationship of the ‘‘a’’ and ‘‘c’’ waves in the jugular curve. When
the heart-block is of the first degree, the ‘‘a-c’’ interval merely becomes
lengthened, but when it is of such degree that the normal impulse some-
times fails to be conveyed along the auriculoventricular bundle, isolated
‘a? waves can be detected. In the higher degrees of heart-block there
are regularly recurring ‘a’? waves having no constant time relationship
to the ‘‘c’’ waves. For the purpose of exact analysis of the curves in
suspected cases of delayed conduction, it is often advantageous to draw
vertical lines below the tracing representing the beginning of auricular
and ventricular systole. This has been done in the tracing reproduced
in Fig. 99. :
The line joining these two verticals indicates the conduction time
or ‘‘a-c’’ interval. When it exceeds one-fifth of a second, there is
delay in the conduction time.
’
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’ POLYSPHYGMOGRAMS 277
A tracing showing a higher degree of heart-block is given in Fig. 100.
Sinus Arrhythmia—In this condition the radial pulse is markedly
‘irregular, but the ‘‘a,’’ ‘‘e’’ and ‘‘v’’ waves of the jugular tracing oceur
with the usual time relationship to one another, and there is no delay
in the ‘‘a’’-‘‘c’’ interval.
Sel
Soe NN RS
Nene (peg ee
: ACC Mewal VV4
Fig. 99.—Delayed conduction time. First stage of heart-block. The A-C intervals measure more
than 0.2 second. (From E. P. Carter.)
Sinus Bradycardia—The beat originates at long intervals in the
sinus; the ‘‘a-c’’ interval is normal, and the radial pulse very slow but
practically regular.
Premature Beats—These may be either ventricular or auricular in
origin. In the former case the ‘‘a’’ waves on the jugular tracing space
regularly throughout, but the ‘‘c’’ waves at the point of disturbance
Fig. 100.—Dropped beats. Second stage of heart-block. (From E. P. Carter.)
coincide with the ‘‘a’’ waves, giving therefore a more pronounced wave.
This is due to a premature contraction of the ventricle occurring about
-the time of the ‘‘a’’ wave, so that the latter finds the ventricle in a re-
fractory state (see page 178). The premature contraction is therefore
followed by a compensatory pause, which is evident on the tracing. An
example of such a case is given in Fig. 101. In doubtful cases the exact
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278 THE CIRCULATION OF THE BLOOD
site of origin of the premature beats can be determined only by careful
measurement of the distances between the various beats of the ventricle.
Whenever an irregularity repeats itself and the duration of one cycle
of the arrhythmia accurately corresponds to another, the irregularity
tnt fo
Nao KX NCGS K
‘ew. | [kot kee Lt ket ff >
KA 3 AY ke’ oa BEh
Fig. 101—Premature beats (extrasystoles) ventricular in origin at PB. Compare the duration of
the intervals marked 4 and B’ with those marked C and D. (From E. P. Carter.)
may be due to: (1) premature auricular or ventricular contractions;
(2) the occasional occurrence of dropped beats (a failure of ventricular
response) ; or (3) a high degree of heart-block with a wide variation in
the ventricular response. The important point to note here is that, no
matter how irregular such a tracing may appear, if the irregularity re-
peats itself it can not be due to auricular fibrillation.
KR.
ee et
eal
are eed
| I
LT TTT ot lotokel hale ald Foss sot
,
Eh PS
: | ]
Fig. 102.—Paroxysmal tachycardia. The paroxysms start at xx following normal beats and
lasting for seven beats, The clue to “a,” which falls with “v’’ after the first premature contrac-
tions, is found in the initial beat of the new rhythm. (From E. P. Carter.)
.
a ' to
Xe e Vou Eveynve
Paroxysmal Tachycardia.When the rate of a regular pulse is sud-
denly altered but the change in rate bears a simple ratio to the previous
rhythm, the change may be due to (1) premature ventricular contrac-
tions which do not reach the radial, or (2) to the sudden development
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POLYSPHYGMOGRAMS 279
of a two-in-one heart-block. When on the other hand, there is no exact
ratio between the slow and the rapid rate, the change is due to the sud-
den appearance or disappearance of paroxysmal tachycardia. The
paroxysms during which the auricle is beating very rapidly may last for
a variable time, such attacks persisting off and on for hours or even days.
The tracing in such a case is given in Fig. 102.
Be
omen co tanmnennecacoan
B
~* | ey | | fe
“wr Sv best Wardy x.
“War COUT TT
Vale Tete belle kre ke]
Fig. 103.—Auricuiar flutter. In this case the ventricular rate varied from 82 to 98 per minute.
(From E. P. Carter.)
Auricular Flutter—It is impossible to diagnose the not infrequent
existence of this cardiac condition without the use of either the poly-
sphygmogram or the electrocardiogram. The jugular curve may be of
two types, one made up of rapid, more or less uniform waves, the other
of waves that are paired with a constant time interval between the pairs.
pr |
—————e—ee _c—n nn — eee
“Kun rake 00 HIT LUT EET
XN a, a
Ril eug a Klan bet lease]
Fig. 104.—Auricular flutter. Note the relative rates of A and V, and also that the ventricular
rate is regular. (From E. P. Carter.)
All of the frequent beats of the auricle do not reach the ventricle in this
condition, so that the ratio between auricular and ventricular beats
may be 1:3 or 1:4. The condition must therefore not be confused with
heart-block, the main point of distinction being that in the latter condi-
tion the ventricular pulse is slow and the auricular about normal. The
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280. THE CIRCULATION OF THE BLOOD
radial pulse may be regular or irregular. The cause for the failure of
every auricular beat to travel to the ventricle during auricular flutter
is partly the refractory condition of the bundle, and partly the refrac-
tory phase of ventricular contraction. The bundle may be considered
as a narrow bridge which will transmit the impulses across it only at a
certain rate. If the impulses arrive too rapidly, only some of them can
cross the bridge, and even of those that do cross, a limited number only
will find the ventricle in a condition of excitability because of the re-
fractory period (see page 178). Tracings showing auricular flutter are
c 7
tA Oe ee EE ev ey CV tvey
Ayo id.
Nertxs dyin v
y
TEE Ise [uy [a dag Ts [ay [sv fos few fag Tan |
Fig. 105.—Auricular fibrillation. Note the absence of all “a”? waves from the jugular tracing, the
marked irregularity of the radial pulse, and the occurrence of “c” and “vy” during the sphygmic
period. (From E, P. Carter.)
given in Figs. 103 and 104. In one of them the radial pulse is regular;
in the other, irregular.
Auricular Fibrillation—The contractions of the auricle, as already ex-
plained, are entirely irregular, so that the jugular tracings show an en-
tire absence of all ‘‘a’’ waves, the radial tracing being characterized by
the complete absence of a dominant rhythm and by great variation in the
length of the individual beats from one cardiac cycle to the next. This
irregularity does not repeat itself, and the long pauses are not simple
multiples of the shortest pause. Tracings from a ease of auricular
fibrillation are shown in Fig. 105.
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CHAPTER XXXII
CLINICAL APPLICATIONS OF CERTAIN PHYSIOLOGIC
METHODS (Cont’d)
THE MEASUREMENT OF THE MASS MOVEMENT OF THE BLOOD
Method.—The apparatus used for this purpose consists essentially of
a vessel containing a known quantity (3,000 ¢.c.) of water and a ther-
mometer from which a change of temperature of a hundredth of a de-
gree centigrade can be read. In order to diminish as much as possible
the loss of heat between the vessel and the outside air, the walls are
double, the space between being stuffed with broken cork. The top of
the vessel is closed with a thick cork plate, having suitable openings in
it for the hand or foot and for the thermometer and a stirrer (feather)
with which to keep the water in constant motion. The apparatus is called
a calorimeter.
After the hand or foot has been in the calorimeter, with the water a
few degrees below that of the body, for a certain time (ten minutes), the
temperature of the water will of course become raised, and the degree
to which this occurs, multiplied by the volume of the water in cubic
centimeters, will give in calories the amount of heat dissipated. By the
application of a very simple formula it is now an easy matter to calculate
how much blood must have passed through the blood vessels of the part
in order to give out the observed amount of heat; for, if we divide the
calories by the difference in temperature between the inflowing and out-
flowing blood of the part, the result must indicate the volume of blood, in
cubic centimeters, that has passed through it (since by définition a calorie
equals volume multiplied by difference in temperature). It remains to
explain the equation by which the results are arrived at. If Q equals the
amount of blood, H the calories of heat given out to the calorimeter, T
the temperature of the arterial blood and 7’ the temperature of the
e
venous blood, then we have the equation: Qa. It has been shown
*For the determination of H we must multiply the cubic centimeters of water plu.
: : s the wat
equivalent of the hand and calorimeter (because both of these will absorb some ao by the dif.
ference in temperature plus the self-cooling of
e ; : the calorimeter (because some heat is lost t
re oe ae pen The soap equivalent of the hand is equal to its volume mae
y 0.8; that o e calorimeter must be determined for each inétrument i
The self-cooling of the calorimeter is determin fl pee
ed by observing the fall in tem i
equal to that of the actual observation without the hand in the calorimeter. ene LEE
281
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282 THE CIRCULATION OF THE BLOOD
by Stewart that 7 may be taken as the same as that of the mouth, or 0.5°
C. below that of the rectum, and T’ as the average temperature of the
water in the calorimeter during the observation. To allow for the specific
heat of blood, the result is multiplied by , the reciprocal of the specific
heat of blood.
Theoretically, then, the method is very simple, and there are no un-
usual technical difficulties in applying it. The only special precaution
is that the air surrounding the calorimeter should be kept fairly con-
stant in temperature, so that we may be enabled to allow in our caleula-
tions for the loss of heat from the calorimeter itself, this value being
obtained by observing the change of temperature in the calorimeter for
a certain period of time after the hand has been removed from it.
The Normal Flow
The results are calculated on the basis of grams of blood flowing
through 100 ¢.c. of tissue in one minute. The volume of the hand or foot
is ascertained by placing it in water contained in a small-sized irrigation
can, the tube of which is connected with a burette. The height to which
the water rises in the burette is noted, and after withdrawing the hand,
water is added from a graduate to the irrigation can until the same_ .
height is reached on the burette. The number of cubic centimeters re-
quired gives the volume of the hand. In a normal, healthy individual
the average flow in the hand is from 12 to 13 gm. for the right hand,
and about half a gram less for the left. This difference between the two
hands corresponds, of course, with their relative degree of development.
The average foot flow is much less, and varies according to whether the
patient is sitting up or lying down while the measurement is being made.
In a normal individual, while lying down, it was 5.11 gm. in the right
foot and 5.23 gm. in the left, per 100 c.c. of foot; but only 2.96 gm. for
the right and 4.1 gm. for the left foot, while sitting up. The results have
been found to undergo only a slight variation from month to month in a
given healthy individual, provided the air temperature during the dif-
ferent observations is the same and the person has been some time in the
room before the observations are begun. This precaution is especially
important if he is a dispensary patient and has been in the open air with
bare hands. The flow varies in different individuals both with regard
to absolute amount and the ratio between the two hands or feet. When
the total flow in the hands is compared with that in the feet, a ratio of
about 3 to 1 is usually obtained.
The Physiologic Causes for Variations in Bloodflow.—As above indicated,
the most marked of these is probably the temperature of the room. The
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MEASUREMENT OF MASS MOVEMENT OF BLOOD 283
temperature of the water in the calorimeter has likewise a great influence,
and for the comparison of different cases it is always important that the
room and calorimeter temperatures be stated alongside the results. Muscular
contractions, produced by moving the fingers in the calorimeter, cause a
marked increase in flow, accompanied by a diminished flow in the hand
that was at rest. A great diminution of flow results from constriction of
the arm of sufficient degree to obstruct the venous circulation; and when
the constriction, as that caused by a blood pressure armlet, is increased to
between the systolic and diastolic pressures, extremely little blood flows
through the hand.
By immersing the opposite hand or foot in hot or cold water, the blood-
flow through the observed hand increases or decreases, respectively.
The change may be of a temporary character, or it may persist through-
out the whole period of immersion of the hand. These reactions are due
to a vascular reflex, and observations of its sensitiveness are of value in
the study of the effects of lesions either of the nerve or of the nerve
centers concerned in vascular reflexes.
Clinical Conditions which Affect the Bloodflow.
Even in cases where there is plenty of other evidence of curtailment
of flow, the measurement may be of importance either for detecting
an alteration in the vascular reflex or, by comparison of the two
hands, for demonstrating the relative degree of alteration in flow. In
acute inflammatory conditions affecting one hand, there is an increase
in flow on the affected side accompained by a marked curtailment on
the other side. This indicates that an increased flow in the infected
area is accompanied by a reflex vasoconstriction elsewhere, particu-
larly in the symmetrically placed part of the opposite side of the
body. In eases of nonbacterial inflammation of the hand, as in gout,
no sign of vasoconstriction may be observed.
There are many clinical conditions in which Stewart’s method re-
veals an alteration in bloodflow that would be unsuspected by the use
of ordinary clinical methods. It is for the investigation of these that
the method is of greatest value. The most important findings are as
follows:
Anemia.—The bloodflow in the hand may be much less than normal
in pernicious anemia and secondary anemia, and distinctly curtailed
im chlorosis. Since the minute volume of the heart is also increased
in these conditions, the vasoconstriction at the periphery will assist
in compelling more blood to pass through the lungs, so as to make up
for deficiency of blood.
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284 THE CIRCULATION OF THE: BLOOD
Fever.—Since changes in the cutaneous circulation probably con-
stitute the chief factor in the derangement of the temperature-regu-
lating mechanism in fever (cf. page 723), it is evidently of great ad-
vantage to be able to measure such changes quantitatively. This has
been done by Stewart in several cases of typhoid fever and in one case
of pneumonia. In general it was found that the flow in the feet never
exceeded the normal flow, and was usually much below it. This ten-
dency to vasoconstriction seems to be earried into convalescence. For
practical reasons the handflow has not been so extensively studied.
This hyperexcitability of the vasoconstrictor mechanism at the periph-
ery is most naturally interpreted as a defensive reaction of the or-
ganism by which an increased supply of blood is imported to those
internal organs which bear the brunt of the infection. When we con-
sider that in spite of this constriction of the periphery the blood pres-
sure is low and the pulse dicrotic, we must conclude that there is con-
siderable dilatation of other vascular parts, especially the splanchnic
area. A very practical application of these facts presents itself in con-
sidering the rationale of the cold-bath treatment for fever. If, for
example, we conclude that the cutaneous constriction is in the inter-
ests of an increase in the bloodflow to the organ on which the stress
of the infection falls, it will evidently be more rational to lower the
temperature by methods which will not diminish, and may even in-
crease, the cutaneous constriction than to do so by causing the vessels
to dilate. In other words, the use of antipyretics seems to be contra-
indicated, since they diminish the body temperature by causing vaso-
dilatation at the periphery with a consequent withdrawal of blood
from the seat of infection.
Cardiovascular Diseases——In cardiac cases the handflow is far more
apt to be markedly deficient where there is evidence of serious impair-
ment of the myocardium than in cases where a gross valvular lesion
exists but the heart action is strong and orderly. This indicates that
it is more serious for the force of the heart pump to be interfered
with than for its valves, particularly the mitral, to be leaky. Even
where there is considerable venous engorgement, the flow may be lit-
tle diminished. In untreated cases of auricular fibrillation the blood-
flow is subnormal. After the administration of digitalis the bloodflow
in such eases is often promptly and decidedly increased.
As would be expected, arteriosclerosis is associated with a small blood-
flow, and the vasomotor reflexes are weaker than in normal persons.
In aortic aneurism, when the aneurism is of such a size as to cause
pressure on the subclavian artery or vein, there is a diminution in flow
of the corresponding hand, but aortic aneurism itself, although it may
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MEASUREMENT OF MASS MOVEMENT OF BLOOD 285
cause great. changes in the character of the pulse beat, does not decid-
edly affect the mass movement of the blood. In aneurism of the sub-
clavian artery, the bloodflow may be much greater in the corresponding
than in the opposite hand, even though the amplitude of the pulse is
very obviously diminished and the difference between the systolic and
diastolic pressures (the pressure pulse) is much less on the affected
than on the normal side. By ordinary clinical measurements, there-
fore, false estimates of bloodflow are quite likely to be made. These
results are no doubt owing partly to vasodilatation brought about by
pressure of the aneurism on the brachial plexus and partly to the
lower resistance to the flow of blood into the dilated subclavian.
In Raynaud’s disease, as would be expected, the flow is small, the
diminution being more or less proportional to the duration of the
disease. The contralateral vasomotor reaction to cold is also pecu-
liarly intense.
In diabetic gangrene of the feet there is # very subnormal flow in both
the hands and the feet. The vasomotor reflexes are also feeble.
It is sometimes difficult to tell whether an observed curtailment of
flow is a nervous (reflex) effect or is due to some mechanical interfer-
ence. There are two ways by which the exact cause may be diagnosed:
(1) by observing the flow from day to day; if it remains unchanged,
any alteration must be dependent on mechanical causes; (2) by observ-
ing the change in flow brought about by altering the temperature of the
room or calorimeter and seeing whether the ratio between the two hands
remains unchanged or becomes altered. If the latter occurs, the in-
equality in flow must be due to nervous causes.
Diseases of the Nervous System.—The effect of neuritis on the flow
varies with the duration of the disease. In cases of early peripheral
unilateral neuritis there may be an increase of flow altering the ratio be-
tween the two hands with the greater flow on the diseased side. In
neuritis of long standing the flow is cut down, the greater flow occurring
on the healthy side. The changes here are probably due to anatomic
alterations in the lumen of the tube, perhaps a thickening of the intima.
In motor-neuron disease without any involvement of the sensory skin
nerves the flow seems to remain normal and the reflexes to be well-
marked. This indicates that involvement of the motor nerves does not
interfere with bloodflow to anything like the same degree as involvement
of the skin nerves.
Hemiplegia.—A deficiency of bloodflow of the paralyzed side is usually
observed, and the vasomotor reflexes are altered, the most usual change
being that vasoconstriction is more easily produced than vasodilatation.
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286 THE CIRCULATION OF THE BLOOD
In some cases an abnormal tendency to vasoconstriction is a conspicuous
feature.
Tabes Dorsalis—The flow is distinctly diminished, especially in the
feet, although also in the hands, and the vasomotor reflexes are feeble.
Sometimes there is inequality in the flow of the two hands, which how-
ever need not necessarily indicate a unilateral lesion of the cord in the
cervical region.
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CHAPTER XXXITII
‘SHOCK
Shock may be due to a variety of causes. In general it may be de-
scribed as a condition in which there is more or less paralysis of the,
sensory and motor portions of the reflex are, along with profound dis-
turbances in the circulatory system, subnormal temperature, frequent
and shallow respiration, and more or less unconsciousness. Certain of
these symptoms may be considered as primary and others as secondary,
an important step in the investigation of this difficult and important
problem being to distinguish between the two groups. Before attempt-
ing to do this, however, it will be profitable to differentiate as sharply
as possible the various conditions in which one or another of the many
varieties of shock is said to occur.
The following varieties of shock have been described:
1. Gravity Shock.—This is caused by the stagnation of blood in the
splanchnic vessels and the consequent inadequate filling of the heart in
diastole. It occurs, when the erect position is assumed, in animals in
which the mechanism which ordinarily compensates for the tendency of
gravity to make the blood flow to the dependent parts is inadequate.
Thus, when a domesticated rabbit with a large pendulous abdomen is
held in the vertical tail-down position for any length of time, the animal
gradually passes into a shocked condition and may die in a short time
(20 to 30 minutes). Observation of the blood vessels of the ear or a
record of arterial blood pressure will show that the cause of shock in
this ease has been a great curtailment of the blood supply to the upper
part of the body, and therefore to the nerve centers (Fig. 244). The
shock is entirely dependent upon the laxity of the abdominal muscula-
ture, for if a binder is applied to the abdomen, or if the experiment is
performed on a rabbit whose abdominal musculature is in good condi-
tion, gravity shock does not develop. Nor can fatal gravity shock be
produced in a dog, although in a deeply anesthetized animal a
marked fall in arterial blood pressure occurs when the vertical tail-
down position is assumed. In man, in whom compensation for the erect
posture is highly developed, shock from gravity occurs only when there
has been some other considerable upset in the circulatory mechanism
(see also page 245).
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288 THE CIRCULATION OF THE BLOOD
2. Hemorrhagic Shock.—Free hemorrhage produces a typical condi-
tion of shock, but the extent to which different individuals react to the
same degree of hemorrhage varies considerably. The essential factor in
the production of hemorrhagic shock is of course similar to that of grav-
ity shock—namely, a deficient diastolic filling of the heart with blood.
Details concerning the effect of hemorrhage will be found elsewhere
(page 135):
3. Anesthetic Shock.—So far as blood-pressure reflexes are concerned,
an animal can be kept in a perfect condition when ether is administered
in just sufficient amount to produce light anesthesia. When larger
quantities of ether are employed, a typical condition of shock is almost
certain to supervene after a time. In such instances the arterial blood
pressure remains low and can not be restored even after an hour or two
of artificial respiration. There is, however, a difference between ether
shock and the variety which we shall discuss later under the title of
surgical shock: in the former, removal of the anesthetic causes all reflexes to
return, whereas in surgical shock most of these are subnormal. The danger
of anesthetic shock has been considerably diminished in the clinie by
the more careful administration of ether or by the use of other anesthet-
ies, such as nitrous oxide gas. A condition closely simulating shock
may also be induced in the earlier stages of the administration of anes-
thetics when these are badly given, but paralysis of the heart or of the
respiratory center is a usual contributory cause. m3
4, Spinal Shock.—Spinal shock is produced by section of the spinal
cord, but it is to be carefully distinguished from all other forms of shock
because of its local character, as it affects only those parts of the body
which lie below the level of the lesion in the cord. Above this level the
animal may be in a perfectly normal condition, except in cases where
the section has been at so high a level that it has severed the vasocon-
strictor pathway and thereby produced a fall in blood pressure from
vasodilatation. Even when this has happened the part of the animal
anterior to the spinal lesion is by né means in a condition of shock. Thus,
Sherrington observed in a monkey whose spinal cord had been cut far
forward that, although the posterior part of the body was in profound
spinal shock and the blood pressure very low, the animal amused him-
self by catching flies with his hands. A sufficient description of the con-
dition of spinal shock has been given elsewhere, but here it may be noted
that it consists essentially in a paralysis involving at first all of the re-
flex mechanisms, including the control of the sphincters, in the part of
the cord posterior to the section. In the course of a few days or weeks,
according to the position of the animal in the scale of development, the
reflexes gradually return, until ultimately in a couple of months—in a
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SHOCK 289
dog, for example—they may all have reappeared. The cause of this
shock is no doubt the sudden interruption of the nervous pathways
which reflex action ordinarily takes in the higher animals (see page 803).
5. Nervous Shock; ‘‘Shell Shock.’’—Considerable attention has been
paid to the nervous shock that has frequently been observed in men who
have been subjected to the harrowing sights and the constant noise and
nerve strain incurred in modern warfare. The symptoms may appear
suddenly at the front or they may develop in men who have comported
themselves in an apparently normal manner until removed to the rear,
when they pass into a condition more or less simulating that of shock.
Severe conditions may also result to soldiers from injuries which in nor-
mal individuals would not in themselves be sufficient to produce sur-
gical shock. The characteristic symptoms in such cases are entirely
different from those of other forms of shock, and, as has been shown by
Elliot-Smith and T. H. Pear,”* the condition must be treated from the
neurologic or psychopathic point of view.
6. Surgical Shock.—It is this variety that is usually referred to when
one speaks of shock. It may be produced either by severe mechanical
injury to a healthy person or by extensive manipulation and rough
handling on the operating table. It is common in trench warfare, be-
ing therefore an important variety of ‘‘shell shock,’’ which term must
be used only in a general sense. However produced, the symptoms of
surgical shock are very much the same. The patient lies in a quiet,
apathetic condition, caring little for what is going on around him, and
answering questions only when repeatedly and importunately questioned.
His skin, lips and gums are very pale and more or less cyanotic; the skin
feels cold and is moist with sweat; the reflexes are greatly diminished,
and it is usually only after applying a very painful stimulus that any
movement of defense is elicited or resentment is shown on the part of the
patient. The postural reflexes are also abolished, so that if a limb is
lifted it falls back limp and toneless. The puls® at the wrist is very
rapid, thin and almost imperceptible, and the arterial blood pressure is
abnormally low. The respirations are frequent and shallow. The rec-
tal temperature is 1° C. or more below normal. The pupils are dilated
and react slowly to light. When he can be induced to speak, the pa-
tient’s voice is hoarse, and he complains of cold and numbness in the -
extremities. The symptoms are not unlike those of cholera.
Experimental Investigations of Shock
For inducing shock experimentally, two general methods have been
employed: either rough manipulation of the abdominal viscera, or re-
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290 THE CIRCULATION OF THE BLOOD
peated stimulation of large afferent nerves. Since the experiments’ are
usually performed on anesthetized animals, the effect of the anesthetic
‘has to be discounted in experimental work on the causes of shock.
The first step in such an investigation is naturally to classify the
symptoms into primary and secondary, for on the success of the classi-
fication must depend the outcome of further investigation into the
problem. 3
The earlier investigators were naturally attracted to the pronounced
fall in blood pressure as the primary cause of shock. It-is true that a
pronounced lowering will ultimately produce symptoms that are not
unlike those of shock, but on the other hand it can readily be shown that
this is a result only—a symptom and not a cause of the condition. It
was believed by Crile that the fall in blood pressure depended on a
universal dilatation of the blood vessels caused by exhaustion of the tone
of the vasoconstrictor center. It has been clearly shown, however, that
the tone of this center is practically normal in shock, and that the arte-
rioles are maintained not in a dilated but in a contracted state, indicat-
ing clearly therefore that the low blood pressure must be dependent
upon inadequate output of blood from the heart. The evidence for this
conclusion is as follows: (1) W. T. Porter®* and his collaborators have
shown that both pressor and depressor reflexes are perfectly normal
in a rabbit that is in a condition of extreme shock. It is particularly im-
portant that depressor effects were still obtained, since this indicates
that tonic activity of the center must still have been present. (2) The
blood vessels in a shocked animal are in a contracted state. On opening
a vessel and observing the outflow of blood, an increase occurs when the
nerve to the blood vessel is cut. (3) This same fact has been shown by
Seelig and Joseph,?? who cut the vasomotor nerve proceeding te the
vessels of one ear of a white rabbit and thus caused a local paralytic
dilatation of the vessels. Intense shock was then produced in the animal
in the usual way, after which the blood pressure in the anterior part of
the animal was suddenly raised by applying a clamp to the abdominal
aorta just below the diaphragm. This increased blood pressure caused
the vessels of the denervated ear to become engorged with blood, but
. not those of the opposite normal ear, which retained their tone (Fig.
106). (4) The volume of blood expelled by the ventricles has been
shown by Henderson” to be distinctly diminished in the early stages of
shock, the lack of pronounced fall in blood pressure indicating that there
must be a compensatory constriction of the arterioles. Lastly (5), it
has been found by Morrison and Hooker” that the outflow of blood
from the perfused organs of a shocked animal is less than that from the
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Fig. 106.—Illustration showing the appearance of the blood vessels in the ears of a rabbit
“in a state of deep shock.” The marked vasoconstriction is very plain in the left ear, the ves-
sels of the right ear being dilated because the cervical sympathetic, which carries the constrictor
fibers, has been cut. (From Seelig and Joseph.)
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SHOCK 291
same organs under normal conditions. Furthermore, severing of the
nerve of such an organ causes an increased outflow.
To these various pieces of evidence of a constricted condition of at
least certain of the vessels in shock, may be added the less direct evi-
dence furnished by the pallor of the shocked patient and the indications
. that the sympathetic nervous system, instead of being paralyzed, is
in an excited state, as shown by the sweating and the dilated pupils.
Furthermore, we know from the experiments of Pike, Guthrie and
Stewart®? on the resuscitation of the nerve centers aftcr interference
with the cireulation to the brain, that the vasomotor center is remark-
ably resistant to anemia. It can withstand this condition without losing
its tone or reflex activity better than any of the other cardinal centers.
Those who have maintained that a deficiency in the tone of the vaso-
constrictor and other nerve centers is responsible for shock have based
their evidence partly on histologic examination of nerve cells of shocked
. animals, it being assumed that the chromatolysis shown by these cells
indicates an exhausted condition. The assumption is, however, entirely
unwarranted, and no regard is given to the well-established fact that
similar histologic changes may be produced by other conditions. It
is certainly safe to conclude that the changes in the nerve cells in shock
are the result and not the cause of the low blood pressure of this
condition.
Since the fall in arterial blood pressure occurs with contracted ar-
terioles, it must be dependent on a diminished discharge of blood from
the heart. Interference with the heart action itself (independently of
the blood carried to this organ), or a deficiency in the filling of the ven-
tricles during diastole,—that is, a stasis of blood in the venous or cap-
illary areas,—are the possible causes for the diminished output. The
possibility that the heart action itself has been interfered with, as by
paralysis of the vagus mechanism, causing-a rapid beating of the heart,
has been shown to be untenable by various experiments. After stimulat-
ing the central end of an uncut vagus nerve in the neck in shock, the
reflex vagus mechanism is still operative. Furthermore, when the arte-
rial blood pressure is artificially raised, either by epinephrine injection
or by cerebral compression, the heart promptly responds to the in-
creased blood pressure by contracting more slowly and vigorously.
Evidently, therefore, as the cardiac mechanism itself is*normal, the de-
ficient discharge of blood must be dependent upon improper diastolic
filling. After this condition has set in, it becomes progressively worse
because of weakening of the heart muscles consequent upon the failing
blood supply through the coronary vessels.
The question therefore narrows itself down to the cause of the ineffi-
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292 THE CIRCULATION OF THE BLOOD
cient return of venous blood to the heart. In the first place, let us see
whether shock can be produced experimentally in animals by mechanical
interference with the bloodflow in the vena cava. That such is the case
was shown by H. H. Janeway and Jackson, who found that mechanical ob-
struction of the inferior vena cava for a short time was followed by the usual
signs of shock. Such interference with the venous return to the heart |
may also be produced by excessive movements of the thorax as a re-
sult of artificial respiration. That this in itself may cause shock is known
to all experimental investigators on the subject, although the interpre-
tation has not always been that which is given above. Yandell Hen-
derson® ‘thought that the excessive ventilation caused a blowing off
of carbon dioxide from the blood (see page 293), thus producing a
low tension of this gas in the blood (acapnia), which he believed to be
the responsible factor.
As in gravity shock, so in surgical shock, stagnation of blood in the
splanchnic area is common; the animal bleeds into his own (splanchnic)
blood vessels (capillaries and venules), because these have lost their tone.
As we have noted above, one of the most certain ways of producing
shock is by exposure and rough handling of the abdominal viscera. It
is therefore of importance to study the effects that can be noted on
the blood vessels of this area under such conditions. When the viscera
are first exposed to air, there may be a short period during which vaso-
constriction is evident. This is soon followed by a dilatation of the
arterioles in the exposed area, causing the capillaries and veins to be-
come markedly distended as during the first stage of inflammation. This
accumulation of blood in the mesenteric veins has been shown by Mor-
rison and Hooker to cause an increase in the weight of an isolated loop
of intestine as an animal passes into a state of shock.
Splanchnic engorgement alone does not, however, suffice to explain all
the loss of blood, and we are driven to conclude that the capillaries of
the tissues outside the abdomen must entrap much of it. As a matter of
fact, Cannon, and others, have found that concentration of the blood
occurs in these capillaries as indicated by comparisons of the corpuscles
and hemoglobin in blood drawn from veins and from capillaries. Nor-
mally the values are equal; in shock the capillary blood is much con-
centrated.
In so far as the circulatory disturbances are concerned, we may there-
fore sum up the conditions occurring in shock as follows: The blood
accumulates -in the veins and capillaries—that is, in a part of the vas-
cular system that is beyond vasomotor control. The consequent with-
drawal of this blood from the circulation produces a diminution of the
bloodflow in the vena cava and consequently an inadequate filling of the
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SHOCK 293
heart. The consequent curtailment in the systolic discharge does not,
however, at first cause any marked fall of arterial blood pressure be-
cause of a reciprocal constriction of the peripheral arterioles of the
body. Meanwhile, however, the stagnation of blood in the capillary areas is
progressively increasing, so that less and less blood remains available
for the systemic circulation. Consequently, after a while, in spite of
the arterial constriction, the blood pressure falls to the dangerous shock
level, and the secondary symptoms of fall in temperature, dulling of the
reflexes, etc., supervene. Increasing viscidity of the blood also retards
its flow.
The fundamental question in the pathogenesis of shock concerns there-
fore the cause of the stagnation of circulatory fluid in the capillaries and
venules. Two hypotheses have been offered, one being that the stimulation
of afferent nerve fibers to the respiratory center causes excessive alveolar
ventilation with a consequent washing out of carbon dioxide from the
blood (acapnia), which causes a veno-capillary atonia, and the other,
that a bombardment of the vasoconstrictor and other nerve centers
by afferent impulses brings these centers into a condition of exhaus-
tion, which is the essential cause of shock. The acapnia hypoth-
esis may be at once dismissed, since, on the one hand, it has been
shown that in typical shock there is no deficiency of carbon dioxide in
the venous blood (Short), and on the other hand, conditions of shock
are often produced without excessive breathing.
Nor is there any evidence to support the view that shock is caused by
fatigue of the cardinal centers as a result of excessive sensory stimu-
lation. In the first place, it has been shown by Mann* that during han-
dling of the abdominal viscera the nervous impulses transmitted up the
spinal cord are much less marked than those transmitted when the cen-
tral ends of sensory nerves are stimulated by operative processes on the
limbs and joints, although shock is much more readily produced by the
former procedure. The method employed by Mann for detecting the
existence of these afferent impulses was that of Forbes and Miller, in
which electrodes are placed on the brain stem in decerebrate animals,
and the current of action which accompanies the passage of nerve im-
pulses registered by a string galvanometer. Although this method is
simple and direct in principle, it has been found by Mann to require
great care in practice because of the fact that.the slightest movement
of the head end of the animal produces deflections of the galvanometer.
If the further results of this investigation should show, as the early
ones have done, that shock may be produced in an animal without any
observed deflection of the galvanometer, it will disprove once and for all
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294 THE CIRCULATION OF THE BLOOD
the theory that shock is dependent upon an impairment. of a higher
nerve mechanism as a result of overstimulation by afferent impulses.
Cannon* has recently suggested that the engorgement of the splanch-
nie blood vessels may be the result of a constriction of the portal rad-
icles in the liver, which dams back the blood in the portal circulation.
He points out that these radicles have vasoconstrictor nerve. fibers, as
evidenced by the fact that the rate of flow of fluid through the per-
fused liver decreases during asphyxia, as well as when the hepatic
nerve plexus is stimulated electrically or when epinephrine is injected
into the portal vein. He argues that, since the blood vessels in other
areas of the body are constricted in shock, so also will be those of the
liver, with the result that the blood of the portal vein, in which ordinarily
there is a very low blood pressure (10 mm.Hg), will become dammed
back in these veins and therefore removed from the systemie circulation.
It does not seem to the writer, however, that this explanation is likely
to be the correct one, for, although it is true that vasoconstrictor in-
fluences have been shown to exist in the hepatic radicles of the portal vein,
yet, since it is only under special experimental conditions that this can
be done, they must be very feeble in nature. As we have seen else-
where, portal vasoconstriction can not be demonstrated by stimulation
of the hepatic plexus with stimuli which are sufficient to produce marked
constriction of the hepatic artery radicles (see page 255).
The engorgement of the splanchnic capillaries and venules is much
more likely to be dependent upon local influences acting on the vessels
themselves. When shock is produced by manipulation of the abdominal
viscera, it is easy to see how this local disturbance might be set up.
When shock is caused in other ways, as by violent stimulation of sen-
sory nerves, the atony of the splanchnic vessels is not so easily accounted
for unless we assume that it is a type of abnormal reciprocal vascular
innervation. For example, when stimuli are applied locally to sensory
surfaces under ordinary conditions, a distribution of the blood of the
body takes place, more being sent to the irritated region and less to
other parts of the body (see page 238). During the sensory stimula-
tion preceding shock, it is conceivable that this reciprocal innervation
acts in a faulty manner, causing at first a dilatation of the splanchnic
arterioles and thus allowing more blood to enter the splanchnic capil-
laries and venules, which being possessed of little tone are incapable
of responding by increased tonicity, so that they become overdistended
and the blood in them stagnates.
In any ease there is no doubt that the initial change is the stagnation
of blood in these vessels, and when once such stagnation has occurred,
the process goes on spontaneously probably on account of the accumula-
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SHOCK 295.
tion in the stagnant blood of incompletely oxidized metabolic products,
which raise the hydrogen-ion concentration of the blood, and produce
a further relaxation of the muscle fibers in the vessel walls. That acid
has such an effect is well known (page 937). Dildtation or atonicity thus
progressively increases and is meanwhile further encouraged by the de-
privation of oxygen, for it has been shown that isolated artery strips do
not exhibit their usual tonicity when deprived of oxygen.
Treatment
Whatever may be the cause of the atony of the capillaries and
venules in shock, the existence of this condition indicates that the most
hopeful line of treatment is to cause the vessels to reacquire their tone.
It will be remembered that in gravity shock in a rabbit recovery may
be accomplished by the application of a tight binder to the abdomen,
or by placing the animal in a head-down position. Such measures ap-
plied in the case of man have not, however, been found of much value.
Pressure thus applied is evidently not brought to bear sufficiently on
the atonic vessels. Cannon has therefore made the interesting suggestion
that a hopeful procedure may consist in injecting directly into the ab-
domen a saline solution containing pituitrin, a hormone which, it will
be remembered, acts directly on involuntary muscle fiber.
Two other methods have been advocated for the treatment of shock—
namely, saline or blood transfusion and injection of epinephrine; but
neither method has proved of practical value. Epinephrine injections
do indeed temporarily raise the arterial blood pressure, but the subse-
quent condition of shock is possibly worse than that originally present.
After the injection of blood or saline solution containing gelatin or
mucilage, the blood pressure, although temporarily raised, very quickly
falls again. In this regard surgical shock differs from the shock follow-
ing severe hemorrhage, in which, as explained elsewhere, recovery of the
blood pressure as well as of the general condition of the animal can
be accomplished by transfusion with blood or with saline solution con-
taining mucilage or gelatin. This would indicate that there is some
essential difference between these two forms of shock (see page 140).
The only treatment of avail appears to be to keep the patient warm and
to remove causes of excessive pain.
Causes of Secondary Symptoms
It remains to consider the cause of some of the secondary conditions
developing in shock—namely, the disturbances in sensation and motion
and the fall in body temperature. All of these are undoubtedly depend-
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296 THE CIRCULATION OF THE BLOOD
ent upon the low arterial blood pressure, although some authors have
suggested that the loss of sensation may be dependent upon an increased
resistance or block at the synapses of the receptor neurons (page 803).
This suggestion depends on the fact, demonstrated by Sherrington, that
repeated stimulation of the receptors of a reflex are produces fatigue
of that particular reflex, and that this fatigue must be resident in the
synapsis and not in the motor neuron, since the same motor neuron
that participated in the fatigue can still be called into activity by afferent
stimuli transmitted to its nerve cell through other sensory pathways
(see page 825). It is thought that in shock the frequent afferent stimula-
tion produces synaptic fatigue and therefore dulls the sensory responses
of the animal. The researches of Mann above referred to, in which he
shows that shock may occur without any demonstrable afferent stimuli
in the brain stem, would seem, however, to negative the above hypothesis.
The raised threshold of sensory stimulation is no doubt an effect of the
low blood pressure. It has been shown, for example, by EH. L. Porter**
that when the arterial blood pressure is maintained at a uniform level,
the threshold stimulus for spinal cord reflexes remains practically uni-
form, but becomes promptly increased when the arterial blood pressure
is made to fall. Why a lower blood pressure should have this effect is,
however, difficult to understand in the light of the researches of Stewart
and his coworkers, who, as remarked above, found that the cells of the
central nervous system may endure total anemia for many minutes and
still recover their physiologic condition. It may be, however, that the
low blood pressure affects the conductivity of the synapsis.
The muscular weakness is probably also dependent on low blood
pressure, for it has been found in animals that, when the arterial blood
. pressure is lowered to about 90 mm. Hg, the muscles contract much less
efficiently than ordinarily. The fall in body temperature is dependent
upon the muscular inefficiency.
In conclusion, it should be pointed out that W. T. Porter, in the inves-
tigation of acute shock met with at the front, has found that, in many
cases at least, the circulatory disturbance is due to a condition of fat
embolism. The fat is derived from the marrow of long bones, such as
the femur, by injuries which smash the bones. Porter’s observations
are at least very. suggestive.
CIRCULATION REFERENCES
(Monographs)
Wiggers, C. J.: The Circulation in Health and Disease, Philadelphia, 1915.
Mackenzie, J.: Diseases of the Heart, Oxford Medical Publishers, ed. 2, 1910.
Lewis, Thomas: Mechanism of the Heart Beat, 1911, Shaw & Son, Fetter Lane,
London.
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SHOCK 297
Lewis, Thomas: Harvey Lectures, 1913-1914, J. B. Lippincott Co.
Lewis, Thomas: Clinical Disorders of the Heart Beat, P. B. Hoeber, New York, 1912.
Hill, Leonard: The Mechanism of the Circulation of the Blood, in Schifer’s
Physiology, ii, 1900. Young J. Pentland. : .
Gaskell, W. H.: The Contraction of Cardiac Muscle, in Schifer’s Physiology, ii,
1900, Young J. Pentland.
Flack, M.: Further Advances in Physiology, 1909. Ed. by Leonard Hill, E. Arnold,
London.
Porter, W. T.: American Text Book of Physiology, W. B. Saunders Co., 1900.
(Original Papers)
1MacWilliam, J. A., et al.: Heart, 1913, iv, 393; ibid, 1914, v, 153; Brit. Med.
Journal, Nov., 1914; VII Internat. Congress of Medicine, London, 1913, See.
II, Physiology.
?Hill, Leonard, F, R. S., et al.: Proc. Roy. Soc., 1914, B, Ixxxvii, 344; ibid., 1915, B,
Ixxxviii, 508 and 516.
3Erlanger, J.: Am. Jour. Physiol., 1916, xxxix, 401; ibid., 1916, x1, 82.
4Downs, A. W.: Am. Jour. Physiol., 1916, xl, 522.
5Bayliss, W. M.: Proce. Roy. Soc., 1916, lxxxix, B, 380.
6Knowlton, F. P.: Jour. Physiol., 1911, xliii, 219.
7Milroy, T. H.: Jour. Physiol., 1917, li, 259.
8Eyster and Meek: Heart, 1914, v, 119; ibid., 194, v, 137; Am. Jour. Physiol., 1914,
xxxiv, 368.
®Porter, W. T.: Art. on Circulation in an American Textbook of Physiology, W. B.
Saunders Co., 1900.
1Brodie, T. G.: Proc. Physiol. Soc., 1905, Jour. Physiol., 1905, xxxii.
11Stewart, G. N.: Heart, 1911, iii, 33.
12Garrey, W.: Am. Jour. Physiol., 1912, xxx, 451.
13Mines, G. R.: Jour. Physiol., 1913, xlvi, 188.
14Cohn, A. E.: Jour. Exper. Med., 1912, xvi, 732; Robinson, G. Canby: Ibid., 1913,
Xvii, 429; Cohn and Lewis, T.: Ibid., 1913, xviii, 739.
15Mathison, G. C.: Jour. Physiol., 1910, xli, 416.
16Porter, W. T.: Am. Jour. Physiol., 1911, xxvii, 276; ibid., 1915, xxxvi, 418.
ivMartin, E. G., and co-workers: Am. Jour. Physiol., 1914, xxxii, 212; xxxiv, 220;
1915, xxxviii, 98; 1916, xl, 195.
1sBayliss, W. M.: Proc. Roy. Soc., 1908, lxxx, B, 339.
19Hill, Leonard: The Physiology and Pathology of the Cerebral Cireulation, J. and
A. Churchill, 1896. _
20Hill, L., and Macleod, J. J. R.: Jour. Physiol., 1900, xxvi, 394.
21Macleod, and Pearce, R. G.: Am. Jour. Physiol., 1914, xxxv, 87.
22Porter, W. T.: Am. Jour. Physiol., 1898, i, 144. :
23Hill, L., and Barnard, H.: Jour. Physiol., 1887,°xxi, 323.
24Carter, E. P.: Jour, Lab. and Clin. Med., 1916, i, 719.
25Elliot-Smith, G., and Pear, T. H.: Shell Shock, Longmans, Green & Co., 1917.
26Porter, W. T.: Am. Jour. Physiol., 1907, xx, 399.
27Seelig, M. G., and Joseph, D. R.: Jour. Lab. and Clin. Med., 1916, i, 283; also See-
lig and Lyon, E. P.: Surg., Gynec., and Obst., 1910, ii, 146.
28Henderson, Yandell: Am. Jour. Physiol., 1908, xxi, 155; also Mann: Bull. Johns
Hopkins Hosp., 1914, p. 210; Markwald, J., and Starling, E. P.: Jour. Physiol.,
1913, xlvii, 275.
29Morrison, R. A., and Hooker, D. R.: Am. Jour. Physiol., 1915, xxxvii, 86.
30Pike, F. H., Stewart, G. N., and Guthrie, C. C.: Jour. Exper. Med., 1908, x, 499;
see also Dolley, D. H.: Jour. Med. Research, 1909, p. 95, and 1910, p. 331.
siJaneway, H. H., and Jackson, H. C.: Proc. Soc. Exper. Biol. and Med., 1915, xii,
193; Erlanger, J.: Gesell, Gasser, Proc. Am. Physiol. Soc., Am. Jour. Physiol.,
1918, xlv.
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298 THE CIRCULATION OF THE BLOOD
32Henderson, Y., and Haggard, W. H.: Jour. Biol. Chem., 1918, xxxiii, 333, 345-355-
365 (gives older references). See also Macleod, J. J. R.: Jour. Lab. and Clin.
Med., (editorial), 1918, iii. ,
33Short, Rendel: Lancet, London, 1914, p. 131.
34Mann: Jour. Am. Med. Assn., 1918, lxx, 611. Also Boston Med. and Surg. Jour.,
1917.
35Cannon, W. B.: Papers by Cannon and Collaborators in Jour. Am. Med. Assn., 1918,
Ixx, 520, 526, 531, 611, 618.
3éPorter, E. L.: Proc. Am. Physiol. Soe., Am. Jour. Physiol., 1916, xlii, 606.
37TWiggers, C. J.. and Dean, A. L.: Am. Jour. Physiol., 1916, xlii, 476; Am. Jour.
Med. Se., 1917, clii, 666. é
»
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PART IV
THE RESPIRATION
CHAPTER XXXIV
RESPIRATION
For convenience, the physiology of respiration may be considered un-
der its mechanics, its control, and its chemistry.
THE MECHANICS OF RESPIRATION
Of the many factors concerned in maintaining the normal functioning
of the animal body, the respiratory act is probably the most important.
On this account and also because we are conscious of the respiratory
movements, the physiology of respiration has been studied from the
earliest times. Much of the earlier work naturally concerned itself
with the study of the air that enters and leaves the lungs at each respi-
ration—the ventilation of the lungs, as it may be called. Two obvious
properties of the respired air are: (1) its pressure and (2) its volume.
The Pressure of the Air in the Respiratory Passages—the Pulmonary
or Intrapulmonic Pressure
This is readily measured by inserting a tube into one nostril and con-
necting the tube with a manometer; at each normal inspiration the
manometer registers a negative pressure of 2 or 3 mm. Hg, and at each
expiration, a positive pressure of about the same degree. Although
normally of small magnitude, the intrapulmonie pressure may become
very great when any obstruction is offered to the free passage of the
air. The greatest possible expiratory pressure can be measured by sim-
ply blowing into a mercury manometer, when it will be equal to that
which all the muscles of the thorax and abdomen can exert in compress-
ing the lungs. In a strong man it may amount to more than 100 mm.
Hg. Similarly, the greatest possible negative pressure on inspiration
may be measured by attempting to inspire through a tube connected
with a manometer. It represents the force with which the musculature
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300 THE RESPIRATION
of the thorax and abdomen can open up the thoracic cage, and may
equal -70 mm. Hg. These measurements in themselves are not of much
importance, except as a measure of muscular development.
Intrapulmonic pressures that are intermediate between the two ex-
tremes will be acquired in the lower air passages in cases in which there
is partial obstruction of the upper respiratory passages, as in bronchitis,
spasm of the glottis, diphtheria, ete. During coughing also, the intra-
pulmonic pressure may become very high. In this act the thorax is first
filled with air by a deep inspiration; the glottis is then closed, and a
* forced expiration is made. When a sufficiently high intrapulmonie pres-
sure is attained, the glottis opens and the sudden change in pressure
causes so forcible a blast of air that the offending foreign substance is
frequently carried with it out of the air passages. It is often assumed
that during coughing the sudden increase in pressure in the alveoli will
tend to cause their walls to rupture. This, however, is not the case.
The alveoli do not alone support the increase of pressure; they merely
act as the inner layer of a practically homogeneous structure com-
posed of lung, pleura and thoracic cage. When the tissues of the lung
are partially degenerated or atrophied, as in old people, then it is pos-
sible that a rupture may take place, but under ordinary conditions it
is not likely to occur.
Amount of Air in the Lungs
Measurements of the amount of respired air have recently assumed a
considerable interest on account of the various applications which can
be made of them in the study of lung conditions. The tidal air is that
which enters and leaves the lungs with each respiration (about 500 ¢.¢.) ;
the complemental air is that which we can take in over and above an
ordinary tidal respiration (about 1500 e¢.c.); and the supplemental air,
is that which we can give out after an ordinary tidal expiration (about
1500 ¢.c.). Taking these three together, we have what is known as the
vital capacity. It is usually about 3500 c.c., and is represented by the
amount of air which we can expel from the lungs after as deep an inspi-
ration as possible. The vital capacity is diminished: in certain pulmo-
nary diseases (see page 314). After all the supplemental air has been
expelled, there still remains in the lungs a large volume of air which
can not be voluntarily expelled. This is known as the residual air. To
measure it in a dead animal it is necessary to clamp the trachea, open
the thorax, remove the lungs to a vessel of water, and then allow the air
to collect from the opened trachea in an inverted graduated cylinder.
One part of the residual air is sometimes called the minimal air; it is
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RESPIRATION 301
represented by that which is not expelled from the lungs of a dead
animal when the thorax is opened. In the collapse of the lungs thus
produced, the alveoli are not completely emptied of air, because some
becomes pocketed within them and is expelled only when the lungs are
compressed under water.
The volume of the residual air can readily be measured during life
by causing a person, after a forced expiration, to take two or three
breaths in and out 6f a rubber bag containing a measured quantity of
an indifferent gas such as hydrogen. Suppose the bag to contain at
the start 4000 ¢.c. of hydrogen, and after a few breaths 3000 c.c. of
this gas and 1000 ¢c.e. of other gases (the total volume of hydrogen and
expired air in the bag being still 4000 ¢c.c.); then the residual air will
Maximum inspiration -7-~
2000 120
Complemental air .. ce or cub.in
Ordinary inspirati : - Vital capacity
TIDAL AIR-4 — |500cc. or 30 cub.in.
Ordinary expiration -->—-
Supplemental air -
Capacity of equilibrium
Maximum expiration -—-
Residual air -
Fig. 107.—Amounts of air contained by the lungs in various phases of ordinary and of forced
respiration, (From Waller.)
be 1333 ¢.c., for it is evident that after a few breaths the composition of
the expired air in the bag will be the same as that in the lungs. This
calculation is based upon the assumption that no hydrogen is absorbed
by the blood during the experiment, which is not strietly the case.
The amount absorbed is, however, so small in two or three breaths as to
make it permissible to disregard it. The measurement can also be made
by taking a few breaths in and out of a bag containing pure 0,. By
_ascertaining the proportion of nitrogen that collects in the bag, the
quantity of residual air can be calculated. We shall see later that the
measurement of the residual air during life has some practical impor-
tance in connection with the measurement of the bloodflow through the
lungs.
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302 THE RESPIRATION
Alveolar and Dead Space Air
In addition to these moieties of respired air, we have to consider the
division of the air in the lungs into what is called alveolar air and
dead-space air. The former is the air which comes in contact with the
epithelium through which gas diffusion between the blood and the air
occurs, the latter being the air which fills the respiratory passages. The
dead space can not be defined anatomically with exactitude; it is func-
tional rather than morphologic.
Measurement of the volume of the alveolar and dead-space air can be
made in an animal breathing under normal conditions by taking ad-
vantage of the fact that, while it is in the lungs, the air has added to
it CO, gas, which is present in the inspired air only in negligible traces.
The necessary data are: (1) the volume of the tidal respiration; (2) the
percentage of CO, in alveolar air; (3) the percentage of CO, in the tidal
air. Suppose the values to be 500 cc., 6 per cent and 4 per cent, re-
spectively; then the volume of alveolar air must be 500 <5 — 233 c.¢.,
and the dead space 167 c.c. The measurement so made is accurate only
when certain precautions are taken. Because of the practical impor-
tance of this part of our subject we shall, however, defer its further
consideration until we have become familiar with the general features
of pulmonary physiology. Since the first air to move into the alveoli
at the beginning of inspiration is that present in the dead space,—the
last air expelled from the alveoli on the previous expiration,—it is of
no value in purifying the air already present in the alveoli. If we take
a tidal inspiration as amounting to 500 ¢.c. and the functional dead space
as 150 ¢.c.; it is plain that only 350 ¢.c. of the outside air gains the
alveoli, and that the subsequent expiration is composed of 150 c.c. of
outside air that had lodged in the dead space plus 350 ¢.c. of alveolar air.
These facts deserve a certain amount of emphasis because of their
practical importance in many phenomena connected with respiration.
One seldom thinks, for example, that out of the 500 c.c. of air inspired
with each breath, only 350 c.c. reaches the alveoli, where it comes in
contact with the 2500-3000 c¢.c. of air already present in this part of the
lungs. :
There must-therefore be a sort of interface somewhere in the alveoli
between the fresh outside air that comes in with each breath through
the bronchioles and the air which is more or less stagnant in the alveoli.
This interface must move backward and forward somewhat with each
breath, and a rapid diffusion of oxygen and of CO, must take place
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RESPIRATION 303
aeross it between the inspired air and that in the alveoli. It is impossible
to fix any anatomic point at which the interface occurs.
The above described mechanism for the ventilation of the alveoli in-
sures the maintenance of slight but constant changes in the composition
of the air next the alveolar epithelium. It helps to prevent sudden varia-
tions in the amount of gases in the blood, particularly of CO,. Should
such variations occur, irregular stimulation of the respiratory and other
important centers that are influenced by the amount of this gas present
in simple solution in the blood, would be the result. The mechanism
serves as a sort of mechanical buffer by diminishing the sudden changes.
im gas concentration produced by inspiration and expiration.
Respiratory Tracings
The measurements of air for the determination of the foregoing val-
ues are made by the use of meters of various types. Sometimes, how-
ever, it is necessary to obtain an inscribed’ record of the respirations.
Fig. 108.—Pneumograph. The straps (b, b) are held around the thorax, and the tube of the
tambour connected by rubber tubing with a recording tambour.
This may be either qualitative or quantitative. A qualitative record is
taken by attaching some sort of receiving tambour to the thoracic wall
(the best type is shown in Fig. 108), and connecting this with a record-
ing tambour arranged to write on a blackened surface. When it is
desired merely to count the respirations or to observe their. regularity,
such a tracing is all that is required, but obviously it does not tell us
how much air has entered and left the lungs at each respiration. To
obtain a quantitative tracing, we must either connect a recording instru-
ment with the trachea or inclose the body of the animal in what is
known as a body plethysmograph. In observations on laboratory an-
imals the best type of recording instrument to connect with the respira-
tory passages is the Gad or Krogh pneumograph. A body plethysmograph
as used in the case of man is shown in Fig. 109.. Ail these instruments
must of course be calibrated, which is done by pouring a definite num-
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304 THE RESPIRATION
ber of ¢.c. of water from a graduate into a bottle with which the record-
ing instrument is connected by tubing. The displacement of the writing
point gives us the necessary data for standardization.
The Intrapleural Pressure
The air which we have just been considering depends for its move-
ment in and out of the air passages upon changes occurring on the outer
aspect of the lungs in the space between them and the thoracic wall.
This is called the intrapleural space. It does not really exist as an
actual space in the living animal, for the visceral pleura which covers
the lungs is in accurate and intimate apposition with the parietal pleura
on the inner aspect of the thorax.
Fig. 109.—Body plethysmograph for recording respiration. (From J. S. Haldane and J. G.
Priestley.)
If the thoracic walls are punctured in a living animal or in one which
has recently died, the air will rush into the thorax, the two layers
of pleura separate, and the lungs collapse, causing temporarily a space
to be formed between the two layers of pleura and indicating that a
certain subatmospheric or negative pressure must exist in the intact
thorax to prevent the lungs from collapsing. The degree of this nega-
tive pressure may be measured by connecting a tube and a manometer
with the thoracic cavity. While the thorax is at rest, as in expiration
or immediately after death, this pressure amounts to about -5 milli-
meters.* On inspiration it increases to -10 millimeters. There are there-
fore two problems to be considered: (1) the cause of the negative pres-
sure in the quiescent thorax, and (2) the cause of the increase of the
negative pressure during inspiration.
*The minus sign indicates that the pressure is negative or subatmospheric. It is a suction pressure.
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RESPIRATION 305
The Permanent Negative Pressure—Let us start with the changes
that occur in the thorax when the first breath is drawn. While the an-
imal is still in utero, the lungs completely fill the thorax. When the
first breath is drawn the thoracie cage expands more quickly than the
lungs, so that the latter become stretched, the stretching force being
the air that is introduced into them from the outside through the tra-
chea and bronchial tubes. On becoming stretched the lungs fill the
increased space created in the thorax by the greater expansion of the
thoracie cage. This in itself, however, would not explain the cause of a
subatmospherie pressure in the intrapleural space. Another factor must
come into play—namely, the elastic tissue of the lungs, which by the
expansion will become stretched and, therefore, tend constantly to re-
lax to its previous condition and so exert a pull on the structures be-
tween it and the thoracic wall. It is this elastic recoil which we really
measure when we connect a manometer with the intrapleural space.
Throughout life the lungs remain of smaller size than the thoracic wall,
and therefore to fill the thoracic cavity they are constantly more or
less distended and the elastic tissue somewhat stretched. The lungs
are, however, not the only structures in the thorax which’ become ex-
panded; all thin-walled vessels and viscera, like the veins, the esopha-
gus, the auricles, ete., must also become opened out a little.
When the thoracic wall is punctured and the outside air allowed free
entry to the intrapleural space, differences in pressure no longer exist
‘on the inner and outer aspects of the lungs, so that they collapse into
the postmortem condition on account of the elastic recoil. If a puncture
in the thoracie wall of a living animal is immediately occluded, the
lungs will expand again, because the blood absorbs the gases from the
intrapleural space and recreates the partial vacuum required to expand
the lungs. This absorption of gas in the pleural cavity is usually quite
rapid; but if the pneumothorax, as the condition is called, is allowed to
persist for any length of time, the lungs will not become properly ex-
panded again.
The Greater Negative Pressure on Inspiration. The cavity of the tho-
rax becomes increased in all diameters during inspiration, with the re-
sult that a greater space in the pleural cavity has to be filled. All the
thin-walled structures in the thorax therefore become still more stretched,
the lungs of course participating to the greatest extent because of the
entrance of outside air. The stretching of the elastic structures causes
a greater pull, or negative pressure, to be exerted in the pleural cavity.
Instead of being -5 mm. Hg, as in expiration, the intrathoracic pressure
now comes to be above -10 mm. Hg.
When any obstruction exists in the air passages, the changes in intra-
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306 THE RESPIRATION
thoracic pressure produced by the movements of respiration become
more pronounced than under normal conditions. When the thorax ex-
pands with the trachea blocked, the lungs are not able to open up suffi-
ciently to fill all the space so that there is excessive dilatation of the
veins, auricles and esophagus, as well as drawing in of the intercostal
spaces and bulging upwards of the diaphragm. If a manometer is con-
nected with the pleural space under these conditions, a very large
negative or suction pressure will be observed, amounting often to —70
or -80 mm. Hg. It is possible that under such conditions some space
might temporarily exist between the parietal and visceral layers of the
pleura, but it could not remain long, for it would very soon be filled
by fluid exuding from the blood vessels. In the opposite condition, in
which the respiratory passages are blocked and a forced expiration is
made, as for example in the first stage of coughing or during such acts
as defecation and parturition, the thoracic cage is compressed upon the
viscera, with the result that the air in the lungs assumes a positive
pressure, amounting often to nearly 100 mm. Hg. If a puncture wound
is made in the thorax under these conditions, the lungs instead of col-
lapsing will bulge out of the wound, for what is really occurring is
that the thorax is forcibly contracting on occluded sacs of air.
It is the alternating changes in intrapleural pressure that are respon-
sible for the changes in intrapulmonic pressure and these for the move-
ment of air in and out of the lungs with each respiration. In other
words, the thorax does not expand on inspiration because air rushes
in, as the uninitiated imagine, but air rushes in because the thorax
expands.
The Influence of Intrapleural Pressure on the Blood Pressure.—The
movements of respiration produce effects on the vascular system that
are of considerable importance in maintaining the circulation of the
blood. If an arterial blood-pressure tracing is examined, it will be
observed that aside from the cardiac pulsations large waves exist on it that
are approximately synchronous with the respiratory movements, the
upstroke of each of these waves corresponding in general with inspira-
tion, and the downstroke with expiration (Fig. 22). These respiratory
variations in blood pressure might be due either to changes in heart
rhythm or to a purely mechanical cause. Regarding the first possi-
bility, it is indeed the case in most animals that the pulse is quicker on
inspiration than on expiration, but that this alone is not an adequate
explanation of the rise is shown by the fact that it still persists after
the vagus control of the heart has been eliminated, either by cutting
the nerve or by the action of atropine.
The cause must therefore be a mechanical one. Bearing in mind the
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RESPIRATION 307
effects which we have seen are produced on the movement of air in and
out of the lungs by the changes in capacity of the thorax with each res-
piration, we naturally assume that the increase in blood pressure may
be due to the fact that on inspiration more blood is sucked out of the
systemic veins into those of the thorax, that this excess when it is pro-
pelled by the heart into the arteries raises the blood pressure, and that
on expiration the opposite condition obtains. That the movements of
the thorax on inspiration do accelerate the speed with which the venous
blood is traveling towards the heart can easily be shown by measure-
ments of bloodflow.
This explanation, however, does not suffice to account for | all the
changes of blood pressure which occur in respiration, for if we take
very accurate tracings of blood pressure and of the respiratory move-
ments side by side, we shall find that, although, in general, the blood
pressure rises with inspiration, yet the beginning of the rise is consid-
erably delayed; that is, immediately following the beginning of the
inspiratory act the arterial blood pressure continues for some time to
fall, and at the beginning of expiration it continues for some time to
rise (Fig. 22). Moreover, it will be found,.if tracings taken from dif-
ferent animals are compared, that frequently the general effect of ex-
piration is to cause more rise than fall, and of inspiration more fall
than rise. It will be found that these differences are dependent largely
on the type of respiration, whether thoracic or abdominal (Lewis).**
Let us consider first of all exactly what will happen in an animal
breathing entirely by the thorax (e.g., the rabbit). The first effect of
the inspiration is to cause the veins leading to the auricles, the auricles
themselves and the blood vessels of the lungs to become suddenly ex-
panded. More blood therefore will flow into them. For a moment or
two this blood will, however, tend to stagnate in the more capacious
vessels, and it will be some time until it finds its way to the left side
of the heart; therefore the initial effect of inspiration is a distinct fall
in arterial blood pressure. When the extra space created in the blood
vessels has been filled with blood,—that is, when inspiration has prac-
tically ceased,—the blood will flow on in increased volume to the left
side of the heart, and, therefore, raise the arterial blood pressure. On
expiration the first effect is that the diminishing negative pressure will
cause the thin-walled vessels mentioned above to constrict and thus
squeeze the blood inside them into the left side of the heart and raise
the pressure; but the ultimate effect in the later stages of expiration
will be that the vessels, being constricted, will allow less blood through
them and the arterial blood pressure will fall.
Take now the case of abdominal respiration. In inspiration the dia-
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308 THE RESPIRATION
phragm descends and crowds the viscera against the vena cava, with
the result that at first more blood is squeezed into the thorax and the
blood pressure tends slightly to rise. After this initial effect, how-
ever, the compression of the vena cava causes less blood to reach the
thorax, and the arterial blood pressure falls. The conditions will be
exactly reversed on expiration. The initial effect of thoracic inspira-
tion is, therefore, to make the arterial blood pressure fall, and the in-
itial effeet of abdominal inspiration, to make it rise. The net effect
—— Se a |) 2
Tuatha is aR,
A. ABDOMEN.
a ie cee
A AMIRIKLA Panis
io ae
——_— Insp.
at P line
D. cHest.
Fig. 110.—Effect of abdominal and chest breathing on the pulse and blood pressure of man.
Abdominal inspiration raises the pressure and diminishes the amplitude of the pulse curve. Thoracic
inspiration less clearly lowers the pressure. Expiration has the cpposite effects. (From Lewis.)
produced will be the algebraic sum of these two opposing influences
(see Fig. 110).
Another factor that comes into play in determining the effect of the
respiratory movements on the cardiac output acts through the changes
in the pericardial pressure. When this is lowered, as early in inspira-
tion, it encourages diastole, thus causing better filling and therefore
better discharge from the heart.
These considerations taken together make it easy to understand the
changes in blood pressure, particularly in the veins, which occur when
a foreed inspiratory or expiratory movement is made with the glottis
closed. A forced expiration of this nature occurs during the acts of
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RESPIRATION 309
defecation and parturition, as well as in the first stages of coughing; it
is also produced by blowing into a tube, or against some resistance.
On account of the positive pressure that is brought to bear on the véins
as they enter the thorax, the venous pressure suddenly rises, slowing
down the flow of blood through the capillaries and causing bulging of
the veins and, if the effect is sustained, cyanosis. On the arterial
side of the vascular system, after a momentary rise caused by the
squeezing out into the left side of the heart of the blood in the capil-
laries of the lungs, there is a more permanent fall in pressure due’ to
the fact that less blood is now getting from the right side to the left
side of the heart. After some time the pressure begins to rise again,
partly on account of the back pressure through the capillary vessels
and partly because of vasoconstriction as a result of asphyxial
conditions.
In the opposite condition, during a forced. inspiratory movement with
the glottis closed or with the mouth attached to some tube through
which the attempt is made to suck air, the thoracic cavities open up
without the lungs being able to oecupy completely the extra space.
The dilatation of the veins and other thin-walled structures in the tho-
rax thus causes an immediate fall in both the venous and the arterial
pressure—in the venous, because the blood is sucked toward the large
vessels in the thorax and lungs, and in the arterial, because the blood is
now delayed in its passage from the right to the left side of the heart.
If this condition is maintained, the arterial pressure may recover some-
what, but that in the veins is permanently lowered.
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CHAPTER XXXV
THE MECHANICS OF RESPIRATION (Cont’d)
VARIATIONS IN THE DEAD SPACE, THE RESIDUAL AIR AND
MID-CAPACITY, AND THE VITAL CAPACITY IN VARI-
OUS PHYSIOLOGIC AND PATHOLOGIC CONDITIONS
By R. G. Pearce, B.A., M.D.
Dead Space
Under ordinary conditions of breathing the dead space is fairly con-
stant in volume. Haldane* and Henderson® believe that it may be in-
creased by 400 per cent in maximal deep breathing, and that the in-
crease is due to the passive stretching of the lower air sacs. Although
such large variations in the capacity of the dead space has not been ob-
served by Krogh and Lindhard’ or by R. G. Pearce,’ it is undoubted
that moderate rhythmic variations may occur. Even in deeper breath-
ing (1500 ¢.c. or over), a slight increase, which with maximum breaths
may amount to 100 ¢c., can be demonstrated. This is not surprising
when we remember that the walls of the bronchi and bronchioles are
made up largely of readily expansible tissue (elastic and smooth-muscle
fibers). As the respirations become deeper and the expanding force of
the inspiratory movements of the thorax becomes more pronounced, the
diameter of the bronchi and bronchioles will enlarge proportionately—
that is, the diameter or circumference will increase in direct proportion
to this force; but the area of the cross section of the bronchi (i.e., the
capacity) will increase as the square of the diameter. This depends on
the fact that the area of a circle is increased by 125 per cent when the
diameter is increased by 50 per cent, and by about 300 per cent when
the diameter is increased by 100 per cent.
The capacity of the dead space has a certain clinical significance.
Siebeck® has estimated that the dead space may increase by 100 ¢.c. in
asthma, but others believe that the increase may be greater. One rea-
son for the discordant results lies in the fact that the percentage of
CO, found in the alveolar air obtained by the Haldane-Priestley method
has been used as one of the basic figures in the determination of the
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THE MECHANICS OF RESPIRATION 311
capacity of the air passages. Ag explained elsewhere (page 344), the pro-
longation of expiration required to obtain the sample of alveolar air by this
method gives figures that are too high even. under normal conditions,
and it is plain that this error will be exaggerated in asthma, where the
expiration is greatly prolonged. An increase in the capacity of the
dead space must be accompanied by an increase in the respiratory vol-
ume if the alveoli are to be adequately ventilated. It has been thought
by some clinicians that the difficulty in asthma, emphysema and ear-
diac decompensation may lie in part in an increase in the dead space.
Careful estimations of the dead space in these conditions, however,
fail to demonstrate any great variation.
An explanation of the fact that the dead space in emphysematous
patients has been found to be generally large when determined by the
Haldane-Priestley method (sce page 340), and also for some of the clin-
ical phenomena accompanying the condition, may be as follows: In
emphysema the walls of the alveoli, especially about the lateral and
lower borders of the lungs, have lost their elasticity and fail to expand
or relax properly during the respiratory cycle. As a result the air in
these alveoli remains relatively unchanged except when forced respira-
tions are made. When a sample of alveolar air is taken directly, this
dead air is pushed out of the distended and diseased alveoli by the
forced respiration required in the direct sampling of the alveolar air.
Since the air in these alveoli has been in contact with the blood enter-
ing the lungs, it has a high CO, content, which results, when compared
with the uniformly low CO, content found in the tidal air, in giving a
large figure for the dead space. Since the capacity of the dead space
is not increased, the blood in the normal alveoli is probably being super-
ventilated in order to compensate for the high CO, tension in the blood
entering the left heart from the diseased alveoli. However, the O,
content of the blood leaving the sound alveoli is practically normal (be-
cause superventilation can not cause it to take up more), and can not
compensate for the low O, content in the blood coming from the dis-
eased alveoli, the net effect being therefore a low tension of O, in the
blood leaving the heart, which accounts for the cyanosis often seen in
emphysema (Pearce). A somewhat similar explanation can be given
for the cyanosis present in pulmonary edema, if we assume that all the
alveoli in this condition do not share alike in the edema (Hoover).
The Residual Air and Mid-capacity of the Lungs
During muscular exercise the residual air of the lungs is increased,
and the vital capacity decreased (Bohr). This causes the lungs to as-
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312 THE RESPIRATION
sume a more inflated condition between breaths or, as it has been clum-
sily styled, a greater mid-capacity. These changes may serve as a
physiologic method for ingreasing the efficiency of alveolar ventilation
so as to meet the greater needs of the body. This is partly because the
pulmonary vessels become dilated and the bloodflow through the lungs
is favored, and partly because of the influence of the reserve and sup-
plemental airs on the tension of the arterial blood gases during the res-
piratory cycle. For example, if the lungs were completely depleted
* of air during expiration, the blood leaving them at the end of this act
would be entirely venous. On the other hand, if the amount of air left
in the lungs at the end of expiration were above the normal amount,
each increment of CO, given off from the blood, or of O, absorbed by
it would produce less change in the pressure of the CO, or Oy.
The importance of these influences will be seen from the following
figures. If the residual and supplemental air amounts to 2000 ¢.c., and
the percentage of CO, in the alveolar air at the end of expiration is
5 per cent, then 100 e.c. of CO, must be present in the lungs. In a con-
dition of bodily rest about 20 ¢.c. of this gas is excreted during a res-
piratory cycle, so that if the breath were held during this period, the
percentage of CO, would rise from 5 to 6 per cent, and an inspiration of
400 ¢.ec. would be required to bring the air in the lungs back to 5 per
cent of CO,. On the other hand, if the residual and supplemental air
amounted to 3000 ¢.c. with 5 per cent of CO, in the alveolar air at the
end of the expiration, there would be 150 e.c. of CO, in the lungs at
the end of the expiration, so that holding the breath for the time of the
respiratory cycle would raise the percentage of CO, only to 5.66 (pro-
vided the production of CO, was the same as in the first case), and an
inspiration of 600 ¢.c. would be necessary to reduce it to the normal
expiratory figure. Or, putting it another way, the production of CO,
ean be increased 50 per cent in the time of a respiratory cycle without
affecting the tension of gases in the lungs, provided the residual and
supplemental air and the volume of the respiration are increased 50
per cent. If only one of the factors is changed, however, then the bal-
ance of the respiration must be disturbed, and the greater variation
in the tension of the gases in the arterial blood must occur at the dif-
ferent phases of the respiratory cycle. Bohr and Siebeck have shown
that the residual air is invariably increased in emphysema and that the
mid-eapacity of the lungs is likewise increased; and it would appear
from Siebeck’s data that a similar condition must be present in cases of
decompensated heart.
Patients suffering from dyspnea, particularly those suffering from
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THE MECHANICS OF RESPIRATION 313
cardiac dyspnea, can not breathe as comfortably when lying as when
sitting. This condition is known as orthopnea. The advantage of the sit-
ting over the lying position for breathing can not be satisfactorily ex-
plained. The greater vital capacity in the upright position; the favor-
ing of the return of the venous blood from the cerebral vessels by
gravity; the increased caliber of the pulmonary vesselg because of the
enlarged thoracic cavity (see page 318); and the increase in the reserve
air of the lungs—are all factors to be considered.
The Vital Capacity —At one time it was thought that the vital capacity
of the lungs was related to their ventilatory capabilities, but for years
the determination of this value in patients has been considered unimpor-
tant. Recently Peabody and Wentworth” have called attention to the
fact that patients with heart disease become dyspneic more readily than
do healthy subjects, and that this tendency seems to depend largely
on their inability to increase the depth of the respiration in a normal
manner. They find that this inability to breathe deeply corresponds to
a diminished vital capacity of the lungs as measured in a spirometer,
by the volume of the greatest possible expiration after the deepest in-
spiration. They believe that any condition which limits the possibility
of increasing the minute volume of air breathed must be an important
factor in the production of dyspnea.
In normal adults the following averages (Table I), were secured from
a large series of clinical cases. The subjects are grouped into two
classes, each group being subdivided according to height.
Taste I
Tue ViraL Capacity oF THE Luncs or NorMAL MALES
NORMAL NUMBER NUMBER
HEIGHT IN *R picHest | Lowest | HIGH-
“GROUP SED FEFT AND Bee 10% oF VITAL VITAL gst | LOWEST ann
INCHES es woamar, | cAPacity| capacrty| 9% To sd
I 14 : 6+ 5,100 9 7,180 5,030 141 99 0
II 44 Over 5’ 4,800 41 5,800 4,300 121 90 0
8%" to 6’ .
III 38 Ae 4 to 4,000 31 5,080 3,450 127 86 1
‘2
Tue VitaL Capacity oF THE Luncs of NorMAL FEMALES
I 10 Over 5° 3,275 5 4,075 2,800 124 86
It 13 Over 5’ 3,050 9 3,425 2,660 112 : 88
4” to 5’ ;
Ill 21 5” its or 2,825 16 3,820 2,500 135 89 1
ess.
(Peabody and Wentworth.)
It would appear that in normal people the vital capacity is at least
85 per cent, and almost always 90 per cent or more, of the standard
adopted for each group. In elderly persons a slight decrease from these
standards may be expected.
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314 THE RESPIRATION
TasBLe IL
Tur RELATION OF THE VITAL CAPACITY OF THE LUNGS TO THE CLINICAL CONDITION IN
Patients wirH Hrart DisrasE*
GROUP VITAL NUM- MOR- SYMPTOMS WORK- REMARKS
CAPACITY BEROF TALITY OF DECOM- ING
% ‘CASES % PENSATION % -
. 2 %
I 90 - 25 9 0 92 Few symptoms ref-
erable to heart.
II 70 to 90 41 5 2 54 History of dyspnea
with exertion, yet
ablé to do moder-
; ate work.
IIT 40 to 70 67 17. . 89 7 Dyspnea with mod-
erate exercise.
Few able to work.
IV Under 40 23 61 100 0 Bedridden, with
marked signs of
cardiac insuf-
ficiency.
(Peabody and Wentworth.)
*Certain cases were tested several times and, owing to changes in the vital capacity they appear
in more than one group. In the “mortality” column they are-included only in the lowest group into
which they fell. “Symptoms of decompensation” indicate dyspnea while at rest in bed or on very
slight exertion. Under “working” are included only those actually at work, and able to continue.
Many other patients in Group II were able to work, but they are not included as they were still in
the hospital.
Table II shows that there is a remarkably close relationship between
the clinical condition of cardiac patients, particularly as regards the
tendency to dyspnea, and the vital capacity of the lungs. Peabody and
Wentworth believe that the determination of the vital capacity affords
a clinical test as to the functional condition of the heart, since compen-
sated patients who do not complain of dyspnea on exertion have a nor-
mal vital capacity. Patients with more serious disease in whom dyspnea
is a prominent symptom, have a low vital capacity; and the decrease in
vital capacity runs parallel with the clinical condition. As a patient
improves, his vital capacity tends to rise; as he becomes worse, it tends
to fall. In other diseases in which mechanical conditions interfere with
the movements of the lungs, the tendency to dyspnea corresponds closely
to the decrease in the vital capacity. The cause of the decrease in the
vital capacity of the lung in cardiac decompensation is difficult to ex-
plain satisfactorily. It may be the limitation in the movements of the
lungs produced by engorgement of the pulmonary vessels, by the weak-
ness of the intercostal muscles, the rigidity of the bony thorax,
emphysema, or accumulation of fluid in the pleural cavities.
In cardiac disease the air in the lungs at the end of a normal expiration
is usually increased. This is similar to the condition which attends exer-
cise, and is probably a physiologic adaptation to give optimum aeration
to the blood, as explained above. ute
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CHAPTER XXXVI
THE MECHANICS OF RESPIRATION (Cont’d)
THE MECHANISM BY WHICH THE CHANGES IN CAPACITY OF
THE THORAX AND LUNGS ARE BROUGHT ABOUT
By R. G. Pearce, B.A., M.D.
The changes that take place in the form and the dimensions of the
thorax during respiration are brought about by movements of the ribs,
diaphragm, sternum, and vertebre. The share which each plays must
be considered separately.
The Movements of the Ribs
The first seven pairs of ribs progressively increase in length, and are
attached directly to the sternum by cartilaginous bands. The eighth to
the twelfth pairs progressively decrease in length, and as far as the
tenth they are indirectly attached to the sternum by cartilages which join
the seventh. The eleventh and twelfth have their anterior ends free, and
may be considered a part of the abdominal wall and not an intrinsic part
of the thoracic cage.
Each pair of ribs, together with its articulating cartilage and vertebre.
forms a ring, the plane of which is directed forward and downward.
The spinal articulations of the upper ribs differ from those of the lower
ones. In the former the articulations on the tubercle exist as convex
ovoid facets, which fit into corresponding hollow facets on the transverse
processes of the vertebre, while the corresponding facets of the lower
ribs are flat. Hach transverse process from above downward is tilted a
little more backward than the one above, so that the angle at which the
ribs are set to the spine increases from above downward. This manner
of articulation of the upper ribs with the vertebre prevents any rotation
in the spinosternal axis, so that there can be no so-called bucket-handle
movement in this region (Keith). The articulation, however, allows the
neck of the rib to rotate in an axis approximately transverse to the body.
The angle which the shaft of the rib makes near its neck, together with
the arch of the shaft, which is directed downward and forward, has the °
effect of causing the transverse rotation of the neck of the rib to be
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316 THE RESPIRATION
converted into an upward movement, which is greatest in that part of the
shaft lying parallel to the axis of rotation of the neck (Fig. 111).
The upper ribs from the first to the fifth form a cone-shaped top to the
thorax, whereas the lower ones form a vertical series, each being situated
almost directly above its neighbor. The upper set is arranged for the
expansion of the conical upper lobe of the lungs, the lower for the ex-
pansion of the more or less cylindrical lower lobes. During inspiration
the’ anteroposterior diameter of the conical portion of the thorax in-
creases, because the ribs, together with the sternal connections, move
through progressively increasing arches, and each lower rib tends to over-
ride the rib just above. The maximal rise of the ribs from the first to the
Axis of
rotation
Fig. 111.—A, first dorsal vertebra; B, sixth dorsal vertebra and rib. Axis. of rotation shown in
each case. “
tenth during inspiration shifts more and more from the anterior to the
lateral aspects of the thorax, because the angle formed by the shaft near
the neck of the rib approaches nearer to the articulating joints on the
vertebra.
An examination of the shape of the first rib, its relationship to adjacent
structures and its movements, shows that it differs from the others in
its respiratory function. The first pair of ribs and the manubrium sterni
are bound closely together by short, wide costal cartilages, and form a
structural unit which Keith! calls the thoracic operculum. This lid is
articulated behind with the first thoracic vertebra by a joint, which is
more nearly transverse than that of the rest of the costal series; and in
front with the manubrium, which is also articulated with the clavicles
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THE MECHANICS OF RESPIRATION 317
above and with the body of the sternum below. The freedom of move-
ment at the angle which the manubrium makes with the sternum at this
joint is related to the type of breathing. When the lower portion of: the
sternum is élevated during inspiration, the movement of the joint is not
free, but when the sternum is retracted, the movement at the angle may
amount to 16°. Lack of movement of the sternal manubrial joint has
been considered by some physicians as one of the predisposing causes of
pulmonary tuberculosis. During inspiration, the first rib and its anterior
attachments are raised by the scaleni, and serve as a point towards which
the second, third, fourth and fifth ribs are elevated. During expiration,
they are depressed toward the lower ribs, which form a more or less
fixed base.
The combined effect of these influences is to produce a motion of the
upper ribs which is described by the clinician as being undulatory. This
movement is more apparent in the upper part of the thorax, because
here the relative difference in the length of the ribs is greatest. Hoover
attributes a certain diagnostic significance to loss of the undulatory
movement, diminution in the extensibility of the underlying lungs causing
it to become less or to disappear. The phenomenon is elicited by placing
the tip of the ring finger on the second rib in the midclavicular line, the
tip of the middle finger on the third rib midway between the midclavicu-
lar and anteroaxillary line, and the tip of the index finger on the fourth
rib in the midaxillary line. The patient is then instructed to make a
moderately rapid and deep inspiration. The finger on the third rib will
be observed to move farther than that on the second rib, and the finger
on the fourth rib will move farther than that on the third. The move-
ment of each rib from above downward succeeds and exceeds that in
the rib just above.
‘When there is a moderate degree of impairment in the ventilation of
the upper lobe, the three ribs move in unison and through the same dis-
tance, so that the undulatory movement is lost although the ribs involved
may exhibit a considerable excursion. The undulatory movement is also
impaired by any disease which encroaches on the air spaces, invades the
interstitial tissue of the lung, or displaces the lung as in the case of an
enlarged heart or a distended pericardial sac. Another possible factor
in this phenomenon is that any inflammatory process in the lung or ad-
jacent tissue will produce a reflex inhibition of the muscles of the ribs,
and thus limit the expansion of the thorax.
The axis of movement of the lower ribs, as of the upper ribs, accurately
corresponds with that indicated by their articulation with the vertebre,
because the muscles attached to them, as well as the diaphragm, influence
their movements to a large extent. Anteriorly the lower ribs from the
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318 THE RESPIRATION
sixth to the tenth are joined to the sternum by the cartilages which unite
the sixth, seventh, eighth, ninth, and tenth, so that any movement in
which the ribs are raised is accompanied by an anterior movement of the
sternum (Fig. 112). The ribs are so articulated to the spinal column that
the inspiratory act causes the lateral and anterior part of each rib arch
to move forward and outward more than the one above it.
In natural breathing in the standing or sitting posture there is a
slight extension.of the spine during inspiration. This serves to increase
all diameters of the thorax and its absence is undoubtedly an important
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Git
Fig. 112.—Lower half of the thorax from the 6th dorsal to the 4th vertebra, seen from the
front. c, ensiform process; d,d’, aorta; , esophagus; f, aperture in tendon of diaphragm for
passage of vena cava inferior; I, 2, 3, trilobate expansions of tendinous center of diaphragm; 4, 5,
costal portions, right and left, of diaphragm muscle; 6, right crus of diaphragm; 8, 9, internal
intercostal muscles, which are absent near the vertebral column, where it joins 9 and 9, the ex-
ternal intercostals; ro, 10, subcostal muscles of left side. (From Luschka.)
contributory factor in reducing the vital capacity of an individual when
lying on the back. Figures given by Hutchinson for the effect which
posture has on the vital capacity are of interest because of their bearing
on the cause of orthopnea. In the same individual he found the following
vital capacities:
Standing 4300 ¢.c.
Sitting 4200 c.e.
Supine 3800 e¢.e.
’ Prone 3620 c.c.
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THE MECHANICS OF RESPIRATION 319
The Action of the Musculature of the Ribs
In a general way, the external intercostal muscles may be considered
as a broad extension of the scalene muscles over the thoracic walls, with
the ribs as intersections. The scaleni serve to fix the position of the
Fig. 113.—Intercostal muscles of 5th and 6th spaces. A, side view; B, back view; IV, 4th
dorsal vertebra; V, Sth rib and cartilage; 1, J, M. levatores costarum, 2, 2, external intercostals;
3, 3, internal intercostals, exposed by removal of the external muscles. In A, there are no external
intercostals in the intercartilaginous spaces; in B there are no intercostals near the vertebral
column. (From Allen Thomson.)
first rib so that it forms an anchorage for the action of the external
intercostal muscles in raising the lower ribs. They also raise the upper
three pairs of ribs along with the manubrium and sternum.
The function of the intercostal muscles has been the subject of much
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Fig. 114.—Hamberger’s schema to demonstrate the functi i ; .
ternal intercostals. tional antagonism of penn and ex-
When the ribs ac and bd pass into the inspiratory positions a, i
, ¢ g and bf, the inter
dilates (bh is greater than ab); the sternum gf moves away from the em ai (BF is
greater than be); the fibers of the external intercostals ak shorten (ak is greater than al); and
those of the internal intercostals ck lengthen (ck is greater than /g). The reverse occurs : hi
the inspiratory position is taken. (From Luciani’s Human Physiology.) er
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320 THE RESPIRATION
“debate, and ean not be said to be definitely settled. The direction of the
‘fibers in the internal intercostals indicates that they are expiratory in
function, since they can not shorten in the inspiratory position; while,
on the other hand, the fibers of the external intercostals can not shorten
in the expiratory position, and hence must be considered inspiratory in
character (Fig. 113). In 1751 Hamberger showed that mechanically this
is the case, and gave the schema shown in Fig. 114.
The function of the intercartilaginous muscles, however, must be
inspiratory, as is shown in Fig. 115.
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Fig. 115.—Schema to demonstrate that the function of tlie internal intercartilaginous intercos-
tals is identical with that of the external interosseous intercostals.
The ribs and costal cartilage may be regarded as rods bent at the angles acd and bef, in
which the articular points c and e represent the symphysis between the bony and the cartilaginous
parts on which traction is made. During inspiration the fibers of the intercartilaginous muscles,
which have the direction gh, move the sternum df away from the vertebral column ab, like the
fibers of the external intercostals, which run in the direction k/. During this double action the
angles c and e must be decreased, because the muscles of the upper intercostal spaces work simul-
taneously, and the entire thorax is slightly elevated during inspiration. From this scheme it is
apparent that the external intercostals and the intercartilaginous muscles must be the same. (From
Luciani’s Human Physiology.)
The Action of the Diaphragm
It is possible, however, that the main function of both the intercostal
muscles is to regulate the tone of the intercostal spaces and so prevent
their suction inwards when the negative pressure in the thorax increases
(i. e., suction becomes greater). The ascent of the ribs, while producing an
increase in the anteroposterior and transverse diameters of the thorax,
would decrease the vertical diameter if this was not counteracted by the
fixation of the lower ribs and the descent of the diaphragm. The periph-
eral edges of the diaphragm are attached behind to the lumbar vertebre,
laterally to the lower edges of the six lower ribs and their cartilages,
and in front to the tip of the ensiform cartilage. The fibers converge to
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THE MECHANICS OF RESPIRATION 321
enter the central tendon, and the lateral sheets are pressed upward by
the intraabdominal positive and intrathoracic negative pressures, so that
they form’a dome-shaped vault, with the liver in the right side and the
stomach and the spleen in the left. ‘
During expiration the lateral edges of the diaphragm are in contact
with the parietal pleura of the thoracic cavity, forming what are known
as the pleural sinuses. During inspiration the fibers of the diaphragm
shorten; this straightens out the arch of the diaphragm and pulls the
lateral edges of the diaphragm away from the parietal pleura, thus open-
ing up the pleural sinuses, into which the lungs descend. Usually the
opening up of the sinuses is accompanied by a slight retraction of the
external chest wall, which is known as Litten’s diaphragm phenomenon.
The descent of the diaphragm may produce a movement of from 10 to
15 mm. on each side, which accounts for a rather important fraction of
the volume of air exchange by the lungs. The central portion of the
diaphragm does not move much in normal respiration, but in forced
respiration its movement may be considerable.
Because of its attachments to the lower six ribs, the contraction of the
diaphragm tends to pull the margins of the ribs towards the median line,
but under normal conditions this movement is opposed by the action of
the external intercostals in raising the ribs and expanding the horizontal |
diameters of the thorax, and by the lower vertebral muscles, which fix
the position of the lower ribs.
The relative part which the diaphragm and the external intercostal
muscles play in the widening of the lower part of the thorax is of some
importance from the standpoint of diagnosis. It has generally been held
that the contraction of the diaphragm produces a widening of the lower
part of the thorax, because in its descent it presses upon the abdominal
viscera and so distends the abdomen and pushes out the lower ribs.
That this might occur seems not improbable, but Hoover? has recently
shown by experimental and clinical observations that the flaring in the
costal margins seen in normal inspiration depends on other factors. He
ealls attention to the fact that the contraction of the intercostals raises
the ribs and increases the angular divergence of the subcostal borders.
This widening of the angle made by the costal margins at the tip of the
sternum is very pronounced in paralysis.of the diaphragm while in
paralysis of the intercostal muscles, the costal borders are drawn towards
the median line and the subcostal angle is decreased. This shows that
the diaphragm must tend to diminish the angle.
The line of traction of the diaphragm is a straight one joining the cen-
tral tendon with the edge of the ribs. When the diaphragm forms a
well-defined arch, it exerts its traction at a disadvantage, and the ex-
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322 THE RESPIRATION
Te.
Fig. 116.—Diagram to show the effect of high and low positions of the diaphragm on the
costal angle.
Line 1. Normal position of diaphragm. Costal margins move out during inspiration.
Line 2. High .position of diaphragm. Normal outward movement of costal margins accentuated.
Line 3. Low position of diaphragm. Costal margins move in during inspiration.
Line 4. Very low position of diaphragm. Costal margins move out during inspiration.
Line 5. Actual line of traction of diaphragm. (From T. Wingate Todd.)
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THE MECHANICS OF RESPIRATION 323
TH
Fig. 117,—Diagram to show the effect of clinical displacements of the diaphragm on the costal
angle. :
Line 1. Normal position of diaphragm. Costal margains move out during inspiration.
2. Position of diaphragm in general cardiac enlargement. Costal margin from ensiform to ninth
rib moves toward median line.
3. Position of diaphragm in left-sided cardiac enlargement. Left costal margin is fixed or
moves in during inspiration. .
4. Position of diaphragm in right-sided cardiac enlargement. Right costal margin is fixed or
moves in during inspiration.
5. Costal margin. (From T. Wingate Todd.)
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324 THE RESPIRATION
ternal intercostals have the mastery and cause the costal borders to
spread. When the arch of the diaphragm is depressed, as in pleurisy
with effusion, emphysema, and empyema, the line of traction and the
line of the muscular fibers of the diaphragm correspond more closely,
so that the diaphragm is able to use its full force against the intercostal
muscles, with the result that the costal border moves towards the median
line. The curves of the different fibers of the diaphragm vary greatly;
the arch is much less marked in the portion attached to the costal margin
near the median line than in that attached in the axillary line. For this
reason the anterolateral part of the diaphragm requires less depression
to give it a horizontal position than is required for parts occupying a
more lateral position. A small pericardial effusion or an increase in the
size of the heart may therefore depress the diaphragm sufficiently to give
it mastery over the intercostals in the front portion, so that the costal
border may here move towards the midline, while the lower borders
move in a perfectly normal manner (see Figs. 116 and 117).
During forced breathing several muscles are brought into play, among
the most important of which are the scaleni, sternomastoid, trapezius,
pectorals, rhomboids, and serratus magnus.
There has been considerable debate as to whether expiration is normally
an active or a passive process. Undoubtedly the expiratory phase under
normal conditions does not require the same muscular effort as does that
of inspiration, but there are many observations which indicate that ex-
piration is partly under muscular control. The abdominal musculature,
for example, increases in tone during expiration, so as to bring about a
rise in the abdominal pressure, with the result that the relaxed diaphragm
is pushed up into the thoracic cavity. To this extent at least, expiration
is accompanied by increased muscular activity.
Before leaving the subject of the diaphragmatic movements, reference
must be made to the recent observations of Lee, Guenther and Meleney*
bearing on the general physiologic properties of the diaphragmatic
muscle. They point out that most skeletal muscles in the living body
contract with varying degrees of intensity and at irregular intervals,
between which relatively long periods of rest occur, but the diaphragm
from birth to death performs a continuous succession of brief contrac-
tions of fairly regular rhythm and uniform extent, alternating with brief
periods of rest. Its muscle fibers, together with those of the other
respiratory muscles, therefore hold a unique position among skeletal
muscles, which suggests a crude analogy with that of the heart. They
have compared the physiologic properties of the diaphragm with those
of the extensor longus digitorum, the sartorius, and the soleus, and found
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THE MECHANICS OF RESPIRATION 325
that the diaphragm is composed of a much more efficient muscular tissue
than that of the other muscles.
The Effects of the Respiratory Movements on the Lungs.—The changes
produced in the dimensions of the lungs by the inspiratory expansion of
the thoracic cavity are not uniform, since different parts of these struc-
tures are not equally extensible. From an anatomic. standpoint, the
lungs may be divided into three zones: (1) The inner or root zone contain-
ing the bronchus, artery and vein, and their main subdivisions. The
large amount of fibrous tissue in this region offers great resistance to
any expanding force. (2) The intermediate zone, containing the vascular
and bronchial ramifications radiating towards the surface of the lungs,
with pulmonary tissue implanted between the rays. This part of the
lungs has varying degrees of extensibility, the pulmonary tissue having
the most and the vaseular.and bronchial the least. (3) The outer zone.
perhaps 25 to 30 mm. in depth, composed of pulmonary tissue and
equally extensible throughout (Keith'). The expansion of the lung is
accomplished by a moving apart of the less extensible rays of tissue so
as to permit the expansion of the more extensible pulmonary tissue be-
tween them. Keith compares the mechanism to that seen in the opening
of a Japanese fan.
Because the lung expands in the direction of least resistance, study
of the inflated dead lung does not reveal the normal expansion brought-
about by the thoracic movements. In the living body expansion is more
limited in some regions than in others. Of the five areas which may be
distinguished on the surface of the lungs, three are in contact with rela-
tively immovable parts of the chest wall, and therefore can not be ex-
panded directly. These are: the mediastinal, in contact with the pericar-
dium and the structures of the mediastinum; the dorsal surface, in contact
with the spinal column and the posterior aspect of the thoracic cage, and
the apical surface. The motions of the first pair of ribs and the manu-
brium expand chiefly the anterior and ventrolateral part of the apex
of the lung, and have only an indirect influence on the dorsal part of the
apex—i.e., the part lying directly in front of the necks of the first and
second ribs, the most common site of pulmonary tuberculosis. The two
surfaces of the lungs which are directly expanded are the diaphragmatic
and the ventrolateral or sternocostal. Meltzer* found that the negative
pressure in the thorax during inspiration was least along the relatively
stationary walls of the thorax, and greatest in the regions nearest the
diaphragm. From this he concludes that some of the expanding force
is lost as it passes through the lungs to the surfaces of indirect expansion.
Many observers have claimed that the expansion of the lung does not
take place throughout instantaneously and equally. This is illustrated
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326 THE RESPIRATION
by the fact that, in the region immediately surrounding a localized con-
solidation, a fluid has inereased resonance, which would not be the case
if the relaxation produced was equally distributed throughout the lung.
The root of the lung, which has generally been regarded as more or
less fixed, undergoes in normal breathing a definite forward, downward
and outward movement, and the heart shares in this movement (Keith).
The movements of the lower ribs and diaphragm are responsible for the
expansion of the lower lobes and dorsal portion of the upper lobes of the
lungs, whereas the movement of the upper five ribs expands the anterior
portion of the upper lobes. The relative infrequency of pleuritic fric-
tion-sounds and ‘pain over the upper lobes as compared with their fre-
queney over the lower lobes is explained by the fact that the expansion
of the upper lobes is accomplished with little displacement of the pleural
surfaces, whereas in the lower lobes expansion is accompanied by a glid-
ing of the lungs across the ribs.
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CHAPTER XXXVII
THE CONTROL OF THE RESPIRATION
The participation of such widespread groups of muscles in the respira-
tory act demands that some mechanism be provided to insure its adequate
control. With every inspiration, for example, the muscles of the ale
nasi act so as to cause dilatation of the nares, the vocal cords are ab-
ducted, and the intercostal muscles, along with the scalenes and the
diaphragm are contracting while the muscles of the abdominal wall are
relaxing; and all these events occur at exactly the proper time so as to
bring about the most efficient opening up of the thoracic cavity. Evi-
dently there must be some mechanism to insure this perfect control. This
is effected through the nervous system.
THE RESPIRATORY NERVE CENTERS
The efferent fibers to the various groups of muscle originate in their
respective motor neurons, which in most cases are situated in the gray
matter of the spinal cord. The harmonious action of these motor neu-
rons, or subsidiary centers, is brought about by the transmission to them
of impulses from a higher or master center placed in the medulla ob-
longata, the pathway of transmission between this master center and the
subsidiary centers being in the lateral columns of the spinal cord.
The evidence that the chief respiratory center is in the medulla is fur-
nished by observing the effects produced on the respiratory movements
by serial destruction of the cerebrospinal axis from above. downward.
By this method the approximate position of the center is found, its exact
location being then determined by punctiform destruction or stimulation
of the supposed locus of the center. If we destroy the cerebrum from
before backward, piece by piece, we shall find that no marked effect is
produced on the respirations until we arrive at about the middle of the
medulla, when immediate paralysis of the respiratory movements occurs.
If we now proceed to puncture various areas on the floor of the fourth
ventricle in another animal, we shall find an area called the noeud vital,
located about the tip of. the calamus scriptorius, destruction of which
causes immediate cessation of respiration. It is believed that the center
resides in the group of nerve cells known to neurologists as the fasciculus
solitarius. It is bilateral.
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328 THE RESPIRATION
The subsidiary centers are entirely dependent upon the master center
for their harmonious action, as is shown by the fact that if the phrenic
motor neuron—which is situated in the cervical spinal cord between the
fourth and sixth spinal segments—is isolated from the medulla by a
lateral hemisection of the cord just above the fourth segment and by
mesial section of the cord opposite the center, the corresponding half of
the diaphragm no longer participates in the inspiratory act (see Fig. 118).
The chief center on either side of the midline of the medulla is con-
nected with the motor neurons of both sides of the spinal cord, as is
proved by the following experiment. When the central end of the vagus
nerve is stimulated, the respiratory center becomes excited and the respi-
rations more pronounced, the participation of the muscles on both sides
of the body being equal in extent. If now we bisect the medulla down the
Medulla
Spinal cord
& roots
Fig. 118.—Diagram to show cuts required for isolation of the phrenic center.
midline and repeat the stimulation of one vagus, the muscles on both sides
will still participate in the increased respiration, which they will likewise do
if the cervical cord is bisected or hemisected but the medulla left intact
(Fig. 119). The simplest interpretation of these results is that commis-
sural fibers connect both halves of the respiratory center in the medulla
and that each half is also connected with the motor neurons of both sides
of the spinal cord. Often, especially in young animals, a hemisection of
the cord causes cessation of the movements of the diaphragm on the same
side; but this paralyzed side at once begins to’contract again when the
phrenic of the opposite side is cut, probably because the respiratory
impulse descending from the chief center, on finding its way along the
motor center of the same side of the cord blocked, is forced to follow the
crossed path. The crossing in the cord is believed to take place at the
same level as that at which the subsidiary center is located (W. T.
Porter??). é
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THE CONTROL OF THE RESPIRATION 329
The question now arises as to how the chief center functionates. Is it
purely reflex in the sense that it depends for its activity entirely on the”
transmission to it of nervous impulses from elsewhere, or is it automatic
‘in the sense that it can work independently of such impulses? The au-
tomaticity of the heart makes it seem not improbable that the center
which controls the co-ordinate action of the respiratory muscles would
also have an inherent or automatic power. The activity of such an auto-
matic respiratory center would, of course, be subject to great variation
as a result of changes in the composition of the blood supplying it, and
the fact that it was automatic would not remove it from the influence of
nervous impulses. Indeed it is possible to conceive of the automaticity
of the center as being of a low order, with its normal functioning
dependent upon afferent nerve impulses. Its automaticity might, then,
Spinal cord
& roots
Fig. 119.—Diagram to show certain positions in the medulla and upper cervical cord, where
sections may be made without seriously disturbing the respirations. Sections made separately will
not disturb the respiration, nor interfere with the effect of vagus stimulation. If both sections
are made at once, however, breathing will be seriously interfered with on the side of the
hemisection, and this side will not respond to vagus stimulation.
be merely a factor of safety called into play only when the influences
ordinarily controlling the center were. for some reason removed.
The question which at present confronts us, however, is whether the
center may or may not act automatically. Many experiments have been
undertaken to test this point, the nature of all of them depending upon
the isolation of the center as completely as possible from afferent nerve
paths. The most successful experiment has been performed as follows:
The influence of the higher nerve centers was removed by cutting across
the peduncles of the cerebrum or the pons. The influence of afferent im-
pulses traveling up the spinal cord was removed by completely severing
the spinal cord below the level of the phrenic nerves and sectioning all
the posterior or sensory spinal roots of the cervical cord above the level
of this section. The vagi were also cut to remove the impulses traveling
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830 THE RESPIRATION
by them to the respiratory center. By such an operation the only lower
respiratory neurons left intact are those of the phrenic nerve, so that the
respiratory movements that alone are possible are those in which the
diaphragm participates and the muscles of the ale nasi and larynx. It
was found that the animal after the operation went on respiring, though
imperfectly, and that the respirations soon became more marked and
asphyxial in character, indicating that the blood was not becoming
N. Phrenicus
Diaphragm.
Fig. 120.—Diagram te show where cuts are made to isolate the chief respiratory center from
afferent impulses.
properly aerated and that the chemical changes occurring in it were
acting directly on the center, stimulating it to greater activity. The
conclusion seems warranted that the respiratory center can act auto-
matically, for the only possible afferent nerves left.in the above prepara-
tion were those carried to the center by the fifth nerve (Fig. 120).
That the respiratory center is extraordinarily sensitive to changes in
the composition of the blood flowing through it is a fact that has been
known for a long time, but it is only within recent years that the exact
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THE CONTROL OF THE RESPIRATION 331
nature of this control and the remarkable sensitivity of the center towards
it have been thoroughly established. We shall return to this important
subject later. Meanwhile we shall proceed to examine the manner in
which the center is affected by sensory impulses transmitted to ‘it.
THE REFLEX CONTROL OF THE RESPIRATORY CENTER
The afferent nerve fibers going to the respiratory center may conven-
iently be divided into two groups: those which act on it only occasionally,
and those which act on it more or less continuously.
The Occasionally Acting Impulses
To the first group belong afferent nerves from practically every part of
the body. That impressions from the skin affect the respiratory center
is well known by the increased breathing caused by applications of cold
water. The influence of these afferent impulses is often very marked,
and is frequently taken advantage of in stimulating a newborn infant to
take the first breath. Stimulation of the terminations of the fifth nerve
in the mucous membrane. of the nose, as by inhaling a pungent odor,
immediately inhibits respiration. To these occasionally acting afferent
impulses may be added the impulses that are conveyed to the respiratory
center from the higher nerve centers of the cerebrum. These impulses
are largely valuntary in nature, and enable us to hold our breath at will.
Some of the cerebral impulses are however also involuntary, their exist-
ence being seen by observing the respirations of an animal before and
after sectioning the pons or peduncles. The respirations for a time at least
become distinctly affected, but they later return with perfect regularity.
They may become very irregular, however, if the vagi as well as the pons
are cut. Other experimental evidence of the existence of cerebral respir-
atory fibers is furnished by cerebral localization experiments. During
stimulation of the cerebral cortex, for example, a marked effect on the
respiratory movements is often noted.
Respiratory rhythm, unlike that of the heart, has often to be modified
in order that the respiratory mechanism may be used for other purposes
than the ventilation of the lungs. This alteration in rhythm may take
the form of a mere inhibition, such as the act of swallowing; or the
respiration may be altered, as in phonation and singing. More consid-
erable alteration in the expiratory discharge occurs in coughing and
sneezing, and still more in the acts of micturition, defecation, and parturi-
tion. We must conclude therefore that the rhythmic stimuli sent out
from the respiratory center are so weak that stimuli from other sources
may instantly inhibit or change their form at any stage of the cyele.
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332 THE RESPIRATION
Stimulation of the endings of the glossopharyngeal nerve inhibits res-
piration, which explains the holding of the breath that occurs in swal-
lowing.
The Continuously Acting Afferent Impulses
The continuously acting afferent impulses are transmitted to the chief
respiratory center by the vagi and their branches, the superior laryngeal
nerves. If the vagus nerves are cut or their continuity severed by
freezing a portion of them, the respiratory movements become markedly
slower. Evidently, the vagus nerves in some way hurry up the respira-
tory movements. Again, if the central end of either vagus is stimu-
lated with the ordinary interrupted faradie current, a profound effect
on the respiratory movements is usually observed. This effect is how-
ever not strictly predictable. Usually there is a quickening of respira-
tion, and if the stimulus is a strong one, there may be a standstill of the
thorax in the inspiratory position. On the other hand, if the central
end of the nerve is stimulated with other types of stimuli, as by slow,
weak faradic shocks or by the stimulus produced by the closure of an
ascending voltaic current, the effect may be to stimulate expiration
rather than inspiration. Such results would seem to indicate that the
vagus contains two kinds of afferent fibers to the respiratory center, one
kind stimulating inspiration, the other, stimulating expiration.
Supposing that such fibers exist, the next question is, how do they
become stimulated at their terminations in the lungs? The most nat-
ural assumption is that the mechanical distention and collapse of the
alveoli which occurs with each respiratory act, serves as the stimulus—
an hypothesis to which support is offered by the observation that, when
air is blown into the lungs so as to distend the alveoli, the animal im-
mediately makes a forced expiratory movement, whereas when the air
is sucked out, the thorax assumes the inspiratory position.
Of the many methods that have been employed to produce disten-
tion of the alveoli, the best is undoubtedly that recently employed by
Haldane and Boothby.** The person or animal is made to respire through
a tube in which is inserted a three-way stopcock, which communicates
either with the outside air or with a side-tube leading to a spirometer
or bag containing air under slight pressure, so that when the stopcock
is turned breathing takes place against a definite positive pressure.
Such a method is obviously much more physiologic than one in which
the air-tube is suddenly clamped at the end of inspiration and the lungs
left in a distended condition.
The term used to designate the cessation of breathing is called apnea.
The extent to which it occurs varies very considerably in different an-
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THE CONTROL OF THE’ RESPIRATION 333
imals and, in the case of man, in different individuals. Thus, when a
man is made suddenly to breathe into compressed air, the apnea often
lasts for about half a minute, the pause being then broken by a deep ex-
piration followed by a further pause, then again an expiration, and so
on with progressively shorter pauses. Disregarding for the present
any influences which changes in the composition of the air in the lungs
or of the gases in the blood might have in producing the apnea, we may
consider the possibility that it is the result of afferent fibers in the
vagus. This is an old view, but the most recent experimental evidence
does not lend support to it. It was shown by Boothby and Berry,** for
example, that a similar apnea, though indeed of shorter duration, could
be produced in dogs in which the pulmonary branches of. both vagus
nerves had been severed two months previously. The apnea is, there-
fore, not a reflex of the vagus, and must be interpreted as due to nery-
ous impulses passing to the respiratory center from some other part of
the nervous system, perhaps from centers higher up, or to stimuli trans-
mitted to the respiratory center possibly through afferent fibers in the
respiratory muscles.
The formerly very popular theory that respiration is controlled au-
tomatically by alternate distention. and collapse of the alveoli, acting
through the afferent fibers of the vagus nerve on the respiratory center
in such a way as to bring the opposite act with each expiration and
inspiration, must, therefore, be abandoned. But we can not. deny that
the vagus plays a most important role in the control of the function of
the respiratory center, for apart from the effect which we have seen to
follow the severence of continuity of the nerve, there is the important
observation of Alcock and others’® that when nonpolarizable electrodes
are placed on the vagus nerve and connected with a galvanometer, a
current of action occurs toward the end of each inspiration in quiet
breathing; and when the respirations are forced, a current of action
appears during both inspiration and expiration. Another reason for
believing that the vagi have some important function to perform in con-
nection with the control of respiration is the fact, observed by F. H. Scott,"
that in an intact animal, when atmospheres containing increasing percent-
ages of carbon dioxide are respired, the respirations become both deeper
and quicker, whereas in one whose vagi have been eut the carbon diox-
ide causes only a deepening of the respirations. From this result it
would appear that the vagi exert an influence on the rate of the respira-
tions but not on their depth, this effect, as we shall see later, being de-
pendent primarily on changes in the composition of the blood supplying
the respiratory center. It is probable that both controlling agencies act
together, the one serving to maintain the center in a proper state of
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334 THE RESPIRATION
excitability, and being active to a greater or less extent all the time;
while the other acts only occasionally on the ‘‘tuned up’’ center. There
is, of course, no doubt that it is through the nerves that the occasional
alterations of respiration occur. They appear also to have a certain
. influence on the rhythm, for Stewart, Pike and Guthrie?’ observed that,
after resuscitation from acute brain anemia, the respirations when they
returned were of the same rhythm as that of the artificial respirations
employed during the resuscitation.
The usually accepted hypothesis as to the mechanism by which the
nerve impulses hasten the respiratory movements is that an afferent
impulse is transmitted to the respiratory center towards the end of each
inspiration, which has the effect of inhibiting the inspiratory discharge
from the center and thus cutting short the act of inspiration so that ex-
piration automatically supervenes. This explanation is in agreement
with the fact that quiet inspiration involves activity on the part of the
respiratory muscles, whereas expiration is usually almost entirely pas-
sive, being due to the return to a resting position of the stretched and
displaced structures. On the other hand, in forced respiration and in
certain animals under normal conditions, expiration becomes active, in
which event a current of action becomes evident in the vagus nerve dur-
ing the expiratory, phase.
The superior laryngeal branch of the vagus should really be classified
as one of those nerves that have an occasional influence on the respiratory
center, its particular function being in connection with the act of cough-
ing. When a foreign body irritates the mucous membrane of the larynx,
the nerve fibers transmit impulses to the respiratory center which ex-
cite a violent expiration and at the same time cause the glottis to close.
The closure of the glottis lasts, however, only during the first part of
the expiration; it then opens, with the result that the sudden release of
intrapulmonic pressure causes the expulsion of the foreign substance
in the air passages.
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CHAPTER XXXVIII
THE CONTROL OF RESPIRATION (Cont’d)
THE HORMONE CONTROL OF THE RESPIRATORY CENTER
Just as the rhythmical activity of the heart is readily influenced by
changes in the composition of the blood supplying it, so also is that of.
the respiratory center. In the case of the heart it is the cations—ceal-
cium, potassium and sodium—that have the most pronounced effect,
whereas in the case of the respiratory center it is largely the relative con-
centration of hydrogen and hydroxyl ions—the H-ion concentration
(Cx) of the blood. This influence can be shown in a general way by
injecting acid or alkaline solutions into the peripheral end of the carotid
artery of an anesthetized animal, or better still of one that has been
decerebrated. Acid injections stimulate the respiratory activity; alka-
line injections tend to depress it. When the acid or alkaline solutions
are injected intravenously in other parts of the body, so that they be-
come thoroughly mixed with the blood before the respiratory center is
reached, the effects are not nearly so pronounced, because the buffer in-
fluence of the blood has time to develop (see page 36).
From the results of such injection experiments, however, one could
not draw the conclusion that under normal conditions the activity of
the respiratory center is affected by measurable changes in Cy of the
blood, for, as we have seen, constancy of Cy is one of the most remark-
able properties of the animal fluids. To justify the conclusion that the
respiratory center is affected by changes in Cy, it is necessary to observe
the behavior of some easily measurable acid or alkaline constituent of
the blood that undergoes changes in amount that are proportional to an
alteration in Cy. In order to understand what this acid or basic
substance may be, it will be advisable to recapitulate the main factors
concerned in maintaining Cy at a constant level. This value is obviously
dependent upon the balance between basic and acid substances, so that
any variations which it undergoes must be caused by changes in the
relative amount of one of these. Changes in base may occur, exoge-
nously, by altering the alkali content of the food, or, endogenously, in
various ways but particularly by variations in the amount of ammonia
produced during the course of metabolism of protein. Thus, when sud-
den demands are made by the organism for an increased amount of base,
335
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336 THE RESPIRATION
the amino groups—split off from the amino bodies—become converted
into ammonia instead of into the neutral substance, urea. But the chief
variations seem to concern acids rather than the basic substances. These
acids may be divided into three groups: fixed inorganic acids, represented
by phosphoric; fired organic acids, represented by lactic; and volatile
acids, represented by carbon dioxide. Of these three groups, the first
shows the least tendency to change, and the third, the greatest. Changes
in the second group (fixed organic acids) are effected partly by exeretion
through the urine and partly by oxidation into volatile acid. The sud-
den and rapid changes in the third group are brought about by the dif-
fusion of the CO, of the blood into the alveolar air. Gross changes in
the acid content of the blood are therefore mainly effected through al-
teration in the excretion of the fixed acids, whereas sudden changes are
effected by excretion of the volatile acid. It is important to note here
that the fixed organie acids do not participate to any great extent in
the makeup of the acid content of normal blood: they appear only under
unusual conditions, as in dyspnea. The variations in Cy that ordinarily
affect the activity of the respiratory center are therefore dependent
upon changes in the volatile acid, a direct measure of which is found
in the tension of CO. in the blood. The correlation between Cy of the
blood and respiratory activity must be a very close one if Cy is to be
maintained.
The Laws of Gases.—In order to understand the principles upon which
alterations in CO, tension are dependent, it will be necessary for us to
review briefly some of the gas laws. Among these laws the first in im-
portance is the law of pressure, which states that, other things being
equal, the pressure of a gas is inversely proportional to its volume; if
a gas occupying a certain volume is compressed by a pump so that it oc-
cupies one-half of its previous volume, its pressure will become doubled.
The second is the law of partial pressure, which states that the partial
pressure of a gas in a mixture of gases, having no action on one another,
is equal to that which this particular gas would exert did it alone oc-
cupy the space occupied by the mixture. Thus, atmospheric air consists
roughly of 79 volumes per cent of nitrogen and 21 of oxygen; the par-
tial pressure of the oxygen is therefore equal to atx 760mm. Hg,
this last figure being the barometric pressure of air at sea level. The
third is the law of solution of gases, which is to the effect that the amount
of gas which goes into solution in a liquid having no chemical attraction
for the gas, is proportional to the partial pressure of gas. If water is
exposed to air, the amount of oxygen which it dissolves will be the same
as if the water had been exposed to oxygen at a pressure equal to that
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THE CONTROL OF THE RESPIRATION + 337
of the partial pressure which it produces in air. The same will be
the case with the nitrogen of the air. The actual amount of gas which
becomes dissolved in the fluid, pressure and temperature being constant,
depends partly on the nature of the gas and partly on the nature of the
fluid. For example, the solubility of oxygen in water is considerably
different from that in a neutral oil; or, taking the same solvent, nitro-
gen and CO, do not dissolve to the same extent in water. It becomes
necessary, therefore, in calculating what amount of a particular gas
will dissolve in a particular fluid to use a figure known as the coefficient
of solubility of the gas—that is, the amount of gas taken up by a unit
volume of fluid at standard temperature and pressure; for example, to
say that the coefficient of absorption of nitrogen in water at 0° C. is
0:0239 means that, at this temperature and at normal barometric pres-
sure, 1 ¢.c. of water will dissolve 0.0239 ¢.c. of nitrogen when exposed to
a pure atmosphere of this gas. Obviously, then, if water were exposed
to 79 per cent of an atmosphere of nitrogen (as in air) the amount which
would become dissolved in each e.c. would be ae 0.0239 — 0.0189 c.c.
In solutions containing no chemical substances with which the gas can
enter into combination, it is evident that the tension of the gas will be
proportional to the amount of gas that can be displaced or pumped out
from the fluid. On the other hand, when a chemical compound is formed,
the combined gas will exercise no direct influence on the tension, so that
this will be independent of the amount; in such cases separate methods
will have to be used for the determination of amount and tension. Let
us take the case of pure water exposed to an atmosphere of CO,: the
amount of CO, which goes into solution will depend entirely on the
pressure. If a trace of alkali is dissolved in the water, however, some
of the CO, will become combined to form carbonate, so that a much
larger quantity of CO, will be displaceable from the solution (as by
adding a mineral acid to it) than corresponds to the tension of CO, in
the atmosphere surrounding it. Since blood contains alkali the eondi-
tions are analogous with those of a weak alkaline solution.
The Tension of CO, and O, in the Arterial Blood.—If we were to
pass blood at body temperature in a very thin film over the walls of a
confined space containing a mixture of gases one of which was CO,, it
is evident that the percentage of CO, in the atmosphere contained in
this space would remain unchanged only when the tension of this gas in
the blood was the same as that in the confined atmosphere. If, on the
other hand, the tension of CO, in the blood should correspond to a per-
centage that is higher than that in the atmosphere, then CO, would dif-
fuse from the blood, and at the end of the experiment an analysis of the
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338 THE RESPIRATION
atmosphere in the space would show that the CO, percentage had been
raised. If the blood contained a lower tension than that corresponding
to the percentage of CO, in the space, some of the CO, would diffuse
into the blood, and its percentage in the atmosphere would be lowered.
By successively exposing blood to gas mixtures that contain slightly
different percentages of CO,, we should ultimately find one with which
the free CO, in the blood was in perfect equilibrium, and we should be
able to state that the tension of this gas in the blood was equal to a
certain percentage in the atmosphere surrounding the blood (see Fig.
121). '
Many forms of apparatus based on the above principle have been in-
vented for the examination of the tension of the gases in the blood.
The most accurate is that devised by Krogh,’* the principle of which
6% ct stort
co, 52% atend
5.3 at start
CO, 5.1 atend
5:15 at start
S15 at and
if
fig. 121.—Diagram to show principle for measurement of the tension of COz in blood. The
COz tension of blood is supposed to be 5.75,
differs slightly from that just described in that a bubble of air is
exposed to a relatively large quantity of blood, so that after a time
actual equilibrium of gas tension becomes established between the bub-
ble and the gases of the blood. This apparatus is shown in Figs. 122
and 123. It consists of a graduated tube of narrow bore sur-
rounded by a water jacket. To the upper end of the graduated tube
a small syringe is attached. The lower end of the graduated tube ex-
pands into a thistle-shaped bulb, closed below by a cork, through which
is inserted a tube (inflow tube) ending near the top of the bulb in a
fine opening and connected outside with an artery. An outflow tube is
also connected with the thistle-shaped bulb.
At the beginning of the experiment the thistle-shaped bulb and the
graduated tube are filled with physiologic saline. -By means of the
syringe a small bubble of air is then introduced, so that it lies at the
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THE CONTROL OF THE RESPIRATION 339
junction of the thistle-shaped bulb and the graduated tube. As the blood
is allowed to enter through the inflow tube, it is ejected in a fine stream
around the bubble of air, which moves about in the stream. The blood
displaces the saline out of the bulb into the side tube. After the bub-
ble has been subjected to the influence of the blood for some minutes,
the gases in it come into perfect equilibrium with those in the blood.
The percentage of O, and CO, in the bubble will therefore correspond
to the tension of these gases in the blood. The analysis is effected by
drawing the bubble into the graduated tube by means of the syringe,
yj
A
,
a.
Fig. 122. Fig. 123.
Fig. 122.—The gas analysis pipette for the microtonometer shown in Fig. 123. For description
see context. (From A. Krogh.)
Fig. 123. .—Microtonometer, to be inserted into a blood vessel. The small circle represents the
fiwbale of air. For further description see context.. (From A. Krogh.)
measuring its capacity, transferring it into a bulb containing KOH,
which absorbs the CO,, then taking it back into the capillary tube and
again. measuring. The shrinkage obviously corresponds to the amount
of CO,. The bubble is then transferred into potassium pyrogallate solu-
tion, where the O, is absorbed.*
The Tension of CO, and O, in Alveolar Air.—Having seen how we
may determine the tension of the gases in blood, we must now consider
*Since the above was written, a more efficient tonometer deviscd by the late T. ee Brodie has
been described by O’Sullivan (Am. Jour. Physiol., Sept., 1918).
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340 THE RESPIRATION
the method by which the tensions of these gases in alveolar air can be
determined. The simplest and until recently the most accurate method
is that of Haldane and Priestley.*° This consists in having an individual,
with his nostrils clamped, breathe quietly through a piece of hose pipe
about a meter long, which has at the mouth end a short side-tube lead-
ing to an evacuated gas-sampling bulb of about 50 ¢.c. capacity.* After
the subject has become accustomed to breathing through the tube, he
is asked to make a forced expiration and at the end of it to close the
mouthpiece with his tongue. At this moment the operator opens the
tap of the sampling tube, allowing the air from the tubing through
which the individual has made the forced expiration to rush in and fill
it. This sample represents the air from the alveoli (see page 302), and
is analyzed for percentages of CO, and O,. Since each normal inspira-
tion dilutes the alveolar air somewhat, it is necessary, for constant re-
Fig. 124.Apparatus for collection of a sample of alveolar air by WHaldane’s method. It is
better to use a mouthpiece than a mask.
sults, to make two analyses of alveolar air from each subject, one taken
at the end of a normal inspiration and the other at the end of normal
expiration. The average of the two results is taken as the composition
of the alveolar air.
On account of the difficulty in sécuring intelligent cooperation in the
application of this method, particularly with children, others have been
devised. One of the simplest is that of Fridericia, which is a modifica
tion of the Haldane-Priestley method, the apparatus for which is shown
in the figure (Fig. 125), and the manipulation of which is outlined in
the legend. Another is to take a mixed sample of the very last portion
of several normal expirations. On account of the extended use which is
being made of measurements of alveolar air composition, both in lab-
*In place of the gas-sampling tube it is much more convenient and ‘equally accurate to employ one
of the modern ground glass piston syringes (Luer). The piston should, of course, be well smeared
with a good mineral grease.
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THE CONTROL OF THE RESPIRATION 341
Fig. 125.—Fridericia’s apparatus for measuring the COz in alveolar air. The person expires
forcibly through the tube with the stopcocks as in I. A is closed and the tube placed in water to
cool the air, after which B is turned as in II. The entrapped column of_air equals 100 cc. A
solution of caustic alkali is now sucked into C with stopcocks as in II. B is then turned as in
I but with 4 still closed, and the alkali solution allowed to enter b, after which B is turned off,
the excess of alkali solution in C allowed to run out and the burette shaken. The burette is
then submersed up to a in a cylinder of water, with B as in III. After allowing for cooling,
the level at which the water stands gives the per cent of COs.
1h 2095 147 % %
in inspired
air
{30 40 50 g 10 20 30 40 50 o 10
Fig. 126.—Curves to show the relationship between the Og and COz tensions in alveolar air
(dotted lines) and arterial blood (continuous lines). It will be observed that the tension of COzg
in blood is slightly above that in alveolar air, but that the reverse relationship obtains for Oz. In
the upper part of the curve the Oe tension in the alveolar air was experimentally altered, causing
a corresponding alteration in the Oz tension of the blood. This result is of practical significance
in connection with Og alterations in gas poisoning, pneumonia, ete. (Irom A. and M. Krogh.)
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342 THE RESPIRATION
oratory and in clinical work, a special chapter has been devoted to the
subject, giving in detail the more recent methods devised by R. G. Pearce.
Lastly, it should be noted that several observers believe that a more
reliable estimate of the alveolar tension of CO, (and of O,) can be made
by analyzing a sample of ordinary expired air and calculating the per-
centages of CO, and O, in the alveolar air by allowing a constant dead-
space capacity of 140 c.c. (Krogh, etc.).
If we compare the CO, tension of arterial blood, as measured by the
Krogh method, with that of alveolar air, we shall find that there is a
remarkable correspondence, indicating, therefore, that, when the arterial
20
Qh Por rr aree nen “ae Bgesee > Ri eee 5
18r
wt
6t ee
15, 02
at
et
on
5 J
eK
37 Tae |
2/ “esa
29 005 29 995 - __ 44% C02 ia mspired air
220 30 te SO 3 10 20 30 40 50 4 20 20 30 40
Fig. 127.—Same as Fig. 126, except that in this case the tension of COz in the alveolar air was
experimentally altered. (From A. and M. Krogh.)
blood leaves the alveoli, its partial pressure or tension of CO, is exactly
equal to that in the alveolar air. This is shown in the accompanying
curves of experiments performed by Krogh. The dotted line in these
curves represents the tension of CO, or O, in alveolar air, and the con-
tinuous line, these tensions in arterial blood. Close correspondence
will be observed between the CO, curves even when sudden changes in
alveolar CO, were induced by artificial means. In the case of the O,
tensions, however, that of the blood is always lower than that of the
alveolar air, the differences being especially marked when the O, ten-
sion in the alveoli is raised (Figs. 126 and 127).
Tension of CO, in Venous Blood.—If we examine the CO, tension of
the venous blood coming to the lungs, we shall find that it is distinctly
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THE CONTROL OF THE RESPIRATION 343
higher than that in the alveolar air. The earliest method for measuring
it consisted in passing a lung catheter into the right bronchus and then
blocking the passage above the open end of the catheter by inflating a
rubber collar or ampulla. The renewal of air in the right lung is thereby
prevented, and a sample of the stagnant air can be removed and analyzed.
In such a case, however, the blood will have circulated several times
round the body, and with only one lung operating the risk is incurred
that more CO, is being discharged into the blocked lung than cor-
responds to the tension of CO, of venous blood under normal conditions.
Much more practical methods are those of Haldane, Yandell Hender-
son and R. G. Pearce, which are much the same in principle. In Pearce’s
method, the person first of all inspires from a gas meter containing a
gaseous mixture with about 10 per cent of CO,. Immediately after fill-
ing the lungs, he makes a rapid forced expiration into a tube provided
with a valve having four openings. This valve is turned through a
complete circuit during the expiration, so that four fractions of the ex-
pired air can be collected in rubber bags connected with side tubes
opening opposite the four openings in the valve. The first fraction will
contain a little less than 10 per cent CO,, the second distinctly less,
while the fourth will contain the same as the third, indicating that equi-
librium between the CO, of the alveolar air and the blood must have been
attained. This figure therefore gives us the tension of CO, in the venous
blood of the lungs. In Henderson’s method the rebreathing is per-
formed into gas receivers containing 6 per cent CQ,.
These results then indicate that the whole process by which CO, is
exchanged in the lungs is dependent on the law of gas diffusion; the gas
diffuses from a place of lower to a place of higher pressure, and does
so until equilibrium is attained.
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CHAPTER XXXIX
THE CONTROL OF RESPIRATION (Cont'd)
THE ESTIMATION OF ALVEOLAR GASES
By R. G. Pearce, B.A., M.D.
Methods such as that of Haldane and Priestley, which calculate the
mean percentage composition of the alveolar air by analysis of a sample
taken from the end of a prolonged forced expiration, give values which
are too high for CO, and too low for O,. There are several reasons
for this: (1) In the time taken for the prolonged deep expiration an
appreciable amount of CO, will be given off by the blood to the alveolar
air, and oxygen will be absorbed—that is, the sample will not contain
the same percentages of CO, and O, at different stages of expiration.
(2) The portion of the tidal air which reaches the alveoli dilutes the
alveolar air and thus causes the amount of CO, given off by the blood to
vary during the different phases of respiration. If we bear in mind that
the tensions of CO, in the alveolar air and in the blood leaving the lungs
are always the same (page 343), and that the entire fall in CO, tension
in the alveolar air occurs during inspiration, then it is clear that the
blood in the pulmonary capillaries must have a maximum tension and
load of CO, at the end of expiration, and a minimum tension and load
of CO, at the end of inspiration. Accordingly, the average of the per-
centage of CO, and O, at the end of inspiration and expiration, as de-
termined by the Haldane-Priestley method or by any of its modifications,
must fail to give the correct mean tension of these gases in the alveolar
air during expiration. The error which makes the CO, higher than it
should be, makes the percentage of O, less than it should be. These in-
fluences taken along with the fact, which will be shown later, that the
evolution of CO, from the blood is relatively more rapid at low than at
high tension of CO,, indicates that the blood in the pulmonary eapil-
laries during inspiration must contribute a greater part of the CO,
excreted during a respiratory cycle than that in the pulmonary capil-
laries during expiration, and moreover that a greater part of the CO,
excreted must be evolved at a tension which is below the mean tension
of the CO, present in the entire time of ‘the expiration. We conclude,
therefore, that the average tension of CO, in the alveolar air, determined
ee 344
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THE CONTROL OF THE RESPIRATION 345
by the actual tension under which the gas is evolved from the blood, is
less than the average tension of CO, in the alveolar air during the time
of a respiratory cycle.
In the case of O, the conditions are different. While the diluting
effect of the alveolar tidal air is marked in altering the amount of CO,
given off during the different phases of a respiration, it can have little
influence on the amount of O, taken up by the blood under normal con-
ditions. This is evident from a study of the dissociation curve of hemo-
globin (page 383), which shows that at tensions above 65 mm. Hg the
hemoglobin is practically saturated with O,. Since the tension of O,
in the alveolar air under normal conditions is greater than 65 mm.
(95-100 mm.), the rate of absorption of O, must be practically maximal
during the respiratory cycle—that is, it will not change at different
phases of it.
While the relationship of the alveolar gases is continually changing
at different stages of the respiratory cycle, their mean relationship for
periods including several respirations or for complete respirations is
more or less constant, being controlled by the type of the metabolism,
and mathematically expressed by the respiratory quotient (page 547).
The average relative percentages of the two gases in the alveolar air
must therefore be the same as in the tidal air. In the alveolar air col-
lected by the Haldane method, however, the above factors cause the
respiratory quotient to be less than that in the tidal air.
These points have been insisted upon because much of the knowledge
of the gaseous exchange between the blood and the air in the lungs, as
well as the control of respiration, has been built upon data obtained by
the Haldane-Priestly method, and in considering this work, which we
shall do in subsequent pages, it is advisable that we be aware of the
limitations of the method emploved. The method has been an invaluable
one for opening up a hitherto entirely unexplored field of research, but
now, tlie pioneer work having been done, we must employ methods
which will enable us to explore more exactly.
An Accurate Standard Method for Normal Subjects.—The most accu-
rate method, and one free from many of the theoretic errors present in
the others, depends on the relationship found to exist between the dilut-
ing effect of the air in the dead space (see page 302) and the known per-
centage composition of the alveolar air in expirations which are of vary-
ing depths but of equal and normal duration and which follow normal
inspirations (R. G. Pearce).
In this method the subject is made to breathe through valves, which automatically
separate the inspired from the expired air. The expired air is led into a tube con-
nected with two spirometers by two three-way stopcocks. The spitometers are of the
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346 THE RESPIRATION
Gad-Krogh type, one being capable of holding ten liters, and the other one and a half.
The exact time during which air enters is recorded by the small spirometer by means of
a grooved dial on the axis of the lid, on which a thread works over a svstem of pulleys,
and any movement is accurately recorded by a writing point on the smoked paper of a
drum. The spirometers are connected so that the air current may be directed in the
three following ways: (1) through Cocks 1 ard 2 outside; (2) directly through both
cocks into the large spirometer for the purpose of collecting a series of expirations;
and (3) through Cock 1 directly into the small spirometer for catching a single expira-
tion. In all experinvents the first filling of the spirometer is rejected, so that the dead
space of the spirometers is filled with air of approximately the same composition as in
the succeeding expirations. The time is marked in seconds by a time clock. The respira-
tory movements are recorded by a pneumograph. (Fig. 128.)
The subject is brought into respiratory equilibrium by having him breathe through
the valves for a period of time before the observation. The respiratory movements
during this time are recorded while the cocks are in Position 1. When the observation
is started, the cocks are turned into Position 2 during the time an inspiration is being
t
Amall Ppiromeler
:
y
ition, pttton.
Fig. 128.—Arrangement of meters and connections of Pearce’s method for measurement of CO2
of alveolar air in normal subjects.
made, so that the expirations which follow may be collected in the large spirometer.
After about ten respirations (a counted number) have been collected, the cocks are
turned tv Position 3 during an inspiration, aud a single deep expiration is collected
in the small spirometer. In order that the time of this may be the same as the normal
expiration, it is necessary to quicken it. This is more or less a chance procedure, but
with a little training, the operator can close the stopcock with sufficient accuracy to
interrupt the deep expiration at the end of the normal expiratory time. Should
there be any gross variation from the normal expiratory time, the sample must be col-
lected again. Not infrequently the inspiration immediately preceding the expiration
into the small spirometer is varied involuntarily by the subject on account of his being
aware that the following expiration has to be deepened and quickened; this can be
partially overcome by giving him the signal to breathe out deeply after he has actually
begun to expire.
Determinations are made of the average volume of the tidal air (c.c. air in large
spirometer divided by number of breaths), of the volume collected from the deep ex-
piration, and of the percentage composition of the tidal air and that of the deep
expiration. A criterion for determining whether or not the procedure has been carried
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THE CONTROL OF THE RESPIRATION 347
out correctly is the respiratory quotient (ratio of CO, excreted to O, absorbed). For
reasons which are set forth above, the quotients should be approximately equal in the
air collected in the large and in the small spirometers; if they are not so, the condi-
tions of the method have not been correctly carried out.
Since the déad space and the average composition of the alveolar air under these
conditions may be considered constant, the percentage composition of the deep expira-
tion will differ from that of the mixed sample of several normal expirations in propor-
tion as the dead space exerts a greater diluting effect in the small than in the large
expiration. This being the case, the data obtained can be combined algebraically to
give either the capacity of the air passages or the percentage composition of the
alveolar air.
Let A = amount of air in large expiration (small spirometer),
Ai = amount of air in small or normal expiration (tidal air),
B = the percentage of CO, or O, in the expired air of large expiration,
Bi — the percentage of CO, or O, in the expired air of small expiration,
xX = the capacity of the dead space,
y = the average percentage of CO, or O, in the alveolar air; then,
AxB= (A-x)y and Aix Bi = (Ai-x)y.
Solving this for x, y remaining constant under the same physiologic conditions, we
have: x == Axdie (BED the dead space. Or solving for y, we have:
AXB-Aix Bi .
_ AxB-AixBi
de
dead space for O, is desired, B and Bi must be made to equal the O, absorbed.
, the mean percentage of CO, in the alveolar air. In case the
Clinical Method.—The use of the kymograph and pneumograph, and
other complicating factors, make the method as just described quite im-
practicable for clinical procedure, but the use of the same apparatus
with the following modification will yield satisfactory results for most
clinical purposes. The patient is made to respire through the valves for
a short time, after which the observer collects a single ‘expiration in a
small spirometer by turning the stopeock from Position 1 to 2. A sam-
ple of this is taken for analysis, and the spirometer is again emptied
and a series of successive samples of deeper expirations taken. This is
done by directing the patient, after he has started to breathe normally
into the spirometer, to breathe more deeply. The amount of air col-
lected in each expiration is controlled by the observer by closing the
stopcock when the desired volume is obtained. By this means one can
collect several expirations differing from one another by increasing
amounts but all occupying the same time. The saniples of the various
expirations are collected in a series of numbered sampling syringes, and
the gaseous composition of each is determined. When the percentage
of CO, or O, in each expiration is plotted on cross section paper on the
ordinates, with the volume of the expirations in ¢.c. on the abscisse, a
hyperbolic curve should be obtained. Any marked deviation from such a
curve indicates that some error has been made in taking a sample, and
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348 THE RESPIRATION
this observation should be discarded. The different observations are
then combined in the formula given on page 347. The determination
of the CO, percentage of expired air is so simple that a number of speci-
mens of varying depths of expiration can be taken and thus many points
on the curve determined. For the most accurate results it is in general
best to compare only those expirations which differ from one another
by at least 0.38 per cent in CO, and by at least 200 ¢.c. in volume. This
depends on the fact that the diluting effect of the dead space in reduc-
ing the percentage of CO, in the expired air from that in the alveolar
air is greater-in relatively small expirations. If more exact work is de-
sired, the O, content can be determined on each specimen, the respiratory
quotient calculated, and only those expirations which show the same
respiratory quotient combined.
In the table each observation is compared with each of the others in
all possible combinations.
PrR CENT . ALVEOLAR CO, DZAD SPACE
NO. OF TOTAL co, IN
OBSERVA- EXPIRED EXPIRED
TION AIR AIR t 2 ‘ .
1 450 3.10 :
2 637 3.66 4.99 170
3 750 4.00 5.34 189
4 960 4.28 5.30 5.48 5.27 189 183 214
5 1120 4.30 5.11 5.15 4,92 161 140 | 184
6 1440 4.40 5.16 4.98 4,82 171 127 171
General average for CO in alveolar air, 5.13.
_ General average for dead space, 172, Dead space in valves in this experiment was
about 30 ¢.e.
Another method which has been suggested for clinical purposes is
that of Plesch; this consists in having the subject breathe several times
in and out of a small bag. It is assumed that after such respiration
the composition of the air in the bag will become the same as that in the
alveoli. Although this is no doubt true, it has been shown that the
method is-fallacious, because the CO, tension determined in this way
is not that of the arterial blood alone, but is the average between it and
that of the venous blood.
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CHAPTER XL
THE CONTROL OF RESPIRATION (Cont’d)
THE NATURE OF THE RESPIRATORY HORMONE
The practical importance of the observations described in the foregoing
chapters in the investigation of the relationship between Cy of the
blood and respiratory activity will now be plain, and it remains for us
to consider the physiologic evidence that such a relationship exists. In
the first place, let us consider the behavior of the acid-base equilibrium
during conditions of abnormal breathing—hyperpnea and dyspnea.*
As CO, accumulates and O, becomes used up in a confined space, the
breathing becomes intensified. In searching for the exact cause of this
effect, we must first of all ascertain whether the hyperpnea is due to the
deficiency of O, or to the accumulation of CO,. Many of the experi-
ments bearing on these problems can be more satisfactorily performed on
man than on laboratory animals, because anesthesia is not necessary and
the subjective symptoms experienced are of great value in the inter-
pretation of the results. If an individual is placed in a large air-tight
chamber (2000 liters’ capacity), and the depth and rate of breathing ob-
served as the CO, accumulates and the O, becomes used up in the air of
the chamber, no distinct change in respiration will be observed until the
CO, percentage of the air has risen to almost 3. Above this point, how-
ever, the hyperpnea becomes more and more pronounced, until finally,
when the CO, percentage has risen to about 6 and the O, percentage has
fallen to 13.5, it becomes unbearable (dyspnea). From the results of the
foregoing observation alone we could not, however, decide whether the
excitation of the respiratory center is’due to the deficiency of O, or to
the increase of CO,. If the experiment is repeated with the difference
that the CO, as it accumulates is absorbed by soda lime, no hyperpnea
will develop even when the O, is as low as in the previous experiment.
We may conelnde, therefore, that in the first experiment CO, accumulation
must have acted as the respiratory stimulus.
‘The same conclusion is arrived at as a result of observations on indi-
viduals caused to breathe in a more confined space as into a rubber bag
of about 225 liters’ capacity. Under these conditions hyperpnea de-
*Hyperpnea means slightly increased breathing; dyspnea, labored breathing, but yet with suffi-
cient ventilation to maintain life; asphyxia, the results of insufficient breathing.
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350 THE RESPIRATION
velops more rapidly than in the large cabinet, and a higher percentage
(10 per cent) of CO, can be tolerated. That in this case also deficiency of
O, is not responsible for the hyperpnea can be shown by repetition of the
experiment either with an excess of O, in the bag or with absorption of the
CO, by soda lime. In the former case hyperpnea will develop as usual,
while in the latter it will not supervene until the percentage of O, has
fallen below 10, when cyanosis becomes marked. In fact, some people
become cyanosed and unconscious, and collapse under these conditions
before there is any respiratory disturbance. A peculiarity of the effect
of O, deficiency is that the person may be unaware of the seriousness
of his condition ; indeed he may be somewhat stimulated. The conclusion
may be drawn that deficiency of O, per se can serve as a respiratory
stimulus only when it is so extreme as to cause other serious symptoms.
This conclusion does not rule out an important influence of O, deficiency
in increasing the excitability of the center towards CO,. Under ordi-
nary conditions, however, the center is far more sensitive towards slight
changes in the CO, percentage.
There is an obvious reason why the adjustment of pulmonic ventila-
tion should not depend upon changes in O, supply to the respiratory cen-
ter. If it were so, many other tissue activities and other nerve centers
would suffer from the O, deficiency before there was time for the breath-
ing to become stimulated sufficiently to make good the loss of O,. As a
matter of fact, headache, dizziness, nausea and even fainting are almost
certain to be caused whenever any muscular exercise is attempted in an
atmosphere containing a deficiency of 0, but no excess of CO, (ef. moun-
tain sickness). An adequate O, supply of the body is, therefore, insured
by changes in CO, tension of the blood.
Quantitative Relationship between CO, of Inspired Air and Pulmonary
Ventilation. These results suggest, as the next step in the investigation
of our problem, the determination of the quantitative relationship be-
tween the CO, percentage of the respired air and the amount of air
breathed (pulmonic ventilation).* ‘ That there is such a relationship has
been most successfully demonstrated by R. W. Scott, who used for his
purpose decerebrate cats.t The trachea was connected, through a T-tube
provided with valves, with tubing leading to a large bottle and a Gad-Krogh
spirometer, so that the animal breathed out of the bottle into the
spirometer, these two being also connected together. The spirom-
*A distinction is somewhere drawn between pulmonic ventilation and alveolar ventilation, the
former being the total amount of air that enters and leaves the lungs, and the latter, that which en-
ters and leaves the alveoli. This distinction is based on the assumption that the capacity of the dead
space may vary considerably from time to time, which, as pointed out elsewhere, is erroneous. For
practical purposes pulmonic ventilation is the safer value to give.
+Decerebrate animals must be used in these experiments, since anesthetics markedly depress the
activity of the respiratory center.
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THE CONTROL OF THE RESPIRATION 351
eter was made to record its movements on a drum, so that an accurate
record of the depth and frequency of the respirations was secured. Sam-
ples of air were removed from the bottle by ground-glass plunger syringes
at frequent intervals during the time that the animal was respiring into
the tubing.
Fig. 129.—Composite curve obtained from the “data _on sixteen experiments, showing the re-
spiratory response to COz in the decerebrate cat. Abscissae = percentage of COz in the inspired
air. Ordinates = the percentile increase the tidal air per minute. (From R. W. Scott.)
The results are given in the accompanying curve (Fig. 129), which shows
that there is a perfect correspondence between the CO, percentage in the
air of the bottle and the pulmonary ventilation. Moreover, when the
bottle was filled with O, instead of air to start with, the same results
were obtained, showing that the CO, accumulation alone was responsible
for the hyperpnea. In these cases the percentage of O, remaining in the
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352 THE RESPIRATION
system after hyperpnea had hecome.extreme, was far above that at which
direct excitation of the center from O, deficiency is possible.
Experiments of a similar type had previously been performed by Por-
ter and his pupils,?* but their object was not so much to show the close
parallelism between the CO, content of the respired air and the pulmonic
ventilation as to demonstrate the changes produced in the sensitivity of
the respiratory center in pneumonia.
Possibility that CO, Specifically Stimulates Center—After showing
that CO, acts as an excitant of the respiratory center, the question arises
whether we are justified in the assumption that has been made tentatively
that the action depends on the raising of the Cy of the blood, or whether
it may be a specific action of the HCO, anion itself. Many attempts have
been made to decide this question experimentally, the general principle
of the experiments being to determine whether Cy of the blood runs
parallel with the CO, content of the respired air and with the hyperpnea.
Using the gas-chain method (page 31), Hasselbalch and Lundsgaard??
found that the hyperpnea produced in rabbits by breathing in CO,-rich
air runs approximately parallel with the increase in the Cy of the blood,
but on account of the experimental difficulties encountered they could not
decide whether changes in Cy are alone responsible for the effect. These
authors had previously demonstrated that changes in Cy can be induced
in blood removed from the body by alterations in the CO, tension within
the physiologic limits. An increase of one millimeter in CO, tension
was found to cause an increase in Cy of 0.0065 x 107 (see page 27).
R. W. Scott’s experiments, above referred to, have, however, yielded
more definite results. By using the colorimetric method for determining
Cu of the blood (see page 32), it could be readily shown, as is evident
from the table (col. 8 in table), that a marked rise in Cy became evident
when the inspired air contained 5 per cent or more of CO,. That this
rise was due to increase in the CO, tension was shown not only by finding
a greater percentage of CO, (col. 15) in the blood, but also by being able
_to demonstrate that when CO,-free air was bubbled through the blood
removed during the dyspnea, Cy immediately returned to the normal,
which it also did when the blood rémoved after the animal had breathed
for a few minutes in outside air (col. 16). The CO, content likewise re-
turned (col. 17). Had the increase in acidity been caused by nonvolatile
acids—lactic, for example —these results, particularly the latter, could
not have been obtained.
Although there is therefore no doubt that the Cu of the blood ma~
be raised because of an increase in CO, in solution in the blood plasma—
a CO, acidosis, as we may call it (see page 354)—this does not prove that
the stimulation of the respiratory center is brought about solely by Cu:
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THE CONTROL OF THE RESPIRATION 853
The increase in the carbonate ion—HCO, ion—itself might also serve as
a stimulus. That such is actually the case was demonstrated by
finding that, if the Cy of the blood was first of all lowered by injecting
alkali intravenously, hyperpnea still developed in proportion as the CO,
accumulated in the inspired air; and that Cy of the blood, when the
hyperpnea was at its highest, was below that of normal blood. Some
other factor than Cy must obviously be responsible for this result. This
must be the HCO, anion.
THE EFFECT OF REBREATHING CARBON DIOXIDE ON THE MINUTE VOLUME AND ON THE
H-IoN CONCENTRATION AND TOTAL CARBONATE CONTENT OF THE ARTERIAL
Boop IN THE DECEREBRATE CaT
FIFTEEN
MINUTES
PRELIMINARY PERIOD* REBREATHING PERIOD AFTER RE-
BREATHING
PERIOD
8 2) Se le) 2 (2 S55 35
2 s |. |] 8 [8s ly | @ EE] o | B lee | B lee
B & o 12) ./ 5 (88 (8 | . fos] & | S/es | 2 #8
Bly 7 Time fle alg |e le Be 8 |S ee | jes
z\ 8) 28 |'s | & loaciS | = lee] 2 | 2 loaul 2 loge
Zi) E] - fis | @] 2 jOsss |e ES] 2 | 2 JOE 8 loge
B\e| ® eelee|/a| * |geslas| = [8s] = | * sts] * lads
b\w | B/235/ co] we |gsales] —o Es zs |Sea) 2 [fas
61612 ee }elele le bs ie le bee | ae
kg. ce. cc. cc. ee a c.c. ce
27| 36} 2.2 |10.30 a.m. /38.5/48 | 864 | 7.45 | 30.3 | 64 |3,968/540| 6.45}7.25| 53.4 | 7.4 | 34.2
28/33] 2.2| 1.50 p.m. [38.8)22 | 616| 7.40 | 40.2 | 40 |2,880/367) 5.02/7.20) 58.0 | 7.35 | 42.0
29135) 2.0 10.30 p.m. |38.6/28 | 784 | 7.40] 38.1 | 40 |3,840\389] 5.35| 7.25] 51.1 | 7.4 | 40.0
30|32{| 2.3 11.00 p.m. |38.7/32 | 768 | 7.45 | 30.2 | 54 |4,536/489| 6.40|7.20| 51.1 | 7.4 | 35.0
31] 31] 2.0 11.15 a.m. |38.8/32 | 896 | 7.40 | 32.0 | 60 | 4,800)436| 5.95]7.20) 50.0 | 7.4 | 35.2
1 t
Average ------------- seal 785 |7.42| 34.1 at
*Pu is the actual value given in the table. This is inversely proportional to Cu.
Further corroboration of the claim that the HCO, anion has a specific
stimulating effect on the respiratory center that is independent of Cy,
has been furnished by Hooker, Wilson and Connett.22 These authors
succeeded in keeping the centers of the medulla alive by perfusion with
defibrinated blood through the blood vessels of the brain, and found
that, although the respiratory movements of the diaphragm became de-
pressed with a decrease and excited with an increase in Cy of the per-
fused fluid, a greater activity of the center was produced when this con-
tained a high tension of CO, than with another fluid of the same Cy
but with a low tension of CO,. We conclude that, although the Cy is the
important respiratory hormone, the carbonate ion (HCO, anion) also has
a stimulating influence.
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354 THE RESPIRATION
Relationship Among Acidosis Conditions, Alveolar CO, and Respir-
atory Activity.—It will be plain that variations in the respiratory hor-
mone, whatever this may be, are associated with changes in the CO,
content of the alveolar air. Closer examination has shown, however,
that this relationship is by no means always so simple as in the instances
just described, where increased respiration was found to be associated
with an increase in alveolar CO,. There are many cases where the re-
verse relationship obtains—namely, decreased alveolar CO, and hy-
perpnea. The whole question is very closely linked with that of the con-
trol of the reaction of the body fluids and with the etiologic factors in
acidosis. When it is fully answered, many obscure clinical conditions in
which respiratory disturbances occur will be much better understood than
they are at present. On account of its great importance, considerable
attention will be devoted in the next few pages to some of the researches
which have been made bearing on the relationship between the CO, of
the alveolar air and the various modified types of breathing that can be
produced experimentally or become developed under altered physiologic
conditions.
We shall consider these conditions in the following order: (1) Con-
stancy of the alveolar CO, under normal conditions and during moderate
variations in barometric pressure. (2) The quantitative relationship
between an artificially induced increase in alveolar CO, tension (as by
breathing CO,-rich air) and the increased respiration. (3) The results
of these observations will demonstrate a very precise relationship to exist
between alveolar CO, tension and respiration, but if we proceed to repeat
the latter observations under conditions where the accumulation of CO,
in the inspired air is accompanied by oxygen deficiency (as by breathing
in a confined space), we shall see that the relationship no longer holds,
indicating that the oxygen deficiency has caused something to happen
which disturbs it.
We shall find that the disturbing factor is accumulation of unoxidized
acids in the blood, and this will naturally lead us to study the conditions
in which such acids might develop; namely, (4) Breathing in rarefied
air (mountain sickness). (5) Apnea. (6) Muscular exercise.
In succeeding chapters, we intend to review the work in these fields in
considerable detail, partly because of its very important bearing on the
general question of the control of the respiratory center and partly be-
cause of the light the observations throw on the nature of the mechanism
involved in the adjustment of the Cy of the blood and tissues.
As we have seen, much work concerning the physicochemical principles
involved in the control of the reaction of the blood has been contributed
during recent years by physical and biological chemists, but much of this
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THE CONTROL OF THE RESPIRATION 355
work in our judgment fails to pay sufficient regard to the extraordinarily
complicated conditions existing in the animal body, and more particu-
larly, to correlate the purely physicochemical data with the numerous
observations that have from time to time been recorded by physiologists
regarding the behavior.of the respiratory center. Physical chemists have:
recently, for example, gone so far as to define acidosis as a condition in
which there is a diminution in the bicarbonate content of the blood in-
duced by the discharge into it of fixed acids. This is going too far, for
it fails to recognize acidosis due to an increase in the CO, of the blood.
[waarce: | which determines the tension of CO,.
When CO, is added to the blood, either experimentally by respiring the
gas, or naturally owing to muscular exercise or to pathologic conditions in
which there is a deficient excretion of CO,, as in heart disease, the ten-
dency of the equation to change, by increase of the numerator, is pre-
vented partly by stimulation of the respiratory center, which gets rid of
CO,, and partly by an increase in the denominator. The respiratory
center is so sensitive to slight increases in Cy that it becomes excited
before a sufficient increase in H,CO, has occurred to disturb the normal
: H,CO, ;
ratio [Narco | . When fixed acids are added to the blood the denom-
inator of the equation, NaHCO,, is lowered and consequently Cy rises,
and increased respiration occurs, lowering H,CO, and thus reestablishing
the ratio.
It is the molecular ratio
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CHAPTER XLI
THE CONTROL OF RESPIRATION (Cont’d)
THE CONSTANCY OF THE ALVEOLAR CO, TENSION
UNDER NORMAL CONDITIONS
Since a close relationship exists between the alveolar CO, tension and
the respiratory activity, it is to be expected that the two would bear a
strict proportionality to each other, and since the breathing under normal
conditions does not vary much, the CO, tension should also be constant.
Many observations show this to be the case. The tension is remarkably
constant from day to day and even from month to month in the same
individual, provided the physiologic conditions are the same. A slight
seasonal variation is said to exist, a rise in the temperature of the en-
vironment of the individual causing a slight depression in the CO, ten-
sion, while a fall in temperature causes a slight rise (Haldane). These
changes are independent of any demonstrable change in rectal temper-
ature and, therefore, are probably due to the influence of the temperature
on the skin.
Since it is the number of molecules of CO, in a given volume of alve-
olar air (i.e., the partial pressure or tension) that is of importance, it
is only when the barometric pressure is the same that the percentage of
CO, in the sample of alveolar air can be constant. To allow for this,
all results are reduced to standard barometric pressure (760 mm. Hg).
If the barometric pressure is lowered, there will have to be a higher
percentage. of CO, in the sample in order that there may be the same
tension of this gas in the air of the alveoli; and vice versa when the bar-
ometric pressure is raised. The equation by which this tension, ex-
pressed in millimeters of mercury, is determined is: 100:760::a:p, where
a is the percentage actually found in the air of the sampling tube and p
the tension. A correction must also be introduced in this equation to
allow for the vapor tension of the air in the alveoli, for of course H,O
molecules will behave like CO, molecules in causing a partial pressure.
When reduced to this standard, it has been found that the tension of
CO, in the alveolar air remains constant under the different barometric
conditions that obtain at the top of a mountain or at the foot of a deep
mine. This is shown in the following table:
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THE CONTROL OF THE RESPIRATION 357
(1) (2) (3)
BAROMETRIC CO, ACTUALLY FOUND PARTIAL PRESSURE
PRESSURB IN. DRY ALVEOLAR OF CO, IN MOIST
(MM. HG) AIR ALVEOLAR AIR AFTER
(PER CENT) CALCULATING FOR
BAROMETRIC PRESSURE
Top of Ben Nevis 646.5 6.62 5.23*
Oxford 755 2 5.95 5.53
Foot of Dolcoath Mine 832 5.29 5.48
Compressed air cabinet 1260 3.52 5.64
5 7 ; B’- Ax P’ a
*The figures in this column are arrived at by the formula: — = P, when P = figures in
last column; B’ = figures in first column; A = aqueous tension of alveolar air; P’ = figures of
second column; B = barometric pressure at sea level. A is obtained from tables giving the aqueous
tension at different temperatures.
Changes in the frequency of breathing that are within physiologic
limits have no influence on the tension of alveolar CO,, provided that
exactly the same*time is taken in performing the forced expirations
during which the samples of alveolar air for analysis are removed.
The Degree of Sensitivity of the Respiratory Center to Changes in the
CO, Tension of the Alveolar Air
This can be determined by observing the alterations produced in the
volume of air that actually enters the alveoli (alveolar ventilation) dur-
ing breathing in atmospheres containing different percentages of CO,.
In man an increase of from 0.2 to 0.3 per cent in the alveolar CO, is
sufficient to double approximately the alveolar ventilation; or, more pre-
cisely, an increase of ten liters in the air entering and leaving the alve-
oli per minute is caused by raising the alveolar CO, tension by from 2.2
to 3.1 mm. Hg (Douglas, etc.)?*.
THE ALVEOLAR CO, TENSION DURING BREATHING IN A
CONFINED SPACE
We have already employed similar experiments in ascertaining whether
CO, accumulation or O, depletion is responsible for the hyperpnea pro-
duced under these conditions. We concluded for the former, but now
on closer examination we shall see that, although our conclusion was
correct, the deficiency in O, also has an indirect effect on the respiratory
center. This is revealed by the fact that the tension of the CO, in the
alveolar air does not increase in proportion to the observed increase in
pulmonary ventilation. We must conclude that the decrease in O, has
some effect. How may this be explained? Two possibilities exist: (1)
that the O, want has caused organic acids to accumulate in the blood
and so raise the Cy; and (2) that in the absence of a certain tension of
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358 THE RESPIRATION
O, the excitability of the center is raised (i.e¢., its ‘‘threshold’’ lowered),
so that it becomes stimulated by Cy, to which ordinarily it does not re-
spond. We shall now proceed to examine the experimental evidence
bearing on these possibilities.
By examination of the alveolar air of an individual confined in a pneu-
matic cabinet in which the barometric pressure is gradually lowered,
it has been found that although the CO, tension remains constant for
a considerable range (cf. page 356), it begins to fall when the barometric
pressure has reached about 550 mm. Hg. At this pressure the tension
of O, in the alveolar air will be 62 mm. instead of its normal of about
105 at atmospheric pressure. Below it the alveolar CO, tension quickly
falls, and at the same time hyperpnea becomes evident, although the
person himself may be unaware that he is breathing more deeply. If
this experiment is repeated with the difference that, as the pressure is
lowered, an excess of O, is introduced into the chamber, the hyperpnea
does not supervene until a barometrie pressure has been reached that is
distinctly lower than when no excess of O, is present, and at the same
time the CO, tension in the alveolar air remains unchanged. The ex-
planation of this result is that by lowering the O, tension in the alveolar
air and, therefore, in the blood and tissues, oxidative processes become
depressed so that unoxidized acids, such as lactic, accumulate in the
blood and by adding their effect to that of the CO, serve to raise the Cu
of the blood. As a result, the respiratory center becomes excited, hy-
perpnea supervenes, and the volatile CO, is removed from the blood into
the alveolar air. On supplying O, artificially, this failure of proper
oxidation does not set in and breathing goes on normally.
There should be a stage in the above experiment during which the
CO, tension of the alveolar air is increased—namely, when the fixed acids
first appear and decompose the carbonates of the blood. This stage has,
however, not been detected. When a person is kept in such a chamber
for some time at a pressure which causes a diminution in the alveolar
CO, tension, the tension does not immediately return to its normal level
when atmospheric air is again breathed, indicating that the fixed acids
are only slowly got rid of.
The second hypothesis—namely, that the O, deficiency directly raises
the excitability of the respiratory center—has many advocates, among
them Lindhard,”® who found that, when the percentage of O, in the alve-
olar air was raised, a higher percentage of CO, was necessary to cause
an increase in the ventilation of the lungs, and conversely, that a distinct
inerease in the excitability of the center occurred when the inspired air
contained less than the normal percentage of O,. Although it is ad-
mitted by Haldane and his school that such alteration in the excitability
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THE CONTROL OF THE RESPIRATION 359
of the respiratory center to the Cy of the blood may occur after long-
continued exposure of the center to the changed tension of O,, yet they
deny that such alteration can occur as a temporary condition. These
workers found that, in order to raise the pulmonic ventilation by 100
per cent, the increase in the alveolar CO, tension required was practically
the same (0.3 per cent) when the inspired air contained 20 per cent of
O, as when it contained 54 per cent. .
In the observations already referred to on the decerebrate cat, R. W.
Scott?° has secured some evidence that would seem to support Haldane’s
contention. He found that the response of the respiratory center to the
percentage of CO, in the respired air was exactly the same whether the
latter contained a low (13-14) or a high (80 and over) percentage of O,.
The possibility that the excitability of the respiratory center is affected
directly by the O, tension is to be considered as one of the most im-
portant problems awaiting solution.
Even if it may have a certain influence on the excitability of the re-
spiratory center, O, deficiency per se can serve as a direct stimulus of
the center only when it is of extreme degree. Much light has been
thrown on the relationship of O, to respiratory activity by observing
the respirations during breathing in and out of rubber bags through
soda lime absorption bottles of sufficient size to remove the CO,. We
have already seen that even the general results of such observations
(page 349) show. clearly how much more potent a respiratory stimulant
is accumulation of CO, than deficiency of O,. More particular investi-
gation in which the alveolar air is analyzed bears out these conclusions
and at the same time indicates the exact conditions under which organic
acids become developed.
With a very small bag (a few liters’ capacity) hyperpnea of a dis-
tressing type but without cyanosis supervenes in a few minutes, and the
alveolar air contains perhaps as low as 6 per cent O, and 4 per cent CO..
Of still greater interest and significance, however, is the fact that the
ratio between the volume of CO, excreted and of O, absorbed (respira-
tory quotient) during the hyperpnea is raised considerably above unity,
indicating that an excessive excretion of CO, must be occurring. This
result is explained by assuming that the deprivation of O, causes large
quantities of fixed acids to be produced, and that these expel CO, from
the blood more quickly than the O, is absorbed. In corroboration of
this explanation, it has been observed that, after outside air is breathed
for some time following the above experiment, the respiratory quotient
becomes very low, so that CO, must now be accumulating in the blood.
If the above experiment is repeated with a larger bag (about 200
liters), so that the O, falls slowly, the breathing can be maintained for
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360 THE RESPIRATION
a much longer period without any evident symptoms of hyperpnea, even
though the O, percentage in the alveolar air may fall as low as in the
previous experiment, and there are marked symptoms of O, want, such
as cyanosis, twitching of the muscles of the hands, lips, ete. The re-
spiratory quotient does not become abnormal in this experiment indicat-
ing that no expulsion of CO, from the blood can have occurred as in
the previous experiment. The -cause for the virtual absence of hyper-
pnea in this experiment is no doubt that the more gradual reduction in
O, of the alveolar air and therefore of the blood did not bring about the
accumulation of lactic acid at a rate that was greater than that at which
the CO, was got rid of into the alveolar air.
BREATHING IN RAREFIED AIR; MOUNTAIN SICKNESS
In considering the part played by fixed organic acid in the control
of the Cy of the blood, the most important results have been secured
by observations on the condition of individuals living at high altitudes.
As is well known, under these conditions certain symptoms are likely
to develop, the condition being known as mountain sickness. The great
interest which physiologists have taken in this subject has been owing,
not so much to the importance of the observations in connection with
the condition itself, as to the light which they throw on the mechanism
of respiratory control and on the cause for abnormal types of breathing.
More or less hyperpnea, especially on exertion, soon appears in a
rarefied atmosphere, and the alveolar CO, tension assumes a value con-
siderably below the normal. For example, at sea level the minute vol-
ume of air breathed in one individual was 10.4 liters, and the alveolar
CO, tension 39.6 mm. Hg. After being some time on Pike’s Peak, where
the barometer registers only 459 mm. Hg, Douglas® found the minute
volume of air to be 14.9 liters, and the alveolar CO, tension 27.1 mm. Hg.
At first sight the above statement may seem to contradict one pre-
viously made, to the effect that the alveolar CO, tension remains constant
at different barometric pressures. This applies, however, to the imme-
diate effects, whereas we are now considering the later effects. The im-
portant point is: How are we to reconcile with the above hypothesis the
fact that a diminution in the alveolar CO, tension should be accompanied
by hyperpnea? A solution of the seeming contradiction will not only
be of importance in connection with our present problem, but will assist
us in the investigation of the clinical conditions of hyperpnea, in which
likewise a diminished CO, alveolar tension is often observed. Mountain
sickness may indeed be considered as an intermediate condition between
the physiologic and the pathologic.
From what we have learned we should expect the above result to be
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THE CONTROL OF THE RESPIRATION 361
dependent upon an increase in the nonvolatile acid content of the blood
That such is really the case has been conclusively shown both by titra-
tion methods and by observing the dissociation curve of hemoglobin,
which, as will be explained later (see page 386), may be made to serve
SEA
LEVEL
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Fig. 130.--The horizontal interrupted lines represent the mean normal alveolar COzg and O»
pressures at sea level (i. e., Oxford and New Haven); the thick line, alveolar COs pressure; and
the thin line, alveolar O, pressure. (Irom Douglas, Haldane, Henderson, and Schneider.)
as an index of the H-ion concentration of the blood. The exact chemical
nature of the nonvolatile acids that accumulate in the blood is not as yet
known. Two types of acid can be thought of, either unoxidized organic
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362 THE RESPIRATION
acids, of which lactic acid may be taken as the representative, or inor-
ganic substances, like the acid phosphates. That it is not lactic acid is
shown by both direct and indirect evidence. The direct evidence has
been furnished by Ryffel, who was unable to find any increased per-
centage of this substance either in the urine or in the blood of persons
who had been living for some time in the famous Regina Margherita
hut on Monte Rosa.?” The indirect evidence has been furnished by ob-
serving the time that it takes after the individual has started breathing
the rarefied air for the alveolar CO, tension to fall, as well as that re-
quired to bring about the recovery to the normal when he descends to
sea level. The following curve, which is self-explanatory, will illustrate
these points. . :
Thus, on Pike’s Peak, where the barometric pressure is 459 mm. Hg,
the CO, tension after an initial fall took about seven days before it
came to its permanent level for that barometric pressure, and fourteen
days elapsed after descending from the mountain before the sea-level
tension had been regained. The slow nature of these changes, when com-
pared with the rapid changes observed in the experiment with the bags
already alluded to (page 358), shows clearly that lactic acid can not be
responsible for the increase in H-ion concentration in mountain sickness.
By exclusion it would appear that the increase in Cy is the result of an
excess of fixed inorganic acid (H,PO,) in the blood dependent on a dis-
proportionate excretion of bases by the kidneys during the period of
acclimatization to the rarefied air.
Other observers aver that the acidosis does not really exist, but that
the excitability of the respiratory center itself becomes raised (its
threshold lowered), so that it responds more readily to the normal Cx
of the blood. It has been stated that the increase in excitability of the
center is dependent upon the action of ‘the intense light rays at high
altitudes—the erythema of the skin, etc., being evidence of this excit-
ing action of light. The constant irritation of the skin, these authors
say, Serves by stimulation of afferent nerves to maintain a hyperexcit-
ability of the respiratory center. Others believe that the hyperexcit-
ability of the center is a direct result of the maintained O, deficiency.
The balance of evidence, however, stands in favor of the view that the
phenomena of mountain sickness depend on changes occurring in the in-
organic nonvolatile acids of the blood. The other phenomena of this
interesting condition will be discussed elsewhere (page 399).
APNEA
If a man breathes forcibly and quickly for about two minutes, he
will experience no desire to breathe for a further period of about the
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THE CONTROL OF THE RESPIRATION 363
same duration—he becomes apneic. When the desire to breathe re-
turns, the breathing is at first very shallow, but gradually becomes more
marked, until at last normal respiration is reestablished. If a sample of
alveolar air is removed at the time when the desire to breathe returns,
it will be found to contain a very small percentage of O, indicating
that for some time previous to the onset of breathing there had been in
the alveolar air, and therefore in the blood, so low a percentage of O,
that if O, deficiency could stimulate breathing, this would have started
much earlier than it actually did. A curve showing the results of such
an experiment by Haldane is given in Fig. 131. The person may begin
to show symptoms of O, want, such as cyanosis, before the desire to
breathe returns, which furnishes’ strong proof that O, want itself can
not serve as a stimulus to the respiratory center. The failure of the
center to act must rather be due to the lowering of the Cy consequent
upon the removal of CO, from the blood by the forced respiration which
preceded the apnea—washing out of the CO,, as it is called. That this
has really oceurred can readily be shown by estimating the CO, con-
tent of a sample of alveolar air collected by having the subject make a
forced expiration early in ‘apnea. Extremely low values along with a
respiratory quotient (page 547) of about 0.2 are often found, whereas,
during the preceding forced breathing while the CO, is being washed
out, the quotient is often ten times as great—viz., 2.0.
As would be expected, the low O, percentage present in the alveolar
air toward the end of the apneic pause is not without some effect, indi-
rect though it may be, on the excitability of the respiratory center.
This accounts for the fact that the alveolar air, at the moment the de-
sire to breathe returns, usually contains a lower percentage of CO, than
the normal, indicating that some nonvolatile acid must have accumulated
in the organism so as to raise the Cy of the blood, and thus require a
lower tension of CO, to overstep the threshold of excitability of the re-
spiratory center. In agreement with this explanation it has been found
that, if the last two or three forced respirations preceding the apnea
. are made in an atmosphere of O, instead of air, so as to fill the alveoli
with O,, the apnea can be maintained for a very much longer period;
and when the natural desire to breathe returns, the CO, tension of the
alveolar air, instead of being below the normal, is above it. The effect
of O, in prolonging apnea must, therefore, be dependent on the fact that
it prevents the accumulation in the organism of the unoxidized acids,
leaving to CO, alone the function of raising the Cy in the blood to the
level required to excite the respiratory center. By this means the period
during which the breath can be held after breathing O, is sometimes
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364 THE RESPIRATION
oy opru0IeD
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Fig. 131.—Curves showing variations in alveolar gas tensions after forced breathing for two
minutes. Thin line = Os tension; thick line = COg tension. Double line = normal alveolar
CO, tension. Dotted line shows the alveolar COz tension at which breathing would recommence
at the end of apnea with the alveolar O2 pressures shown by the thin line. ‘The actual breathing
is indicated at the lower part of the figure. It is periodic to start with. (From Douglas and
Haldane.)
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THE CONTROL OF THE RESPIRATION 365
phenomenal; in one individual, for example, after breathing forcibly for
a few minutes and then filling the lungs with O,, apnea lasted for eight
minutes and seventeen seconds.
The Supposed Nervous Element in Apnea
It is necessary to point out that, prior to the elaboration of accurate
methods for the investigation of the chemistry of respiration, many
physiologists interpreted the apnea following forced breathing as the
result of a sort of inhibition of the respiratory center brought about by
its repeated stimulation by afferent nervous impulses transmitted to it
along the vagus nerves, these impulses being set up by the frequent col-
lapse and distention of the alveoli acting on the terminations of the
nerve. In justification of the nervous interpretation of apnea, it was
claimed by the earlier observers that it could not readily be produced
in animals after severing both vagus nerves. More recent work has
shown that this is not an accurate observation, for if the severing of
the vagi is accomplished not by cutting but by freezing, then apnea is
as readily produced as in an intact animal (Milroy).”®
That chemical and not nervous factors cause the apnea is further
demonstrated by the well-known experiment of Fredericq, who, after
ligating the vertebral and one of the carotid arteries in two dogs, anas-
tomosed the central end of the remaining carotid of the one to the
peripheral end of the carotid of the other animal, thus establishing a
crossed circulation. He then found that by applying forced artificial
respiration to the one animal, the apnea which supervened affected the
other animal and not that to which the artificial respiration had
actually been applied. Another proof of the chemical theory of
apnea is furnished by the observation that if forced breathing is per-
formed in an atmosphere containing CO, in about the same partial pres-
sure as in the alveolar air, no apnea supervenes, and if the experiment
is repeated several times with progressively declining percentage of
CO, in the air each time, the length of the apneic pause proportionally
inereases as the CO, pressure in the inspired air diminishes.
Although in the foregoing account we have adopted Haldane’s view
that oxygen deficiency per se can act as an excitant of the respiratory
center only when it is of extreme degree, it should nevertheless be pointed
out that studies by A. S. Loevenhart on the action of evanides on the
respiratory center have led him to conclude that interference with oxida-
tive processes may be a more potent factor in its stimulation than the
experiments in which oxygen-poor atmospheres are respired would lead
us to expect. We must await further evidence before a final verdict is
pronounced on this most perplexing problem of modern physiology.
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CHAPTER XLII
THE CONTROL OF RESPIRATION (Cont’d)
THE EFFECT OF MUSCULAR EXERCISE ON THE
RESPIRATION
During muscular exercise the pulmonic ventilation increases to an
extraordinary extent. At rest an average man respires 6 to 8 liters of
air per minute, but during walking on the level at the rate of 5 kilometers
an hour, this figure may increase to about 20 liters.
The first investigations as to the cause of the relationship between
muscular activity and pulmonic ventilation were made by animal ex-
periments in which tetanus of the muscles of the hind limbs was pro-
duced by electric stimulation of the spinal cord. The problem was to
find out what serves as the means of correlation (nerve reflex or hormone
control) between the muscular activity and the respiratory activity.
By cutting the spinal cord above the point of stimulation, it was found
that the. tetanus was still accompanied by as marked a hyperpnea as
before. On the other hand, when the spinal cord was left intact but the
blood vessels of the limb were ligated, no hyperpnea followed the teta-
nus. Evidently therefore the pathway of communication is the blood.
The next step was to seek in the blood for the substance or hormone that
acted as the respiratory excitant, and naturally the first possibility con-
sidered was a change in the gases of the blood, either a deficiency
of O, or an increase in CO,. Direct examination of the blood for the
quantity of these gases, however, yielded results which were quite con-
trary to such an hypothesis. It was found that the percentage of O,,
if anything, was slightly increased, and that of the CO,, if anything,
diminished. Moreover, when the expired air was analyzed during the
hyperpnea, the percentage of CO, contained in it was distinctly below
the normal average, and the percentage of O, above it. Evidently, there-
fore, the amount of gases in the blood has nothing to do with the excita-
tion of the respiratory center, and the conclusion drawn by the earlier
investigators was to the effect that the exciting substance carried from
the active muscles to the respiratory center must be some unusual meta-
bolic product, possibly the lactic acid produced by contraction.
It was further found, by examination of the respiratory quotient, that
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THE CONTROL OF THE RESPIRATION 367
an excess of CO, was being expired during the work and immediately
after it, but that this was subsequently followed by a much lower quo-
tient, indicating that CO, was being retained. Such a result would be
in conformity with the view that an acid such as lactic is discharged
into the blood, on the carbonates of which it would act as explained on
page 355. Breathing in and out of a small rubber bag causes the same
alterations in the respiratory quotient (see page 358).
That lactic acid is actually produced by contracting muscle could not,
however, be shown by all investigators, and it was not until some years
later that Fletcher and Hopkins” clearly demonstrated the conditions
under which it may appear in active isolated muscle. These observers
found that lactic acid is produced in excised muscles only when the
muscular contraction occurs in a deficiency of O,. When it occurs in an
adequate supply of O,, CO, instead of lactic acid is produced.
Taking these facts together with what we already know concerning
the conditions under which the respiratory center reacts to conditions
which presumably cause a change in the Cy of the blood, we may formu-
late the hypothesis that respiratory activity during muscular exercise
is due to a slight increase in the Cy of the blood, and that this increase
is owing partly to an actual increase in CO, production by the acting
muscles and partly to the production of lactic acid. Such an hypothesis
would satisfactorily explain why the actual amount of CO, in the blood
might be below the normal during muscular exercise, for the CO, would
be ‘‘washed out’’ from the blood by the hyperpnea induced by the in-
crease in Cy.
The obvious method of putting this hypothesis to the test is to ex-
amine the alveolar CO, tension and the respiratory quotient under various
conditions of muscular activity. The results of such observations are
given in the accompanying table.
Q) (2) (3)- (4) (5)
O, used CO, pro- R. Q. CO,in Total alveolar
ine.ec. duced inc.c. vol. CO, alveolar ventilation in
permin. per min. vol. O, air liters per min.
1. During rest, standing 228 264 0.804 5.70 5.80
2. Walking at the rate of
3 kilometers per hour 780 662 0.849 6.04 13.6
3. Walking at the rate of
5 kilometers per hour 1065 922 0.866 6.10 18.8
4, Walking at the rate of
6 kilometers per hour 1595 1898 0.876 6.36 27.6
5. Walking at the rate of
7 kilometers per hour 2005 1788 0.891 6.20 35.6
6. Walking at the rate of
8 kilometers per hour 2543 2386 0.938 6.10 48.2
T
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368 THE RESPIRATION
In the first column is given the O, used in ¢.c. per minute. Among other
things these figures indicate the actual amount of work done. In the
second column is given the CO, production in ¢.c. per minute. By divid-
ing the figures of the second column by those of the first, we obtain the
figures of the third column, representing the respiratory quotient. The
fourth column gives the CO, content of the alveolar air, and the last
column the total alveolar ventilation in liters per minute.
Taking for the present the figures in the first and fourth columns and
postponing a consideration of the respiratory quotient, it will be noted
that, as the muscular work increases up to a total consumption of about
1600 c.e. of O, per minute, the CO, percentage in the alveolar air
steadily increases. The question arises, does the alveolar ventilation
increase in proportion to the increase in CO, tension? If it does so,
increase in CO, tension in the blood ean be held solely responsible for
the hyperpnea (i. e., a pure CO, acidosis); whereas if the hyperpnea is
greater than can be accounted for by the increase in CO, tension, other
acids must’ be partly responsible for the acidosis. By making this same
individual breathe atmospheres containing different percentages of CO,
it was found that to produce a doubling of the alveolar ventilation it
required an increase amounting to 0.33 per cent of an atmosphere of CO,
in the alveolar air (see also page 357). When we examine the above
figures during muscular, exercise, however, we find that a rise in alveolar
CO, from 5.70 to 6.36 (i. e., 0.66 per cent) multiplied the normal alveolar
ventilation by considerably more than four times, whereas had it been
entirely due to an increase in CO,, it should not have been more than
twice as much. Evidently therefore, some other factor than CO, tension
must have been responsible for the increased respiratory activity. This
conclusion is further confirmed by examination of the alveolar CO,
during very strenuous muscular effort, when a relative decrease in the
CO, percentage becomes apparent. :
If it is true that the exciting agency has been dependent partly on an
increase in the CO, tension of the blood, and partly on the production of
nonvolatile organic acids (lactic acid), we should expect that imme-
diately after discontinuing the muscular exercise the CO, tension of the
alveolar air would fall to a level distinctly below normal, that it would
only slowly recover thereafter, and that further exercise before the re-
covery had occurred would produce only a slight increase in alveolar
CO,. These results we should expect because of the much slower rate at
which the nonvolatile organic acid is got rid of from the organism, com-
pared with the volatile CO,. By actual experiment these suppositions
have been found to be correct, as is shown in the following table.
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THE CONTROL OF THE RESPIRATION 369
TIME AFTER DISCONTINUING ALVEOLAR CO, TENSION
_ A BRIEF PERIOD OF IN MM. HG
MUSCULAR EXERCISE
1st Period: 10” 49.2
3” 0” 35.4
6’ 30” 35.3
12’ 30” 35.8
2nd Period: 10” 38.9
3” 0” 33.7
6’ 30” 34.4
3rd Period: 10” 36.9
3” 0” 34.4
8’ 30” 32.4
18’ 30” 33.7
24’ 0” 36.2
Normal resting: 39.0
(Douglas.)
In this table the figures of Period 1 represent the alveolar CO,
tension in mm. Hg immediately following a period of strenuous work.
The figures in Period 2 are for the same individual again performing
the same amount of work with, however, only a short period of rest in-
tervening, and the figures of the third period are a repetition of the same
conditions. It will be observed that the muscular exercise at first raised
the alveolar tension of CO, from the normal of 39 mm. to 49.2 mm., but
that in three minutes after the work had been discontinued the tension
was considerably below the normal. During the second period of mus-
cular exercise the CO, in the alveolar air collected immediately after the
effort did not increase above the normal level, and in the third period
the increase was still less—results which are entirely in conformity with the
view that as a consequence of the first period of muscular exercise non-
volatile organic acids had accumulated in the blood, so that to produce
the required respiratory activity in the second and third periods a
much less increase in CO, tension was required.
We may sum up the conclusions which these observations justify by
stating that during muscular exercise the Cy of the blood becomes slightly
increased because of the liberation into it of CO, and of lactie acid from
the acting muscles. The respiratory center is, however, so sensitive to
the slightest increase in Cy that it immediately responds and produces
hyperpnea, with the result that the volatile CO, is so washed out of the
blood that the Cy is held down in spite of the continued production of
acid substances by the muscles. The more strenuous the exercise, the
less able is the O, content of the blood to keep pace with the metabolic
activity of the muscles, so that relatively more and more lactic acid is
produced, necessitating therefore a greater and greater washing out
of CO,.
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3870 THE RESPIRATION
The readiness with which CO, can be got rid of prevents the hormone
which excites the respiratory activity from continuing to act after it is
no longer required. Provision for the removal of a hormone after its
activity has been displayed is of course essential to efficient correlation
of function, and is seen in the case of other hormones, such as epinephrine
and secretin, whose discontinuance of action is effected by their de-
struction in the blood (see page 745). ;
Direct evidence that lactic acid is formed during strenuous muscular
exercise in man has been furnished by Ryffel.*° Blood removed from a
person immediatey after running at full speed for about three minutes
contained 70.8 milligrams of lactic acid per 100 ¢.c. of blood, the normal
amount being 12.5 milligrams. Much of the lactic acid accumulating in
the blood is no doubt got rid of by oxidation, but a large part of it is
also excreted by the urine, in which it was found by Ryffel in consider-
able amount after strenuous muscular exertion.
Finally, let us consider for a moment the behavior of the respiratory
quotient. This ratio rises early in the muscle work (Table on page 367),
indicating that more CO, is being excreted than O, absorbed. After the
work is discontinued, it usually falls below the normal because of retention
of CO, to take the place of the lactic acid that is being gradually used up
or excreted. A similar fall may sometimes occur in the respiratory
quotient during muscular exercise, if this is continued for a long time.
It probably indicates that a balance has been struck between the produc-
tion of lactic acid in the muscles and the loss of this substance by oxida-
tion. In any case it is a significant occurrence, for it coincides with the
great improvement in the subjective sensations accompanying muscular
exercise. It occurs, for example, at the same time as the appearance of
the ‘‘second wind,’’ when the circulatory and respiratory distress expe-
rienced during the earlier stages of strenuous muscular exertion disap-
pear. The stages prior to the second wind correspond to the period when
considerable quantities of free CO, are being got rid of from the blood
and are probably creating a temporary maladjustment of the Cu which
acts on the various medullary centers. If by forced breathing much of
this CO, is discharged before the muscular exercise is undertaken, the
initial hyperpnea is not nearly so marked.
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CHAPTER XLIII
THE CONTROL OF RESPIRATION (Cont'd)
PERIODIC BREATHING
Types of Periodic Breathing
In the best known of these, called Cheyne-Stokes respiration, a period
of hyperpnea supervenes upon one of apnea, each period following in
regular sequence. After an apneic period, the breathing begins at first
faintly, gradually becomes more pronounced until it is markedly exag-
gerated, and then fades off again to the apneic pause. Sometimes the
apneic period is immediately followed by one of intense hyperpnea, there
being no gradual increase in the respiratory movements. Between these
two types all varieties of the condition are met (Fig. 182).
The conditions in which periodic breathing occurs may be divided into
physiologic and pathologie groups. Of the physiologic conditions the
following may be taken as examples: (1) Breathing in an atmosphere
containing a deficiency of O,; thus, periodic breathing is very readily
produced in persons living in rarefied air. (2) The initial breathing fol-
lowing an apnea induced by forced ventilation of the lungs. In this post-
apneic periodicity, the apneic periods may at first be quite marked, but
as breathing returns they become gradually shorter and the breathing
intervals gradually longer, until normal respiration is restored (Fig.
131). (3) Breathing through a long tube having a small vessel contain-
ing soda lime inserted between the tube and the mouth, the whole capacity
of this vessel and tubing being about a liter. This will cause periodic
breathing in persons that are susceptible to oxygen deficiency. Even
breathing through the tube without soda lime will sometimes cause a
periodic type of breathing in such individuals.
The pathologic conditions in which periodic breathing becomes devel-
oped are particularly those associated with renal disease and cerebral
hemorrhage. In many of these cases, the periodic breathing does not
appear to depend on the same factors as are concerned in the experi-
mental types. The symptoms would rather appear to depend on some
influence of the higher cerebral (supranuclear) centers on the respiratory
center. At least some other evidence of disturbance of the cerebral func-
tions is always forthcoming, such as a slight paralytic shock, and the
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372 THE RESPIRATION
periodic breathing is nearly always aggravated during sleep. Many of
these cases are greatly benefited by administration of caffeine.
In both the physiologic and the pathologie groups, the breathing may
develop a periodic character only when the person is asleep, and even
normal people, particularly infants or very old people, may exhibit it to
a certain degree.
ce i ae MM (Ml we
I
NAVA ORAV AVA Aaa ee
OIG AN aes SAV AVA AN
Tig. 132.—Various: types of periodic breathing. (From Mosso’s “Life of Man in the High. Alps.’’)
Causes of Periodic Breathing
Great interest attaches to an investigation of the causes of periodic
breathing, but it can not be claimed that any perfectly satisfactory ex-
planation has as yet been offered. Pembrey* attributes it to a diminished
excitability (a raised threshold) of the respiratory center due to faulty -
blood supply, the supposition being that, when thus suppressed, the
normal Cy of the blood is unable to excite the center, so that breathing
stops. During the resulting apnea, CO, again accumulates until it has
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THE CONTROL OF THE RESPIRATION 373
raised the Cy sufficiently to excite the depressed center. Hyperpnea
follows, causing a washing out of the CO, and a resulting diminution of
the effective stimulus, so that again the center fails to be stimulated and
apnea supervenes, and so on. Support for this explanation would appear
to be furnished by the fact that, when patients exhibiting periodic breath-
ing are made to breathe an atmosphere containing a high percentage of
CO,, the periodicity of the breathing may give place to regular breath-
ing; a result which may also be obtained by making such patients
breathe in atmospheres rich in oxygen. In the former case, the stimulus is
raised to meet the depressed excitability of the center; in the latter, the
excitability of the center is increased because of better blood supply
so that it is enabled to react to the diminished stimulus. But even
granted that the excitability of the center is depressed, it is difficult to
see why this should occasion a periodic type of breathing unless we as-
sume that it is only when stimulus (i.e., Cy of blood) and threshold of
excitability of the center are adjusted at a certain physiologic level that
smooth and continuous action can go on.
Haldane and his school aver that there is no permanent alteration in
the excitability of the center, but that the periodicity is due to several
causes, which do not always operate to the same degree in the different
conditions in which such periodicity: exists. To study these causes the
exact conditions existing in the various types of periodic breathing that
ean be produced experimentally in man have been investigated.
The most simple to consider first is the periodic breathing that is
produced in a person susceptible to O, want, by breathing through a tube
and bottle (of a total capacity of 1 liter), containing soda lime.
In such a case no outside air enters the lungs, for what we have really
done, besides providing for the absorption of CO,, is greatly to prolong
the dead space. The oxygen tension of the rebreathed air, therefore,
quickly falls, until at last a point: is reached at which the respiratory cen-
ter is directly stimulated by O, deprivation, as we have seen it to be
when this falls to a sufficiently low level (see page 350). The deep
breaths (hyperpnea) which follow, being of greater volume than 1000
c.c., cause outside air to be inspired so that the O, want is made good
and the hyperpnea again disappears, possibly to the extent of apnea, for
now, in consequence of a coincident ‘‘washing out’’ of CO,, there has
been a lowering of the Cy of the blood below the threshold value. During
the apnea the O, is rapidly used up, till a point is reached at which the
center again becomes excited. In such an experiment the effect of O,
want becomes very marked, as shown by the intense cyanosis which
develops.
That breathing under these conditions should be periodic and not
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374 THE RESPIRATION
merely show a steadily increasing hyperpnea is probably due to the un-
equal rates at which the O, and CO, tensions change in the blood. Be-
cause of a ‘‘buffer action’’ the latter fluctuates much less than the for-
mer. Another cause for the periodicity is no doubt the delay between
the gas exchange in the lungs and the arrival of the blood in the brain.
When the O, tension of the blood supplying the respiratory center falls
to so low a level that excitation of the center occurs, the resulting in-
creased breathing aspirates outside O, into the alveoli. After a moment
or so, the O, is carried by the blood to the center, so that its stimula-
tion by O, deficiency is removed, and it is left in a condition in which
it fails to discharge any impulses, since there is a subnormal Cy of the
blood as a consequence of the lowering of the CO, tension produced by
the hyperpnea. A little time must now elapse before the CO, again
rises or the O, falls sufficiently to excite the center.
Fig. 133.—-Quantitative record of breathing air through a tube 260 em. long and 2 cm. in diameter.
(From Douglas and Haldane.)
A similar although less marked degree of periodic breathing can
sometimes be obtained by merely respiring through a long tube without
any provision for the absorption of CO,. In this case it is more difficult
to explain the cause of the periodic breathing, but that the main factor
concerned is one of O, deprivation is evidenced by the fact that in this
as in the previous experiment, the periodic nature of the respiration is
immediately changed to the regular breathing if O, is introduced into
the tube. The interest of the experiment lies in the fact that a similar
relative elongation of the dead space is probably accountable for the
periodic breathing seen in the winter sleep of hibernating animals. Dur-
ing this condition, on account of the depression of metabolism less O,
is required and less CO, is produced, so that the exchange of gases
through the pulmonary endothelium is greatly diminished. The dead
space, however, remains of the same capacity, which amounts to the
same thing as if the latter had been prolonged under unchanged con-
ditions of pulmonary gas exchange.
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THE CONTROL OF THE RESPIRATION 375
The explanation for other types of experimental periodic breathing is
much less satisfactory. Important evidence that changes occurring in
the tensions of O, and CO, in the alveolar air and therefore in the
arterial blood of the respiratory center are largely responsible for periodie
breathing has been secured by studying the condition that develops after
a period of apnea produced by voluntary forced breathing. The results
of such observations are given in the curve shown in Fig. 131.
The thin line represents the O, tension of the alveolar air, the thick
line the CO, tension. The double line running across the chart repre-
sents the average tension of CO, during quiet normal breathing. The
respiratory movements are represented by the tracing at the foot of
the curve along the abscissa. It will be observed that the oxygen ten-
sion falls very rapidly during the apneic period, until just before breath-
ing recommences it may be as low as 30-35 mm. Hg instead of the nor-
mal of about 95. Meanwhile the CO, tension rises from the very low
level of 12 mm., at first very rapidly, then more gradually, although,
when breathing recommences, it has not yet gained the normal level.
As a result of the first periods of breathing, the O, tension suddenly
shoots up, but the CO, falls only slightly. During the next apneic stage
the O, quickly comes down again, and the CO, rises so as almost to at-
‘tain normal tension before breathing again supervenes. As the apneic
periods subsequently become less pronounced, the CO, tension comes to
stand almost at its normal level, whereas considerable variations in the
O, tension continue to occur.
Several interesting features of these results demand attention. In
the first place, it is plain that the body is possessed of some mechanism
by which it can prevent great fluctuations in the CO, tension of the
blood, whereas towards O, no such ‘‘buffer action’’ is displayed. It will
further be observed that the CO, tension of the alveolar air rises very
rapidly during the first part of the apneic period, and then more grad-
ually, the explanation being that during the forced breathing the CO,
has been washed out from the blood but not from the body as a whole.
At first sight one might attribute the periodicity to the same cause
as that operating during breathing through a long tube with soda lime—
namely, to oxygen deficiency. But this explanation is untenable, be-
cause the periodicity remains evident for some time after all possibility
of direct stimulation of the center of O, deficiency is over. A possible
clue is furnished by the fact that breathing returns while the CO, ten-
sion is still considerably below its normal level. The return, as we have
seen, is accounted for by the appearance of lactic acid, and if we assume
that this has occurred particularly in the respiratory center itself, a
slight degree of hyperpnea will be excited, which by supplying O, will
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376 THE RESPIRATION
quickly oxidize the lactic acid, so that the still slightly subnormal Cy
of the blood is unable to excite the center. Apnea therefore supervenes
and lasts until lactic acid has again accumulated in the center. To ex-
plain why local accumulation of lactic acid in the center should produce
a periodic type of breathing, we must further assume that there is con-
siderable delay between the moment at which equilibrium of the gases
in the blood and alveolar air becomes established and that at which
the blood arrives at the respiratory center. This delay is caused by
the slowing of the bloodflow on account of the absence of respiratory
movements.
Emphasis is placed on the fact that it is in the center itself and not
in the blood that the lactic acid becomes oxidized by the excess of O,,
because lactic acid is known to disappear slowly under these conditions
from isolated blood, but to do so very quickly from tissues such as muscle,
and presumably therefore also from nervous tissue.
In support of the above explanation it has been found that, if toward
the end of the forced breathing the lungs are filled with sufficient O,
so that the tension of this gas in the alveoli is not lower than 120 mm.
Hg, breathing is regular in type when it returns, and the CO, tension
of the alveolar air is several millimeters above instead of below the nor-
mal stimulating level.
To sum up, the periodic character of the breathing supervening on
a period of apnea may be explained as follows: Under ordinary condi-
tions of breathing and barometric pressure the O, tension of the blood
is sufficient’ between normal respirations to prevent any accumulation of
lactic acid in the respiratory center, so that the stimulus afforded by the
Cy of the blood produces a constant effect. During the apnea which
supervenes upon forced breathing, lactic acid accumulates in the center,
causing this to respond to the gradually rising Cy of the blood before the
latter has reached its physiologic level. The hyperpnea thus excited
does not, however, bring about a prompt oxidation of the lactie acid
in the center or a lowering of the Cy of the blood circulating through it,
because more time than usual is taken for the blood to get from the
lungs to the brain on account of the absence of respiratory movements.
When the aerated blood does reach the respiratory center, the excess of
O, which it contains oxidizes the lactic acid so that apnea supervenes,
and the lactic acid again accumulates, although not now so much as
before because of the gradually rising Cy of the blood itself. The essen-
tial factor in the causation of periodic breathing is therefore a delayed
mass movement of the blood from the pulmonary capillaries to the re-
spiratory center. The delay may be caused by cessation of the respira-
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THE CONTROL OF THE RESPIRATION 377
tory movement, as in postapneic periodicity, or by some pathologic cir-
eulatory disturbance.
Periodic breathing is produced by forced respiration more readily in
rarefied air than at sea level. It was found by Douglas,” after breath-
ing forcibly for one minute at sea level, that the breathing when it
returned showed 8 to 10 different periods of apnea and hyperpnea. On
repetition of the experiment at an altitude giving a barometric pres-
sure of 600 mm., 25 such periods followed the apnea; at a height cor-
responding to 520 mm., 40 periods. Indeed, at high altitudes periodic
breathing may be brought about by the slightest alteration in normal
respiration ; even taking a deep breath may be sufficient to cause distinct
periodicity in the succeeding respirations, and in many persons living
at high altitudes periodic breathing is very apt to occur during sleep.
As in pathologic cases exhibiting Cheyne-Stokes respiration, the peri-
odic breathing at high altitudes can be immediately removed by inspir-
ing oxygen. ;
We have devoted considerable space to a discussion of these extremely
difficult problems in the hope that clinical observers, by becoming ac-
quainted with the purely experimental work, may be in a position to
conduct more searching investigations as to the cause of Cheyne-Stokes
and other pathologic forms of periodic breathing.
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CHAPTER XLIV
RESPIRATION BEYOND THE LUNGS
Up to the present our studies in respiration have concerned the various
mechanisms involved in bringing about a constant change in the com-
position of the alveolar air. We must now consider the nature of the
means by which the oxygen is conveyed to the tissues and the CO, re-
moved from them.
In the first place, it is important to note that it is not for purposes
of oxidation in the blood itself that the O, is required. In its respiratory
function this fluid serves as a transporting agency between the lungs
and the tissues, in which reside the furnaces of the body that con-
sume the O, and produce the CO,. This does not imply that there is no
oxidation in the blood itself; indeed, we should expect a certain degree
of oxidation because of the fact that the blood contains some living
cells—the leucocytes. It is scarcely necessary nowadays to offer evi-
dence for the foregoing conclusion. One well-known experimental proof
consists in replacing the blood in a frog with physiologic saline solution
and then subjecting the frog with the saline in its blood vessels to an
atmosphere of pure O,, when it will be found that the animal continues
to absorb the normal amount of O, and exhale the normal amount of
CO,. It respires normally without any blood in the blood vessels.
In order that this transportation of gases between the lungs and the
tissues may be efficiently performed, the blood must be provided with
means for carrying adequate amounts of gases to supply the requirements
of the tissues, both during rest and during their varying degrees of
activity. Not only, therefore, must the O, and CO, capacity of the
blood be very considerable, but it must be capable of very rapid adjust-
ment from time to time.
Our problem naturally resolves itself into three parts: (1) the eall
of the tissues for oxygen (Barcroft) ; or, as it is styled, tissue or internal
respiration; (2) the mechanism by which the blood transports the proper
amounts of gases to meet the requirements of the tissues; and (3) the
mechanism by which the blood gases are exchanged in the lungs—ex-
ternal respiration. For convenience, however, we shall change this nat-
ural order and consider the transportation of the gases first.
378
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RESPIRATION BEYOND THE LUNGS 3879
THE TRANSPORTATION OF GASES BY THE BLOOD
The Transportation of Oxygen
It is plainly not by mere solution in the plasma of the blood that the
transportation of O, occurs, for at the partial pressure of this gas ex-
isting in the alveolar air at the temperature of the body the amount that
could be dissolved in the blood would be only one-fortieth of that which
is actually found to be present. If there were only plasma in the blood
vessels, it would require a volume of fluid amounting to 150 kilograms
or more in order to convey the necessary amount of O, from the lungs
to the tissues; that is, the contents of the vascular system would weigh
twice as much as the average weight of a man.
The substance that carries the O, in the blood is the hemoglobin, which
may be described as a highly complex iron compound of protein espe-
cially evolved for the purpose of transporting O,. In some of the lower
animals other compounds exist in the blood for this purpose, but none
of them is to be compared in its efficiency with hemoglobin. They are
merely poor imitations ‘of it.
Regarding the conditions under which hemoglobin combines with or
delivers up O,, the first question that presents itself is whether or not
the reaction is a strictly chemical one. If so, a definite amount of O,
must be capable of combining with a definite amount of hemoglobin. It
is impossible to secure hemoglobin of sufficient purity to test this rela-
tionship directly on hemoglobin itself, so that we must test it indirectly
by examining the combining equivalent between O, and that portion of
the hemoglobin molecule upon which the combining power depends. This
is the part of the molecule containing iron. Now, if we compare the
amount of O, which hemoglobin can take up with the amount of iron
present in the hemoglobin, we shall find that one atom of iron becomes
combined with two atoms of O,. Evidently, then, we are here dealing
with a definite chemical reaction occurring between the O, and the iron
of the hematin portion of the hemoglobin. This relationship is known
s ‘‘the specific oxygen capacity of hemoglobin. ”’
In showing that the union of O, and hemoglobin occurs according to
chemical laws, we throw into prominence consideration of the mechanism
by which the O, combined with hemoglobin in the blood is rapidly de-
livered up in the capillaries so as to supply the tissues with their require-
ment, and is then as rapidly recombined again in the lungs. Moreover,
we must reconcile facts implied by the idea of a specific O, capacity with
the well-known observation that the hemoglobin in the circulation is
usually united with considerably less O, than the total amount possible.
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880 THE RESPIRATION
In other words, we must recognize that, although it is essentially a
chemical reaction, the combination of O, with hemoglobin is greatly in-
fluenced by other factors, and ann it is these that are likely to be of
physiologic importance.
In order to understand the conditions under which hemoglobin will
take up and give off O, in the animal body, we must study the combining
power of hemoglobin when it is exposed to different partial pressures
of O, (for laws governing this, see page 336). In the blood, the ex-
tremes of the partial pressure of O, are represented, at the one end, by
that in the alveolar air, which we have seen to be about 100 mm. Hg,
and at the other, by that existing in the tissues, such as muscle, which
has been shown to be not more than 19 or 20 mm. Hg. We must further
bear in mind that the O, in its passage from the alveolar air to the hemo-
globin and from the hemoglobin to the tissues, is transmitted in solution
through the plasma; that is, so far as the supply of O, to the tissue cells
is concerned, the plasma serves as the immediate source. Since the tis-
sues are using up O, at a very great speed, especially when active, and
are thus tending to lower the tension of O, in the plasma, favorable con-
ditions have to be created whereby the hemoglobin liberates O, at the
same rate as that at which it is leaving the plasma. In brief, it is the
O, tension of the plasma in the tissue capillaries that is the important
factor, the hemoglobin merely serving as a storehouse, which delivers
its O, at just such a rate as to maintain the plasma-oxygen tension at
a constant level. It is obviously of the greatest importance that we
should understand how this mechanism of an adequate plasma-oxygen
tension is maintained.
Methods of Investigation—_We must remember that the combination
of O, and hemoglobin, being a definite chemical reaction, will be re-
versible, and must, therefore, obey the laws of mass action (see page
23) according to the equation: Hb + O,<@HbO,. In order to ascertain
the position of the balance of this equation at different partial pressures
of O,,—that is, the relative quantities of oxy- and reduced hemoglobin
formed in a solution of hemoglobin when this is shaken with O, at differ-
ent pressures,—we may proceed as follows: A few c.c. of the hemoglobin
solution are placed in each of a series of vessels called tonometers, like
those shown in Fig. 134. In addition to the hemoglobin solution, each
tonometer contains a mixture of nitrogen and O, in different propor-
tions. Suppose we use six vessels and in No. 1 have pure nitrogen; in
No. 2, nitrogen containing 5 mm. partial pressure of O,; in No. 3, 10
mm.; in No. 4, 20; in No. 5, 50; and in No. 6, 100. We now rotate the
tonometers in a water-bath at body temperature for about twenty min-
utes, so that, by the formation of a thin film of hemoglobin solution over
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RESPIRATION BEYOND THE LUNGS 381
the walls of the vessel, perfect equilibrium between the atmosphere and
the fluid may be attained (see page 338). A measured quantity of hemo-
globin solution (0.1 or 1.0 ec.) is then removed from each tonometer
\ |
Fig. 134.—Barcroft’s toncmeter for determining the curve of absorption of oxygen by hemoglobin
or blood. (From Starling’s Physiology.)
and placed, together with some very dilute ammonia to lake the blood.
in one of the small bottles of the differential manometer, shown in Fig.
135.* This manometer consists in principle of a graduated U-shaped
tube of narrow bore, containing clove oil, the free end of the U-tube
Fig. 135,—Barcroft’s differential bicod gas manometer. The capillary U-tube contains clove oil.
The pockets on the sides of the blood bottles should be deeper. For manipulation see context.
being connected with small bottles provided with some device so that
two fluids can be placed in each of them but kept unmixed until the
bottle is violently shaken. The three-way stopcock between -the small
*The blood-gas manometers are made in two sizes for use with 1 c.c. and 0.1 c.c. quantities of
blood, respectively. The results with these small quantities are as accurate as with larger amounts.
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382 THE RESPIRATION
bottles and the manometer serves to permit communication of. the
manometer with the outside air.
An equal quantity of hemoglobin solution that has been saturated
with oxygen—i. e., oxyhemoglobin—is placed in the bottle on the other
end of the manometer tube from that containing the bottle with the un-
saturated hemoglobin solution. The bottles having been attached to
the manometer with the stopcocks open to the outside, the apparatus
is placed in a water-bath until the temperature conditions are constant.
The manometers are then closed to the outside air and the bottles are
shaken in order that the hemoglobin solution that is unsaturated with
O, may take up O, from the atmosphere in the bottle until it becomes
‘
‘
.
4
=,
‘
pee Perse tenant en Ts,
we
wv
Saad?”
Fig. 136.-—Barcroft blood gas manometer. This form can be used either as a differential
manometer (page 390) or for direct measurement of pressure. For the latter purpose one bottle
is removed and the pressure of gas generated in the other bottle is measured by the height to
which it raises the clove oil in the distal tube of the manometer, the meniscus in the proximal
limb being readjusted to its original level by compression with the brass screw of the rubber tube
shown in the center.
saturated. The resulting shrinkage in the volume of the atmosphere
on the side of the unknown hemoglobin solution causes the clove oil
meniscus to move towards that side, the degree of movement being pro-
portional to the initial unsaturation of the hemoglobin. The manometer
tubes are then again brought into communication with the atmosphere
so that the meniscus of clove oil may move back to its old level, and the
bottle with saturated hemoglobin is removed from the manometer and a
drop or two of a saturated solution of potassium ferricyanide placed
in the separate compartment of the bottle without allowing it to mix
with the hemoglobin. The bottle is then reattached, the temperature
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Reduced Haemoglobin 100 p.c.
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: 3, oo $ Bie oe
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si ON : °
= S = =
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2 Q 3
3 SS
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g N 3 a
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g = = $ = Q = = a
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ry < i 3 = iy 3s 8 =
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6 Ss
\O 10 20 40 100
mm, pressure
Percentage saturation
with oxygen
100
90
80
70
60
50
40
30
20
10
o
Oo 10 20 30 40 50 60 70 80 90 100
Oxygen pressure
mm,
Fig. 137.—Upper left hand, percentage saturation of hemoglobin with oxygen at 37° C. cor-
responding to oxygen pressures of 0, 10, 20, 40 and 100 mm. of oxygen, respectively.
Upper right hand, the same spaced with the oxygen pressure as the abscissae.
Lower figure, dissociation curve representing the equilibrium between oxygen, oxyhemoglobin
(red) and reduced hemoglobin (purple). (From Joseph Barcroft.)
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RESPIRATION BEYOND THE LUNGS 383
conditions readjusted, the manometer closed off from the ouside air,
and the apparatus again shaken so that the ferricyanide mixes with the
hemoglobin solution. This drives off all the O, from the oxyhemoglobin
solution, and, therefore, raises the pressure in the atmosphere of that
bottle so that the clove oil moves to the opposite side of the manom-
eter, the degree of displacement being proportional to the amount of
oxyhemoglobin.
We have now all the necessary data for estimating the relative amounts
of reduced hemoglobin in the hemoglobin solution as removed from the
tonometers, for it is plain that the second estimation, as described above,
tells us how much oxyhemoglobin might have been formed had all the
hemoglobin been saturated and the first one, how much O, had yet to be
taken up by the original hemoglobin solution to produce saturation.
The Dissociation Curve.—The next step is to plot the results obtained
from the various hemoglobin solutions in the form of a curve. This is
known as the dissociation curve of hemoglobin. It is plotted with the
relative percentages of reduced and oxyhemoglobin in each of the solu-
tions along the ordinates, and the partial pressures of O, in millimeters
of mercury to which they were exposed along the abscisse. The curve
thus drawn is exactly of the same shape as that which would be pro-
duced if we were to place the tonometers in a row at distances from one
another corresponding to the partial pressure of O, which each con-
tained, and then to mark on each tonometer the relative amounts of
reduced and oxyhemoglobin found in the solutions after shaking. A
line joining these marks on the tonometers would then exactly corre-
spond to the curve drawn by the method described above. This will be
clear from the accompanying figure from Barecroft’s book (Fig. 187).
In such a chart the space below the curve can be taken to represent
the percentage of oxyhemoglobin (red in chart), and that above it of
reduced hemoglobin (blue in chart), at the varying partial pressures of
O, which are indicated along the abscisse as being contained in the at-
mosphere of the tonometers, and which must be proportional to the
partial pressure of O, in the solution in which the hemoglobin is dis-
solved.
Difference between Curves of Blood and Hemoglobin Solutions—The
eurve obtained from pure hemoglobin solutions is very far, however,
from clearing up the problem as to how the blood absorbs and
discharges O,. On the contrary, it makes this problem appear
all the more difficult, for, according to the curve (Fig. 137) the hemo-
globin is already more than half combined with O, at a partial pressure
of this gas of no more than 10 mm. Hg, which means that in the low
partial pressure of O, existing in the capillaries the oxyhemoglobin, in-
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384 THE RESPIRATION
stead of readily yielding up its load of O,, would greedily retain prac-
tically the whole of it. The curve, in other words, would satisfactorily
explain why hemoglobin should readily absorb O, from the alveolar air,
but would fall far short of explaining how this O, is readily released
when it is required in the tissues. Obviously there is some artificial con-
dition present in the above experiment which can not obtain in the nat-
ural environment of the blood.
q0 EDS =
80 4
70 L A L
eb LL
Hay.
wo LAY
ot LLL.
Y/
WL
N\
oll
10 20 30 40 50 60 70 80 40 100
Fig. 138.—Average dissociation curves.
Ordinates—Percentage saturation of hemoglobin with oxygen.
Abscisse—T'ension. of oxygen in mm. of mercury.
Curve A—Degree of saturation of pure hemoglobin solutions at varying pressures.
Curve B—Disregard this curve.
Curve C—Effect of 20 mm. COz pressure on above solution.
Curve D—The saturation curve in normal blood at 40 mm. carbon dioxide pressure.
Since hemoglobin takes up O, in proportion to its iron, it can not be
because of changes in the O, combining part of the hemoglobin itself
that blood and pure hemoglobin solutions have dissimilar dissociation
curves, but rather because of differences in the environment in which the
hemoglobin acts. That this is so can be readily shown by plotting the
dissociation curve, not for a hemoglobin solution, but for blood itself
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RESPIRATION BEYOND THE LUNGS 385
(D in Fig. 188). The results are very different. At a partial pressure
of O, of about 60 mm. Hg—that is, a lower pressure than exists in the
lung alveoli (100 mm.)—the blood becomes nearly saturated with 0.,,
whereas at pressures below 50 mm. it readily loses O,, so that at 10 mm.
there is nearly complete reduction.
The question is: What are the environmental conditions under which
the hemoglobin in the blood so alters its combining power for Q, as to
Fig. 139.—Dissociation curves of hemoglobin.
Ordinates—Percentage saturation of hemoglobin.
Abscissa—Tension of oxygen in mm. of mercury.
1. Dissociation curve of hemoglobin dissolved in water.
II. Dissociation curve of hemoglobin dissolved in 79 NaCl.
III. Dissociation curve of hemoglobin dissolved in 9% KCl.
Temperature 37-38° C. (From Joseph Barcroft.)
produce such a difference in the dissociation curve? By experimenting
with hemoglobin solutions, three such factors have been found to come
into play: (1) the presence of inorganic salts, (2) the hydrogen-ion eon-
centration (CO, tension) of the solution, and (3) the temperature. If
hemoglobin is dissolved in water containing the various salts of plasma
in the same proportion as in blood (artificial plasma), the dissociation
curve will be found to change so as to resemble that of blood (Fig. 139).
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386 THE RESPIRATION
Since the plasmas of different animals contain different proportions of
salts, the artificial plasma required to secure the result is not always the
same. It differs, for example, for the dog and man. Potassium salts
are particularly efficient in causing hemoglobin to absorb O,. The in-
fluence of varying hydrogen-ion concentrations of the solution may
be conveniently studied. by adding varying percentages of CO, to the
gas mixture in the tonometers, when it will be found that the curve be-
comes lowered in proportion to the amount of CO, present. This is shown
in Fig. 140.
The effect of temperature on the dissociation curve is twofold: (1) on
the rate with which equilibrium is established at the given partial pres-
| AA
+ WAS
“HA
“(Hf
WV
10 =620 30 40 50 60 70 80 90 100
Fig. 140—Dissociation curves of human blood, exposed to 0, 3, 20, 40 and 90 mm. CO». Ordinate,
percentage saturation. Abscissa, oxygen pressure. (From Joseph Barcroft.)
sure of O,, and (2) on the position of the curve; the lower the tempera-
ture, the higher the curve.
The Rate of Dissociation —Though it is now clear that the three con-
ditions—namely, saline content, Cy, and temperature—are capable of
altering the dissociation curve of a pure hemoglobin solution so as to
make it correspond with that of blood, this does not entirely solve our
problem, for we have yet to show how the cooperation of these forces
renders it possible for the rate at which hemoglobin takes up O, in
the lungs to correspond exactly with that at which it gives up its O,
to the tissues. To study this problem a somewhat different kind of
experiment must be undertaken. The hemoglobin solution is placed in
a tube and the gas mixture slowly bubbled through it, samples of the
solution being removed at intervals for analysis in the differential blood-
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RESPIRATION BEYOND THE LUNGS 387
gas apparatus. To obtain the rate of oxidation, a mixture of N, or H,
and O, is bubbled through the blood with the partial pressure of the
O, the same as that which obtains in alveolar air—namely, about 95-100
mm. Hg; and to obtain the rate of reduction pure N, or H, gas is bub-
bled through.
The rates of reduction or of oxidation as thus determined are then
plotted in curves constructed with the percentage saturation of the
Oxidation
17°5° C. no CO,
Reduction
Oxidation
37°5° C. no CO,
Reduction
Oxidation
37-5° C..
+40 mm. pressure”
of CO,
Reduction
Fig. 141.—Curves showing relative rates of oxidation and reduction of blood as influenced by
temperature and tension of COs.
* Ordinates—Percentage saturation.
Abscissae.—Time in minutes.
Reducing gas, hydrogen.
Oxidizing gas, oxygen.
A, temperature 17.5° C., with no COs.
B, temperature 37.5° C., with no COs.
C, temperature 37.5° C., but the Oz and H contained 40 mm. Hg pressure of COe. (From
Joseph Barcroft.)
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388 THE RESPIRATION
hemoglobin on the ordinates and the time in minutes along the abscissx
(Fig. 141). Even if we use blood in this experiment and therefore make
certain that the hemoglobin is acting in the presence of the proper pro-
portion of salts, we shall find, as Fig. A shows, that at room temperature
the rate of oxidation is very much greater than the rate of reduction.
If now we repeat the observation at a temperature of 37° C., the two
curves come more nearly to correspond, but still ‘the rate of reduction is
slower than that of oxidation. If in a third experiment, besides having
proper temperature and chemical conditions, we produce the oxidation
and reduction in the presence of a partial pressure of CO, of 40 mm.,
which corresponds to that of the arterial blood, we shall find that oxida-
tion becomes a little slower, whereas reduction is further quickened.
Indeed the two curves, as seen in C in the figure, come practically to
correspond, indicating that the environmental conditions under which
hemoglobin combines and gives off O, in the blood are exactly adjusted.
One word more with regard to the influence of Cy. Its effect in flat- .
tening out the curve, especially at the lower partial pressures of O,,
indicates that when a high Cz is present, the blood will very readily part
with its O, supply. Now, the most significant application of this fact
is that high concentrations of H ion will occur just exactly where it
will be of benefit—namely, in the capillaries (because of the CO, and
lactic acid produced by the tissues). Some doubt has, however, recently
been thrown on the importance of this factor.
Since, as we have seen, hemoglobin absorbs O, according to chemical
laws, it will naturally be asked not only why the dissociation curve flat-
tens out while yet maintaining the shape of a right-angled hyperbola,
as by the action of acids or an inerease in temperature, but also why it
should change its shape when salts are also present. The explanation
offered by Barcroft and his pupils is that the changes depend on the
fact that hemoglobin being a colloidal substance, its molecules undergo
processes of aggregation under the conditions referred to above, and
therefore cause the reaction to become of a different type from that
represented by the equation HbO, = Hb+0,. As has been pointed out
by Bayliss, although such an explanation might suffice to explain the
flattening out of the curve, it fails to explain the change in its shape;
for, according to the laws of mass action, such a change could occur
only if molecules of a different type came to take part in the reaction.
Dissociation Constant.—Notwithstanding these criticisms, it is of econ-
siderable practical importance to know that an equation exists from
which the entire dissociation curve can be plotted by making only one
determination of the relative amounts of oxy- and reduced hemoglobin
at a particular tension or partial pressure of oxygen. This equation is as
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RESPIRATION BEYOND THE LUNGS 889
gg goo
follows: 100 ~ Wake’
hemoglobin with O,, x the O, tension, and K and are constants, K
being the equilibrium constant and n the average number of molecules
of hemoglobin supposed to exist in each aggregate.
When this equation is applied to human blood, the value of » remains
unchanged and is given as 2.5, so that by transposition we are enabled
to find the value of K as follows: K ——___* __. Jf we find the value
x"(100 -y)
of K by measuring the relative saturation of the blood with O, at one pres-
sure of this gas, then by changing the value of x to correspond to other
O, pressures, we can find all positions of the curve for a given sample of
blood.
An important practical application of this method is found in the
determination of the Cy of blood, for, as we have seen, the dissociation
curve becomes lowered in proportion to the concentration of hydrogen
ions. The acidity of a sample of blood can therefore be found by com-
parison of its dissociation curve, as plotted from the values found for
K, with that of normal blood to which known quantities of acid have
been added. When the curves correspond, the bloods must contain the
same amounts of acid, other things being equal. In brief, then, the re-
action of the blood is proportional to the value of K. When this is low,
it indicates that the blood is taking up an abnormally low percentage
of its possible load of O, at a given pressure of O,, and that the acidity
is greater than normal; when K is high, for the same reason the acidity
must be low.
In determining K for the blood as it exists in the body, it is necessary
that it should be subjected to the same tension of CO, as obtains in the
blood vessels. K will then be proportional to the Cy of the living blood.
This condition would be impossible to fulfil in drawn samples were it
not for the fact that we can place in the tonometer an atmosphere con-
taining the same partial pressure of CO, as is found in the alveolar air.
Since this value varies in different individuals, it must be separately
ascertained in each case (see page 344). As determined with these
modifications, K has been found to vary in healthy men between
0.000212 and 0.000363 (ten individuals). When acid substances appear
in the blood, as in acidosis, K becomes extremely low; thus, in oné case
suffering from acidosis with dyspnea, it was found a few hours before
death to be only from 0.000082 to 0.00011. Similarly K becomes low
in the acidosis associated with mountain sickness, and it is said to be
raised after taking food that is rich in alkali.*
where y equals the percentage saturation of
*When K is found to be normal, the blood is said to be mesectic; where K is low, it is said to
be myonectic; and when K is high and the acidity is therefore small, it is said to be pleonectic.
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CHAPTER XLV
RESPIRATION BEYOND THE LUNGS—Cont’d
THE MEANS BY WHICH THE BLOOD CARRIES THE GASES
In the foregoing account ofthe physiology of the blood gases, empha-
sis is placed on the tension under which the gases exist rather than on
the total amount of each gas present in the blood. This has been done
because the exchange of gases between alveolar air and blood and be-
tween blood and tissues proceeds according to the laws of gas diffusion,
which are of course dependent upon differences in gas pressure or
tension.
Something must now be said regarding the amount of the gases. This
may be measured either by physical or by chemical methods. In the
former, a measured quantity of blood is received into an evacuated glass
vessel, which is then attached to a mercury pump, by which the gases
are sucked out of the blood and transferred, by suitable manipulations
of stopcocks, to a graduated tube, in which they are then analyzed by
chemical means. The principle of the chemical method has already been
deseribed in connection with the measurement of oxygen in hemoglobin
solutions (see page 382). A measured quantity of blood, kept free from
contact with the air, is transferred under some weak ammonia solution
to one of the blood-gas bottles of the blood-gas differential manometer,
and a few drops of a saturated solution of potassium ferricyanide is
placed in the pocket of the bottle. After the blood has been laked and
temperature conditions adjusted, the ferricyanide is mixed with the
blood solution, thus causing the O, to be quantitatively displaced. From
the increased pressure produced in the manometer the amount of O, can
readily be computed. To determine the CO, of the blood, the bottle is
now removed from the manometer and a few drops of a saturated solu-
tion of tartaric acid placed in the pocket. When this is mixed with the
deoxygenated blood mixture, after the usual adjustment for tempera-
ture, the pressure caused by the evolved CO, is recorded and the amount
present calculated.
The results of the analysis are expressed as the number of cubic centi-
meters of gas present in 100 c.c. of blood—the volume percentage, as it
is called. The following are approximate percentage values:
390
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RESPIRATION BEYOND THE LUNGS 391
OXYGEN CARBON DIOXIDE TOTAL GAS
Venous blood 12 48 60
Arterial blood 20 40 60
The estimation of the amounts of the gases, although of little value
in connection with the physiology of gas exchange, is very important in
supplying information regarding the respiratory activities of the various
organs and tissues. Just as we determine the total respiratory exchange
of an animal by measuring the differences in 0, and CO, in inspired and
expired air, so may we determine the degree of tissue respiratory ex-
change by analysis of the gases in blood removed from the artery and
vein of the tissue. It should be clearly understood, however, that it is
not the percentage but the total amount of the gases that must be con-
sidered, and that it is therefore necessary to know the volumes of blood-
flow as well as the percentage .of the gases. Something will be said later
of the results of such investigations (see page 393).
At present we are concerned with the manner in which gases are
carried in the blood. The O,, as we have seen, is carried by the hemo-
globin, some being also in a state of.simple solution in the plasma. The
CO,, which it will be noted is present even in arterial blood in con-
siderably greater amount than the O,, is partly combined with alkali to
form bicarbonates. The alkali available for this purpose varies from
time to time according to the amount of other acid substances present.
Since these are stronger acids than carbonic, any increase in their
amount (acidosis) causes displacement of some of the CO,, thus bring-
ing about, as we have seen, a relative increase in free CO, in the blood
and therefore raising the Cy.
What particularly interests us here is the agency by which the com-
bined CO, is carried in the blood. If blood is exposed to a full atmos:
phere of CO,, it will take up as much as 150 per cent of the gas—that
is, between two and three times the amount ordinarily present in it.
It has therefore a great reserve capacity for CO,. A greater propor-
tion of the CO, is carried in the plasma than in the corpuscles; but if
plasma (or serum) is exposed in a vacuum, all of the CO, present in it
will not be evolved. When blood itself is similarly exposed, on the
other hand, all the CO, is given off. To liberate all of the CO, from
plasma in vacuo, some acid must be added, from which it has been in-
ferred that blood corpuscles act like weak acids. It is commonly stated
that hemoglobin or some constituent of blood is capable of freeing CO,
from solutions of sodium carbonate, but the recent work of Buckmaster”
shows that this is not the case. The decomposing power of blood is
caused by the development of acidity in the shed blood and any similar
power that the corpuscles may exhibit is due to a discharge from
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392 THE RESPIRATION
them into the plasma of acid radicles. At least it has been found that
the alkalinity of the plasma increases when CO, is bubbled through
blood, this increase in alkalinity being interpreted as the result of the
migration of acid radicles into the corpuscles. This would lead us to
expect that under the opposite conditions (i.e., in vacuo) acids would
leave the corpuscles.
Proteins are amphoteric substances—that is, they combine with acids
or alkalies—which would lead us to expect that they would be capable
of absorbing some CO,. That this is the case, particularly for hemo-
70
tLood.
‘
00 volume
o
ao
a
oa
Bb
a
bh
Oo
Volumes of CO, aboovbed by 1
30.440 2«2502~=C«0”COsté=“‘i‘“CSC who found that blood collected from the finger of a man living on
a high mountain is brightly arterial, whereas if this same blood is
shaken in a flask with alveolar air from the man from whom it was
taken, it will become darkly venous. To account for this difference it is
believed that the pulmonary epithelium forces O, into the blood contrary
to the laws of diffusion.
A more exact proof was sought for by comparing the relative amounts
of O, and CO that blood would take up (1) when exposed outside the
body and (2) while in the blood vessels. Carbon monoxide has a very
great avidity for hemoglobin, so that if blood is shaken in a flask with
air containing 0.07 per cent of this gas, colorimetric measurement will
show an equal mixture of oxy- and carboxy-hemoglobin. Since carbon
monoxide is destroyed with extreme slowness in the body, it is possible
by causing a man to breathe a mixture of it in air to determine, in a
sample of drawn blood, whether as much carboxy-hemoglobin has been
formed as in vitro. If so, the O, tension in the blood must equal that in
the alveoli; if less carboxy-hemoglobin should be formed, it would indi-
cate that a higher tension of O, exists in the blood. This latter is the re-
sult which Haldane states he has secured. In one experiment, for ex-
ample, when blood was shaken outside the body with 0.04 per cent CO,,
the amount of carboxy-hemoglobin formed was 31 per cent of the whole
hemoglobin. When the same mixture was inhaled for three or four hours
the percentage of carboxy-hemoglobin in the blood rose only to 26 per
cent, which would correspond to an O, tension of 25 per cent of an atmos-
phere, whereas even at sea level the tension of O, in the alveolar
air can not be above 15 per cent of an atmosphere.
The constant low tension of O, in the plasma stimulates the red blood
corpuscles and the percentage of hemoglobin to become markedly in-
creased after residence for some time in high altitudes. At first this is
due to a concentration of the blood by a diminution in plasma, but grad-
ually the blood-forming organs become excited and an actual increase
in the total amount of hemoglobin occurs. In the light of these facts it
is interesting to compare the average number of red corpuscles in the
blood of inhabitants living at different altitudes.
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402 THE RESPIRATION
HEIGHT ABOVE SEA RED CORPUSCLES
(METERS) (PER C.MM. BLOOD)
Christiania 0 4,970,000
Zurich ’ 412 5,752,000
Davos 1560 6,551,000
Arosa 1800 7,000,000
Cordilleras 4392 8,000,000
(From Starling.)
COMPRESSED-AIR SICKNESS; CAISSON DISEASE;
DIVER’S PALSY
Divers and caisson workers are susceptible to peculiar symptoms.
These are frequently of sufficient severity to cause death, but may be so
mild as almost to escape notice. They first appear, not when the worker
is subjected to the high pressure, but after he has come back to atmos-
pheric pressure.*
While in the compressed air the worker as a rule suffers no discom-
fort. A stuffiness may be felt in the ears and temporary giddiness; the
respiration and pulse rate may become slow and frequency of micturition
may be noticed, but none of the symptoms of disease appear until after
the caissonier or diver has been decompressed (after he has returned to
atmospheric pressure), the exact time of their onset being either imme-
diately after decompression or af the end of several hours. The worker
may have returned home and spent the evening feeling perfectly well
until he went to bed, when symptoms supervened which may include mus-
cular and joint pains, vertigo, embarrassed breathing, subcutaneous em-
physema and hemorrhages, pains in the ears and deafness, vomiting,
perhaps hemoptysis and epigastric pain. These symptoms usually pass
off after some hours but the arthralgia and myalgia sometimes persist
for a considerable time.
In the more severe cases the first symptom is severe pain in the mus-
cles and joints, quickly followed by motor paralysis, so that the patient
falls and is likely to become unconscious. The pulse is almost imper-
ceptible, the respiration is labored, sometimes even asphyxial, the face
cyanosed, and the surface of the body cold. Many of the cases are fatal;
indeed, death may be almost instantaneous. Such cases are common in
careless diving when the divers, to return the more quickly, screw up the
outlet valve in their helmets so as to fill their suits with air, which car-
*A caisson is a steel or wooden chamber sunk in water and prevented from filling by means of
compressed air. For the passage of the workmen and of material, into and out of the caisson, the
latter is connected with a second smaller chamber fitted with air-locks and decompressing cocks. A
diver works in a waterproof suit, the head being enclosed in a copper helmet connected by hose with
air pumps. Every 10 meters or 33 feet of water corresponds to one atmosphere pressure (15 pounds
to the square inch), so that at this depth the total air pressure in a caisson, or in a diver’s helmet,
would amount to 30 pounds to the square inch, that is, + 1 atmosphere.
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BREATHING IN COMPRESSED AND IN RAREFIED AIR 403
ries them to the surface, where they decompress themselves by opening
the valve.
Autopsies of persons dead of caisson disease have shown, asa rule,
intense congestion of the viscera, hemorrhages in the spinal cord and
brain, and ecchymoses on the pleura and pericardium. In some eases
interlobar emphysema of the lungs and laceration of the spinal cord and
brain have been noted.
The Cause of the Symptoms
The cause for the symptoms is not, as was at one time supposed, that
the pressure drives the blood from the peripheral into the deep regions
of the body, including the nerve centers. Such a process is impossible,
because the fluids of the body—and all tissues, even the bones, are full
of fluid—are incompressible. Pressure applied to any part of the body
will be immediately distributed equally to every other part. If this were
not so, life would be impossible during any variation of atmospheric pres-
sure. It is now clearly established that all-the symptoms of caisson disease
are due to decompression, and not, in the slightest degree, to the mechan-
ieal effect of the pressure itself (Paul Bert, Leonard Hill and Macleod*‘).
When an animal is under pressure, its tissue fluids dissolve a large
amount of gas. They absorb it in obedience to the law of solution of a
gas in a fluid, which states that the amount of gas dissolved in water is
directly proportional to the partial pressure of that gas in the atmos-
phere; at two atmospheric pressures twice as much gas will pass into
solution as at zero pressure (Dalton’s law). So long as the gas is in
simple solution, it does not in any way change the physical condition of
the blood and tissue fluids. If, however, the animal is suddenly decom-
pressed (i. e., the pressure of air surrounding it is reduced to zero), the
dissolved gas will be so quickly thrown out of solution that bubbles of
it are set free. These bubbles act as air emboli, sticking in the pulmonic
eapillaries or blocking up a terminal artery in the brain; or they may be
large and tear the capillary wall and so lead to hemorrhage. If these
bubbles are produced in the posterior spinal roots, intense pain results;
if in the anterior, motor paralysis. Frothing of the blood in the heart im-
pedes the action of the organ and death soon follows.
The following experiments furnish proof of this explanation: A frog
was placed in a small steel chamber connected with a cylinder of com-
pressed air and provided with two windows by which a strong are light
could be passed through the chamber. The web of the foot was stretched
on a wire and fixed so that the small blood-vessels could be seen by apply-
ing a microscope to the outside of the window. After carefully observing
the circulation of the blood in the vessels at atmospheric pressure, a posi-
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404 THE RESPIRATION
tive pressure, amounting in some experiments to + 50 atmospheres, was
introduced but no effect could be noted on the circulating blood. By
opening a tap in the chamber, decompression to zero pressure was quickly
effected and, immediately, large bubbles were seen to develop in the
blood, blocking the vessels and producing stasis. The bubbles were de-
rived from the gas that had gone into solution under pressure. On re-
applying the pressure the bubbles of gas again went into solution and
the blood circulated normally. When the pressure was subsequently very
gradually lowered to zero, the circulation went on undisturbed, and the
frog was removed from the chamber in normal condition.
The process involved in causing caisson disease is evidently the same as
that which can be observed in a bottle of aerated water; if the cork in
such a bottle is drawn, the dissolved gas escapes as bubbles and effer-
vescence results; if the bottle is recorked, the gas reenters solution and
the fluid becomes quiet. If a pin hole is made in the cork, the gas will
gradually escape and no effervescence will result. ,
Confirmatory results have been secured by observations on mammals.
The arterial blood pressure of rabbits was not found to become altered
by exposure to compressed air, and various animals placed in a large,
strong steel chamber at pressures far in excess of those to which man
ever subjects himself did not show any symptoms like those of caisson
sickness, unless the pressure was suddenly lowered. Many times also, if
symptoms had appeared they could be removed by again subjecting the
animals to the compressed air.
Investigations were also carried out iy. determine exactly how much
gas the blood of an animal subjected to high pressures contains, and how
long it takes to absorb the maximal amount of gas and to release it. It
was found that the gases that increased in amount were nitrogen and
oxygen, and that these become dissolved in the blood according to Dal-
ton’s law.
The Prevention of the Symptoms
The most important practical application of these observations con-
cerns the length of time required for the saturation and desaturation to
occur, for the results serve as a basis upon which the safe regulation of
work in compressed air by man can be conducted. The most significant
outcome of the above experiments from this standpoint is that it takes
considerable time for the blood to absorb its full quota of gas at a given
atmospheric pressure and to liberate it again when the animal is decom-
pressed. The cause of delay is that the tissue fluids other than the blood
take much longer than would be expected to reach equilibrium with the
partial pressure of gas in the blood plasma.
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BREATHING IN COMPRESSED AND IN RAREFIED AIR 405
To understand why this delay should occur, let us suppose that the
only gas coneerned is nitrogen. As the pressure rises, the blood in the
capillaries of the lungs must dissolve nitrogen in proportion to the pres-
sure of this gas in the alveoli; the blood carries the dissolved gas to the
tissues and these dissolve it until the pressure is again equalized between
them and the blood. The blood, after giving up its excess of dissolved
nitrogen, returns to the lungs and again becomes saturated and this goes
on until blood and tissue have become saturated with gas at the external
pressure. The tissues are two-thirds water and they contain (in man)
from 15 to 20 per cent of fat. Fat, however, dissolves five times more
nitrogen than water (Vernon) ; consequently, it takes longer for a given
volume of tissue than of blood to become saturated at a given pressure.
The blood in man constitutes one-twentieth of the body weight; so
that if the tissues were all liquid they would dissolve 20 times as much
nitrogen as the blood. On account of the fat which they contain, however,
the tissues take up more than this proportion—namely, in an average
man about 35 times more than the blood. All the blood in the body takes
about one minute to complete a round of the circulation, so that in this
time, after being suddenly subjected to an increased pressure—assuming
that the blood circulates equally throughout the body—the tissues will
be one-thirty-fifth saturated; in the next minute another thirty-fifth of
thirty-four thirty-fifths will be saturated, and so on. After five minutes
the body will be about 22 per cent, and in 25 minutes about one-half,
saturated; but it will take about two hours before saturation is complete.
These calculations assume that the blood is evenly distributed through-
out the body; but this is not the ease, for its mass movement varies
considerably in different parts, being much greater in the active muscles
and in the glands than in passive structures, such as fat. These less vas-
cular parts will therefore lag behind the others in taking up their full
quota of gas, and therefore prolong the time necessary for complete
saturation of the body as a whole.
We see therefore that, after some time in compressed air, the blood
and active tissues will be saturated and eontain volumes of dissolved
gas in proportion to their relative bulks; the fat, although not saturated,
will yet contain up to five times more gas than an equal volume of
blood, and the passive tissues will be incompletely saturated.
These considerations regarding the saturation of the different parts
of the body apply also in its desaturation. Suppose, for example, that
the external pressure is suddenly lowered: the blood, on leaving the
lungs, will contain no excess of gas; when it reaches the tissues it will
remove gas until the pressure is equalized, discharge this into the alveoli
and return again for more. Other things being equal, it will take the
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406 THE RESPIRATION
same number of minutes to desaturate that it took to saturate, and the
parts of the body that will lag behind the others, in being desaturated,
are those with a sluggish circulation.
When the mass movement of the blood is increased by muscular exer-
cise, the rate of saturation and desaturation with nitrogen is increased
in proportion. During active work the increase in movement of the
blood may be four or five times over the normal, so that the tissues of
the caisson worker become much more quickly desaturated during decom-
pression than the above figures would lead one to expect.
Application of Foregoing Laws in Practice
With regard to the application of these principles in the decompression
of caisson workers, it is impracticable to occupy as much time as it takes
to saturate the body even at comparatively low pressures. If the great
dangers attending work in compressed air are to be avoided, we must
either insist on very gradual decompression or we must show how the
dissolved gases may be got rid of by some modification in the decom-
‘pression procedure. With this object in view, we must determine what
difference of pressure may be allowed between the external air and the
- body without the formation of bubbles. Actual experience shows that
there is no risk of bubble-formation, however quick the decompression,
after exposure to + 15 pounds pressure ( i.e., 2 atmospheres absolute).
“‘Now, the volume of gas capable of being liberated on decompression
to any given pressure is the same, if the relative diminution of pressure
is the same’’—(Haldane**). On reduction from 4 to 2 atmospheres,
the same volume of gas will-tend to be liberated as on reduction from 2
to 1 atmospheres—that is to say, no bubbles will form. The practical
conclusion is ‘‘that the absolute air pressure can always be reduced to
half the absolute pressure at which the tissues are saturated without
risk.’’ Thus, after saturation at 90 pounds absolute pressure (+ 5 atmos.
pheres), a man can be immediately decompressed to 45 pounds (+ 2
atmospheres) in a few minutes without risk, but from this point on the
decompression must be conducted slowly, so as to insure that the nitrogen
pressure in the tissues is never more than twice the air pressure. The
great advantage of this method is that it makes the greatest possible use
of difference of pressure between tissues and blood in order to get rid of
the gas that these contain.
When the decompression from the start is gradual, the desaturation
of the tissues will progressively lag behind that of the blood, and the
tendency to the liberation of free gas will become greater. In such a
ease the decompression is far too slow at first and far too rapid later.
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BREATHING IN COMPRESSED AND IN RAREFIED AIR 407
Theoretically, therefore, the decompression should be rapid at first and
very slow later.
Before recommending the adoption of this principle of stage de-
compression in caisson work, Haldane and his coworkers made numerous
observations on the incidence of decompression symptoms in laboratory
animals. They assert that the stage method is decidedly safer than the
uniform method, the advantage being particularly after short exposures.
On the other hand, Leonard Hill could make out no definite advantage
for the stage method. The two methods have also been compared in
actual caisson work at the Elbe Tunnel, where the pressure was + 2
atmospheres. Very little advantage could be demonstrated for the
stage as compared with the uniform method at this comparatively low
pressure. The general conclusion which we may draw is that the stage
method should be employed, although it is not to be expected that it
will absolutely insure absence of decompression symptoms. Of course
the great advantage of the stage method is the saving of time, making
it possible to persuade the workmen to adopt it.
There are two other factors that are to be considered in hastening the
desaturation of the tissues; these are muscular exercise, and the breath-
ing of an indifferent gas.
It is clear, from what has already been said, that the gas dissolved in
the tissues will become removed in proportion to the mass movement
of the blood, and it is probably true that muscular exercise, performed
in the decompression chamber, is of as great importance in preventing
the subsequent development of symptoms as a much prolonged decom-
pression. In a man at rest, the circulation through the central nervous
system and the viscera is constantly influenced by the pumping action
of the respiratory movements, but in the capillaries of the muscles,
joints, fat, etc., this influence is not felt and the blood flows more slowly.
It is consequently in these parts that bubble formation is likely to oc-
cur, especially some time after decompression. The bubbles cause the
neuralgic pains—the ‘‘bends’’ and ‘‘screws’’ so well known to caisson
workers. These could no doubt be entirely prevented by muscular
exercise and massage of the limbs during decompression. In illustration
of these facts the following experiment by Greenwood may be cited:
During decompression from + 75 pounds pressure in 95 minutes ‘‘Green-
wood flexed and extended all the limb joints at frequent intervals, with
the exception of the knees. Subsequently pain and stiffness were ex-
perienced in the knees and nowhere else.’’ In another experiment the
knees also were flexed and no pain was felt.
But even in the parts with active circulation, the gas in the tissues
may lag considerably behind that in the blood, although the decompres-
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408 THE RESPIRATION
sion has been properly controlled. This has been shown by Leonard
Hill in the case of the kidney. The ‘‘tissue’’ gas in this case can be
taken.as the gas dissolved in the urine, by analyzing which, therefore,
at different stages of decompression, the excess of nitrogen over what
it should be at the external pressure, can be ascertained. On decom-
pression from +30 pounds by two stages to zero, a considerable super-
saturation was found to exist. The excess of nitrogen can, however, be
cleared out of the kidneys rapidly and completely by breathing oxygen,
which should therefore be administered during decompression in cases
where great care has to be exercised (Leonard Hill). :
When symptoms do appear, they can, in most cases, be relieved by
recompression, and all modern caisson works are provided with a special
chamber for this purpose. We need scarcely say anything about this
treatment here, as its value is so well known. Suffice it to say that,
although it is most likely to afford relief when applied as soon as pos-
sible after the appearance of the symptoms, yet it is often efficacious
when applied several days after their onset.
Quite apart from the dangers of decompression, it must of course be
remembered that the working conditions in a caisson are somewhat dif-
ferent from those at atmospheric pressure, as the air, owing to its com-
pression, is warmer and is loaded to saturation point with moisture.
This hot, wet air interferes with the heat-regulating mechanism of the
body, making hard muscular work very uncomfortable because of the
tendency of the body temperature to rise. The reaction of the body
against this tendency to hyperthermia consists in dilatation of the su-
perficial capillaries and increased heart action.
When such working conditions are repeated day by day, the appetite
is likely to fail, partly because of the tendency of the body to suppress
the activity of the metabolic processes, so as to keep down heat produc-
tion, and partly, no doubt, because the digestive processes are working
below par on account of there being less blood circulating through the
visceral blood vessels, it having been sent to the surface of the body to
be cooled off. The worker therefore tends to take less food, his metabo-
lism becomes depressed, and his factors of safety against bacterial
infections become lessened.
The risk of the appearance of symptoms on decompression is also
greater when the air in the caisson has been moist and hot, for the heart
has been overworking to maintain the bloodflow in the dilated vessels;
it gets fatigued and is consequently unable to maintain, during decom-
pression, a rate of bloodflow that is adequate for carrying the gas-
saturated blood to the lungs, where the excess of gas becomes dissi-
pated.
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BREATHING IN COMPRESSED AND IN RAREFIED AIR 409
The criterion of proper working conditions in the caisson is there-
fore the wet-bulb temperature. This should stand below 75° F. To
maintain this condition it is necessary to ventilate the caisson, pref-
erably with air that has been cooled by cold-water radiators; in any
case, the ventilation should be adequate to keep down the wet-bulb
temperature. The increased expense of ventilation with cooled air
would soon be balanced by the greater working efficiency of the men.
Constant circulation of the air in the caissons by means of fans assists
also in improving the conditions, for it helps to increase dissipation of
heat from the body.
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’ CHAPTER XLVII
THE CIRCULATORY AND RESPIRATORY CHANGES ACCOM-
PANYING MUSCULAR EXERCISE*
During activity the muscles require many times more blood than dur-
ing rest. When the activity is widespread the greater blood supply is
provided by increased heart action accompanied by dilatation of the
muscular arterioles and constriction of those of the splanchnic area, so
that the entire available blood supply of the body is made to circulate
more rapidly. When, on the other hand, the activity is confined to a
limited group of muscles, the increased blood supply is mainly provided
by a local dilatation of the blood vessels of the active muscles accom-
panied by a reciprocal constriction of those of inactive parts. Under
these conditions there may therefore be no quickening of the bloodflow
as a whole. In order that this accurate adjustment of blood supply to
tissue demands may be promptly and adequately brought about, all
available types of coordinating mechanism are called into play; that is
to say, mechanical, nervous and hormone factors cooperate to an extent
which is dependent upon the type of work being performed.
Besides the changes in pulse rate and blood pressure which are evi-
dently designed to supply more blood to the acting muscles, changes
dependent upon a secondary effect of the muscular movements have also
to be considered. Although the various factors work together and are
more or less interdependent, the final effect can be understood only after
we have studied the relative influence of each separately.
The Mechanical Factor.—It is particularly with regard to this factor
that the circulatory changes may be an unavoidable consequence of,
rather than a useful adjustment to, the muscular effort. The effects vary
with the type of exercise performed. In repeatedly lifting and lowering
dumbbells from the floor to above the head, the contracting muscles of
the back and extremities and of the abdomen compress the veins and
cause the blood to flow more rapidly into the heart, so that the arterial
pressure suddenly rises. So long as this compression exists, the veins
remain relatively empty and the arteries overfilled, but whenever it
ceases and the muscles relax, the veins fill up again and the arterial pres-
*This chapter is placed here rather'than following circulation because of the interdependence of
the circulatory and respiratory adjustments.
410
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CHANGES ACCOMPANYING MUSCULAR EXERCISE 411
sure markedly falls, until the extra space in the veins has been occupied
by blood. It is for this reason that the arterial blood pressure is always
found to be little, if any, above normal when taken within a few seconds
after such exercise. It subsequently rises because the other factors
responsible for the increased pressure (quick heart and arteriole constric-
tion) are still in operation at the time the veins again become filled with
blood. The purely mechanical influence outlasts the exercise for a com-
paratively short time, whereas the nervous and hormone influences con-
tinue acting. This interpretation is supported by the observation that
the fall of blood pressure is greater when the subject is left standing
‘after a given amount of dumbbell exercise than when he is allowed to sit
with his elbows resting on his knees. In the standing position the pres-
sure on the abdominal veins is less and the hydrostatic effect of gravity
causes more blood to collect in the large veins (Cotton, Rapport and
Lewis**). Being purely mechanical in its causation, the preliminary fall
following dumbbell exercise can always be demonstrated if the observa-
tions are made at close enough intervals of time.
The mechanical response of the circulation to exercise acts therefore
through the rate of filling of the right heart with blood, and if this organ is
in a healthy condition, it will respond to the greater inflow by correspond-
ingly increased discharge. Like every other physiologic mechanism, the
heart works with a large factor of safety—a reserve power—and it is
the rate of venous filling that determines how much of this reserve must
be called upon to maintain the circulation. In isolated heart-lung prep-
arations Starling and his coworkers have very clearly demonstrated the
close dependence of cardiae output upon rate of venous filling and the
enormous range through which the systolic discharge can be made to
vary by altering this factor. As explained elsewhere, when the reserve
power of the heart is lessened, the rise in blood pressure following exer-
cise is longer in attaining its maximum, which is set at a higher level and
persists for a longer time. Observation of the extent of these changes
furnishes a most useful functional test of cardiac efficiency.
Other mechanical factors that augment the cardiac output depend on
the increased respiratory movements. ‘During each respiration the in-
crease in capacity in the thorax causes both an opening up of the thin-
walled veins, so that blood is aspirated towards them from the extra-
thoracic venous system, and a dilatation of the blood vessels of the lungs,
so that the blood finds its way from right to left heart more readily.
Although this dilatation will at first tend to cause more blood to collect
in the intrathoracic vessels and less to be pumped out of them, the expira-
tory act when it supervenes will, by compressing the veins, cause the
extra blood to be expelled into the left ventricle and thence into the
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412 THE RESPIRATION
arteries. It is obvious that increased depth and frequency of the respira-
tory movements will accelerate the bloodflow and tend to raise the arte-
rial blood pressure.
The above factors will come into play during most kinds of muscular
exercise such as walking, running, or swinging dumbbells, ete. There
are certain types of muscular effort, however, in which the mechanical
factors produce decidedly disturbing effects on the circulation. During
a sustained effort as, for example, in pulling against a resistance or in
attempting to lift a heavy load, the respirations are suspended, often after
a deep inspiration, and the contracted abdominal muscles press the dia-
phragm up into the thoracic cavity. After a preliminary squeezing out
of blood first of all from the veins of the abdomen into the thorax and
then from those of the latter into the systemic arteries, with a consequent
rise in arterial pressure, there comes to be a damming back of blood into
the peripheral veins, causing them to swell and, if continued, marked
cyanosis may develop. When such efforts are maintained for long, the
arterial pressure begins to fall, and this fall is very pronounced indeed
at the end of the effort, because, the compression being removed from the
abdominal and thoracic veins, these open up and form a large unfilled
blood reservoir.
A similar mechanism comes into play during expulsive acts such as
defecation, parturition, ete. In these the glottis is closed, usually after
a preliminary inspiration, and a powerful expiratory movement is per-
formed, with the consequence that the intrathoracic and intraabdominal
pressures rise considerably, greatly augmenting the systolic discharge
and causing the blood pressure to rise. Because of the obstruction to
the bloodflow in the large veins of the abdomen and thorax, however,
the later effect of the effort is to diminish the systolic discharge, but the
fall in blood pressure which this would be expected to occasion is masked.
The pressure remains high because other factors increasing the peripheral
resistance come into play. The fall in blood pressure following these acts
may be very marked indeed. Similar mechanical effects are produced
in the acts of coughing, sneezing, ete.
The capacity of the veins varies considerably with the position of the
body, and it is in order that we may cause alterations in this capacity
and therefore encourage a more rapid bloodflow that we stretch the body
after sitting for some time in a cramped position.
The Nervous Factor.—The vagus, vasoconstrictor and respiratory cen-
ters are all excited during muscular effort. In the earlier stages the
excitation depends entirely on nervous impulses transmitted to the cen-
ters, but later it depends on changes in the composition and temperature
of the blood flowing through them—the hormone factor. The initial
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CHANGES ACCOMPANYING MUSCULAR EXERCISE 413
stimulation of the centers must be due to cerebral impulses independ-
ently transmitted to the above centers, since the quickening of the pulse
and respirations may be observed to begin before the actual muscular
contractions. ; ;
The Hormone Factor—We have to consider first the nature of the
hormone, and secondly the mode of its action.
The Nature of the Hormones—The most important hormone is ear-
bonie acid, but when the exercise is strenuous and continued, or from
the very start is of such a nature that it uses up oxygen more quickly
than the blood can supply it to the muscles, lactic acid also appears.
Evidence for these statements can readily be supplied in man by analy-
sis of the expired air (for carbon dioxide) and of the urine (for lactic
acid) before and during muscular work. The real hormone in both cases
is believed to be an increase in the H-ion concentration of the blood.
There is, however, no direct proof of this assertion—that is to say, no
one has actually shown that a measurable change in the H-ion concentra-
tion of the arterial blood (for of course a change in the venous blood
would be of no significance) does occur before the changes believed to
be dependent upon acid production make their appearance. The well-
known buffer action of the blood (that is, its ability to take up con-
siderable quantities of acid or of alkali before any perceptible change
occurs in H-ion concentration) furnishes another-reason why doubt
must be cast upon the H-ion hypothesis. The most delicate means for
demonstrating a change in H-ion concentration of the blood consists in
finding the dissociation constant for hemoglobin and the results have
shown that acidosis develops during exereise at least at high altitudes
(Barcroft?). So far as we are aware, however, it has not been possible by
direct measurement (page 29) to detect a rise in H-ion concentration,
Of course it may well be that the sensitiveness of the various nerve
centers and other structures towards the H-ion concentration is very
much greater than our most refined and sensitive laboratory methods
can reveal. Such is at least commonly believed to be the case for the
respiratory center (see page 351), and it may also be so for those of
vascular tone and cardiac action. It is nevertheless possible that an
inerease in the free carbonic acid itself—the carbonate anion (-HCO,),
in other words—is the effective hormone. In the first stages of muscular
work, this increase would be due to greater production of CO,, whereas
later, especially when the work is strenuous, lactic acid would decom-
pose the NaHCO, of the blood, liberating -HCO,, which would become
added to that still being produced by the active muscles, and as the
NaHCO, (buffer substance) became gradually used up, would cause a
relatively greater and greater proportion of -HCO, to exist in a free
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414 THE RESPIRATION
state. That the CO, tension of the alveolar air should be found to be
lowered by prolonged muscular exercise in no way detracts from this
explanation, for it is dependent upon the greatly increased rate of
movement of air into and out of the alveoli (see also page 366).
One serious difficulty in accepting the.HCO, ion as the exciting hor-
mone of the nerve centers during muscular exercise depends on the ob-
servation that the alveolar CO, after some time is lower than normal.
If we accept Haldane’s teaching that there is accurate correspondence
between the tensions of CO, in arterial blood and alveolar air not only
during rest but also during muscular activity, then obviously we must
discard the HCO, hypothesis. Leonard Hill and Flack,?’ however, have
shown quite clearly both in experimental animals and in man that equi-
librium between the blood and alveolar tensions of CO, may fail to
oceur. When blood with excess of CO, is injected into the jugular vein
of dogs, the respiratory center is stimulated, as shown by the increased
breathing, which indicates that the CO,-rich blood must have passed
through the lungs without the excess of CO, being removed from it.
Hill believes that the diffusion of CO, out of the blood into the alveolar
air may be depressed in muscular exercise, and that this rather than the
appearance of lactic acid in the blood is responsible for the low CO, ten-
sions usually found present (see page 369). He points out in support
of this view that a person after exercise can hold his breath for a much
shorter time than is usual, and the CO, meanwhile mounts in the alveolar
air very rapidly. i
The only way by which progress may be made in a problem like that
under discussion is, however, to adopt some hypothesis and then to
gather evidence for or against it. At the present stage of our knowl- .
edge, the hypothesis usually adopted is that a slight change in H-ion
concentration of the blood is the effectual hormone. It is an hypothe-
sis which is supported by the parallelism between the effects observed
during muscular exercise and those produced by experimental increase
in H-ion concentration.
The Effects of the Hormone.—These may be classified as follows: (1)
strictly local effects on the muscles themselves; (2) effects on the heart;
and (3) effects on the nerve centers. The local production of acids in
the muscles will cause dilatation of the arterioles, for it has been shown
by various observers that acids cause relaxation of vascular muscle.
Even the capillaries themselves are said to be dilated by carbonic acid
(Severini). The effects produced on the heart by changes in H-ion con-
centration of the blood have been particularly studied by Starling and
Patterson,“ who, working on isolated heart-lung preparations, have
shown that the heart relaxes more and more and discharges less blood
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CHANGES ACCOMPANYING MUSCULAR EXERCISE 415
as the H-ion concentration of the perfusion fluid is increased by adding
CO, to the air ventilating the lungs.
The influence of changes in H-ion concentration of the blood on the
vagus and vasomotor centers is usually believed to. be stimulatory.
There is no doubt that an increase in Cg stimulates the vasoconstrictor
centers, not only of the medulla, but also, although much more feebly,
of the spinal cord. But it is a question whether any part of the rise in
systolic pressure during muscular exercise can be attributed to this
cause, for the enormously increased bloodflow which is known to occur
makes it problematical whether any vasoconstriction really occurs. If
it does so, it must be confined to the splanchnic area, where it would
have the effect of bringing about a redistribution of the total available
blood by expressing it from the viscera and sending it to the active
muscles.
The effect of increased H-ion concentration on the vagus center must
be insignificant. It is commonly believed that it would cause not what
is actually observed, a quickening, but rather a slowing of the heart rate.
But even this is doubtful. The slowing of the heart that is observed in
asphyxia, for example, is in part at least due to the increased intra-
cranial pressure, for when the carotid artery is connected with a mer-
cury valve so that the blood escapes as the pressure rises above the
normal level, no slowing of the heart is said to occur in asphyxia. As
Leonard Hill and Flack®" have shown, however, a part of the slowing is
due to the direct effect of CO,. If increase in the H-ion concentration
does affect the heart during muscular exercise, it must act by inhibiting
the vagus tone, which is opposite to the action which it is usually be-
lieved to have. The activity of the respiratory center is of course ex-
cited by increase in H-ion concentration, and this, as we have seen, will
cause important changes in the circulation because of the mechanical
effects which follow.
Along with hormones we must consider the effect of change in the
temperature of the blood. That this rises during muscular exercise is
well known, but that it should be responsible for many of the cardio-
vascular adjustments that occur is quite commonly overlooked. It is,
for example, very likely that rise in blood temperature is responsible
for the acceleration of the heart that occurs during exercise when both
vagi have been severed, and it no doubt is responsible for a part at
least of the vasodilatation and respiratory acceleration.
Finally, it is interesting to speculate as to the nature of the changes
that occur when the ‘‘second wind”’ is acquired during strenuous mus-
cular exercise. In running, for example, considerably more distress is
experienced a short time after the start than some time later. Three
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416 THE RESPIRATION
very definite changes occur at the time the relief is experienced—namely,
a slowing and steadying of the previously much quickened and irregu-
lar pulse, sweating, and a marked fall in the respiratory quotient. The
last mentioned change possibly gives a clue to the cause of the others.
In the early stages R.-Q. is raised, which indicates that relatively more
CO, is being expelled from the blood into the alveolar air than oxygen
is being absorbed, perhaps because of inadequate movement of blood
through the lungs. At the time of the adjustment it is possible that a
pronounced vasodilatation occurs in the muscles and coronary arteries.
The former change by lowering the arterial blood pressure will relieve
the pumping action of the heart, and the latter will improve its power of
contraction by supplying it with more oxygen.
RESPIRATION REFERENCES
(Monographs)
maaan a The Respiratory Function of the Blood, University Press, Cambridge,
Borrutau, H.: Nagel’s Handbuch der Physiologie, 1905, i, 29.
Douglas, C. G.: Die Regulation der Atmung beim Menschen, Ergebnisse der Physiol-
ogie, 1914, p. 338.
Hill, Leonard: Caisson Sickness, International Mcdical Monographs, E. Arnold,
London, 1912.
Keith, Arthur: The Mechanism of Respiration in Man, Further Advances in Physi-
ology, E. Arnold, London, 1909.
Schenck, F.: Innervation der Atmung, Ergebnisse der Physiologie, 1908, p. 65.
(Original Articles)
1Keith, Arthur: Cf. Further Advances.
2Hoover, C. F.: Arch. Int. Med., 1913, xii, 214; ibid., 1917, xx, 701.
3Lee. F. S., Guenther, A. E., and Meleney, H. F.: Am. Jour. Physiol., 1916, xl, 446.
4Meltzer, S. J.: Jour. Physiol., 1892, xiii, 218.
sHaldane, J. S., and Priestley, J. G.: Jour. Physiol., 1905, xxxii, 225.
Haldane and Douglas: Ibid., 1913, xlv, 235.
éHenderson, Y., Chillingworth and Whitney: Am. Jour. Physiol., 1915, xxxviii, 1.
Henderson and Morriss: Jour. Biol. Chem., 1917, xxx, 217.
7Krogh, A., and Lindhard: Jour. Physiol., 1913, xlvii, 30; ibid., 1917, li, 59.
sPearce, R. G.: Am. Jour. Physiol., 1917, xliii, 73; ibid., 1917, xliv, 369.
*Siebeck, R.: Skand. Arch. f. Physiol., 1911, xxv, 87; Carter, E. P.: Jour. Exper.
Med., 1914, xx, 21.
10Peabody, F. W., and Wentworth, J. A.: Arch. Int. Med., 1917, xx, 443.
11Lewis, T.: Jour. Physiol., 1908, xxxiv, 213, 233.
12Porter, W. T.: Jour. Physiol., 1895, xvii, 455.
1aChristiansen and Haldane, J.: Jour. Physiol., 1914, xlviii, 272.
14Boothby, W. M., and Berry, F. B.: Am. Jour. Physiol., 1915, xxxvii, 433; also
Boothby, W. M., and Shamoff, V. N.: Ibid., p. 418.
15Alcock, N. H., and Seemann, J.: Jour. Physiol., 1905, xxxii, 30.
1sScott, F. H.: Jour. Physiol., 1908, xxxvii, 301.
i7Stewart, G. N., and Pike, F. H.: Jour. Physiol., 1907, xx, 61.
17aCoombs, H. C., and Pike, F. H.: Proc. Soc. Exper. Biol. Med.,.1918, xv, 55. ;
isKrogh, A.: Skand. Arch. f. Physiol., 1910, xxiii, 248; and A. Krogh with Marie
Krogh, ibid., 179.
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CHANGES ACCOMPANYING MUSCULAR EXERCISE 417
19Haldane, J. S., and Priestley, J. G.: Jour. Physiol., 1905, xxxii, 225.
20Scott, R. W.: Am. Jour. Physiol., 1917, xliv, 196.
21Newburg, Means, and Porter, W. T. : Jour. Exper. Med., 1916, xxiv, 583.
22Hasselbalch, K. A., and Lundsgaard, Chr.: Biochem. Ztschr., 1912, xxxviii, 77, and
Skand. Arch. f. Physiol., 1912, xxvii, 13.
23Hooker, D. R., Wilson, D. W., and Connett, H.: Am. Jour. Physiol., 1917, xliii, 357.
24Campbell, J. M. H., Douglas, C. G., and Hobson, F. G.: Jour. Physiol., 1914, xlviii,
303.
25Lindhard, J.: Jour. Physiol., 1911, xxxviii, 337; Haldane, J. S., and Douglas, C. G.:
Thid., 1913, xlvi.
26Douglas, C. G.: Art, Ergebnisse der Physiologie, see Monographs.
27Barcroft, J.; see Respiratory Fuaction of Blood. ,
28Milroy, T. H.: Quart. Jour. Physiol., 1913, vi, 373.
29Fletcher, W. M., and Hopkins, F. G.: Jour. Physiol., 1907, xxxv, 247; also Fletcher,
W.M.: Jour. Physiol., 1913, xlvii, 361.
soRyffel, J. H.: Proce. Physiol. Soc. in Jour. Physiol., 1909, xxxix, 29.
31Pembrey, M. S., and Allen, R. W.: Jour. Physiol., 1909, xxxii, 18.
32Buckmaster, G. A.: Jour. Physiol., 1917, li, 105.
33Douglas, C. G., Haldane, J. 8., Henderson, Y., and Schneider, E. C.: Phil. Trans.
. Roy. Soe., 1913, 203, B, 185.
34Hill, Leonard, Macleod, J. J. R.: Jour. Physiol., 1903, xxix, 507; Hill, Leonard,
Greenwood, M., Flack, M., ete.: see Hill’s Caisson Sickness.
s5Haldane, J. S.: Deep Water Diving, Committee of the Admiralty (British), see
Hill’s Caisson Sickness.
3éCotton, T. F., Rapport, and Lewis, T.: Heart, 1918.
37Hill, Leonard, and Macleod, J. J.R.: Jour. Physiol., 1908, xxxvii, 77.
38Patterson, S. W., Piper, H., and Starling, KF. H.: Jour. Physiol., 1914, xlviii, 465.
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PART V
DIGESTION
CHAPTER XLVIII
GENERAL PHYSIOLOGY OF THE DIGESTIVE GLANDS
The function of digestion is to bring the food into such a condition
that it can be absorbed through the intestinal epithelium into the blood
and lymph. Carbohydrates are broken down as far as monosaccharides;
neutral fats are split into fatty acids and glycerine; and proteins are
broken down into the amino acids. The agencies which effect these
decompositions are the digestive enzymes, or ferments, contained in the
various digestive fluids or juices. The digestive juices are produced by
glands, which are most numerous in the upper levels of the gastro-
intestinal tract, the lower levels having as their main function that of
absorption of the digested products. In order that the masses of food
may be kept in a state of proper consistency, and that they may move
readily along the digestive canal, numerous mucous glands are also
seattered along the whole extent of the canal. Some of the digestive
glands, such as the main salivary glands, the pancreas, and the liver,
discharge their secretions into the digestive canal by special ducts,
whereas others, such as the isolated salivary gland follicles in the mouth,
the gastric glands and the crypts of Lieberkiihn in the intestine, do not
have an anatomically distinct duct, but discharge their secretions directly
into the digestive tube.
It will be. convenient to consider, first of all, certain properties that are
common to the digestive glands, and then, the conditions under which
each gland functionates during digestion.
MICROSCOPIC CHANGES DURING ACTIVITY
Structurally the active part of the glands, represented by the acinus
or tubule, is composed of a basement membrane lined internally with the
secreting epithelium. Outside the basal membrane are the lymph spaces
and blood eapillaries. After the gland has been at rest, the cells become
418
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 419
filled with granules or small globules, which are often so numerous as
almost entirely. to obliterate the nucleus. When the gland becomes active,
on the other hand, the granules or globules leave the cells, except for a
few which remain toward the lumen border. (Figs. 143 and 144.)
Fig. 143.—Cells of parotid gland showing zymogen granules: A, after prolonged rest; B, after a
ne moderate secretion; C, after prolonged secretion. (From Langley.)
These observations indicate that the granular or globular material must
represent part at least of the secretion of the glands. Sometimes, even
before they are extruded, the granules become changed into some differ-
ent material, as is indicated by the fact that they stain differently from
Fig. 144.—-Parotid gland of rabbit in varying states of. activity examined in fresh state. The
upper left-hand acini are resting. The upper right-hand acini are from a gland stimulated to
activity by injecting pilocarpine, and the two lower acini from one after stimulation of its sym-
pathetic nerve. (After Langley.)
those of the resting gland. It must not be thought, however, that an
extrusion of granules necessarily accompanies secretory activity, for
under certain conditions a copious secretion of water and inorganie salts,
- as well as a certain amount of organic material, may be produced with-
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420 DIGESTION
out any change in the arrangement of the granules. In such cases it has
been observed, as in the pancreas, that fine channels develop in the
protoplasm of the cell (see page 429).
From this histologic evidence it would appear that the gland cell
during rest is endowed with the property of building up out of the pro-
toplasm, as granules or globules, the material which is to serve as one of
the main organic constituents of the secretion. It is commonly believed
that this is the precursor of the active ferment of the secretion; hence its
name, zymogen. It has been shown that the process of separation of the
zymogen granules starts around the nucleus with the production of a
basophile substance, which in hardened specimens sometimes takes the
form of filaments. From this basophilic ergastoplasm, as it is called, the
granules are gradually formed, and then for some time continue to
undergo slight further changes, as is evidenced by the fact that the
staining reaction of those near the base of the cells differs from that of
those at the free margin. When the gland cell is excited to secrete,
the granules before being extruded, as noted above, often undergo a
definite change, becoming swollen and more globular in shape.
MECHANISM OF SECRETION
These histologic studies merely tell us that active changes, associated
with the production and liberation of certain of the constituents of its
secretion, are occurring in the gland cell, but they throw no light on the
mechanism whereby the gland eells secrete water and inorganic salts.
This may be dependent, to a certain extent at least, on differences in
osmotic pressure (see page 11). A possible explanation of the flow of
water is as follows: If a watery solution of some osmotically active sub-
stance is put in a tube, which is closed at one end by a membrane
impermeable to this substance and at the other by one permeable to it,
and the tube immersed in water, a continuous current will be
found to issue from the permeable end so long as there remains any
osmotically active substance in the tube. If we assume, then, that the
membranes at the two ends of the secreting cell are of such a nature that
the one next the basement membrane is impermeable to some osmotically
active substance manufactured by the cell, and the other toward the
lumen is permeable, it will be clear that, so long as this substance
exists in the cell, it will attract water from the blood, and the water
together with the osmotically active substance will be discharged into
the Wmen.
It“is‘possible that when anything excites the cell to secretory activity,
such as a nerve impulse or hormone, it does so by causing a change in
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 421
the permeability of the lumen border of the cell. This change in permea-
bility may be dependent upon alterations in surface tension brought
about by the migration of electrolytes to the border. That such a migra-
tion of electrolytes does actually occur has been demonstrated by A. B.
Macallum® who developed a microchemical test for potassium, and was
able to show that this electrolyte accumulates at the lumen border of the
cell during secretory activity. Potassium may be taken as a prototype
of electrolytes in general. Support is given to this view by the fact
that potassium always accumulates at the border of the cell through
which the secretion takes place. In the epithelium of the small intes-
tine, for instance, where the current goes in the opposite direction to that
in gland cells, the accumulation of potassium occurs at the portion of
the cell next the basement membrane.
Another possibility is that, when the gland becomes more active, the
molecules present in the cell become broken down into smaller molecules
and so raise the osmotic pressure of the cell content, with the result that
water is attracted from the blood.
When the gland is excited so that the zymogen granules, as well as
water and salts, are secreted, the primary change appears to involve the
granules only. Those near the lumen swell up by absorbing water, and
become converted into spheres in which salts are dissolved in smaller
proportions than exist in the lymph bathing the cells. These swollen
structures are then ruptured at the periphery of the cell and discharged
into the lumen. This discharge of a fluid containing fewer saline con-
stituents than the cell or surrounding blood plasma brings about in-
creased concentration in the remaining parts of the cell, a process which
possibly is assisted by a breaking up of molecules in the protoplasm itself,
and which causes an increase in osmotic pressure with a consequent
flow of water from the lymph to the cells and therefore from the blood
to the lymph.
OTHER CHANGES DURING ACTIVITY
Whatever may be the nature of the physiological changes that
are responsible for the secretory activity of the cell, the fact stands out
prominently that a considerable expenditure of energy is entailed. This
is indicated by the fact that considerably larger quantities of oxygen
are taken up by the gland when it is in an active state than when at
rest. Thus, the oxygen consumption of the resting submaxillary gland
of the cat may be increased five times during active secretion. On
account of this increased oxygen consumption it is not surprising that
it should be found that the secretory activity of the cell is greatly im-
paired by a deficiency in oxygen.
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422 DIGESTION
These active processes occurring in the gland when it is excited to
secrete are associated with changes in electric reaction and in the
volume of the gland. The electric changes have been most extensively
studied in connection with the salivary gland. Cannon and Cattel,® by
connecting a galvanometer with nonpolarizable electrodes, one placed
on the gland and the other on neighboring connective tissue, were able
to show that with each period of active secretion a current of action was
set up. This was first discovered by Rose Bradford and Bayliss, and
has been carefully studied by Gesell.“* That the electric current is
definitely associated with the secretion of saliva and is not caused by
the vascular changes which usually accompany this act was shown by
its occurrence when the blood supply was shut off from the gland,
and by its absence when there was no secretion even though the vascular
changes were brought about; neither is the electric change due to the
movement of fluid along the duct, as evidenced by its persistence after
ligation of the duct.
With regard to change in volume, it sis be expected, on account. of
the greater vascularity of the gland accompanying activity, that this
would increase. On the contrary, however, it has been shown to de-
crease, because of the large quantity of fluid secreted from the gland cells.
The action of two drugs on the gland cells is of considerable physio-
logie importance: that of atropine, which paralyzes the secretion, and
that of pilocarpine, which stimulates it. We shall see later how this
information may be used in working out the exact mechanism of the
different glands.
Important observations concerning the relationship of glandular activ-
ity to the blood supply have been made by experiments in which glands
were artificially perfused outside the body. When the submaxillary
gland of the dog is perfused with oxygenated Ringer’s solution, stimula-
tion of its nerve supply does not produce the usual secretion, but if the
Ringer’s solution is mixed with blood plasma, the nerve stimulation has its
usual effect for a short time. Although no secretion occurs when
oxygenated Ringer’s solution is perfused alone, the usual vascular
changes still occur in the gland. The results seem to indicate that the
presence of some constituent of the blood plasma is essential for the
change in the permeability of the cell wall necessary for the process of
secretion. Similar results have been obtained during artificial perfusion
of the pancreas when secretin was used as the stimulus.
CONTROL OF GLANDULAR ACTIVITY
Having outlined the general nature of the changes occurring in gland
eells during their activity, we may now proceed to study the nature of
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Center for :
cranial secre Py fi
Ves
: odlgior ne Otic ganglion
Ui igemina// 5emilunar Gasserian) ganglion
ZChorda tympani nerve
Z/ mall superficial
5 ZS petrosal nerve parotid duct
LS Inf, max. div. N.V (Stenson's)
Cs )
n’5)
( duct (Whar:
Medulla gan g ublin ual
Parotid gland—4Vaso constrictor fibers
f Mauaneiene secretary fibers
Out going sympathetic
Tami communicantes
Post ganglionic fibers are
dotted thus ----
Fig. 145.—Diagrammatic representation of the innervation of the salivary glands in the dog. (From
Jackson.)
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 423
the process by which this glandular activity is controlled. Two mechan-
isms of control are known: (1) by the nervous system, and (2) by means
of hormones.
Nervous Control.—Control through the nervous system is most marked
—indeed it may be the only means of control—in glands which have to
produce their secretion promptly, whereas hormone control pre-
dominates in those in which prompt changes in secretory activity are not
required. Thus, nervous control alone is present in the salivary glands,
whereas hormone control is predominant in the pancreas, intestinal
glands and liver. The gastric glands are partly under nervous control,
and partly under hormone control. It should be pointed out here that
the glands of the body other than the digestive glands are also subject to
nervous or hormone control according to the promptness with which they
are required to secrete. The lachrymal and sweat glands, and the venom
glands of reptiles, for example, are practically entirély under nervous
control, whereas most of the ductless glands, with the exception of the
adrenals, are mainly under the influence of hormones.
The exact nature of the nervous control of glandular function has,
therefore, been most extensively studied in the salivary glands, and that
of the hormonic in the pancreas. With regard to the salivary glands,
the following points are of importance: Their nerve supply comes from
two sources: the cerebral autonomic, and the sympathetic autonomic
(see page 877). These two nerve supplies have usually an opposite influ-
ence on the secretory activity of the glands, and very frequently also on
the vascular changes that accompany secretory activity.
On account of its ready accessibility, the submaxillary gland in the
dog and eat has been most thoroughly investigated. The cerebral auto-
nomic nerve in this case is represented by the chorda tympani, and the
sympathetic autonomic by postganglionic fibers that run from the
superior cervical ganglion to the gland along its blood vessels (Fig. 145).
After tying a cannula into the duct of the gland, it will be found in the
dog that stimulation of the chorda tympani produces an immediate and
abundant secretion of thin watery saliva accompanied by a marked
dilatation of the blood vessels of the gland.
That this secretion is not dependent on the vasodilatation is easily
shown by repeating the experiment after administering a sufficient dose
of atropine to paralyze the secreting cells. Stimulation of the nerve then
produces a vasodilatation but no secretion. The same conclusion is
arrived at by an experiment of an entirely different nature; namely, by
observing the pressure produced in the duct when the chorda tympani is
stimulated. This pressure rises considerably above that in the arteries,
so that no such physical process as mere diffusion can be held accountable
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424 DIGESTION
for the secretion, and therefore vasodilatation alone can not be respon-
sible for it. If the sympathetic nerve supply is stimulated, a very scanty,
thick secretion takes place accompanied by vasoconstriction.
Repetition of these experiments in the cat yields different results,
particularly with regard to the influence of the sympathetic, a copious
secretion being produced by such stimulation. The histologic changes
produced in the gland cells are marked after sympathetic stimulation,
but very slight, if present at all, after chorda stimulation.
The outstanding conclusion which may be drawn from these results
is that two kinds of secretory activity are mediated through the nerves;
one causing a thin watery secretion, containing only a small percentage
of organic matter, and the other, a thick viscid secretion with a large
-amount of organic material. To explain these differences the hypothe-
sis has been advanced that: there are really two kinds of secretory
fibers, called secretory and trophic, the former having to do with the
secretion of water and inorganic salts, and the latter with the secretion
of organic matter; i.e., with the extrusion of the zymogen granules.
Certain authors (Langley) believe that such an hypothesis is unneces-
sary, and that the different results are dependent upon the concomitant
changes in the blood supply produced by stimulating one or other nerve.
That there are really different kinds of true secretory fibers is, however,
evident from the following experiment. If the duct of the gland is
made to open through a fistula in the cheek, secretion of saliva through
the fistula can be induced by placing various substances in the mouth, such
as meat powder or hydrochloria acid. If the experiment is performed in
such a way that the bloodflow through the gland can be observed, it will
be found that the saliva produced by the stimulation with, the meat powder
contains a very much higher percentage of organic material than does that
produced when hydrochloric acid is the stimulant, whereas the vascular:
changes. in the gland and the inorganic constituents of the saliva are the
same in both cases. Since stimulation of the chorda tympani causes the
secretion of a watery saliva, while that caused by stimulation of the
sympathetic is thick, it might be thought that the secretory fibers were
contained in the former and the trophic fibers in the latter nerve; that
this is not the case can be shown by a repetition of the above experiment
in animals from which the superior cervical ganglion has been removed.
The same results are obtained, indicating that the chorda tympani con-
tains both secretory and trophic fibers.
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CHAPTER XLIX
PHYSIOLOGY OF THE DIGESTIVE GLANDS (Cont’d)
THE HORMONE CONTROL
This is exhibited best in the case of the pancreas. The crucial experi-
ment demonstrating that this gland is not primarily dependent upon
nervous impulses for the control of its activity was performed by Bay-.
liss and Starling.? Starting with the well-known fact that the application
of weak acid to the duodenal mucous membrane excites secretion of pan-
creatic juice, these workers carefully severed all the nerve connections of
a portion of the duodenum, and found on again applying acid to the mucous
membrane that the secretion persisted. To explain this result they postu-
lated that the acid must cause some substance to be liberated into the
blood stream, which carries it to the pancreas, the cells of which it then
excites to activity. To test this hypothesis they scraped off the mucous
membrane of the duodenum and ground it in a mortar with weak hydro-
chloric acid (0.6 per cent), and, after boiling the solution so as to remove
the protein and nearly neutralizing it, they obtained a fluid which, when
injected intravenously, immediately caused a copious secretion of pan-
ereatic juice.
Accompanying the secretion, however, a marked fall in arterial blood
pressure was observed, making it possible that the secretion might have
been due to a vasodilatation occurring in the pancreatic blood vessels. To
eliminate this possibility they prepared an extract that was free of the
depressor substances by extracting intestinal epithelium without any of the
submucous tissue. The resulting extract had merely the secretory effect
and produced no fall in blood pressure. This secretagoguary substance
they named secretin.
Further evidence that the action of secretin is independent of the
depressor substances has been obtained by taking advantage of the fact
that the depressor substance is more soluble in alcohol than the secretin.
If an acid decoction of duodenal mucous membrane is poured into abso-
lute alcohol, a precipitate is formed. If this precipitate is redissolved
in water and reprecipitated several times by absolute alcohol, then after
drying a white powder is obtained, which is easily soluble in water. The
resulting solution injected intravenously has a powerful secretory action,
but produces no effect on blood pressure. The concentrated alcoholic
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426 DIGESTION
liquor, on the other hand, when similarly injected produces a marked fall
in blood pressure. It is believed that this effect is due to the action of
8-imidazolylethylamine. A very strong preparation of secretin can also
be prepared by the method of Dale and Laidlaw,’ which depends on pre-
cipitation by mercuric chloride.
Secretin does not exist preformed in the epithelial cells, as is shown by
the fact that an extract, made with neutral saline solution, does not as a
rule, have any secretory action when injected intravenously. Sometimes
a slight secretion may be produced, but this is probably to be explained
by the fact that some secretin remains behind in the cells ag a result of a
preceding phase of activity. If, on the other hand, the above neutral or
_Slightly alkaline opalescent solution of the mucous membrane is boiled
with acid, secretin may become developed in it. The interpretation put
upon these results is that a substance, called prosecretin, exists in the
epithelial cells, and that this becomes converted into secretin by the action
of acid on the cells. The secretin thus produced is then taken up by the
blood, none of it passing into the intestinal canal, because the free borders
of the cells are impervious to secretin. That this is actually the case has
been shown by finding that the introduction of neutralized secretin solu-
tion into the duodenum, or other parts of the small intestine, does not
cause a secretion of pancreatic “juice.
We know practically nothing concerning the chemical nature of secretin.
Being soluble in about 90 per cent alcohol and in fairly weak acids, it can
not belong to any of the better known groups of proteins. As it is
readily diffusible through parchment membrane, it can not be of very
complex structure, and as it withstands heat, it can not be an enzyme.
It rapidly deteriorates in strength in the presence of alkalies.
Any acid when applied to the mucous membrane is capable of producing
secretin, and so are certain other substances, such as mustard oil. Watery
solutions of saccharose or urea, when rubbed up with the duodenal mucosa
in a mortar, produce secretin solutions of varying activity, but they do
not in the living animal excite pancreatic secretion when applied to the
duodenum. Secretin is very susceptible to destruction by such digestive
enzymes as those present in the pancreatic, gastric, and intestinal juices.
That secretin is present in the blood when acid is in contact with the
duodenal mucosa has been shown by the fact that injection into a normal
dog of blood from one in which secretin formation is going on (as a
result of acid in the duodenum), excites pancreatic secretion.
The pancreatic juice produced by the injection of secretin, like that
which is produced under normal conditions, does not contain any active
trypsin, but instead contains its precursor, trypsinogen. This becomes
converted into trypsin in the intestine, being activated by contact with
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 427
enterokinase, an enzyme present in the intestinal juice. By such a mechan-
ism the mucosa of the pancreatic duct is protected against autodigestion
by trypsin.
NERVOUS CONTROL OF PANCREAS
Prior to the discovery of secretin, Pavlov! and his pupils had published
numerous experiments purporting to show that the secretion of pancreatic
Fig. 146.—Pancreatic acini stained with hematoxylin. The acini at the top and to the left
of the figure are from a resting gland, those to the right being from one that had been secreting
for over three hours as a result of acid in the duodenum. The lowermost figure is from a gland
the vagus nerve supply of which had been stimulated off’and on for several hours. Note that
the zymogen granules are extruded only after vagus activity but not after secretin activity. (From
Babkin, Rubaschkin and Ssawitsch.)
juice is controlled through the vagus nerve. The amount of secretion
produced by nervous stimulation was, however, never found to be so large
as that produced by secretin, and for several years after the discovery of
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428 DIGESTION
the latter hormone, much doubt existed as to the correctness of Pavlov’s
claim. As in many other fields of physiologic science,. investigators at-
tempted to show that one or the other mechanism obtained, and they were
not inclined to consider the possibility that both mechanisms might exist
side by side. That such is the case, however, is clear from the most recent
work, in which it has been found that if proper precautions are taken,
repeated stimulation of the vagus nerve does call forth a secretion of
pancreatic juice which, besides being less copious than that following
IL. Tit,
‘Fig. 147.—Three preparations of pancreatic acini stained by eosin orange toluidin blue. The
acini of Fig. I were from a gland after vagus stimulation, and it is noted that besides free ex-
trusion of the granules, globules staining with orange (and appearing in deep black in the photo-
graph) have formed and may be present in the ductules. Some of the globules, however, change
in their staining properties, becoming light red (dark gray in photograph). The acini in II and III
were from glands excited by secretin. No globules appear; the granules remain, and fine canaliculi
‘appear in the clear protoplasm. (From Babkin, Rubaschkin and Ssawitsch.)
secretin injection, differs from it in the important fact that it contains
not trypsinogen but active trypsin. Since the normal pancreatic juice
contains trypsinogen, this last mentioned fact would appear to indicate
that vagus control of the normal secretion can not be an important affair.
The vagus secretion of pancreatic juice is, moreover, paralyzed by atro-
pine, which has no action on the secretin mechanism (cf. Bayliss).
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PHYSIOLOGY OF THE DIGESTIVE GLANDS ° 429
The copious secretion of pancreatic juice produced by secretin, on the
one hand, and the scanty, thick secretion produced by vagus stimula-
tion, on the other, calls to mind similar differences observed in the secre-
tion of saliva as the result of chorda-tympani or sympathetic stimulation.
It will be remembered that from these latter results it was concluded
that there must be secretory and trophic fibers concerned in the control
of the activities of gland cells. Interesting corroboration of this conclusion
has recently been obtained by histologic examination of the pancreas fol-
lowing secretin or vagus activity. After the repeated injection of secre-
tin, it is difficult to observe any signs of fatigue in the cells; the zymogen
granules remain practically as numerous as in a resting gland, but in the
clear protoplasm of the outer third of the cell, it is said that fine channels
of fluid can be seen. Through these channels water is believed to pass
from the blood towards the lumen and in its course to carry with it some
of the zymogen granules, without, however, changing them. Thus, when
the gland cells are stained with eosin and orange, after secretin activity
some of the zymogen granules can occasionally be seen’ in the lumen of
the acini stained with eosin like those in the cell itself. After vagus
stimulation the appearances are different; not only are the granules more
freely extruded from the cells, but they undergo a preliminary change;
they lose the property of staining with eosin and become stained with
the orange, at the same time increasing in size so as to form vacuoles.
These vacuoles may wander into the ductules, and when they are present
here they are stained by orange (Figs. 146 and 147) (Babkin, ete.74).
Why there should be both a nervous and a hormone control of the pan-
creatic secretion is not clear. This gland, unlike the gastrie and salivary
glands, is not called upon to become active all of a sudden, and it is dif-
ficult to see what could serve as the normal stimulus operating through
the nervous pathway. Taking it all in all, it is probably safe to con-
clude that the nervous mechanism is relatively unimportant, and that
under normal conditions it seldom if ever is called into operation. Cor-
roboration for this view is afforded by the fact, above mentioned, that
the pancreatic juice produced by vagus stimulation contains active tryp-
sin, which is not the case with normal pancreatic juice.
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CHAPTER L
PHYSIOLOGY OF THE DIGESTIVE GLANDS (Cont’d)
Up to the present we have been concerned with the physiologic activi-
ties of digestive glands in general, but now we must study each of them
separately in order to find out the conditions under which they become
stimulated to activity in the normal process of digestion. The secretion
of each gland has a definite role assigned to it in the complex and lengthy
process of digestion. It takes up its work where the preceding secre-
tion left off; e.g. the pepsin of gastric juice digests protein so far as
proteoses and peptone; the trypsin of pancreatie juice then attacks the
proteoses and peptone, and the resulting lower degradation products
are finally attacked by the erepsin of the intestinal juice. The secre-
tions of the various glands are, therefore, required in a certain definite
order—they are correlated; and we must now give some attention to the
precise condition upon which the activity and correlation depend.
THE NORMAL CONDITIONS UNDER WHICH THE GLANDS
BECOME STIMULATED TO INCREASED ACTIVITY
To make possible such observations on the normal activities of the
glands, a preliminary operation has to be performed so as to bring the
duct of the gland to the surface of the body and permit of the observa-
tion of its secretory activity after the animal has recovered from the
immediate effects of the operation. We owe to Pavlov: the surgical
technic by which these conditions can be fulfilled. The general principle
of the operation, in the case of glands provided with ducts, consists in
making a circular cut through the mucous membrane surrounding the
opening of the duct and then, after dissecting the duct free, stitching
the edges of the cut to the skin wound. Healing then takes place without
the formation in the duct of any stricture due to the cicatricial tissue. After
the wound has healed, the secretion can readily be collected in a receiver
attached over the duct fistula, the animal being in every other way in a
perfectly normal condition. In the case of glands not provided with a
duct, other methods must be adopted to collect the secretions. These
will be described elsewhere.
dobbs staal
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 431
THE NORMAL SECRETION OF SALIVA
The duct fistula can in this case be made either for the submaxillary
gland, representing a mucous gland, or for the parotid, representing a
serous gland. Under ordinary conditions there is very little secretion
from either duct. When secretion occurs, it is, of course, caused by
influences acting on a nerve center or centers in the medulla oblongata,
the exact location of which for the different glands has been worked out
in recent years by Miller.2 The impulses acting on these centers may be
transmitted along afferent nerves coming from the mucous membrane of
the mouth, nares, ete., or by impulses which we may call psychic, trans-
mitted from the higher nerve centers. The reflex secretions caused by
impulses traveling by the afferent nerve from the mouth, eté., have been
called «unconditioned, and those from the higher nerve centers, condi-
tioned. With regard to the former, there is considerable discrimination
in the type of stimulus that will be effective. Thus, if the dog—for most
of the experiments have been performed on this animal—is given meat,
a secretion of thick, mucous saliva will be observed to occur (submaxil-
lary gland). On the other hand, if the meat is dried and pulverized,
the secretion which it calls forth will be very copious and watery (par-
otid gland). There is, then, an obvious association between the nature
of the secretion and the function it will be called upon to perform when
it becomes mixed with the food. The mucous secretion called forth by
meat will serve to lubricate the bolus of food and thus facilitate its
swallowing, whereas the thin watery secretion produced by the dry
powder will have the effect of washing the powder from the mouth.
It is evident that the mechanical condition of the food partly deter-
mines its exciting quality. Mechanical stimulation of the mucosa alone is,
however, not an adequate stimulus, for if pebbles are placed in the mouth,
little secretion occurs, but if sand is placed in the mouth, secretion immedi-
ately becomes copious. The nerve endings also respond to chemical stimuli.
Thus, weak acid causes a copious secretion, while alkali has no effect;
disagreeable, nauseous substances also excite secretion. The above dif-
ferences in the response of the glands according to the mechanical condi-
tion of the food has been observed only in the case of the parotid gland,
increase in the submaxillary secretion being obtained only when actual
foodstuffs are placed in the mouth. :
The investigations that have been made on the conditions of psychic
secretion of saliva are still more interesting and important. Their im-
portance depends not so much on the information they give us concern-
ing the secretion of saliva as such, as on the methods they furnish us for
investigating the various conditions that affect the psychic processes
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432 DIGESTION
associated with the taking of food. It is from the psychic rather than
from the physiologic standpoint, therefore, that these observations are
of importance, for they permit us, by objective methods, to study on
dumb animals problems that would otherwise be beyond our powers of
investigation. Many of the results, with their bearing on the functions
of the higher nerve centers, have been discussed elsewhere. Meanwhile,
however, even at the risk of repetition it may not be out of place to cite
a few of the most interesting experiments.
If we tease a hungry animal with food for which he has a great appe-
tite, a copious secretion of saliva immediaiely occurs. If we go on teas-
ing him without giving him food, and repeat this procedure on several
succeeding days, it will be found that gradually he no longer responds
to the teasing by inereased salivation. Evidently, therefore, the reflex
is conditioned upon the animal’s afterward receiving the food.
The experiment may be performed in another way. If, for example,
we offer the animal some food for which he has no appetite, no secre-
tion of saliva will oceur; but, if at the end of the process we give him
appetizing food, it will be found after repeating this procedure on
several successive days that the presentation of the unappetizing food
calls forth a secretion. He has learned to associate the presentation of
unappetizing food with the subsequent gratification of his appetite. The
experiment can even be performed so that a definite interval of time
elapses between the application of the stimulus and the salivation: if
the animal is teased on successive days with food for which he has an
appetite but is not given the food until after ten or twenty minutes,
presentation of this food will come to be followed by salivation—not
immediately, but after the exact interval of time that had been allowed
to intervene in the training process. During this interval there must be
an inhibition of psychic stimulation of the salivary centers by other nerve
centers. It is of great interest that this inhibition may itself be inhib-
ited by various forms of stimulation of the nervous system (see page 858).
THE SECRETION OF GASTRIC JUICE
Methods of Investigation
There being no common dutt, the secretion of the gastric glands is a
much more difficult problem to investigate than is that of glands which,
like the salivary, are supplied with ducts. One of the most interesting
chapters in the history of physiology concerns the methods which from
time to time have been evolved for the collection of this juice and for
studying the digestive processes in the stomach. Prominent among the
problems confronting the earlier investigators was the question whether
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 433
the main function of the stomach is to crush or triturate the food or to
act on it chemically. The great French scientist Reaumur and a little
later the Italian Abbé Spallanzani (1729-1799) attacked this problem by
methods that anticipate those of Rehfuss and Einhorn. Spallanzani ulti-
mately devised the method of swallowing small perforated wooden tubes
containing foodstuffs and covered by small linen bags. After the bags
were passed per rectum, he found that considerable erosion or digestion
of the food had occurred, but that the wooden tubes, however thin-
walled they might be, were not crushed. In order to secure samples of
the gastric juice free from food, the only method available to the older
investigators consisted in swallowing sponges attached to threads, which
after being for some time in the stomach were withdrawn and- squeezed
dry of juice.
The next great contribution came from this country, where, in 1833,
Dr. Beaumont, while a surgeon in the service of the American troops
located at Mackinaw, made observations on a Canadian voyageur by the
name of Alexis St. Martin, who by the premature discharge of his gun
had wounded himself in the stomach, the wound never healing but leav-
ing a permanent gastric fistula. Beaumont arranged to keep Alexis St.
Martin in his service for several years, during which time he made
numerous observations on the process of digestion in the stomach—
observations many of which are of great value even at the present day.
By none of these methods, however, could a sample of pure gastric
juice be secured while the digestive process was actually in progress.
To make the collection of such a sample possible, Heidenhain devised a
method of isolating portions of the stomach wall as pouches opening
through fistule on the abdominal wall. The results of Heidenhain’s
experiments are, however open to the objection that the secretion in
the isolated pouches may not really correspond to that oceurring in the
main stomach, since the connections of the pouches with the central
nervous system must have been severed. In order that these connec-
tions might remain as nearly intact as possible, the Russian physiologist,
Pavlov, devised an ingenious operation in which the pouch, or ‘‘minia-
ture stomach,’’ remains connected with the main stomach through a con-
siderable width of mucous and submucous tissue: The essential nature
of this operation will be evident from the accompanying degra,
(Fig. 148).
The most recent investigations have been made by Cannon? and by
Carlson.* The former fed animals food impregnated with bismuth sub-
nitrate, and then exposed the animal to the x-rays. A shadow is
produced by the food mass in the stomach, and from the changes in the
outline of this shadow facts have been collected, not only concerning the
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434 DIGESTION
movements of the viscus, but also concerning the rate of discharge of
food into the intestine and therefore the duration of the gastric digestive
process. Carlson’s contribution has been rendered possible by his good
fortune in having in his service a second Alexis St. Martin, a man with
complete closure of the esophagus and a gastric fistula large enough to
permit of direct inspection of the interior of the stomach. Seizing the
opportunity thus presented, Carlson during the last four or five years
has devoted his attention exclusively to a thorough investigation, not
only of the movements of the stomach, but also of the rate of secretion
of the gastric juice under different conditions. He has also, with praise-
worthy enthusiasm and keen scientific spirit, extended his observations
both on laboratory animals and on himself and his coworkers, so as not
~/
ny
Fig. 148.—Diagram of stomach showing miniature stomach (,S) separated from the main stomach
(V) by a double layer of mucous membrane. 4A.4., is the opening of the pouch on the abdominal
wall. (Pavlov.)
to incur the error, which is all too frequently made, of confining the
observations to one animal.
The Nervous Element in Gastric Secretion
The first stimulus to the secretion of gastric juice is nervous in origin,
and is dependent on the gratification of the appetite and the pleasure of
taking food. This fact, after having been suggested by observations
made in the clinic, was first thoroughly investigated by Pavlov, who for
this purpose observed the gastric secretion flowing either from a fistula
of the stomach itself, or from a ‘‘miniature stomach,’’ in dogs in which
also an esophageal fistula had been established. When food was given
by mouth to these animals, it was chewed and swallowed in the usual
manner, but before reaching the stomach, it escaped through the esopha-
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 435
geal fistula. This experiment is known as that of ‘‘sham feeding.’’
Within a few minutes after giving food the gastric juice was found to
be secreted actively, and if the feeding process was kept up, which could
be done almost indefinitely since the animal never became satisfied, the
secretion continued to flow. Thus, in one instance Pavlov succeeded in
collecting about 700 c.c. of gastric juice after sham feeding an animal
for five or six hours in the manner above described.
After the stomach has emptied itself of the food taken with the pre-
vious meal, it is said by Pavlov to contain only a little alkaline mucus.
The more recent work of Carlson, however, shows that this is not strictly
the case, there being more or less of a continuous secretion of gastric juice
in the entire absence of food. The amount varies from a few c¢.c. up to
60 ¢.c. per hour, more secretion being produced when it is collected every
five or ten minutes than if it is collected every thirty or sixty, thus
indicating that, ordinarily, some escapes through the pylorus into the
duodenum. The secretion contains both pepsin and hydrochloric acid.
As to the cause of this continuous secretion, little is known. It may be
an example of the periodic activities of the digestive glands described by
Boldyreff, or it may in part be due to a psychic stimulation dependent
upon the thought of food. That the latter is probably not the cause, is
indieated by the fact that, at least in Carlson’s patient, the psychic juice
could not be made to flow, short of giving food.
The sham feeding causes stimulation of the gastric secretion through
impulses transmitted to the stomach along the vagus nerves; for it has
been found, in animals in which the vagus nerve has been eut, that the
sham feeding no longer induces a secretion of gastric juice. The ques-
tion therefore arises as to how the nerve center is stimulated. Three
possible causes may be considered: (1) mechanical stimulation of the
sensory nerves of the mouth; (2) chemical stimulation of the nerves;
(3) the agreeable stimulation of the taste buds and olfactory endings
concerned in the tasting of food. In investigating these possibilities,
mechanical stimulation was readily ruled out by showing that mere
taking of solid matter in the mouth did not excite any secretion, although
it might cause a flow of saliva. Mere chemical stimulation could not be
the cause, for no secretion was induced by placing substances such as
acetic acid or mustard oil in the mouth. By exclusion, then, it would
appear that the adequate stimulus must consist in the agreeable stimula-
tion of the taste buds, ete—that is to say, in the gratification of appetite.
Further justification for this conclusion was readily secured by noting
that foodstuffs for which the animal had no particular desire or appe-
tite failed to excite the secretion. Most dogs, for example, although
they may take it, are not particularly fond of bread, and when fed with
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436 DIGESTION
it, these animals did not produce any appetite juice. In one animal that
showed considerable liking for bread, active secretion occurred when he
was fed with this foodstuff.
Pavlov further noted that usually it was not necessary actually to
allow the animal to take the food into his mouth, but that mere teasing
with savory food was sufficient to cause the secretion, and that in
highly sensitive animals even the noises and other events usually asso-
ciated with feeding time were sufficient to excite the secretion. In the
case of a hungry animal, the mere approach of the attendant with food,
or some other noise or action definitely associated with feeding time,
was sufficient to excite the secretion. The appetite juice when started
was found to persist for some time after the stimulus causing it had
been removed.
Carlson has succeeded in confirming in man most of these observa-
tions. He noted, however, that the secretion produced by seeing or
smelling or thinking of food is much less than would be expected from
Pavlov’s observations on dogs. Even when his subject was hungry,
Carlson did not observe that the bringing of a tray of savory food into
the room caused any secretion of gastric juice. It is, of course, to be
expected that the quantity of the psychic secretion will not be the same
in different individuals. It has been observed, for example, by Pavlov
to vary considerably in the case of dogs, and it.is very likely that it will
vary still more in man, with his more highly complicated nervous system.
In no case could Carlson observe any secretion of gastric juice produced
by having his patient chew on indifferent substances, or by stimulating
the nerve endings in the mouth by substances other than those directly
related to food.
In man the rate of secretion is proportional to the palatability of the
food, the smallest amount, during twenty minutes’ mastication of pal-
atable food, being 30 ¢.c. and the largest 150 ¢.c., in a series of 156 obser-
vations. A typical curve showing the amount of the secretion is given
in Fig. 149. To construct this curve the gastric juice was collected dur-
ing five-minute intervals while the man was chewing a meal of average
composition and of his own choice. An interesting feature depicted on
this curve is that the secretion rate was highest in the last five-minute
period, this being the time during which the dessert was being taken,
for which this man had a great relish. Quite clearly there was a direct
relation between the rate of the secretion of the appetite juice and’ the
palatability of the food. It will further be observed that it took only
from fifteen to twenty minutes after discontinuing the chewing before
the juice returned to its original level. :
The practical application of these facts in connection with the hygiene
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 437
of diet and the feeding of patients during convalescence, is obviously
very great. However perfect in other regards a diet may be, it will
probably fail to be digested at the proper rate unless it is taken with
relish. Frequent feeding with favorite morsels is more likely to be fol-
lowed by thorough digestion and assimilation than occasional stuffing
with larger amounts. We see too in these experiments an explanation
of the well-established practice of starting a meal with something
savory. A hors d’oeuvre is nothing more than a physiologic stimulant
to appetite. It is also interesting from a practical standpoint to observe
that with those who have a keen relish for sweetmeats the taking of des-
sert has a real physiologic significance, for, as in Carlson’s patient, it
stimulates toward the end of a meal a further secretion of the gastric
20
/ Zz Ihe
wy \
SS
os
0 5 10° 15 20° 25° 30° 35 40° 45 50° 55
Chewing food
Fig. 149.—Typical curve of secretion of gastric juice collected at 5-minute intervals on mas-
tication of palatable food for 20 minutes. The rise in secretion during the last 5 minutes of
opens is due to chewing the dessert (fruit) for which the person had great relish. (From
‘arlson.
juice, and thus insures a more rapid digestion of the food. Good cooking,
it should be remembered, is really the first stage in digestion, and it is
the only stage over which we can exercise voluntary control.
The Hormone Element in Gastric Secretion
Although gastric digestion is initiated by the appetite juice, it is
clear that this alone can not account for all the secretion that occurs
during normal gastric digestion. After an ordinary meal gastric diges-
tion lasts usually about four hours, whereas we have seen, particularly
from Carlson’s observations, that the appetite juice lasts only for some
fifteen or twenty minutes after the exciting stimulus has been removed.
The appetite juice, in other words, serves only to initiate the process of
secretion, and the question arises, What keeps up the secretion during
the rest of gastric digestion? The answer was furnished by Pavlov, who
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438 DIGESTION
observed animals in which not only a miniature stomach had been made,
but a fistula into the main stomach as well. The behavior of the secre-
tion of gastric juice as a whole could be followed by collecting that
which was secreted in the miniature stomach, for it was shown, in con-
trol experiments, that this secretion runs strictly parallel with that in
the main stomach, being quantitatively a definite fraction of it—accord-
ing to the relative size of the miniature stomach—and qualitatively
identical. The miniature stomach, in other words, mirrors the events
of secretion in the main stomach.
It was observed that when the animal was allowed to take the food
into the main stomach by the mouth and esophagus, the secretion from
the miniature stomach continued to flow until the process of gastric
digestion had been completed, a result which was quite different from
that obtained after sham feeding. The only possible explanation for this
result is that the food in the stomach sets up secretion as a result of
local stimulation. To investigate the nature of this local stimulation,
whether mechanical or chemical, food and other substances were placed
in the main stomach through the gastric fistula without the animal’s
knowledge so as to avoid possible psychic stimulation, and the secretion
observed from the miniature stomach. When the mucous membrane of
the main stomach was stimulated mechanically, as by placing inert
objects such as a piece of sponge or sand in the stomach, no secretion
occurred. Evidently, therefore, the stimulus is dependent upon some
chemical quality of the food.
By introducing various foods it was found that there is considerable
difference in the degree to which they can excite the secretion. Water,
egg white, bread and starch, were all found to have very little if any
effect. On the other hand, when protein that had been partly digested
by means of pepsin and hydrochloric acid was introduced into the
stomach, it immediately called forth a secretion. The conclusion is that
the partly digested products, even of insipid food, are capable of directly
exciting the secretion. These include proteoses and peptenes, and it
was, therefore, of great interest to find that a solution of commercial
peptone is also an effective stimulus. This is a result of deep significance,
for it indicates that the food which has been partially digested by the
appetite juice will serve as a stimulus to continued secretion.
The psychic juice has been aptly called the ‘‘ignition juice,’’- because
by producing partial digestion it serves to ignite the process of gastric
secretion. Experimental evidence of its great importance in gastric
digestion was secured by Pavlov in experiments in which he placed
weighed quantities of meat attached to threads in the stomach through
a gastric fistula, and after some time removed them and determined by
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 439
the difference in weights the extent to which they had become digested.
It was found that when the appetite juice was excited by sham feeding
at the same time that food was placed directly in the stomach, its diges-
tion was much more rapid than in cases in which it was placed in the
stomach without the animal’s knowing, as when he was asleep.
Other foods having a direct stimulating effect on the gastric secre-
tion are meat extracts and, to a certain extent, milk. This effect of meat
extract is interesting in connection with the practice of taking soup as
a first or early stage in dining. It not only excites the appetite juice,
but also serves as a direct stimulus to the gastric secretion.
As to the nature of the mechanism by which this direct secretion takes
place, it was shown by Popielski?™ that the secretion still occurs after all
the nerves proceeding to the stomach are cut. Evidently, therefore, it
is independent of the extrinsic nerve supply of the viscus. As a result
of his experiments Popielski concluded that the secretion must depend
on a local reflex mediated through the nerve structures present in the
walls of the stomach itself. Another explanation of the result has,
however, in recent years been given more credence by the experiments of
Bayliss and Starling on the influence of hormones on the secretion of
panereatic juice (ef. page 425). Edkins'®? suggested that a similar
process in the stomach might account for the continued secretion of
gastric juice. To test the possibility this investigator, after ligating the
cardiac sphincter in anesthetized animals, inserted a tube into the
pylorie end of the stomach, through which he placed in the stomach
about 50 c.c. of physiologic saline. After this had, been in the stomach
for an hour, he found that no water was absorbed, and that if the fluid
was removed after this time, it contained neither hydrochloric acid nor
pepsin. On the other hand, if during the time the saline was in the
stomach a decoction of the mucous membrane of the pyloric end, made
either with peptone solution or with a solution of dextrine, was injected
intravenously in small quantities every few minutes, it was found that
the saline contained distinct quantities of hydrochloric acid and pepsin.
Furthermore, it was found that, if the peptone solution or the dextrine
solution alone was injected intravenously, there was no such evidence
of gastric secretion. The conclusion which Edkins drew from his experi-
ments is to the effect that the half-digested products of the earlier stages
of gastric digestion act on the mucous membrane of the stomach so as to
produce a hormone, which is then carried by the blood to the cells of
the gastric glands, upon which, like secretin, it directly develops an
exciting effect. This hormone has been called gastrin. These observa-
tions of Edkins have been confirmed, and they explain very simply how
gastric secretion is maintained after the cessation of the secretion of the
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440 DIGESTION
appetite juice.” By such a mechanism gastric juice would continue to be
secreted so long as any half-digested food remains in the stomach.
The action of gastrin is the first instance of a hormone control of the
digestive glands. In the earlier stages of digestion, the secretion of saliva
and appetite juice is mediated through the nervous system, because these
juices must be produced promptly. In the later stages of gastrie diges-
tion, such promptitude in response on the part of the gland is no longer
necessary, so that the slower, more continuous process of hormone con-
trol is sufficient.
Quantity of Gastric Juice Secreted
According to Carlson, the total amount of gastric juice secreted in
man on an average meal composed of meat, bread, vegetables, coffee or
Hours 1234567812345678910123456
12
10
8
8
4
Juice inc
2
Flesh, 200 gm. Bread, 200 gm. Milk, 600 c.c.
Fig. 150.—Cubic centimeters of gastric juice secreted after diets of meat, bread, and milk. (From
. Pavlov.)
milk, and dessert, amounts to about 700 c.c., being divided into 200 c.c.
in the first hour, 150 in the second, and 350 c.c. during the third, fourth
‘and fifth hours. These figures were estimated partly on the basis of
observations made on the man with the gastric ‘fistula, and partly from
the data supplied by Pavlov’s observations on dogs. Carlson believes
that Pavlov overestimated the relative importance of the appetite juice
in gastric digestion. He found, for example, that after division of both
vagus nerves in dogs normal gastric digestion might be regained a few
days after the operation, although, of course, under such cireumstances no
appetite juice could have been secreted. Moreover, he observed that cats
when forcibly fed with unpalatable food may digest that food as rapidly
as when they eat voluntarily. In support of his contention, Carlson
states that he has frequently removed all of the appetite juice from his
patient’s stomach before the masticated meal was put into it without
any evident interference with the digestive process.
Fat has a distinct inhibiting influence on the direct secretion of gas-
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 441
tric juice; cream takes considerably longer to be be digested than milk,
and the presence of oil in the stomach delays the secretion of juice poured
out on a subsequent meal of otherwise readily digestible food. By col-
lecting all of the gastric juice from the miniature stomach after feeding
by mouth with quantities of different protein-rich foods containing the
same quantities of nitrogen, interesting observations have been recorded
concerning the amount of juice secreted and its proteolytic power. The
results of some. of the experiments are shown in the accompanying
curves (Figs. 150 and 151).
‘Tt will be seen that the most abundant secretion occurs with meat, that
of milk being not only smaller but also slower in starting. The digestive
power is greatest in the case of bread.
i '
Hours! 23456782345678923456
Mm of: Protein
Column
0
<= SN ww w}
~~ w
Flesh, 200 gm. Bread, 200 gm. Milk, 600 c.c.
Fig. 151.—Digestive power of the juice, as measured by the length of the protein column digested
in Mett’s tubes, with diets of flesh, bread, and milk. (From Pavlov.)
vate
“ THE INTESTINAL SECRETIONS
Pancreatic Juice
Regarding the natural secretion of pancreatic juice, little need be added
to what has already been said (see page 425). The secretion begins when the
chyme enters the duodenum, and attains its maximum when the outflow
of this is greatest. By collecting the juice from a permanent fistula of the
pancreatic duct, it has been found that the amount varies with different
foods. When quantities of food containing equivalent amounts of nitro-
gen are fed, the greatest secretion is said to occur with bread and the least
with milk. Such differences are probably dependent upon the amount of
acid secreted in the stomach and passed on into the duodenum. It was
thought at one time that, besides variation in quantity, the nature of the
enzymes in the pancreatic juice might vary according to the kind of
food. This, however, has been shown not to be the case.
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Bile
The secretion of bile runs practically parallel with that of pancreatic
juice. The liver is producing bile more or less continuously, since besides
being a digestive fluid it is also an excretory product. The bile produced
between the periods of digestion is mainly stored in the gall bladder.
When the acid chyme comes in contact with the duodenal mucous mem-
brane, it excites afferent nerve endings that cause a reflex contraction of
the gall bladder, and this expresses some of the bile into the duodenum.
The secretin, which the acid at the same time produces, besides affecting
the pancreas, qcts on the liver cells, stimulating them to the increased
secretion of bile. Thus, by a nervous reflex operating on the gall bladder
and later by a harmone mechanism operating on the liver cell, the increased
secretion of bile is insured throughout digestion. Of the bile discharged
into the intestine, a certain proportion of the bile salts is reabsorbed into
the portal blood. When these arrive at the liver they also excite secre-
tion of bile, thus assisting secretin in maintaining the secretion through-
out the process of intestinal digestion.
Fig. 152. —Loop of intestine after tving off the portions, cutting the nerves running to the middle
portion, ‘and returning the loop to the abdomen for some time. (From Jackson.)
Intestinal Juice
The secretion of intestinal juice, or succus entericus, can obviously be
studied only after isolating portions of the intestine and connecting them
with fistule of the abdominal walls. It appears here again that both a
nervous and a hormone mechanism exist. Mechanical stimulation of the
intestinal mucous membrane causes an immediate outflow of intestinal
juice, the purpose of which under normal conditions is evidently to assist
in moving forward the bowel contents. This mechanically excited juice
does not contain any enterokinase and only small amounts of the other
enzymes. Further evidence for nervous control of the secretion of intes-
tinal juice has been obtained by isolating three pouches of intestine be-
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PHYSIOLOGY OF THE DIGESTIVE GLANDS 443
tween ligatures, and then denervating the central pouch by carefully
cutting all the nerves without wounding the blood vessels. On returning
the pouches to the abdomen and leaving them several hours, it has been
found that the middle pouch becomes distended with secretion, whereas
the two end pouches remain empty (Fig. 152). If the pouches are left for
several days in the abdomen, however, the secretion from the denervated
portion disappears again. The explanation of the result is possibly that
the nerves under ordinary conditions convey impulses to the intestinal
glands, which tonically inhibit their activity.
The existence of hormone control is evidenced by the fact that *no
enterokinase is present in the intestinal juice unless pancreatic juice is
placed in contact with the mucous membrane. Injection of pancreatic
juice into the blood, however, does not cause any secretion of intestinal
juice; whereas the injection of secretin has such an effect.
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CHAPTER Ll
THE MECHANISMS OF DIGESTION
MASTICATION, DEGLUTITION, VOMITING
Mastication
By the movements of the lower jaw on the upper, the two rows of
teeth come together so as to serve for biting or crushing the food. The
resulting comminution of the food forms the first step in digestion. The
up and down motion of the lower jaw results in biting by the incisors,
and after the mouthful has been taken, the side to side movements enable
the grinding teeth to crush and break it up into fragments of the proper
size for swallowing. The most suitable size of the mouthful is about
5 cc., but this varies greatly with habit. After mastication, the mass
weighs from 3.2 to 6.5 gm., about one-fourth of this weight being due to
saliva. The food is now a semifiuid mush containing particles which
are usually less than 2 mm. in diameter. Some, however, may measure
Torevenl12mm. —
Determination of the proper degree of fineness of the food is a func-
tion of the tongue, gums, and cheeks, for which purpose the mucous
membrane covering them is supplied with very sensitive touch nerve
endings (see page 794). The sensitiveness of the tongue, etc., in this
regard explains why an object which can scarcely be felt by the fingers
seems to be quite large in the mouth. If some particles of food that are
too large for swallowing happen to be carried backward in the mouth,
the tongue returns them for further mastication.
The saliva assists in mastication in several ways: (1) by dissolving
some of the food constituents; (2) by partly digesting some of the
starch; (3) by softening the mass of food so that it is more readily
crushed; (4) by covering the bolus with mucus so as to make it more
readily transferable from place to place. The secretion of saliva is
therefore stimulated by the chewing movements, and its composition
varies according to the nature of the food (page 431). In some animals,
such as the cat and dog, mastication is unimportant, coating of the food with
saliva being the only change which it undergoes in the mouth. In man
the ability thus to bolt the food can readily be acquired, not, however,
without some detriment to the efficiency of digestion as a whole. Soft
444
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THE MECHANISMS OF DIGESTION 445
starchy food is little chewed, the length of time required for the mastica-
tion of other foods depending mainly on their nature, but also to a
certain degree on the appetite and on the size of the mouthful.
It can not be too strongly insisted upon that the act of mastication is
of far more importance than merely to break up and prepare the food
for swallowing. It causes the food to be moved about in the mouth so as
to develop its full effect on the taste buds; the crushing also releases
odors which ‘stimulate the olfactory epithelium. On these stimuli depend
the satisfaction and pleasure of eating, which in turn initiate the process
of gastric digestion (see page 435).
The benefit to digestion as a whole of a large secretion of saliva, brought
about by persistent chewing, has been assumed by some to be much
greater than it really is, and there has existed, and indeed may still
exist, a school of faddists who, by ‘deliberately chewing far beyond
the necessary time, imagine themselves to thrive better on less food than
those who occupy their time with more profitable pursuits.
Deglutition or Swallowing
After being masticated the food is rolled up into a bolus by the action
of the tongue against the palate, and after being lubricated by saliva is
moved, by elevation of the front of the tongue, towards the back of the
mouth. This constitutes the first stage of swallowing, and is, so far, a
voluntary act. About this time a slight inspiratory contraction of the
diaphragm occurs—the so-called respiration of swallowing—and the
mylohyoid quickly contracts, with the consequence that the bolus passes
between the pillars of the fauces. This marks the beginning of the
second stage, the first event of which is that the bolus, by stimulating
sensory nerve endings, acts on nerve centers situated in the medulla
oblongata so as to cause a coordinated series of movements of the
muscles of the pharynx and larynx and an inhibition for a moment of
the respiratory center (page 332). .
The movements alter the shape of the pharynx and of the various
openings into it in such a manner as to compel the bolus of food to pass
into the esophagus (see Fig. 153): thus, (1) the soft palate becomes
elevated and the posterior wall of the pharynx bulges forward so as to
shut off the posterior nares; (2) the posterior pillars of the fauces ap-
proximate so as to shut off the mouth cavity, and (8) in about a tenth of
a second after the mylohyoid has contracted, the larynx is pulled up-
wards and forwards under the root of the tongue, which by being
drawn backwards becomes banked up over the laryngeal opening. This
pulling up of the larynx brings the opening into it near to the lower
half of the dorsal side of the epiglottis, but the upper half of this struc-
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446 DIGESTION
ture projects beyond and serves as a ledge to guide the bolus safely past
this critical part of its course. (4) As a further safeguard against any
entry of food into the air passages, the laryngeal opening is narrowed by
approximation of the true and the false vocal cords.
So far the force which propels the bolus is mainly the contraction of
the mylohyoid, assisted by the movements of the root of the tongue.
When it has reached the lower end of the pharynx, however, the bolus
readily falls into the esophagus, which has become dilated on account
of,a reflex inhibition of the constrictor muscles of its upper end. This so-
called second stage of swallowing is, therefore, a complex coordinated
movement initiated by afferent stimuli and involving reciprocal action
of various groups of muscles: inhibition of the respiratory muscles and
ess Sea
Fig. 153.—The changes which take place in the position of the root of the tongue, the soft
palate, the epiglottis and the larynx during the second stage of swallowing. The thick dotted line
indicates the position during swallowing.
of those that constrict the esophagus, and stimulation of those that
-elevate the palate, the root of the tongue, and the larynx. It is purely
an involuntary process.
The third stage of deglutition consists in the passage of the swallowed
food along the esophagus. The mechanism by which this is done de-
pends very much on the physical consistence of the food. A solid bolus
that more or less fills the esophagus excites a typical peristaltic wave,
which is characterized by a dilatation of the esophagus immediately in
front of and a constriction over and behind the bolus. This wave travels
‘down the esophagus in man at such a rate that it reaches the cardiac
sphincter in about five or six seconds. On arriving here the cardiac
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THE MECHANISMS OF DIGESTION 447
sphincter, ordinarily contracted, relaxes for a moment so that the bolus
passes into the stomach. In many animals, including man and the cat,
the peristaltic wave travels much more rapidly in the upper part of the
esophagus than lower down because of differences in the nature of the
muscular coat, this being of the striated variety above, and of the non-
striated below. The purpose of more rapid movement in the upper part
is no doubt that the bolus may be hurried past the regions where, by
distending the esophagus, it might interfere with the function of neigh-
boring structures, such as the heart. In other animals, as the dog, the
muscular fiber is striated all along the esophagus, and the bolus of food
correspondingly travels at a uniform, quick rate all the way. It takes
only about four seconds for the bolus to reach the stomach in the dog.
The peristaltic wave of the upper part of the esophagus in the cat and
presumably in man, unlike that of the intestines (see page 466), is trans-
mitted by the esophageal branches of the vagus nerves. If these are
severed, but the muscular coats left intact, the esophagus becomes dilated
above the level of the section and contracted below, and no peristaltic
wave can pass along it; on the other hand, the muscular coat may be
severed (by crushing, ete.) but the peristaltic wave will continue to
travel, provided no damage has been done to the nerves.
In the lower part of the esophagus, however, the wave of peristalsis,
like that of the intestines, travels independently of extrinsic nerves.
This has been observed in animals in which all of the extrinsic nerves
have been cut some time previously. This difference between the upper
and the lower portions is associated with the difference in the nature of
the muscular fibers above noted (Meltzer).
The propagation of the wave by the nerves in, the upper part of the
esophagus indicates that the second stage and the first part of the third
stage of deglutition must be rehearsed, as it were, in ‘the medullary
centers from which arise the nerve fibers to the pharynx and the upper
levels of the esophagus. It is thought that the discharges from these
local centers are controlled by a higher swallowing center situated in the
medulla just above that of respiration, the afferent stimuli to which
proceed from the pharynx by the fifth, superior laryngeal, and vagus
nerves. The exact location of the sensory areas whose stimulation is
most effective in initiating the swallowing reflex varies considerably
in different animals. In man-it is probably at the entrance to the
pharynx; in the dog it is on the posterior wall. A foreign body placed
directly in the upper portion of the esophagus of man has been observed
to remain stationary until the individual made a swallowing movement.
The afferent fibers in the glossopharyngeal nerve exercise a powerful
inhibitory influence on the deglutition center as well as on that of respira-
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448 DIGESTION
tion. Thus, if swallowing movements are excited by stimulating the cen-
tral end of the superior laryngeal nerve, they can be instantly inhibited
by simultaneously stimulating the glossopharyngeal, and the respiratory
movements stop in whatever position they may have been at the time.
When the glossopharyngeal nerves are cut, the esophagus enters into a
condition of tonic contraction, which may last a day or so. This shows
that the inhibitory impulses are tonic in nature.
This inhibition of the esophagus is indeed a most important part of
the process when liquid or semiliquid food is swallowed. By the contrac-
tion of the mylohyoid muscle, fluids are quickly shot down the distended
esophagus, at the lower end of which, on account of the closure of the
cardiac sphincter, they accumulate until the arrival of the peristaltic
wave which has meanwhile been set up by stimulation of the pharynx.
If the swallowing is immediately repeated, as is usually the case in
drinking, the esophagus remains dilated because peristalsis is inhibited,
and the fluid lies outside the cardiac orifice until the last mouthful has
been taken.
The Cardiac Sphincter
The passage between the esophagus and the stomach is guarded by
the cardiac sphincter or cardia. This exists in a permanently con-
tracted state, or tonus, superimposed on which from time to time are
rhythmic alternations of contraction and relaxation. This tonus is never
very pronounced. In man it-is said that a water pressure of from 2 to 7
em. applied to the esophageal side of the sphineter will drive air’ or
water into the stomach, this pressure being less than that of a column
of fluid filling the thoracic esophagus in the erect position. During
repeated deglutition the tonus becomes less and less marked, and after.
a number of swallows the sphincter may become completely relaxed.
When this relaxation disappears, however, the sphincter becomes more
contracted than usual and remains so for a longer time.
The tonic condition of the sphincter is controlled by the vagus nerve,
stimulation of which causes relaxation with an after-effect of strong
contraction. Mechanical or chemical stimulation of the lower end of the
esophagus increases the tonus of the sphincter. Forcing of the sphincter.
from the stomach side requires a higher pressure than from the esopha-
geal. Eructation of gas, for example, does not take place until intra-
gastric pressure has risen to about 25 cm. of water. In deep anesthesia,
however, intragastric pressure may rise considerably higher without.
forcing the sphincter.
In animals fed with starch paste impregnated with subnitrate of bis-
muth and then examined by means of the x-rays, the variation in degree
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THE MECHANISMS OF DIGESTION 449
of tone of the sphincter has been observed to be responsible for occasional
regurgitation of some of the gastric contents into the esophagus up to the
level of the heart or even to the base of the neck. The presence of the
gastric contents in the esophagus starts a peristaltic wave, which pushes
the material back again into the stomach. This peristaltic wave starts
in the absence of any other phases of the deglutition process, indicating
that it has been excited by the presence of the material in the esophagus
itself, and belongs, therefore, to the lower order of peristaltic wave, as
seen in the intestines but not in the upper half of the esophagus. Regur-
gitation of food into the esophagus occurs only when the intragastric
pressure is fairly high. It may last for a period of from twenty to thirty
minutes after the meal is taken, and disappears when the tonus of the
sphincter becomes increased as a result of the presence in the gastric
contents of free hydrochloric acid.
Much information has been secured by listening with a stethoscope to
the sounds caused by swallowing and by observing with the x-ray the
shadows produced along the course of the esophagus when food impreg-
nated with bismuth subnitrate is taken. When a solid bolus is swal-
lowed only one sound is usually heard, but with liquid food there are
two, one at the upper end, due to the rush of the fluid and air, and
the other at the lower end (heard over the epigastrium), four or six
seconds later, due to the arrival here of the peristaltic wave with the
accompanying opening of the cardiac sphincter and the escape of the
fluid and air into the stomach. Sometimes, when the person is in the
horizontal position, this second sound may be broken up into several,
indicating that, unassisted by gravity, the fluid does not so readily pass
through the sphincter. The x-ray shadows yield results in conformity
with the above. After swallowing milk and bismuth, for example, the
shadow falls quickly to the lower end of the esophagus and then passes
slowly into the stomach. When the passage of a solid bolus is watched
by the x-ray method, its rate of descent will be found to depend on
whether or not it is well lubricated with saliva; if not so, it may take as
long as fifteen minutes to reach the stomach; if moist, but from eight
to eighteen seconds.
Vomiting
Vomiting is usually preceded by a feeling of sickness or nausea, and
is initiated by a very active secretion of saliva. The saliva, mixed with
air, accumulates to a considerable extent at the lower end of the esopha-
gus, which it distends. A forced inspiration is now made, during the
first stage of which the glottis is open so that the air enters the lungs,
but later the glottis closes so that the inspired air is sucked into the
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450 DIGESTION
esophagus, which, already somewhat distended by saliva, now becomes
markedly so. The abdominal muscles then contract so as to compress
the stomach against the diaphragm and, simultaneously, the cardiac
sphincter relaxes, the head is held forward and the contents ofthe
stomach are ejected through the previously distended esophagus. The
compression of the stomach by the contracting abdominal muscles is
assisted by an actual contraction of the stomach itself, as has been clearly
demonstrated by the x-ray method. After the contents of the stomach
itself-have been evacuated, the pylorie sphincter may also relax and
permit the contents (bile, etc.) of the duodenum to be vomited.
The act of vomiting is controlled by a center located in the medulla,
and the afferent fibers to this center may come from many different
regions of the body. Perhaps the most potent of them come from the
sensory nerve endings of the fauces and pharynx. This explains the
tendency to vomit when the mucosa of this region is mechanically stimu-
lated. Other afferent impulses come from the mucosa of the stomach
itself, and these are stimulated by emetics, important among which are
strong salt solution, mustard water and zine sulphate. Certain other
emetics, particularly tartar emetic and apomorphine, act on the vomit-
ing. center itself, and can therefore operate when given subcutaneously.
Afferent vomiting impulses also arise from the abdominal viscera, thus
explaining the vomiting which occurs in strangulated hernia, and in
other irritative lesions involving this region. X-ray observations have
been made on the movements of the stomach of cats after the admin-
istration of apomorphine (Cannon). The first change observed is an
inhibition of the cardiac end of the stomach, which becomes a perfectly
flaccid bag. About the midregion of the organ, deeper contractions then
start up, which sweep from the pylorus, each contraction stopping as a
deep ring at the beginning of the vestibule, while a slighter wave con-
tinues. A very strong contraction at the incisura angularis: finally
develops and completely divides the gastric cavity into two parts. On
the left of this constriction the stomach remains completely relaxed, but
at the right of it waves continue running over the vestibule. It is while
the stomach is in this condition that the sudden contraction of the dia-
phragm and abdominal muscles shoots the cardiac contents into the
relaxed esophagus. As these jerky contractions are continued, the gastric
walls seem to reacquire their tone.
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CHAPTER LIT
THE MECHANISMS OF DIGESTION (Cont’d)
THE MOVEMENTS OF THE STOMACH
The Character of the Movements
Even from the earliest days it has been recognized that the stomach
performs two important functions: (1) receiving the swallowed food
and then discharging it slowly into the intestine, and (2) initiating the
chemical processes of digestion. In order to understand the mechanism
by which the stomach collects and then discharges the food, it is neces-
sary first of all to recall certain anatomic facts concerning the organ,
and for this purpose it is most convenient to accept the description
given by Cannon, which is illustrated in the accompanying figure. The
organ is divided into a cardiac and a pyloric portion by a deep notch in
the lesser curvature, called the incisura angularis. The cardiac portion
is further subdivided into two by the cardiac orifice. The part which
lies, in man, above a line drawn horizontally throtigh the cardia is the
fundus. The part lying between the fundus and the incisura angularis
is known as the body of the stomach, which, when full, has a tapering
shape. The pyloric portion lying on the right of the incisura angularis
is further divided into two parts: the pylorie vestibule and the pyloric
eanal, the latter of which lies next the pyloric sphincter and in man
measures about 3 em. in length (see Fig. 154).
The filled stomach of a person standing erect is so disposed that the
greatest curvature forms its lowest point, which may be considerably
below the umbilicus. As digestion proceeds and the stomach empties,
the greater curvature becomes gradually raised, so that ultimately the
pylorus comes to be the most dependent part of the stomach. From
these and many other observations it is certain that the emptying of the
stomach does not at all depend on the operation of the force of gravity.
Indeed, that this can not be the case is perfectly clear when we con-
sider the disposition of the stomach in quadrupeds.
Exact observation on the movements which the stomach performs from
the time it is filled with food till it empties, have been made by the
x-ray method, first introduced by Cannon.??_ The method consists in feed-
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452 DIGESTION
ing the animal with food that has been impregnated with bismuth sub-
nitrate, then exposing him to the x-ray and either taking instantaneous
photographs of the shadows or observing them by means of a fluorescent
sereen. The descriptions of the original observations made by Cannon
Fig. 154.—Schematic outline of the stomach. At C_is the cardia; F, fundus; JA, incisura an-
gularis; B, body; PC, pyloric canal; P, pylorus. (From Cannon.)
on the stomach of the cat have been so little modified by observations
on man that we may take them as a convenient type. In the accompany-
ing figure (Fig. 156) the outline of the shadow cast by the stomach is
shown at intervals of an hour each during digestion. Soon after the
Fig. 155.—Diagrams of outline and position of stomach as indicated by skiagrams taken on
man in the erect position at intervals after swallowing food impregnated with bismuth subnitrate.
A, moderately full; 8, practically empty. The clear space at the upper end of the stomach is due
to gas, and it will be noticed that this ‘stomach bladder” lies close to the heart. (From T. Win-
gate Todd.)
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THE MECHANISMS OF DIGESTION 453
stomach has become filled, peristaltic waves are seen to take their
origin about the middle of the body of the stomach, and to course
towards the pylorus. Above the region at which these waves originate—
that is, the cardiac half of the body of the stomach and all of the
fundus—there are no waves, but as digestion proceeds the walls slowly
and steadily contract on the mass of food. This so-called cardiac pouch
does not, however, diminish in size so rapidly as the part of the body of
the stomach over which the peristaltic waves are passing. The circular
fibers of the walls of this part of the stomach—sometimes called the
gastric tube—contract tonically, so that it becomes tubular in form,
with the full cardiac pouch at the left and above and the pyloric por-
DDS
ive
Fig. 156.—Outlines of the shadows cast by the stomach at intervals of an hour each after feeding
a cat with food impregnated with bismuth subnitrate. (From Cannon.)
tion at the right. The latter portion meanwhile does not diminish much
in size, although the peristaltic waves traveling over it are very pro-
nounced. As will be clear from the figure, these changes in outline go
on until the cardiac pouch has become practically empty and the food
has been all moved along the now tubular portion of the body into the
pyloric vestibule. ;
From this description it is evident that the function of the cardiac
end is to serve as a reservoir for the food, which, by a slow contraction
of the walls, is gradually delivered into the gastric tube, where by
peristalsis it is carried towards the pyloric vestibule.
The time required for the peristaltic waves to travel from their place
of origin to the pylorus is considerably longer than the interval between
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454 DIGESTION
the waves, so that several of these are always seen on the stomach at
the same time. They sometimes become so pronounced in the pyloric
region, especially in a half-empty stomach, that they appear almost to
obliterate the cavity. They always stop at the pylorus, never going on
to the duodenum. The rate of recurrence of the waves varies somewhat
in different animals, being about’ six per minute in the cat and about
three in man. Their initiation does not seem to depend on the presence
of acid in the gastric contents, for, when food is introduced into the
stomach, they do not wait for the gastric contents to become acid in
reaction (see page 482). Nevertheless, acid does seem somewhat to stim-
ulate the depth and frequency of the waves, and they recur oftener with
carbohydrate than with fatty food.
The pressure in the stomach contents—the intragastric pressure—is
low and_constant at the cardiac end and fairly high and variable in the
pyloric end (in the former from 6 to 8 em. of water, and in the latter
from 20 to 30). Constancy of pressure in the cardiac end indicates
that the stomach wall must adapt itself very promptly to the amount of
food in the organ. The higher and more variable pressure in the pylorie
end is, of course, due to the peristaltic waves, and it is interesting to note
that it is sufficient to propel the gastric contents through, the pylorus for
several centimeters into the duodenum.
.
The Effect of the Stomach Movements on the Food
This has been studied: (1) by dividing the food into portions that.
are differently colored and, after some time, killing the animal, freezing
the stomach and making sections of it (see Fig. 157); (2) by mak-
ing little pellets of bismuth subnitrate with starch and observing their
behavior under the x-rays; or (3) by removing samples of the stomach
contents by means of a stomach tube (Rehfuss tube) inserted so that
its free end lies in either the cardiac or the pyloric region. By the
first of the above methods it has been found that the first mouthfuls
of food lie along the greater curvature, where they form a layer over
which that subsequently swallowed accumulates, with the last por-
tions next the cardia. The pepsin and hydrochloric acid of the car-
diac end, therefore, act soonest on the first swallowed portion of a
meal, and the more recently swallowed central masses are not affected
by the secretions for some time, so that opportunity is given for the
saliva mixed with the food to develop its digestive action.
As has been shown by removing the stomach contents with a tube at
various periods after feeding with starchy food, considerable amylolysis
may occur for some time. When separate samples are removed in this
way from the, cardiac and pyloric parts, it has been found that after
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THE MECHANISMS OF DIGESTION 455
half an hour the contents of both have about the same percentage of
sugar, but that for. some time after this: interval the cardiac contents
contain considerably more sugar than the pyloric. Later the percentages
of sugar again become about equal, no doubt on account of diffusion.
The diastatic action in the fundus is finally brought to an end when
the contents become completely permeated by the hydrochloric acid.
In this connection it is worthy of note that the addition of hydrochloric
acid up to the point of neutrality greatly accelerates the rate of diastatic
digestion.
As the outer layers of food in the stomach become partly digested on
account of the action of the pepsin and hydrochloric acid, the food is
slowly pressed into the active right half of the stomach, where by the
action of the peristaltic waves it is moved on to the pylorie vestibule.
By observing the x-ray shadows cast by two pellets of bismuth subni-
trate it has been noted by Cannon that, as the peristaltic wave approaches
Fig. 157.—Section of the frozen stomach (rat) some time after feeding with food given in three
differently colored portions. (From Howell’s Physiology.)
a pellet, it causes it to move forward more rapidly for a short distance,
but soon overtakes it and in doing so causes the pellet to move back a
little towards the fundus. This backward movement is less than the
forward movement, so that after the wave has passed, the position of
the pellet is a little forward of that which it would have occupied had
there been no wave. The behavior of the pellet, and, therefore, of the
stomach contents, is very like that of a cork floating at the edge of the
sea; as each wave approaches, it hurries the cork on a little, but after
its passage the cork recedes again until the second wave carries it still
a little farther forward. As the peristaltic wave approaches the pyloric
vestibule and becomes more powerful its effect on the pellets becomes
more marked. They are carried rapidly along this part of the stomach,
until the pylorus is reached. If this remains closed, they are shot back
into the vestibule. From nine to twelve minutes may elapse before they
are transferred to the pylorus from the place where they are first affected
by the peristaltic wave.
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These observations made on eats and other laboratory animals no
doubt also apply in the case of man. Removal of the contents of the
cardiac and pyloric regions separately with a stomach tube after feeding
with a test meal part of which was colored with carmine or charcoal,
has shown that none of the coloring material was present in the contents
of.the pyloric end up to twenty minutes or so after the food had been
taken. It then appeared but at first only in traces. Another important
distinction between the food in the two portions of the stomach relates
to its consistency. In the pyloric end it is semifluid and homogeneous
in character; in the cardiac end, on the other hand, it is a lumpy, rather
incoherent mass.
The gastric movements must greatly facilitate the digestive processes
in the stomach. In the cardiac part the undisturbed condition of the
food will, as we have seen, facilitate the digestive action of ptyalin,
whereas in the body of the stomach the peristaltic waves, besides mov-
ing the food onward, will tend to bring fresh portions of mucous mem-
brane and food in contact, so that the latter becomes more thoroughly
mixed with the pepsin and hydrochlorie acid. In the pyloric part, where
no hydrochloric acid is secreted, the contents, already sufficiently acid
in reaction, beconie more thoroughly churned up with the local pepsin
secretion, so that proteolytic action progresses very rapidly.
The peristaltic waves also facilitate absorption from the stomach of such
substances as glucose in concentrated solution and, probably, of hydro-
lyzed protein; water, however, is not absorbed. One effect of such
absorption is the production of gastrin, which we have seen is the hor-
mone concerned in maintaining the gastric secretion after the psychic
flow. The fact that the mucosa of the vestibule has, relatively to the
eardiae end, few secreting glands is in harmony with the view that
absorption is an important function of this part of the stomach.
THE EMPTYING OF THE STOMACH
The Control of the Pyloric Sphincter
When digestion has proceeded far enough in the stomach to bring the
food into a homogeneous, souplike fluid (chyme), portions of this, as they
are driven against the pyloric sphincter by the peristaltic waves, instead of
being returned as an axial stream into the stomach, are ejected into the
duodenum.
We must now consider the mechanism by which the pyloric sphincter
opens to permit the passage of the chyme. Bombardment by the peri-
staltie waves is evidently not the cause of its opening, for, as we have
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THE MECHANISMS OF DIGESTION 457
seen, many such waves may arrive at it without this result. Since it is
‘evidently in order that the intestine may not suddenly become over-
whelmed with large masses of food that the pylorus only occasionally
opens, it might be thought that its opening depends upon the disten-
tion of the upper part of the intestine. It is true that excessive disten-
tion of the upper part of the intestine does hold the pyloric sphincter
closed, but this can not, be the physiologic stimulus, because considerable
quantities of chyme are never found here.
The first clue to the real nature of the mechanism was afforded by
observing the behavior of the sphincter when solutions are introduced
into the duodenum through a fistula. Acid solutions were found to
cause a complete inhibition of gastric evacuation, whereas alkaline solu-
tions had no effect. This difference indicates that acids in contact with
the duodenal mucous membrane reflexly excite contraction of the sphine-
ter, and that it relaxes only after the acid has become neutralized
by mixing with the pancreatic juice and bile.
On account of the great importance of the pyloric mechanism in insur-
ing that the chyme shall enter the intestine only in such quantities that
it ean be properly acted upon by the intestinal digesting juices, it will
be of interest to consider briefly some of the experimental observations
‘by which this mechanism has been studied. We may consider first the
evidence that acid on the stomach side of the pylorus causes a relaxation
of the sphincter: (1) When carbohydrate food is fed, it ordinarily leaves
the.stomach fairly rapidly, but if its acid-absorbing power is increased
by mixing it with sodium bicarbonate, exit from the stomach is greatly
delayed. (2) Proteins ordinarily leave the stomach more slowly than
carbohydrates, but if acid proteins are fed, their exit is much more
rapid. (3) If a fistula is made into the pyloric vestibule through which
some of the contents can be removed, it will be found that just prior to
the opening of the pyloric sphincter, a distinctly acid reaction develops
in the food; and furthermore if acid solutions are injected through this
fistula, they cause the pyloric sphincter to open, whereas alkalies retard
its opening. (4) A similar effect of acid in opening the sphincter can
be demonstrated by applying it to the pyloric mucosa of an excised
stomach kept alive in oxygenated Ringer’s solution.
The evidence that acid on the duodenal side causes closure of the
sphincter is as follows: (1) When acid is placed in the duodenum through
a fistula, the sphincter will not open; (2).when the pancreatic and bile
ducts are ligated, the stomach empties much more slowly than normally;
and (3) the discharge of protein is considerably hastened if the pylorus
is sutured to the intestine below the duodenum. After such an opera-
tion it was observed that the protein began to leave the stomach through
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458 DIGESTION
the pyloric sphincter about the same time as normally, but the subse-
quent evacuation was very much accelerated, because no acid came in’
contact with the duodenal mucosa. Water and egg white may leave
the stomach independently of any acid reflex control of the pylorus. By
observations made through a duodenal fistula it has been found that,
after a quantity of water has been swallowed, most if not all of it very
soon enters the duodenum in a more or less continuous stream. It is no
doubt on this account that drinking contaminated water is especially
dangerous on an empty stomach.
The nervous pathway through which these acid reflexes take place has
been shown to be the myenteric plexus. Indeed, the whole mechanism
is quite analogous with that which we shall see occurs in the intestine
during peristalsis: the stimulus, that is, the acid, causes a contraction
of the gastric tube behind it and a dilatation in front.
Fig. 158.—Outlines of shadows in abdomen obtained by exposure to x-rays 2 hours after
feeding with food containing bismuth subnitrate. ‘The food in A was lean beef, and in B boiled
rice. The smaller size of the stomach shadow and the much greater total area of the intestinal
ay B than in A show that carbohydrate leaves the stomach earlier than protein. (From
‘annon,
Rate of Emptying of Stomach
The relationship of these facts to the rate at which different foodstuffs
leave the stomach is very readily explained. The method for investigat-
ing this problem, which again we owe to Cannon, consists in feeding ani-
mals with a strictly uniform amount of different foods made up, as
nearly as possible, of equal consistency and containing bismuth subni-
trate in the proportion of 5 gm. to each 25 ¢.c. By feeding such mix-
tures to cats previously starved for twenty-four hours, and examining
the abdomen by the x-ray at regular intervals, the shadows cast by the food
after passage into the intestine can be outlined on tracing paper, and
the total length* measured (Fig. 158). In taking this as an estimate of
the amount of food im the intestine, several errors are no doubt incurred
*This is permissible since the shadows are practically all of the same width.
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THE MECHANISMS OF DIGESTION 459
on aceount of the crossing and foreshortening of the loops, etc., but, as
their constancy testifies, there is no doubt that the results are sufficiently
close for the purpose of finding out how quickly food gains access to the
small intestine; and the method has a great advantage over all others
in that digestion is allowed to proceed practically without interruption.
The points we have to determine are: (1) when the food first leaves the
stomach; (2) the rate at which different foods are discharged; (3) the
time required for the passage through the small intestine.
Let us consider first of all the results obtained by feeding with prac-
tically pure fat or carbohydrate or protein. By plotting the length of
the shadows in centimeters along the ordinates, with hours along the
abscisse, curves such as those shown in Fig. 159 have been secured.
When fats were fed (dash line in chart), the discharge began rather
slowly, and continued at a slow rate. Even after seven hours some fat
still remained in the stomach, and at no time was any large quantity
Centimetres
0 4 6 7
Hours
Fig. 159.—Curves to show the average aggregate length of the food masses in the small
intestine at the designated intervals after feeding. The curve for various fat foods is in the
dash line, for protein foods in the heavy line, and for carbohydrate foods in the light line.
(From Cannon.)
present in the intestine, indicating that almost as quickly as it is dis-
charged into this part of the gastrointestinal tract fat becomes digested
and absorbed. The discharge of carbohydrates was quite different (light
line in chart) ; it began often in ten minutes, and soon became abundant,
reaching a maximum, as a rule, at the end of two hours, after which it
fell off, the stomach being empty in about three hours. Protein left at a
rate intermediate between that for fats and that for carbohydrates
(heavy line). Little left before the first half hour; the curve then
slowly rose, attaining a maximum in about four hours, and then gradu-
ally declining at about the same rate as it rose. It is interesting to note
that at the end of half an hour about eight times as much carbohydrate
had left the stomach as protein; at the end of an hour, five times as much.
These results are clearly dependent upon the rates at which the dif-
ferent foodstuffs assume an acid reaction in the stomach. Carbohydrate
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has no combining power for acids, so that the acid secreted with the
psychic juice remains uncombined and on gaining the pyloric vestibule
excites the opening of the sphincter. Protein, on the other hand, as is
well known, absorbs considerable quantities of free hydrochloric acid,
so that for some considerable time after it is taken, none of the acid exists
in a free state. Fats owe their slow discharge partly to inhibition of
gastric secretion, and partly to the longer time it takes for them to become
neutralized in the duodenum, because of the fatty acid split off by the
action of lipase.
Interesting observations have also been made on the rate of discharge
when various combinations of foodstuffs were fed. This has been done
by feeding one foodstuff before the other, or by mixing the foodstuffs.
When carbohydrates were fed first and then protein, the discharge be-
gan much earlier than with protein alone, because the carbohydrate food
first reached the pyloric vestibule (see page 454). However, at the end
of two hours, when the carbohydrate curve should begin to come down,
it remained high, indicating that the protein had by this time reached
the pylorus and was being discharged at its own rate. When the meat
was fed: before the carbohydrate, the curve to start with was exactly
like that for protein, becoming, however, considerably heightened later-
when the carbohydrate reached the pyloric vestibule. The presence of
protein near the pylorus, therefore, distinctly retards the evacuation of
carbohydrate from the stomach. These facts, it will be remarked, all
fit in admirably with the observations which we have already detailed
concerning the disposition of food in the stomach.
When mixtures of equal parts of different foods were fed, the results
indieated that the emptying of the stomach occurred at a rate which
was intermediate between those of the foods taken separately. Mixing
protein with carbohydrate, for example, accelerated the rate at which
protein left, and mixing fats with protein caused the protein to leave
the stomach considerably more slowly than if protein alone had
been fed.
Influence of Pathologic Conditions on the Emptying
An important surgical application of these facts concerns the behavior
of food after gastroenterostomy. It has been thought that this operation
would cause the food to be drained from the stomach into the intestine
and thus leave the region of the stomach between the fistula and the
pylorus inactive. This assumption is based on the idea, which we have
seen to be erroneous, that gravity assists in the emptying of the stomach.
As a matter of fact, it has been found that, if the gastroenterostomy is
made when there is no obstruction at the pylorus, the chyme takes its
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THE MECHANISMS OF DIGESTION 461
normal passage through the sphincter and, almost without exception,
none leaves by the fistula. When the pylorus is partly oceluded, the
food sometimes passes in the usual way, and sometimes by the stomach.
The cause for this predilection for the pyloric pathway depends on the
pressure conditions in the gastric contents. Gastroenterostomy, there-
fore, is efficient only when gross mechanical obstruction exists at the
pylorus. The operation should never be performed in the absence of
demonstrable organic pyloric disease.
Another objection to gastroenterostomy in the presence of a patulous
pyloric sphincter rests on the fact that the food, after passing the sphinc-
ter and moving along the intestine, may again enter the stomach through
the fistula. This is most likely to oceur when the stomach is full of
food, for under these conditions the stretching of its walls separates the
edges of the opening, the intestine being drawn taut between the edges,
so that the opening between the stomach and the intestine assumes the
form of two narrow slits, which act like valves permitting the food to
enter but preventing its escape from the stomach. Only seldom under
these circumstances can any food pass into the intestine beyond the
stomach opening. Repeated vomiting after gastroenterostomy has been
observed in experimental animals only when obstructive kinks or other
demonstrable obstacles were present in the gut, the obstruction being lo-
cated in that part of the intestine beyond its attachment to the stomach.
When the pyloric obstruction is complete, food must, of course, leave
by the fistula, digestion by the pancreatic juice and bile being still ear-
ried on because of the fact that for a considerable distance down the
intestine, secretin, which we have seen is essential for the secretion
of these fluids, is still produced by the contact of the acid chyme with
the intestinal mucosa. Further provision for adequate digestion of
food in such cases is secured, as some of the food after leaving the
fistula passes back for some distance into the duodenum, where, however,
it soon excites peristaltic waves, which again carry it forward.. This
insures thorough mixing with the digestive juices. From their experi-
mental experience Cannon and Blake’? recommend that, when the
fistula has to be made, it should be as large as possible and near the
pylorus, and that the stomach afterwards should not be allowed to
become filled with food. To avoid kinking of the gut, they also recom-
mend that several centimeters of the intestine should be attached to the
stomach distal to the anastomosis.
The effect of hyperacidity of the contents on the emptying of the
stomach has been studied by feeding animals with potatoes containing
varying percentages of hydrochloric acid. With an acidity of 0.25 per
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462 DIGESTION
cent, the rate of discharge was increased, but it became slower when the
acidity rose to 1 per cent. With an acidity of 0.5 per cent, the rate of
discharge was about the normal. Hyperacidity, therefore, causes a retar-
dation of the emptying of the stomach.
The consistency of the food appears to have little influence on its rate of
discharge from the stomach—at least in the case of potatoes. Dilution
of protein food, however, increases the rate. Distinetly hard particles
in the food retard the stomach evacuation.
There is usually a considerable amount of gas in the part of the stomach
above the entrance of the cardia, on account of which this part of the
stomach has sometimes been called the stomach bladder. In the upright
position this gas forms a bright area in the x-ray plate (Fig. 155), but
when the person reclines it spreads to a new location. Its presence may
influence gastric digestion by preventing the contact of the food with
the mucous membrane, and by interfering with the efficiency of the peri-
staltic waves in moving the food. Considerable gas therefore retards the
emptying of tlie stomach, as has been shown experimentally by x-ray
observations on animals fed with the standard amount of food followed
by the introduction of air. It was noted that the air did not diminish
the frequency or strength of the peristaltic waves, but that these could
not efficiently act on the food. When along with gas there is also atony
of the stomach walls, the retardation in the discharge will, of course, be
still more pronounced. The temperature of the swallowed food does
not appear to have much influence on the stomach movements or on the
the rate of discharge from the organ.
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CHAPTER LIII
THE MECHANISMS OF DIGESTION (Cont’d)
THE MOVEMENTS OF THE INTESTINES
The length of the small intestine and the size of the cecum of the
large intestine vary considerably in different animals. In the carnivora,
such as the cat, the small intestine is relatively short; in the herbivora,
relatively long. Thus, it is three times the length of the body in the cat,
and four to six times in the dog; whereas in the goat and sheep, it may
be nearly thirty times the length of the body. In the carnivora the
cecum is either absent or rudimentary, whereas in those herbivora which
do not have a divided stomach the cecum is very large and sacculated,
as is also the colon. The reason for the great size in herbivora is that
practically the whole of the digestion of cellulose takes place in this
part of the gut. This digestion, as we shall see later, does not depend
on any secretion poured forth by the animal itself, but upon the action
of bacteria and of certain enzymes (cytases) that are taken with the
vegetable food.
Movement of the Small Intestine
The movements of the small intestine have been studied (1) by the
bismuth subnitrate and x-ray method, (2) by observing them after open-
ing the abdomen of an animal submerged in a bath of physiologic saline
at body temperature, (3) by observing the changes in pressure produced
in a thin-walled rubber balloon inserted in the lumen of the gut and
connected with a recording tambour (Fig. 160), and (4) by excising
portions of the intestine and keeping them alive in a bath of saline solu-
tion at body temperature, through which oxygen is made to pass.
THe Se@MENntTING MovEeMENTS
When a suitably fed animal is placed on the holder for examination
by the x-ray method, no movement in, the intestinal, shadows is generally
observed for some time. The first movement to appear is-the breaking of
one of the columns of food into small segments of nearly equal size.
Each of these segments again quickly divides, and the neighboring
halves suddenly unite to form new — and so on, in a manner
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464 DIGESTION
which will be made clear by consulting Fig. 161. This rhythmic seg-
mentation, as Cannon has ealled it, continues without cessation for more
than half an hour, and the food shadow does not meanwhile seem to change
its position in the abdomen to any extent. The splitting up of the seg-
ment and the rushing together of the neighboring halves proceed as a
rule with great rapidity; thus, if we count the number of different seg-
6 f
Tube to
tambour
Flastic rutber
band to attach
Fig. 160.—Apparatus for recording contractions of the intestine. (From Jackson.)
ments during a definite period, we may find the rate of division in the
cat to be as high as 28 or 30 a minute. In man the divisions occur at a
frequency of approximately 10 per minute, which corresponds to the fre-
queney with which sounds can be heard when the abdomen is auscultated.
Although half an hour is the period which this process usually oe-
‘eupies, it may last considerably longer. In certain animals, such as the
rabbit, segmenting movements have not been observed, but instead
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THE MECHANISMS OF DIGESTION 465
of them a rhythmic to-and-fro shifting of the masses of food along the
lumen of the gut, rapidly repeated for many minutes.
When the intestines are floated out in a warm bath of saline solution,
it is seen that the rhythmic segmentation is caused by narrow rings of
contraction. Under such conditions also it is often noted that the
loops of intestine sway from side to side. The balloon method also re-
veals the presence of slight waves of contraction that pass rapidly along
the gut, and follow each other at the rate of twelve to thirteen per minute.
Both of the muscular coats of the intestine are involved, and it is believed
that the contractions are responsible not only for the pendular move-
ments but for the rhythmic segmentation observed by the x-ray method. °
According to this view these movements are constantly passing along
the intestine, and become exaggerated by the mechanical stimulus which
is offered by the masses of food to such an extent that they divide the
masses into portions. The evidence for this belief rests on the fact that
. C=
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Fig. 161.—Diagrammatic representation of the process of segmentation in the intestine. An
unbroken shadow is shown in 1 and its segmentation in 2. The dotted lines across each mass
show the position of division and in 3 is shown how new masses are formed by the split portions
coming together. (From Cannon.)
when the contraction is studied by the balloon method, it becomes marked
over the middle of the b lloon; where the greatest tension exists.
Several functions ane assigned to these movements. They cause
intimate mixture of the food with the digestive juices, and by bringing
ever new portions of food in contact with the mucosa, they encourage
absorption. They also have an important massaging influence on the
blood and lymph in the vessels of the intestinal walls. Indeed, the pas-
sage of lymph from the lacteals into the mesenteric lymphatics seems to
depend very largely upon these movements.
THE PERISTALTIC MOVEMENTS
The other movement observed in the small intestine is that known as the
peristaltic wave. It occurs in two forms: (1) as a slowly advancing con-
traction (1 to 2 em. per minute), preceded by an inhibition of the walls,
and proceeding only through a short distance in a coil (4 to 5 em.); and
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466 DIGESTION
(2) as a swift movement called the peristaltic rush, which sweeps with-
out pause for much longer distances along the canal.
Further analysis of the peristaltic wave can readily be made by the
balloon method (Fig. 162). If the gut is pinched above the balloon, a
marked relaxation occurs over it, and this relaxation extends for about
two feet down the intestine. If, on the other hand, the gut is pinched
a little below the situation of the balloon, a long-continued contraction
occurs over the latter. The conclusion that we may draw from this result
is that the stimulation of the gut causes contraction above the point of
the stimulus and relaxation below, this being known as ‘‘the law of the
intestine’’—(Bayliss and Starling). We have seen that it applies also in
the case of the cardiac and pyloric sphincters.
Fig. 162. —Intestinal contractions (balloon method) after excision of the abdominal ganglia and
section of both vagi. Mechanical stimulation above (1) and below (2) the balloon causes relaxa-
tion and contraction respectively. (From Starling.)
THE PHysioLocic NaTuRE oF THE RHYTHMIC AND PERISTALTIC MoveMENTS
Interesting information in this connection has been gained by obser-
vation of the behavior of the movements after the application of drugs
to the gut or after cutting the nerve supply. The rhythmic movements
are not affected by the application of nicotine or cocaine. Since these
drugs paralyze nervous structures it has been concluded that the rhythmic
movements are myogenic in origin. The question is not a settled one,
however, for it has been found by Magnus that, although strips of the
longitudinal muscle, isolated in oxygenated saline solution, will continue
to beat, they do not do so if the adherent Auerbach’s plexus of nerves
is stripped off from them. The nature of the peristaltic contractions is
more definite; they must clearly depend upon a local nervous struc-
ture, since they are paralyzed by the application to the gut of cocaine or
nicotine. This local nervous system no doubt also resides in Auerbach’s
plexus, which must therefore be considered as complex enough to be (see
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THE MECHANISMS OF DIGESTION 467
page 796) endowed with the power of directing nervous impulses so as to
bring about relaxation of the gut-in front of the stimulus and contrae-
tion over it.
Nervous Controt of MovEMENTS
’ The influence of the central nervous system on the intestinal movements
has been studied by the usual methods of cutting and stimulating the
extrinsic nerve supply. Through the splanchnic nerves tonic inhibitory
impulses are conveyed to the intestine (except the ileocolic sphincter),
for after these nerves are severed the movements become more distinct.
Indeed, in many animals after opening the abdomen no intestinal move-
ment can be observed until these nerves have been cut. Stimulation of the
peripheral end of the nerve also inhibits any movement which may mean-
while be in progress. The impulses through the vagus nerve are of an
|
Fig. 163.—The effect of excitation of both splanchnic nerves on the intestinal contractions. (From
c arling.)
opposite character. Section of these nerves has little effect, but stimula-
tion causes contraction. (Figs. 163 and 164.)
By observing the rhythmic contractions of an isolated strip of the small
intestine suspended in a bath of oxygenated saline solution at body tem-
perature, it can readily be shown that the presence of even a minute trace
of epinephrine is sufficient to produce complete inhibition of the movement.
The parallelism between the effects of splanchnic stimulation and those of
epinephrine injection is very significant, for in this way the marked inhi-
bition of intestinal movement which occurs during fright may possibly
be explained (see page 736).
The circular muscular coat of the last two or three centimenters of
the ileum before it joins the cecum is definitely thicker than the rest of
this coat, indicating that it has a sphincter-like action. This ileccolic
sphincter, as it is called, opens when food is pressed against it from the
ileum, but remains closed when food is pressed against it from the cecum.
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It therefore obeys the law of the intestine. That it is physiologically
distinct from the musculature of the rest of the ileum is indicated by the
fact that the splanchnic and vagus nerves do not affect it in the same
way; thus, stimulation of the splanchnic causes a strong contraction of
the sphincter, whereas it is unaffected by stimulation of the vagus.
Peristalsis is much more rapid in the duodenum than in other parts of
the small intestine. During the first stages of digestion, the food ordi-
narily lies mainly in the right half of the abdomen, and later in the left
half. There is considerable variation in the time that elapses before it
enters the colon. In the cat, carbohydrates reach this part of the gut in
about four hours.
Fig. 164.—The effect of stimulation of right vagus nerve on the intestinal contractions. (From
Starling.)
Movements of the Large Intestine
On account of the great differences which we have already seen to
exist in. the size and relative importance of the colon as a digestive organ
in different classes of animals, it is not surprising that the movements
observed are very different according to the dietetic habits of the animal.
Apparently the movements are much the same in thé cat as in man. As
the food passes through the ileocolic sphincter into the cecum and
accumulates there, it gradually sets up, by its pressure, a contraction of
the muscular walls of the gut somewhere about the junction between
the ascending and transverse colon. This wave of contraction then
begins to travel slowly toward the cecum, without, however, being pre-
ceded by any relaxation of the wall of the gut, as is the case with a true
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THE MECHANISMS OF DIGESTION 469
peristaltic wave. This first wave is soon followed by others, with the
result that the food is forced up into the cecum, against the blind end
of which it is crowded, being meanwhile prevented from passing into
the ileum by the operation of the ileocolic sphincter and by the oblique
manner in which the ileum opens into the cecum.
As the result of the distention of the cecum set up by these so-called
antiperistaltic waves, a true coordinated peristaltic wave is occasionally
initiated, and passes along the ascending colon preceded by the usual
wave of inhibition. These waves, however, disappear before they reach
the end of the colon, so that the food is again driven back by the so-
230Pm
5P
1230
eM
0 AM,
Fig. 165.—Diagram of time it takes for a capsule containing bismuth to reach the various parts
of the large intestine.
called antiperistaltic waves. The effect of the movements is to knead
and mix the intestinal contents, and thus encourage the absorption of
water from them. The resulting more solid portions then collect toward
the splenic flexure, and become separated from the remaining more fluid
portion by transverse waves of constriction, which develop into peri-
staltic waves carrying the harder masses into the distal portions of the
colon, where they collect chiefly in the sigmoid flexure. The descending
colon itself is never distended with contents and merely serves as a tube
for transferring the masses from the transverse colon to the sigmoid
flexure. The time taken for a capsule of bismuth to reach the various
parts of the large intestine is shown in Fig. 165.
After a certain mass has collected in the sigmoid flexure and rectum,
the increasing distention causes a reflex evacuation of this portion of the
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470 DIGESTION
gut through centers located in the spinal cord. The impulses from these
centers, besides contracting the rectum, etc., also coordinate the contrac-
tion of the abdominal muscles and the relaxation of the. sphincter ani
so as to bring about the act of defecation. By the skiagraphic method it
has been found that the pelvic colon gradually becomes filled with feces
from below upward, and that the rectum remains empty until just before
defecation.
Errect or CiinicaL ConpItions ON THE MovEMENTS
Observations of practical value have been made on the behavior of the
peristaltic wave after various intestinal operations. After an end-to-end.
anastomosis of the gut, no evidence can be obtained by the x-ray method
that any hesitation occurs in the movement of the shadows at the anas-
tomosis. On the other hand, when a lateral anastomosis is established,
stagnation of the food in the region of the junction may occur, this
having been found, on opening the gut, to be caused by the accumu-
lation of hair and undigested detritus at the opening between the op-
posed loops. Another objection to lateral anastomosis is thé fact that
in performing the operation a considerable amount of the circular muscle
is cut, which interferes with peristaltic activity. Moreover, the end of
the proximal loop beyond the opening is in danger of becoming filled up
with hardened material, and the end of the distal loop may become
invaginated and induce obstruction in the region of the anastomosis.
Observations have also been made by the x-ray method on the be-
havior of the intestinal contents following intestinal obstruction. It has
been observed that, as the material collects in the gut just above the
obstruction, strong peristaltic waves are set up, which move the food
toward the obstruction so powerfully as to cause the walls of the canal
in front to become bulged, until at last the pressure causes the con-
tents to be squirted back through the advancing ring of peristaltic con-
traction. These waves were observed to succeed one another rapidly.
When a portion of gut is reversed in position, the peristaltic waves con- .
tinue to travel in their old direction toward the duodenum. The effect of
this is to produce a partial obstruction at the upper end of the re-
ceptive gut.
The type of peristalsis known as the peristaltic rush can be induced
experimentally in animals by intravenous injection of ergot. It prob-
ably also occurs in conditions of abnormal irritation of the gut in man,
and is believed to be the characteristic activity of the gut after a
strong purge.
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CHAPTER LIV
HUNGER AND APPETITE
Hunger and appetite are distinct sensations, the former being definitely
correlated with contraction of the empty stomach, and the latter being
a complex of sensations operating in the nervous system along with
memory impressions of the sight, taste, and smell of palatable food.
Appetite is therefore a highly complex nervous integration, whereas
hunger is a much simpler process. It is particularly with hunger that
we shall concern ourselves at present.
When a thin-walled rubber balloon of proper size is placed in the
stomach and connected by a rubber tube with a water, bromoform or
chloroform manometer (made of wide glass tubing 1.5 em. in diameter
and provided with a suitable float on the free limb) a tracing may be
taken of the movements of the stomach. For use on man the capacity of
the balloon should be from 75 to 150 cubic centimeters. The record thus
obtained when the balloon is placed in the empty stomach of a normal
person shows four types of wave. Two of these may be discounted,
being due to the arterial pulse and the respiratory movements. The
third is known as the tonus rhythm, and is caused by tonie contractions
of the fundus of the stomach of varying amplitudes and occurring at a
rate which varies from 18 to 22 per second. The periods of tonus in-
erease during the powerful rhythmic contraction to be immediately
described. While these changes in tone are occurring, no subjective sen-
sation of hunger is experienced. (See Fig. 167.)
The fourth and most significant type consists of powerful rhythmic
contractions, alternating with periods of quiescence. These contrac-
tions occupy a period of about twenty seconds, and are superimposed
upon the tonus rhythm. They gradually increase in amplitude and fre-
quency; and, in the case of young and vigorous persons, may gradually
pass into a condition of incomplete tetanus, after which they suddenly
subside, leaving only a faint tonus rhythm. These rhythmie contrac-
tions are definitely associated with the sensation of hunger, and are
more marked the more intense the sensation is. When tetanus occurs
the hunger sensation is continuous, but it instantly disappears when
the tetanus gives place to relaxation. When the contractions are com-
paratively feeble, the length of the period during which they occur is
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about twelve minutes, being shorter than when the contractions are power-
ful, in which case the periods are always initiated by weaker contrac-
tions with long intervening pauses. Finally, the pauses disappear and
the contractions become more and more pronounced until, as above men-
tioned, a virtual tetanus, lasting from two to five minutes, may super-
vene. The duration of the hunger period varies from one-half to one and
a half hours, with an average of from thirty to forty-five minutes, and
the number of individual contractions in a period varies from twenty to
seventy. Between the hunger periods, intervals of from one-half to
two and one-half hours of quiescence may supervene. (See Fig. 168.)
Similar contractions, often passing into incomplete tetanus, have been. .
observed in the stomach of healthy infants, some of the observations hav-
ing been made before the first nursing. The intervals of motor quies-
T
LD
WN
Fig. 166.—-Diagram of method for recording stomach movements. B, rubber balloon in stomach.
D, kymograph. F, cork float with recording flag. 4, manometer. LZ, manometer fluid (bromo-
form, chloroform, or water). , rubber tube connecting balloon with manometer. S, stomach.
T, side tube for inflation of stomach balloon. (From Carlson.)
cence between the hunger periods are shorter than in adults. In obser-
vations made during sleep, it was observed that, when the contractions
were very vigorous, the infant would show signs of restlessness and
might awake and cry. As in the adult, the contractions are evidently
associated with subjective sensations of hunger. Contractions. of the
empty stomach have also been recorded on a large variety of animals,
including the dog, rabbit, cat, guinea pig, bird, frog and turtle. They
vary somewhat in type in different animals.
With regard to the time of onset of the tonus and hunger contractions,
it has been observed that the only period during which the fundus is
free of them is immediately after a large meal. After a moderate meal
the tonus rhythm begins to appear in about thirty minutes. It gradually
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HUNGER AND APPETITE 473
increases in intensity, until by the time the stomach has nearly emptied
itself the tonus has become conspicuous, and the stronger hunger con-
tractions usually begin to appear. Superimposed upon those of the
tonus rhythm, hunger pangs may appear in man when the stomach still
contains traces of food.
Fig. 167.—Tracing of the tonus rhythm of he pomeee (man) three hours after a meal. (From
arlson.)
By studying the shadow of the outline of the stomach produced by
having a person or animal swallow two balloons, one inside the other
and with a paste of bismuth subnitrate between them, it has been ob-
served that the weaker type of hunger contraction begins as a con-
Fig. 168.—Tracings from the stomach during the culmination of a period of vigorous gastric hunger
contractions. One-half original size. (From Carlson.)
striction involving the cardiac end of the stomach, and moving toward
the pyloric end as a rapid peristaltic wave. When the contractions are
very vigorous, this wave spreads so rapidly over the stomach that it is
difficult to determine whether it really occurs as a very rapid peristalsis
or as a contraction involving the fundus as a whole. These contractions
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resemble very closely the movements that have sometimes been observed
after a bismuth meal, and which have been thought by clinical observers
to indicate a hyperperistalsis of the stomach. The fundus is therefore
not entirely passive during digestion; for, although early in this act
there may be no evidence of contraction, yet the contractions of the tonus
rhythm may appear and become pronounced before the stomach is en-
tirely empty. In other words, the digestion contractions of the filled
stomach (see page 451) pass gradually over into the hunger contractions
of the empty organ.
It appears that the stomach contractions produce the hunger sensa-
tions by causing stimulation of afferent nerve endings in the muscle
layers of the viscus. Mere pressure on the mucosa itself does not originate
such a sensation; thus, sudden distention of the balloon or rubbing the
mucosa with the closed end of a test tube, inserted through a gastric
fistula, did not cause any sensation of hunger, unless the stimulus was
so strong as to cause a contraction of the musculature of the stomach.
It has been thought by some observers that, during hunger, contrac-
tions similar to those of the stomach also occur in the lower end of the
esophagus. It is believed by Carlson, however, that these contractions
are not at all responsible for the hunger sensation, although they may
give rise to a feeling that something has stuck in the esophagus. Con-
tractions of the intestine have also been observed in hunger, but it is doubt-
ful whether they have anything to do with the cause of the hunger
sensation.
REMOTE EFFECTS OF HUNGER CONTRACTIONS
It is well known that during hunger certain general subjective symp-
toms are likely to be experienced, such as a feeling of weakness and a
sense of emptiness, with a tendency to headache and sometimes even
nausea in persons who are prone to headache as a result of toxemic
conditions. Headache is likely to be more pronounced or perhaps only
present in the morning before there is any food in the stomach. These
symptoms indicate that hunger contractions are associated with hyper-
excitability of the central nervous system, and ‘it is of considerable
interest that objective signs of this association can be elicited. If the
knee-jerk be recorded along with a record of the gastric contractions, it
will be found that it is markedly exaggerated simultaneously with the
strong hunger contractions of the empty stomach, this augmentation
being greatest at the height of the stomach contractions, when the hun-
ger pangs are most intense, and falling off again to normal when these
disappear (Fig. 169). Further changes occurring during the hunger
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HUNGER AND APPETITE 475
period include an increase in the pulse rate and vasodilatation. By
comparing plethysmographic tracings of the arm volume (see page 230)
and stomach contractions, it has been found that the increase in volume
occurs part passu with the increasing tonus of the stomach, but that it
begins to shrink before the stomach contraction has reached its maximum.
Occasionally, however, as in acute hunger, a somewhat different rela-
tionship obtains, vasoconstriction being more prominent. During each
hunger contraétion there is also increased salivation, the degree of
which varies with different individuals. This salivation is independent
of the more copious ‘‘watering of the mouth’’ that accompanies the
thought or sight of appetizing food.
moh ob bh
Fyadh aal
Fig. 169.—Showing augmentation of the knee-jerk (upper tracing) during the marked hunger con-
tractions (lower tracing). (From, Carlson.)
HUNGER DURING STARVATION
During enforced starvation for long periods of time, it is known
that healthy individuals at first experience intense sensations of hunger
and appetite, which last however only for a few days, then become less
pronounced and finally almost disappear. It is of interest to know the
relationship between these sensations and the hunger contractions in
the stomach. This has been investigated by Carlson and Luckhardt, who
voluntarily subjected themselves to complete starvation, except for the
taking of water, for four days. During a great part of this time records
of the stomach contractions were taken by the balloon method, and it
was found that the tonus of the stomach and also the frequency and
intensity of the hunger contractions became progressively more pronounced
as starvation proceeded. ‘Towards the end of the period it was also noted
that incomplete hunger tetanus made its appearance where ordinarily,
as in Carlson’s case, this type of hunger contraction was infrequent.
Sensations of hunger were present more or less throughout the period,
being therefore probably due to the persistently increased tonus. The
onset of a period of hunger contraction could usually be foretold by an
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inerease in the hunger sensation, and as these contractions became more
marked, the hunger sensations became more intense. On the last day of
starvation a burning sensation referred to the epigastrium was added to
that of hunger. The appetite ran practically parallel with the sensa-
tion of hunger, and both of these sensations became perceptibly dimin-
ished on the fourth or last day of starvation, this diminution being,
however, most marked in the sensation of appetite. Indeed, instead of an
eagerness for food, there developed on the last day a distinct repugnance
or indifference towards it. Accompanying these sensations of hunger
and appetite a distinct mental depression and a feeling of weakness were
experienced during the latter part of the starvation period.
On partaking of food again the hunger and appetite sensations very
rapidly disappeared, and also practically all of the mental depression
and a great part of the feeling of weakness. Complete recovery from
the latter, however, did not take place until the second or third day
after breaking the fast. From this time on both men felt unusually
well; indeed they state that their sense of well-being and clearness of
mind and their sense of good health and vigor were as greatly improved
as they would have been by a month’s vacation in the mountains. They
further point out that, since others who have starved for longer periods
of time unanimously attest the fact that, after the first few days, the
sensations of hunger become less pronounced and finally almost dis-
appear, they must have experienced the most distressing period during
their four days of starvation. Although the hunger sensation was
strong enough to cause some discomfort, it could by no means be ealled
marked pain or suffering, and was at no time of sufficient intensity to
interfere seriously with work. Mere starvation can not therefore be
designated as acute suffering. It is of further interest to note that dur-
ing the starvation period a continuous flow of secretion of acid gastric
juice was found to oceur in the stomach, the presence of this acid prob-
ably explaining the acid or burning sensation experienced in the epigas-
trium on the last days.
CONTROL OF THE HUNGER MECHANISM
The control of the hunger mechanism, like that of any other mechan-
ism in the animal body, may be effected through the nervous system or
it may depend on the presence of chemical substances or hormones in
the blood. As a matter of fact, it can readily be shown that both those
methods of control are operative, and we will now consider briefly some
of the facts upon which this conclusion depends.
Although many facts are now known with regard to the nervous con-
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HUNGER AND APPETITE 477
trol of the hunger mechanism, it is difficult to piece these together in
such a way as to formulate a simple theory which fits in with all the
observed facts. We know that the stomach possesses in itself a local
nervous mechanism by which, like the heart or intestine, it can auto-
matically perform many of the movements which are exhibited in the
intact animal. These local movements may, however, be considerably
influenced by impulses transmitted to the stomach along the vagus and
splanchnic nerves. We have therefore to seek for evidence indicating
the relative importance of the local nervous mechanism in the stomach
itself and of the impulses transmitted to this organ by the extrinsic
nerves. We must then seek the position of the center which perceives
the sensation of hunger. ,
It will be simplest to consider first the effect of section of the extrinsic
nerves in observations made on lower animals. Section of the splanchnic
nerves increases gastric tonus and augments the gastric hunger contrac-
tions. Section of both vagus nerves, performed of course below the level
of the heart, leaves the stomach in a more or less hypotonie condition.
The tonus is not entirely abolished; it varies somewhat from day to day,
and may become, quite pronounced even though the vagi are cut. In
this hypotonic state the hunger contractions are diminished in rate
and regularity. Section of both the splanchnie and vagus nerves throws
the stomach into a permanent hypotonus, except in prolonged starva-
tion, when hunger contractions develop that are usually of great ampli-
tude and with particularly long intervals between the contractions.
The general conclusion to be drawn from these experiments is that,
although completely ‘isolated from the central nervous system, the
stomach still exhibits typical hunger contractions, which must therefore
be essentially dependent upon an automatic mechanism in the stomach
wall itself. Over this mechanism, extrinsic nerve impulses have merely a
regulatory control.
Variations and Inhibitions of the Hunger Contractions
The afferent stimuli that may set up impulses traveling by the extrin-
sic nerves to the stomach are conveyed by the nerves of sense or are of
psychic origin. Stimulation of the gustatory end organs in the mouth,
as by chewing palatable food, always causes an inhibition of the tonus
and a diminution or disappearance of the hunger contractions. Even the
chewing of indifferent substances, such as paraffin, suffices to produce
distinct inhibition, unless in a case in which the contraction has passed
into a tetanus. It is of interest that swallowing movements, in the ab-
sence of any food substance in the mouth, are sufficient to produce a
transitory inhibition of the gastric tonus—a receptive relaxation of the
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478 DIGESTION
stomach, as it has been aptly called. The diminution in tonus and
hunger contractions in these various ways is accompanied by a diminu-
tion in the hunger pains.
Afferent nerve stimulation affecting the hunger contractions may also
originate in the stomach mucosa itself, as has been shown by Carlson on
his patient by introducing the various substances to be tested through
a tube into the stomach. A glassful of cold water introduced in this
way inhibits the tonus and the hunger contractions for from three to five
minutes unless these are severe, this inhibition being followed by no
augmentation either of the tonus or of contractions. Ice-cold water has
a greater effect than water at body temperature. This result is some-
what different from that which most men experience as the result of
drinking a glass of cold water.
Weak acids of strengths varying up to that found present in the
gastric juice itself—0.5 per cent—cause a marked inhibition of the
hunger movements, but this inhibition does not persist until all the acid
has escaped from the stomach or been neutralized, which explains why
hunger contractions should still occur when an acid secretion is present
in the stomach, as in starvation. Normal gastric juice itself produces
an inhibition, which is no doubt dépendent upon the acid which it. con-
tains, and it is probable that, at the same time that it leads to inhibition
of the hunger contractions, the acid initiates peristalsis of the pyloric
region (see page 453). Weak alkaline solutions have no greater effect on
the hunger contractions than an equal volume of water. Weak solu-
tions of local anesthetics, such as phenol or chloretone, are without effect.
With regard to alcoholic beverages interesting results were obtained.
Wine, beer, brandy, and diluted pure alcohol inhibit both the tonus and
the contractions. The duration of this inhibition varies directly with the
quantity of the beverage introduced into the stomach and with its alco-
hol percentage. These observations are apparently not in harmony with
the experience of most men that the taking of alcoholic beverages serves
to awaken or increase the appetite, the difference being no doubt due to
the fact that appetite and hunger contractions of the stomach are not
dependent on each other, appetite being, as we have seen, a complex
psychic affair, whereas the hunger contractions depend upon a local
mechanism in the stomach wall. itself.
As the inhibition produced in one or other of these ways passes off,
the hunger contractions are resumed at their previous intensity and not
in an augmented form. From the promptness of the inhibition, it would
appear that the stomach contractions are affected, not reflexly through
the central nervous system or by changes in the chemical composition
of the blood, but by a direct action on the neuromuscular mechanism
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HUNGER AND APPETITE 479
in the stomach walls, and it is important to bear in mind that the
inhibitory effects on the stomach contractions of the fundus may proceed
quite independently of the changes in the pyloric region that are con-
cerned with the mechanical processes of digestion. After one or both
of the extrinsic nerves of the stomach were severed in dogs, a certain
degree of inhibition could still be induced by the above methods, indicat-
ing that, although section of the extrinsic nerves depresses the inhibitory
reflex, it does not abolish it. ,
Various mitigations of the hunger contractions have been discovered.
Smoking has this effect, and compression of the abdomen by tightening
the belt also inhibits the contractions provided they are not of marked
intensity. Considerable muscular exercise, such as brisk walking or
running, causes inhibition, which usually persists until after the exer-
cise is discontinued. When the tonus and contractions return, in this
case, they seem to be somewhat more pronounced. Application of cold
to the surface of the body—as by placing an ice pack on the abdomen
or taking a cold douche, procedures which are well-known to induce
increased neuromuscular tonus, in general—causes an inhibition of the
gastric tonus and hunger contractions, the degree of which is roughly
proportional to the intensity of the stimulation. There is certainly never
an increase in the gastric tonus or hunger contractions. If such stimula-
tion is maintained, the inhibitory effects on the stomach gradually
diminish, even though the individual be shivering intensely.
With regard to the nerve centers concerned in these phenomena, little
that is definite is known. The sensory nuclei of the vagus nerve in the
medulla must be considered as the primary hunger center, and through
this center, not only influences affecting the stomach contractions, but
also those associated with the hunger sensations, must be mediated. It
would appear from observations on the hunger behavior of decerebrate
animals that there can be no hunger center located on the cerebral cortex
itself, for such animals exhibit practically the same hunger effects as
normal animals. It is interesting to note that, at least in the case of
decerebrate pigeons, this hunger behavior entirely disappears on removal
of the optic thalami, where important nerve centers having to do with
the bodily responses of the animal to hunger impulses would therefore
appear to be located. These observations support the suggestion that
has been made by several neurologists that the sense of pain is located
somewhere in the thalamic region.
Concerning the influence of psychie states, Carlson says that in his
own case the hunger contractions became weaker and the intervals
between them greater when he was suddenly awakened during his
fast and saw two of his friends partaking at his bedside of a ‘‘feast of
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porterhouse steak with onions, potatoes, and a tomato salad.’? These
results are no doubt due to local inhibition dependent upon the psychic
secretion of appetite gastric juice. When no such juice is produced,
the sight and smell of good food does not appear to affect materially
the hunger contractions of the stomach. No doubt it stimulates the
appetite, but that, as we have seen, is a psychic affair.
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CHAPTER LV
THE BIOCHEMICAL PROCESSES OF DIGESTION
In a book designed primarily for clinical workers, it would be out of
place to enter into details concerning the biochemical processes taking
place during the digestive process. There is, however, a certain amount
of fundamental knowledge which it is essential that we should consider.
In the first place it should be borne in mind that in the digestion of
carbohydrates and proteins, various intermediate stages are passed
through before the final absorption products are formed. The highly
complex molecule of which protein, for example, is composed, is first
of all broken down into several smaller but still highly complex mole-
cules, each of which then undergoes further disruption, until ultimately
the amino acids are set free. Certain enzymes, such as trypsin, can
carry this process from the beginning through the greater part of its
course without the assistance of other enzymes, but in the natural proc-
ess of digestion, as it occurs in the gastrointestinal tract, the different
stages of the disruption are controlled by different enzymes. One enzyme
prepares the food for action by the next. This interdependence of the
actions of the enzymes demands that some provision should be made
whereby each enzyme is secreted at the proper time; that is, when the
foodstuff has already been prepared for its action by that of its prede-
cessor. Thus, it would be useless after food is taken for the gastric and
pancreatic juices to be secreted at the same time. Instead, the gastric
juice is secreted first, and the pancreatic only after the food has been
prepared for its action. This correlation in function we have already
seen to be dependent largely on the action of hormones.
DIGESTION IN THE STOMACH
The gastric juice contains two important digestive agencies: (1) the
enzyme, pepsin, and (2) hydrochloric acid. It is particularly in juices
secreted in the cardiac end of the stomach that these two substances are
found present; towards the pyloric end the hydrochloric acid entirely
disappears, and the pepsin content becomes distinctly less.
; 481
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482 DIGESTION
The Functions of Hydrochloric Acid
The functions of hydrochloric acid may be conveniently divided into
physiological and biochemical. The former functions have to do with
the control of the movements of the stomach, including the opening
of the pyloric sphincter, and, after the chyme has entered the duodenum,
with the secretion of pancreatic juice and bile. The biochemical functions
are concerned: (1) in assisting the pepsin in the digestion of proteins,
(2) in bringing about a certain amount of inversion of disaccharides,
and (3) in having an antiseptic action on the stomach contents. Re-
garding the last mentioned of these functions, it may be said that the
chyme, as it is ejected from the stomach, is usually sterile, although it
may contain spores and certain bacteria that are protected against the
digestive agencies of the stomach. This protection is afforded by an
outer covering of a chitinous nature (spores), or, as in the case of the
tubercle: bacillus, by a covering of waxlike material. It is believed that
persons with ‘strictly normal digestion are much less liable to infection
by such bacteria, as those of typhoid and cholera, than persons with less
active gastric secretion. When the acid of the gastric juice falls below
the level at which it develops an antiseptic action, various bacteria and
yeasts grow in the stomach contents, producing by the resulting fermen-
tation irritating organic acids and gases. It is-under these conditions
that yeasts, sarcine, and lactic and butyric acid bacilli find in the gastric
contents a suitable nidus on which to grow.
Tre AMOUNT oF ACID
It has long been known that considerable variations in the amount of
hydrochloric acid in the gastric juice are associated with symptoms of
indigestion. On this account a more or less elaborate technic has been
developed for the purpose of determining the amount of hydrochloric
acid in the gastric contents.* There are three things in connection with
this activity that we may measure: (1) the total titrable hydrochloric
acid; (2) the free hydrochloric acid; and (3) the actual hydrogen-ion
concentration. The determination of the total available acids is made
by titrating a measured quantity of gastric juice against a standard
alkali, using phenolphthalein as an indicator. By this method about
75 ¢.c. of decinormal alkali solution are required to neutralize 100 c.c.
of normal gastric juice. The determination of the free hydrochloric acid
is made by using special indicators, such as those of Giinzberg and
Tépfer, which change color at a hydrogen-ion concentration of about
10° (see page 27). To produce this hydrogen-ion concentration, a con-
*The methods can be found in any volume on clinical diagnosis.
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THE BIOCHEMICAL PROCESSES OF DIGESTION 483
siderable quantity—0.05 per cent or more—of an organic acid is neces-
sary, whereas it requires only a trace of hydrochloric acid. Normal.
human gastrie juice, when titrated with one of these indicators, gives
a figure which corresponds to about 0.03 N. hydrochloric acid (see page
22). For the accurate determination of the hydrogen-ion concentration,
it is necessary to use the gas-chain method (see page 29).
When gastric juice is eollected through a fistula from an empty
stomach, very little difference will be found between the free hydro-
chloric acid and the total acid; that is, between the results obtained by
the second and the first of the methods described above. This is because
in such juice there is no organic matter capable of combining with the
hydrochloric acid, and there are no other acids, such as lactic or butyric,
which might be produced by fermentative processes. The difference
between the two titrations, however, becomes quite marked when pro-
tein food is undergoing digestion in the stomach, because at its different
stages of digestion protein combines with increasing quantities of the
hydrochloric acid. The pathologie condition in which there is most
definitely a diminution of the hydrochloric acid is cancer, either of the
stomach itself or occasionally of some other part of the body. An in-
crease is particularly marked in ulcer of the stomach. A considerable
variation in hydrochloric acid may however be the result merely of func-
tional (neurotic) conditions.
Tur Source or THE ACID
A question that has puzzled physiologists for many years concerns the
mechanism by which hydrochloric acid is secreted. The percentage of
hydrochlorie acid in the gastric juice is considerably above that at which
any animal cells can live, and yet this acid is secreted by the lining
membrane of the stomach, its source being, of course, the sodium
chloride of the blood plasma. How then do the cells of the gastric
glands bring about the separation of this powerful acid from the per-
fectly neutral blood plasma? In the first place, it is significant that the
mucous membrane of the stomach contains a higher percentage of
chlorine than the average of other organs and tissues, indicating that it
has the power of abstracting chlorine from the blood. The excess of
chlorine in the mucosa must, moreover, be but a very small fraction of
that actually secreted into the the gastric juice. The chlorine content
of the mucosa of the cardiac end is considerably greater than that of the
pyloric. These facts indicate that chlorine is attracted by the gastric
cells, but they throw no light on the question as to where the hydro-
chlorie acid is really formed. Is it in the cells, or only in the lumen of
the gland tubes? That is to say, is it formed before or after the gastric
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484 DIGESTION
juice has been secreted from the cells? After intravenous injection of
solutions of potassium ferrocyanide and some inert salt of iron, such as
one of the scale preparations, examination of the gastric glands has
shown that the prussian blue reaction, which requires the presence of
free mineral acid, is most pronounced in certain of the parietal cells. A
considerable amount of the precipitate is, however, also visible in the
lumen of the glands and in the stomach itself. Certain observers affirm
that, although some of the parietal cells may take the stain, the vast
majority of them do not do so; and, moreover, that cells incapable of
forming hydrochloric acid (e. g., of the liver) may also become stained,
and that the precipitation may occur in the blood and lymph.
The confusion in the results by these methods prompted A. B. Macal-
lum™“ and Miss M. P. Fitzgerald to investigate the distribution of the
chlorine in the cells by a microchemical method, in which the chlorides
were precipitated with silver nitrate and the silver chloride then reduced
by exposing the section to light. It was found that both kinds of gas-
trie-gland cell, chief and parietal, but particularly the parietal, gave the
chloride reaction. Using as a stain a substance (eyaninine) which reacts
blue with acid and red with alkali, Harvey and Bensley,® however, aver
that the secretion of the glands is practically neutral until the foveola is
reached, where the stain becomes blue, indicating an acid reaction.
This seems to show that the acid is not really secreted by the cells of
the gastric gland, but is formed after secretion.
According to the latter investigators, the chlorine is secreted by the
cells into the fovea as some weak chloride, such as ammonium chloride,
or it may be as an ester. Shortly after its secretion this weak chloride
undergoes a hydrolytic or other dissociation, during which free hydro-
chloric acid is liberated and ammonia or some other weak base set free.
Of these two products of the reaction the weak base is reabsorbed by
the gland cells, but the hydrochloric acid is left behind because the
cells are impervious to it. Indirect evidence in support of this view is
afforded by certain other instances in which hydrochloric acid is pro-
duced by the action of cells; thus, the mold Penicillium glaucum when it
is grown in a medium containing ammonium chloride absorbs the am-
monia but leaves the hydrochloric acid. The high penetrating power
of the ammonia ion in practically all cells, and the fact that the mucosa
of the stomach contains a higher percentage of ammonia than any other
tissue in the body, must also be considered as circumstantial evidence
in favor of this view.
Whatever be the mechanism by which hydrochloric acid is produced,
there is no doubt that the’ epithelium is impenetrable to it. When the
vitality of the epithelium becomes lowered, as in anemia or after partial
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THE BIOCHEMICAL PROCESSES OF DIGESTION 485
ocelusion of the arteries, the acid may penetrate the cells and cause
digestion of the stomach walls. Hyperacidity may on this account
become dangerous, as it lowers the resistance of the cell.
The digestive action of hydrochloric acid is closely linked with that of
pepsin, with which it will, therefore, be considered.
The Action of Pepsin
It is commonly believed that before its secretion pepsin exists in the
cells of the gastric glands as zymogen granules. The chief evidence for
this belief appears to be that after considerable activity the amount of
zymogen granules in the gland cells is found to be decidedly dimin-
ished. By such an hypothesis it is easy to explain certain interesting
results concerning the effect of weak alkali on the activities of extracts
of the mucous membrane of the stomach. When the mucous membrane
is extracted with weak acids, the extract is very active proteolytically.
If this so-called pepsin solution be made faintly alkaline, or even only
neutralized, and again made acid, it will be found to have lost much,
if not all, of its activity. On the other hand, an aqueous extract may be
rendered slightly alkaline for a short time and still display its digestive
activity on subsequent acidification. The extract made with water is
therefore much more resistant toward alkali than that made with weak
acid, and the difference is explained on the supposition that the watery
extract contains pepsinogen, whereas the acid extract contains pepsin.
It is believed that there are several varieties of pepsin, because the
optimum concentration of acid in which pepsin derived from the stomachs
of different animals acts is not always the same. Pepsin of the dog, for
example, acts best in a hydrogen-ion concentration corresponding to
that of a 0.05 N. hydrochloric acid solution, whereas that of the human
stomach works best at a concentration of 0.03 N. Different pepsin
solutions also show a difference with regard to the optimum tempera-
ture at which they act, and with regard to the nature of the protein
which they most readily attack. Thus, the pepsin of a calf’s stomach
digests casein very rapidly, but coagulated egg white only slowly,
whereas the pepsin of the pig’s stomach acts on both these proteins at
about the same rate.
It is well known that the activity of pepsin can proceed only in the
presence of acids, but this action of acids does not appear to depend on
the hydrogen-ion concentration alone, for when equal quantities of the
same pepsin are mixed with quantities of different acids so that the
hydrogen-ion concentration of the mixtures is uniform, it is found that
digestion proceeds most rapidly with hydrochloric acid and least rapidly
with sulphuric acid. The SO, ion seems, therefore, to be unfavorable
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486 DIGESTION
for peptic activities. The acid seems-to combine with the protein before
the pepsin attacks the latter; for, if we first combine the protein with
acid and then wash away all traces of free acid, the protein can be
digested in a neutral pepsin solution without the liberation of any free
acid.
There is evidence to show that pepsin itself also becomes combined
with the protein during the digestive process. If a piece of protein such
as fibrin be immersed in a solution of pepsin and then taken out and
washed thoroughly to get rid of all adherent pepsin, it will be found, on
placing it in a hydrochloric acid solution of the proper strength, that
peptic digestion proceeds. Advantage may be taken of this fact to
separate pepsin from a solution, but the best protein to use for this pur-
pose is not fibrin but elastin. By such a method it has, for example,
been shown that there is some pepsin in the intestinal contents, proving
thus that when the chyme passes into the intestine, the pepsin is not, as
used to be thought, immediately killed by the proteolytic enzyme.
Propucts oF Peptic DigESTION
With regard to the products of gastric digestion, little can be said
here. The first product is a metaprotein known as acid albumin or
syntonin. It is precipitated from the digestion mixture by neutraliza-
tion. The next product is known as primary proteose, being precipi-
tated by half saturation with ammonium sulphate. The third product
is secondary proteose, produced by complete saturation with the above
reagent; and after all these bodies have been separated out, there re-
mains in solution the fourth product—peptone—which among other
things is characterized by the fact that with the biuret test it gives not
a violet but a rose-pink color.
It has often been claimed that along with these products a certain
amount of free amino acids may also appear in a peptic digestive mix-
ture. This, however, may be due to the action of erepsin, which is
usually present in pepsin preparations. It is important to note that the
term proteose is a general one, and that there are probably many varieties
of this substance, differing from one another according to the protein
from which they are derived.
The change produced by pepsin and hydrochlorie acid is of the nature
of an hydrolysis, for it has been found that the amount of hydrogen and
oxygen in the digestive products is greater than that in the original
protein. It is by a similar process of hydrolysis that the other proteolytic
enzymes, such as pancreatin and erepsin, operate, but this does not
imply that the exact grouping that is split apart: by the hydrolytic proc-
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THE BIOCHEMICAL PROCESSES OF DIGESTION 487
ess is the same for each of these enzymes. Indeed, there is considerable
evidence that pepsin does not, like the other enzymes, break up the long
chain of amino acids that are linked together to compose the polypep-
tides, but that it only splits the big molecule of albumin or globulin
into several large groups, each of which is composed of long amino-acid
chains. Its action appears to be analogous with that of amylase on
starch, by which, it will be remembered, the big polysaccharide mole-
cule is split into smaller polysaccharide molecules, which then become
attacked by the dextrinase and split into disaccharide molecules (see
page 656). The evidence in support of this view is: (1) that pepsin is
unable to digest polypeptides, and (2) ‘that it is able to. digest certain
proteins upon which erepsin (see page 490) has no action.
The hydrolytic splitting of large into smaller protein molecules, like
that by which the chains of amino acids in the polypeptides are subse-
quently broken up, consists in a breaking of amino-carboxyl linkings
(NHCO) (see page 598), with the consequent liberation of a large num-
ber of unattached amino groups. The number of these free amino groups
can be determined quantitatively by the formaldehyde titration method
of Sérensen.* By this method it can be shown that from the very start
of peptic digéstion the number of free amino groups increases, and pari
passu the power of the digestive products to combine with free hydro-
- chlorie acid. Indeed, when the experiments are done quantitatively and
the digestion allowed to proceed for a considerable time, the increase in
- the formol titration is practically equal to the decrease in the free acids
as determined by the Giinsberg reagent.
The rate of peptic digestion is usually estimated by the law of Schiitz
and Borissow, according to which the amount of coagulated albumin
that is digested in a Mett’s tube is proportional to the square root of the
amount of pepsin.t
The pepsin which leaves the stomach in the chyme is not all destroyed
in the intestine, as was at one time believed to be the case, for, as we
have seen above, some pepsin can be detected in the gastrointestinal con-
tents. A part of the pepsin may be absorbed into the blood and carried
back to the gastric glands to be used again. This would account for the
presence of antipepsin in the blood, and also for the presence of pepsin
in the urine. It is probable, however, that most of the pepsin is de-
stroyed after it enters the intestine.
*In this method the basic character of the amino acids is destroyed by the formaldehyde, so
that a higher degree of acidity develops in the mixture. By determining the increased acidity by
titration with alkali, an estimate is obtained of the number of amino groups. (See page 599.)
fThe amount of coagulated egg albumin digested is ascertained by measuring the length digested
away from the end of a column of coagulated egg white contained in a glass tube (Mett’s method).
(See Cobb, P. W.: Am. Jour. Physiol., 1905, xiti, 448.)
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Clotting of Milk in the Stomach
Besides its power of digesting protein, the gastric juice is also endowed
with the property of clotting milk. This action is commonly attributed
to the presence of another enzyme besides pepsin, namely, rennin; but
in recent years considerable controversy has raged around the question
as to whether pepsin and rennin are not the same thing. One strong
argument in favor of this view is that all digestive juices that are capable
of digesting protein can also clot milk. In any case, when gastric juice
acts on milk, it splits the casein* of the milk into two portions, one of
which, called paracasein, immediately combines with calcium to form an
insoluble colloidal compound, which is precipitated and, by entangling
the fat of the milk, forms the clot; the other protein remains in solution
and is known as whey albumose. From studies on molecular weight it
‘is believed that the paracasein is produced from casein by the splitting
of the molecule of the latter into two, from which it would, appear that
the action of this enzyme is nothing more than the first stage in the
hydrolysis of the casein molecule. The whey albumose, according to this
view, is a by-product. /
There are many investigators, however, who believe that rennin and
pepsin are not identical, since an infusion of the stomach of a calf has a
powerful clotting action on milk but a very weak digestive one on egg
white, whereas a similar infusion from the stomach of a pig shows exactly
the reverse properties. This question is one of so controversial a na-
ture that it- would be out of place to go into it further here. It
should be pointed out, however, that, when the gastric contents are acid
in reaction, milk will become clotted by the action of the acid itself
quite independently of any pepsin or rennin the juice may contain.
This acid clotting of milk is probably of a different chemical nature
from that produced by the enzymes.
On other foodstuffs than proteins the action of the gastrie juice is
relatively unimportant, although polysaccharides may be. considerably
broken down in the sardiac end of the stomach on account of the action
of swallowed saliva (see page 454), and disaccharides, as we have seen,
may become split by the hydrolyzing effect of the hydrogen ion. Fat
digestion also takes place in the stomach when the fat is taken in an
emulsified condition, as in milk and egg yolk, but not when in masses,
as in meat or butter. This action is due to the presence of a fat-splitting
enzyme, or lipase, in the gastric juice.
*In the above nomenclature casein is the same as caseinogen, and paracasein the same as casein,
of the English physiologists.
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CHAPTER LVI
THE BIOCHEMICAL PROCESSES OF DIGESTION (Cont’d)
DIGESTION IN THE INTESTINES
The further changes which the half-digested foodstuffs in the chyme
undergo in the intestinal canal depend on the enzymes present in the
secretion of the various glands and on the presence of bacteria. The
most important of the digestive juices are the pancreatic juice and bile.
The latter, however, does not contain any enzyme, its influence on diges-
tion being entirely adjuvant.
Pancreatic Digestion
When we were considering the mechanism of secretion of the pan-
creatic juice, we saw that the juice produced by the action of secretin on
the gland cells does not contain any active proteolytic enzyme, although
it contains one capable of acting on polysaccharides and another, on fat.
Tue ActTION oF TRYPSIN
When this juice is mixed with the secretion of the duodenum or of
the upper part of the small intestine, it immediately develops powerful
proteolytic power. The same result may also be obtained by mixing it
with an extract of the mucous membrane of the duodenum made with
dilute bicarbonate solution. A very small amount of the extract is
capable of increasing the digestive activity of a very considerable quan-
tity of pancreatic juice, showing that the action depends on the presence
of an enzyme which has been called enterokinase. This influence of the
intestinal secretion is readily destroyed by heating.
Large quantities of alkali are contained in the pancreatic juice and
bile, so that in the upper reaches of the intestine the acidity of the
chyme is practically neutralized. A little lower down, however, an acid
reaction may. again develop (see page 505). On account of these facts it
has been concluded that the activity of trypsin is most rapid in the pres-
ence of a slight excess of hydroxyl ions; i.e., in a weakly alkaline solu-
tion. It is interesting to note that, as a result of the great secretion of
alkali by the pancreas, extracts of this organ after death show a very
high degree of acidity in comparison with extracts from other organs
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490 DIGESTION
and tissues. It has also recently been shown that the activity of trypsin
does not depend on the presence of free hydroxyl] ions, but that it may
proceed in the presence of free acid, even up to a strength of Cy=1.5.
If pepsin is present together with trypsin in a distinctly acid solution,
the pepsin seems to destroy the trypsin, unless the mixture contains a
considerable quantity of protein, when the tryptic activity may persist
even for several hours. A practical conclusion that we may draw from
‘these results is to the effect that preparations of trypsin—the so-called
pancreatin, for example—if given with the food, may pass in an active
condition into the duodenum, where, in the more favorable environment
created by the neutralization of the excess of acid, it will develop its
proteolytic power. The therapeutic administration of pancreatin is,
therefore, justified (Long).
The activated trypsin acts on proteins in very much the same way as
pepsin, except that the decomposition of the peptone and proteoses into
polypeptides is the chief feature of the process. Thus, after tryptic
digestion has proceeded for some time, only a trace of primary proteoses
but considerable quantities of leucine, tyrosine and other amino acids
will be found present. Some investigators believe that the thorough
nature of the digestive action of activated pancreatic juice may depend
on its also containing erepsin, an enzyme which we shall see to be pres-
ent in considerable amount in the mucous membrane of the intestine and
other tissues, and whose particular function is to split polypeptides into
the amino acids. From the autolytic digestion which takes place in
organs kept in a sterile condition after death, tryptic digestion differs
in that it produces only small quantities of ammonia. The large quanti-
ties of ammonia produced in autolytie digestion no doubt have a rela-
tionship to the acids simultaneously set free during this process.
In the products of tryptic digestion it is usually found that, although
there has been considerable splitting of the protein into amino acids,
there are still a good many amino-carboxyl (NHCO) linkages left un-
broken, indicating that certain polypeptides are left intact in the mix-
ture. To split the polypeptides requires the aid of the erepsin, which is
present in the mucous membrane of the intestine. Interesting inves-
tigations have been made on the exact degree to which trypsin-entero-
kinase can split up the various known polypeptides. This seems to
depend on the structure of the polypeptide molecule and on the number
of amino acids present in the chain. For example, analylglycine, but
not glycylalanine is hydrolyzed, although both contain the same amino
acids but linked together in a different way; and tetraglycylglycine,
which contains five glycine radicles, is hydrolyzed, whereas diglycylgly-
eine, which contains only three, is not.
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THE BIOCHEMICAL PROCESSES OF DIGESTION 491
The importance of the presence of erepsin in the mucous membrane
of the intestine is that it serves as a barrier to the passage of any unsplit
amino acids from the intestinal contents into the blood. It insures the
breaking up of the protein molecule into its ultimate units before absorp-
tion. The further fate of the absorbed amino acids will be considered
under the subject of protein metabolism.
Tur Action or LIPASE
Neutral fat is decomposed into fatty acids and glycerine by the lipase
present in the pancreatic juice. This enzyme may also be extracted from
the glands by means of 60 per cent aleohol. Its action is remarkably
accelerated by the presence of bile, and considerably depressed by inor-
ganic salts. It is also very dependent on the degree of alkalinity, the
optimum being a hydrogen-ion concentration of Hx 10°. The favoring
action of bile is undoubtedly owing to the bile salts (see page 493), and
it is probable that this action is dependent upon the influence which
these -have in lowering surface tension and therefore bringing about a
more intimate contact between fat and water.
Tor ACTION oF AMYLOPSIN
The action of pancreatic juice on carbohydrates depends on the
amylolytic enzyme called amylopsin. In animals having no active ptyalin
in the saliva, amylopsin serves as the only diastatie enzyme concerned
in the digestive process. In any ease, at least for the first stages of the
disruption of the starch molecule—that is, its conversion into dextrines—
amylopsin is a more powerful enzyme than ptyalin. It does not appear
to be so efficient as ptyalin in the final stages of the hydrolysis, for it
does not produce so much reducing sugar as ptyalin does. Indeed ex-
tracts of pancreas will sometimes convert starch into soluble starch and
dextrine with great speed, but produce scarcely any reducing sugar.
On this account it is believed by many investigators that there are at least
two distinct and separate enzymes in amylopsin and also perhaps in
ptyalin, one a true amylase, which converts starch into dextrine, and
the other a dextrinase, which converts dextrine into maltose. In the
ease of both ptyalin and amylopsin digestion proceeds best in a very
weak acid reaction. Amylopsin, as it is secreted in the pancreatic juice,
is fully activated; bile, apart from the alkali which it contains, having
no influence on its digestive power.
Besides amylopsin the pancreatic juice also contains maltase, and in
the case of young animals or of those that take milk with their food
throughout their lives, lactase also. After the suckling animal has dis-
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492 ; DIGESTION
continued taking milk, the lactase disappears from the pancreatic juice.
Attempts have been made to bring it back by feeding the adult upon
milk, but without success. Occasionally the pancreatic juice also con-
tains invertase.
The Bile
Associated with the pancreatic juice in all its functions is the bile.
When this fluid is prevented from entering the intestirie, the digestive
process becomes very imperfect, the absorption of fat being particularly
interfered with (see page 691). Bile is also an excretory product, and
its composition therefore is much more complex than that of the other
digestive fluids. This varies very much, however, according to the
method of collection. Bile from the gall bladder after death contains
much more solid material, particularly bile salts and mucin, than that
collected from a fistula of the bile duct or gall bladder during life.
These differences will be evident from the accompanying table.
Bile from
Gall bladder Fistula
100 parts contain— ‘
Wiad OR sevaie sd iesteneore oauete. butnaisaree eraiac 86 97
SOlids: wind ated wee aie ages 14 3
Organic salts (bile salts)......... 9 0.9-1-8
Mucin and bile pigment.......... 3 0.5
ChOlestEPOl? ceases cee venue eyes 0.2 0.06-0.16
Lecithin and fat..........-...-. - _0.5-1.0 0.02-0.09
Inorganic salts ...........00.00- 0.8 0.7-0.8
In general it may be said that bile obtained from a fistula in man
contains only about 3 per cent of total solids, of which from one-fourth
to one-half are inorganic, whereas bile from the gall bladder contains
10 to 20 per cent of total solids, of which only about one-twentieth are
inorganic. The chief cause for this difference appears to be that when
the bile goes to the intestine, a considerable proportion of its bile salts
is reabsorbed into the portal blood and reexcreted by the liver. Some
of the difference may also be caused by the fact that absorption of
water takes place from the gall bladder, and that mucin and possibly
cholesterol are secreted by this organ. These striking differences be-
tween fistula and gall-bladder bile are observed only when the com-
mon bile duct is occluded. If the bladder fistula is made with the com-
mon duct left open, some of the bile gains entry to the duodenum and
therefore becomes reexcreted. It is well known that a fistula of the gall
bladder in man after a time closes up and the bile again takes its usual
course along the bile duct into the duodenum.
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THE BIOCHEMICAL PROCESSES OF DIGESTION 493
Interesting observations have been collected on the amount of the secre-
tion from a fistula both in man and in the lower animals. In man it is
commonly stated that about 500 ce. of bile are secreted daily, the
amount varying considerably during the different hours of the day. The
secretion of bile is greatly reduced by hemorrhage. It is greater on a
meat diet than on one of carbohydrates. It is reduced during starva-
tion, but continues to be secreted up to the moment of death.
FuNcTIONS OF BILE
One of the main functions of the bile salts is that they greatly assist,
not only in the digestion, but also in the absorption of fats. When bile
is excluded from the intestine, the feces are loaded with fatty acids
which have been split off partly by the now less effective lipase and
partly by the action of bacteria. The fatty acid thus liberated in the
absence of bile salts is not absorbed, because the bile salts serve as the
earriers of fatty acids into the epithelial cells and lacteals. They com-
bine with the fatty acids, probably by forming some chemical compounds,
in which they carry them into the endothelial cells where the compounds
become disrupted, the fatty acid combining with glycerine to again form
neutral fat and the bile salts being carried to the liver and reexcreted.
The influence of bile salts in assisting the action of lipase is probably
due to a lowering of the surface tension, thus bringing water and fat
into closer union. This accelerating influence has also been demonstrated
when synthetic bile salts have been used, showing clearly that it is really
these and not any other constituent of the bile that are responsible for
its accelerating influence.
Bile also functionates as a regulator of intestinal putrefaction. This
it does apparently because of its slight laxative properties, by which
the intestinal contents are expelled before the bacteria have grown to
any great extent.in them. Bile itself is a favorable culture medium for
certain bacteria, so that it can have no antiseptic action. Its assistance
in the action of trypsin and amylopsin depends very largely upon the
alkali which it contains.
As an excretory vehicle bile is important, because it possesses the
power of dissolving cholesterol. Toxins and metallic poisons of various
kinds are also excreted in it.
Although not directly concerned with the digestive function, it will be
convenient to say something here concerning the chemical nature and
derivation of the various biliary constituents.
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494 DIGESTION
THE CHEMISTRY OF BILE
The Bile Salts
In most animals the bile salts consist of the sodium salts of glyeocholic
and taurocholie acids. Each of these acids is composed of a part called
cholic acid which is more or less related to cholesterol, and of glycine
(CH,NH,COOH amino-acetie acid) or taurine (C,H,NSO,), a derivative
of cysteine, which is a-amino-8-thiopropionic acid (CH,HS.CHNH,.
COOH). The exact form of cholic acid varies in different animals, that
of the pig, for example, being different from that of man. Bile salts are
an exclusive product of liver metabolism; i.e. they are not formed in
any other part of the animal body. They give a very sensitive color
reaction known as Pettenkofer’s, which however is not specific of bile acids,
since it is also given by oleic acid and by many aromatic substances and
alcohols. It mtist be remembered that the part of the bile salts that is
chafacteristic of the liver is the cholic acid, the taurine and glycine
being present in other tissues and organs.
When cholic acid is given to animals mixed with the food, the amount
of taurocholic acid excreted with the bile is increased, indicating that
there must be a store of taurine available in the organism. This store
can not, however, be large, for if the feeding with cholic acid is repeated
several times, it will be found that the taurocholic acid diminishes and
glycocholic acid takes its place; and this increased excretion of glyco-
cholic acid goes on just as long as cholie acid is fed. The reserve of
taurine in the animal body appears therefore to be limited, although it is
used in preference to glycine when there is an excess of cholic acid to be
neutralized. On the other hand, the store of glycine seems to be inexhaust-
ible. That there is no reserve of cholic acid itself in the body is indicated by
the fact that no increase in taurocholic acid excretion by the bile results
when eystine, the mother substance of taurine, is given with the food.
If both taurine and cholic acid be fed, however, the excretion of tauro-
cholic acid increases. ;
The relative amounts of taurocholic and glycocholie acids in the bile of
different animals differ considerably. Human bile contains relatively
a small amount of taurocholic acid; on the other hand, the bile of the dog
contains a large &xcess of it.
Cholesterol
In human bile the percentage of this important substance is not high
(1.6 parts per 1000), but it is of great clinical importance because of the
fact that it may separate out as a precipitate forming gallstones. The
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THE BIOCHEMICAL PROCESSES OF DIGESTION 495
percentage of cholesterol in these varies from 20 ‘to 90; the remainder
being organic material such as epithelial cells, inorganic salts, pigment,
ete. The origin of cholesterol is partly endogenous and partly exoge-
nous. In the former case it comes from the envelope of red blood cor-
puscles and from the nervous tissues, where it is present in considerable
amount. The latter source is, of course, the food. The increase in
cholesterol esters in the blood after feeding with food rich in this sub-
stance has been shown, particularly in rabbits.
That the bile should be the pathway through which cholesterol is
excreted depends no doubt on the fact that it contains bile salts, which
along with their other properties have a remarkable solvent action on
cholesterol. This solvent property depends on the cholic acid part of
the bile salts, which, as already remarked, is chemically very closely
related to cholesterol; indeed, the relationship is so close that some have
suggested that cholic acid is derived from cholesterol. This would mean
that the cholesterol of blood is excreted in two ways, as cholesterol and
as cholic acid. Other observers, however maintain that the cholesterol
is excreted mainly by the lining membrane of the gall bladder, and
that this explains why gall-bladder bile contains more of it than fis-
tula bile. This evidence is, however, not very strong, for the greater
excretion of cholesterol under conditions where the circulation of bile
is going on may be explained as due to the presence of bile salts, which
serve to carry the cholesterol out of the blood.
Many problems remain to be elucidated in connection with the metabolic
history of cholesterol. That some of it is absorbed when cholesterol is
contained in the food might seem to indicate that its source is entirely
exogenous. Against this view, however, stand two facts: (1) that the
cholesterol in the feces of herbivorous animals is of the same variety as
that present in those of carnivorous animals and not the phytosterol
which is present in plants; and (2) that the universal presence of
cholesterol in cells indicates that it must be manufactured there.
The Bile Pigments
The pigments of bile are bilirubin and biliverdin. The latter is pro-
duced from the former by oxidation. If the oxidation be carried a
stage further, a blue pigment called bilicyanin is formed. This process
of oxidation can be observed in the ring test for bile pigment with
fuming nitric acid. When bilirubin is reduced, urobilin, one of the
pigments in urine, is formed. Bilirubin must therefore be considered
as. the mother substance of all these pigments, and it is of interest in
connection with its derivation to know that it has the same formula
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496 DIGESTION
as iron-free hematin or hematoporphyrin, which is produced by treating
hemoglobin with concentrated sulphuric acid.
Chemical investigation has shown that bilirubin is built up from sub-
stituted pyrrols, probably four such being contained in the molecule.
The pyrrol group is also present in indole and tryptophane, and con-
sists of four carbon atoms and an NH group linked together as a ring
(see page 604). Similar pyrrol derivatives can be produced by decom-
posing chlorophyl, the green coloring matter of plants. It is important
to remember that bilirubin is acid in nature, and, therefore, can com-
bine with alkalies to form salts. The relative amounts of bilirubin and
biliverdin vary in the bile of different animals.
When these pigments enter the intestine they are reduced to urobilin,
part of which passes out with the feces, another part being absorbed into
the blood and exereted in the urine. Part of that excreted in the urine
exists, however, as a so-called chromogen named urobilinogen. The
urobilinogen is converted into urobilin by the action of oxygen.
The method by which urobilin is produced from blood pigment has
been studied by histologic examination of the liver particularly of birds
and amphibia, in which destruction of blood pigment goes on rapidly.
Increased destruction of blood pigment can be induced by poisoning
with certain substances such as arseniureted hydrogen. From such
studies it is usually believed that the bile pigments are a peculiar product
of hepatic activity, being produced from blood pigments that are de-
rived from erythrocytes which have been broken down either in the liver
itself or in some other viseus (e. g., the spleen). Whipple and Hooper”
have brought forward seemingly incontrovertible evidence against such
a view. They have found, for example, that the bile pigments are
formed just as readily in animals in which the circulation of the liver
was greatly curtailed. by anastomosing the portal vein with the vena
cava (Eck fistula) as in normal animals. Even when the circulation
was limited to the anterior end of the animal (head and thorax) bile
pigment appeared in the blood when hemolyzed erythrocytes were in-
jected, and it was also formed when hemoglobin was placed in the pleural
and peritoneal cavities. The endothelial cells of the blood vessels and
elsewhere can evidently form the pigments, at least when the liver is
absént. When such a process occurs under normal conditions, it is quite
probable that the liver acts merely as an excretory organ for the pig-
ments in the same way as the kidney does for urea. Possessed of endo-
thelial cells, the liver might itself also produce some of the pigments,
but no more than other organs with a similar number of those cells.
Even the derivation of bile: pigments from: hemoglobin is called in
question, for the same workers have observed that, whereas the excre-
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THE BIOCHEMICAL PROCESSES OF DIGESTION " 497
tion of pigment from a biliary fistula.is remarkably constant in a dog
fed on a fixed mixed diet, it became increased, sometimes by 100 per
cent, when the diet was changed to one of carbohydrates, and depressed
on a diet of meat. The question arises as to whether, after all, the bile
pigments are really derived from broken-down hemoglobin. May they
not be manufactured de novo out of other materials?
Whipple and Hooper have also shown that bile is a most important
secretion, for dogs rarely survive on an ordinary diet if bile is perma-
nently prevented from entering the intestine. Intestinal symptoms
soon supervene, and become progressively more severe until the death
of the animal. Feeding with bile does not relieve the condition, but
feeding with cooked liver seems to have a beneficial effect.
After extravasation of blood in the subcutaneous tissues, as in a bruise,
for example, a decomposition of hemoglobin proceeds quite like that
occurring in the liver, and leads to the production of blue and brown
and green pigments like those of the bile. When hemolysis is produced,
as by inhalation of arseniureted hydrogen or the injection of inorganic
or biological hemolysins, there is an immediate increase in the amount
of bile pigment in the bile. Even the injection of hemoglobin solutions
has this effect. Under these conditions of hemolysis, besides an increase
in urobilin, there may be considerable quantities of hemoglobin secreted
in the urine.
Bile salts and pigments usually accompany each other when any-
thing occurs to interfere with the free secretion of bile. For example,
after ligation of the bile duct both bile pigments and bile salts accumu-
late in the blood, in the serum of which they may be recognized by the
ordinary chemical tests in from four to six hours after the operation.
If the accumulation ‘be allowed to proceed further, the bile pigments
become deposited in the tissues, giving them the peculiar yellowish ap-
pearance known as jaundice. Under these conditions the bile salts and
pigments also appear in the urine. The accumulation of bile salts in
the body affects certain physiologic processes; for one thing, it causes
a great lengthening in the clotting time of the blood.
If the blood supply to the liver is interrupted by ligation of the portal
vein and hepatic artery at the same time that the bile ducts are occluded,
not a trace either of bile salts or of bile pigment appears in the blood
during the six to eighteen hours that the animals survive the operation.
The amount of obstruction of the bile duct necessary to produce these
symptoms is very slight, since bile is secreted at a very low pressure.
Even a clot of mucus or a swollen condition of the mucous membrane
of the duct is sufficient to produce obstruction. .In the discharge of bile
from the gall bladder into the duodenum it is claimed by Meltzer”! that a
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reciprocal relationship exists between the contraction of the bladder
musculature and the relaxation of the muscular fibers surrounding the
duct in the duodenum. If this reciprocal innervation fails to operate
properly, discharge of bile into the duodenum may become obstructed °
so that a certain amount passes back into the blood, as in eases of bile-
duct obstruction.
Bile also contains a certain amount of lecithin and other phospholipins.
‘The amount varies considerably in the bile of different animals, even in
animals of the same species. It is probably derived, as already men-
tioned, like the cholesterol, from the breaking-down of red blood cor-
puscles that goes on in the liver. It is no doubt digested by the ferments
of the intestinal tract, the liberated cholin, since it is toxic if absorbed,
being further attacked by bacteria so as to become converted into cer-
tain substances of a nontoxic nature.
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CHAPTER LVII
BACTERIAL DIGESTION IN THE INTESTINE
On an average diet, in twenty-four hours the feces of man weigh
about 100 grams, or after drying, about 20 grams. About one-fourth of
the dry matter consists of the bodies of bacteria. If plated out by the
ordinary bacteriologic methods, however, it will be found that only a
small proportion of these bacteria are living. The greater number have
been destroyed, probably by the action of the mucin in the large intes-
tine. The nitrogen content of the feces amounts to about 1.5 grams a
day, of which about one-half is bacterial nitrogen. If the diet contains
large quantities of cellulose material, as in green vegetable food and
fruit, the mass of feces as well as the bacterial content may be consid-
erably greater.
The foregoing facts indicate that very extensive bacteriologie proc-
esses must be going on all the time in the intestinal contents, and the
question arises as to whether such action is beneficial or otherwise to the
animal economy. To answer this question interesting observations have
been made on the growth and well-being of animals excised from the
uterus under strictly sterile conditions and maintained thereafter on
sterile food. Such observations made on guinea pigs have shown that
the animals thrive and grow perfectly for a considerable time. Experi-
ments carried out on chicks have not, however, yielded similar results.
Chicks hatched out from the egg under strictly sterile conditions and
then fed on sterile grain, do not thrive, but do so if with the grain is
mixed a certain amount of fowl excrement. These experiments, appar-
ently contradictory in their results, show that for certain groups of
animals bacteria are required, but not for others:
The difference is probably dependent on the nature of the foods. It
will be remembered that the size of the large intestine varies consider-
ably according to the nature of the diet (see page 463). Animals taking
great quantities of cellulose foodstuffs have very large ceca and very
long large intestines; whereas those which, like the cat, live practically
entirely on cellulose-free food, have a rudimentary large intestine. The
size of the lower intestine is obviously dependent on the presence or
absence of cellulose in the food. It will be remembered also that the
forward movement of the contents of the large intestine is very slow;
indeed, special provision is made, by the presence of the so-called anti-
4
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peristaltic wave, to delay its movement. This suggests that an important
digestive process must be proceeding in this part of the gut. In these
ways conditions become established in the ceeum for the active opera-
tion of bacteria. They attack the cellulose, and liberate the more diges-
tible foodstuffs contained in the vegetable cells, also producing out of
the cellulose itself materials of nutritive value. The acids that are also
produced by this process are neutralized by the carbonates secreted
by the mucosa.
In certain herbivorous animals—the ruminants—this process in the
cecum is not relatively of such importance, because it takes place in the
paunch. The animals swallow the food and it mixes in this part of the
stomach with the saliva, so that bacteria and ferments contained in it,
called cytases, attack the cellulose, liberating the more easily digested
foodstuffs inclosed within the cell walls. As this process goes on acids’
accumulate in the digestive mixture. The food is then returned to the
mouth, chewed over again, and swallowed again into the main stomach,
where it is digested. The aid which bacteria render to digestion depends
therefore on the nature of the diet. Man, being omnivorous, stands mid-
way between the two groups of animals discussed above. Although the
cellulose contained in his food is not itself sufficiently digested to furnish
nutriment, yet it is so far acted upon as to permit the rupture of the
cell, the contents of which are then digested. The cellulose is, however,
of value in furnishing bulk to the intestinal contents—‘‘ intestinal bal-
last,’’ it is sometimes called.
In the small intestine in man there are bacteria capable of acting on
carbohydrates and producing from them organic acids, such as lactic,
acetic, etc. So long as a sufficiency of carbohydrate exists to encourage
the action of these bacteria, others having an action on protein do not
seem to thrive. It may be that this is to be accounted for partly by the
production of acid substances by the carbohydrate fermentation, and
partly by the fact that, as soon as the protein molecule is broken
down by the digestive enzymes, its building-stone amino acids are ab-
sorbed. ‘There are probably also bacteria in the small intestine capable
of splitting fat into fatty acid and glycerine, but practically nothing is
known of their action. In the large intestine of man, along with the
cellulose-digesting bacteria already mentioned, protein-digesting bac-
teria are also present. These bacteria belong to the class, Bacillus coli
communis, the various members of which are known as faculative anae-
robes beeause they can grow in the presence or absence of oxygen.
If bacterial growth is excessive or there is an insufficiency of carbohy-
drates in the small intestine, the bacteria attack the amino acids pro-
duced by the digestive enzymes and decompose them into products
that may be toxic if absorbed into the blood.
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BACTERIAL DIGESTION IN THE INTESTINE 501
Bacterial Digestion of Protein
From a pathologic standpoint, the most important action of bacteria
is that which takes place on protein. Under anaerobic conditions the
intestinal bacteria have in general the power of splitting off the amino
group whereas under aerobic conditions they split off the carboxyl
group. This splitting off of the carboxyl group as carbon dioxide is per-
formed by the so-called carboxylase bacteria, and it may take place either
before or after deamidization (see page 615): If it happens after this
process, the products are not highly toxic and include phenol, eresol,
indole and skatole, which are partly absorbed into the blood and partly
excreted with the feces.
The fractions of those substances that are absorbed into the blood
have their toxicity removed by conjugation mainly with sulphuric acid
to form the so-called ethereal sulphates. A part is also combined with
glycuronie acid (see page 632). In the case of phenol and cresol this
conjugation occurs immediately after absorption, but in the case of
indole and skatole it is preceded by an. oxidative process, converting
these substances into indoxyl and skatoxyl respectively. The detoxica-
tion process occurs in the liver, as has been shown by experiments in
which this organ was artificially perfused outside the body. They are
then removed from the blood by the kidneys and excreted in the urine.
The proportion of ethereal sulphates in this fluid is therefore an indica-
tion of the extent of intestinal putrefaction of protein (see page 632).
The indican, being readily detectable by the well-known color reaction
of Jaffé, serves as an indicator of the extent of intestinal putrefaction.
The indole and skatole which are not thus absorbed and detoxicated are
excreted with the feces, to which they give the characteristic odor.
The source of the phenol is tyrosine and that of the indole is trypto-
phane. The chemical processes involved are shown in the following
equations, in which the by-products of the reactions are in brackets.
C.0H COH COH COH COH
\ an an \
ud Non un? CH HC CH HC CH no” CH
I bit | | | |
HC CH HC CH HC CH HC bn nd Ua
\Z \Z \Z \Z% \
Cc Cc Cc C CH
| —_ | — | —_ | —
CH, CH, CH, CH, (CO, + H,O)
| (NH;) | (CO,+H,0) | (CO,)
: ol : CH, COOH
|
COOH COOH
(tyrosine) (p-oxyphenyl- (p-oxyphenyl- (paracresol) (phenol)
propionic acid) acetic acid)
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502 DIGESTION
Putrefaction of tryptophane is probably preceded by deamidization:
CH CH
a, un
HC C—cC—CH,.CHNH,.COOH HC C—C—CH,CH,.COOH
EY ae) ol —— | i od —
HC Cc CH (NH,) HC C CH (CO, + H,O)
Nt Ne
CH NH CH NH
(tryptophane) (indole-propionic acid)
ON CH CH
a
HC C——C—CH,.COOH uc” Noon ud Nooo,
Po =F ah | ll El ah! aif
oe sae ae (CO, + H,O) HO Cc CH HC Cc Cc
ile é\f
CH NH CH MA (+ CH,) we hi
(indole-acetic acid) (indole) (skatole)
If, however, the carboxylase bacteria remove the carboxyl group be-
fore the amino group has been removed, highly toxic substances called
amines are produced. They are the so-called ptomaines. From alanine,
ethylamine is formed; from tyrosine, phenolethylamine; from histidine,
which it will be remembered is an important protein building-stone,
imidazylethylamine, and so on. The process of formation is illustrated
in the accompanying formule:
1. CH,.CH(NH,).COOH = CO, -- CH,.CH,(NH,)
’ Alanine Ethylamine
2. C,H,(OH).CH,.CH (NH,).COOH — CO, + C,H,(OH).CH,.CH,.NH,
Tyrosine Phenylethylamine
3. C,N,H,.CH,.CH (NH,).COOH — CO, ++ C,H,N,. CH,.CH,.NH,
Histidine. Imidazylethylamine.
Similar substances are very common in the metabolic products of
plants; for example, they constitute the active principle of ergot. They
are also no doubt produced in the tissues of mammals, imidazylethyla-
mine, commonly ealled histamine, being thus produced, as well as the
closely related epinephrine, which is the active principle of the supra-
renal gland (see page 737), and may be described as a methylated ethyla-
mine derivative of tyrosine.
Phenylacetic acid produced by a similar process from tyrosine may
be excreted in the urine, where it forms the mother substance of homo-
gentisic acid, to which the dark brown color of the urine in alkaptonuria
is due.
The great importance attached. to these decomposition products of
proteins depends on the fact that they have powerful pharmacologic
actions. These actions are developed very largely upon the vascular
system; histamine, for example, produces marked vasodilatation and
lowers the coagulability of the blood, whereas other substances of the
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BACTERIAL DIGESTION IN THE INTESTINE 503
same class, like epinephrine, have the property of raising the blood pres-
sure. In larger doses, serious nervous symptoms and a condition of pro-
found collapse are produced. These observations have led several inves-
tigators to believe that the persistent occurrence of bacterial fermen-
tation and the absorption of the resulting decomposition products of
protein into the blood ultimately cause arteriosclerosis and the other symp-
toms that accompany senescence. It is difficult at the present time to
know how much of this one ought to believe, although it can not be
doubted that putrefaction has an unfavorable action on the arteries,
and that an excessive degree of it causes the symptoms of ptomaine
poisoning. :
If the ptomaines have formed in the food before it is eaten, the symp-
toms develop in from one to five hours after the meal, but if the decomposi-
tion occurs in the intestine on account of bacteria that are taken at the same
time as the food, the ptomaines may not have developed sufficiently to
cause symptoms until from twelve to forty-eight hours; sometimes, how-
ever, they develop in an hour or so. Prominent among the symptoms is
usually diarrhea, which develops for the purpose of getting rid of the
offending bacteria and ptomaines.
Actual infection of food with bacteria of the paratyphoid-enteritidis
type is much more common than poisoning by substances (ptomaines) that
have been generated in food before it is taken (Jordan’*). Meat, milk
and other protein foods are usually the carriers of the bacilli, and: in most
of the accurately recorded cases the meat or milk was found to be
derived from animals suffering from enteritis or some other infection.
Sometimes, however, perfectly good food may become infected by
handling. Although the symptoms are usually acute, they may closely
simulate those of typhoid fever, and the effects of the attack may linger
for weeks or months.
BotuLism
The commonest type of poisoning by substances actually present in the
food is that known as botulism. In this the gastrointestinal symptoms
are not pronounced,—indeed, paralysis of the intestinal tract with con-
stipation is the rule,—but those affecting the nervous system, dizziness,
diplopia and other visual disturbances, with difficulty in swallowing,
are very prominent. The temperature and pulse are usually normal.
In practically all of the reported cases of botulism, the source of infection
has been food which after having been subjected to some preliminary treat-
ment, such as smoking, pickling, or canning, had been allowed to stand
for some time and then eaten without cooking. The Bacillus botulinus,
which is responsible for the production of the poisons or toxins, is a
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504 DIGESTION
strict anaerobe and is readily destroyed by cooking, as are also the
poisons. Antitoxins are formed by sublethal injections. Another but
now very rare example of poisoning by products formed in food is
that caused by ‘‘ergotoxin.’’
The treatment in such cases is to encourage diarrhea by giving pur-
gatives. If the intoxication is of a more chronic character, the symptoms
are vague, consisting of drowsiness, lassitude, headache, and general de-
pression. The treatment here also is to clear out the intestines by a
good purge. There can be little doubt that many of the unhealthy condi-
tions of the skin leading to the formation of pimples, acnes, and boils,
are also caused by chronic intoxication with protein decomposition prod-
uets. Again, purgation is the proper treatment.
It is unnecessary in a work of this character to go further into these
highly important questions. It is probable, however, that the importance
of the relationship of excessive protein putrefaction in the intestine to
many of the so-called minor diseases can not be overemphasized. On the
other hand, we must be careful not to attribute every sort of chronic
condition to this putrefaction. Toxemia is often a shibboleth of the
profession. When a chronic disease can not be diagnosed, it is put down
as a toxemia. This, however, is not medical science—it is medical shirk-
ing. It is certainly unsafe at the present time to conclude that the
ordinary symptoms of senescence, such as hard arteries or increased blood
pressure, are invariably to be attributed to this cause. It will be re-
membered that Metchnikoff is largely responsible for such a view, and
also that he suggested, as the surest way to ward off the chance of such
intoxication, the taking of buttermilk, which would supply bacteria
through whose growth in the intestine the protein-destroying bacteria
would not be able to thrive. It is probable that the same result could be
attained in patients showing undoubted signs of suffering from intestinal
putrefaction by a change in diet in the direction of giving more carbo-
hydrate, for, as we have seen, if there is a plentiful supply of this food-
stuff in the small intestine, the bacteria do not tend to attack the protein.
Before leaving this subject it is interesting to consider for a moment
the cause of the severe symptoms that follow intestinal obstruction.
This question has recently been diligently investigated by Whipple,**
who found that the nonprotein nitrogen of blood (page 606) becomes greatly
increased in intestinal obstruction. The cause for this increase in non-
protein nitrogen is found to be an excessive breakdown. of tissue protein
caused by the absorption into the blood of a proteose. When this pro-
teose isolated from obstructed loops of intestine was injected into fast-
ing dogs, profound symptoms of depression were produced, followed, in
cases in which the dose was sublethal, by recovery.in from twenty-four
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BACTERIAL DIGESTION IN THE INTESTINE 505
to forty-eight hours. Along with these symptoms the nitrogen elimina-
tion by the urine increased by 100 per cent.
the total volume of air shrinks. This shrinkage is indicated by the meter, and a Ebcepenaice
amount of Os is delivered from the weighed Oos-cylinder. The increase in weight of bottles
II and III gives the COs; that of I, the water vapor.
tary blower or pump to maintain a constant current of air, as indicated by
the arrows. Following the air stream as it leaves the chamber, we note
a side tube connecting with a meter to indicate changes in volume of the
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544 METABOLISM
air in the system. Beyond this and the pump is a specially constructed
bottle containing concentrated H,SO,, then one containing soda lime, and
lastly another H,SO, bottle. The first H,SO, bottle absorbs all the water
vapor contained in the air coming from the chamber; the soda lime bottle
absorbs the CO,, and the second H,SO, bottle absorbs water that is pro-
duced in the chemical reaction involved in the absorption of the CO, by
the soda lime (2NaO0H+CO,—H,0+Na,CO,). By weighing these ab-
sorption bottles before and after an animal has been for some time in the
chamber, the weight of H,O and of CO, given out can be determined. An-
other side tube leads to an oxygen cylinder, the valve of which is manip-
ulated so as to cause oxygen to be discharged into the system at such a
rate as to compensate exactly for that used up by the animal, as indicated
by the behavior of the meter. The amount of oxygen required is de-
termined either by weighing the oxygen cylinder before and after the ob-
servation or by measuring the volume of oxygen used by passing it through
a carefully calibrated and very sensitive water meter inserted on the side
tube that connects the O, cylinder with the main tubing of the system.
Since muscular activity causes pronounced changes in the rate of me-
tabolism, means are usually taken to secure graphic records of any move-
ments made during the observation. k
The growing importance in clinical investigations of measurements of
the respiratory exchange and the necessity for having methods that are as
simple as is consistent with accuracy, have led to the introduction of
several other forms of apparatus, of which those of F. G. Benedict and of
Tissot* are the most important. In the former a tightly fitting mask,
applied over the nose and mouth is connected, by a short T-piece, with
the same tubing as that used in the respiration calorimeter. The patient
thus breathes in and out of the air stream that is passing along the tubing
without any of the obstruction experienced when the breathing has to be
performed through valves, as in the older (Zuntz) forms of portable
respiratory apparatus. It is particularly for studies on man that this
apparatus has been devised. The Tissot and Douglas methods are shown
in Figs. 179 and 180.*
To complete the investigation, it is necessary that the urine and feces
be collected and the nitrogen excretion measured. When the respiratory
excreta are measured over a considerable period of time, as in the large
calorimeter, the urine is collected for the same period, but when shorter
respiratory measurements are made, the. urine of the twenty-four hours
is usually taken.
Principles Involved in Calculating the Results——Provided with the an-
alyses furnished by the above methods, we proceed to ascertain the total
*The Tissot method will be found described in full elsewhere (page 554).
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METABOLISM 545
amounts of nitrogen and carbon excreted and to calculate from the
known composition of protein how much protein must have undergone
metabolism. We then compute how much carbon this quantity of pro-
tein would account for; and we deduct this from the total carbon excre-
tion. The remainder of carbon must have come from the metabolism of
fats and carbohydrates, and although we can not tell exactly which, yet
we can arrive at a close approximation by observing the respiratory quo-
tient (R. Q.), which is the ratio of the volume of carbon dioxide exhaled.
to that of oxygen retained by the body in a given time, i. ee. By ob-
2
serving this quotient, therefore, we can approximately determine the
souree from which the nonprotein carbon-excretion is derived.
Having in the above manner computed how much of each of the proxi-
mate principles has undergone metabolism, we next proceed to compare
intake and output with a view to finding whether there is an equilibrium
between the two, or whether retention or loss is occurring.
It may serve to make clear the methods by which these calculations are
made to study the following example:
Example of wu Metabolism Investigation—It is desired to know whether a diet con-
taining 125 grams protein, 50 grams fat, and 500 grams carbohydrate is sufficient for a
man doing a moderate amount of work.
INTAKE
Carbon Nitrogen Calories
Protein, 62 gm. 20 gm. 512.5
Carbohydrate, 200 ad 2050.0
Fat, 38 —_ 465.0
Total, 300 gm. 20 gm. 3027.5
OUTPUT
Carbon Nitrogen
In urine, 11 gm. (16.5 x 0.67) 16.5 gm.
In feces, 5 1.0
In the breath, 254 _—
Total, 270 gm. 17.5 gm.
Retained in Body.—30 gm. carbon and 2.5 gm. nitrogen. This amount of nitrogen repre-
sents 2.5 x 6.25 — 15.6 gm. protein or 75 gm. muscle. Now, this amount of protein will
account for 8.25 gm. carbon; so that 30-8.25— 21.75 gm. carbon represents 21.75 x
1.3 = 28.3 gm. fat. On this diet, therefore, the subject retains in his tissues 15.6 gm.
protein and 28.3 gm. fat per diem.
Furnished with these data we may now proceed to compute how much
energy must have been liberated in the body.
To express the above result in terms of energy liberated, we know that
3027.5 C. were supplied and that all these have been used except 15.6
4.1—64 retained as protein, and 28.39.3=263.2 retaimed as fat; or in.
toto 327.2 C. We find, therefore, that 3027.5 — 327.2 =2,700 C. have been
required. ;
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546 METABOLISM
This is called the method of indirect calorimetry, and it has been clearly
established by numerous observations that the results agree exactly with
those secured by the method of direct calorimetry described above. For
most purposes the indirect method is quite satisfactory, and it is espe-
cially valuable in cases in which there are considerable and sudden
changes in body temperature. That the results by the two methods should
agree shows clearly that the law of the conservation of energy must apply
in the animal body, for it is evident that if any energy were derived from
outside the body other than that taken with the food, the results by the
direct method would be higher than those by the indirect.
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CHAPTER LXI
THE CARBON BALANCE
Before proceeding to discuss the special metabolism of proteins, fats
and carbohydrates, it will be advantageous to consider briefly some gen-
eral facts concerning the excretion of carbon dioxide and the intake of
oxygen. In the first place, it is important to note that the extent of the
combustion process in the animal body is proportional to the amount of
oxygen absorbed and of carbon dioxide produced, whereas the nature of
the combustion is indicated by the ratio existing between the amounts of -
carbon dioxide expired and of oxygen retained in the body. An investi-
gation of the carbon balance, in other words, is partly quantitative and
partly qualitative—quantitative in the sense that it indicates how in-
tensely the body furnaces are burning, and qualitative in the sense that
it tells us what sort of material is being burned at the time.
THE RESPIRATORY QUOTIENT
Influence of Diet—The respiratory quotient is determined by com-
parison of the volume of carbon dioxide expired with the volume of oxy-
gen meanwhile retained in the body or, as a formula,
, Vol. CO, expired
vol, reuined
For the sake of brevity the respiratory quotient is often written R. Q. That
it serves as an indicator of the kind of combustion occurring will be evi-
dent from the following equations:
1. Carbohydrate: C,H,,0,+ 60, = 6CO, + 6H,O
(Dextrose.)
co 6
ae R. a = 2 —— = 1,
e 0, 6
2, Fat: C,H, (C,,H,,0,), + 800, = 57CO, + 52H,
(Olein.) :
Rigs a = =0.71
3. Protein: C,,H,,,N,,0,,8 + 770, = 63C0, + 38H,0 + 9CO(NH,). + SO,
[Empirical formula for
albumin (Lieberktihn).]
co, = 63
-. BQ. —-2 7 — = 0.82
Q 0, 77
547
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548 METABOLISM
4. Conversion of fat into carbohydrate:
20,H, (C,,H,,0,); + 640, = 16C,H,,0, + 18CO, + 8H,O
(Olein.)
R652
= = 0.281
oO, 64 M28
/ 5. Conversion of carbohydrate into a miaed fat: '
13C,H,,0, = C,,H,,.,0, + 23CO, + 26H,0.
(Oleostearopalmitin. )
Taking carbohydrates first, the general formula may be written CH,O,
from which it is plain that, to oxidize the molecule, oxygen will be re-
quired to combine with the carbon alone, according to the equation,
CH,0 + 0, = CO,+H,0. In other words, the volume of carbon dioxide pro-
duced by the combustion will be exactly equal to the volume of oxygen
used in this process, in obedience to the well-known gas law that equi-
molecular quantities of different gases occupy the same volume. The
respiratory quotient is therefore unity (Equation 1). With fats and pro-
teins, however, the general formula must be written CH,--O, indicating
therefore that for its complete oxidation the molecule must be supplied
with oxygen in sufficient amount to combine not only with all of the car-
bon, but also with some of the hydrogen, forming water; so that the vol-
ume of CO, produced will be less than the volume of oxygen retained,
and the respiratory quotient will be less than unity. As a matter of fact,
as the above equations show (2 and 3), the respiratory quotient for fats
and proteins lies somewhere between 0.7 and 0.8, being usually nearer
0.7 in the case of fats, and nearer to 0.8 in the case of proteins.
That the conditions hypothecated in the equations exist in the animal
body during the combustion of the foodstuffs can easily be shown’ by ob-
serving the respiratory quotient of animals on different diets. An her-
bivorous animal, such as a rabbit, when it is well fed gives invariably a
respiratory quotient of about 1, whereas a strictly carnivorous animal,
such as the eat, gives a respiratory quotient of about 0.7. Even more
striking perhaps is the comparison of the respiratory quotients in an
herbivorous animal while it is well fed and after it has been starved for a
day or two. In the latter case the respiratory quotient will fall to a low
level because, by starvation, the animal has been compelled to change its
combustion material from the carbohydrate of its food to the protein and
fat of its own tissues.
As already explained (page 545), it is from the respiratory quotient
that we are enabled to tell what proportions of fat and carbohydrate,
respectively, are undergoing metabolism. A useful table showing the
percentage of calories produced by each of these foodstuffs, after allow-
ing for protein, is given by Graham Lusk (see page 565).
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THE. CARBON BALANCE 549
Influence of Metabolism—Apart from diet, the respiratory quotient
may often be altered by changes in the metabolic habits of the animal.
These are most. conspicuously exhibited in the case of hibernating
animals. In the autumn months, when the animal is eating voraciously
of all kinds of carbohydrate food and depositing large quantities of
adipose tissue in his body, the respiratory quotient may be considerably
greater than unity, indicating therefore either that relatively more
carbon dioxide is being discharged or less oxygen retained. As a matter
of fact, it can easily be shown that it is the former of the causes that
is responsible for the higher quotient, the explanation for the increased
production of CO, being that, as the carbohydrate changes into fat, the
relative excess of carbon in the former is got rid of as CO,, as indicated
in Equation 5. On the other hand, if the animal is examined while in
his winter sleep, it will be found that the respiratory quotient is now
extremely low, often not more than 0.8 to 0.4, which may be interpreted
as indicating either an excessive absorption of oxygen or a markedly
decreased excretion of carbon dioxide. As a matter of fact, there is a
great diminution in both the excretion of carbon dioxide and the intake
of O,, because the whole metabolic activity of the animal is extremely
depressed, but this diminution affects the oxygen to a much less degree,
indicating therefore a relative increase in the oxygen retention. The
explanation is that the oxygen is being used in the chemical process in-
volved in the conversion of the fat back into carbohydrate.
Whatever may be the relationship between fat and carbohydrate in
the nonhibernating animal, there is no doubt that during hiberna-
tion, before the fat stores are burned, fat is converted into something
closely related to carbohydrates, the equation for the process being rep-
resented as given above (No. 4).
In man and the higher mammalia, the only condition apart from diet
which can affect the nature of the combustion process is disease; thus
in total diabetes (page 678) the organism loses the power of burning
carbohydrate, so that whatever the diet may be, the respiratory quotient
is very low, never higher than that representing combustion of fat and
protein. It has been claimed by certain investigators that in diabetes
the respiratory quotient may fall considerably below 0.7, indicating, as
in hibernating animals, that fat is being converted into carbohydrate.
The most recent and carefully controlled observations, however, deny
this claim, and for the present we must assume that in the body of man
fat is not converted into carbohydrate (see page 664). In numerous other
diseases investigated by Du Bois and others* no qualitative change in
the combustion processes in man has been brought to light.
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550. METABOLISM.
THE MAGNITUDE OF THE RESPIRATORY EXCHANGE
It is evident that the amount of carbon dioxide expired and of oxy-
gen retained will be proportional to the energy liberation in the animal
body. Even at the risk of repetition it should be noted that the
energy exchange can be very accurately calculated from the result of
the material balance sheet—indirect calorimetry, as it is called (page
562). On account of the comparative simplicity of measuring the carbon
dioxide output and oxygen intake, it is natural that many of the obser-
vations that have been made on energy production in the animal body
depend on the use of this method, justification for which is found in the
complete agreement between the results of direct and indirect calorim-
etry in a great variety of diseases and conditions in man (Du Bois*).*
In the first place, it is interesting to compare the respiratory ex-
changes of different animals computed per kilo body weight. This is
shown in the following table.
OXYGEN AB- |CARBON DIOXIDE
WEIGHT |SORBED PER KILO| DISCHARGED VOL. CO, .| TEMPERA-
ANIMAL GM. AND HOUR PER KILO — TURE OF
GM. AND HOUR VOL. 0, AIR
GM.
Insecta
Field cricket 0.25 — 2.305 — _—
Amphibia
Edible frog 0.063 0.060 0.69 15°-19°
(44.2 ¢.¢.) (30.76 ¢.c.) :
0.105 0.1134 0.78 _
(73:4 ¢@.¢.) (57.7 ¢.¢.)
Aves :
Common hen 1280 1.058 1.327 0.91 19°
(740 ¢.c.) (675 c.c.)
Pigeon 232-380 — 3.236 = _
Sparrow 23 9.595 10.492 0.79 18°
(6710 e.c.) (5334.5 ¢.c.)
Mammalia
Ox 638,000 — 0.389-0.485 _— a
660,000
Sheep 66,000 0.490 0.671 0.99 16°
; (343 ¢.¢.) (341 ee.)
Dog 6213 1,303 1.325 0.74 15°
(911 ¢.c.) (674 ¢.c.)
Cat 2464 1.356 1.397 0.75 -3.2°
3047 (947 ¢.c.) (710 ¢.c.)
ae 0.645 0.766 0.86 29.6°
(450 c.c.) (389 ¢.c.)
Rabbit 1433 1.012 1.354 0.97 18°-20°
Guinea pig 444.9 1.478 1.758 0.86 22°
Rat (white) 80.5 == 3.518 = 7°
(1789 ¢.c.)
Mouse ‘‘ 25 —_— 8.4 — 17°
Man 66,70 0.292 0.327 —_ —
(Modified from Pembrey.)17
*For the convenience of those who may desire to know more about the methods of analysis
that are suitable in the clinic, a chapter on the subject will be found beginning on page 554.
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THE CARBON BALANCE 551
Several factors operate to explain these differences, and of these the
following are of importance:
1. The Body Temperature—lIncrease in body temperature entails in-
creased combustion. This explains why the metabolism of a bird is
greater than that of'a mammal of the same size, for, as is well known, the
temperature of a bird is two or three degrees centigrade above that of
other animals. Rise in body temperature also explains, in part at least,
the increased metabolism observed in fever.
2. The Temperature of the Environment.—In considering this we must
distinguish between the effect produced on warm-blooded and on cold-
blooded animals. Since the body temperature of a cold-blooded animal
is only one or two degrees Centigrade above that of its environment, it
follows that the metabolic activity will be directly proportional to the
temperature of the latter. In a warm-blooded animal, on the other hand,
the body temperature remains constant whatever changes may occur
in that of the environment, this constancy of body temperature being
dependent on the fact that the intensity of the combustion processes is
inversely proportional to the cooling effect of the atmosphere. Thus,
suppose the external temperature should fall, then the loss of heat from
the body will tend to become greater, and to maintain the body tempera-
ture at a constant level, the body furnaces must burn more briskly, with
the result that an increased excretion of carbon dioxide and intake of
oxygen will occur.
This influence of the surrounding atmosphere on the metabolic activ-
ity of warm-blooded animals has, as already pointed out, been used by
several investigators to explain the greater combustion per kilo body
weight of small as compared with large animals. The argument is that,
since the surface of small animals relatively to their mass is much greater
than in large animals, the cooling of the small animals will be proportion-
ately greater. The relationship between surface and mass is shown by tak-
ing two cubes and putting them together; the mass of the two cubes is
equal to double that of either cube, whereas the surface is less than
double, since two aspects of the cubes have been brought together. To
prove the contention, the respiratory exchange has been computed per
square meter of surface instead of per kilo body weight, with the result
that a very close correspondence in the metabolism of different animals
has been observed; but this question has already been discussed, and we
now know that the law of cooling can not be the only one that determines
extent of the respiratory exchange (see page 541).
3. Muscular Exercise.—This has a most important influence on the ex-
change and it is particularly in connection with it that studies in earbon-
dioxide output and oxygen intake have been of great practical value, par-
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552 METABOLISM
ticularly when the investigations are undertaken on men doing ordinary
types of muscular exercise, such as walking or climbing. It is true
that the influence of muscular exercise on the energy metabolism may
also be studied by having a person in the calorimeter do exercises on an
ergometer, but the results thus obtained are in many ways not nearly so
valuable as. those which can be secured by observing the respiratory
exchange of persons doing ordinary types of muscular exercise in the
open. The following table of observations on horses is of interest in this
connection.
CONDITION AIR EXPIRED CARBON DIOXIDE OXYGEN ABSORBED co,
IN LITERS DISCIHARGED IN IN LITERS PER 0,
PER MINUTE LITERS PER MINUTE
MINUTE
Rest 44 1.478 1.601 0.92
Walk 177 4.342 4.766 0.90
Trot 333 7.516 8.093 0.93
It will be observed that the metabolism increases extraordinarily for
even a moderate degree of work, but that at the same time the respiratory
quotient remains constant. From observations on the respiratory ex-
change of working men and animals, extremely important facts concern-
ing the efficiency of muscular work have been secured. The form of
respiratory apparatus (Zuntz or Douglas) employed for this purpose
must be capable of being strapped on the man’s back without causing
any embarrassment to his bodily movements. By a comparison of the
respiratory exchange with the amount of work done, the efficiency of the
work can readily be determined. It has been found, for example, that
the efficiency is much greater after the man or animal has got into the
swing of the work, his energy expenditure per unit of work being much
greater during the first half hour’s work in the morning than it is
later on. This indicates that after a little practice the muscles can ex-
ecute a given movement and perform a given amount of work much
more smoothly than when they are not in training. Another interesting
outcome of the investigations has been to show that work done under ab-
normal conditions that tend to produce any kind of muscular strain is
done inefficiently. It has been found in marching soldiers, for example,
that the slightest abrasion of the foot greatly increases the energy
expenditure, for the man, in trying to avoid the pain produced by the
abrasion, brings into operation muscular groups that are really not
required for the efficient performance of the movement, but are used
instead to avoid pressure on the sore. Fatigue also causes inefficient
performance of work; that is to say, the fatigued person, on attempting
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THE CARBON BALANCE 553
the same amount of work as he performed before becoming fatigued,
will do so at a much greater expenditure of energy.
There is a diurnal variation in the respiratory exchange, which is in
general parallel with the body temperature; it rises during the day, the
time of activity and work, and falls during the night, the time of rest
and sleep. Food also affects respiratory exchange, but it will be unnec-
essary to go into this further after what has been said on page 547.
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CHAPTER LXII*
A CLINICAL METHOD FOR DETERMINING THE RESPIRATORY
EXCHANGE IN MAN
By R. G. Pearce, B.A., M.D.
Principle.—Since the determination of the respiratory exchange in
man is of some importance in the study of certain diseases of the respira-
tion, circulation and metabolism, and also because directions for carry-
ing out the necessary procedures are not generally available, we have
thought it might be of assistance to include here brief directions for the
Tissot and the Douglas methods. These methods have been found to
compare favorably in accuracy with others in use at present,t and be-
cause of their adaptability and simplicity they are specially suited for
clinical work.
By these methods the energy metabolism of the body is calculated from
oxygen consumption or carbon dioxide excretion per minute (indirect
calorimetry) (page 546), the figures for which are determined from the
volume and percentile gaseous composition of the expired air.
The subject breathes through valves which automatically partition the
inspired and expired air. The expirations from a number of respirations
are collected in a spirometer or bag, and the volume of the respirations
per minute is determined. The gaseous composition of the expired air
is determined by gas analysis, and the oxygen consumption and energy
output of the body are calculated from the data obtained.
Description and Use of Parts of the Apparatus: 1. Tae MoutTHriecE
AND VaLves.—The mouthpiece is made of soft pure gum rubber, and con-
sists of an elliptical rubber flange having a hole in the center 2 em. in
diameter, to which on one side a short rubber tube is attached. On the
opposite side of the hole, at right angles to the rubber flange, are at-
tached two rubber lugs. The rubber flange is placed between the lips,
and the lugs are held by the teeth. The rubber tube of the mouthpiece is
connected to the tube carrying the valves. The nose must be tightly
closed if nrouth breathing is used. This is accomplished by a nose clip,
which consists of a V-shaped metal spring, the ends of which are pro-
vided with felt pads. A toothed rachet is attached to the ends of the
*This chapter is added for the convenience of workers in this subject.
Carpenter: Carnegie Institution of Washington Reports, No. 216, 1915.
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METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN 555
spring, and serves to hold the spring tightly clamped on the nostrils in
the proper position (see Fig. 177).
- Some individuals experience great distress when made to breathe
through the mouth. For these it is best to use a face mask. Unfortu-
nately at the present time no mask is entirely satisfactory. Perhaps the
best is one sold by Siebe, Gorman & Co.,* which is pictured in the cut.
Fig. 177.—A, Nose clip; B, Face mask; C, Mouth piece.
After being placed in position the face mask should be tested for leaks,
which can be done by putting soap around the edges.
2. Tue Vatves.—The valves of Tissot are probably the best for the
purpose, but they are expensive and difficult to obtain. We have made
perfectly satisfactory valves from the prepared casings used in the
manufacture of bologna sausage. These can be obtained preserved in
salt, and they will keep indefinitely on ice. When needed a short piece
*This mask has been used extensively by Carpenter. The agent in this country is H. N. I\lmer,
1140 Monadnock Bldg., Chicago.
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556 METABOLISM
is taken, washed free from salt by allowing water from the tap to run
through it, and softened in a weak glycerine solution. The gut becomes
very soft and pliable, and does not dry quickly. A piece of the casing
about 10 em. long is threaded through a glass tube of about 15 mm. bore
and 4 to 6 cm. long. One end of the easing is brought around the outside
of the tubing and secured by means of a thread. The lower end of the
membrane is pinched off and the casing is then cut a little more than
half way across its middle, so that the opening will lie just within the
free end of the tube when the casing is drawn back through it. The
loose end of the casing is slightly twisted—an essential procedure—and
is then secured by a thread on the outer side of the tube. If properly
made, the valve will work freely without vibration, and the opening be
sufficiently large to allow a good current of air to pass. It should col-
lapse instantly and be air-tight when the current of air is reversed. The
back lash, or lag of closure, of these valves is extremely small, and
they will open or close with a pressure of air not exceeding the pressure
Fig. 178.—Diagram of respiratory valves.
changes in normal respiration. When not in use, the valves should be
kept in glycerine water on ice. Valves prepared in this way have been
in use a month without loss of efficiency. They are, however, made with
so great ease that new valves are provided for each subject, and they are
therefore especially adapted to ward work (Fig. 178).
The valves are inserted in reverse order into a supporting metal
T-piece, and the joints made air-tight by tape. The stem of the T is
connected with the mouthpiece. Through a rubber tube of about 3/4
inch bore, the expired air is collected in the spirometer, or Douglas Bag.
3. THE Tissot SPIROMETER is pictured in Fig. 179. We have found the
100-liter size to be very serviceable in the clinic. This instrument is
mounted on a platform having rubber wheels, and can be moved about
the wards with ease. The bell of the spirometer is made of aluminum
and is suspended in a water-bath between the double walls of a hollow
cylinder made of galvanized iron. The height of the bell is 72 em.
and the diameter 42 cm. An opening at the bottom of the
cylinder connects through a three-way stopcock with the rubber tube
leading from the expiratory valve of the mouthpiece (see Fig. 177).
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METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN MAN 5oT
The bell is counterpoised by means of a weight. In the original Tissot
spirometer an automatic adjustment permitted water in amount equal
to the water displaced by the bell to flow from the spirometer exlinder
into a counterpoise cylinder as the bell ascended out of the water.
Fig. 179.—The Tissot spirometer. In actual experiment, subject is reclining or lying down and
the valves and mouthpiece are held with a clamp.
The bell, being heavier out of water than when it is immersed, is accord-
ingly counterpoised in any position, although Carpenter has shown that
this refinement is unnecessary. An opening in the top of the spirometer
permits the insertion of a rubber stopper, through which are passed a
thermometer, a water manometer, and a stopcock with tube for drawing
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558 METABOLISM.
the sample of air. dC — 89.2 liters.
760 (1+ 0.003665 x 20)
The latter calculation can be considerably simplified by using standard
tables which give constants for corrections of gas volumes. These are
easily obtainable and are given in part in Table IV.
According to these tables for 20° C. and 727.21 mm. Hg B.P., the
factor is 0.89124; therefore:
0.89124 x 100 = 89.124 liters, 0°C. and 760 mm. Hg.
0.89124 x 4.57 — 40.7 liters of O, in 15 min., or 16.28 L. per hour.
The Caloric Value Calculated from the Gas Exchange——By reference
to Table V giving the heat value of 1 liter of O, at various respiratory
quotients, it is found that at a R.Q. of 0.87, 4.888 calories are expended ;
16.28 liters of O, is therefore equivalent to 18.4 x 4.888 —79 calories.
The results must be calculated for surface area as well as body weight.
Suppose the subject weighed 85 kg. and was 170 em. in height; by refer-
ence to the chart for determining the surface area of man (page 540),
this would be found to be 1.96 square meters. "Fhe caloric expenditure
per square meter in the above case is therefore a = 40.3 calories.
TABLE I
Tur PERCENTAGE OF OXYGEN WHICH IS EQUIVALENT TO THE NITROGEN FOUND IN THE
EXPIRED AIR
To obtain the nitrogen in the expired air, add the percentage of CO, and O, found
and subtract the sum from 100. The table gives the percentage for O, corresponding to
_ this figure:
%N, 78.7 78.8 78.9 79.0 79.1 79.2 79.3 794 79.5 79.6 79.7 79.8
%O, 20.86 29.88 20.90 20.93 20.96 20.98 21.01 21.04 21.07 21.10 21.12 21.14
79.9 80.0 80.1 80.2 803 804 80.5 80.6
21.16 21.19 21.22 21.25 21.28 21.31 21.35 21.38
TaBLe II |
TENSION OF AQUEOUS VAPOR IN MILLIMETERS OF MERCURY
To obtain the dry barometer pressure, subtract the mm. Hg. corresponding to the
temperature of the air from the barometer pressure at the time of the experiment:
Temp. 15° 16° 17° 18° 19° 20° 21° 22° 23° 24° 25°
Mm. 12.7 1385 144 154 163 174 18.5 19.7 209 22.2 23.5
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METHOD FOR DETERMINING RESPIRATORY EXCHANGE IN M4N 565
Tasie IIT
TEMPERATURE CORRECTIONS TO REDUCE READINGS OF A MERCURIAL BAROMETER WITH A
Brass SCALE To 0°C.
Subtract the appropriate quantity as found in table from the height of the barometer.
The table is for a barometer with a brass scale, and the values are a little lower (about
.2 mm.) than for the glass scale. ‘The corrections for intermediate temperatures can be
approximated.
Temp. 700 710 720 730 740 750 760 770
mm, mm, mm. mm, mm. mm. mm. mm.
15° 1.69 1.72 1.74 1.77 1.79 1.81 1.84 1.86
20° 2.26 2.22 2.32 2.36 2.39 2.42 2.45 2.48
25° 2.83 2.87 2.91 2.95 2.99 3.03 3.07 3.11
TaBLE IV
TABLE FOR REDUCING GASEOUS VOLUMES TO NORMAL TEMPERATURE AND PRESSURE
The observed volume, when multiplied by the factor corresponding to the temperature
and pressure, will give the volume of the expired air reduced to 0° and 760 mm.
Mm. 15° 16° 17° 18° 19° 20° 21° 22° 23° 24° 25°
720 «4.898 =. «894—s «B91 88 885 =. ««882—iwB880 877) «873870 «BGT
730 «6.910 3=—.907 904 —Ss 6901—Ss—(«iwS97~—S( 8940S 91888885 C88 879
740 4.922 ~—.919 916 913 910 907 904 901 .897 .894 891
750 «6.935 = 69382 928925922 919 Ss 916 Ss 9138S 910 907 ~— 904
760 4.947 = .944— 94:1 938 934 931 928 925 .922 919 916
770 «=.960 957 953 950 948 945 .940 .936 .933 930 .927
TABLE V
RB. Q. CALORIES FOR 1 LITER O, RELATIVE CALORIES CONSUMED AS
Number Carbohydrate Fat
per cent per cent
0.707 4.686 0 100
0.71 4.690 1.4 98.6
0.72 4.702 4.8 95.2
0.73 4.714 8.2 91.8
0.74 4.727 11.6 88.4
0.75 4.739 15.0 85.0
0.76 4.752 18.4 81.6
0.77 4.764 21.8 78.2
0.78 4.776 25.2 74.8
0.79 4.789 28.6 714
0.80 4.801 32.0 68.0
0.81 4.813 35.4 64.6
0.82 4.825 38.8 61.2
0.83 4.838 42.2 57.8
0.84 4.850 45.6 54.4
0.85 4.863 49.0 51.0
0.86 4.875 52.4 47.6
0.87 4.887 55.8 44.2
0.88 4.900 59.2 40.8
0.89 4,912, 62.6 37.4
0.90 4.924 66.0 34.0
0.91 4.936 69.4 30.6
0.92 4.948 72.8 27.2
0.93 4.960 76.2 23.8
0.94 4.973 79.6 20.4
0.95 4.985 83.0 17.0
0.96 4.997 86.4 13.6
0.97 5.010 89.8 10.2
0.98 5.022 93.2 6.8
0.99 5.034 96.6 3.4
1.00 5.047 100.0 0.0
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CHAPTER LXIII
STARVATION
In order to furnish us with a standard with which we may compare
other conditions, we shall first of all study the metabolism during starva-
tion. A valuable chart compiled from observations made in the Carne-
gie Institution of Washington on a man who fasted for thirty-one days
is reproduced in Fig. 182.
The Excretion of Nitrogen—When an animal is starved, it has to
live on its own tissues, but in doing so it saves its protein, so that the
excretion of nitrogen falls after a few days to a low level, the energy
requirements being meanwhile supplied, so far as possible, from stored
carbohydrate and fat. Although always small in comparison with fat,
the stores of carbohydrate vary considerably in different animals. They
are much larger in man and the herbivora than in the carnivora. Dur-
ing the first few days of starvation it is common, in the herbivora, to find
that the excretion of nitrogen is actually greater than it was before
starvation, because the custom has become established in the metabolism
of these animals of using carbohydrates as the main fuel material, so
that when carbohydrates are withheld, as in starvation, proteins are
used more than before and the nitrogen excretion becomes greater. We
may say that the herbivorous animal has become carnivorous. The same
‘thing may oecur in man when the previous diet was largely carbohy-
drate; thus, almost invariably in man the nitrogen output is larger on
the third and fourth days of starvation than on the first and second.
Another factor influencing the nitrogen excretion during the early
days of the fast is the amount of previous intake of nitrogen; the greater
this has been, the greater the excretion. By the seventh day, however, a
uniform output of nitrogen will usually be reached irrespective of the
individual’s protein intake. During the greater part of starvation, most
of the energy required to maintain life is derived from fat, as little as
possible being derived from protein. This type of metabolism lasts until
all the available resources of fat have become exhausted, when a more
extensive metabolism of protein sets in, with the consequence that the
nitrogen excretion rises. This is really the harbinger of death—it is often
called the premortal rise in nitrogen excretion. It indicates that all the
ordinary fuel of the animal economy has been used up, and that it has
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STARVATION 567
[nutrition LABORATORY OF THE CARNEGIE INSTITUTION OF WASHINGTON, BOSTON, MASSACHUSETTS]
METABOLISM CHART OF A MAN FASTING 31 DAYS
APRIL 14-MAY 15, 1912
OXYGEN AND CARBON
DIOXIDE, c.c.:
ALVEOLAR CO, TENSION, mm
36.0 3.
34.0
32.0
30.0 3.1
28.0
132
128
124
120
116
BLOOD PRESSURE, mm.
EAT PER 24 HRS. CALS.-
. sa 1081
1041
100
«p96!
Bovy TEMPERATURE, °C——P®
364 |
36.0 |
424
1.08 1
1.041
1.00 1
WEAT PER KILO, PER HR, CALS.
RESPIRATORY QUOTIENT:
92
RESPIRATION RATE 88
T
PULSE RA a. i
i}
CHLORINE (Ci), GMS.
60
TOTAL NITROGEN, GHS.—$5 Hs
PHOSPHORUS (P,03), GMS.
CARBON IN URINE, GMS.
B-OXYBUTYRIC ACID, oat ae, Zo
10.0 t
8.0
6.0
RIC ACID
URIC ACID-N, GM, 20.1
0.1
SULPHUR M.——."*
aM URG)G 0.70 .1
-60 .0
UAMMONIASH, GMS. on
? Fig. 182.—Curve constructed from data obtained from a man who fasted for thirty-one days.
The days of the fast are given along the abscissae, and the various measurements along the or-
dinates. (From F. G. Benedict.)
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568 METABOLISM
become necessary to burn the very tissues themselves in order to obtain
sufficient energy to maintain life. Working capital being all exhausted,
an attempt is made to keep things going for a little longer time by liq-
uidation of permanent assets. But these assets, as represented by pro-
tein, are of little real value in yielding the desired energy because, as
we have seen, only 4.1 calories are available against 9.3, obtainable
from fats.
These facts explain why during starvation a fat man excretes daily
less nitrogen than a lean man, and why the fat man can stand the starva-
tion for a longer time. The premortal rise is, however, not prevented by
feeding oil, which would seem to indicate that death may be due not so
much to the absence of fuel as to serious nutritional disturbance of es-
sential organs; e.g., there may be no available material to supply the
glands of internal secretion with the building stones they must have
(see page 580).
Not only is there this general saving of protein during starvation,
but there is also a discriminate utilization of what has to be used by the
different organs, according to their relative activities. This is very
clearly shown by comparison of the loss of weight which each organ un-
dergoes during starvation. The heart and brain, which must be active if
life is to be maintained, lose only about 3 per cent of their original
weight, whereas the voluntary muscles, the liver and the spleen lose
31, 54 and 67 per cent, respectively. No doubt some of this loss is to
be accounted for as due to the disappearance of fat, but a sufficient
remainder represents protein to make it plain that there must have been
a mobilization of this substance from tissues where it was not absolutely
necessary, such as the liver and voluntary muscles, to organs, such as the
heart, in which energy transformation is sine qua non of life. The vital
organs live at the expense of those whose functions are accessory.
The energy output per square meter of body surface steadily declines.
In the man examined by Benedict, it was 958 C. per square meter of
surface at the end of the first twenty-four hours, but only 737 on the
thirty-first day of the starvation period. The oxygen intake and carbon-
dioxide output correspondingly diminish.
The behavior of the nitrogenous metabolites in the urine is of par-
ticular interest, the following facts being of significance: Urea nitrogen
relatively falls and NH,-—WN rises. For example, on the last day of feeding
the percentage output of NH,-N in relation to total nitrogen was 3.16;
on the eighth day of the fast it was 14.88 (Catheart).? Acidosis is the
cause. The total amount of creatinine and creatine shows. only a slight
fall, but creatinine relatively decreases and creatine increases (Cathcart).
Since creatine is a substance peculiar to muscle tissue, it is possible by
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STARVATION 569
comparing the creatine and creatinine output with that of nitrogen to
determine whether all of the nitrogen liberated by the breakdown of
muscle has been excreted, or whether some has been retained either for
resynthesis in the muscle itself or for use elsewhere. If the muscle
. breakdown as caleulated from the creatine-creatinine output is greater
than that calculated from the nitrogen, synthesis of the noncreatine
remainder must be occurring; whereas if the breakdown calculated from
nitrogen is greater than that calculated from creatine, ete., other tis-
sues than muscle must be contributory. Stored nitrogen or free nitro-
gen in transit from tissue to tissue for utilization is the most Likely
source of such excess nitrogen.
That transference of nitrogenous substances from place to place in the
body in starvation is proved (1) by the constant presence of amino ni-
trogen in the blood and tissues (Van Slyke); and (2) by the effect of
copious water drinking. The latter causes a decided increase in the out-
put of nitrogen, but it does not appear that the extra nitrogen is due to
increased protein breakdown. It is probable, however, that in such cases
there would also be an increase in endogenous protein metabolism, since
the washed-out free nitrogen would have to be replaced.
Excretion of Purines.—Although at first they fall somewhat, the total
amount increases as the fast progresses. Perhaps the first decline is
due to general using up of hypoxanthine of muscle and the later rise
to the breakdown of nuclei (page 638).
Excretion of Sulphur.—It is important to compare the excretion of
sulphur and nitrogen. In the early days of starvation a ratio of 17 N: 18
has been found, but later one of 14.5:1, which is practically the same
as that in muscle (i.e., 14; 1), indicating that late in fasting the main
source of protein supply is muscle.
Several of the changes observed during starvation can be attributed
to, the condition of acidosis which supervenes. The acids are derived
from incomplete combustion of fat (see page 683), and are represented
by B-oxybutyric, the amount being sometimes considerable (10-15 grams
a day), especially in obese individuals. The large ammonia excretion
(sometimes 2 grams a day) is evidently for the purpose of neutralizing
the excess of acid. Another consequence of the acidosis is the decline
in the alveolar tension of CO, (page 354), and it is possible that some of
the circulatory changes shown in the chart may also be dependent on
it. The method of repeated fasting used for reducing obesity is quite
safe if the acidosis is carefully watched.
Many secondary changes also occur in the starving organism. Thus,
the mobilization of fat is often responsible for a pronounced increase in
the fat content of the blood (see page 698), and that of protein explains
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570 METABOLISM
the presence of an amount of amino nitrogen not much below that of
normal animals (viz., 4 mg. per 100 ec. of blood). Similarly with
carbohydrates, early in the condition the blood sugar becomes much
lower than normal, but then remains steady. This is significant when
we remember that after two or three days of starvation all of the avail-
able glycogen has been used up. It indicates that carbohydrate must
be essential for life, and that it is produced in starvation from proteins
(see page 667).
Starvation ends in death in an adult man in somewhat over four
weeks but much sooner in children, because of their more active metab-
olism. At the time of death the body weight may be reduced by 50 per
cent. The body temperature does not change until within a few days
of death, when it begins to fall, and it is undoubtedly true that if means
are taken to-prevent cooling of the animal at this stage, life will be
prolonged.
Death from starvation must be due either to a general failure of all
the cells or to injury of certain organs that are essential for life. Since
the loss of protein from the body as a whole may vary between 20 and
50 per cent at the time of death by starvation, it is unlikely that general
failure can be its cause. If it were so, death would always occur when
some fixed loss of protein had occurred. Certain organs evidently cease
to perform their function, either because they-are deprived of raw mate-
rial for the elaboration of some substance (hermone) necessary for life,
or because the organs themselves wear out from want of nourishment.
NORMAL METABOLISM
Apart from the practical importance of knowing something about the
behavior of an animal during starvation, such knowledge is of great
value in furnishing a standard with which to compare the metabolism
of animals under normal conditions. Taking again the nitrogen balance
as indicating the extent of protein wear and tear in the body, let us
consider first of all the conditions under which equilibrium may be re-
gained. It would be quite natural to suppose that, if an amount of pro-
tein containing the same amount of nitrogen as is excreted during
starvation were given to a starving animal, the intake and output of
nitrogen would balance. We are led to make this assumption because we
know that any business balance sheet showing an excess of expenditure
over income could be met by such an adjustment. But it is a very differ-
ent matter with the nitrogen balance sheet of the body; for, if we give
the starving animal just enough protein to cover the nitrogen loss, we
shall cause the excretion to rise to a total which is practically equal to
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STARVATION 571
the starvation amount plus all that we have given as food; and although
by daily giving this amount of protein there may be a slight decline in
the excretion, it will never come near to being the same as that of the
intake. The only effect of such feeding will be to prolong life for a
few days. ‘
Nitrogenous Equilibrium.—tTo attain equilibrium we must give an
amount of protein whose nitrogen content is at least two and one-half
times that of the starvation level. For a few days following the estab-
lishment of this pure protein diet, the nitrogen excretion will be far in
excess of the intake, but it will gradually decline until the two practically
correspond. Having once gained an equilibrium, we may raise its
level by gradually increasing the protein intake. During this progres-
sive raising of the ingested protein, it will be found, at least in the car-
nivora (cat and dog), that a certain amount of nitrogen is retained by
the body for a day or so immediately following each increase in pro-
tein intake. The excretion of nitrogen, in other words, does not immedi-
ately follow the dietetic increase. The amount of nitrogen thus retained is
too great to be accounted as a retention of disintegration products of
protein; it must therefore be due to an actual building up of new pro-
tein tissue—that is, growth of muscles.
Nitrogenous equilibrium on a protein diet alone is readily attainable
in the eat, and less readily in the dog. But in man and the herbivorous
animals, it is impossible to give a sufficiency of protein alone to: maintain
equilibrium; there will always be an excess of excretion over intake.
Indeed it scarcely requires any experiment to prove this, for it is self-
evident when we consider that there are less than 1000 C in a pound of
uncooked lean meat, and that there are few who could eat over three
pounds a day, an amount, however, which would scarcely furnish all of
the required calories. A person fed exclusively on flesh is therefore
being partly starved, even although he may think that he is eating
abundantly and be quite comfortable and active. This fact has a prac-
tical application in the so-called Banting cure for obesity.
Protein Sparers.—Very different results are obtained when carbohy-
drates or fats are freely given with the protein to the starving animal.
Nitrogen equilibrium can then be regained on very much less protein,
so that we speak of fats and carbohydrates as being ‘“‘protein sparers.’’
Carbohydrates are much better protein sparers than fats; indeed they
are so efficient in this regard that it is now commonly believed that car-
bohydrates are essential for life, and that when the food contains no
trace of carbohydrates, a part of the carbon of protein has to be con-
verted into carbohydrate. This important truth is supported by evi-
dence derived from other fields of investigation (e. g., the behavior of
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572 METABOLISM
diabetic patients, in whom the power to use carbohydrates is greatly
depressed). The marked protein-sparing action of carbohydrates is il-
lustrated in another way—namely, by the fact that we can greatly
diminish the protein breakdown during starvation by giving carbo-
hydrates. In this way we can indeed reduce the daily nitrogen excre-
tion to about one-third its amount in complete starvation. Carbohy-
drate starvation is said to entail a failure of the muscles to use again in
their metabolism certain of the products (e.g., creatine) which result
from their disintegration. At any rate it has been found that creatine
is excreted in the urine when no carbohydrates are available.
In the case of man living on an average diet, although the daily nitro-
gen excretion is about 15 grams, it can be lowered to about 6 grams
provided that in place of the protein that has been removed from the
diet enough carbohydrate is given to bring the total calories up to the
normal daily requirement. If an excess of carbohydrate over the energy
requirements is given, the protein may be still further reduced with-
out disturbing the equilibrium. It has been found that it is not the
amount of carbohydrate alone that determines the ease with which the
irreducible protein minimum can be reached; the kind of protein itsélf
makes a very great difference. This has been very clearly shown by
one investigator, who first of all determined his nitrogen excretion while
living exclusively on starch and sugar, and who then proceeded to see how
little of different kinds of protein he had to take in order to bring him-
self into nitrogenous equilibrium. He found that he had to take the
following amounts: 30 gm. meat protein, 31 gm. milk protein, 34 gm.
rice protein, 88 gm. potato protein, 54 gm. bean protein, 76 gm. bread
protein, and 102 gm. Indian-corn protein. The organism is evidently
able to satisfy its protein demands much more readily with meat than
with vegetable proteins.
This variability in the food value of different proteins depends on their
ultimate structure—that is, on the proportion and manner of linkage - -
of the various amino acids that go to build up the molecule. In no two
proteins are these building stones, as they are called, present in exactly
the same proportions, some proteins having a preponderance of one or
more and an absence of others, just as in a row of houses there may be
no two that are exactly alike, although for all of them the same build-
ing materials were available. Albumin and globulin are the most im-
portant proteins of blood and tissues, so that the food must contain the
necessary units for their construction. If it fails in this regard, even to
the extent of lacking only one of the units, the organism will either be
unable to construct that’ protein, and will therefore suffer from partial
starvation, or it will have to construct for itself this missing unit. It
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STARVATION 573
is therefore apparent that the most valuable proteins will be those that
contain an array of units that can. be reunited to form all the varieties
of protein entering into the structure of the body proteins. Naturally,
the protein which most nearly meets the requirements is meat protein,
so that we are not surprised to find that less of this than of any other
protein has to be taken to gain nitrogen equilibrium.
The most exact information regarding the ‘‘food value’’ of different
proteins has been secured by observations on the rate of growth of young
animals. This method yields more reliable information than can be
secured by studies on the nitrogenous balance, because it is not usually
possible to keep up the latter observations for a sufficient period of
time, or to secure an adequate number of data. During growth the
building-up processes are in excess of the breaking-down, so that the
effect is an increase in bulk of the tissues, thus permitting us, by the sim-
ple expedient of observing the body weight, to draw conclusions as to
the influence of various foodstuffs on tissue construction,
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CHAPTER LXIV
NUTRITION AND GROWTH
‘In the growth of animal tissues two factors are concerned, one being
the property of the cell to grow, the growth factor; and the other, the
availability of suitable material to grow upon, the food factor. Concern-
ing the growth factor little is known; its variability in different species
of animal, its irregularity despite proper adjustment of the food factors,
its abnormality leading to tumor formation, etc., are all well-known but
apparently inexplicable facts (Mendel*).
THE FOOD FACTOR OF GROWTH
Our knowledge is constantly increasing concerning the food factor of
growth, and many facts of extreme practical importance have been ac-
cumulated in recent years. In seeking for the relationship of food to
growth, we must first of all consider whether this process entails a
greater expenditure of energy than is necessary for mere maintenance
in adult life. Important results bearing on this question have been se-
cured by observations on the basal metabolism of young children. In
computing the energy supply of fasting adult animals of different sizes,
it will be remembered that the smaller the animal, the greater is the
energy exchange in relationship to the body weight, although when
-computed in relationship to body surface tolerably constant values are
obtained. When the calorie output per square meter is determined in
growing children, there is, as we have already seen, clear evidence of
greater energy expenditure (see page 541), particularly marked in boys
just before puberty. An increased energy metabolism has also been de-
seribed in the case of infants, but the uncontrollable muscular activity,
the psychic disturbances, etc., may explain the result. Even after dis-
counting these factors, however, it is possible that there may be a cer-
tain influence, depending probably on the active mass of growing proto-
plasmic tissue, which stimulates the energy expenditure. The question
is not yet finally settled.
The Relationship of Proteins to Growth and Maintenance of Life.—
Since protein constitutes the fundamental chemical basis of the cell, it
is natural to devote attention in the first place to this food principle.
574
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NUTRITION AND GROWTH 575.
In the pioneer investigations, studies on the nitrogen balance in young
animals yielded results from which it was concluded that the conditions
for the disintegration of protein are less developed in young animals
than in adults, so that the growing organs rapidly withdraw circulating
protein and build it into tissue protein.
In consideration of the accumulation of data extending over several
decades, Rubner denied these conclusions, and showed that the diet of
the growing infant is by no means relatively rich in protein. He con-
eluded that ‘‘ growth is not proportional to the quantity of protein in the
diet.’? Important though this pioneer work may have been in the de-
velopment of our present-day conception, the viewpoint of the men who
carried it out was very much narrowed on account of the paucity of
knowledge concerning the structure of the protein molecule. No allow-
ance was made for the fact, which has recently been firmly established,
that the protein molecule may vary extremely in regard to the units
of which it is composed, and that the growing tissues may demand, not
so much an abundance of protein as such, but rather a proper supply of
all the building stones which are required for growth (Mendel).
, QUANTITATIVE COMPARISON OF AMINO ACIDS OBTAINED BY HYDROLYSIS of ProTzixs*
(Compiled by T. B. Osborne, 1914) t
OVAL- ox
CASZIN BUMIN GLIADIN ZEIN EDESTIN LEGUMIN MUSCLE °
Glyeocoll 0.00 0.00 0.00 0.00 3.80 0.38 4.0
Alanine 1.50 2.22 2.00 13.39 3.60 2.08 8.1
Valine 7.20 2.50 3.34 1.88 6.20 ? 2.0
Leucine 9.35 10.71 6.62 19.55 14.50 8.00 14.3
Proline 6.70 3.56 13.22 9.04 4.10 3.22 8.0
Phenylalanine 3.20 5.07 2.35 6.55 3.09 3.75 4.5
Glutaminie acid 15.55 9.10 43.66 26.17 18.74 13.80 10.6
Aspartic acid 1.39 2.20 0.58 1.71 4.50 5.30 22.3
Serine 0.50 ? 0.13 1.02 0.33 0.53 g
Tyrosine 4.50 1.77 1.61 3.55 2.13 1.55 4.4
Cystine g ? 0.45 g 1.00 q q
Histidine 2.50 1.71 1.84 0.82 2.19 2.42 4.5
Arginine 3.81 4.91 2.84 1.55 14.17 10.12 11.5
Lysine 5.95 3.76 0.93 0.00 1.65 4.29 7.6
Tryptophane, about 1.50 present 1.00 0.00 [resent present present
Ammonia 1.61 1.34 5.22 3.64 3.28 1.99 1.07
65.49 48.85 85.68 88.87 82.28 57.43 102.87
‘ weeds analyses are combinations of what appear to be the best determinations of various
chemists.
{The figures for the more recent. analyses of gliadin are inserted,
From the accompanying table giving the percentage of the various
amino acids, etc., present in certain proteins, it will be evident that there
are very marked variations in the units of which different proteins are
composed. If any one of these units should be essential for growth and
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576 METABOLISM
the organism be unable to manufacture the missing unit for itself, it
is clear that growth could not proceed however much protein not contain-
ing the necessary unit we might feed to the animal. It is an application
of the law of the minimum, and is analogous with the failure of growth
which has long been known to ensue when certain inorganic substances
are withheld from the growing animal. A diet might be perfectly bal-
anced as judged by compariscn of the nitrogen intake and output, and
yet if it should fail to contain even one of the essential units and the
organism should be incapable of supplying this unit, then would the
diet be inadequate for growth.
These important facts are the outcome of modern work, and they
have been established by observations on the growth of young animals
fed with a ‘‘basal ration’’ to which were added mixtures of amino acids
v
oo
x . A.
si 4
ww
Zein + Tryptophan
Each division = 20 days.
Gliadin
aa
Days Each division=20 days.
Fig. 183.—Curves of growth of rats on basal rations plus the various proteins indicated. The
normal curve may be taken as that with casein (I). (Adapted from Lafayette B. Mendel and
T. B. Osborne.)
or various proteins which differ considerably from one another in the
nature of the units entering into their make-up. In such experiments
the periods during which growth is observed must be prolonged, since
a transient increase in weight might depend merely on repair processes
occurring in tissues which had previously for some reason been brought
below par.
Among the most important observations have been those of Lafayette B.
Mendel and T. B. Osborne® and of McCollum and his collaborators. The
animals chosen for Mendel and Osborne’s experiments were young white
rats. Large batches of these animals were fed-on a basal ration consisting of
protein-free milk (containing the inorganic salts, the sugars, traces of
protein, and unknown substances having an important influence on
Digitized by Microsoft®
NUTRITION AND GROWTH 577
growth—vitamines?), to which were added more carbohydrate, purified
fat, and the protein whose influence on growth it was desired to study.
The same diet was fed at regular intervals to a given batch of rats, and
the weight of each rat was periodically taken, the observation being pro-
longed until the animals grew to maturity and produced young, and these
again grew to maturity, reproduced, and so on. By plotting the re-
sults in curves, with the time periods along the abscisse and the average
weight of the rats of each batch along the ordinates, the extent of the
influence of a given diet on the curve of growth was obtained. A normal
curve of growth is shown in No. 1 of Fig. 183. It was obtained from re-
sults secured by adding liberal amounts of casein to the basal diet.
wr
of T T T rf
300}-
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260 ©
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tol [Ss
iNYS
120, Ke 2
em
100 1X
0
60
40 4 . 1 F L 7 1 4 i 1 1 1 1
Oays Each division ~ 20 days Days Each division «20 days
Fig. 184.—Curves of growth of rats on basal rations plus the proteins indicated. In curve
III the effect of the addition of zein to an inadequate allowance of the perfect protein, lactalbumin,
is shown; and in IV the effect of the addition of cystine to a deficient casein allowance. (From
Lafayette B. Mendel and T. B. Osborne.)
Similar curves were obtained with lactalbumin of milk and ovalbumin
and ovovitellin of egg. Perhaps the most interesting substances capable
of producing the normal curve of growth are certain of the proteins that
T. B. Osborne has succeeded in separating in crystalline form from
vegetable foodstuffs. These are edestin (hempseed), globulin (squash
seed), excelsin (Brazil nut), glutelin (maize), globulin (cottonseed),
glutein (wheat), glycinin (soy bean), cannabin (hempseed).
That growth proceeds normally with any one of these proteins when
fed abundantly does not, however, necessarily indicate that each con-
tains in adequate proportion all of the necessary units to meet the pro-
tein demands of growing tissues. In the case of casein, for example,
one of the units, namely, glycocoll, which is the simplest of all the
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578 METABOLISM.
amino acids, is entirely missing, and another, cystine, which is a sul-
phur-containing amino acid, is present only in small’amount. The ab-
sence of glycocoll, however, is not of importance, because the organism
can manufacture it for itself (see page 630). In the case of cystine,
which the tissues can not manufacture themselves, the deficiency has to
be made up for by feeding an excess of casein so as to cover the needs
of the tissues for this amino acid. By so doing a superabundance of
most of the other units will be ingested, and this superabundance will
entail the destruction and excretion of the useless amino acids, a process,
however, which is conducted in such a way as to permit of the utilization,
by the organism, of a part of the energy which the cast-off amino acids
contain (see page 667). It is, therefore, not entirely a wasteful process.
When the supply of casein is limited, on the other hand, the curve
of growth becomes subnormal, because an insufficient supply of cystine
is thereby offered (Fig. 184). Similar results have been obtained in the
ease of edestin, a protein from hempseed. This contains an insufficiency
of the diamino acid, lysine. Fed in abundance, edestin gave a normal
curve of growth, but when fed in insufficient amount the curve failed to
ascend properly, which, however, it could be made to do by adding some
lysine to the edestin.
There is a large group of proteins which fail to permit of any growth
no matter in what amounts they may be added to the basal ration. These
include: legumelin (soy. bean), vignin (vetch), gliadin (wheat or rye),
legumin (pea), legumin (vetch), hordein (barley), conglutin (lupine),
gelatine (horn), zein (maize), phaseolin (kidney bean). The adequacy
to maintain growth of any of these pure proteins varies according to
the deficiency in their amino acids. In the case of gliadin of wheat or
rye, glycocoll is lacking, and lysine is present only in small amount (see
table). The absence of glycocoll can not, however, as we have already
seen in the case of casein, explain the inadequacy of gliadin as a foodstuff
for growth (Curve II in Fig. 183). It must be the lysine that is at fault. A
still more deficient protein is the zein of maize. With this as the only
protein added to the basal diet, the curve of growth actually descends
(Curve III of Fig. 183), thus indicating that the animal is starving and
must soon succumb. The missing units in this protein are glycocoll,
lysine and tryptophane (see table on page 575), and it is very signifi-
cant that if the latter two amino acids are‘supplied along with zein, an
almost normal curve of growth will result. Some improvement can
even be brought about by giving tryptophane alone; that is to say, the
curve assumes a horizontal line instead of descending, indicating that, .
although inadequate for growth, the diet. is now sufficient for the main-
tenance of life. :
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NUTRITION AND GROWTH : 579
The important fact demonstrated by these experiments, is that cer-
tain diets are adequate for the maintenance of life although they are
inadequate for growth. In conformity with this conclusion, it was found
when young white rats were fed with gliadin alone for periods of time ex-
° .
Fig. 185.—Photographs of rats of same brood on perfect diet (uppermost picture); on a main-
tenance diet but inadequate for growth (middle picture); and on a diet that was inadequate both
for maintenance and growth. (From Mendel and Osborne.)
ceeding those in which they should have become full grown, that
they remained in an ungrown stunted condition. The capacity to grow
had not, however, been lost, for when the gliadin was replaced by milk,
the animals resumed growth at a very great rate. The capacity to grow
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580 ° METABOLISM
had only been inhibited by the inadequate diet, and there was nothing
really abnormal about the stunted animals. For example, the reproduc-
tive function developed normally, as was shown in the case of a young
female rat which, after being fed with gliadin as the sole protein sup-
ply for 154 days, was mated and produced four young. Although the
mother was still maintained on the gliadin diet, the young rats pre-
sented normal growth, for they were living on the milk supplied by the
mother, and this milk, because it contained either casein or some other
necessary accessory factor (vide infra), was an adequate food.
After removal from the mother, three of these rats were fed on an arti-
ficial diet of casein, edestin and the basal ration, and continued the nor-
mal course of growth, but when one of them was placed on the gliadin
food mixture it immediately failed to grow properly. It would appear
from these experiments that, of the two amino acids that are missing or
deficient in gliadin—namely, glycocoll and lysine—it must be the lysine
that is essential for growth. This very important conclusion was fully
corroborated by finding that, in young rats stunted by previous gliadin
feeding, growth immediately started when lysine was added to the diet
and ceased again when the lysine was removed, and so on, the experi-
ments being often repeated in various modifications. Mendel and Os-
borne call attention to the relatively high percentage of lysine in all
those proteins that are concerned in nature with the growth of young
animals; thus, it is present in large amounts in casein, lactalbumin and
egg vitellin.
It is particularly in protein of vegetable origin that indispensable units
are likely to be missing, the best known of these units being the aromatic
amino acids, tyrosine and tryptophane; the diamino acid, lysine; and
the sulphur-containing acid, cystine. Some animal proteins, such as
gelatine, also fail to contain aromatic groups, and are therefore utterly
inadequate as protein foods.
That the absence of one or two units should render a protein utterly
incapable of maintaining life suggests that a specific role may be taken
by certain amino acids in the maintenance of nutritional rhythm; thus,
they may be necessary for the elaboration of some hormone or other in-
ternal secretion essential to life, such as epinephrine, the active principle
of the suprarenal gland. This is an aromatic substance not far removed
in its chemical structure from tyrosine (see page 734). It ‘is
therefore natural to suppose that the absence of the tryptophane unit
in zein is the reason that this protein is incapable of maintaining the in-
itial body weight.
In attacking the problem from this viewpoint, Hopkins and Willeock”
made. observations on the survival period of. young mice; that is, the
period during which the animals survived when fed on a basal diet
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NUTRITION AND GROWTH 581
mixed either with zein alone or with zein plus small quantities of tryp-
tophane. It was found that, with zein alone, the mice were unable to
maintain growth; they lost in weight and died in from about a week to
about a month. Other mice fed on the same amount of basal diet and
zein, but to which was also added some tryptophane, although they did
not grow, were capable of maintaining their body weight and lived in
some instances for nearly a month and a half. There were other indica-
tions of the difference in the efficiency of the two. diets. The mice fed
on the zein alone were very inactive, and remained for a considerable
period of the time in a condition of torpor. The hair was ruffled, the
eyes were half closed, and the ears, feet and tail were cold. The ani-
mals, however, gave evidence of having a good appetite. On the other
hand, the mice to which tryptophane was also given manifested a strik-
ingly different behavior, being active and more or less normal until
just before death. That both groups of animals failed to live more than
forty-four or forty-eight days is probably to be accounted for by the
absence in the zein of the other unit, lysine. Had this been added along
with the tryptophane it is probable, in the light of Mendel and Osborne’s
observations, that the animals would have survived much longer.
To supply the missing unit, besides using the pure amino acid, we
may employ other proteins which contain the required amino acid (Curve
III of Fig. 184). That mixtures of protein foodstuffs are desirable has long
been apparent to those who have studied practical dietetics. We must com-
bine the unsuitable protein with others which, although in themselves
perhaps also unsuitable, yet furnish us with a mixture which contains all
the essential units both for maintenance and growth. As Mendel points
out, these considerations suggest that we may be able to utilize certain
of the low priced protein by-products of the cereal, meat and milk in-
dustries. The test of the adequacy of the corrected diet must, however,
be determined by experiments of the type which we have just described.
It is probably in stock-raising rather than in connection with human nu-
trition that these facts will prove of practical value; for, not only is the diet
‘ of man more varied, but it contains animal proteins in which the deficien-
cies are hot so common.
Most important work of this character is being conducted by McCol-
lum and his collaborators.12, It would take us beyond the confines of
this book to discuss the results in detail, but it may be mentioned that
they have shown that, since the adequacy of the diet depends on a
multiplicity of factors besides the amino-acid make-up of proteins,—
some of which we shall discuss immediately,—very extensive observa-
tions with various food mixtures must be conducted over long periods
of time. The nutritive values of the common cereals added to a stand-
ard diet that had brought, the animals, (rats) to the threshold of death,
582 METABOLISM
were found to be as follows: With cornmeal there was immediate recov-
ery and rapid growth, both of which were also secured in considerable
degree by wheat embryo and entire wheat kernel; with rye and oats, on
the other hand, there was little if any improvement.
Much work is, of course, yet to be done before we can determine the
exact role which each unit plays in the physiologic development of
young animals. To swum up what we already know, it may be said that
glycocoll is not essential, since it can be manufactured by the animal
itself; that tryptophane is essential for maintenance, probably because
it is required for the production of certain essential hormones, for the
make-up of which in its absence other tissues must become disintegrated,
leading therefore to a diminution in body weight; and that lysine ap-
pears_to be essential for growth. Tissues can be maintained without
lysine, but they can not grow, for the slight trace which most food con-
tains of this important amino acid may be sufficient for maintenance
purposes, but utterly inadequate for growth. That the young rats in
the experiments of Mendel grew normally while living on milk supplied
by the stunted mother indicates that the requisite lysine must have been
produced in the mother’s body.
In the application of the foregoing principles to human dietetics, it
is undoubtedly safe to follow Bayliss’s advice to take care of the calo-
ries and allow the proteins to take care of themselves. For example,
in the case of milk the deficiency of cystine in its chief protein, casein,
is corrected by the presence of lactalbumin, which, though present in
only small amounts, contains sufficient quantities of this amino acid to
meet the demands of the growing tissue.
These observations on maintenance and growth suggest very interest-
ing applications in connection with the growth of tumors. Is it possible
that we might retard the growth of tumors by a diet that was insufficient
for growth while sufficient for maintenance. In an experiment devised
to test this proposition mice were fed on a diet of starch, lard, lactose
and gluten on which they could merely maintain existence but failed
to grow. Some of these rats were inoculated with a rapidly growing
tumor at the same time as another batch of mice kept on normal diet, and
it was found that the tumor grew much more slowly in the stunted mice
than in the others. One mouse, for example, on the restricted diet had
a scarcely visible tumor 52 days after the inoculation. When this mouse,
however, was placed on a normal diet of bread, milk, etec., the tumor
immediately began to grow at a very great rate.* Too much importance
should not be placed on this experiment.
We shall now pass on to consider some of the factors besides the pro-
tein content which have an important bearing on dietetic efficiency. .
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CHAPTER LXV
NUTRITION AND GROWTH (Cont’d)
THE RELATIONSHIP OF OTHER FACTORS THAN PROTEINS
The Relationship of Carbohydrates.—As we have seen elsewhere, car-
bohydrates are almost certainly essential for normal metabolism. If they
are not given with the food, they must be manufactured out of protein by
the organism itself. It is not surprising, therefore, that their absence
from the diet of growing animals should lead to abnormality in the
rate of growth. Pediatrists have not infrequently insisted that one
form of earbohydrate is more advantageous for growth than another.
This no doubt in the main is true, but the whole question of adequacy
probably depends on the digestibility of the carbohydrate and not upon
its essential chemical nature. It is likely that the only carbohydrate
required by the tissues is glucose. The readiness with which the ecar-
bohydrate of the food becomes converted into this monosaccharide is
probably the only determinant of its efficiency as food material.
The Relationship of Fats— Although fats are an invariable constit-
uent of practically every diet, it is yet a debatable question as to
whether they are essential to the maintenance of a healthy normal
organism. Difficulties standing in the way of a solution of this problem
are that it is not only technically very difficult to remove fat entirely
from the common foodstuffs, but also that the simple fats are usually
associated with substances having similar solubilities and physical
properties: namely, the lipoids, phosphatides, cholesterol, pigments, etc.
Since these substances are present in practically every cell, it is almost
certain that they can be manufactured by: living protoplasm. Indeed,
experimental evidence is not wanting to show that this is actually the
case. Although the cell can manufacture lipoids, a young animal can
apparently not grow when these substances, as well as simple fat, are
entirely absent from the diet. This has been shown by feeding young
mice on a diet from which all traces of fat and lipoids had been removed
by extraction with alcohol and ether (Stepp)!*. On such a diet the mice
lived only a few weeks. They could be kept alive much longer when
some of the alcohol-ether extract was mixed with the diet, but not so
-when neutral fat instead of the alcohol-ether extract was added. The
583
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584 METABOLISM
addition of the ash of the lipoid extract failed to maintain the mice, so
that the lacking substance could not be inorganic in nature.
More recent and extended observations, however, have shown that neutral
fat is also necessary for the adequate and continued growth of the
animal. For a period of two months or so an animal may, as we have
seen from Osborne and Mendel’s experiments, grow in apparently nor-
mal fashion on an artificial fat- and lipoid-free diet composed of casein,
carbohydrate and inorganic salts, but sooner or later the great majority:
of these animals begin to show failure of adequate growth... The in-
adequately growing animals often manifest indications of malnutrition
other than the failure to increase in weight; for example, inflammation
of eyes, roughening of the fur, ete. When certain fats are added to
the inadequate diet, normal growth is immediately resumed. Fats pro-
ducing this normal growth are such as butter fat, or the fat extracted
from egg yolk, or cod-liver oil, added to the extent of 5 per cent of the
ration. On the other hand, vegetable oils, such as olive oil or almond
oil, are inefficient in promoting growth. That all oils or fats do not
suffice to produce growth, and that one dose of an adequate oil or fat may
be sufficient to stimulate it, indicate that something other than the mere
presence of the comparatively simple fat molecule—that is, some acces-
sory material—must be the agency responsible for the growth.
This conclusion is further supported by the interesting observation of
McCollum and Davis that vegetable oils can be rendered efficient for
growth by shaking them with a solution of soap prepared by com-
pletely saponifying butter fat with potassium hydroxide in the absence
of water.
ACCESSORY FOOD FACTORS, VITAMINES
In searching for the nature of the accessory food factors, the im-
portant observations which have been made in recent years concerning
the so-called vitamines must be considered. These are substances essential
in the diet for the proper maintenance of nutrition in adult animals.
The existence of such substances was suggested by observations on
the disease beriberi, which is caused by exclusive feeding on polished
rice; that is, on rice from which the pericarp had been removed by the
process of polishing. When patients suffering. from this disease were
given unpolished rice, the symptoms immediately disappeared. Further
investigation of the exact nature of these substances was greatly facil-
itated by the discovery that a similar condition is readily induced by
feeding fowls on polished rice. The birds develop a polyneuritis, from
which, however, they very promptly recover if some rice polishings or, .
Digitized by Microsoft®
NUTRITION AND GROWTH 585
better still, an extract of rice polishings, is added to the polished rice
diet. The extract is made by means of slightly acid 91 per cent alcohol,
and from it Funk has succeeded in separating a substance in erystal-
line form apparently related to the pyrimidines, which it will be re-
membered are a characteristic constituent of the nucleins. Doses as
small as 0.02 to 0.04 gm. of this material given by mouth were adequate
to eure the polyneuritis of fowls in from six to twelve hours; indeed, in
some cases the bird seemed quite well after three hours. H H H H H H
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igitize
THE METABOLISM OF PROTEIN 603
by the Van Slyke method; but in other cases, as in arginine, it fails to
give this and the other characteristic reactions of the amino group. That
the extra amino group in lysine reacts directly with nitrous acid explains
why various proteins when examined for amino nitrogen yield an amount
that is equal to half of the lysine nitrogen.
It will further be observed that the amino acids are arranged in three
main groups: one basic, another neutral, and the third acid. The acids
of the basic group are three in number and have an alkalinity similar to
that of ammonia. They have been called the hexone bases, because each
contains six carbon atoms. They are alone present in certain forms of pro-
tein called protamines. The neutral amino acids contain one amino group
and one earboxyl group, which exactly neutralize each other. This is
the largest group of amino acids, and is further subdivided into three:
one containing aromatic or benzene rings and including the very im-
portant amino acids, tyrosine and tryptophane; another containing the
so-called pyrrolidine ring; and the third, the largest of-all, containing
the so-called aliphatic chains; that is, the chains characteristic of the
fatty acids and which may be either straight or branched. When the chains
are branched, the substance is called an isosubstance, as in isoleucine.
The acid amino acids, including glutamic acid and aspartic acid, are
characterized by containing two carboxyl groups and only one amino
group. They therefore resemble acetic acid in acidity.
It may be of assistance to some if we restate these chemical facts
from a slightly different standpoint as follows:
Glycine, or glycocoll, is aminoacetic acid, CH,NH,COOH.
NH,
Alanine is glycine plus a methyl group, CH,CH 3 it is therefore amino-
COOH
OH
propionic acid and is closely related to lactic acid, which is cH,cR” . Many of
COOH
the other amino acids may be considered as derivatives of alanine, thus:
1. Serine is alanine with an ‘‘OH’’ (hydroxyl) group in place of one of the ‘‘H’’
2
atoms of the methyl group, CH,OH -CH
\
COOH
2. Cysteine is alanine with an ‘‘SH’’ (thio) group in this position,
CH,SH ~CH
\
COOH
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604. METABOLISM.
, Two cysteine molecules united at the ‘‘S’’ groups give cystine.
NH,
x
CH,S - CH
®
COOH }.,.
NH,,
3. Phenylalanine has a C,H; (phenyl) group, CH,C,H,- CH
COOH
NH,
4. Tyrosine has a C,H,OH (phenol) group, CH,C,H,OH - CH
CcooH
Cc
rN :
5. Tryptophane has a C,H, CH (indole) group:
. a.
NH
C— CH, --CH - NH, - COOH.
NH
6. Histidine has a bos = d — (imidazole) group:
CH
no ya
| |
CH = C.CH, . CH. NH,- COOH.
The last two are also called heterocyclic compounds, of which there
is another, viz.;
Proline (and oxyproline), which is a-pyrrolidine carboxylic acid:
c CH, — CH,
| |
CH, CH.COOH
Net
NH
Other amino acids are:
CH, CH, CH, CH, CH, C,H,
fo wwe a
CH CH CH
(1) Valine | | |
Leucine CH.NH, CH, CH.NH,
Isoleucine | | |
thus: COOH CH.NH, COOH
| :
(valine) COOH (isoleucine)
(leucine) :
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THE METABOLISM OF PROTEIN 605
(2) The amino dibasic acids:
Aspartic, which is aminosuccinic acid,
CH,COOH
|
CHNH,COOH; and
Glutaminic, which is aminoglutaric acid,
|
CH, - COOH
|
CHNH, COOH.
Lastly there are the diamino acids, in which two groups exist:
Lysine a e-diaminocaproic acid, ;
NH,
: x
NH,CH, - CH, — CH, — CH, - CH, - CH
COOH.
Arginine a-amino—é-guanidine-valerianic acid,
NH,
7
HN=C NH,
-
NH.CH, - CH, - CH, - CH :
\
COOH \
The guanidine group in this acid is of interest because of its close relationship to
NH,
urea, which is O—C
\
NH,
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CHAPTER LXVIII
THE METABOLISM OF PROTEIN (Cont’d)
AMINO ACIDS IN THE BLOOD AND TISSUES
In the Blood.—Furnished with the general facts concerning the chem-
istry of proteins, we may now proceed to consider the more precise
knowledge recently acquired concerning the history of this substance
in the animal economy. Although no one has succeeded in separating
amino acids in pure condition from drawn blood even during the height
of digestion, it has nevertheless been possible to do so from circulating
blood by a method of dialysis, known as vividiffusion, elaborated by
Abel** and his pupils. The method consists in connecting a long tube
of collodion with the two ends of a cut artery in an anesthetized animal.
The tube, coiled many times, is then immersed in a solution containing
approximately the same salt content as the blood plasma of the animal.
The diffusible constituents of the blood plasma dialyze into the saline
solution; or any one of them may be prevented from dialyzing by adding
that particular substance to the saline in such amounts as will make its
concentration in plasma and saline alike. In some ways, it will be seen,
the apparatus may be considered as an artificial kidney. Its possible
clinical application for the purpose of removing poisons from the blood
is under investigation. It has been possible in this way to isolate several
of the amino acids and other ammonia-yielding substances from blood.
Thus, alanine and valine have been obtained as crystalline salts, and
histidine. and creatine (see page 622) shown to be present by their reac-
tions. All of the amino substances, however, do not dialyze, and these
exceptions are further characterized by the fact that they do not readily
give up their ammonia on the addition of sodium carbonate, as do the
diffusible substances (Rohde). Although amino acids can thus be sepa-
rated in a pure state from circulating blood, their concentration in a
drawn specimen is too low to make direct quantitative estimation possible.
By the methods of Van Slyke and Sorensen, already described, however,
it has been shown among other things that the blood always contains a
certain concentration of amino acids; thus, in that of fasting animals from
3 to 5 mg. per 100 c.c. of blood are usually found present. During the
absorption of a protein’ meal, the amino content of the blood undergoes
606
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THE METABOLISM OF PROTEIN 607
a marked increase, becoming doubled or more; and a similar result has
been obtained by placing:'pure amino acids in the small intestine. After
10 grams of alanine, for example, the amino nitrogen of the mesenteric
blood rose from 3.7 to 6.3 mg. per cent.*
In the Tissues.—After entering the circulation, the amino acid very
quickly disappear from it again. This has been demonstrated by ob-
serving the amount of amino acids in the blood after intravenously
injecting a solution of amino acid into an anesthetized animal. After
injecting 12 gm. of alanine into the vein of a dog, 90 per cent was found
Fig. 186.—Vividiffusion apparatus of J. J. Abel.
to have disappeared from the circulation within five minutes. The ques-
tion is, What becomes of the amino acids that rapidly disappear? Are
they decomposed in the blood, or do they become absorbed by the tis-
sues? This problem has been attacked by analyzing portions of various
organs and tissues removed before and some time after the injection
into an animal of amino acid solutions. In the case of the muscles it
has been found that the amino-acid content increases until from 60 to
80 mg. per cent of amino acid has accumulated. Beyond this point,
however, the muscles do not seem to be able to take up any more amino
acid. The capacity of the intestinal organs, however, is more elastic;
“This is « convenient way of stating per 100 c.c. of blood.
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608 METABOLISM
for example, the amino nitrogen of the liver ha8 been observed to become
increased to 125 or 150 mg. per cent of the original amount. Although
this absorption of amino acids by the tissues is extremely rapid, it never
proceeds to such a point that the blood becomes entirely free of them.
Even after many days’ starvation the blood contains its normal quota
of from 3 to 10 mg. per 100 gm. of moist tissue (Fig. 188). This indicates
that a certain equilibrium must become established between the amino-acid
content of the blood and that of the tissues, the concentration in the tissues
being approximately from five to ten tines gréater than in the blood.
150
Injection
100
a
Mj
Mg. Amino WN per*100 gm. fresh tissue.
Blood Urea
L 2 4
Hours
Fig. 187.—Curves showing the amount of amino nitrogen taken up by different tissues after
the cutaneous injection of amino acids. The lowermost curve shows the urea concentration, of the
blood. (From D.'D. Van Slyke.)
The absorbed amino acids are very loosely combined with the tissues,
for they can be extracted by such feeble reagents as water or dilute al-
cohol. Their presence can not, however, be merely due to diffusion;
for if it were, the concentration could not become greater in the tis-
sues than in the blood. The further fate of the amino acids is difficult
to follow. We know that they do not remain in the body for a long time,
because most of the protein nitrogen in the food is excreted as urea
within twenty-four hours after ingestion; and when single amino acids
are fed, they quickly reappear in the urine as urea.
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THE METABOLISM OF PROTEIN 609
The tissues can therefore be only a stopping-place for the amino
acids. When the latter are determined in blood collected. from different
parts while absorption of protein from the intestine is in process, it
has been found, as shown in Fig. 188, that during the passage of the
blood through the liver there is a greater fall in the concentration of
amino acids than during its passage through the entire remainder of
the body. :
It will be seen that the above conclusions are drawn from estima-
tions made on blood taken from the vena cava, portal vein, and hepatic
eG |
a g 5
z 3 a
0 -
DIGESTING
8
2
0
vy So OS
oO ‘
ae g 8
o oO
a a a 8 a
§ 2 a
2 5 4 o 2
° oO = do
2 3 a &
a
ei) i : a
wo
Fig. 188.—Curves showing the concentration of amino-acid nitrogen in the blood during fasting
and protein digestion. (From D. D. Van Slyke.)
artery, the upper curves in the chart being from animals during digestion
and the lower from fasting animals. The results show that the liver must
be particularly greedy of amino acids, which, however, must rapidly be-
come transformed into other substances, since no conspicuous varia-
tion has been found to occur in the amino-acid content of the tissues
according to whether the animal is fasting or is digesting protein food.
This result, it is to be noted, is quite different from that which is ob-
tained after the intravenous injection of amino acids, and the results of
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610 C METABOLISM
the two experiments taken together, indicate that the amino acids after
their absorption can not remain in the tissues in a free condition for a-
long time. It means that the amino acids during natural digestion must
be disposed of at a rate which is practically the same as that at which ab-
sorption is proceeding.
THE FATE OF THE AMINO ACIDS
To follow the metabolism of the amino acids further we must deter-
mine the end product into which they are converted. This is urea,
whose estimation can nowadays be made with considerable accuracy on
account of the discovery, by Marshall, of the action of urease in con-
verting its nitrogen into ammonia, which can then be estimated by com-
paratively simple methods (Folin).
When the viscera are compared before and at various periods after
the intravenous injection of amino acids, the immediate increase in
amino nitrogen remains undiminished in all of them except the liver, in
which a very rapid reduction is observed to occur. At the same time
the percentage of urea in the blood steadily rises. These facts are illus-
trated in Fig. 187.
The simplest interpretation of these results is that the liver converts
the amino acids into urea and discharges this urea into the blood. This
conclusion, however, it must be observed, is not inevitable; for it is pos-
sible that the amino acids may be condensed into polypeptides in the
liver, just as sugar is condensed by this organ into glycogen, and that
the increase in urea is merely coincident (Fiske).
It must not be imagined that the conversion of the amino acids into
urea is exclusively a function of the liver. On the contrary, it is well
known that this process may oceur in animals from which the liver hag
been entirely removed. It is probably safe to conclude, however, that
the liver is the most active center for amino-acid transformation and
urea formation.
When urea is estimated in samples of blood removed at short inter.
vals of time after the ingestion of a large amount of protein, it is found
that the increase becomes very early established. In one experiment,
before the food was taken the concentration of urea nitrogen in the blood
was a little over 10 mg. per cent; one hour after taking 500 grams of
meat, it had risen to about 18, and in two hours to nearly 25. Evidently
the increase had occurred about the same time as the passage of food
from the stomach into the duodenum. These facts indicate that urea
formation in the liver becomes stimulated long before the other tissues,
such as the muscles, have had time to take up their full quota of amino
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THE METABOLISM OF PROTEIN 611
acids. During digestion of protein the liver does not appear to wait
until the other tissues have become saturated with amino acids before it
begins to destroy the unnecessary excess by conversion into urea; on
the contrary, this process sets in with the very first installment of amino
acid that reaches the liver by the portal blood. This conclusion is in
harmony with the well-established fact that, when protein is given to a
starving animal, the greater part of its nitrogen is soon excreted as
urea, leaving only a small fraction to be used for rebuilding the wasted
tissues (see page 643).
The amino acids that are absorbed by the extrahepatic tissues become
very quickly converted into formed protein, as is evident from the fact
that the concentration of free amino acids in the tissues of an animal
during absorption of protein is not perceptibly greater than in those of
a fasting animal, and the question remains to be considered, What be-
comes of the protein thus formed? The answer is, that it is gradually
used up in the metabolic processes, so as to liberate again the amino
acids, which add themselves to those absorbed from the intestine and be-
come used again or excreted, according to the demands of the tissues at
the time for amino acid. _
This process of liberation of amino acid from the breakdown of body
protein goes on of course irrespective of absorption of amino acid from
the intestine. It goes on, for example, during starvation; indeed, in
this condition the percentage of free amino acids in the muscles is, if
anything, somewhat higher than that observed in an ordinarily fed an-
imal. In starvation also the migration of amino acid is going on among
the various organs, of which those whose activity is essential to the
maintenance of life, such as the heart and the respiratory muscles, are
supplied with amino acids from tissues that are less vital, such as the
skeletal muscles (see page 568). These experiments further show that
free amino acids can not serve to any significant extent as food reserves
in the same way as glycogen and fat. If amino acids were of value as
food reserves, we should expect the store of them to be depleted
by starvation. As to how long a period of time elapses between the
incorporation of the absorbed amino acids into tissue protein and their
subsequent liberation again by autolysis, we are entirely ignorant.
The researches which we have just been considering do not throw any
light on the relative value of different proteins in tissue metabolism.
They do not inform us as to which of the amino acids must be absorbed
ready-made from the digested food, and which of them may be dispensed
with since the organism can manufacture them for itself. We know that
the higher animals can synthesize some amino acids, such as glycocoll,
but not others, such as tryptophane; but which amino acids belong to
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612 - METABOLISM
the glycocoll and which to the tryptophane groups, can not as yet
be definitely stated. The investigation of this problem has to be under-
taken by experiments of an entirely different type—namely, by observing
the welfare and growth of animals fed on proteins of varying amino-
acid composition. A full discussion of these experiments is given in
the chapters on Nutrition and Growth.
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CHAPTER LXIX
THE METABOLISM OF PROTEIN (Cont’d)
THE END PRODUCTS OF PROTEIN METABOLISM
Introductory—So far we have approached the problem of protein
metabolism by studying the behavior of the absorbed products of pro-
tein breakdown, and we have seen that these become gradually assimilated
by the tissues and used by them in their metabolie processes. We have
been unable, however, to offer any facts regarding the exact chemical
changes which each amino acid undergoes during this process of tissue
metabolism. At first sight it might appear an easy matter to collect
such information by direct examination of the tissues themselves, either
by searching in them for amino derivatives which might be derived from
absorbed amino acids, or by studying the changes which occur when
the amino acids are subjected to the action of the isolated tissue en-
zymes that must be responsible for the change. Such methods of in-
vestigation are, however, fraught with technical difficulties so great that
very little can be learned from them, and for the present at least we
must be content to piece our information together from facts derived
by less direct methods. Such a method is offered by investigating
the behavior of the end products of protein metabolism.
The main end product is urea along with traces of its precursor am-
monia, but these are not the only ones, for some amino acids after being
incorporated with the tissue proteins break down into products that
are no longer members of the amino-acid series, although they may be
closely related to certain amino ocids. Such substances are creatine and
its anhydrid creatinine. A part of the amino acids during their pres-
ence in a free state in the blood may also be excreted unchanged by
the kidney. Our list so far therefore includes urea, ammonia, creatine,
creatinine, and amino nitrogen, of which the last is usually included in
metabolism investigations in the fraction designated undetermined
nitrogen.
Another group of closely related substances coming, not from the
general protein metabolism of the tissues, but from the metabolism
which is peculiar to the nuclei, consists of the so-called purine bodies.
Furthermore, so as to serve as a check on results obtained by examining
these nitrogenous metabolites, it is important to observe the manner of
613
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614 METABOLISM
excretion of the sulphur moiety of the protein molecule; for it will be
remembered that it is in protein alone that sulphur is usually taken into
the animal body. The excretion of sulphur therefore runs more or less
parallel with the intensity of protein metabolism.
After selecting the end products that are most likely to be of signif-
icance, the first question concerns the amount of each of them excreted
during twenty-four hours on diets that are either rich or poor in pro-
tein. The possibility of conducting such investigations obviously de-
pends on the use of quick and yet reliable methods for the estimation
of the nitrogenous metabolites. Such methods have been furnished by
the painstaking and careful work of Folin, an example of whose results
are given in the accompanying table. ;
NITROGEN-RICH DIET - WITROGEN-POOR DIET
Volume of urine 1170 ee. 385 c.c.
Total nitrogen 16.8 grams 3.60 grams
Urea nitrogen 14.7 grams = 87.5% 2.20 grams — 61.7%
Ammonia nitrogen 0.49 pram = 3.0% 0.42 gram 11.3%
Uric-acid nitrogen 0.18 gram = 1.1% 0.09 gram = 2.5%
Creatinine nitrogen 0.58 gram = 3.6% 0.60 gram —17.2%
Undetermined nitrogen 0.85 gram — 4.9% 0.27 gram —= 7.3%
Total SO, 3.64 grams 0.76 gram
Inorganic SO, 3.27 grams — 90.0% 0.46 gram = 60.5%
Ethereal SO, 0.19 gram = 5.2% 0.10 gram — 13.2%
Neutral SO, 0.18 gram = 4.8% 0.20 gram = 26.3%
(Folin.)
The general conclusions which may be drawn from these results are
as follows:
1. With a protein-rich diet much more urine is excreted in twenty-
four hours than with one that is protein-poor. Evidently the nitrogenous
metabolites act as diuretics.
2. The total or absolute amounts of nitrogen and of all the other
nitrogenous metabolites, save creatinine, become diminished during the
starvation period. The same is true of the sulphur derivatives, except
in the case of the neutral sulphur, which behaves like creatinine.
3. The decrease in the portion of nitrogen excreted as urea is relatively
greater than-the decrease in total nitrogen, this fact being shown in the
table by the percentage figures, which were secured by calculating
the proportion of nitrogen in the various substances as a percentage
of the total nitrogen excreted during the periods. The inorganic sul-
phate behaves in a manner similar to the urea—that is, the percentage
of total sulphate excreted in the inorganic form becomes much less
during starvation.
4. The relative amount of all the other nitrogenous metabolites, as
well as that of the ethereal and neutral sulphates, becomes increased
during starvation.
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THE METABOLISM OF PROTEIN 615
The most striking results of the above investigation are that creatinine
remains unchanged during starvation, but that urea becomes relatively
increased. The former must be derived from metabolic processes going
on in the tissues independently of the supply of foodstuff carried to
them, whereas the latter must depend, if not entirely, yet very largely,
on the protein content of the food. Creatinine may therefore be called
an end product of endogenous metabolism, and urea an end product of
exogenous metabolism.
Other metabolites—namely, ammonia, uric acid and the undetermined
nitrogen, as well as the ethereal sulphates—must represent processes
of metabolism that are partly exogenous and partly endogenous.
Having made ourselves acquainted with the general nature of the
changes that occur in the nitrogenous metabolites when protein metab-
olism is stimulated by the taking of food or depressed by starvation,
we may now proceed to take up each of the metabolites Separately and
see what other information can be obtained regarding their source and
origin in the animal body.
UREA AND AMMONIA
For various reasons it is important to consider these two metabolites
together. During the intermediary metabolism of the majority of the
amino acids, the amino group becomes broken off as ammonia, which
immediately combines with the available acids to form neutral ammonium
salts. The most available acid for this purpose is carbonie acid; there-
fore ammonium carbonate is formed in large amounts. A small propor-
tion of the ammonia may combine with other acid radicles, such as
chlorine, to form ammonium chloride. The fate of these two types of
salt is very different. The ammonium carbonate becomes quickly trans-
formed into urea, whereas the ammonium chloride is excreted in the
urine. The process of urea formation may therefore be considered as
having the function of preventing the accumulation of ammonium ear-
bonate in the animal body. It is the means by which a harmful substance
is converted into an innocuous substance—a detoxication process, in
other words.
Regarding the nature of the chemical process involved in this trans-
formation of ammonium carbonate into urea, reference to the formulas
below will show that the ammonium carbonate that is formed by the
union of carbonic acid with ammonia, by losing one molecule of water
becomes ammonium carbamate, which by repetition of the process be-
comes transformed into urea:
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616 METABOLISM
OH ONH, ONH, NH,
é a ra
co +2NH, = CO -H,O@Cco -H,O=CO
% \ x
OH ONH, NH, NH,
(carbonic (ammo- (ammonium (ammonium (urea)
acid) nia) carbonate) carbamate)
Some of the urea may come from metabolic processes of an entirely
different type. One of these at least is known; namely, the splitting-off
of urea from arginine, which it will be remembered is guanidine-amino-
valerianic acid (see page 605). An enzyme ealled arginase, having this
action, has been isolated from various organs and tissues. The diamino-
valerianie acid, or ornithine, which remains after the urea is split off,
may be further used in protein metabolism. The reaction is shown in
the following equation:
NH, ae - NH - CH, - CH, - CH, - CHNH, - COOH + H,O
NH (arginine)
== NH,-CO
| + NH, —- CH, - CH, - CH, - CHNH, - COOH
NH,
(urea) (ornithine)
On an ordinary diet, as we have seen, a man excretes somewhat more
than 90 per cent of his total nitrogen as urea and about 3 per cent as
ammonia, the remainder of the nitrogen appearing in the other nitrog-
enous metabolites.
Influence of Acidosis on Ammonia-Urea Ratio It sometimes happens
that a large proportion of the ammonia is not converted into urea, but
is used for the purpose of neutralizing abnormal acids present in the
organism. When mineral acids are given to an animal, or when acids
are produced in the organism itself by some faulty type of metabolism,
the ammonia excretion by the urine immediately rises. In diabetes, for
example, where considerable quantities of B-oxybutyrie acid are ‘pro-
duced (see page 683), a decided increase in the ammonia excretion by
the urine is observed. A milder type of acidosis may also be induced
in normal persons by withholding carbohydrates from the diet, and
here again the ammonia excretion is relatively increased.
Tn such cases it is quite evident that ammonia is used as an alkaline
reserve of the body; that is, as a substance which is capable of prevent-
ing acidosis by neutralizing the acids. It does not appear, however,
that all types of acidosis entail the utilization of ammonia as reserve
alkali, and an increase in the relative amount of ammonia in the urine
does not necessarily indicate a condition of acidosis. In the pernicious
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THE METABOLISM OF PROTEIN 617
vomiting of pregnancy, for example, a relatively high excretion of am-
monia has been found associated with no greater a degree of acidosis, as
determined by the power of the plasma to absorb carbonic acid, than in
normal cases of pregnancy.
Influence of Liver on Ammonia-Urea Ratio.—Experimental Observa-
tions: (1) Removat or Liver.—There are several facts which indicate that
other causes than acid-production may interfere with the conversion of am-
monia into urea. What are these causes? Since, as we have seen,
the liver is the organ which most actively converts amino acids
into urea, it would be natural to expect that, when the functions of
this organ were interfered with, relatively more of the nitrogen excre-
tion would oceur as ammonia and relatively less as urea. In order to
determine the exact significance of the liver as a urea-forming organ,
two types of investigation have been used; namely, (1) observation of
the changes produced in the ammonia-urea ratio in the urine by partial
or total removal of the liver; and (2) observation of the urea-forming
power of a liver perfused outside the body:
To remove the liver from the circulation the portal vein is brought
in apposition with the vena cava, the two are sewed together, and a
passage opened between them, after which the portal vein is ligated above
the anastomosis (forming the so-called Eck fistula). The portal blood
then passes directly into the vena cava, and the liver is now supplied
only by the hepatic artery. The animals live for a considerable time
after the operation, and the urine frequently contains relatively less
urea and more ammonia than normal. The results are, however, not
nearly so striking as would be expected if the liver were the main seat
of urea formation. The experiments have nevertheless brought to light
a fact of considerable clinical interest—namely, although the animals
may thrive if kept on a diet not containing an excess of flesh, they im-
mediately begin to develop peculiar symptoms, not unlike those of ec-
lampsia or uremia, when they are fed with large amounts of flesh food.
Most of the symptoms can be referred to abnormal stimulation of the
central nervous system, and examination of the urine has shown a large
increase in the excretion of ammonia and a change from the normal
acid reaction to an alkaline one.
At one time it was assumed that the toxie symptoms were caused by
the presence in the blood of ammonium carbamate, since large quantities
of the calcium salt of this substance could be separated from the urine.
It is now known, however, that the ammonium carbamate is present only
because of the excess of ammonium carbonate, the two salts existing to-
gether in solution according to the laws of mass action. That the intox-
ication is not due to ammonium carbamate does not exclude the pos-
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618 METABOLISM
sibility that it may be due to ammonia itself, although it is more likely
that other nitrogenous metabolites, produced when excess of flesh food
is taken, are the responsible agents.
If the liver is entirely removed by ligating the hepatic arteries in an
animal with an Eck fistula, a more pronounced decrease in urea and
increase in ammonia occur during the short period of time that the
animal survives the operation.
The results observed after the removal or diminution of liver function
fail to occur when other viscéra are removed from the animal, which
would at least tend to indicate that the liver is very important in the
manufacture of urea out of ammonia. This does not, however, warrant
the conclusion that the liver is the only place in the animal body in which
such a process occurs.
In corroboration of these observations on mammals, it may be of in-
terest to note that when the liver is removed from birds, which is a com-
paratively simple operation on account of a natural anastomosis between
the portal and renal veins, there is a marked decrease in the excretion
of uric acid and a corresponding increase in the excretion of ammonia
during the twelve hours or so that the birds survive. In birds and
reptiles urea is excreted as uric acid, being produced by a synthetic
process in the liver (see page 644). The changes in this experiment are
of considerable magnitude; thus, before the operation the amount of
ammonia nitrogen relative to total nitrogen has been found to vary be-
tween 10 and 18 per cent; after the operation it may be increased to
between 45 and 60 per cent. The uric-acid nitrogen normally varies be-
tween 60 and 70 per cent of. the total nitrogen; after the operation it may
fall to between 3 and 6 per cent.
In animals with an Eck fistula and with the hepatic artery ligated,
an increase in the urea output occurs when amino acids are injected under
the skin. This result corroborates the conclusion that the liver can not
alone be responsible for the conversion of ammonia into urea.
(2) Perrusion or Organs.—This method consists in removing the or-
gan into a warm chamber or bath and perfusing it, through cannule
inserted in its main artery and vein, with a solution of defibrinated blood
or of defibrinated blood mixed with saline solution. The perfusion
liquid is kept at body temperature and is saturated with oxygen. By
means of a pump it is made to circulate in a pulsatile flow, and the total
amount of urea or other metabolite in the circulating fluid is determined
before and after the fiuid has been circulated several times through the
organ. When the liver is perfused, urea gradually accumulates in the
fluid, particularly after the addition of one of its known precursors—
for example, ammonium carbonate. When other organs or viscera are
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THE METABOLISM OF PROTEIN 619
perfused, no urea is formed. The evidence shows that the liver is an
important seat of urea formation, but not necessarily that other organs
are unable to form it in the intact animal, for there are many sources
of inaccuracy in perfusion experiments. Even though we exercise the
greatest care, we can not hope to maintain the organ in other than a
slowly dying condition. It is certainly far removed from the normal
state, as is revealed not only by histologic examination, but by the fact
‘that edema almost invariably sets in and the blood vessels become ex-
tremely constricted, thus necessitating a gradual increase in the per-
fusion pressure as the perfusion goes on. Furthermore, the organ being
isolated from the nervous system, there can be no control of the rela-
tive blood supply of different parts. In the intact animal the circula-
tion is more or less distributed according to the particular needs of the
different viscera, and such conditions obviously can not be simulated in
a perfusion experiment. Another objection depends on the fact that
the well-being of the organs in the intact animal is largely dependent on
hormones conveyed to them from other organs. Such hormones are
frequently quite labile in nature, and soon disappear from the perfusion
fluid.
Notwithstanding these objections, there can be no doubt that many
of the functions of an organ are retained much longer than they would
be if the organ were not perfused; for example, the contractility of the
muscle or the power of forming urea in the liver. Perfusion experiments.
are of value therefore when they yield positive results. Negative re-
sults may indicate either that the organ does not perform the particular
function that is being investigated or that it has lost this function as a
result of partial death. That a perfused muscle retains its power of
contraction does not necessarily indicate that it maintains all of its
metabolic functions; neither does the fact that the liver forms urea
prove that it is capable of performing its other functions. It is easy to
show that the liver dies piecemeal; some functions, such as glycogen-
formation, die early, while others, such as urea-formation, remain for a
long time intact. The use of perfusion experiments for the settling of
questions of metabolism should therefore always be very carefully con-
trolled and never used as the sole line of evidence on which to base impor-
tant conclusions.
(3) Before leaving this subject it may be well to point out that the
method which at first sight might appear to be the simplest for settling such
questions—namely, the examination of the inflowing and outflowing blood
of different parts or organs—is not applicable in most cases. This is be-
cause of the extremely small changes in concentration which may occur
even although large amounts of the particular substance in question
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620 METABOLISM
are being absorbed or produced. As we shall see later, this criticism is
particularly applicable in the case of sugar. Even during the injection
of considerable quantities of sugar into the portal vein, no difference
in percentage can be demonstrated between the blood of the two sides
of the liver, although we know that sugar is being retained to form
glycogen. For the same reasons, differences in the percentage amounts
of amino acids or of urea are often difficult to demonstrate in the blood
entering and leaving the liver even when we know that large quantities
of them are being added to or removed from it.
Clinical—Since the liver is an important seat of urea formation, the
question arises as to whether the relative percentage of urea and am-
monia in the urine will become altered by disease of the liver. Many
observations with this point in view have been undertaken, but it can
not be said that the results are very striking. In extreme destruction,
such as that produced by phosphorus poisoning, there may indeed be
a great increase in the relative amount of ammonia and a decrease in
that of urea. The same is true in acute yellow atrophy of the liver, in
which disease the nitrogen excreted as ammonia may amount to as much
as 70 per cent of that excreted as urea. In milder forms of liver dis-
turbance, however, such as cirrhosis, the figures are much less striking.
When an increased ammonia excretion is observed in such cases, we
must be cautious in drawing the conclusion that it is due primarily to
abolition of the hepatic function. It may just as well be caused by the
development of acids in the organism that require the ammonia for
their neutralization. It is significant, for example, that considerable
quantities of acids are produced in phosphorus poisoning.
Although the urea and ammonia excretions become altered by exten-
sive destruction of liver tissue, it is a remarkable fact that very little if
any change occurs in the amino nitrogen, either of the urine or of the
blood. In experimental necrosis of the liver produced by chloroform
or by phosphorus, it is only in the latest stages of the condition and
when it is of the very severest type that an amino-acid increase has been
found to occur in the blood and urine. The conditions seem to be some-
what different in man, abnormally high amounts of amino nitrogen hay-
ing been observed in the blood in a considerable proportion of patients
with impaired liver function. In very severe cases of diabetes, for ex-
ample, figures that are distinctly higher than normal have been observed
(Van Slyke, ete.). In eclampsia the marked pathologie changes in the
liver might be expected to be associated with an upset in the metabo-
lism of amino acids. Losee and Van Slyke have, however, recently
shown by the most accurate methods that neither in the blood nor in the
urine is any excess of amino acids to be found in this condition, although
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THE METABOLISM OF PROTEIN 621
in cases of pernicious vomiting of pregnancy, there was a relative in-
crease in the ammonia excretion. We have already seen that this.
increase did not bear any relationship to the acid-absorbing power of
the blood plasma (see page 617).
The importance of the kidneys in removing the urea from the blood
is readily seen from the change in the percentage of urea in this fluid
after the partial or complete removal of the kidneys. Animals sur-
vive nephrectomy for about three days, and during this time urea rapidly
accumulates in the blood and begins to make its appearance in the
saliva and the intestinal secretions. In man also where the kidneys
are extensively diseased, a similar accumulation of urea occurs in the
blood, some of the excess being got rid of through the sweat and to a
certain extent through the intestine. The importance of encouraging
perspiration and a free movement of the bowels in cases of nephritis is
thus indicated. It must not be concluded that the accumulation of
urea in the organism is the direct cause of the symptoms. Urea itself
is comparatively inert, and it is generally believed that other metabolic
products with which the urea runs parallel in amount are the toxic
agents. Hewlett has found, however, that very large injections of urea
do produce symptoms in animals.**
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CHAPTER LXX
THE METABOLISM OF PROTEIN (Cont’d)
CREATINE AND CREATININE
Creatine and creatinine are very largely products of endogenous metab-
olism; they are mainly derived from chemical processes occurring in
the tissues although some of the creatine and creatinine present in the
food may appear as creatine in the urine.
Essential Chemical Facts
Before we proceed further with a discussion of the metabolism of
these important substances, it will be necessary to refer briefly to some
points in their chemistry. The simpler of the two bodies is creatine,
which is methyl-guanidine-acetie acid; creatinine is its anhydrid, being
formed from creatine by the removal of a molecule of water, so that the
NH, groups become joined together in the same way as they do in the
formation of peptides from amino acids (page 599). The relationships
are illustrated in the following formulas:
(methyl)
CH,—_N
fos
fo CH,COOH
“NH=C -H,O—
\ (aectie acid)
24s a
(guanidine) NH,
(creatine)
CH, -N-CH-Co
|
|
NH—C |
\ |
\ |
Suu
NH
(creatinine)
It should be noted that guanidine is closely related to urea
NH,
(O—C ), and that when creatinine is formed from creatine a ring
NH,
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THE METABOLISM OF PROTEIN 623
formation occurs, giving what may be regarded as an imidazole deriva-
tive (see page 604). Creatine is also related to one of the important
diamino acids, arginine, since both contain guanidine radicles,
NH,
(NH=C ), and to histidine and the purines (see page 634), both
*
NH,
of which contain the imidazole ring. The close relationship which
creatine bears to urea is illustrated by the fact that urea is formed
when creatine is subjected to the action of boiling barium hydrate. When
it is oxidized by means of potassium permanganate, urea is also formed,
the remainder of the molecule, more or less intact, being split off as
NH-CH,
methyl-amino-acetic acid (CH, ), also known as sarcosine.
COOH
The conversion of. creatine to creatinine goes on slowly in aqueous
solutions, but is much accelerated by heating with acid. Heated in an
autoclave at a temperature of 117° C. for thirty minutes, with half nor-
mal hydrochloric acid, the creatine goes over almost quantitatively into
creatinine. It will be noted that the creatinine ring is partly oxidized.
This renders it unstable, so that creatinine in the presence of alkalies
has the power of reducing metallic oxides. Like glucose it can reduce
alkaline solutions of copper, silver and mercuric salts; it also reduces
picrie acid in weakly alkaline solution to picramic acid, which, being red,
furnishes us with a solution the strength of which can be estimated
colorimetrically.
Quantitative Estimation. Although the presence of creatinine in the
urine has been known for many years, there being from 1 to 2 grams of
it in the twenty-four-hour urine, little progress was made in the study
of its metabolism because of the absence of a reliable method for its
estimation. The elaboration by Folin of a colorimetric quantitative
method for creatinine, depending on the reduction of picrie acid, has
furnished the starting point for the modern work which has been done.
To estimate the creatine by this method, it is usual to proceed as fol-
lows: The creatinine content is first of all determined, another portion
of urine being then heated with acid in the autoclave until all of its
creatine has been converted into creatinine. A second determination of
creatinine is then made, and the difference between the two is calculated
as creatine.
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624 METABOLISM
It should be pointed out that, since the creatine is estimated by an
indirect method, there are considerable chances for inaccuracy. Indeed,
it has been shown that errors may have been incurred in some of the
recent work on account of the fact that when acetoacetic acid is present
in the urine it prevents the creatinine from developing its full reducing
power on picric acid in the cold, so that when subsequently the urine is
heated with acid for the purpose of converting the creatine into creati-
nine, the destruction of acetoacetic acid allows the reducing power of the
creatinine to develop to full intensity. It is obvious that this would make
it appear as if creatine had been converted into creatinine. It is par-
ticularly in the urine of diabetic patients, in which acetoacetic acid is
present that mistakes are likely to be made.
Metabolism
When we come to consider the metabolism of creatine and creatinine,
we find that there are remarkably few facts definitely known concerning
it. The average amount excreted daily, expressed as the number of milli-
grams of creatinine in twenty-four hours per kilogram body weight,
is known as the creatinine coefficient (Shaffer).*° For a lean person this
is about 25 mg.; for a corpulent person, about 20 mg., the difference in-
dicating that muscle mass, and not body weight, is the important factor
determining the coefficient. Further evidence that this relationship ex-
ists is furnished by the fact that in the muscular atrophies creatine ex-
eretion is distinctly below normal. It must be the mass of the muscles
rather than.their activities that is the determining factor, for the creatine
excretion does not become increased by muscular exercise.
Influence of Food, Age, and Sex.—Although creatine and creatinine are
endogenous metabolites, it must be remembered that, under ordinary
dietetic conditions, a part of each is derived from these substances pres-
ent in the food. It is important therefore to consider the conditions
under which.the creatine and creatinine in the food appear in the urine.
Regarding creatinine, it is pretty well established that practically all
that is taken with the food reappears as creatinine in the urine. Shaffer
has, for example, succeeded in recovering 76 per cent of ingested creat-
jnine in the urine excreted during twenty-one hours following the in-
gestion of 0.7 gm. creatinine.
The conditions for the excretion of creatine are more complex. It is
present in the urine of children in considerable amount, but in that of
adults only as traces. In the first years of life the creatine in boys’
urine may amount to one-half of the total creatine and creatinine, ‘but
it becomes gradually less and practically disappears at about seven”
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THE METABOLISM OF PROTEIN 625
years of age. Girls, on the other hand, continue to excrete creatine until
about puberty, after which, although ordinarily absent, it reappears in
the urine at each monthly sexual cycle, and is present during pregnancy
and for some days after delivery. Feeding creatine to children causes
it to appear in the urine, accompanied usually by a slight increase in
the creatinine. The same results can be observed in women during the
monthly periods, when as much as 0.1 gm. may be present, and during
pregnancy. Creatine is also present in the urine of most if not all of
the other mammalia. Some of these facts are shown in the following
table:
AGE CREATININE-N CREATINE-N EXCRETED
IN 24-HR, URINE
2 0.025 0.023
3 0.057 0.022
5 0.112 0.025
Bore) 4 0.163 0.0
11 0.157 0.0
15 0.378 0.0
5 0.069 0.005
6- 0.032 0.003
Girls; 7 0.157 0.066
10 0.147 0.020
12 0.201 0.011
(From Mathews.)
When creatine is given to an animal that has been kept in a starved
condition, most of it seems to disappear. It can not be recovered in the
urine either as creatine or as any other nitrogenous metabolite. It seems
to functionate more as a food than as a useless substance. The possi-
bility that some of it can be destroyed by the intestinal bacteria being
admitted, there is nevertheless some justification for the view that the
creatine finds a useful function in the anabolic process of the muscles.
Influence of Complete and Partial Starvation— Although, as we have
seen, the creatinine excretion remains constant when the amount of pro-
tein in the diet is greatly reduced, yet it does not remain constant during
complete fasting or when carbohydrates are entirely withheld from the
diet. In fasting it has been found that creatine appears in place of the
creatinine which has disappeared, so that if both creatine and creatinine
are determined, very little if any diminution will be found to have oc-
curred. Fasting, therefore, causes the adult creatine and creatinine
metabolism to become like the juvenile metabolism. As pointed out by
Mathews, it would be interesting in the light of this observation to see
whether other substances, passed in the urine of young animals but ab-
sent in that of the adult, would reappear in the urine when the animals
were made to fast. In the case of man, for instance, allantoin would be
worth investigating in this regard (page 641).
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626 METABOLISM
A similar replacement of some of the creatinine by creatine appears
when carbohydrate is entirely withheld from the diet, or in diabetic
animals, either in the disease diabetes mellitus in man or in the experi-
mental condition induced in animals by giving phlorhizin. ,Unfortu-
nately, in a considerable part of the work that has been done on this
phase of the subject a method of estimation was employed which did not
take sufficiently into account the influence of acetoacetic acid on the
creatine estimation; but even after allowing for this possible source of
error, there can be no doubt that creatine appears in the urine when
carbohydrates are improperly metabolized. If carbohydrates are given
to a starving animal, for example, the creatine is replaced in its urine by
creatinine, although this will not occur when either protein or fat is fed.
The general conclusion which may be drawn from these observations is
that carbohydrates in some way are required for the proper conversion
of creatine into creatinine in the animal body (Cathcart) *’.
Origin of Creatine and Creatinine
Notwithstanding the amount of excellent work that has recently been
done on the metabolism of creatine and creatinine, we know very little
indeed regarding the origin of these bodies in the animal organism. It
would be profitless to discuss this problem to any great extent, but a
few of the most important facts so far established may be of interest and
of value. The first step in attacking such a problem is to compare the
amounts present in the various organs and tissues, in the blood, and in
the excreta. Of the approximately 120 grams of creatine and creatinine
in the body of an average adult, a very large proportion is in the muscles,
the voluntary muscles containing the largest percentage, the heart con-
taining a medium percentage, and the involuntary (intestinal) muscles
containing relatively a small amount (Myers and Fine)**. Next to the
skeletal muscles, and containing more than the involuntary mus-
cles, come the testis and brain. The liver, pancreas, thyroid, kidneys,
spleen, ete., contain traces, the smallest amount of all being found in the
blood.
In all these places by far the greatest proportion of the total creatine-
creatinine exists as creatine, which is exactly the reverse of the condi-
tion obtaining in the urine of adults, where practically all is excreted as
creatinine. The close chemical relationship between creatine and creat-
inine, considered along with the above facts regarding their quantitative
distribution in the body, indicates that the creatinine of the urine is de-
rived from the creatine of the tissues. The question is, How does the
creatine come to be converted into creatinine? Such a transformation is
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THE METABOLISM OF PROTEIN ‘ 627
probably effected by many of the tissues of the body and certainly by
the blood, the active agency in all cases being no doubt an enzyme. That
the blood contains such an enzyme is indicated by the fact that creatine
is transformed to creatinine by blood serum more quickly than it is
when merely dissolved in water. Even heated blood serum possesses
some of this power. The liver also probably brings about the transfor-
mation, as has been shown by perfusion experiments, and by the fact
that in cases of phosphorus or hydrazine poisoning creatine displaces
creatinine in the urine.
The problem therefore narrows itself down to the question of the
origin of creatine. In the light of chemical knowledge there are several
precursors from which creatine might be formed. One, for example, is
arginine, which it will be remembered is guanidine-amino-valerianic acid
(see page 605). By oxidation this might become changed into guani-
dine-amino-acetic acid, which by methylation would then be changed into
creatine. That such a process of methylation may actually occur in the
animal body is definitely known, for it happens when such substances as
pyridine or naphthalene are given with the food. They appear in the
urine as methyl derivatives. The possibility of the derivation of creatine
from arginine is not, however, borne out by the result of the injection of
arginine, for such injection does not increase the creatinine in the urine.
The closely related substance, guanidine-acetie acid, when fed to animals
(rabbits) does cause a slight increase in the excretion of creatine (Jaffé),
and also, it is said, an increase in the creatine content of the muscle.
Even in this case, however, by far the largest proportion of the admin-
istered guanidine-acetic acid is excreted in the urine unchanged.
The large percentage of creatine in muscle tissue leads one to expect
that some relationship must exist between muscular metabolism and the
amount of creatine present either as such in the muscles or as creatinine
in the urine. Regarding the latter point it is definitely established that
muscular exercise leads to no increase in the creatinine excretion, al-
though it is said that an increase occurs following a tonic contraction
of the muscles. With regard to the creatinine in the muscles, no definite
results indicating that muscular metabolism changes its amount are on
record. In the light of the fact already stated regarding the presence
of creatine in other organs than the muscles, it seems probable that the
substance has really little to do with muscular contraction as such, but
rather is concerned in some way in the formative metabolism of the cell,
with its general growth or maintenance. Indeed, it is a question whether
creatine is an actual constituent of the living tissue. It may rather, as
has been suggested by Folin, be a postmortem product, represented dur-
ing life by creatinine.
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628 METABOLISM
Creatine appears in the urine in phosphorus poisoning, in carcinoma of
the liver and during postpartum involution of the uterus. It is not de-
rived from the disappearing uterine muscle, however, for creatinuria also
oceurs after cesarean section with removal of the uterus. Creatine
elimination is not an index of cellular destruction, for it has been found
large in a dog injected with phlorhizin and maintained in constant weight
by feeding with washed meat (S. R. Benedict). Muscular fatigue also
leaves the creatine content of muscle unchanged. In late stages of
nephritis, creatinine accumulates in the blood and serves as an index of
the gravity of the condition (page 651).
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CHAPTER LXXI
THE METABOLISM OF PROTEIN (Cont’d)
UNDETERMINED NITROGEN AND DETOXICATION
COMPOUNDS
In the present chapter we shall refer briefly to the groups of urinary
substances styled undetermined nitrogenous compounds and to the com-
pounds that are excreted in the urine as the result of the combination in
the body of certain toxie bodies with chemical substances that render
them harmless (detoxication compounds).
Undetermined Nitrogen
Included under undetermined nitrogen are amino acids, peptides and
basic substances. The amount of amino acids and peptides in normal
urine is very small but may become considerable in disease, especially
of the liver, when leucine and tyrosine may appear. The presence of
traces of amino acid and peptone in normal urine is to be expected,
for although the actual concentration of amino acids in the blood is
never very great, a certain leakage of amino acids must occur into the
urine.
The peptide is sometimes known as oxzyproteic acid. It becomes dis-
tinctly increased in phosphorus poisoning and in such conditions as are
accompanied by excessive protein metabolism. The basic constituents
include such substances as trimethylamine, ethylamine, putrescine and
cadaverine (page 502), and there are probably many more of a similar
nature. Many of these substances are similar to the so-called ptomaines
found in meat, etc., and they have been called the ptomaines of urine,
from which they can be isolated by rendering the urine alkaline and
shaking out with ether. It is probably to the presence of these sub-
stances that urine mainly owes its toxic action.
The Detoxication Compounds
Certain nocuous substances are produced in the intestine during the
digestive process (see page 501), and others may result from the meta-
bolic processes in the tissues. To guard against the harmful action of
these substances on the organism, they become detoxicated in various
629
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630 METABOLISM
ways, mainly by forming inert compounds with other substances, par-
ticularly with glycocoll, sulphurie acid or glycuronic acid. The com-
pound thus formed is then excreted in the urine.
Hippuric Acid.—Glycocoll is used mainly to detoxicate the benzoic
acid which results from the oxidation of the aromatic substances pres-
ent in large quantities in vegetable food and fruit (particularly in cran-
berries). Some benzoie acid may also be produced by the breakdown
of the aromatic group of the protein molecule; phenylalanine, for ex-
ample, gives rise to benzoic acid by bacterial decomposition. The com-
pound formed is hippuric acid, this name indicating that it is present in
large quantities in the urine of the horse, as it is also in the urine of
all herbivorous animals,
‘Hippuric acid is benzoyl-glycine (C°H®.CO. NH. CH. ,COOH), and it
ean readily be produced in the laboratory by bringing together benzoyl
‘ehloride with eae, thus:
(henzagl chiovide), ethencell). Gapoute ‘wed
Under ordinary dietetic conditions only a trace of hippurie acid is
present in the urine of man, but much larger quantities, 2 grams a day
for example, may appear when the diet contains a large proportion of
fruit or vegetables. It is not known to undergo any characteristic varia-
tions in disease. The benzoic acid which is contained in certain canned
foods as preservative also combines in the body with glyeocoll, so that-
any toxic effect which it might produce is practically negligible. There
is certainly no very evident reason why canned foods containing benzoic
acid should be tabooed, for in so far as the benzoic acid is concerned, they
can be no more toxic than a diet composed largely of vegetables and
fruit.
This detoxication of benzoic acid requires the presence in the organ-
ism of a constant supply of glycocoll, which, it will be recalled,
is the lowest in the series of amino acids, being aminoacetic acid
(CH,NH,COOH). It is present in greatest amount in the protein of the
connective tissues. It is said, however, that not more than from 2 to
3.5 per cent of glycocoll is available in the proteins of the body. Al-
though this amount of glycocoll would amply suffice to detoxicate the
benzoic acid produced by the metabolism. of.the food in carnivora, it
is quite inadequate for this purpose in the ease of herbivora, and the
question naturally presents itself as to where the glycocoll in these
animals comes from. It is said, for example, that of the total nitrogen
excretion in herbivora 50 per cent may- appear as glycocoll under cer-
tain conditions. These facts indicate that the organism is capable of
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THE METABOLISM OF PROTEIN 631
producing new glycocoll for itself, and it is interesting to consider how
this glycocoll may be derived. A very probable source is by synthesis
between ammonia and glyoxylic acid (CHO. COOH). That glyoxylic acid
or its aldehyde, glyoxal, is readily produced during metabolism from car-
bohydrates and that ammonia is always available would seem to lend
some support to this view (see page 665). The synthesis of glycocoll
from glyoxal and ammonia occurs thus:
H.COCHO + NH, — CH,NH,COOH.
(glyoxal) (glyeocoll)
The linking up of glycocoll with benzoie acid occurs in the kidney.
If the kidney is removed from the circulation in the majority of animals
that produce hippuric acid in large amount—the rabbit being an excep-
tion—no hippurie acid will accumulate in the blood. On the other hand,
an isolated perfused preparation of the kidney produces hippuric acid
provided benzoic acid is added to the perfusion fluid, and the latter also
contains an abundance of oxygen, which is best secured by using de-
fibrinated arterialized blood instead of artificial serum (Locke’s solu-
tion). The necessity of a plentiful supply of oxygen is further shown
by the fact that, if the hemoglobin of the blood is rendered incapable
of carrying O, by bubbling carbon monoxide gas through it, no synthe-
sis of hippuric acid will result from perfusing the blood through the
kidney. The actual chemical process by which the synthesis occurs (de-
hydration) is similar to that by which polypeptides are formed by the
union of amino acids, or creatinine from creatine. ‘
(C,H,CO |OH +H! HNCH,COOH).
Glycocoll may be used for detoxicating other substances than benzoic
acid, particularly cholic acid, forming the glycocholic acid of the bile
(see page 494) and phenylacetie acid. In birds the benzoic acid be-
comes combined with diamino-valerianic acid or ornithine (NH, — CH, -
CH, - CH, - CH - NH, - COOH) in place of glycocoll, so that in the urine
of these animals in place of hippuric acid a compound called ornithuric
acid occurs.
It is of importance to point out here that this pairing of aromatic toxic
substances with certain of the metabolic products of the organism has
frequently been found an excellent experimental method for demon-
strating the presence of intermediary metabolic substances that other-
wise would not have appeared in the excreta. These substances are
thus diverted from their normal course in metabolism so as to form
neutralization or detoxication compounds. Glycuronic acid is an example.
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632 METABOLISM
Ethereal Sulphates and Glycuronates.—The other substances used for
detoxication purposes are sulphuric and glycuronie acids. Phenol, and
its derivative cresol, after being absorbed from the intestine, in the
contents of which they are produced by the bacterial decomposition of
protein (see page 501) become combined in the body, probably in the
liver, with sulphuric acid or with glycuronic acid to form the sulphate
or glycuronate. The aromatic sulphate further combines with potassium
to form the so-called ethereal sulphates, as which the substance is excreted
in the urine. Adenine
{3) (4) (5) (6)
(Action of deaminizing enzymes)
Xanthosine Inosine
(9) (Action of hydrolyzing enzymes) ca
Uric Acid€— (11) Xanthine < (11) > Hypoxanthine
(Aotion of xanthine oxidase)
(Jones.)
The next step in the disintegration process is that the amino group
is removed and the corresponding oxypurine is produced. To bring this
about, there exists a specific deaminizing enzyme for each of the above
amino compounds, and each enzyme is named according to the exact
amino purine upon which it acts; thus, guanase (3), guanosine-deaminase
(4), adenosine-deaminase (5), and adenase (6) have all been identified.
The free base may then be split off from the nucleosides by specific
hydrolyzing enzymes (7) (8) (9) (10).
The joint action of these enzymes leads to the formation of oxypurines,
xanthine and hypoxanthine, which are oxidized to uric acid by xanthine-
oxidase (11).
In man and the anthropoid apes uric acid is the end product of the
above changes, but in other mammals most of the urie acid is further
oxidized into allantoine. It has also been found, except in man and the
chimpanzee, that extracts of organs such as the liver, are capable of
decomposing uric acid into allantoine. The identification of these specific
enzymes is sought by a determination of the free amino-purine bases
and the phosphoric acid produced by allowing an aqueous extract of
the tissue in question to act on nucleic acid (of yeast)* at body tempera-
ture. Another portion of the digested mixture is then hydrolyzed by
means of boiling sulphuric acid and the constituents again determined.
From the results it is often possible to draw conclusions as to the exact
nature of the enzymes present.
The most remarkable outcome of this work has been to show that
the distribution of the enzymes is not the same in the tissues and organs
of different animals. Very briefly, some of the most important results
that have so far been obtained are as follows: Gastric and pancreatic
juices do not contain a trace of any of the enzymes. Intestinal juice,
*Yeast nucleic acid is used because it is less resistant to disintegration than thymic nucleic acid.
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640 METABOLISM
on the other hand, contains a nuclease capable of splitting the poly-
nucleotides into mononucleotides. The two pyrimidine nucleotides split
off do not undergo further change, but the purine nucleotides are con-
verted into nucleosides (the enzyme being designated ‘‘nucleotidase’’).
Extract of the intestinal mucosa, besides having the same action as the
intestinal juice, can also decompose the purine, but not the pyrimidine
nucleosides, into carbohydrate and purine groups (specifie action of
““nucleosidase’’). A similar action is produced by extracts of kidney,
heart muscle, and liver. Blood serum, hemolyzed blood, and extract of
pancreas, on the other hand, are capable only of carrying the decompo-
sition as far as the mononucleotides.
Regarding the other enzymes mentioned in the above list, it is im-
portant to note that they appear at different stages in embryonic develop-
ment, and that their distribution varies considerably in different species
of adult animal, the spleen, liver, thymus, and pancreas containing them
most abundantly. The distribution of enzymes in the organs of the
monkey resembles that in the lower animals considerably more than it
does that in man. Some remarkable facts have come to light regarding
guanase and adenase, particularly that guanase is deficient in the organs
of the pig, in the urine of which animal it has also been found that the
purine bases are in excess of the uric acid. This absence of guanase
no doubt accounts for the fact that deposits of guanine may occur in the
muscles, and that these may be so large as to constitute the condition
known as guanine gout found in this animal. Adenase, on the other
hand, is absent from the organs of the rat, which again corresponds with
the fact that, when adenine is injected subcutaneously into these ani-
mals, it undergoes oxidation without the removal of its amino group.
In the human organism, adenase appears to be absent from all of the
organs, whereas guanase is present in the kidney, lung and liver, but
not in the pancreas or spleen. Xanthine-oxidase exists only in the liver.
The distribution of uricase is perhaps the most interesting. It is pres-
ent in most of the lower animals. On account of its presence extracts
of the liver, spleen, etc., in all breeds of dogs, with the exception of
Dalmatians, rapidly destroy uric acid; and practically no urie acid
when injected subcutaneously can be recovered unchanged in the urine,
but appears as allantoine. Uricase, however, is absent in man. This has
been demonstrated by finding (1) that when uric acid is injected sub-
cutaneously, nearly all of it appears in the urine, and (2) that uric acid
is not destroyed when extracts of the organs are incubated at body
temperature with uric acid or its precursors. It must of course be kept
in mind that, although the uric acid is thus shown not to be destroyed
in vitro, it may nevertheless be destroyed in the living animal.
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URIC ACID AND THE PURINE BODIES 641
The importance of the above described results rests in the fact that
from them we may hope to be able, ultimately, to state exactly in what
organs and tissues the intermediary metabolic processes concerned in
nucleic acid metabolism occur. The work at the present time is of spe-
cial significance, since it represents one type of evidence which we must
have before we can trace exactly every Step in the metabolism of any
other biochemical substance.
The absence of uricase from the tissues of man places him in a unique
position with regard to the metabolism of nucleic acid, and renders the
investigation of the problem particularly difficult, since animal experi-
mentation is useless. Recently, however, S. R. Benedict has discovered
that the Dalmatian breed of dog—also known as the carriage dog, and
having a spotted or mottled skin—has a purine metabolism like that of
man.* When fed on food containing no purine substances, he excretes
large quantities of uric acid, and when the latter substance is injected
subcutaneously, it is eliminated quantitatively as such in the urine. We
shall see later how experiments on this animal have been made use of
in the investigation of problems of purine metabolism as applied to man.
Tn all other animals most of the uric acid is oxidized to allantoine before
being excreted. The degree to which this occurs varies between 79
and 98 per cent of the urie acid in different species. This has been
ealled the uricolytic index (Hunter and Givens).
The Balance between Intake and Output of Purine Substances under
Various Physiologic and Pathologic Conditions—The main purine ex-
cretory product in man is uric acid, but there is also a certain amount
of purine bases. The presence of uric acid has attracted attention for
a great many decades in medical investigation, because of the relative
ease with which it can approximately be determined quantitatively, and
because of the well-known fact that it may be responsible for certain
diseases, such as gout, when it accumulates in the tissues in an insoluble
form. On a diet containing meat, or more particularly on one con-
taining glandular substances, the total daily excretion of uric acid is
very considerably greater than when the diet contains no such food
stuffs. The conclusion which Burian and Schur*? drew from this ob-
servation is that purine must be partly of exogenous and partly of
endogenous origin. In other words, some of it is derived more or less
directly from performed purine substances in the food, and the remain-
der from the purine constituents of the animal’s own tissues.
Endogenous Purines.—It was thought that a definite proportion of
each of the administered purines could be invariably recovered from
the urine. Although this has not been found to be exactly true, there
is nevertheless a certain constaney in the proportion of administered
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642 METABOLISM
purine that is excreted. Thus, Mendel and Lyman have found recently
that about 60 per cent of injected hypoxanthine, 50 per cent of xan-
thine, 19-30 per cent of guanosine, and 30-87 per cent-of adenine were
eliminated as uric acid. When combined purines—i.e., nuclear mate-
rial—are given, only a small proportion of the purine reappears as uric
acid in the urine. There is, therefore, a general parallelism between
the purine content of the food and that of the urine, which indicates that
purine-rich food should be eliminated from the diet of patients who are
suffering from deposition of insoluble urate in the tissues, as in gout.
The fate of the purine that disappears in the body is unknown; some of
it may be decomposed in the intestine, but why so much of the remainder,
after absorption by the blood, should disappear is a mystery, since no
uricase can be discovered in any of the organs or tissues. The destroyed
purines can not be shown to influence any of the other well-known
nitrogenous metabolites of the urine.
The following table of experiments by Taylor and Rose* may serve
to illustrate these points. The subject was placed on a purine-free: diet
consisting of milk, eggs, starch and sugar, for three days. After this
period a part of the total nitrogen (8 grams) was supplied as sweet-
breads—thymus gland, ete.—containing a high percentage (0.482) of
purine nitrogen; for another period of four days still more of the nitro-
gen (6 grams) was replaced by sweetbread nitrogen; and this was fol-
lowed by a final period in which the original diet of milk, ete., without
purine substances, was restored. The following table gives the results:
1ST PERIOD 4TH PERIOD
PURINE-FREE 2NDPERIOD 38RD PERIOD PURINE-FREE
DIET DIET
Total urinary N 8.9 8.7 “9.1 8.8
Urea N and NH, 7.3 7.1 7.1 7.05
Creatinine 0.58 0.55 0.56 0.47
Purine N (total) 0.11 0.17 * 0.26 0.10
Uric acid N 0.09 0.14 0.24 0.07
Remainder N 9.91 0.88 1.18 1.18
The increase of uric acid accounted for less than half of the purine
nitrogen ingested. This appeared as uric acid, the excretion of purine
bases being practically unchanged.
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CHAPTER LXXIIT
URIC ACID AND THE PURINE BODIES (Cont’d)
SOURCE OF ENDOGENOUS PURINES
Even after the entire elimination of all purine substances from the
food in the case of man, purine continues to be exe¢reted in the urine
as uric acid. This, as above femarked, is called endogenous excretion.
At first it was thought by Burian and Schur that the total nitrogen of
the purine-free diet could be considerably varied without causing any
alteration in the amount of the endogenous purine excretion, but a rep-
etition of the work has shown that, when these changes are of consider-
able magnitude, the endogenous moiety does not remain constant. This
has already been demonstrated in the table on Folin’s results (see page
614), and is still better illustrated in the accompanying table, which
shows the excretion of uric acid and coincidently of urea from hour to
hour in the urine after taking food which is free from nuclein or purine
substances. After a fast of six hours, a diet consisting of bread and
potatoes was taken at 1:30, and the urea and uric acid measured in the
urine each hour thereafter.*
TIME UREA URIC ACID AMOUNT OF URINE
GM. MG. . c.c.
10-11 1.07 26 175
11-12 1.13 27 118
12-1 P.M. 1.07 24 164
1-2 (meal) 0.64 21 60
2-3 1.12 22 43
3-4 1.16 38 41
4-5 0.84 40 53
5-6 1.16 56 59
6-7 1.20 39 56
7-8 1.37 39 95
8-9 1.47 33 183
9-10 1.33 24 155
10-11 1.33 23 180
(Hopkins and Hope.) 46
A postprandial increase of endogenous purine excretion is very dis-
tinet, and it indicates that during the process of assimilation something
must be occurring in the organism which entails the production of purine
*These investigations should be repeated, since there is some question as to whether the method
of analysis employed (Folin-Shaffer) is suitable for determining hourly uric-acid excretion.
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644 METABOLISM
from the organism itself. As to what this may be, it is impossible to
say. It may be associated with the work of the gastric and intestinal
glands, which recalls the interesting suggestion, originally made by
Horbaczewski, that ingested substances increase the exeretion of uric
acid by causing a leucocytosis, the purine being derived from the nucleic
acid set free when the leucocytes become broken down. That this’ is
not the correct explanation, however, is indicated by the fact that in-
gested substances that give rise to an increased number of leucocytes
affect. the excretion of uri¢ acid during the period the leucocytes are
present in the blood, and not after they have disappeared, which would
have to be the case were the uric acid a product of purine substances
liberated by their breakdown. This would indicate that the purine sub-
stance is a metabolic product of the living leucocytes and not a break-
down product of those that are dead. It should be noted that the increase
in the postprandial uric-acid excretion occurs earlier than that of urea.
The most pressing question concerns the origin of the endogenous
purines. Urie acid is the purine with which we are most concerned in
the case of man, and chemistry shows us that it may be produced either
by the oxidation of the lower purines—namely, of those which are the
constituent parts of the nucleic-acid molecule—or by a synthesis of two
urea molecules with a carbon residue containing three carbon atoms.
There are consequently two sources from which the endogenous purine
excretion in man may be derived : (1) synthesis of two urea molecules,
and (2) oxidation of the lower purines.
We will consider first the possibility of synthesis. In birds and
reptiles practically all the nitrogen is excreted in the form of urie acid,
and it is easy to show that this has been produced in the organism by
the synthesis of urea with carbon-rich residues, occurring mainly in the
liver. Minkowski found that by removing the liver from geese, which
is a comparatively simple operation on account of an anastomotic vein
between the portal and the renal veins, the uric acid in the urine became
very markedly decreased and ammonium lactate took its place (page
618). Since we know that ammonium in the animal body is ordinarily
converted into urea, we may conclude from this observation that some-
thing has occurred to prevent the synthesis of urea into uric acid. In
confirmation of this conclusion it was subsequently found that, if am-
monium lactate was added to the blood perfused through the isolated
liver of the goose, uric acid was produced in the perfusion fluid.* Fur-
thermore, when birds and reptiles are fed with ammonium salts or
‘with the degradation products of protein, there is an increase in the ex-
*The reason for the formation of this relatively insoluble metabolite in place of the soluble urea
is connected in some way with the fact that birds and reptiles do not take such large quantities
of water with their food as other animals.
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URIC ACID AND THE PURINE BODIES 645
cretion of uric acid instead of urea. Everything which in a mammal
tends to cause an increase in urea excretion causes in birds and reptiles
a similar increase in the excretion of uric acid.
In the early days of research in the uric-acid problem, not inconsid-
erable mistakes were made on account of failure to recognize the essen-
tial difference in the metabolism of uric acid in birds and mammals,
and the tendency for some time after the exact state of affairs was
discovered was to consider that in mammals none of this synthetic proc-
ess occurs. The latter view, however, is surely incorrect, for a cer-
tain amount not only of uric acid itself but of the lower purine bodies
can be produced by synthesis in the mammalian body. Thus, Ascoli and
Izar*? discovered that uric acid could be made either to disappear or
to be formed when a minced preparation of liver was incubated, depend-
ing upon whether oxygen or carbon dioxide was bubbled through it.
With oxygen uric acid disappeared, whereas with carbon dioxide uric
acid accumulated, indicating that in the presence of this gas the destroyed
uric acid became reformed from the disintegration products of the oxy-
genation process. As similar results were obtained from the livers of
birds, it is clear that no essential difference exists between the purine
metabolic processes occurring in the livers of birds and of mammals.
The difference is a quantitative not a qualitative one.
Regarding the chemical nature of the product into which uric acid is
broken down and from which it may be resynthesized, it has been pos-
sible so far to identify but one substance—namely, dialurie acid. This
is a perplexing result, for from all other investigations it would appear
that in mammals, with the exception of man and the anthropoid apes,
uricase splits uric acid into allantoine (see ‘page 640), which substance,
however, when added to liver extract did not cause any uric acid to be
formed; nor did any of the other known decomposition products of uric
acid have such a result. The chemical reaction involved in the produc-
tion of uric acid from dialurie acid and urea is indicated as follows:
NH—C=0O
x |
~
ce Ci=HOH ” H ' NH
yO
\ | t + ti C=O
\ be
NH—C:=0O H ‘NH
(dialuric acid) pips onde oy (urea)
The synthesis of uric acid is brought about by the combined action
of a thermolabile enzyme in the blood and a thermostable body in the
tissues. An aqueous extract of blood-free liver of the dog ean destroy
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646 METABOLISM
uric acid only in the presence of oxygen; it can not reform it even in
the presence of carbon dioxide. On the other hand, blood serum can
not reform uric acid, whereas a mixture of the bloodless liver extract
and blood serum produces uric acid readily under suitable conditions.
Boiling of the liver extract does not affect the result, but boiling of the
blood serum renders it incapable of exerting its joint action with the
bloodless liver extract.
These experiments with dog’s liver serve only as circumstantial evi-
dence that uric-acid synthesis occurs in mammals as well as in birds.
More direct proof that purine synthesis occurs in mammals is as follows:
(1) It was discovered long ago by Miescher that salmon, after leaving
the sea to ascend the rivers, have a well-developed muscular system, but
that in the upper reaches of the stream the muscular system becomes
considerably atrophied and the testes enormously developed. As the
fish takes no food during the migration, there must be conversion of
the protein of the muscles into the cellular tissue of the sexual glands,
and nucleic acid must be produced. (2) A hen’s egg before its ineuba-
tion contains practically no nucleic acid, whereas after development has
well started nucleic acid increases by leaps and bounds. Similarly the
eggs of insects increase in purine content very markedly as development
proceeds. (3) Milk contains practically no purine derivative, and yet
when it is fed to young growing animals, the organs lay on purine sub-
stances abundantly. In general, indeed, it may be said that the combined
purine increase is in proportion to the increase in body weight on the
milk diet. (4) In Osborne and Mendel’s experiments already alluded
to, it has been shown that adequate growth depends primarily on the
nature of the protein building stones, and not upon the purine content
of the food. (5) An objection might be raised to these results on the
score that they do not apply to the adult mammal, Investigation of
the problem has hitherto been seriously impeded by the fact that no or-
dinary laboratory animals were known in which uric acid is excreted in
the urine. The discovery that this occurs in the Dalmatian dog has,
however, made it possible for 8S. R. Benedict*t to show, not only that
after increasing the amount of nonpurine food there was a very distinct
inerease in the uric-acid excretion, but also that when the animal was
kept for a year on such foods there was excreted a total amount of uric
acid at least ten times greater than could have come from the traces
unavoidably included in the food.
Regarding the chemical nature of the substance from which the purine ©
is synthesized, we know at present practically nothing. No doubt some
of the protein building stones functionate in this capacity, pyrimidine
being probably the product that is first formed. Thus, pyrimidine may
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URIC ACID AND THE PURINE BODIES 647
be produced as a result of the combination of amino-malonic acid with
urea, the amino-malonic acid being produced by condensation of hydro-
eyanic-acid molecules:
3 HCN — H,N - CH(CN),+CO(NH), > NH-CO
co CNH,
el
NH - CNH.
(hydrocyanie (amino-malonie (urea) (oxy-diamino-pyrimidine)
acid) nitrile)
Another possible source of pyrimidine is the oxidation of arginine to
guanidine-propionic acid, which then condenses to form amino pyrimi-
dine.
Purine synthesis undoubtedly occurs in the mammalian body, but it
is difficult to recognize in metabolism investigations because it is a slow,
continuous process. The probability of its occurrence, however, is indi-
cated by such results as those described on page 614, in which increase
in purine excretion is observed after varying the intake of food, even
when this is itself entirely free from purine substances. Whether or not
changes in the activity of purine synthesis occur in conditions of disease
is a question which awaits investigation.
The Influence of Various Physiologic Conditions, of Drugs, and of
Disease on the Endogenous Uric-acid Excretion —Muscular exercise was
thought by Burian to cause an increased excretion of uric acid, from
which he drew the conclusion that the hypoxanthine present in compara-
tively large amount in muscular extract, or its precursor, inosinic acid,
must be an important source of endogenous uric acid. Other observers
(Leathes, ete.) have found that strenuous exercise causes a distinet in-
crease in uric-acid excretion, which, however, is much less marked on
repetition of the same kind of exercise on the next day. If some new
kind of muscular work is performed, another increase in uric acid will
result: There are still other investigators who deny that muscular work
has any influence on uric-acid excretion. ,
It has been observed by several investigators that the endogenous
purine excretion is distinctly higher during the waking hours than during
sleep. This can not be shown to depend on variations in the urinary
function, and since it is decidedly doubtful whether ordinary muscular
activity has any influence, the diurnal variation is most difficult to
account for. The endogenous excretion in man is not the same for
different individuals, even when calculated for the same body weight; it
varies between 0.12 and 0.20 per cent purine nitrogen in an adult man.
It remains remarkably constant for a given individual from time to
time, being unaffected by moderate degrees of variation in the amount
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648 METABOLISM
of food taken provided this be purine-free; when, however, the amounts
are extremely variable, changes are produced (see page 614).
In disease, fever causes an increased excretion. This has been most
clearly shown by Leathes, who took a large enough dose of antityphoid
serum to produce a distinct degree of fever (103° F.), and found that
an increase in uric-acid excretion occurred. That increased combustion
processes occurring in the tissues were responsible for the uric acid,
was shown by the same author, who caused a similar increase by sub-
jecting himself to cold baths for a considerable period of time. The in-
ereased loss of heat thus induced stimulated the combustion processes in
the body so as to maintain the body temperature, and as a result there
was an increase in uric-acid excretion. It has long been known that an
excessive amount of uric acid is excreted in leucocythemia. The nuclein
of disintegrated leucocytes is commonly held responsible for the increase.
Naturally, much work has been done on the endogenous and exogenous
purine excretion in gout. No very striking anomalies of excretion have,
however, been brought to light, except perhaps that after the ingestion
of purine-rich foodstuffs it takes longer for the resulting exogenous ex-
cretion to develop and pass away.
Certain drugs affect the excretion of uric acid. Salicylic acid is said
to cause an increased excretion, and citrates certainly have this effect.
In both eases the increase is followed by a compensatory fall, which
indicates that these drugs act by facilitating the excretion rather than
by influencing the metabolic processes that are the source of the uric
acid. The effect of caffeine has been very carefully investigated. Given
to the Dalmatian dog, referred to above, 8. R. Benedict found that a
small dose caused a slight decrease, but that a larger dose had practically
no effect, although there was a notable retention of nitrogen. On man,
however, different results were secured, for it was found that when 1
gram of caffeine was given daily for several days, a slight but definite
progressive increase in the endogenous uric-acid excretion occurred, and
it lasted for 10 days after the caffeine administration was discontinued.
Liberal allowance of this alkaloid may, therefore, not be quite so innocu-
ous as it is assumed to be.
Uric Acid of Blood.—In all of the investigations considered above,
the behavior of uric acid is judged from the amount of it excreted in
the urine. Valuable though such results must be, their interpretation is
always difficult, since two factors that are quite independent of. each
other have to be kept in mind—namely, the production of the uric acid
in the organs and tissues and its excretion by the kidneys. In connection
with the latter factor, we must also consider the method of transporta-
tion of urie acid by the blood from its place of production (or absorp-
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URIC ACID AND THE PURINE BODIES 649
tion) to the kidneys. These problems have recently been very consider-
ably simplified by the elaboration of an accurate method for the estima-
tion of the uric-acid content of blood.
By observing changes in the amount of uric acid in the blood rather
than in the urine, the excretory factor is partly controlled, and it can
be completely so if urine and blood are both investigated. Thanks to
the work of Folin, it is now possible to determine with an extreme de-
gree of accuracy the uric acid in as little as 10 ¢.c. of blood. The impor-
tance of this achievement will be appreciated when we state that prior
to Folin’s work no method existed by which uric acid could be approx-
imately measured even when large quantities of blood were available.
Much of the work that has been done by the use of this new method
has so far applied to the amount of uric acid in the blood of man in
various diseases. We shall refer to these results immediately; but
meanwhile it is important to call attention to some very suggestive
observations concerning the condition of uric acid in the blood. For
many years there have been investigators who have thought that uric
acid can not be simply dissolved in the blood plasma, like sugar or some
inorganic salt. It is believed by many that at least a portion of the uric
acid circulates in combination with nucleic (thymic) acid (see page 637),
which would account for the fact that some purines are catabolized in
the body when they are given in a combined state, as thymic acid, but
are excreted unchanged when ingested in a free state. When given freely,
certain purines—adenine, for example—may moreover cause inflamma-
tion and calculus formation in the kidneys of dogs, a result not obtained
when thymic acid is fed.
Other observers have concluded that uric acid exists as two isomeric
varieties, lactam and lactim, the monosodium salts of which are of un-
equal stability. The less stable a-salt is much more soluble in blood
serum than the stable @-salt. It is the a-salt that becomes increased in
the blood in gout, the deposition of urates in the tissues, which is the
most characteristic symptom of this disease, being caused by conversion
of the a-salts into B-salts. The structural formulas of the two isomers
are as follows:
H.N-C:0 N-C.OH
hy UI
O:C C-NII *HO.C C-NH
| fl \ | | \
| q co | C.OH
eel /
H.N-C-NH Net v
[lactam modification forming [lactim modification forming
unstable a-urates] stable B-urates] :
(relatively soluble) (relatively insoluble)
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650 METABOLISM.
The most recent work of 8. R. Benedict has shown that uric acid ex-
ists, chiefly in combination in the blood of most mammals but not in
that of the bird. It was found, for example, that fresh ox-blood exam-
ined by the Folin method contains only 0.0005 gm. free uric acid per 100
gm. of blood; after boiling the protein-free blood filtrate with hydro-
chlorie acid, however, the uric acid increased by about ten times. This
larger amount was also found present in whole blood that had been
allowed to stand for some time, indicating that the uric-acid compound
can be split by means of an enzyme. The compound exists in the cor-
puscles and not in the plasma. It is of some significance that after thus
setting free the uric acid, there should be about 50 per cent more of it
present in the blood of the ox than in that of the bird, where most exists
in a free state in the serum, although the urine of the ox contains only
the smallest trace of urie acid, and that of the bird is loaded with it.
Investigation of the condition of uric acid in human blood is at present
in progress.
Uricemia in Gout and Nephritis
The practical application of these observations is particularly impor-
tant in connection with the etiology of gout. In typical cases of this dis-
ease, the uric acid of the blood increases from its normal value of 1 to
3 mg. per cent to nearly 10 mg., indicating a considerable degree of
renal insufficiency. This uricemia can not in itself, however, be the cause
of the deposition of urates in the joints, because it also occurs in other
diseases with renal retention, such as nephritis. Moreover, the blood
serum is capable of dissolving much larger quantities of uric acid than
are ever found present in it in gout. The real cause for the gouty deposits
must depend on some change affecting the blood so as to alter the form
in which uric acid exists therein, with the result that it is excreted into
the joints and deposited there.
Other diseases showing uricemia are lead poisoning and nephritis. In
the latter disease the damaged excretory function of the kidney is
manifested first of all by an increase in the uric-acid content of the
blood, accompanied later by a retention of urea and later still by one
of creatinine. The severity of the renal involvement may therefore be
gauged .by determining the percentage of these three metabolites. On
account of the importance of these facts from a clinical standpoint, we
append a table containing results secured by Myers and Fine, in which
the behavior of the metabolites in the blood is shown in relationship
to the severity of the case as gauged by the blovd pressure.
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URIC ACID AND THE PURINE BODIES
651
Uric Actp, UrEA N AND CREATININE OF BLOOD IN GOUT AND EaRLy AND LaTE NEPHRITIS
URIO
“AC UREAN CREATININE SYSTOLIC
Benge MG. TO 100 C.c. scien
BLOOD PRESSURE,
Typical Cases of Gout 9.5 13 1.1 230
8.4 12 2.2 164
7.2 17 2.4 200
: 6.8 14 1.7
Typical Early Interstitial Nephritis 9.5 25 2.5 185
8.0 37 2.7 150
5.0 37 3.9 130
7.1 16 2.0
6.6 24 3.3 _ 185
6.3 18: 2.1
8.7 20 3.6 100
7.0 33 2.6 117
6.3 31 2.1
6.3 23 2.4 150
Chronic Diffuse and Chronic Inter- 8.0 80 4.8 240
stitial Nephritis 4.9 17 2.9 170
8.3 72 3.2 238
5.3 21 1.9 145 .
9.5 44 3.5 210
2.5 19 1.9 120
7.7 67 3.1
6.7 17 1.6 165
8.3 39 2.9
‘ 6.5 24 3.0 200
Typical Fatal Chronic Interstitial 22.4 236 16.7 210
Nephritis 15.0 240 20.5 225
14.3 263 22.2 220
13.0 90 11.1 265
8.7 144 11.0 225
(Myers and Fine:
Arch. Int. Med., 1916.)
Lastly, regarding the influence of drugs on the blood uric acid in dis-
ease, it has been found by Fine that both atophan and salicylates cause
a pronounced decrease in the amount, but that it gradually rises to the
old level even while administration of the drugs is being continued.
Important contributions to the behavior of‘uric acid in blood are
constantly appearing at present, mainly from the laboratories of Folin
in Boston, of 8. R. Benedict, and of Myers and Fine in New York.
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CHAPTER LXXIV
THE METABOLISM OF THE CARBOHYDRATES
The healthy animal organism is capable of rapidly oxidizing large
quantities of carbohydrate, as is evident from the following facts: If
carbohydrate is given to a starving animal, (1) -the energy output very
shortly afterward increases; (2) the respiratory quotient also increases,
indicating that, relatively to oxygen intake, more carbon dioxide is being
excreted (see page 647); and (3) none of the ingested carbohydrate
makes its appearance in the excreta. Indeed, of the three proximate
principles of food, carbohydrate is the most available for combustion
in the animal body. It may therefore be considered as the quickly
available fuel for the body furnaces.
CAPACITY OF THE BODY TO ASSIMILATE CARBOHYDRATES
Assimilation Limits—When the limit to the amount of carbohydrate
that the organism can metabolize is overstepped, some of it appears in
the urine. The amount that can be tolerated without causing glycosuria
is commonly called the assimilation or saturation limit. The use of the
term ‘‘limit’’ is, however, very unfortunate, for it implies that beyond
this point the organism is capable of dealing with no more carbohy-
drate, which is far from being the case, for if a larger amount is taken,
only a small trace of the excess will appear in the urine. When the
urine is allowed to collect for twenty-four hours, the mixéd specimen
shows no trace of glucose in the majority of healthy individuals after
the ingestion of 200 gm.; after 300 gm. a somewhat higher percentage
of cases develop a mild glycosuria, but frequently none is evident even
after 500 gm. Beyond the last mentioned amounts the limit of ingestion
‘is reached, on account of nausea, etc., and it is improbable that, even
if larger amounts could be tolerated, any more of the dextrose would
be absorbed than with 300 or 400 gm. The testing of the so-called
assimilation limit has been considered an important aid in the diagnosis
of early cases of diabetes, the characteristic feature of such cases being
the inability of the organism to assimilate properly the usual quantity
of carbohydrate contained in the diet.
It has been found:that to make the results of any value, certain
conditions must be fulfilled in applying the assimilation test. The most
652
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THE METABOLISM OF THE CARBOHYDRATES 653
important of these concerns the activities of the gastrointestinal appa-
ratus at the time the sugar is given, for it has been found that if other
foodstuffs are being absorbed at the same time as the sugar, more of
the latter can be tolerated than when the sugar alone is being absorbed.
It has therefore been custcmary to give the sugar dissolved in water,
or in weak coffee, the first thing in the morning after the patient awakes;
i.e., at least twelve to sixteen hours after the last meal was taken. In
making these tests the urine voided before the sugar is estimated should
of course itself be thoroughly examined for reducing substances, and
the urine should be collected every ninety minutes and examined by a
reliable test (Benedict’s or Nylander’s).*
Although a limit is set to the ability of the organism for retaining
sugar (mono- or di-saccharides), this does not seem to apply, in healthy
individuals at least, when starches (polysaccharides) are ingested. Thus,
it is a well-known fact that people can eat enormous quantities of pota-
toes or of bread without the appearance of any trace of reducing sub-
stances in the twenty-four-hour urine. On the other hand, urine collected
and examined at short intervals (every half hour) after taking large
quantities of polysaccharide-rich food will frequently be found to contain
traces of reducing substances.
For practical purposes it has been considered that an individual who
develops glycosuria aften taking 100 gm. of glucose must be considered
as at least a potential diabetic. In the light of the above results and
for many other reasons, there is, however, considerable doubt as to the
value of the assimilation test. Thus, when a solution of glucose is
given orally, its rate of absorption will depend very largely on the
motility of the stomach. If this is normal, the solution will very quickly
find its way past the pyloric sphincter into the intestine, where it will
be rapidly absorbed. If, on the other hand, the pyloric sphincter does
not open freely, the passage of the glucose into the intestine may be
so delayed that no more is present in this place at one time than would
be the case after an ordinary diet of polysaccharide. And even after
the sugar solution enters the small intestine, differences in the amount
of the intestinal contents with which it becomes mixed, in the extent of
bacterial growth, and in the absorption process, may very materially
affect the rate at which the glucose gains entry to the blood.
Although often of doubtful diagnostic value, determination of the
assimilation limit is of considerable aid in controlling the treatment of
*Examination of normal individuals’ has shown that the assimilation limit for different sugars
wil Ge semeanees, 1 fae meenagancaied oak gies Ce See Ena
sugar molecule, the assimilation limit is frem 100 to 150 gm.; for cane sugar or saccharose itself
the figures seem to vary considerably, but are given as between 50 and 200 gm.; for lactose, another
disaccharide, and the sugar present in milk, the assimilation limit is distinctly lower—namely, 100 gm.
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654 METABOLISM
diabetes. For this purpose the patient should first of all be instructed
to follow his usual diet, so that, by examination of the amount of sugar
excreted in the urine, an opinion may be formed of the severity of the
ease. The diet should then be changed so as to consist of a part that
contains no carbohydrates and another composed entirely of starchy
food. The former is made up of eggs, fish, green vegetables, fat, ete.,
and the latter, to start with, should consist of 100 grams of bread, dis-
tributed between the two main meals of the day, one of which is break-
fast. This diet should be continued until the glycosuria either disappears
or attains a constant level. If it disappears, the case is classified as a
mild one of diabetes, and the daily allowance of bread may be increased,
by 50 grams a day, until the sugar again makes its appearance in the
urine, indicating that the assimilation limit has been reached. For
therapeutic purposes, the patient should now be instructed to take about
three fourths of this amount of carbohydrate in his daily rations, and
he should be supplied with explicit instructions in the shape of diet
tables as to what variety and quantities of the various carbohydrate
materials his food may contain. His urine should be examined at fre-
quent intervals—once a week—and he should be instructed as to the
nature of his disease and the importance of his remaining aglycosuric.
By further treatment such so-called latent cases of diabetes may be
kept in perfect health for many years.
When, on the other hand, the glycosuria exists with 100 grams of
bread in the daily ration, this must be reduced to 50 grams, and if after
some days the first reduction does not suffice to render ‘the urine free
from sugar, carbohydrates must be withheld entirely from the diet.
If the glycosuria does not now disappear, the case is to be considered
severe, and it may be necessary to undertake the starvation treatment,
which has recently been developed in this country by Allen*® and Joslin’®
with apparent success. By the reduction of carbohydrate, or by the
starvation treatment, it is usually possible to make even the severest
cases of diabetes aglycosuric, and when this has been attained, then
gradually to increase the amount of protein or carbohydrate food until
the assimilation limit has been reached.
Saturation Limits.—To avoid error caused by irregular absorption from
the intestines, some investigators have recommended the determination
of the assimilation limit after intravenous or subcutaneous injections
_ of sugar. But even this refinement in technic has not, as a rule, had the
effect of rendering the results of any very evident value as a criterion
of the utilization of glucose in the animal body. The reason for this
unreliability of the method is mainly that the period of injection of the
glucose solution usually occupies only a few minutes, so that it causes
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THE METABOLISM OF THE CARBOHYDRATES 655
a sudden instead of a very gradual increase in the sugar concentration
of the blood, the conditions being quite unlike those which exist during
the normal absorption of glucose from the intestine. The mechanism
by which the body ordinarily disposes of excessive amounts of glucose
absorbed into the portal blood, is not adjusted to operate when. the sys-
temic blood is suddenly overcharged with this substance. In the one
case the glucose is a foodstuff; in the other, because of its excessive
concentration in the blood, it is more or less of a poison. Such results, in
other words, merely show us how much- glucose can be added at one
time to the organism without any overflow into the urine, but they
furnish us with no information regarding the power of the organism to
utilize a constant though moderate excess of this substance. In the one
case it is the ‘‘saturation limit,’’ in the other the ‘‘utilization limit’’ of
the organism for glucose, that we are really considering.
Consideration of these principles has led Woodyatt, Sansum and Wil-
der?’ to undertake a thorough reinvestigation of the whole problem of
the utilization or, as they prefer to call it, the tolerance of the body for
glucose. They emphasize the obvious fact that the ability of the organism
to utilize glucose ‘‘must depend on the rate at which the tissues are
able to abstract it from the blood by their combined powers, to burn it,
to reduce it into fat or to polymerize it into glycogen.’’ To form any
estimate of the combined effect of these processes, we must take into
account not only the amount of glucose per unit of body weight (grams
per kilogram), but also the rate of injection, for ‘‘tolerance must be
regarded as a velocity, not as a weight.’’
Briefly summarized, the conclusions which Woodyatt, ete., have so far
drawn from their investigations are as follows: In a normal rabbit, dog,
or man, 0.8-0.9 gm. of glucose per kilogram body weight and per hour can
be utilized by the organism for an indefinite time without causing gly-
cosuria. When between 0.8 and 2 gm. are injected, a part of.the excess
appears in the urine, steadily increasing until a maximum is reached,
after which the excreted fraction remains constant (at about one-tenth).
If more than about 2 grams per kilogram an hour are injected, ‘‘a large
percentage of all glucose in excess of the 2 gm. per kilogram an hour
appears in the urine when constant conditions are once established.’’
The fact that so much glucose injected intravenously can be used
without the appearance of any of it in the urine, indicates a method by
which foodstuffs may be supplied to the tissues in eases where, on account
of gastrointestinal disturbances, it is impossible to have food absorbed
by the usual pathways. The possible value of such a method of. treat-
ment in cases of extreme weakness has been tested on laboratory animals
by Allen, who states that such injection seems to have a valuable nutri-
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656 METABOLISM.
tive and strengthening effect. He found, for example, that in cats
starved to extreme weakness the injection of a fraction of a gram per
kilogram of glucose had an unmistakable strengthening effect, and
sometimes appeared to save life. Such results would seem to indicate that
in certain cases where blood transfusion is impracticable, glucose in-
fusions should be tried. Subcutaneous injection of sugar, either for the
purpose of determining the assimilation limit or with the object of sup-
plying foodstuffs parenterally, is impracticable because of the pain and
sometimes sloughing produced at the point of injection.
We have devoted no inconsiderable space to a discussion of assimila-
tion limits because of the great interest in diabetic therapy which this
procedure has aroused during recent years. We may now turn our
attention to a closer analysis of the changes that take place in carbohy-
drates during their passage through the animal body.
DIGESTION AND ABSORPTION
Digestion. All digestible carbohydrate taken with the food is con-
verted by the digestive agencies into the monosaccharides, glucose and
levulose, as which it is absorbed into the blood of the portal system.
To bring about this resolution of carbohydrate into monosaccharides,
several enzymes are employed. The first of these is the ptyalin of saliva.
It is not a very powerful enzyme, being capable of acting only on starches
that are in a free state, i.e., not surrounded by a cellulose envelope;
but even on free starch, ptyalin displays little of its activity during the
time-the food is in the mouth. After the food is swallowed and becomes
deposited in the fundus of the stomach, there is an interval of time—
lasting until hydrochloric acid has been secreted to such an extent as to
permit some of the acid to exist in.a free state—during which the ptyalin
acts on the starch of the swallowed food. During this time the activity
of the ptyalin is actually assisted on account of the fact that a slight.
inerease in hydrogen-ion concentration of the digestive mixture accel-
erates the action of ptyalin.
The product of ptyalin digestion is maltose, a disaccharide composed
of two molecules of glucose. On entering the intestine, the carbohydrates
therefore exist partly as undigested starch, partly as glucose, and partly
as maltose. In the favorable environment of the duodenum a much
stronger diastatic enzyme called amylopsin very quickly hydrolyzes the
starch through dextrine into maltose. The maltose derived from the
starch and the unchanged sugars, such as cane sugar, maltose and lac-
tose, which have been taken with the food, unless they are present in very
high concentration in the intestinal contents, are not immediately ab-
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THE METABOLISM OF THE CARBOHYDRATES 657
sorbed into the blood, but become subject to the action of other enzymes
contributed by the intestinal juice—namely, the inverting enzymes, one
of which exists for each of the disaccharides. By their action maltose
is converted into two molecules of glucose by the enzyme maltase; lac-
tose, into galactose and glucose by lactase; and cane sugar, into levu-
lose and glucose by invertase. It is interesting to note that in animals
whose food does not contain one of those disaccharides, the correspond-
ing inverting enzyme is absent from the intestinal juice. The herbivo-
rous animals, for example, do not take any lactose in their food, and the
intestinal juice contains therefore no lactase, although it is present in
that of the young animals while still suckling.
A certain amount of carbohydrate becomes attacked by the intestinal
bacteria. These split the monosaccharides into lower fatty acids and
gases, such as methane and carbon dioxide. Besides this obviously de-
structive process, bacteria also perform a useful function in the digestion
of carbohydrates, in that certain strains of them are able to digest cellu-
lose, for which no special enzyme is provided. Bacterial digestion is con-
sequently essential in herbivorous animals; it takes place in the cecum,
which is enormously developed for this purpose (page 463).
Absorption—The glucose and levulose produced by digestion are
absorbed into the blood of the portal system. When a very large quan-
tity of a disaccharide, such as cane sugar, is present in the food, a certain
amount of the sugar is absorbed unchanged—that is to say, as cane sugar
—and appears in the blood, from which, since it is an abnormal con-
stituent, it is excreted unchanged in the urine. This alimentary glyco-
suria is particularly evident when the sugar is taken without any other
food; thus, after taking cane sugar in an amount corresponding to 5
grams per kilogram body weight, it was found in one and a half hours
afterward that the urine of ten out of seventeen healthy individuals con-
tained cane sugar. The urine of three of these men, however, also con-
tained invert sugar—that is, dextrose and levulose. Cane sugar con-
tinued to be excreted for from six to seven hours.
The Sugar Level in the Blood.—While no absorption of sugar is going
on, the percentage of this substance in the blood of the portal vein is the
same as that in the systemic circulation. During absorption the former
becomes perceptibly raised—to what extent we can not say—and in the
latter a less marked increase of sugar concentration is usually detectable.
Evidently, then, between the point at which the sugar is absorbed and
the blood of the systemic circulation, some barrier exists which holds
back some of the excess of absorbed sugar. We have very inaccurate
information as to how efficiently these barriers hold back the excess of
absorbed glucose because of the. technical difficulty in collecting blood
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658 METABOLISM
from the portal vein without serious disturbance to the animal. Indeed,
the only way by which the problem has been accurately studied is by
comparing the blood of the portal circulation with that of the systemic
circulation during the injection of a solution of dextrose into one of the
smaller branches of the portal vein.27_ In such experiments it has been
found that the percentage of sugar is a little less in the blood of the
abdominal vena cava than in that of the portal vein, and is still less in
the blood of the systemic veins, such as the femoral—results which justify
the conclusion that the barriers responsible for taking out some of the
absorbed sugar from the blood exist in the liver and in the muscles. The
curve in Fig. 189. will illustrate to what extent the mechanism operates.
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Fig. 189.—Curves showing the percentage of glucose in blood after a constant injection of
an 18 per cent solution into a mesenteric vein. V.C., vena cava, continuous line; P.D., pan-
creaticoduodenal vein, broken line; J, iliac, dotted line.
It will be observed that, so far as can be judged from changes in the
concentration of sugar in the blood, the sugar-retaining power of the
liver is about equal to that of the muscles—a conclusion which is, how-
ever, contrary to the usually accepted one that the liver has such pro-
nounced sugar-retaining powers that under ordinary circumstances it
removes from the portal blood all the excess of sugar added to it by
absorption and which is not required by the organism.
One objection which may properly be made to these observations is
that the animals on which they were made were under anesthesia, and
that the anesthetic had a paralyzing effect on the sugar-retaining power
of the liver. In view of this criticism it is important to examine the
results obtained on animals that are not under the influence of anes-
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THE METABOLISM OF THE CARBOHYDRATES 659
thesia. Such observations have been made on rabbits, and a few on man
himself. By collecting blood from the ear veins of rabbits, it has been
found that, after giving from two to ten grams of glucose by stomach,
the glucose concentration of the systemic blood begins to rise in fifteen
minutes, attaining a maximum in about an hour and then returning to
the normal level in about three hours.
Similar results have been obtained by examination of the venous blood
in man. After giving 100 grams of glucose by mouth, for example, there
is commonly an increase in blood sugar-amounting to from 30 to 34 per
cent of the normal and lasting for from one to four hours. The existence
of this postprandial hyperglycemia, as we may call it, indicates that the
sugar-retaining powers of the liver and muscles are not: sufficiently de-
veloped to prevent the accumulation of some of the absorbed sugar in the
systemic blood. Whenever this increase exceeds a certain limit, some of
the sugar begins to escape through the kidney into the urine, producing
glycosuria—postprandial glycosuria. The percentage of blood sugar above
which glycosuria occurs is, in the case of man, probably about 0.10 to 0.11
gm. per cent. After damage to the kidney, as in nephritis, or in long-stand-
ing cases of mild diabetes, the percentage may probably rise considerably
higher in the blood without evidence of glycosuria.
Value of Blood Examination in Diagnosis of Diabetes.—The determina-
tion of the amount of ingested carbohydrate required to bring about post-
prandial glycosuria constitutes, as we have already seen, the so-called
assimilation limit for sugar, which is often taken as an index of the sugar-
metabolizing power of the organism. It is evident, however, that the time
of onset, and the extent and duration of postprandial hyperglycemia must
serve as a more certain index of the sugar-retaining power of the liver
and muscles; and now that a simple and rapid clinical method exists
(Lewis-Benedict method) for the accurate determination of sugar in small
quantities of blood, there is no reason why this index should not be used
for the detection of failing powers to metabolize carbohydrate.
In no disease, probably not even in tuberculosis, is it more important
than in diabetes that an early diagnosis should be made. Thus, if we find
that the postprandial hyperglycemia after a certain amount of carbo-
hydrate develops to an unusually high degree and persists for an unusual
length of time, we are justified in curtailing the carbohydrate supply so as
to hold these values down to their level in normal individuals. It is almost
certain that the first sign of diabetes is an unusual degree and duration
of postprandial hyperglycemia. At first the excess of sugar leads to no
damage and it is insufficient to cause any evident glycosuria, although it is
quite likely that if the urine in such individuals were collected at very
frequent intervals after eating carbohydrate-rich food, glucose would be
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660 METABOLISM
found present in at least some of the specimens. In incipient diabetes,
however, the condition progresses, until the postprandial hyperglycemia
after one meal has not become entirely replaced before the next is taken,
so that the increase in sugar produced by the second meal becomes super-
added on that following the first meal. The curve of blood sugar rises
ever higher and higher, until at last permanent hyperglycemia is estab-
lished, or rather the normal level from which the postprandial rise occurs
has become permanently raised, so that in blood collected at any time a
higher percentage of sugar is found.
The Relationship Between the Sugar Concentration of the Blood and’
the Occurrence of Glycosuria.—Claude Bernard first pointed out that the
percentage of sugar in the blood may rise considerably above its normal
level without the appearance of any of the sugar in the urine, or at least
without a sufficient amount to give the usual tests for sugar. Even when
this limit is reached, as we have seen, the sugar which appears is not all of
the excess but only a small part*of it. This overflow hypothesis, as it is
called, has not been universally accepted because of the many results
which are not in conformity with it. Many of these exceptional results
have been explained as due to alterations in the permeability of the kidney
for sugar, and in general it is probably safe to accept Claude Bernard’s
hypothesis with certain reservations.
Strong support has been lent to a modified form of the hypothesis by
the recent work of Woodyatt and his collaborators, who have shown by
continuous intravenous glucose injections that as much as 0.8 gm. of
glucose per kilo body weight can be injected during an hour into an
animal without any glycosuria, although under such conditions a very
distinct increase occurs in the percentage of sugar in the blood.
To explain the failure of glucose to pass into the urine under normal
conditions, it has been supposed by several investigators that the glucose
exists in some form of chemical combination in the blood. This compound
is believed to behave like a colloid. One of the recent supporters of this
view is Allen, who has observed that, when glucose is injected intrave-
nously, it causes diuresis as well as glycosuria; whereas glucose injected
subcutaneously or taken by mouth causes neither of these conditions to
become developed ; indeed it causes for some time after the administration
of the sugar a distinct anuria. To explain these differences in behavior
between glucose administered intravenously and that taken in other ways,
it is supposed that the glucose molecule in passing through the intervening
wall of the capillaries combines with some substance to form a compound
which becomes available for incorporation into and utilization by the
tissues, glucose in a free state being incapable of utilization. This com-
pound is supposed to be of a colloidal nature, and the substance which
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THE METABOLISM OF THE CARBOHYDRATES 661
combines with glucose to form it is believed to be related to the internal
secretion of the pancreas (see page 676).
The difficulty in explaining why the glucose of the blood does not con-
stantly leak into the kidney is, however, the only evidence upon which the
hypothesis of a blood sugar compound rests. No chemical evidence can
be offered in support of such a view. On the contrary, all experimental
work indicates that the sugar exists in a free state; but unfortunately even
this evidence is not convincing. Thus, it has been found that, when speci-
mens of perfectly fresh blood are placed in a series of dialyzer sacs sus-
pended in isotonic saline solutions, each solution containing a slightly dif-
ferent percentage of glucose, diffusion of glucose, in one or other direction,
occurs in all of them save one—namely, that in which the percentage of
glucose in the fluid outside the dialyzer is exactly equal to the total sugar
content of the blood. Such a result can be explained only by assuming that
all of the sugar in the blood exists in a freely diffusible state. In its general
nature this experiment is analogous to that by which the tension or partial
pressure of CO, is determined in blood (see page 338).
It has been assumed by many clinicians that glycosuria may sometimes
become developed because the kidney fails to hold back the blood sugar
even when the percentage is not above the normal—so-called renal dia-
betes. For the diagnosis of this condition a comparison must be made be-
tween the sugar concentration. of the blood and that of the urine. In order
to do this at least two samples of blood must be taken, one of them at the
beginning and the other at the end of a period during which urine is being
collected. Merely to find that one sample of blood collected before or after
or during the period of urine collection contains a normal percentage of
sugar, does not necessarily indicate that at some other period while the
urine was being produced a temporary hyperglycemia may not have ex-
isted.
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CHAPTER LXXV.
THE METABOLISM. OF THE CARBOHYDRATES (Cont’d)
FATE OF ABSORBED GLUCOSE. GLUCONEOGENESIS
‘We may now consider what becomes of the sugar that is retained by
the liver and muscles. Two things may happen to it: It may become
stored, or it may become oxidized or split up. Of these processes, storage
occurs in both the liver and muscles, whereas oxidation occurs mainly if
not entirely in the muscles, although a certain amount of splitting of the
glucose molecule may also occur in the liver.
Storage of Sugar.—For the present we shall consider the process of
storage of sugar and defer a consideration of its utilization until after we
have studied, not only the nature of the process by which the storage
occurs, but also the immediate destiny of the stored sugar. The storage
of sugar by the liver is brought about by its conversion into a polysac-
charide called glycogen. After an animal has been absorbing large quan-
tities of glucose, an acidified watery extract of a portion of liver made
immediately after death will be found to contain no more sugar than that
of anormal liver. On the other hand, it will be observed that the extract
is highly opalescent and yields on the addition of aleohol a copious precip-
itate, which on further purification can readily be shown to consist of a
polysaccharide—that is to say, of a starch-like substance which on hydrol- ©"
ysis with mineral acid becomes entirely converted into sugar. If instead
of removing the liver immediately after death, it is allowed to stand for
some time, the yield of glycogen will greatly diminish, and in its place
will appear large quantities of glucose, indicating that some enzyme must
exist which attacks the glycogen after death and converts it into sugar.
This enzyme is called glycogenase. The existence of postmortem glyco-
genolysis, as it is called, would seem to indicate that during life also there
is a constant tendency for the glycogen in the liver to be attacked by
glycogenase, but that this is prevented by conditions which depend on the
‘vital integrity of the liver cell. It is evident that if anything should
happen during life to interfere with this inhibiting influence, the glycogen
will become converted into glucose, which on escaping into the blood
will produce hyperglycemia and glycosuria.
Sources of Glycogen.—In studying the source of sugar in the animal
body it is important therefore that we should first of all know exactly the
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THE METABOLISM OF THE CARBOHYDRATES 663
conditions under which glycogen may be formed in the liver; that is,
whether it is formed from absorbed sugar alone, or from other substances
also, such as protein and fat. The importance of such knowledge rests
in the fact that in. severe diabetes, sugar still continues to be added to
the blood, although no sugar is being taken with the food. To check the
hyperglycemia in such cases it becomes necessary, therefore, to curtail
the diet not only with regard to its carbohydrate content, but also with
regard to whatever other foodstuff may be capable of causing glycogen
formation. The practical question therefore is, What are these foodstuffs?
There are two methods by which the problem may be investigated. The
first, which we may call the direct method, consists in rendering the liver
free of glycogen and then some time afterward feeding the animal with
the foodstuff in question, afterward killing it and examining the liver
for glycogen. The other, which we may call the indirect method, con-
sists in first of all rendering the animal incapable of oxidizing glucose—
that is, making it diabetic—and then proceeding to see whether the in-
gestion of a giveri foodstuff causes an increase in the sugar excretion in
the urine. The methods for rendering an animal experimentally diabetic
will be considered later; for the present it is important to note that, if
a diabetic animal excretes more glucose while fed on a given foodstuff,
we may infer that the normal animal would convert it into glycogen.
The results of the direct method are much less reliable than those of
the indirect for the reason that it is extremely difficult to remove all
traces of glycogen from the liver. The methods employed for this pur-
pose have consisted in: (1) starvation of the animal; (2) muscular ex-
ercise; (3) exercise and starvation combined; and (4) the production of
certain forms of experimental diabetes—for example, that produced by
phlorhizin. ‘Starvation alone is unsatisfactory, for it has been found
that, although at certain stages of this condition the liver may become al-
most entirely free from any trace of glycogen, at a later stage glycogen
may again make its appearance. It is therefore most difficult to decide
at what stage in starvation the animal should be considered as glycogen-
free.
If the starving animal is made to perform muscular exercise, complete
removal of glycogen from the liver can be depended upon. The exercise
may be produced by the administration of strychnine in such dosage as
just to produce convulsions of the voluntary muscles without permanent
contraction of those of respiration. The most useful method, however,
consists in starving the animal for a few days and then placing it in a
cold, damp room, after giving it a cold bath. The evaporation of mois-
ture from the surface so cools the body down that the glycogen store all
becomes used up in the attempt to supply fuel for the production of
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664 METABOLISM
sufficient heat to maintain the body temperature. This method can be
rendered still more certain in effecting a removal of all carbohydrate
from the body by giving the animal phlorhizin every eight hours. This
drug, as we shall see, renders the animal diabetic.
After removing the glycogen, further deposition in the liver can be
readily shown to occur when any of the ordinary sugars or starches are
given as food. It does not occur, however, when chemical substances
closely related to ordinary sugar, such as the wood sugars (pentoses)
or the alcohols and acids corresponding to dextrose, are contained in the
diet. Nor does it occur with cellulose or with inulin, a polysaccharide
built up from pentose sugar. When proteins are fed the results are not
so definite, although many observers have claimed that glycogen is
formed. With fat, on the other hand, no glycogen formation can be
shown to occur, although we know that a trace of carbohydrate must be
formed out of the glycerine of the fat molecule.
The results of the direct method, even when the conditions are per-
fectly controlled, are very unreliable, because any new sugar produced
by the ingested substance instead of being stored as glycogen may be
directly used by the tissues as it is formed. Where only a slight degree
_of gluconeogenesis is occurring, it is not likely that any of the glucose
will be retained in the body as glycogen.
The methods employed for producing experimental diabetes in investi-
gation of these problems by the indirect method are (1) the entire removal
of the pancreas, and (2) the continuous administration of the drug
phlorhizin. The animal rendered diabetic by either of these methods is
first of all observed for several days to determine the normal daily ex-
eretion of sugar. At the same time the nitrogen excretion for the day
is determined, the ratio between the total nitrogen and the glucose being
known as G to N ratio, and being about 1 to 3.65 when complete diabetes
has become established. The foodstuff in question is then fed to the
animal, and the amount of extra glucose excreted thereby is taken to
represent that which has been derived from the ingested food. By this
method it has been possible to show that, not only the above mentioned
carbohydrates, but protein as well produce a very considerable quan-
tity of glucose in the animal body. Fats, however, yield only negative
results. :
The indirect method has another great advantage over the direct in
that the results are much more quantitative in character; for example,
Lusk and his pupils have been able to determine the amount of glucose
which can be produced by feeding certain of the building stones of the
protein molecule. The great. practical importance of such results in
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THE METABOLISM OF THE CARBOHYDRATES 665
the therapy of diabetes makes it advisable for us to go into the subject
a little more in detail here.
Dogs are rendered diabetic by phlorhizin followed by a cold bath
and exposure in a cold room. When all ‘of the original glycogen in the
body has been got rid of, as evidenced by the constancy of the G to N
ratio in the daily quantities of urine excreted, the substance under in-
vestigation is fed. If this substance contains no nitrogen and causes no
change in the nitrogen excretion, any increase in that of glucose must
obviously represent the extent to which the substance has become con-
verted into this sugar. On the other hand, if the substance itself con-
tains nitrogen, or if it causes a change in the excretion of nitrogen, it
becomes necessary to calculate how much of the excreted glucose might
have been derived from the body protein, assuming that this can form
glucose, and how much from the administered substance.*
From the results of this method it has been an easy matter to show
that the following substances are converted in the animal body into
glucose: (1) Glycol aldehyde (CH,OH-CHO). By placing three mol-
ecules of this substance together, a hexose molecule will be produced,
a synthesis which can be accomplished in the chemical laboratory.
Glycol aldehyde may be formed in normal metabolism out of glycocoll
(CH,NH,COOH).
(2) Glycerol (CH,OH -CHOH-CH,OH) may also readily be con-
verted into hexose in the laboratory, the possible intermediary products
being dioxyacetone (CH,OH-CO-CH,OH) and glyceriec aldehyde
(CH,OH - CHOH-CHO). Two molecules of either of these may be
polymerized to form a hexose molecule, and when this process occurs
in the animal body, the hexose formed is glucose.
(3) Lactic acid (CH,CHOH - COOH) is completely converted to dex-
trose in the diabetic animal, and the process must involve both a re-
arrangement of the molecule and subsequent polymerization. The related
substance, propyl aleohol (CH,-CH,-CH,OH) is also converted into
glucose in the phlorhizinized dog. Ag to the exact nature of the chemical
changes which occur as intermediary steps in the conversion of these
substances into glucose, we are not as yet certain, but a step has been
made in the discovery that a substance called methylglyoxal (CH,COCHO)
may be obtained from lactie acid and also from glucose, and that this
substance is converted into glucose when it is administered to phlorhi-
zinized dogs. We shall find later an important role for this substance
*This calculation is made as follows: The amount of nitrogen in the administered substance is
deducted from the nitrogen excretion, and the difference, which must represent the nitrogen of the
to N ratio which prevailed on the day previous to that on
body protein, is multiplied by the G
which the substance was fed. We obtain in this way the glucose derived from the body. The
glucose coming from the administered substance can then be ascertained by deducting that derived
from the body protein from the total glucose excretion. 5
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666 METABOLISM
in the case of fat metabolism, and it can also readily be produced duri
the intermediary breakdown of certain of the protein building-ston
such for example as alanine (CH,CHNH,COOH). ;
These chemical possibilities regarding the nature of the substanc
that serve as stepping stones between the above sugar-forming su
stances and sugar itself may be translated later into probabilities |
account of the discovery that the enzymes exist in various tissues whi
are involved in converting methylglyoxal into lactie acid:
CH, CH,
| |
cO + 4H, 3HCOH
|
CHO COOH
(methylglyoxal) (lactic acid)
These enzymes are called glyoxalases, and since the reactions whi
they mediate are undoubtedly reversible in character, it is probable th.
the conversion into sugar of lactic acid and alanine—to take those tw
as among the commonest of the sugar precursors of the animal body-
oceurs according to the following equation:
CH,CHNH,COOH ,,
(alanine) CH,COCHO — C,H,,0,
CH,CHOHCOOH 7
(lactic acid) (methylglyoxal) (hexose)
The unique position of methylglyoxal, besides explaining the know
resolutions of protein and fat and carbohydrate in intermediary meta
olism, is also of importance in explaining the synthetic production |
glucose from fructose (or levulose). Fructose will first of all becon
converted into methylglyoxal radicles, and these will then become sy.
thesized into glucose.
The theory of the conversion of glucose into lactic acid as a steppir
stone in the metabolism of carbohydrate meets with one objectioi
namely, that the lactic acid is not produced from carbohydrate in t]
organism, except in cases where there is oxygen deficiency or excess |
alkali in the tissue fluids.
Coming now to the amino acids, which, it will be remembered repr
sent the building stones of the protein molecule, it has been found th
glycocoll, alanine, and aspartic and glutamic acids increase the gluco:
excretion when given to phlorhizinized dogs, whereas leucine and tyr
sine have no such action. By the method described above, it is possib
to determine the exact proportion of the earbon of each of those amit
acids which becomes converted to glucose. This is shown in the accor
panying table.
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THE METABOLISM OF THE CARBOHYDRATES 667
TWENTY GRAMS OF THE VARIOUS AMINO BODIES WERE GIVEN TO
PHLORHIZIN-DIABETIC Docs
AVERAGE AMOUNT PROBABLE GLUCOSE THAT
ACID AND FORMULA OF GLUCOSE PRO- CHANGE WOULD BE PRO-
DUCED IN BODY DUCED BY CHANGE
Glycocoll 13.43 (five dogs, All C converted 16.00
CH,NH,COOH one gave 15.77) to glucose
i. alanine 18.77 (two dogs) ee 20.22
CH,CHNH,COOH
Aspartic acid 12.42 (four dogs) Three of the four 13.52
COOH—CH,—CHNH.—COOH C atoms converted
to glucose
Glutamic acid 13.31 Three of the five 12.24
COOH C atoms converted
/ to glucose
CH,
|
CH,—CHNH,
|
COOH
It is of further interest to point out that these four amino acids
constitute about 26 per cent of all the amino acids in flesh protein, and
that the total yield of glucose from them could be 26.3 grams; thus
accounting for nearly one half of the 66 grams which a diabetic animal
produces from 100 grams of flesh.
Gluconeogenesis in Normal Animals.—Although it has been clearly
shown by the indirect method that not only protein but its decomposi-
tion products as well, can be readily converted into glucose, yet this does
not necessarily indicate that a similar conversion occurs-in the nondia-
betic animal. That such is the case, however, can be shown in various
ways. Thus, at the end of a period of long starvation considerable
quantities of glycogen are quite commonly found in the body, and the
blood sugar, although lower than normal, never entirely disappears.
Now, since no carbohydrate is being ingested, and the body stores of this
foodstuff become exhausted early during starvation (ef. page 663), it
is evident that the carbohydrate must be produced from the protein of
the animal’s body. A still more convincing experiment can be con-
ducted by producing strychnine convulsions in a starving animal. If
the animal is killed after the convulsions have lasted for a certain time,
the tissues will be found almost if not entirely free of glycogen,
but if the convulsions are made to disappear by giving chloral and the
animal allowed to sleep for some time before killing it, glycogen again
accumulates in the body. This glycogen must have been manufactured
out of nonearbohydrate material.
Corroborative evidence of a somewhat different nature is furnished by
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668 METABOLISM
an examination ofthe respiratory quotient, which, it will be reme
bered (page 547), varies according to the nature of the foodstuff or bo
constituent that is undergoing metabolism at the time, being about
with carbohydrate and about 0.7 with protein. If the quotient
observed during starvation, it will often be found to fall below 0.7,
figure which can be explained only by assuming that oxygen has be:
retained in the body beyond the quantity which is necessary for imr
diate purposes of oxidation (ef. equations on page 548).
Since it is known that this retained oxygen can not exist in the boc
in a free state it must be concluded that it has become incorporate
into substances having a high oxygen content. Such would be the ca
if protein or fat, which contains only from 12 to 20 per cent of oxyge
were converted to carbohydrate, which contains about 53 per cen
Utilization of inhaled oxygen for this purpose, as we have seen, becom
very striking in the case of hibernating animals during the winter slee
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CHAPTER LXXVI
THE METABOLISM OF THE CARBOHYDRATES (Cont’d)
FATE OF GLYCOGEN
Having become familiar with the sources from which glycogen may
be derived, we may now proceed to study the fate of the glycogen found
in the liver cells and in the muscles. For the present we shall confine aur
attention to the glycogen of the liver. If a portion of liver removed
from a well-fed animal is examined microscopically after staining either
with iodine or with carmine by Best’s method, it will be found that the
cells of the lobules are filled with glycogen except for the nuclei, which
are free from this substance. If, on the other hand, the liver is from an
animal that has not been recently fed, the lobules will contain no glyco-
gen except for an area bordering on the central vein and perhaps a
narrow strip at the periphery of the lobule. When it is present the rela-
tive amount of glycogen in different lobules, as determined chemically,
is the same over the entire liver—that is.to say, no one lobe is richer in
this substance than another. Nothing definite is known as to how the
glycogen is held in the protoplasm of the cells, although some histolo-
gists suggest that it is combined with a sustentacular material especially
provided for this purpose.
The glycogen stored in the liver is gradually given up to the blood of
the hepatic vein at such a rate as to maintain in the blood of the sys-
temic circulation a more or less constant percentage of glucose. Under
ordinary conditions this process of glycogenolysis is relatively slow, but
when the requirements of the organism for fuel become increased, as
during muscular exercise, it becomes very rapid. The glycogenic func-
tion of the liver appears therefore to exist, in part at least, for the
purpose of preventing the flooding of the blood of the systemic circu-
lation with excess of sugar during absorption from the intestine and of
maintaining the normal percentage at other times. This function is
analogous to that occurring in plants, in which the sugar produced in
the leaves, if not immediately required, is transported to various parts
of the plant and there converted into starch, which, when the plant
requires it, as during new growth, may again become transformed into
glucose.
The agency converting the glycogen into glucose is the diastatic
6
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670 METABOLISM
enzyme glycogenase, which is present, not only in the liver cell, °
also in the blood and lymph. It is a difficult matter to explain w
glycogen should be able to exist at all in the liver cells in the prese)
of this powerful enzyme. The following possibilities may be consider
(1) That glycogenase does not really exist in the living liver cells, ]
is a postmortem product; (2) that, although present, glycogenase is p
vented from acting on the glycogen in the living liver cell on account
the latter being protected from its influence by combination with 1
sustentacular substance; or (3) that some chemical substance in the liv
cell prevents the glycogenase from acting on the glyeogen—an ar
glycogenase. Since the removal of any one of these inhibiting int
ences would .cause glycogenolysis to become excessive, and so bri
about hyperglycemia, it is important, in searching for the’ possil
causes of this condition, to examine the evidence that has been broug
forward in support of each of these views.
Against the view that glycogenase is a postmortem product may
cited the very rapid conversion into glucose that occurs when glycog
is added to living blood, as by injecting some into a vein. On account
the active glycogenolytic action of blood, it has been suggested tk
during life glycogen, does not become transformed into glucose un
after it has been discharged into the blood from the liver cell. Wh
increased sugar must be mobilized, glycogen passes unchanged, or p:
haps as some dextrine, into the blood and lymph of the liver capillar.
and lymphatics, the glyecogenase of which converts it into glucose, t
conversion being so rapid that, by the time the blood has traveled fr
the liver through the heart and pulmonary vessels to the arteries, .
the glycogen has already become transformed into glucose. Postmort
glycogenolysis, according to this view, is due to the opposite ocai
rence—the transference of glyecogenase from the blood into the liv
cell. Some facts supporting this view are as follows: (1) It has be
found that the amount of free glucose in the blood of the vena ca
is sometimes less than in that collected simultaneously from the caro!
artery. (2) After giving certain substances, such as phosphorus
peptone, there is distinct diminution in the amount of glycogen in .t
liver, accompanied, however, by no increase in the amount of gluc
in the blood. And (8) if the liver of an animal that has been render
diabetic by stimulation of the splanchnic nerve or by puncture of t
floor of the fourth ventricle is examined microscopically, after staini
by the carmine method, masses of stained glycogen can be found prese
in the capillaries (sinusoids) that lie among the liver cells.
According to the second view, the glycogen is removed from 1
influence of the intrahepatic glycogenase on account of its combinati
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THE METABOLISM OF THE CARBOHYDRATES 671
with a sustentacular material. By disrupting this combination and thus
exposing. the glycogen to the action of glycogenase, glycogenolysis will
oceur. We may call this the mechanical hypothesis and it deserves
serious consideration, for it has been shown that very little postmortem
glycogenolysis occurs in the intact liver of frogs in winter,—even though
at this time the organ contains an excess of glycogen,—but becomes
marked when the liver is broken down by mechanical means.
The third view depends on the well-known fact that enzyme activities
become most markedly altered by slight changes in the chemical nature
of the environment in which they act. Diastatic enzymes are partic-
ularly susceptible to the reaction (Cu) of their environment, a very
slight degree of acidity favoring and a trace of alkalinity markedly
depressing their activities. That a tendency to increasing acidity in
the liver cells may retard the formation of glycogen is suggested by
the depressing effect produced on the assimilation limit of sugars by
administering acids, and by the observation that postmortem glycogen-
olysis becomes marked in proportion as the dying liver becomes-acid in
reaction. It might be thought then that glycogenolysis in the liver cell
could be set up by the local production of a certain amount of acid.
Such a liberation of free acid could be brought about by a curtailment
in the arterial blood supply of the hepatic cell, producing a local accu-
mulation either of carbonic or of other less completely oxidized acids
(e.g., lactic). It may be that asphyxia causes hyperglycemia by such
a mechanism. Vasoconstriction and consequent curtailment of arterial
blood supply occurs in the liver when the hepatic nerves are stimulated,
and it is possible that the glycogenolysis which is also set up by such
stimulation is due to the appearance of acids. The accelerating effect
of epinephrine on glycogenolysis might, also be explained as due to
limitation of blood supply on account of vasoconstriction.
THE REGULATION OF THE BLOOD SUGAR LEVEL
The level at which the concentration of sugar in the systemic blood
is maintained represents the balance between two opposing factors: (1)
the consumption of glucose by the tissues, and (2) the production of
glucose by the liver. Since this is the most readily oxidizable of all
the proximate principles of food (page 652), muscular activity causes
large quantities of it to be consumed, so that its concentration in the
blood tends to fall below the physiologic level, a tendency which is
immediately met by an increased discharge of glucose from the liver.
The question therefore arises as to how the muscles or other tissues
transmit their requirements for glucose to the liver. There are two
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672 METABOLISM.
possible ways by which this could be done: (1) by means of a nerve
reflex, or (2) by changes in the composition of the blood, either w:
regard to the percentage of sugar itself or because of the appearance
it of decomposition products of glucose or of some special exciti
agent or hormone.
In order to ascertain the relative importance of these methods
correlation between the places of supply and demand of glucose in t
normal animal, it is necessary to investigate the conditions under whi
an excessive discharge of glucose occurs as a result of overstimulati
of the nervous control, or because of the presence of exciting substance
(hormones) in the blood. The glycogenic function can be excited throu
the nervous system in a variety of ways so as to produce hyperglycem
and glycosuria. This constitutes one form of experimental diabetes.
laboratory animals mechanical irritation of the medulla oblongata ai
stimulation of the great splanchnic nerves act in this way. Similar stimul
tion may also occur under certain conditions in man. Excitation as a rest
of changes in the composition of the blood can be produced experime
tally by certain drugs (phlorhizin), or by the removal of certain of t!
ductless glands or the injection of extracts prepared from them, su
as epinephrine.
Nerve Control and the Nervous Forms of Experimental Diabetes.
The simplest experimental condition which illustrates the relationsh
between the nervous system and the blood sugar is electrical stimulati
of the great splanchnic nerve in animals in which, by previous feedii
with carbohydrates, a large amount of glycogen has been deposited
the liver. By examination of the blood as it is discharged into the ve:
cava from the hepatic veins, the increase in blood sugar is very evide
in from five to ten minutes after the first application of the stimulu
but it is not until later that a general hyperglycemia becomes, esta
lished. The conclusion which we may draw from these results is th
the splanchnic nerve contains efferent fibers controlling the rate
which glycogen becomes converted to glucose in the liver. The cent
from which these fibers originate is situated somewhere in the medu
oblongata, for the irritation that is set up by puncturing this portion
the nervous system with a needle yields results similar to those whi
follow splanchnic stimulation. This ‘“‘glycogenic’’ or diabetic center,
it has been called, must be provided with afferent impulses. Such i
pulses have indeed been described in the vagus nerves, but their de
onstration is by no means an easy matter on account of the disturbar
in the respiratory movements coincidently produced by the stimulatic
The changes that such disturbances bring about in the aeration of t
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THE METABOLISM OF THE CARBOHYDRATES 673
blood may in themselves be responsible for the hyperglycemia (see page
332). It can at least be said that when the respiratory disturbances are
guarded against, as by intratracheal insufflation of oxygen, vagal hyper-
glycemia is much less marked, if not entirely absent. But this question
awaits more thorough investigation.
The increased glycogenolysis which results from stimulation of the
efferent fibers in the splanchnic nerves may depend either on a direct
control exercised over the glycogenic functions of the hepatic cells, or
on the discharge into the blood of some hormone which excites the
glycogenolytie process. It must furthermore not be lost sight of that
the glycogenolysis may be secondary to local asphyxial conditions in
the liver cells resulting from vasoconstriction. From their anatomic
position, the adrenals are to be thought of as the source of the hormone,
and evidence that splanchnic hyperglycemia is due to hypersecretion
from these glands has seemed to be furnished by the fact that after they
are extirpated splanchnic stimulation no longer produces hyperglycemia,
neither, indeed, does puncture of the medulla. There is also no doubt
that the nervous system, acting by way of the splanchnic nerves, does
exercise a control over the discharge of the internal secretion of the
adrenal glands and that extracts of the gland, which we must suppose
act in the same way as the internal secretion, cause hyperglycemia when
injected intravenously (epinephine hyperglycemia and glycosuria).
But on theoretical grounds alone, certain difficulties immediately pre-
sent themselves in accepting this as the mechanism by which the nervous
system controls the sugar output of the liver, for if increased sugar
formation in the liver is dependent on a discharge of epinephrine, the
question may be asked why this secretion should be caused to traverse
the entire circulation before reaching the liver.
There are, besides, certain experimental facts which do not conform
with such a view. Thus, after complete severance of the hepatic plexus
of nerves, stimulation of the splanchnic nerve does not cause the usual
degree of hyperglycemia, whereas electric stimulation of the peripheral
end of the cut plexus does cause it. On the one hand, therefore, there
is evidence that stimulation of the efferent nerve path above the level of
the adrenals has no effect on the sugar production of the liver in the
absence of these glands; and on the other, we see that when they are
present, stimulation of the nerve supply of the liver is effective, even
though the point of stimulation is beyond them. There is but one econ-
clusion that we may draw—namely, that the functional integrity of the
efferent nerve-fibers that control the glycogenolytic process of the liver
depends on the presence of the adrenals, very probably because of the
hormone which the glands secrete into the blood. This conclusion is
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674 METABOLISM
corroborated by the fact that stimulation of the hepatic plexus, ev:
with a strong electric current, some time after complete removal
both adrenals, is not followed by the usual degree of excitement of t]
glycogenolytic process.
These experiments demonstrate an important relationship betwe
the nervous control, and at least one form of hormone control, of tl
sugar output of the liver. They indicate that when a sudden increa
of blood sugar is required, the glycogenic center sends out impuls
which not only directly excite the breakdown of glycogen in the h
patie cells, but also simultaneously influence the adrenals in such a ma:
ner as to produce more epinephrine in the blood and so augment the a
tion of the nerve impulse.
We are as yet quite in the dark as to the mechanism by which tl
nerve impulses or the hormone brings about increased glycogenolysi
It must consist of a removal of the influence that prevents glycogenolys
from occurring in the normal liver, for it has been shown by direct o.
servation that there is no increase in the amount of glycogenase prese1
in extracts of the liver removed from diabetic animals over that prese1
in extracts of the liver of normal animals. The possible nature. of th
influence has already been discussed (page 669). The change may co
sist either in a loosening of the combination between the glycogen an
the protoplasm of the liver cell, or in a removal of the chemical influen
that ordinarily prevents the glycogenase from attacking the glycoge
In the former case the glycogen liberated from its union with the su
tentacular substances would either become attacked by the glyecogena:
‘present in the liver cell itself or it would first of all migrate, as glye
gen, into the blood capillaries and there be attacked by the bloc
glycogenase. Evidence for the possibility of the occurrence of such
process has already been given (page 670). The chemical change 1
ferred to under the second possibility might consist in an alteration |
the hydrogen-ion concentration of the liver cell, a change, howeve
which for obvious reasons it is impossible to investigate.
Nervous Diabetes in Man.—The main interest attaching to the inve
tigation of these nervous forms of experimental diabetes depends on tl
insight which they afford us into the nature of the mechanism by whi
a prompt mobilization of. glucose may be brought about in the norm
animal. There is also some evidence that a relationship may exist I
tween certain of the clinical varieties of the disease in man and repeat
excitation of glycogenolysis brought about by nerve stimulation. I
creased glucose output from the liver as a result of nerve excitatic
may be a normal process, but there is reason to believe that freque
repetition of this process tends to induce a permanent rise in the gluco
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THE METABOLISM OF THE CARBOHYDRATES 675
level of the blood and therefore a tendency to diabetes. There have
recently been collected several facts which lend some support to this
view. The frequent occurrence of diabetes’ in those predisposed by
inheritance to neurotie conditions, or in those whose daily habits entail
much nerve strain, and the aggravation of the symptoms which is likely
to follow when a diabetic patient experiences some nervous shock, all
point in this direction.
Diabetes is common in locomotive engineers and in the captains of
ocean liners—that is, in men who in the performance of their daily duties
are frequently put under a severe nerve strain. It is apparently in-
creasing in men engaged in occupations that demand mental concentra-
tion and strain, such as in professional and business work. Cannon?
found glycosuria in four out of nine students after a severe examination,
but only in one of them after an easier examination.* In the urine of
twenty-four members of a famous football squad, sugar was found pres-
-ent in twelve immediately after a keenly contested game. Anxiety and
excitement were responsible for its appearance, for five of the twelve
were substitutes who did not get into the game.
Although these nervous conditions, by excitement of hepatie glyco-
genolysis, produce at first nothing more than an excessive discharge of
sugar into the blood—a condition which is exactly duplicated in our
laboratory experiments by stimulation of the nerve supply of the liver—
their repetition may gradually lead to the development of a permanent
form of hyperglycemia. To prevent the repetition of these transient
hyperglycemias must be one of our aims in the treatment of early stages
of the disease.
Although there can be no doubt that the glycogenic function of the
liver is subject to nerve control, it is probable that its control by hor-
mones is of equal if not greater importance. This dual control of a
glandular mechanism is by no means unique for the glycogeniec function,
for we have already seen it to exist in the case of the gastric glands
and the pancreas, and it is probable that it also exists in the case of
the thyroid. It may well be that the nerve control of the glyeogenic
function has to do only with those transitory changes in sugar produc-
tion that would be demanded by sudden activities of muscle, and that
the hormone control has to do with the more permanent process of build-
ing up and breaking down of glycogen to meet the general metabolic
requirements of the tissues.
1
*We have been unable to confirm this observation even theugh the examinations were made
unusually ‘‘nerve-racking.”
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676 METABOLISM.
HORMONE CONTROL AND PERMANENT DIABETES
Nervous excitation can explain only transitory increases in blood sugar,
the more permanent hyperglycemias being dependent upon some dis-
turbance in the hormone control of carbohydrate utilization. This dis-
turbance is a much more serious affair than that produced by nervous
excitation. In the latter case the hyperglycemia ceases whenever all
of the glycogen stores of the liver have been exhausted; whereas a dis-
turbance in the hormone control, besides causing as its first step a
breakdown of all the available glycogen, goes on to cause a production
of sugar out of protein. A process of gluconeogenesis (new formation
of glucose) becomes superadded on one of glycogenolysis.
To ascertain the nature of this hormone and the mechanism of its
action has been the object of most of the researches on those forms of
diabetes produced by changes in certain of the ductless glands. The
following possibilities may be considered: (1) that it is the concentration
of glucose in the blood; (2) that it is the presence in the blood of decom-
position products of glucose; (3) that it is due to a hormone produced
from some ductless gland. Regarding the first of these possibilities, it
is supposed that the mechanism involved in the adjustment between the
blood sugar and the glycogenic function is one explicable on the basis
of the law of mass action; namely, that glycogen becomes: converted
into glucose whenever the blood flowing to the liver contains less than
its normal concentration of glucose, and conversely, when this blood
contains an excess of glucose, as during absorption, a glycogen-building
process takes place. Although there can be little doubt that the process
of glycogen formation or destruction will depend to a certain extent
upon the amount of glucose present in the blood flowing to the liver
cells, yet it is impossible that this can be an important means in the
control that exists between sugar production by the liver and sugar
consumption by the tissues, because the sugar that is added to the portal
blood during absorption would mask any depletion caused by sugar
consumption in the tissues. ,
The second possibility—that the hormone is some decomposition prod-
uct of glucose—woulcé appear to have some support, if we consider this
hormone to be an acid product (carbon dioxide or lactic) produced by
sugar metabolism, for it is known that an increase in the hydrogen-ion
concentration of the blood flowing to the liver cells excites a glycogen-
olysis. As we have already seen, however, it is difficult to secure ex-
perimental evidence, in anesthetized animals at least, that glycogen-
olytic activity is readily excited in this way.
The third possibility—that some specific hormone may exist in the
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THE METABOLISM OF THE CARBOHYDRATES 677
blood exciting the glycogenolytie process—is investigated by producing
disturbances involving various of the ductless glands, particularly the
pancreas, the adrenals, the parathyroids and the pituitary. The influ-
ence of certain of these glands may be closely bound up with that
exercised through the nervous control, as we have seen to be the case
with the adrenal gland. Whether it is by the production of hormones
directly necessary for proper carbohydrate metabolism, or by the re-
moval from the blood of such substances as interfere with this process,
that the ductless glands functionate, is one of the main problems we
have to consider.
Utilization of Glucose in Tissues——Although the experimental diabetes
induced by disturbances in the function of the ductless glands is at
first dependent on an upset of the glycogenic function and later of glu-
coneogenesis, the utilization of glucose in the tissues ultimately becomes
interfered with. It is therefore important that we should digress for a
moment to consider briefly what is known regarding the process by
which sugar becomes utilized in the organism. That glucose becomes
used up by active muscle there can be no doubt. Thus, if the muscles
of one leg in the frog are tetanized, the glycogen content, compared with
that of the other leg, will be found to be diminished.
At first sight it might appear that the easiest way to study the utiliza-
tion of glucose in the muscles would be to compare its concentrations
in the blood flowing to and coming from the muscle. The muscle that
has been most successfully employed in studies of this kind has been the
heart. Some years ago Starling and Knowlton* examined the consump-
tion of sugar by the excised mammalian heart, and in their earlier
experiments seemed to, be able to show that the extent to which this
consumption occurred was 4 milligrams per 100 grams heart muscle
per hour. A more thorough repetition of these experiments later by Pat-
terson and Starling” showed, however, that the results can furnish no
eriterion of the actual consumption of glucose by the tissue on account
of the fact that the tissue itself may store away large quantities of
carbohydrate in an unused state—i.e., as glycogen.
Other investigators have thought to study the utilization of glucose
by observing the rate at which it disappears from drawn blood kept in
a sterile condition at body temperature for some hours after death.
This process is called glycolysis, and it has been assumed that the process
is similar to that which occurs in the tissues themselves—an assumption,
however, for which there is no warranty. Indeed, it may readily be
shown that the glycolysis occurring in blood has very little if anything
to do with the utilization of sugar in the tissues, for it has been found
that glucose disappears from drawn blood very slowly indeed when
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678 METABOLISM
compared with the rate at which it disappears from the blood of animals
in which the addition of glucose from the liver has been prevented by
removal of this viscus (Macleod).?¢
A third method for studying the utilization of glucose consists in
observing the respiratory exchange of animals. In normal animals the
injection of glucose causes an increase in the carbon-dioxide excretion
and a rise in the respiratory quotient, which it will be remembered is
a ratio expressing the relationship between the amount of carbon dioxide
excreted and of the oxygen retained in the organism. When carbohy-
drate is undergoing combustion, the quotient is nearly 1, whereas with
that of protein it is about 0.7 (see page 547). By observing the quotient
under given conditions one can compute the proportions of carbohydrate
and of fat and protein that are undergoing metabolism. In the hands
of Murlin and others,’ this method has proved of some value in settling
certain questions concerning the utilization of glucose in normal and
diabetic animals; but the results must be interpreted with great care on
account of the fact that temporary changes in the blood may cause a
greater or a less expulsion of carbon dioxide from it. Thus, if acids
appear in the blood, they will dislodge carbon dioxide from it, and
apparently cause the respiratory quotient to rise. Alkalies, on the other
hand, apparently cause it temporarily to fall, and unless the observa-
tions are done over a long period of time and with great care, faulty
conclusions are very apt to be drawn from the results.
Diabetes and the Ductless Glands
We are now in a position to consider the forms of experimental dia-
betes produced by disturbances in the ductless glands.
Relationship of the Pancreas to Sugar Metabolism—tIn no other of
the many causes of diabetes has greater interest been shown than in
that due to disturbance in the pancreatic function. Many of the earlier
clinicians that followed cases of diabetes mellitus into the postmortem
room, noted that definite morbid changes in the pancreas were a fre-
quent accompaniment of the disease. Prompted by these observations,
several investigators attempted experimental extirpation of the gland,
but did not succeed in producing glycosuria in the few animals that
survived the operation. Their failure, no doubt, resulted from incom-
plete extirpation. To reduce the severity of the operation, Claude Ber-
nard injected oil into the pancreatic duct, and tied it; but he succeeded
in keeping only two dogs alive for any length of time, and these did
not exhibit glycosuria. Neither were other investigators that adopted
similar methods any more successful. It looked as if the pancreas had
very little to do with the cause of diabetes. In the year 1889 Minkowski
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THE METABOLISM OF THE CARBOHYDRATES 679
and von Mering in Germany, and de Dominicis in Italy, by thorough
extirpation of the gland, succeeded in producing in dogs a marked and
persistent glycosuria, accompanied by many of the other symptoms of
diabetes. The first two authors attributed the condition to removal of
an internal secretion.
The course of the diabetes thus produced is, however, somewhat differ-
ent from that usually observed in man. It is extremely acute from the
start, the G: N ratio being 1:3.6 (see page 664), and it is unaccompanied
by any of the classical symptoms seen in the clinical. condition. Experi-
mental pancreatic diabetes can, however, be made to simulate very closely
the disease in man. This was first of all demonstrated by Sandemeyer,
who found that if the greater part of the pancreas was removed, the
animals for some months, if at.all, were only occasionally glycosurie,
but later became more and more frequently so, until at last the condition
typical of complete pancreatectomy supervened. Similar results have
more recently been obtained by Thiroloix and Jacob, in France, and by
Allen in this country. These investigators point out that different re-
sults are to be expected according to whether the portion of pancreas
which is left does, or does not, remain in connection with the duodenal
duct. When this duct is ligated, atrophy of any remnant of pancreas
that is left is bound to occur, and this is associated with rapid emacia-
tion of the animal, diabetes and death. When the remnant surrounds a
still patent duct, a condition much more closely simulating diabetes in
man is likely to become developed—one, namely, in which there is, for
some months following the operation, a more or less mild diabetes,
which, however, usually passes later into the fatal type.
It is, of course, difficult to state accurately what proportion of the
pancreas must be left in order that the above described condition may super-
vene. Leaving a remnant amounting to from one-fifth to one-eighth
of the entire gland is commonly followed by a mild diabetes, whereas
if only one-ninth or less is left, a rapidly fatal type develops. ‘As in
clinical experience, the distinguishing feature between the mild and the
severe types of experimental pancreatic diabetes is the tolerance toward
carbohydrates. In the mild form, no glycosuria develops unless carbo-
hydrate food is taken; in the severe form, it is present when the diet is
composed entirely of flesh. It is thus shown that ‘‘by removal of a
suitable proportion of the pancreas, it is possible to bring an animal
to the verge of diabetes, yet to know that the animal will never of itself
become diabetic. . . . Such animals, therefore, ecnstitute valuable
test objects for judging the effects of various agencies with respect to
diabetes’’—(Allen"*). It therefore becomes theoretically possible to in-
vestigate, on the one hand, other conditions which will have an influence
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680 METABOLISM.
similar to removal of more of the gland, or, on the other, conditior
which might prevent the incidence of diabetes, even though this extz
portion of pancreas is removed.
From the work which he has already done, Allen believes that he he
sufficient evidence to show that the continued feeding with excess: ¢
carbohydrate food will surely convert a mild into a severe case, and i
one experiment he succeeded in bringing about the same transition b
performing puncture of the medulla—that is, by creating an irritativ
nervous lesion. By none of the other means usually employed to produe
experimental glycosuria could the bordering case be made diabeti:
although one such animal became acutely diabetic after ligature of th
portal vein. To the clinical. worker the value of these results lies in th
fact that they furnish experimental proof that a so-called latent cas
of diabetes—-that is, one that has a low tolerance value for carbohy
drates—may be prevented from developing into a severe case by prope
control of the diet. Attempts to show whether or not there are am
conditions which might bring about improvements in animals that wer
just diabetic have not as yet been sufficiently made to warrant any con
clusions that could help us in the treatment of human cases. The en
couragement of the internal pancreatic secretion by diminution of tha
discharge into the intestine may be of value.
The certainty with which diabetes results from pancreatectomy in dogs
as well as the frequent occurrence of demonstrable lesions in the pan
creas in diabetes in man, leaves no doubt that this gland must be in som:
way essential in the physiologic breakdown of carbohydrates in th
normal animal, but how, we ean not at present tell. All we know i
that the first change to occur after the gland is removed is a sweepin:
out of all but a trace of the glycogen of the liver, although the muscles ma;
retain theirs; indeed, in the cardiac muscle there may be more thai
the usual amount.?® Nor can any glycogen be stored in the liver whe
excess of carbohydrates is fed. After the glycogen has disappeared
gluconeogenesis sets in, so that the tissues come to melt away into sugar
‘and: all the symptoms of acute starvation, associated with certain other.
that are possibly due to a toxic action of the excess of sugar or othe
abnormal products in the blood, make their appearance.
So far it might be permissible to consider an overproduction of glu
cose as the cause of the hyperglycemia of pancreatic diabetes, just a
we have seen it to be of these forms of hyperglycemia that are due t
stimulation of the nervous system; but this can not be the case, fo:
another very definite abnormality in metabolism becomes evident—
namely, an inability of the tissues to burn sugar. This fact is ascer
tained by observing the respiratory quotient. When glucose is adde
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THE METABOLISM OF THE CARBOHYDRATES 681
to the blood in the case of a completely diabetic animal, no change oc-
curs in the quotient.
There are, therefore, two essential disturbances of carbohydrate
metabolism in pancreatic diabetes—overproduction of sugar and aboli-
tion of the ability of the tissues to use it. It becomes important for us
to see whether the tissues exhibit this inability to use sugar when they
are isolated from the animal; for if they should, a much more searching
investigation of the essential cause of their inability would be possible
than is the case when they are functioning along with the other organs
and tissues. The earlier experiments of Lépine and his pupils, which seemed
to show that diabetic blood did not possess the glycolytic power of
normal blood; and those of Cohnheim, from which it was concluded that
mixtures of the expressed juices of muscle (liver) and pancreas, although
ordinarily destroying glucose, failed to do so when they were taken from
a diabetic animal, are now known to be erroneous.
The failure to show a depression of glycolytic power by these methods
prompted Knowlton and Starling’ to investigate the question whether
any difference is evident in the rate with which glucose disappears from
a mixture of blood and saline solution used to perfuse a heart outside
the body, according to whether the heart was from a normal or a dia-
betic dog. In the first series of observations which these workers made,
it was thought that the normal heart used glucose at the rate of about
4 mg. per 100 gm. of heart substance per minute; whereas that of a dia-
betic (depancreatized) animal used less than 1 mg. If such striking
differences in the rate of sugar consumption could make themselves
manifest for so relatively small a mass of muscular tissue as that of the
heart, it is permissible to assume that a much more striking difference
could be demonstrated when the perfusion fluid is made to traverse all
or practically all of the skeletal muscles, as well as the heart. For this
purpose an eviscerated animal may be employed—that is, one in which
the abdominal viscera are removed after ligaticn of the celiac axis and
mesenteric arteries, and the liver is eliminated by mass ligation of its
lobes. Using such preparations, R. G. Pearce and Macleod”? found that
the rate at which glucose disappears from the blood, although very
irregular, is in no way different in completely diabetic as compared
with normal dogs. They were thus unable to confirm any of Knowlton
and Starling’s earlier conclusions. Patterson and Starling subsequently
pointed out that a serious error was involved in the earlier perfusion
experiments, partly on account of a remarkable but irregular disap-
pearance of glucose from the lungs, and partly because the diabetic
heart may contain a considerable excess of glycogen, from which its
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682 METABOLISM
demands for sugar may be met without calling on that of the perfusion
fluid.
In spite of the failure to show that the isolated tissues of diabetic
animals have a lower glucose-consuming power than those of normal
animals, it is important from a practical standpoint that we should
know something regarding the possible nature of the disturbance which
a removal of the pancreas entails. Even if we could not tell exactly
how this disturbance operates, it would be of value to know whether
it depends on the removal from the organism of some hormone that is
essential to carbohydrate utilization, for, if this were proved to be the
case, encouragement would be offered to seek for the chemical nature
of this hormone so that we might administer it with the object of re-
moving the diabetic state. The hope of a fruitful outcome of such an
investigation is encouraged by the success of researches on diseases of
other ductless glands, particularly the thyroid.
The removal of some hormone necessary for proper sugar metab-
olism is, however, by no means the only way by which the results can
be explained, for we can assume that the pancreas owes its influence
over sugar metabolism to some change occurring in the composition of
the blood as this circulates through the gland—a change which is de-
pendent on the integrity of the gland and not on any one enzyme or
hormone .which it produces. It is obvious that the results of removal
of the gland could be explained in terms of either view, and indeed
there is but one experiment which would permit us to decide which of
them is correct. This consists in seeing whether the symptoms which
follow pancreatectomy are removed, and a normal condition reestab-
lished, when means are taken to supply the supposed missing internal
secretion to the organism; if they should be, conclusive evidence would
be furnished that it is by ‘‘internal secretion’’ and not by ‘‘local in-
fluence”’ that the gland functionates. —
The experiments have been of two types: in the one, variously pre-
pared extracts of the glands have been employed, and in the other,
blood which is presumably rich in the internal secretion. The most
recent work with pancreatic extracts has shown that injection of pan-
creatic extracts into a depancreatized animal produces no change in the
respiratory quotient, although injections of extracts of pancreas. and
duodenum may produce a temporary fall in the dextrose excretion in
the urine on account of the alkalinity of the extract. Neither have
experiments with blood transfusions yielded results that are any more
satisfactory. In undertaking these experiments it is of course assumed
that the internal secretion is present in the blood, and that if this blood
is supplied to an animal suffering from diabetes because of the loss of
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THE METABOLISM OF THE CARBOHYDRATES 683
its pancreas, it will restore it to a nondiabetic state. The general con-
clusion that may be drawn from the numerous researches of this nature,
is that there is no evidence that the blood of a normal animal, even
when it is from the pancreatic vein, contains an internal secretion that
ean restore to a diabetic animal any of its lost power to utilize carbo-
hydrates. When the extent of glycosuria alone is used as the criterion
of the state of carbohydrate metabolism, serious errors in judgment are
liable to be drawn. The condition of the blood sugar and the extent
and character of the respiratory exchange are the most reliable indexes.
DIABETIC ACIDOSIS OR KETOSIS
Nature and Cause.—Much confusion has existed in medical literature
over the correct definition of acidosis, mainly because the term was first
used for the particular variety of the condition observed in the later
stages of diabetes mellitus. The acids which accumulate in the tissue
fluids in this disease are acetoacetic and 8-oxybutyric, and they are
oxidation products of acetone, which is again derived from fatty acids
by a faulty metabolism (see page 709). The essential cause of the
acidosis is therefore entirely different from that in nephritis; in dia-
betes foreign acids are added to the blood, whereas in nephritis the
acids of a normal metabolism accumulate because of faulty excretion
through the kidneys. The usual signs of acidosis exist in both cases,
because the surplus of acid depletes the store of bicarbonate and
causes changes in the alveolar CO,, in the CO,-absorbing power of the
blood, in the reserve alkalinity, and in the acid excretion by the kidney.
It is important to recognize the special nature of diabetic acidosis by a
separate name—kKetosis. : : F
The chemical processes by which the ketone bodies are produced is
discussed elsewhere (page 709). It remains for us to consider the
general nature of the metabolic disturbance responsible for their ap-
pearance in diabetes.
For the thorough combustion of fat in the animal body a certain
amount of carbohydrate must be simultaneously burned. Fat evidently
is a less readily oxidized foodstuff than sugar; it needs the fire of the
burning sugar to consume it. If the carbohydrate fires do not burn
briskly enough, the fat is incompletely consumed; it smokes, as it were,
and the smoke is represented in metabolism by the ketones and derived
acids. Such a closing down of the carbohydrate furnaces may be
brought about either by curtailment of the intake of carbohydrates, as
in starvation (page 569), or by some fault in the mechanism of the
furnace itself, as in diabetes. Besides fat, protein may also contribute
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684 METABOLISM
to the production of ketones when carbohydrate combustion is d
pressed. Fundamentally, therefore ketosis in diabetes is due to t!
same cause as in starvation—namely, an improper adjustment betwet
the metabolisms of fat and carbohydrate.
Bearing these principles in mind, it is easy to see how the intensi
of acidosis which develops during starvation will depend upon the r
lative metabolism of carbohydrate, on the one hand, and of fat ar
protein, on the other; it will therefore depend on the amounts of the:
foodstuffs which have been stored in the organism, and this again wi
depend on the nature of the diet previous to the starvation period. F\
the first few days following entire abstinence from food in a health
well-nourished individual, very few if any ketones will be excreted i
the urine, because the carbohydrate stored in the body as glycogen hi
sufficed during this time to maintain the proper proportion between fi
and carbohydrate. Afterwards, however, their appearance is to be e:
pected, because the glycogen stores become exhausted long before thos
of fat. If starvation is still further prolonged, a stage will come whe
the fat, as well as the carbohydrate, is used up so that the organism he
now to subsist on protein alone. When this stage arrives, the ketone
will diminish, for, although they might be derived from certain of tk
amino acids, yet this does not actually occur, because a large part of tk
protein molecule (nearly half) also becomes changed into glucose, whic
by burning, as above explained, prevents the formation of ketones fro:
the other part of the molecule. For the same reasons, marked acidos
will not be expected to occur during any stage of starvation in lea
persons, who from the start must utilize mainly their stored protein {
supply the fuel upon which to live.
In diabetes exactly the same principles apply, but to an organism }
which the ability to metabolize carbohydrate has been depressed, so thi
“‘the maximum rate at which dextrose can be oxidized is fixed at son
level which is absolutely lower than in health.’’?? Therefore, since a ce
tain proportionality must éxist between the rates of combustion of f:
and carbohydrate, the diabetic can thoroughly oxidize less fat; in oth
_ words, an amount of fat which could readily be burned in a healthy boc
is improperly burned by the diabetic, and ketones and their acids a
cumulate.
Starvation Treatment.—‘‘In order to check a diabetic acidosis, it
necessary to restore the proper ratio of fatty acid to glucose oxidation,
which can best be done by starvation, rest in bed and warmth. But th
treatment may not at first suffice, because we have to deal not only wi
the acidosis bodies derived from fat, but with those which can be deriv:
from protein on account of the diabetic organism having lost the pow
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THE METABOLISM OF THE CARBOHYDRATES 685
even of burning the glucose which is derived from this foodstuff. By
persistence in the starvation, however, the ability of the organism to
utilize carbohydrate usually becomes so far restored that enough burns to
prevent acidosis. Every case of diabetes can not, therefore, be expected
Lo react in the same way to starvation, the determining condition being
the relation between the quantities of glycogen and fat stored in the body
at the outset of the fasting period. This relationship depends on the
nature of the previous diet.
To sum up, “‘fasting will lower acidosis either in health or in diabetes,
if it has the effect of stopping a one-sided metabolism and throwing the
tissues on a more nearly balanced ration of fatty acids and glucose’’—
(Woodyatt). A practical point may be noted here—namely, that there
is likely to be more danger of serious acidosis developing during starva-
tion in fat than in lean diabetics. The importance of our appreciation of
these facts in the starvation treatment of diabetes will be self-evident.
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CHAPTER LXXVII
FAT METABOLISM
Before considering the physiology of fats, a few of the most essential
points regarding their chemistry may be of assistance.
THE CHEMISTRY OF FATTY SUBSTANCES
It is usual to classify all substances that are soluble in ether as lipoids.
They include fatty acids, neutral fats, cholesterols, cholesterol esters, and
phospholipins.
The fatty acids belong to two main homologous series, which differ from
each other with regard to whether they are saturated or unsaturated. A
saturated fatty acid is typified by palmitic, whose formula is CH,-CH,-CH.,-
CH,-CH,-CH,-CH,-CH,-CH,-CH,-CH,-CH,-CH,-CH,-CH,-COOH, or CH,-
(CH,),,-COOH; that is to say it is a higher member of the series to which
acetic acid (CH,-COOH) belongs, differing from the latter in having four-
teen extra methyl] radicles, each joined to its neighbor by one bond or satu-
rated linking on either side. Another member of this series is stearic, in
which there are sixteen extra CH, groups (CH,(CH,),,COOH). An un-
saturated fatty acid is oleic (CH,(CH,),—CH®® = CH-(CH,),-COOH).
Its unsaturation is represented in the formula by the double bond or
unsaturated linking, which it will be seen occupies a position in the mid-
dle of the molecule, the other methyl radicles being linked together by
single bonds.
The fatty acids readily combine with alkali to form soaps; thus,
CH,(CH,),,-COOH + KOH=CH,(CH,),,-COOK +H,0, the reaction being
(palmitic acid) (soap)
analogous to that by which acetic acid forms an acetate with alkalies.
In place of being combined with alkali, the COOH (carboxyl) group of
fatty acids may combine with alcohols to form substances called esters.
Thus, acetic acid and ethyl alcohol form ethyl acetate,
(acetic (ethyl (ethyl acetate)
acid) alcohol)
united with fatty acid is glycerol (glycerine), in which there are three
686
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FAT METABOLISM 687
OH (hydroxyl) groups, the resulting ester—called triglyceride—is neu-
tral fat. Tripalmitin has the formula:
CH, - 00C-C,,H,,
ch -00C:C,,H,,
ot -000:0,,H,,.
By boiling neutral fats with alkali the fatty acid radicles are split off
as soaps, leaving the glycerol. This process is called saponification, and
it may be effected in many other ways, as for example by heating with
steam or by the action of special enzymes called lipases, which are widely
distributed in plants and animals.
The natural fats are usually a mixture of triglycerides, and their dif-
ferences in properties are dependent upon the relative amounts of fatty
acids present. The three most important in animal fats are tripalmitin,
tristearin and triolein. It is essential in the study of fat metabolism that
we should know the most important methods by which the proportion of
fatty acids present in a mixed fat is determined. These methods are as
follows:
1. The melting point. Olein is liquid at 0° C.; palmitic acid melts at
62.6° C.; and stearic at 69.3° C. The solidity of animal fats depends on
the proportion of olein, palmitin and stearin present. Mutton fat, for ex-
ample, is much stiffer than pig fat because it contains less olein and more
stearin. The melting points of fats from different parts of the body may
also vary.
2. The acid number indicates the amount of free fatty acid mixed with
the fat, and is determined by titrating a solution of a weighed quantity of
the fat in aleohol with a N/10 alcoholic solution of KOH, phenolphtha-
lein being used as indicator. :
3. The saponification value indicates the total amount of fatty acid
present, both that which is free and that combined with glycerol. It is
determined by heating a weighed amount of fat with an exactly known
amount of alcoholic KOH (determined by titration with standard acid).
After saponification is complete, titration of the mixture shows how much
alkali has been used to combine with the fatty acid. This is the saponi-
fication value. .
4. The ester value indicates the amount of fatty acid combined with
glycerol, and is obtained by subtracting the acid value from the saponi-
fication value.
Besides these there are two values, known as the iodine and the Reichert-
Meissl values, that are of importance because they depend on certain char-
acteristics of the fatty-acid radicles.
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688 METABOLISM
5. The iodine value indicates the amount of unsaturated fatty acids pres-
ent, or the number of double bonds. It depends on the fact that iodine,
like many other substances, is capable of directly attaching itself to the
fatty-acid chain wherever double bonds exist.
6. The Reichert-Meissl value indicates the amount of volatile soluble
acid present in the fat. It is determined by first of all saponifying the
fat, then decomposing the soap by mixing it with mineral acid and dis-
tilling the liberated fatty acid, the distillate being collected in a known
amount of standard alkali and titrated. It is a value that is not of very
great use in physiologic investigations, but it is so in connection with
food chemistry. Since volatile acids are present in butter, the Reichert-
Meissl value helps us to distinguish between butter and margarine.
Fat is insoluble in water but soap is soluble, forming a colloidal solu-
tion which presents the phenomenon of surface aggregation of molecules.
This consists in the concentration of the soap both at the free surface of
the liquid, where a skin may form, and at the interfaces between the
soap solution and any undissolved particles present in it. This pellicle-
formation around the particles prevents them from running together so
that they remain suspended, thus forming an emulsion. An emulsion
may therefore be formed either of neutral fat of any other physically
similar substance. When fat itself is used, there is usually enough free
fatty acid admixed with it to make it unnecessary in forming the emul-
sion to do more than shake the fat with weak sodium-carbonate solution.
With other substances not containing any free fatty acid, some soaps
should be added. To preserve the emulsion it is often useful to add some
mucilage. In the emulsified state, neutral fats are much more readily
attacked by lipases than when they are present in an unemulsified state.
Thus, emulsified fats are ‘‘digested’’ by the relatively small amounts of
lipase present in the stomach, whereas neutral fats themselves are not so.
Fatty acids also exist in nature in combination not with the triatomic
alcohol, glycerol, but with monatomic alcohols such as cholesterol. These
cholesterol fats differ from the glycerol fats in being very resistant to-
wards enzymes and microorganisms. They are therefore used for pro-
tective purposes in the animal economy; for example, they occur in the
sebum, the secretion of the sebaceous glands, where they serve to moisten
the hairs and skin. They are also present in cells, in which it is prob-
able they take an important part in forming the skeleton of the cell.
Cholesterol is absorbed from the intestine; it is always present in the blood
both in plasma and in corpuscles; and it is an important constituent of bile,
from which it may separate out in the bile passages and form calculi
(gallstones). ;
In the cells themselves the lipoids are represented mainly by compounds
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FAT METABOLISM 689
of a somewhat more complex structure—namely, the phospholipins. As
their name indicates, these consist chemically of phosphoric acid combined
with neutral fat and with a nitrogenous base, cholin. The best known of
the phospholipins is lecithin, which is widely distributed in the animal body
(present in blood and bile as well as in all cells). Other phospholipins
present in nervous tissue are cephalin, cuorin and sphingomyelin. There
are various lecithins distinguished from one another by the fatty-acid
radicles which they contain. Distearyl-lecithin, for’example, has the
formula: ;
CH, ~ 0 - 0C(CH,),,— CH,
CH -O-OC(CH,),,—CH,
| (stearic acid)
CH,-O O
oe
(glycerol) P
OH OCH, - CH, - N(CH,),.
(phosphoric
acid) OH
(choline)
This complex molecule can readily be split up by hydrolysis (warming
with baryta water) into:
glycero-phosphorie acid, CH, -OH
|
CH -OH
|
CH,-O O C,H,OH
s¢ ;
P ; choline, N { (CH,), (oxy-ethyl-ammonium
4
OH OH OH
hydroxide) ; and fatty acids.
With hydrochloric acid, choline forms a salt which readily forms a
double salt with platinie chloride. Since this double salt forms charac-
teristic crystals, it is used to identify and separate lecithins. For quan-
titative purposes, however, it is more suitable to determine lecithin in-
directly by the amount of phosphoric acid present in an ethereal ex-
tract of the organ or tissue.
Evidence is constantly accumulating to show that lecithin is an ex-
tremely important constituent of cells; indeed, it seems to be the inter-
mediate stage in the utilization of neutral fats by protoplasm. Its phos-
phorus also probably serves as the source of this element for the con-
struction of nucleic acid (see page 637). In nervous tissues it is often
associated with carbohydrate molecules (galactose), forming the sub-
stance known as cerebrin. It may therefore have some role to play in
carbohydrate metabolism. Some workers also attribute to leeithin an
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690 METABOLISM.
important function in the transference of substances through cell mem-
branes. When mixed with water it swells up by imbibition, and if erys-
talloids or other substances are dissolved in the water, a means is offered
for bringing water-soluble and fat-soluble substances into intimate con-
tact.
DIGESTION OF FATS
A certain amount of fat, especially when it is in an emulsified condi-
tion, can be digested in the stomach by the lipase contained in the gas-
tric juice. Most of it, however, is digested in the small intestine, into
which as we have seen, it is gradually discharged suspended in the chyme.
For this intestinal digestion of fat both pancreatic juice and bile are nec-
essary. This is easily shown in the rabbit, in which the pancreatic duct
enters the intestine at a considerable distance below the bile duct. If the
mesentery is inspected during the absorption of fatty food, no fat in-
jection of the lymphatics will be noted between the bile and the pan-
creatic ducts but only below the latter. In the dog, in which both, the bile
and the main pancreatic ducts enter the intestine at about the same level,
fat injection of the lymphatics starts at this point, but if the bile duct
(or rather the gall bladder) is transplanted at some distance down the
intestine, it will be found that the injection of the lymphatics with fat
oceurs only below the new point of insertion of the bile duct.
Removal of the pancreas interferes very materially with the absorption
of fat. In man, for example, absence of the pancreatic juice alone di-
minishes the absorption of fat by 50 or 60 per cent. If the bile is also
absent, the diminution amounts to 80 or 90 per cent, and in such eases,
as is well known, the administration of bile or pancreas powder greatly
improves fat absorption. In the dog, although ligation of the pancreatic
duet apparently only slightly influences fat absorption, removal of the
pancreas itself greatly interferes with the process; from which fact some
observers have concluded that the pancreas, in addition to its external
secretion into the intestine, must produce an internal sécretion into the
blood which has something to do with the efficient absorption of the
fat (Pratt, McClure and Vincent**). It is, however, improbable that such an
hypothesis is necessary, for it is very likely that the moribund condi-
tion into which an animal is brought by extirpation of the pancreas,
adequately accounts for the suppression of the fat-absorbing function.
As to the relative roles of pancreatic juice and bile in the digestion of
fat, we know of course that in the pancreatic juice there exists a lipolytic
enzyme, lipase, which, under suitable conditions has the power of split-
ting neutral fat into fatty acids and glycerine. If bile is examined, no
lipolytie enzyme will be found in it. It is entirely inactive on fat, but
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if we mix bile with fresh pancreatic juice, which by itself only slowly
digests fat, we shall find that the bile very materially increases the lipo-
lytie activity of the pancreatic juice. It has been found that the salts
of cholalic acid, the so-called bile salts, are the constituents of bile
that are responsible for this activation of lipase, this fact having been
demonstrated with bile salts prepared in such a way that there was no
possible chance of any other biliary constituent being present as an
impurity. It is important to remember, however, that lipase itself be-
comes slowly activated on standing, which explains why it should be
that bile added to pancreatic juice that has stood for some time, has a
less evident activating influence than bile added to fresh juice. It is
probable that the activating influence of bile salts is due to some physico-
chemical change induced in the digestion mixture.
One may ask how it happens that, when bile and pancreatic juice are
both absent from the intestine, the fat which appears in the feces is not
neutral fat but fatty acid. The reason is that the neutral fat that has
escaped digestion in the small intestine becomes acted on by the intestinal
bacteria, particularly in the large intestine. Under these conditions,
however, the fatty acid that is.split off is not absorbed, because the
epithelium of the lower parts on the intestinal tract can not perform this
function.
Besides assisting the action of lipase, bile facilitates fat digestion in
other ways. Thus, by its containing alkali and mucin-like substances
it assists in the emulsification of fat. Although emulsification is no es-
sential part of fat absorption, yet it greatly facilitates the process by
breaking up the fat into small globules on which the lipase can act
much more efficiently. The alkali also combines with the fatty acids,
as they are liberated by the digestive process, to form water-soluble
soaps, which are readily absorbed by the epithelial cells. The bile salts
further assist in the solution of the fatty acids, and they lower the sur-
face tension of fluids in which they are contained and so bring the fat
and lipase into closer contact.
ABSORPTION OF FATS
After its digestion fat lies in contact with the intestinal border of the
epithelial cells as fatty acid and glycerine. The fatty acid is combined
either with alkali to form a water-soluble soap, or with bile salts to
form a compound, which is also soluble. The glycerine and the dissolved
fatty acids are separately absorbed into the epithelial cells of the in-
testine, in the protoplasm of which—after the fatty acid has been set
free from the alkali or bile salt—they become united or resynthesized
to form neutral fat, which gradually finds its way by the central lac-
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692 METABOLISM.
teals into the villi, and then by way of the lymphatics to the thoracic
duct.
The chemical explanation of the absorption of fat is very different from
that formerly held by histologists who maintained that the fine particles of
emulsified fat in the intestine penetrate by a mechanical process through
the striated border of the epithelial cell into its protoplasm. The histologic
evidence for this view seemed very convincing, for fine fat globules can
readily be seen in the epithelial cells of the intestine after fatty food
has been taken, while they are absent during starvation. These par-
ticles seemed to have passed directly from the intestinal canal into the
epithelial cells because, when the fat was stained with characteristic fat
stains before feeding it to the animal, the globules in the epithelial cells
were found to be similarly stained. The supporters of this mechanistic
view of fat absorption maintained that the appearance of the stained fat
globules in the epithelial cells could not be explained in any other way
than by supposing that the fat globules had wandered unbroken into
the epithelial cells. Such a conclusion is, however, unwarranted, for the
stains that are soluble in fat are also soluble in soap, so that when the
fat splits up, the stain will remain attached to the soap and be carried
along with it into the intestinal epithelium.
Absolute proof that the chemical theory is the correct one has been
supplied by a large number of experiments. The following may be
cited: (1) When the lymph flowing from the thoracic duct is examined
after feeding with fatty acids instead of neutral fat, it is found to contain
only neutral fat, indicating that a synthesis must have occurred between
glycerine and fatty acid during the absorption. The glycerine for this
synthesis is furnished from sources which will be described later. (2)
When an emulsion made partly of neutral fats and partly of some hy-
drocarbon, such as albolene, is fed and the feces are examined for these
substances, it has been found that all the fat but none of the hydroear-
bon is absorbed; the feces contain all of the albolene but none of the fat.
This experiment supplies very strong evidence against the mechanistic
theory, for microscopic examination of the above described emulsion
shows the particles of neutral fat and hydrocarbon to be of exactly the
same size. (3) By examining the properties of the fatty substances in
the thoracic lymph collected during the absorption of such an emulsion
as that described above, nothing but neutral fat has been found present.
(4) Similar results are obtained when wool fat, which is an ester of
cholesterol and fatty acid, is fed.
We may conclude that fatty substances which are insoluble in water or
can not be changed by digestion into substances (soap) that are soluble
in water, are not absorbed, however like fat they may be in other particulars.
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FAT METABOLISM 693
The chemical theory of fat absorption further explains why there should
be such large quantities of soapy substances in the intestinal contents,
and also why the globules of fat present in the epithelial cells of the
intestine are so very much smaller than those wnich lie on the surface of
the epithelium.
It might be objected to the conclusions just stated that, although unde-
tectable, there is really some essential physical difference between emul-
sified fat and emulsified hydrocarbon. In order entirely to prove the case
for the chemical theory, it is necessary to feed a neutral fat possessing
some characteristic that depends on the manner of union existing between
fatty acid and glycerine, and then to see whether it appears in.an un-
changed condition in the thoracic duct. If it does so, the fat must have
been absorbed through the intestinal epithelium in an unbroken, unsapon-
ified condition, for it is unlikely that, in the resynthesis which occurs in
the intestinal epithelium, the fatty-acid molecules would recombine with
the glycerine molecules in exactly the same manner as before.
There are, however, but very few qualities of neutral fats, apart from
those of the fatty acids which compose them, by which they ean be char-
acterized. The most likely one is that of optical.activity. None of the
ordinary fats is optically active, although from chemical considerations
it is quite conceivable that some should be so. In order to obtain such a
fat Bloor*® conducted numerous experiments with the esters of stearic
acid.* In a series of experiments Bloor fed isomannid-dilaurate, a syn-
thetic fat of dextrorotatory power and as readily absorbed as natural fats,
and by éxamination of the neutral fat present in the chyle flowing from
the thoracic duct, found no evidence of the dextrorotatory fat. This result
confirms previous work by Frank, who found that the ethyl esters of
fatty acids are not absorbed unchanged, The results of both workers
emphasize the probability that readily saponifiable fatty-acid esters do
not escape saponification under the favorable conditions of the normal
intestine. In other words, had the fats been absorbed unchanged, as
would be required by the mechanistic theory of fat absorption, they
would have appeared in the chyle in optically active conditions.
These most important conclusions lead us to inquire as to the reason
for the change in fat during its absorption. It can not be for the purpose
of preventing the absorption of undesirable fatty substances, such as the
petroleum hydrocarbons or the wool fats, because such substances are
so rarely present in our food. It is most probable that the breakdown
_ “Bloor prepared an optically active mannitan distearate, but found it to have a very high melt-
ing point and to be only half as digestible as the ordinary fats. Its absorption was too slow and
unsatisfactory to make it Suitable for the above purposes. He, therefore, proceeded to prepare the
di-ester of isomannitan with lauric acid, and he found the resulting compounds to be as well-ab-
sorbed as ordinary fat, and yet to possess very marked dextrorotatory power, which, of course,
they lose on saponification. ‘This fat seemed suitable, therefore, for testing the above question.
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694 METABOLISM
and resynthesis of neutral fat occurs for the same reason that similar
processes occur during the absorption and assimilation of protein. It
will be remembered that protein is entirely disintegrated in the intestine
into its so-called building stones. These are absorbed separately into
the blood, which carries them to the tissues, in which they become re-
synthesized to form the body protein. And so it appears to be in the
ease of fats. The process, in other words, permits of the rearrangement
of fatty-acid molecules, as a result of which the newly formed fat is more
adaptable for use in the organism. It comes to be more like the char-
acteristic fat of the animal. There may be another reason for the proc-
ess. It will be remembered that lecithins, which constitute the most
important of the fatty substances of the cell itself, are mixed glycerides—
that is to say, are compounds containing a variety of fatty acids. The
rearrangement of the molecules of neutral fat which occurs during ab-
sorption may be the first step in the transformation of fat into lecithin.
In order to throw further light on the question, Bloor has performed
a number of interesting experiments in which the chemical properties
of fats before and after absorption were compared. The criteria which
he took were melting point, iodine value, and mean molecular weight;
the melting point representing the solidity of the fat, and the iodine
value, its degree of unsaturation—that is, the number of double links in
the fatty-acid chain. It was found that during absorption very con-
siderable changes occur in these two characteristics; for example, when
fat with high melting point and low iodine value was fed, the fat in the
thoracic lymph was of distinctly lower melting point and higher iodine
value. When fat with a low melting point and a high iodine value was
fed, the reverse change occurred, for the melting point of the thoracic
lymph fat was higher and the iodine value lower. These results could
be explained as due in the first case to the addition of oleic acid to the
fat during its synthesis in the intestinal epithelium, and in the second
case to the addition of some saturated fatty acid.
When a fat consisting mainly of glyceride and saturated fatty acid,
but with a low melting point, was fed, the addition of oleie acid was still
found to occur, as judged from the iodine value. This indicates that the
change is, not merely in order that the melting point of the absorbed fat
may be lowered, but also for some chemical reason. In a fourth series
of experiments, a lowering of iodine value occurred after feeding with
cod-liver oil, which contains a high percentage of glycerides of highly
unsaturated fatty acid.
Evidently, then, the intestine possesses the power of modifying the com-
position of fat during its absorption, and this modification is apparently
of such a nature that it causes a change toward the production of a
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FAT METABOLISM 695
uniform chyle fat, presumably characteristic of the animal body. The
changes are probably greater than could be produced by admixture of
the absorbed fat present in the normal fasting chyle, but the source of
the oleic acid or of the saturated acid required for this synthesis is at
present unknown.
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CHAPTER LXXVIII
FAT METABOLISM (Cont’d)
THE FAT OF BLOOD
Methods of Determination.—_Normally the blood contains only a small
percentage of fat, but after a fatty meal it may contain so large an
amount that the fat actually rises to the surface of the blood like a cream.
By means of the ultramicroscope, examination of the blood in the dark
field after a fat-rich meal reveals the presence of glancing particles,
the so-called ‘‘fat dust.’’ These particles are most abundant about six
hours after the meal has been taken, and they gradually disappear by
the twelfth hour. They do not appear after a meal when the thoracic
duct is ligated. ‘They disappear when oxygen is bubbled through the
blood.
Fat dust has also been found abundantly present in the blood of em-
bryo guinea pigs at full time, but not in the mother’s blood. This would
indicate that the placenta must have the power of taking the constitu-
ents of fat from the mother’s blood and building them into fat, which
then passes into the blood of the fetus. The placenta under these condi-
tions acts like the mammary gland. In this connection it is of interest
that there is also much fat present in the blood of pregnant women. The
fat content of the placenta is, however, greater in the early stages of
pregnancy than later.
Although these facts have been known for some time, it has been
impossible, either on account of the large quantities of blood required
for a chemical examination or because of the difficulty in estimating
the amount of fat from the density of the ‘‘fat dust,’’ to follow with any
great degree of accuracy the exact chemical changes that take place in
the fat of the blood. Recently, however, Bloor has suéceeded in elab-
orating methods by which the fat content of the blood ean be determined
with satisfactory accuracy in small quantities of blood, so that a con-
tinuous series of observations can be made over a considerable period.
The fat is extracted from the blood by an alcohol-ether mixture with moderate heat.
An aliquot portion of the filtrate is evaporated in the presence of sodium ethylate, which
saponifies the fat. The residue, consisting of soap, is well washed and then treated
with hydrochloric acid so as to precipitate the fatty acid. The density of the precipitate
696,
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FAT METABOLISM 697
thus produced is compared in an optical apparatus, called a nephelometer, with a
standard solution of two milligrams of oleic acid treated in the same way. The fatty
acids in human blood are mainly oleic and palmitic.
The lecithin and cholesterol may also be estimated in the same blood extract. For
lecithin the above extract of blood, after the removal of the alcohol and ether, is digested
by heating with concentrated HNO, and H,SO,. This decomposes the lecithin, liberating
the phosphorus, a solution of the resulting ash being rendered faintly alkaline to phenol-
phthalein and then slowly added to a silver nitrate solution. The density of the pre-
cipitate thus produced is compared in the nephelometer with that of a precipitate pro-
duced in the same amount of silver nitrate by adding to it a standard phosphoric acid
solution.
For cholesterol an aliquot portion of the above extract is saponified with sodium
ethylate and then saturated with chloroform; the chloroform extract is mixed with acetic
anhydrid and H,SO, (con.) until the bluish color is fully developed (Liebermann reac-
tion), the intensity of which is then compared in a colorimeter with that obtained by
similar treatment from a standard cholesterol solution.
Variations in Blood Fat—In the dog the percentage of fat in the
blood is remarkably constant under normal conditions. After a fatty
meal the increase in fat begins in about an hour, and reaches its maxi-
mum in about six. The increase is not found in animals in which the
thoracic duct has been ligated. Although this result would seem to
contradict the view held by some that part of the fat which can not be
accounted for in the thoracie-duct lymph is absorbed by way of the
portal vein, it does not by itself disprove the hypothesis, for it has been
found that the fat content of the portal blood is always higher than that
of the jugular.
Very interesting results have been obtained following the intravenous
injection of emulsions of oil, either the so-called casein emulsion or col-
loidal suspensions. Up to a dose of 0.4 gram per kilogram of body
weight—which by calculation would suffice to raise the fat content of
the blood. by 100 per cent—there was no increase in fat content. In or-
der to explain this disappearance of fat, it might be imagined that the
injected fat particles formed emboli in the smaller capillaries. Against
such a view, however, is the fact that the particles of fat in these emul-
sions are one-half to one-seventh the size of a red corpuscle. Although
this argument is no doubt of some weight, it should be remembered
that the physical condition of these’ fine fat globules is not the same as
that of the red blood corpuscle. Their surface condition may be such
that they readily agglutinate so as to form small masses, which may
stick at the branching of the smaller arterioles and capillaries. Bloor
himself suggests that the injected fat may be stored, possibly in the liver,
since the fat in this organ, as we shall see later, increases under similar
conditions. When twice the above quantity was fed in the form of egg-
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698 METABOLISM
yolk fat, some of it persisted in the blood for several hours. This in-
.erease may have been owing to the flooding of the temporary storehouse
with fat, or, more probably, to a retarding influence that lecithin may
have on fat assimilation, for lecithin itself persists in the blood for a
long time after intravenous injection.
During fasting, no increase in blood fat was found unless the animal,
by special feeding, had been stuffed with excess of fat prior to the fast-
ing period. The lipemia in this case indicates that fat is being trans-
ported from one place to another to serve as fuel for the starving tissues.
Narcotics were found to produce an increase in blood fat. Ether pro-
duced this increase during the narcosis, whereas morphine and chloro-
form did not do so until after recovery. The explanation given for the
ether effect is that a mixture of blood and ether has higher solvent power
for fat than blood alone. The explanation for the chloroform and mor-
phine effects is that a certain amount of breakdown of the tissue cells,
in which lipins are set free, supervenes upon the action of these narcotics.
The blood fat also becomes enormously increased in about forty hours
after the administration of phlorhizin, and on the second or third day
after the administration of phosphorus. The special significance of
these facts we shall consider later in connection with the relationship of
the liver to fat metabolism.
By comparison of the fatty acid, lecithin, and cholesterol contents of
blood during fat absorption, it has been found that there is a steady but
very variable increase in fatty acid, accompanied by no variation in
cholesterol, but with an increase in lecithin, which varies from 10 to 35
per cent, but does not run strictly parallel with the fatty-acid increase.
It is probable that this increase in lecithin represents that part of the
absorbed fat which is intended for immediate use in the tissues (page
705). The more or less independent increase in lecithin is of significance
in connection with the fact that in many pathologic conditions of so-
called lipemia the increase does not affect the fats of the blood but rather
the lipoids (i.e., lecithin and cholesterol). Separate analyses of blood
plasma and whole blood show the increase of lecithin to be much more
marked in the corpuscles than in the plasma, whereas the fatty-acid
inerease is confined to the plasma.
To illustrate some of these points the following table will be of value.
In it is shown the average distribution of fatty acid, lecithin and choles-
terol in normal individuals and in cases of diabetes, in which disease,
as has been known for long, there is marked disturbance of fat metab-
olism.
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FAT METABOLISM 699
Buoop Lirorps In NoRMAL AND IN DIABETIC PERSONS
MILD MODERATE SEVERE
NORMA DIABETES DIABETES DIABETES
Enos PER CENT PER CENT PER CENT
Fat by Bloor’s Whole Blood 0.59 0.83 0.91 1.41
Method Plasma 0.62 0.90 1.06 1.80
Whole Blood 0.37 0.59 0.65 1.01
Total Fatty Acids; |Plasma 0.39 0.64 0.75 1.28
Corpuscles 0.34 0.45 0.48 0.62
Whole Blood 0.30 0.32 0.33 0.40
Lecithin ‘Plasma 0.21 0.24 0.28 0.40
Corpuscles 0.42 0.42 0.40 0.40
Whole Blood 0.22 0.24 0.26 0.41
Cholesterol { Plasma 0.23 0.26 0.30 0.51
Corpuscles 0.20 0.21 0.20 0.24
Glyceride Plasma 0.10 0.38 0.46 0.84
Pick { Corpuscles 0 0.18 0.23 0.38
Total Lipoids Plasma 0.68 0.98 1.16 1.98
It will be observed that there is about 0.7 per cent of total fatty sub-
stances in normal blood. The fatty acids (palmitic and oleic) amount to
about 0.4 per cent, and are equally distributed between plasma and
corpuscles; the lecithin, about 0.3 per cent, being twice as abundant in
corpuscles as in plasma, and the cholesterol, 0.2 per cent, about equally
distributed. In diabetes all of these substances are seen to be increased
in proportion to the severity of the disease, the increase being mostly
in the plasma. The increase in cholesterol (confined mainly to the
plasma) is particularly interesting, since the substance is unaffected in
amount by excessive feeding with fat.
The Destination of the Fat of the Blood.—In general, it may be said
that the blood fat is transported to three places: (1) the depots for fat; (2)
the liver; and (3) the tissues. The fat present in each of these places
differs from that in the others, as is revealed by chemical examination
by the methods described on page 687. The depot fat usually yields about
95 per cent of its total weight as fatty acid. The tissue fat, on the other
hand, yields only about 60 per cent of its total weight as fatty acid.
This difference indicates that the fatty acid must be combined in the
tissues with a much larger molecule than is the case in the fat of the
depots. This large molecule is probably that of lecithin or other phos-
pholipin, and the smaller molecule in the depots, that of neutral fat.
The liver fat takes an intermediate position between depot fat and tissue
fat in its yield of fatty acid. When no active metabolism of fat is go-
ing on, the liver fat is like that of the tissues; but when fat metabolism
is active, the liver fat occupies an intermediate position between liver
fat and depot fat.
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Another difference among the fats in these three places is with regard
to the degree of saturation of the fatty-acid radicles. This, it will be
remembered, is indicated by the iodine value; the higher the iodine
value, the greater the desaturation of fatty acid. In depot fat this value
is relatively low—for example, about 30 in the goat and about 65 in man;
depending somewhat on the fat taken in the food, compared with which
it is usually a little higher. The fat in the tissues, on the other hand,
has a high iodine value, possibly 110 to 130. The iodine value of the
fat of the liver is remarkably inconstant, being about the same as that
of the tissues when fatty-acid metabolism is not particularly active, but
approximating that of the depots when fat mobilization is proceeding.
The significance of this fact we shall consider later.
The Depot Fat.—The places in the animal body where depot fat is
deposited in greatest amount are the subcutaneous and retroperitoneal
tissues. These fat depots may sometimes become of enormous size, as
in the case of the famous dog of Pfliiger, of whose total body weight
40 per cent was due to fat. Bloor suggests that there may really be two
different types of fat storage: one of a purely temporary character,
which readily takes up and liberates the fat, but which is of limited
capacity and possibly under the control of some quickly acting regulat-
ing mechanism, like that of the glycogenic function of the liver; and
another of a more permanent nature, into which the fat is slowly taken
up, but the capacity of which is very much greater.
Two questions present themselves concerning this depot fat: (1) where
does it come from, and (2) what becomes of it? Regarding the source
of the depot fat, there is no doubt that it comes partly from the fat and
partly from the carbohydrate of the food; in other words, it is either
taken ready-made with the food or manufactured in the organism. That
some of it comes from the fat of food is now a well-established fact, the
evidence for which need not detain us Jong. The best-known experiment
consists in first of all starving an animal until his stores of fat are
nearly exhausted and then feeding him with some ‘‘ear-marked’’ fat—
that is, with some fat having a characteristic property which it will
not lose during absorption. It will be found that the depot fat thereby
deposited presents many of the qualities of the fed fat. The ‘‘ear-
marking’’ of the fat may be secured by using fats of different melting
points, such as mutton fat, which has a high M.P., or olive oil, which has
alow M.P. On feeding a previously starved dog with mutton fat, the
M.P. of the depot fat approaches that of mutton fat—he becomes a
dog in sheep’s clothing; whereas when olive oil is fed, the subcutaneous
fat becomes oily. Or again we may ‘‘ear-mark”’ the fat by combining it
with bromine, when the deposited fat will likewise be brominized fat.
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It must not be imagined, however, that no change takes place in the
fat during its absorption and before it becomes deposited in the tissues.
On the contrary, the stamp of individuality is put upon the fat, for, as
we have already seen, its iodine value may become altered and its melt-
ing point changed during the process of absorption. In other words,
although the absorbed fat does not become entirely adapted to conform
with the ordinary qualities of the depot fat, yet it tends to change in
this direction.
That some of the depot fat comes from carbohydrate is well known to
stock raisers. When, for example, an animal is fed on large quantities
of carbohydrate and kept without doing muscular exercise, its tissues
become loaded with fat. If we desire strict scientific proof for this, we
do not need to go further than the old experiments of Lawes and Gil-
bert, who, it will be remembered, showed that the fat deposited in the
tissues of a growing pig is greatly in excess of the fat that could have
been derived from the fat or protein which was meanwhile metabolized.
The experiment was performed on two young pigs from the same litter
and of approximately equal weight; one was killed and the exact amounts
of fat and nitrogen in the body determined; the other was fed for several
months on a diet the fat and protein contents of which were accurately
ascertained. When after four months this pig was killed and the fat
determined, it was found that much more had become deposited than
could be accounted for by the fat and protein of the food, even suppos-
ing that all the available carbon of the protein had become converted
into fat. The only: conclusion is that the carbohydrate must have been
an important source of the extra fat.
The Destination of the Depot Fat.—The depot fat becomes mobilized
and transported by the blood to the active tissues whenever the energy
requirements of the body demand it. During starvation, as we have
seen, the depot fat is used to supply 90 per cent of the energy on which
the animal maintains its existence. Before the fat is transported, it is
probably broken down into fatty acid and glycerine, as which it passes
through the cell walls to be again reconstructed into neutral fat in the
blood. What agency effects this constant breakdown and resynthesis
of fat it is difficult to say. Two ester-splitting enzymes are present in
blood, one acting mainly on simple esters, the other on glycerides; but
it has been impossible to demonstrate any evident relationship between
either of them and the extent of fat mobilization.
The Fat in the Liver—The physiology of the liver fat has been very
diligently studied, particularly by Leathes and his pupils.°° The out-
come of this work has been to show that the liver occupies an extremely
important position in the metabolism of fat, being, as it were, the half-
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702 METABOLISM
way house in the preparation of the fatty-acid molecule for consumption
in the tissues. Fat is a material containing large quantities of poten-
tial energy. While in the depots this potential energy is so locked away
as to be unavailable for tissue use. To make it available the depot fat
is carried to the liver, where the energy becomes unlocked but not actu-
ally liberated. The fat is then transported to the tissues, and the libera-
tion of the energy occurs. Neutral fat is like wet gunpowder: it con-
tains much potential energy, but not in a suitable condition for explo-
sion. The liver, as it were, dries this gunpowder, whence it is sent to
the tissues to be exploded.
The great importance of the liver in fat metabolism is indicated by
comparison of the percentages of fat—or better of fatty acid—contained
in it under different conditions of nutrition. In the ordinary run of
slaughter-house animals the liver contains from 2 to 4 per cent of higher
fatty acid, but in about one in every eight animals a much higher per-
centage will be found to oceur. The same is true in laboratory animals.
In the case of the human liver as obtained from autopsies in certain
classes of patients, from 60 to 70 per cent of the dry weight. of the
organ, or 23 per cent of the moist weight, may be fatty acid. There is
no other organ in the animal body that is ever loaded with fat to this
extent. As in the depots, this liver fat might be derived either from fat
carried to the organ from elsewhere in the body, or it might represent
a surplus of manufactured fat.
That transportation of fat to the liver occurs is very readily demon-
strable both in the laboratory and in the clinic. About forty hours
after giving phlorhizin to a dog, it has been found that enormous quan-
tities of fat appear in the liver; a few hours later, however, this excess
of fat may have entirely disappeared. Fatty infiltration of the liver
is also observed in phosphorus poisoning, although in this case the fat
usually persists till the death of the animal. In man, in delayed chlo-
roform poisoning and in cyclical vomiting, enormous quantities of fat
may be present in the liver within a very short period of time after the
onset of the condition. There can therefore be no doubt that fat is
transported to the liver under abnormal conditions, but this can not
be taken as evidence that the liver has anything to do with fat metab-
olism in the normal animal. Such evidence has been supplied by Coope
and Mottram, who have been able to show that, at least in rabbits, a
similar invasion of the liver with fat occurs in late pregnancy and early
lactation. They also found that the fatty acid deposited in the liver
in late pregnancy gives an iodine value which lies nearer to that of the
mesenteric fatty acid than is the case in noymal animals. Mottram con-
eludes that ‘‘wherever . . . there is abundant fat metabolism, the
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FAT METABOLISM 703
liver is found to be infiltrated with fats, presumably to be handed on
elsewhere when worked up.’’ It is interesting that the fetus is greedy
of unsaturated fatty acids.
The most likely source of the fat transported to the liver is the fat pres-
ent in the depots, unless when digestion is in progress, when it may be
the fat from the intestine. That much of it comes from the depots is
easily demonstrated. Thus, the more extensive the infiltration of the
liver with fat, the more closely will this fat be found to agree with the
depot fat in its chemical characteristics. This has been very clearly
shown by, first of all, starving an animal so as to clear the depots of fat
as much as possible; then feeding it on some ‘‘ear-marked”’ fat (unusual
melting-point or a brominized fat); and after another day or so of
starvation, so as to clear the liver itself of fat, poisoning the animal
with phosphorus or phlorhizin. The liver will be found shortly after-
wards to be invaded with fat which has all the ear-marks of that on
which the animal had been fed.
Evidence of the same character has been furnished in a series of clin-
ical eases by observations on the amount of fat and the iodine value of
the fatty acid of the liver. This is shown in the accompanying table.
Farry Acips oF LIvER
HIGHER FATTY
CAUSE OF DEATH ACIDS PER CENT IODINE VALUE
OF DRY WEIGHT OF FATTY ACIDS
1. Pernicious anemia 12.1 116.8
2. Lobar pneumonia 13.7 116.8
at 3. Pernicious anemia 14.25 116.0
8 4. Diabetes ‘144 119.6
5. Toxemic jaundice 15.6 109.5
Commencing J 6. Accident 17.2 103.5
fatty 7. Empyema 21.5 96.0
change 8. Phthisis 25.4 96.4
9. Broncho-pneumonia 38.4 84.9
10. Appendicitis 44.0 91.1
Marked 11. Careinoma of bladder 47.2 77.8
fatty 12. Broncho-pneumonia 54.6 71.8
change 13. Ulcerative colitis 60.9 80.3
14. Aecident 66.3 63.0
15. Dysentery 73.5 69.1
This table clearly shows that the more fat there is in the liver, the
nearer this fat approaches in character that stored in the depots.
That some of the fat in the liver may come directly from the fat re-
cently absorbed from the intestine is also very readily demonstrable.
Thus, when cocoanut oil was placed in the intestine of anesthetized an-
imals, along with bile salts and glycerine, it was found by Raper® that
30 per cent of the absorbed oil appeared in the liver.
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704 METABOLISM
The characteristic feature of cocoanut oil is that its fatty acids are volatile in steam
and are saturated. Some of the fatty acids of the liver are volatile in steam, but they
are unsaturated. By distillation in steam of the fatty acids obtained by saponification
of the liver, it is possible to determine how much of the cocoanut oil has passed to the
liver.
Similar results have been obtained when unsaturated fatty acids, such
as those contained in cod-liver oil, are fed. In all these cases the rela-
tionship of the liver fat to that of the food is even more evident than
that between food fat and depot fat, because in the liver the newly absorbed
fat is not diluted by that deposited it may be months previously, as is
the case in the connective tissues. .
The question now arises: What happens to the fat during its stay in
the ler? An indication of the nature of the change is furnished by
observing the iodine value of the fat. This, it will be remembered, in-
dicates the degree to which the fatty acid is unsaturated. It does not
necessarily indicate the number of unsaturated bonds present in the fatty-
acid molecule, for the difference in iodine-absorbing power may depend
not on the number of such bonds but on the position in the chain at
which a given double bond is inserted. Even with this reservation, how-
ever, it is evident that the increase observed in the iodine values shows
that the liver has the power of desaturating fat. The advantage of
this change depends on the fact that the desaturated fatty acid will
be more liable to break up than the saturated fatty acid. In other words,
the double linkage will weaken the chain with the consequence that it is
liable to fall apart at this place; such at least is the natural interpreta-
tion which the chemist would put on the result. It may not, however,
be the correct interpretation, for it has been shown that, although un-
saturated fatty acids are more susceptible to chemical change in the
laboratory than saturated, yet when fed to animals they appear to be
more stable than many saturated acids. It may then be wrong to con-
clude that the introduction of a double linkage in fat necessarily means
the liability of the fatty-acid chain to break at that point. However
this may be, it seems likely that one function of the liver consists in
introducing double linkages at places in the fatty-acid chain, as a result
of which the chain breaks at these places, and the fragments then undergo
further oxidation.
Double linkages may be introduced not only in one place in a fatty-
acid chain, but in several. For example, it has been found in the liver
of the pig, after oxidizing the fatty acids with permanganate, that oxida-
tion products are obtained indicating the existence of unsaturated acid
with four double links. Permanganate (in alkaline solution) is used for
detecting the position of these double bonds, because, when it is allowed
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FAT METABOLISM . 705
to act on unsaturated fatty acids in the cold, it causes hydroxyl groups to
be introduced in the position of the double bonds. When the oxidation is
performed at a moderate temperature, the fatty acid falls apart at the
hydroxyl groups. A fatty acid with eight hydroxyl groups has been
obtained in this way from the liver of the pig. The presence of the hy-
droxyl groups has been confirmed by finding that an octobromide is ob-
tained by treatment with bromine. An acid of the same formula is said to
be present in cod-liver oil. To sum up, we may conclude that there are
certain positions, in the chains of carbon atoms which constitute the fatty-
acid radicle, where the liver introduces double bonds, and that the weak-
ened radicles then circulate to the tissues, where they break up at those
positions.
But this is probably not the only way in which the liver assists in
the metabolism of fat. It may also take part in the building of fatty-
acid radicles into the complex molecule of lecithin. The process of de-
saturation that we have just considered is probably a preliminary step
to this incorporation of the fatty-acid molecule into lecithin, for it is
well known that lecithin contains highly unsaturated fatty-acid radi-
eles. In support of such a view it is interesting to note that in aleohol-
ether extracts from normal and pathologic livers, the lecithins, which are
precipitated by acetone, have higher iodine values (i. e., are more unsat-
urated) than the neutral fats extracted from the same liver, which also
have higher iodine values than the depot fat of the same animal. The
desaturation process must, therefore, involve the fatty acids before these
become built into the lecithin molecule.
The liver is probably not the only place in the animal body where the
desaturation of fatty acids is brought about. The relative activity of
the different tissues in this regard has been studied by feeding cats
with fatty fish and then determining the iodine value of fat from various
places in the body. The absorbed fat was more obvious in the liver than
in the subcutaneous tissues, because it had not become diluted with fat
deposited it may have been months previously, which would be the
ease in the fat of the fat depots; and it was found that, although the
iodine value of the subcutaneous fat was slightly raised, that of the
liver was much more so, indicating that the desaturation process had
been more active in this organ, but had also occurred to a certain extent
in the depots.
Before leaving this subject of fat in the liver, it is important to re-
eall the old observation of Rosenthal, that a more or less reciprocal
relationship exists between glycogen and fat in the liver. When much
glycogen is present there is little or no fat, and vice versa. It is impor-
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706 METABOLISM
tant to note that the exact locations of fat and carbohydrate in the he-
patie lobule are somewhat different in the two eases.
A practical clinical application of the above work is found: in the fact
that fats will be more readily utilized by the body when they contain a
high percentage of unsaturated fatty acids. It is probably for this
reason that Norwegian cod-liver oil is of such undoubted nutritive value.
It is much more so than Newfoundland cod-liver oil, because in the prep-
aration of this variety oxidation occurs, which makes it no longer unsat-
urated. Fish oils in general are more unsaturated than other animal
oils, and are for this reason more nutritious.
The fat in the tissues differs very materially from that of the liver or
the depots. Only 60 per cent of this fat consists of fatty acid, which is
present very largely as part of the lecithin molecule, thus accounting for
the high iodine value. Some is probably also present as simple glyceride,
in a highly unsaturated and therefore very fragile condition.
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CHAPTER LXXIX
FAT METABOLISM (Cont’d)
Two very important questions of fatty-acid metabolism may now be
considered: namely, (1) how is fatty acid formed from carbohydrate?
and (2) what becomes of the fragments into which the fatty-acid molecule
is split as the result of the desaturation process? Although these prob-
lems involve chemical details of a somewhat complex nature, we must
not on this account fail to consider them; for, as we shall see, much of
what is known has an important practical application depending on the
fact that certain of the intermediary substances may accumulate in the
organism and develop a toxic action.
The Production of Fatty Acid out of Carbohydrate—If we place the
formulas for glucose and palmitic acid side by side, thus:
CH,OH - (CHOH),-— CHO (glucose), and
CH, — (CH,),,- COOH (palmitic acid) ;
we shall see that this transformation must involve: (1) a considerable
alteration in the structure of the molecule, (2) the removal of oxygen,
and (8) the fusion of several glucose molecules into one molecule of fatty
acid.
The conversion of carbohydrate to fat therefore involves a process of
reduction, and the resulting molecule must be capable of yielding more
energy when it is oxidized than the original one of carbohydrate, for
obviously the system O,-CH, (which corresponds to fat) will develop
more energy than that of O, - CHO (which corresponds to carbohydrate) ;
just as a piece of wood when it is burned will develop more heat than a
piece of charcoal. This explains why one gram of fat yields 9.3 calories
of heat, and one gram of carbohydrate, only 4.1 (page 535). Fatty
acid therefore contains more potential energy than sugar, and in explain-
ing its synthesis from sugar in the animal body we must indicate the
source of the extra energy. This is dependent on oxidation of some sugar
molecules—which do not themselves become changed to fatty acid—
proceeding side by side with the reduction which affects the others and
represented in the outcome of the reaction by the combustion products
CO, and H,0, thus:
6C,H,,0,+ 13 0, = 20 CO, + C,,H,;,0, + 20 H,0.
(glucose) (fatty acid)
707
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708 METABOLISM
What evidence have we that such a process actually occurs in the body?
If we compare the intake of oxygen with the output of carbon dioxide
in the respired air, we shall find that usually there is less of the latter;
that is to say, the respiratory quotient, as this ratio is called, is usually
less than unity. During the extensive conversion of carbohydrate into
fat, however, which occurs during the fall months in hibernating animals,
the R.Q. has been found to rise as high as 1.4. The great exéess of
CO, - output over O,-—intake which such a quotient indicates conforms
with the above equation.
The entire dissimilarity in chemical structure between the molecules
of fat and carbohydrate suggests that the primary step in the conversion
must be a thorough breakdown of the carbohydrate chain into compara-
tively simple molecules, from which the fat molecules are then recon-
structed and the unnecessary oxygen set free. The problem is to ascer-
tain the chemical structure of these simpler molecules and the manner
of their union into fatty acid.
Of the several suggestions which have been made, that of Smedley53 seems the most
likely. As will be seen from the following equations, the first step is the conversion of
glucose to pyruvie acid (page 600, No. 1 in equations). By enzymic action pyruvic
acid is converted into acetaldehyde (No. 2), which then condenses with another pyruvic-
acid molecule to form a higher ketonic acid (No. 3), from which by the loss of CO.,
as in the case of the production of acetaldehyde from pyruvic acid, an aldehyde is pro-
duced (No. 4). This higher aldehyde behaves like acetaldehyde in again combining with
pyruvic acid, forming a still higher ketonie acid; and so on until at Jast a long fatty-
acid chain is built up, thus:
(1) C,H,,0, + O, = 2CH,COCOOH + 2H,O0
(glucose) (pyruvic acid)
(2) CH,COCOOH = CH,CHO + CO,
(acetaldehyde)
(3) CH,CHO + CH,CCCOOH = CH,CH:CHCOCOOH + H,O
(unsaturated ketonie acid)
(4) CH,CH : CHCOCOOH = CH,CH :CHCHO + CO,; and so on.
(higher aldehyde)
(5) From the ketonic aldehyde formed at any stage, an unsaturated fatty acid (with
one less C-atom) is readily formed, and this by taking up H may become saturated:
CH,CH:CH CO COOH +O—CH, CH:CH COOH + CO,.
During the butyric-acid fermentation of sugar a slightly different process may oceur—
namely, the lactic acid, which we know is readily produced from glucose, may break down
into acetaldehyde (and formic acid), and two such molecules condense to form f-oxy-
butyric aldehyde; and this again condense to form higher fatty acids, thus:
(1) C,H,,0, = 2CH,CHOHCOOH.
(glueose) (lactic acid)
(2) 2CH,CHOHCOOH = 2CH,CHO + H.COOH
(acetaldehyde)
(3) 2CH,CHO = CH,CHOHCH,CHO; and so on.
(B-oxybutyric aldehyde)
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FAT METABOLISM 709
That higher fatty acids, such as caproic (C,H,,O,) and ecaprylic (C,H,,0,), have
actually been isolated from: the products of this fermentation is a very significant fact,
and it is of interest to note that Leathes has sometimes found an increase in higher fatty
acids to occur during the aseptic incubation of liver pulp. Unfortunately, however, the
increase of fatty acid could not be shown to be affected by adding substances to the
liver which, according to the above equations, should yield fatty acid.
The Method by Which the Fatty Acid is Broken Down.—In the chemi-
eal laboratory, ordinary oxidizing agents attack the fatty-acid chain at the
C-atom next the carboxyl (COOH) group (the alpha C-atom). But
this can not oceur in the animal body, because it would leave behind a
smaller chain containing an uneven number of C-atoms, and such chains
are never found present in the animal fats. On the contrary, the com-
moner fats all contain an even number of C-atoms, thus: Butyric, C,H,0,;
palmitic, C,,H,,0,; stearic, C,,H,,0,; oleic, C,,H,,0..
The intermediary substances which are produced during the gradual
breakdown of the fatty-acid molecule in the normal animal are of a very
transitory character so much so indeed that it is impossible for any one
of them to accumulate in sufficient amount to permit of isolation, or even
detection, by chemical means. How then are we to identify the inter-
mediary products? This has been rendered possible by the discovery that,
when anything occurs to disturb the normal course of fat metabolism, as,
for example, when the tissues are deprived of carbohydrates (as in star-
vation or in severe diabetes), the oxidation of the fatty-acid chain stops
short when a chain of four C-atoms still remains unbroken. These last
four C-atoms seem to form a residue that is more resistant to oxidation
than the remainder of the fatty-acid molecule. It is a residue, therefore,
which is quite readily further oxidized to CO, and H,O under normal con-
ditions, but which, although incapable of becoming completely oxidized
when the metabolism is upset, does undergo a partial oxidation, result-
ing in the production of various intermediary products. These accumu-
late in the body in sufficient amount to overflow into the urine, from
which they can be isolated and identified.
The fatty acid with 4 C-atoms is butyric, CH,CH,CH,COOH, and the
first oxidation product formed from it in the body seems to be B-orybuty-
ric acid, CH,CHOHCH,COOH. This then becomes oxidized to form a
body having the formula CH,COCH,COOH, acetoacetic acid, which, on
further oxidation, readily yields CH,COCH,, or acetone. These sub-
stances (8-oxybutyrie acid, acetoacetic acid and acetone) appear in the
urine during carbohydrate starvation, as in diabetes.
It might be objected, however, that a chemical process occurring under
abnormal conditions need not also oceur in the normal animal. That it
probably does, however, is indicated by the results of the experiments
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710 METABOLISM
of Knoop and of Embden and his coworkers. Knoop conceived the idea
of introducing into the fatty-acid molecule some group which is resistant
to oxidation in the body. The phenyl group (C,H,) was found to have
this effect. By feeding an animal with the phenyl derivatives of acetic,
propionic, butyric, and valerie acids, it was found that the urine con-
tained either hippuric (see page 630) or phenaceturic acid. Both of
these are compounds of aromatic acids with glycocoll or aminoacetic
acid (CH,NH,COOH), one of the protein. building-stones and always
available in the organism to form such compounds, thus:
(1) C,H,COOH + CH,NH,COOH — C,H,CONHCH, COOH.
(benzoic (glycocoll) (hippuric acid)
acid)
(2) C,H,CH,COOH + CH,NH,COOH = C,H,CH,CONHCH,COOH.
(phenylacetic (glycocoll) (phenaceturic acid)
acid)
When either benzoic acid (C,H,COOH) or phenylacetic acid (C,H,CH,-
COOH) is formed in the body as a result of the oxidation of phenyl
derivatives of the higher fatty acids, the acid combines with glycocoll
according to the above equations. From this it follows that if oxidation
occurs so that two C-atoms are thrown off at a time (f-oxidation), fatty
acids with an even C-atom chain should yield hippuric acid, and those
with an uneven chain, phenaceturic. This was found to be the case, as
the accompanying table shows.
OXIDATION
ACID FED Rane EXCRETED AS
Benzoie acid, C,H,.COOH Not oxidized Hippuric acid
Phenylacetie acid, C,H,.CH,.COOH Not oxidized Phenaceturic
acid
Phenylpropionie acid, C,H,.CH,.CH,.COOH C,H,.COOH Hippuric acid
Phenylbutyrie acid, C,H,.CH,.CH,.CH,.COOH C,H, .CH,.COOH Phenaceturic
acid
Phenylvalerie acid, C,.H,.CH,.CH,.CH.CH,.CcCOOH C,H,.COOH Hippuric acid
(From Dakin.)
Embden’s experiments are equally convincing. He studied the forma-
tion of acetone in defibrinated blood perfused through the freshly excised
‘liver. Normally only a trace of this substance is formed, but when fatty
acids with an even number of carbon atoms were added to the blood,
they gave rise to a marked increase in acetone, whereas those with an
uneven chain failed to cause any change. The acetone was found to be
derived immediately from acetoacetic acid. The following table shows
the results.
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FAT METABOLISM 711
NORMAL FATTY ACID FORMATION OF
ACETOACETIC ACID
Acetic acid. CH,.COOH
Propionic acid CH,.CH,.COOH
Butyrie acid CH,.CH,.CH,.COOH
Valeric acid CH,.CH,.CH,.CH,.COOH
Caproie acid CH, ‘OH, ‘CH, ‘OH.. CH,.COOH
Heptylic acid CH, ‘CH, ‘CH, .CH,.CH,.CH,.COOH
Octoie acid CH,. CH, ‘CH, _CH, ‘OH, .CH,.CH,.COOH
Nonoic acid CH,. CH, ‘CH, .CH,.CH, ‘CH, ‘CH,. CH,.COOH
Decoic acid CH, ‘CH, ‘CH, .CH,.CH,.CH, ‘CH, ‘CH, .CH,.CH,. COOH +
(From Dakin.)
For a long time it was difficult for chemists to understand how such
a process of oxidation at the B-C-atom could occur, since they were
unable to bring it about in the laboratory by the use of the ordinary
oxidizing agents, but recently Dakin has removed the difficulty by show-
ing that hydrogen peroxide (H,O,) oxidizes fatty acids just exactly in
this way.
We may sum up the results of these experiments and observations by
stating that normal saturated fatty acids and their phenyl derivatives can
undergo oxidation, not only in the animal body, but also in vitro, in such
a manner that the two (or some multiple thereof) termial C-atoms are
removed at each successive step in their decomposition.
But we, must not be too hasty in concluding from these experiments that
the steps in the process are necessarily in the order of first, the produc-
tion of a B-hydroxy acid, and second, the oxidation of this to a ketone
group. The mere presence, side by side, of B-hydroxybutyric acid and of
acetone in the above experiments does not indicate which is the ante-
cedent of the other, and indeed there are several experimental facts that
seem to show that the hydroxy acid may be derived from the ketone.
For example, when acetoacetic acid is added to minced liver and the
mixture incubated, B-hydroxybutyrie acid is formed (a reduction process),
although less usually the reverse action (oxidation) may occur when
B-hydroxy acid is added. A reversible reaction must therefore be capable
of occurring between these two substances, thus:
Deb eb ae
reduction
CH,.CHOH.CH,.COOH <-————— CH,.CO.CH,.COOH.
oxidation
—> (acetoacetic acid)
(f-oxybutyrie acid)
We know practically nothing as to the conditions determining whether
oxidation or reduction shall predominate, but there are two significant
facts that one should bear in mind: (1) that a plentiful supply of oxy-
gen is necessary for the oxidative process, and (2) that the presence of
readily oxidizable material in the liver (e.g., carbohydrates) may deter-
mine the direction which the reaction shall take. It is commonly said
that fats burn in the fire of carbohydrates, and it may be that the un-
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712 METABOLISM
doubted diminution in acidosis which occurs in diabetes when carbo-
hydrate food is given is dependent upon the directive influence which its
combustion in the liver has on the above processes. But we must be
cautious not to transfer results obtained by experiments with minced
liver in judging of the reactions which occur during life. Provisionally,
then, we must assume either that B-hydroxybutyric acid is a necessary
stage in the oxidation of butyric acid or that it is formed by reduction
of acetoacetic acid, which is really the first step in that process.
Of course there is no evidence in the above experiments that the higher
fatty acids are also broken down by the removal of two C-atoms at a
time, nor has it been possible to detect any ketonic or @-hydroxy deriv-
atives of them in the animal body. We can only reason from analogy
that a similar process may occur, although some support is furnished
for such a view by the fact that ketonic fatty acids have been found in
vegetable organisms.
What, then, it may be asked, is the relation of the desaturation of fatty
acids which we have seen occurs in the liver (and probably elsewhere) to
the @ oxidation? There can be no doubt that both processes can oceur
in the animal body, indeed in the same organ, e.g., the liver; and it is
important to ascertain their relationship to each other. The conclusion
which would seem to conform best with the known facts is. that the
desaturation process occurs (in the liver) so as to break up the long
fatty-acid chain into smaller chains, which are then capable of ~ oxida-
tion (in the tissues); desaturation may be the process by which the mole-
cule is rough hewn, and £ oxidation that by which the resulting pieces
are finally split to their smallest pieces—that is, to molecules of the size
of acetic acid, which are finally completely burnt to carbonic acid and
water.
The increase of iodine value observed by Leathes and his coworkers need not, as has
already been pointed out, necessarily indicate that new double linkages have been intro-
duced in the fatty-acid chain; it may merely indicate that structurally isomeric deriva-
tives which absorb iodine more readily have been formed. Direct evidence of desatura-
tion has, however, been offered by Hartley, who isolated the unsaturated fatty acids (by
dissolving the lead soaps in ether) from pig’s liver and then proceeded to oxidize them
with alkaline permanganate. When the olein of the depot fat is thus treated at a low
temperature, two hydroxyl groups become attached where the double linkage existed
(forming dioxystearic acid), and when the mixture is now warmed, the molecule splits
into two at this place, forming two lower acids (pelargonic and azelaic) :
(1) CH, - (CH,),CH :CH(CH,),COOH;
(oleie acid)
OIL
x Rg
(2) CH,- (CH,),-CH 1— CH
(dioxystearie acid)
(3) CH, (CH,),COOH + COOH — (CH,),COOH.
(pelargonie acid) (azelaic acid)
(CH,),COOH;
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FAT METABOLISM : 713
We may conclude from this that the double linkage in the oleic acid of the depot fat
exists between the ninth and tenth C-atoms, But it is otherwise in the case of the un-
saturated acid from the liver (pig’s), for under the above process of oxidation this
yielded caproie acid, which, since this acid has six C-atoms, would indicate that the
double linkage existed between the sixth and seventh C-atoms. Another interesting fact
brought to light by the experiments was that a tetraoxystearic acid was formed, which
fell apart in such a way as to indicate that the hydroxyl groups occurred between the sixth
and seventh and between the ninth and tenth C-atoms. The occurrence of this substance
would be satisfactorily explained by the introduction into the molecule of oleic acid of a
second double bond—i. e., between the sixth and seventh C-atoms. ‘‘The acids found in
the pig’s liver may be accounted for, in other words, by supposing that desaturation
of stearic acid and of the ordinary (depot) oleic acid occurs by the introduction of a
double link between the sixth and seventh carbon atoms in each case’’—(Leathes), Still
other double links may, however, be introduced into the fatty-acid chain, for acids of the
linolic acid series are present in cod-liver oil. Finally, it is of interest to note that caproic
acid is a product of the above oxidation process, for it has an even number of C-atoms
and therefore will form 8-oxybutyrie acid.
To go into these chemical problems any further here would be out of
place. One other fact, should, however, be borne in mind—namely, that
the unsaturated acids may be formed from saturated acids through the
intermediate formation of B-hydroxy and B-ketonic acids. , Their mere
presence, in other words, should not be taken as evidence that the oxida-
tion of fatty acids is initiated by the introduction of an hydroxyl group
at the 8 position in the chain.
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CHAPTER LXXX
CONTROL OF BODY TEMPERATURE AND FEVER
The classification of animals into two groups—warm-blooded and cold-
blooded—according to their ability to maintain the body temperature at
a constant level, is more or less arbitrary. Between.the two groups an-
other exists, represented mainly by hibernating animals, in which at
certain times of the year the animal is warm-blooded and at other times
cold-blooded. The ability of the higher mammals to maintain a constant
body temperature may or may not be present at the time of birth. The
heat-regulating mechanism of the human infant for example remains ill
developed for some time, so that exposure to cold is liable to lower the
body temperature to a dangerous degree.
VARIATIONS IN BODY TEMPERATURE
In animals in which the heat-regulating mechanism is fully developed,
there is not, even during perfect health, entire constancy in temperature
in the different parts of the body or in the same part at different periods
of the day. The average rectal temperature of man is usually stated as
being 37° C. (98.6° F.), but the diurnal variation may amount to 1° C.,
being highest in the late afternoon and lowest during the night. There
are probably several causes for this variation, and they are in part at
least dependent upon the greater metabolic activities of the waking
hours and upon the taking of food. Apart from these influences, how-
ever, others which are less evident appear to operate; for it has been
found that, when the daily program is reversed by night work, the usual
diurnal variation, although much less pronounced, still remains evident
even although this reversal in habit may have been kept up for years.
It is of interest to note in this connection that nocturnal birds have their
maximum temperature at night and their minimum during the day.
Regarding the temperature in different parts of the body, that of the
rectum is usually about 1° C. higher than that of the mouth, and this
again higher than that of the axilla. Of these three the mouth tempera-
ture is the most variable, for many conditions, such as mouth breathing,
talking, drinking cool liquids and even exposure to cold air, are sufficient
to lower markedly the temperature of this region. When the mouth
714
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CONTROL OF BODY TEMPERATURE AND FEVER 715
temperature is carefully taken by leaving the bulb of the thermometer
under the tongue for a minute’ or more, it is practically the same as the
temperature of the arterial blood of the hand when this is exposed to the
ordinary conditions of outside temperature. Greater differences than
1° C. in the temperature of different regions of the body are often ob-
served in feeble individuals and in those with some circulatory disturb-
ance.
FACTORS IN MAINTAINING THE BODY TEMPERATURE
The body temperature represents the balance between heat production
and heat loss. The production is effected mainly in the muscles by the
oxidative processes which are constantly ensuing there. When the
activity of the muscles is abolished by paralyzing the terminations of
the motor nerves with curare, the temperature of warm-blooded animals
immediately falls or rises according to the temperature of the environ-
ment. —*CH(OH) - CH,NHCH,.
It will be noted that it is closely related to tyrosine (see page 604). It
is also closely related to a group of substances (amines) occurring in
putrid meat and to which the active principles of ergot belong. It
contains an asymmetrie carbon atom (asterisked in formula), which
indicates that there must be three varieties of epinephrine, differing
from one another in the effect which they produce on the plane of
polarized light (i.e., a dextro- and a levo-rotatory and a racemic form).
Epinephrine can be prepared by synthetic means, the first product of
this synthesis being the racemic sait, which can then be split by appro-
priate methods into dextro- and levo- varieties. The levo- variety ap-
pears to be identical in its pharmacologic action with the natural product.
The dextro- variety on the other hand has only poorly developed physio-
logie activities (about seven per cent that of the levo- variety), while
the racemic variety comes in between the two in its action. A valuable
assay of the amount of epinephrine in tissue extracts can be made by
the method of Cannon, Folin and Denis,°? in which an acid extract of
the gland is treated with phosphotungstic acid, and the blue color thereby
developed compared colorimetrically with a standard blue.
Physiologic Action
The physiologic effects of the intravenous injection of epinephrine are
markedly excitatory and slightly inhibitory in nature. We will consider
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THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 735
the excitatory action first. Immediately after the intravenous injection
of as small an amount as 0°00008 milligrams per kilogram of body weight,
a distinct rise in arterial blood pressure may be observed. It is quite def-
inite with 000008 milligrams per kilogram, and is accompanied by a
slowing of the pulse. This slowing is caused by stimulation of the vagus
center, as is evidenced by the fact that if the vagus nerves are cut, or
sufficient atropine administered to paralyze them, the same dose of
epinephrine produces not a slowing but a quickening of the pulse, and
consequently a much greater rise in blood pressure. The vagus action
is developed not because of an effect of epinephrine on the vagus center,
but secondarily because of the rise in blood pressure.
These preliminary experiments indicate that the locus of action of
epinephrine, so far as the circulatory system is concerned, is mainly on
the small blood vessels, constricting them and thus raising the peripheral
resistance. This conclusion can readily be confirmed by applying the
epinephrine directly to the blood vessels of the exposed mesentery, or
by enclosing a vascular organ such as the kidney in a plethysmograph
during the injection of epinephrine, when a great diminution in volume,,
accompanying the rise of arterial blood pressure, will be observed. The
vasoconstricting effect of epinephrine does not become developed on the
large blood vessels near the heart on account of the deficiency in muscu-
lar tissue in their walls. Indeed, these vessels may become passively
dilated because of the increased blood pressure. The arterioles of dif-
ferent parts of the circulation are not equally sensitive to epinephrine;
those of the splanchnic area are most sensitive, whereas those of the
heart—the coronary vessels—do not respond at all in most animals (see
page 257). The pulmonary and cerebral vessels have a variable reactivity
to epinephrine.
The effect on the vessels persists after complete destruction, not only
of the central nervous system, but also of the vasomotor nerves; epi-
nephrine still acts, for example, on vessels the nerve fibers of which
have been allowed to degenerate by cutting them several days before the
epinephrine is applied. This would seem to indicate that the epinephrine
acts directly on the muscular tissue in the walls of the blood vessels,
but this does not appear to be the case, for it has been found that epi-
nephrine is incapable of acting on tissues which are devoid of sympathetic
nerve fibers, and is also inactive on those tissues in the embryo which have
not yet received any nerve supply. In brief, then, although epinephrine
acts only on blood vessels that are supplied by the sympathetic nervous
system, it is not on the nerve fibers that the epinephrine unfolds its
action. We shall see immediately that this conclusion is in conformity
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736 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
with the results of observations made on structures other than the blood
vessels.
Other muscular structures excited by epinephrine are as follows:
(1) the dilator muscle of the pupils, especially after the nerve supply has
been destroyed by extirpation of the superior cervical ganglion; (2) the
sphincters of the pylorus and of the ileocecal valve; (8) the muscle fibers
of the spleen, the vagina, the uterus, the vas deferens, and the retractor
penis. Regarding the action on the uterus, however, it should be noted
that a different response may be obtained according to whether the
uterus is pregnant or not. The plain muscles of the orbit and globe of
the eye are sometimes excited by suprarenal extract, causing the eyes to
protrude, the palpebral fissure to become large and the third eyelid to
be retracted, changes which are very like those which develop as a
result of fright.
Inhibitory effects of epinephrine on muscle are exhibited by the follow-
ing: (1) the muscle of the intestine; (2) the stomach; (8) the esophagus;
(4) the gall and urinary bladders. ;
. The effect of epinephrine in inhibiting the rhythmic contractions of
an isolated portion of the intestine in oxygenated Ringer’s solution is a
very striking phenomenon, and one which, as we shall see, may be very
successfully employed for detecting small quantities of epinephrine.
Extremely dilute solutions of epinephrine increase the contractions.
The effects of epinephrine on glandular structures are the same as those
which would be produced by stimulation of the sympathetic nerve supply
of the gland. Thus, the secretions of the lachrymal gland, the salivary
gland (in the cat), the mucous glands of the mouth and pharynx, the
gastric but not the pancreatic glands, can readily be shown to be
excited.
From these results as a whole, it is evident that the effect of epineph-
rine on muscles and glands is exactly the same as that which would be
produced by stimulation of their sympathetic nerve supply. This paral-
lelism of action between epinephrine and the sympathetic nervous sys-
tem becomes still more evident when we consider certain of the changes
in metabolism that follow administration of epinephrine. Injection of
epinephrine excites glycogenolysis in the liver so that hyperglycemia
and glycosuria become established, results which are also obtained by
stimulating the great splanchnic nerve. Epinephrine causes the clotting
time of the blood discharged from the liver to be very materially short-
ened, an effect also produced by stimulating the splanchnic nerve.*
As in the ease of the blood vessels, the above results are obtained even
after the sympathetic nerves to the part have been allowed to undergo
degeneration, from which it is concluded that the tissues elaborate some
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THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 737
substanee which reacts with epinephrine. This substance may be pro-
duced either at the junction between the nerve and muscle—the myo-
neural junction,—or perhaps throughout the protoplasm itself. It is
called the receptor substance of Langley, and is believed to react not
only with epinephrine, but also with various drugs. The receptor sub-
stance seems to increase, if not in amount, at least in sensitivity after
the removal of the nerve control.
Ergotoxin, which is an amine obtained from ergot and also from cer-
tain of the products of histidine, has an action on the receptor substance
which is inhibitory and therefore antagonistic to that of epinephrine.
The antagonistic action of ergotoxin affects the excitatory but not
the inhibitory. actions of epinephrine. By using this drug we are en-
abled to show that, although the main effect of epinephrine on tissue is
excitatory, a less marked inhibitory influence may be simultaneously
developed. The inhibitory effect may also sometimes be evoked by
doses of epinephrine very much smaller than those used to produec
excitatory effects. These facts are well illustrated in the case of the
muscle fiber of the blood vessels. With an ordinary dose of epinephrine -
constriction occurs; after ergotoxin the same dose of epinephrine causes
dilatation. Or this latter result may also be obtained by administer-
ing to a normal animal quantities of epinephrine that are very much
smaller than the usual quantity. The coexistence of inhibitory and ex-
eitatory influence is also well noted in the case of the uterus. In some
animals the effect of epinephrine on this organ is to augment its rhythmic
contractions, in others to inhibit them. In the former case, however, if
ergotoxin is first of all administered, epinephrine in its usual dosage will
invariably produce an inhibitory effect. The ergotoxin no doubt acts on
the receptor substance, and similar effects have also been produced with
apocodeine.
Although it is especially on plain muscular fiber having a sympathetic
nerve supply that epinephrine unfolds its action, yet, according to Can-
non, it increases the contracting power of voluntary muscle and dimin-
ishes the tendency to fatigue.*
*For further details of these effects the papers of Hoskins and Hartman™ should be consulted.
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CHAPTER LXXXII
THE ADRENAL GLANDS (Cont’d)
Variations in Physiologic Activity
Since it is clearly established that the adrenal glands are indispensable
to life and that extracts of them have a very pronounced physiologic action,
it remains to consider whether the glands produce this internal secre-
tion within the body, and if so, whether it is essential for the well-being
of the animal or required only under certain conditions. We must also
endeavor to find out upon which of the bodily functions of the intact
animal the internal secretion acts. These problems have been attacked
by three methods of investigation: (1) by comparing the epinephrine
content of similarly prepared extracts of the resting gland and of one
removed after a period of supposed increased activity; (2) by collecting
the blood as it flows into the vena cava from the adrenal vein and ex-
amining it for epinephrine by physiologic tests. These consist in observ-
ing the behavior of some tissue that is sensitive to the action of epineph-
rine, such as the intestine or uterus, after applying the blood or serum
to it, or by injecting the blood or serum intravenously into another ani-
mal and looking for epinephrine effects; and (3) by allowing the blood
of the adrenal vein to be discharged under certain conditions through
the vena cava into. the blood vessels of the same animal, and observing
the effect’ produced on certain physiologic processes which in one way
or another have been sensitized toward the influence of epinephrine.
This autoinjection method has recently been used successfully by Stew-
art and Rogoff,®* their favorite structure upon which to observe the
epinephrine effect being the denervated pupil.
Assaying the Epinephrine Content of the Gland
With regard to the first mentioned of the methods, either chemical or
physiologic means may be employed to assay the strength of the ex-
tracts. The best chemical method is that of Cannon, Folin and Denis,®”
the principle of which has already been described. The physiologic
method yielding most satisfactory results is that. of Elliott,?? which con-
sists in injecting a portion of the extract intravenously into animals
from which the influence of the nerve centers on the heart and blood
vessels has been removed by decapitation. The rise in arterial blood
738
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THE ADRENAL GLANDS 739
pressure produced by the injection is then a very fair measure of the
amount of epinephrine contained in it. It has been shown that the re-
sults obtained by the chemical method agree very closely with those obtained
by the physiologic, but it should be remarked that it is difficult to see how the
physiologic method could be accurate in all cases, since it has been
shown that with great dilution of epinephrine a reversed effect—a vaso-
dilatation—may be obtained. Attempts to assay the strength of an
epinephrine solution by investigating the effects which it produces on
other preparations, such as isolated loops of intestine or uterus, or the
enucleated eyeball of the frog, must prove unsuccessful, since the effects
are by no means dependent on the concentration of epinephrine in the
extract. When such preparations are used for quantitative purposes,
the strength of the extract must be judged by finding the extent to
which it can be diluted and still remain active. _
Quite apart from the foregoing possible sources of error, it must be
remembered that the results merely give us an idea of how much epineph-
rine may have been contained in the gland at the time of its excision.
They can not tell us how much epinephrine the gland was secreting. Prior
to excision as much of this hormone might have been undergoing a process
of manufacture in the gland as was being discharged from it, so that the
assayed amount would represent merely the balance of production and loss
of hormone by the gland. We might quite well find that the amount of
epinephrine in the excised gland was normal under conditions where
there had been an excessive discharge of it into the blood; that is to say,
loss and production might have been equal. Where, however, a marked
deficiency is found to exist, it probably indicates that exhaustion of the
power of producing epinephrine was taking place.
The Epinephrine Content of the Blood.—The second method, in which
blood from one animal is tested for its epinephrine effect by intravenous
injection into another animal or by applying it to some isolated prepara-
tion on which epinephrine acts, has yielded important results. Since
serum contains all the epinephrine of blood, it can be conveniently used
for the tests (Stewart and Rogoff). The isolated physiologic prepara-
tions that have been used in testing for epinephrine in the animal fluids
are as follows:
1. A segment of the small intestine of a rabbit, suspended in oxygen-
ated Locke’s solution at body temperature.
2. A segment of the uterus of a nonpregnant rabbit similarly prepared.
The apparatus used for observing the contractions of either prepara-
tion consists of a small glass chamber furnished below with a hook to
which one end of the segment is attached, the other end being connected
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740 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
to a muscle lever, so that the regular rhythmic contractions ean be regis-
tered on a drum (Fig. 190).
Epinephrine inhibits the contractions of the intestine but stimulates
those of the uterus of most animals, the intestine preparation being the
more sensitive (Fig. 191). Indeed, it is said that the inhibition in this
case may be obtained with a solution containing 1 part of epinephrine in
20,000,000 of solution. In using this method, however, great care and
judgment must be exercised in drawing conclusions, because other sub-
stances present in the blood are liable to affect the contractions; thus,
Air vent
Stock
solutica
water
Metal waterbath
Harvard muscle -
armer with
graduated scale
Fig. 190.—Arrangement of apparatus for recording contractions of a uterine strip, intestinal
strip, or ring, etc. The metal water-bath is made of a cheap metal water-pail with a heating rod
soldered through the side at the bottom. A short metal tube is soldered into a 1-inch opening in
the bottom to receive a perforated cork for connecting with the Harvard muscle-warmer inside.
(From Jackson.)
certain substances in blood serum which have been produced by the act
of blood clotting may cause augmentation of the beat in both the intes-
tinal and the uterine preparations. patient suffering from endemic goiter may at first exhibit
symptoms which are usually attributed to a hypersecretion of thyroid
material into the blood (the symptoms will be described immediately),.
but later these give place to symptoms not unlike those of myxedema.
It is concluded that the above conditions are due to deficiency of
thyroid function, or hypothyroidism, because: (1) the gland is atrophied,
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756 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
and (2) similar symptoms to those exhibited by the clinical conditions
can be produced experimentally by the removal of the gland in animals.
By observations on the effect of administration of thyroid extract to
cretinous or myxedematous patients, prompt amelioration of the symp-
toms occurs, which certainly suggests that the real cause is the absence
of an internal secretion. There is probably nothing more striking in
the whole domain of therapeutics than this effect from the administration
of thyroid extract or, more so still, of alpha-iodine. If the treatment is
started early enough, the cretinous child ‘from being an ill-developed
idiot quickly catches up with children of his own age and becomes in
every respect normal. Even if this treatment is not undertaken until
the child is several years of age, it is remarkable how quickly the benefit
may show itself. In myxedema and cachexia strumipriva also, the
symptoms very quickly disappear and the person becomes perfectly nor-
mal by the treatment. In all these conditions, however, the thyroid
extract must be administered continuously in order to prevent the reap-
pearance of symptoms.
Quite distinct from the above described conditions of hypothyroidism
are those produced by an excess of thyroid autacoid in the blood, namely,
hyperthyroidism. Such a-condition can be produced experimentally in
normal animals by the administration of thyroid extract or alpha-iodine
(Kendall). In man. large doses are soon followed by great quickening
of the pulse with some irregularity, flushing of the skin, increased per-
spiration, tremor in the limbs, emaciation, and marked nervous excita-
bility. Along with these symptoms, metabolic investigations have shown
that the energy output per square meter of surface is greatly increased,
being sometimes nearly doubled; that the nitrogen excretion is exces-
sive; and that alimentary glycosuria is very commonly present. The
body temperature is not, however, as a rule increased, because although
metabolism is excited, yet heat loss is correspondingly increased. Ex-
ophthalmos is said to develop very occasionally after such administra-
tion, but this is doubtful. Lastly, there are usually digestive disturb-
ances, although the appetite is likely to be increased. The pulse is quick-
ened after administration of alpha-iodine only when protein food is also
taken. This is believed by Kendall to be due to the association between
the thyroid hormone and the metabolism of the amino acids.
The symptoms following the injection of the extract are very similar
to those of the disease known as exophthalmic goiter. Indeed, the symp-
toms are so much alike in the two conditions that it is scarcely neces-
sary to describe them specially for the disease except to mention that
the exophthalmos is much more likely to be present.
Like simple goiter this variety is from three to four times more fre-
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THE THYROID AND PARATHYROID GLANDS 757
quent in women than in men, a fact of significance when we recall the
evidence of association between the thyroid gland and the generative
organs. It is said that the disease is usually coupled with persistence of
the thymus gland. The thyroid gland in exophthalmic goiter is enlarged,
sometimes in one lobe; it is hard and pulpy, and on auscultation a mur-
mur is heard. Histologically the gland presents a picture very like
that which has been described above as hyperplasia; that is to say, the
vesicles have a deficiency of colloid material; their epithelium is colum-
nar and folded up into the vesicles; and the interstitial tissue between
the vesicles is very markedly increased.
Exophthalmic goiter is almost universally claimed to be due to hyper-
secretion of the thyroid, because: (1) the symptoms of the disease are not
unlike those produced by excessive administration of thyroid to a normal
individual; and (2) they are in general opposite in character to the symp-
toms found in eases where the thyroid gland is atrophied. The blood of
a person with exophthalmie goiter when injected into mice increases their
resistance to the toxic action of acetonitrile, which is also the case after
thyroid extract has been injected. In many cases of exophthalmie goiter
partial removal of the gland is said to ameliorate the symptoms. Other
clinicians, however, state that if the patient is given proper medical
treatment, rest, and diet, equally beneficial results can be obtained.
Certain investigators, however, deny that it has yet been conclusively
demonstrated that exophthalmic goiter is due to hypersecretion of the thy-
roid (Marine). It is pointed out that, if hypersecretion were the cause of
the disease, one would expect that the injection into animals of the blood
of patients suffering from it would produce symptoms similar to those
following the injection of thyroid extract. The results of such experi-
ments, however, have been extremely confusing and very indecisive, since
it is difficult to recognize in laboratory animals many of the characteristic
symptoms, especially those affecting the skin and eyes and the general
bodily nutrition. Another difficulty in accepting the hypersecretion hypoth-
esis is the fact that an extract of a gland removed from an exophthalmic
patient has no different physiologic action on a normal animal from an
extract of a normal gland containing the same percentage of iodine.
The evidence is by no means conclusive one way or the other, and it may
well be that the observed changes in the thyroid gland are not the cause
of the symptoms of exophthalmic goiter, but merely, like the other symp-
toms of this disease, a result of some condition located elsewhere.
The Relationship of the Thyroid with Other Endocrine Organs
1. With the Generative Organs.—Evidence of an association between
the female generative organs and the thyroid is very strong; thus, the
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758 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
thyroid becomes enlarged at puberty, during the menses, and during
pregnancy, and in thyroidectomized young animals the sexual glands
‘fail to develop properly.
2. With the Adrenal Glands.—(See page 746.)
" 3. With the Pituitary Body.—After removal of the thyroid, the pitu-
itary becomes greatly altered and enlarged, particularly the pars an-
' terior, in which it is not uncommon to find that a certain amount of
vesicles containing colloid, not unlike those of the thyroid, become devel-
oped. This colloid material, however, does not contain iodine. It is said
that this increase of the pituitary after thyroidectomy does not occur if
thyroid extract: be administered. Increased activity of the pars inter-
media of the pituitary is also quite plain. These facts would at first
sight seem to indicate that the pituitary and the thyroid can act vica-
riously, but this is very doubtful, for it has not been found that pitu-
itary extract has any beneficial effect in the treatment of goiter and myx-
edema. Nevertheless the associatiow in function of the two glands must
be more or less close, not alone for the above reasons, but also because they
are both associated to much the same degree with the sexual organs,
and both act on the higher functions of the nervous system in much the
same manner.
4. With the Thymus Gland.—The persistence of the thymus in ex-
ophthalmic goiter, as well as the anatomie and embryologic relationship
between thymus and thyroid, is taken to indicate some close relationship.
THE PARATHYROIDS
Experimental Parathyroidectomy
Experimental parathyroidectomy yields results which vary in dif-
ferent groups of animals, undoubtedly because of the fact that in some,
such as the rat and rabbit, accessory parathyroids may exist. In gen-
eral, however, it has been found that if more than two of the four
parathyroids be removed, very definite and pronounced nervous symp-
toms soon supervene and if all four glands be removed, a quickly fatal
result is inevitable. The most acute symptoms are exhibited by the
earnivora. They may not be apparent for a day or two after the opera-
tion, although during the period the animal is in a depressed state, re-
fusing food and losing weight rapidly. The muscles are also more or less
stiff during this stage. When more definite symptoms appear, they con-
sist of a marked abnormality of muscular contraction, leading to the
occurrence of fibrillar contractions, or tremors and, later, to cramp-like
and clonic contractions. When spontaneous movements are made, a
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THE THYROID AND PARATHYROID GLANDS 159
peculiar shaking of the foot, like that made by a normal animal to shake
water off its pads, is a characteristic symptom. The slightest stimulation
of the peripheral nerves is sufficient to induce one of these attacks, which
recur with ever increasing frequency, becoming at the same time more
pronounced and accompanied by other disturbances, such as diarrhea,
profuse salivation, rapid pulse, and dyspnea (in the dog but not in the
cat). In cases that are not quickly fatal, the hair tends to be shed, and
the teeth to be improperly calcified (in young animals). Where a certain
amount of parathyroid tissue has been left—for example, one of the four
lobes—the symptoms may not appear except under conditions of special
strain to the animal economy, such as pregnancy or improper diet.
Thus, in a bitch from which three of the four glands had been removed,
no symptoms of tetany occurred until she became pregnant. Under the
same conditions it has been found that a diet of flesh is much more apt
to bring about the condition than one cf vegetables or milk.
Tetany, as the above condition is called, may also become developed
in man either as the result of surgical removal of the parathyroids or
because of their improper development. The symptoms in man are very
similar to those observed in laboratory animals, the only difference being
that the muscular contractions are more likely to be tonic in character.
Certain symptoms that may develop during pregnancy or in the course
of infectious diseases or in newborn infants have also been found to be
associated with degeneration of or hemorrhage into the parathyroid
(idiopathic tetany), and certain obscure nervous diseases in adults,
such as paralysis agitans, may possibly also be associated with changes
in this gland. Chorea, epilepsy, and eclampsia have likewise been
thought to be associated with it.
The parathyroid gland, besides influencing the nerve centers, has also
an influence on metabolism. The symptoms produced are: (1) rapid
emaciation and failure to grow; (2) a tendency to the production of
glycosuria, often detected by finding that the assimilation limit for
carbohydrate is lowered (page 652); and (3) most definitely of all, an
interference with calcium metabolism, as illustrated by the failure of
the teeth and bones to ealcify properly. This interference with normal
metabolism led Kellogg and Voegtlin™ to study the effect produced on
parathyroidectomized animals by the administration of calcium. It was
found that the symptoms were considerably ameliorated. These authors
concluded from their results that the essential cause of tetany is a
deficiency of calcium in the blood. It is possible however that the bene-
ficial action of calcium salts in this condition is that it decreases the
excitability of the nervous system, an action which it is known to
possess.
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760 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
When the tetany is the result of a complete extirpation of all parathy-
roid tissue, the symptoms can be combated by a successful transplan-
tation or graft of parathyroid tissue made from an animal of the same
species. Indeed, it has been found that the success of a graft of parathy-
roid is assured only when the graft is derived from the same kind of
animal as that from which the parathyroid has been removed. Implan-
tation into the subcutaneous tissue of a tetany patient of parathyroid
tissue obtained fresh from the deadhouse has been performed with bene-
ficial outcome.
Noel Paton, Findlay and Watson® have recently contributed greatly
to our knowledge of the physiologic pathology of tetania thyreopriva,
as the above condition is called, The symptoms are not due to any con-
dition affecting the muscles themselves, since they disappear after sec-
tion of the nerves. Nor are they primarily dependent upon the cere-
brum or cerebellum, since ablation of neither abolishes them. This does
not imply that secondary involvement of the higher centers never oc-
curs; on the contrary, the epileptiform convulsions and disturbances of
equilibrium sometimes observed indicate cerebral or cerebellar involve-
ment, respectively. This leaves some part of the lower neuron reflex
ares as the site of involvement. It is not the afferent neuron, since the
tremors and jerkings persist after section of the posterior roots, leaving
the afferent neuron as the affected structure.
The foregoing conclusion led Paton and his co-workers to compare the
response of muscle and nerve to electric stimulation in norma] and
parathyroidectomized animals. Although there are considerable varia-
tions in the responses of a normal animal, they are very definitely ex-
aggerated in tetany when either the motor neuron or the muscle itself
is stimulated, the exaggeration in the latter case being dependent upon
alterations in the neural structures (nerve endings) in the muscle. The
increased electric excitability can not, however, be taken as a measure
of the severity of the condition, for it may be no more marked in cases
in which there is involvement of the cerebral hemisphere (causing epilep-
tiform fits) than in milder eases.
As to the cause of the symptoms, many possibilities have to be con-
sidered. In the first place, no direct relationship exists’ between the
thyroid and parathyroid in this connection. One cause might be the
absence of some substance which checks the activity of the nervous sys-
tem, some chalone in Schafer’s sense. That such is not the case is shown
among other things by the fact that bleeding and transfusing normal
saline immediately removes the symptoms for some time. Moreover,
the metabolic disturbances go on when the nervous symptoms are slight.
It had previously been thought by W. G. Macallum® that, since symp-
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THE THYROID AND PARATHYROID GLANDS 761
toms like those of tetany can be induced by deficiency of calcium in the
body and the symptoms of parathyroidectomy relieved by administration
of this cation, calcium deficiency is the cause of the symptoms. While
not denying that these ions may have some relationship to the symptoms,
Noel Paton ascribes them to intoxication by guanidine (page 605). The
‘evidence is as follows: (1) Guanidine and methyl guanidine admin-
istered to normal animals produce symptoms that are identical with those
following parathyroidectomy. (2) There is a marked increase in the
amount of these substances in the blood and urine of parathyroidec-
tomized dogs and in the urine of children suffering from idiopathic
tetany. (3) In certain cases the serum of parathyroidectomized dogs
acts upon the muscles of the frog similarly to weak solutions of guani-
dine and methyl guanidine. (4) There is a striking similarity in the
relative amounts of the nitrogenous metabolites in the urine of parathy-
roidectomized dogs and of normal animals injected with guanidine.
It is concluded that the parathyroids control the metabolism of guani-
dine ‘‘by preventing its development in undue amounts. In this way
they probably exercise a regulative action upon the tone of the skeletal
muscles.’’? Since it is similar with regard to its characters and metabo-
lism to the condition following thyroidectomy, it is believed that disease
of the parathyroids is the cause of idiopathic tetany.
The Relationship of the Parathyroid with Other Endocrine
Organs
We know very little of the relationship of the parathyroid with other
endocrine organs. Vincent and others have stated that after removal
of the thyroid itself enlargement of the parathyroid may occur with the
formation of colloid material between the rows of cells, but the con-
clusion that this represents a vicarious function between the thyroid and
parathyroid glands is not generally accepted. The supposed relation-
ships among the parathyroid and the pituitary and adrenal glands are
also based upon uncertain evidence.
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CHAPTER LXXXIV
THE PITUITARY BODY
Structural Relationships
Situated at the base of the brain and lying in the sella turcica, the
pituitary body in man does not weigh much more than half a gram. It
is connected with the brain by-a funnel-shaped stalk, the infundibulum.
On account of a natural cleft, which runs across the gland in an oblique
plane, it is an easy matter to split it into two portions, an anterior, or
pars glandularis, and a posterior, or pars nervosa. This cleft in the
case of man is usually found to be more or less broken up into isolated
cysts containing a colloid-like material, and it represents the remains of
the original tubular structure from which the -pars glandularis is de-
veloped ;,namely, a pouch growing out from the buccal ectoderm.
On histologic examination it will be found that the pars glandularis
consists of masses of epithelial cells with large sinus-like blood capil-
laries lying between them. These blood vessels are very numerous, so
that in an injected gland this portion of the pituitary stands out very
prominently. The vessels are derived from about twenty small arterioles
that converge toward the pituitary from the circle of Willis, and enter
the gland by the infundibulum or stalk by which the gland is connected
with the base of the brain. Three types of cell can be differentiated:
nonstaining (chromaphobe) and granular (chromaphil), of which latter
there are cells with acid-staining and others with base-staining granules,
the former being by far the more numerous (Schifer).°° In some
animals such as the cat, the cells of the pars anterior are arranged around
the blood sinuses in rows as in a columnar epithelium. The cells with
acid-staining granules are said to become much increased in number in
pregnancy and also in the enlarged gland of acromegaly (see page 772).
After thyroidectomy it has been observed that colloid-like masses ac-
cumulate in the pars glandularis, the cells sometimes arranging them-
selves around these masses as in the thyroid gland. The colloid, how-
ever, contains no iodine.
The posterior part of the gland, or pars nérvosa, is composed almost
entirely of neuroglia, cells, and fibers, usually with some hyaline or
granular material lying between them, particularly in the neighborhood
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THE PITUITARY BODY | 763
of the infundibulum, into which it may be traced. It is believed that
the active principle of the gland is represented by this material. The
blood supply of the pars nervosa is relatively scanty.
Between the pars nervosa and the intraglandular cleft above referred
to is a layer of cells differing from those of either the anterior or the
posterior lobe. This layer of cells constitutes the so-called pars inter-
media. The cells are somewhat like those of the pars glandularis, except
that they are distinctly granular, the granules being of the neutrophile
variety, that is to say, they stain with neither basic nor-acid dyes. Well-
defined vesicles containing an oxyphile colloid material are often found
Fig. 196.—Drawing from a photograph of a mesial sagittal section through the pituitary gland
of a human fetus (5th month): a, optic chiasma; c, third ventricle; d, pars glandularis; e, infun-
dibulum surrounded by epithelial cells; f, pars intermedia; g, intraglandular cleft; h, pars nervosa.
(Herring, from Howell’s Physiology.)
between them. The blood supply is much less abundant than that of the
pars glandularis. Although well separated by the cleft from the pars
glandularis, the pars intermedia is not well separated from the pars
nervosa, because many of its cells extend for some distance into the lat-
ter between the neuroglial fibers. Certain of the cells in the pars inter-
media may be seen in various stages of conversion into globular hyaline
bodies, or a granular mass of material may appear in them. In either
case, the cells ultimately break down, setting free the hyaline or granular
material, which is believed to be the origin of the similar material al-
ready described as existing between the neuroglial fibers of the pars
nervosa and therefore ultimately finding its way by the infundibulum
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764 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
into the third ventricle of the brain. These hyaline globules are greatly
increased after thyroidectomy. It should be mentioned, finally, that at
the margin of the intraglandular cleft the intermediary and anterior
portions of the pituitary come together, although the cells of each can
readily be distinguished on account of their staining properties. This
_ pars glandularis et intermedia also extends as a thin layer over part of
the pars nervosa and around the neck of the gland at the infundibulum.
These relationships are well shown in the accompanying diagram (Fig.
196).
Functions
Concerning the function of the pituitary, it may be said in general that
the anterior lobe has an important relationship to the nutritive con-
dition of the body during growth, especially of the skeletal structures,
and that the posterior lobe produces a very active autacoid having to do
with the physiologic activity of unstriped muscle fiber. The pars inter-
media seems to be associated with the posterior lobe in the production of
this autacoid. ‘The function of these two parts will therefore be con-
sidered together.
Function of the Pars Glandularis.—The facts concerning the function
of the pars glandularis have been gleaned largely by observing the ef-
fects produced by partial or complete removal of the entire pituitary,
justification for ascribing to the removal of the anterior, rather than
the posterior, lobe the results that are obtained being furnished by control
experiments, in which by removal of the posterior lobe alone similar
effects are not observed.
Complete removal of the pituitary is almost invariably fatal, the con-
dition being called apituitarism. Two operative procedures have been
employed for the removal of the gland. One of these, elaborated by Cushing
and his pupils,®? consists in trephining the skull and elevating the temporal
lobe of the cerebrum so as to expose the gland. The other, elaborated
by Horsley,®* consists in approaching the gland through the orbital
cavity. Although there is some danger of injury to nervous tissues by
the intracranial method, its results are more dependable since the gland
is actually exposed to view before being removed.
Most hypophysectomized animals die within two or three days, unless
they are very young. This longer survival of young animals is ascribed
to the presence of accessory pituitary material situated in the dura mater
lining the sella turcica. The most extensive observations have been made
on dogs. On the day following the operation the animal appears about
normal, but it gradually becomes less active, refusing food and respond-
ing slowly to stimulation. It gradually gets weaker and weaker; muscu-
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THE PITUITARY BODY 765
lar tremors may appear, the respiration and pulse become slow, the back
arched, the. temperature subnormal; and, usually within about forty-
eight hours, coma develops and the animal dies in this condition. When
the symptoms are less acute and death does not occur so early, it is
believed by Cushing either that small portions of the gland have been
left behind or that some vicarious activity of other organs has developed
to replace that of the pituitary.
When only a part of the pituitary is removed either unintentionally
or intentionally, the symptoms are not nearly so acute, and the condition
is known as hypopituitarism. It is by a study of this condition that
most facts concerning the function of the anterior lobe have been learned.
When the operation is performed on young animals, they fail to grow
properly; the milk teeth and the lanugo are retained; the epiphyses
do not ankylose; the thyroid and thymus glands are enlarged; and the
cortex of the suprarenal and the sexual organs fails to develop. The
animal, though small, becomes very fat and may therefore increase in
weight. There is distinct evidence of mental dullness. From these
results it is concluded that the anterior lobe of the pituitary produces
autacoids having to do with the development of the skeletal and other
structures of the growing animal. That this autacoid is not derived from
the posterior lobe is evidenced by the fact that partial injury of this
lobe, or indeed its entire removal, is not followed by similar symptoms.
Closer examination of the metabolic function in hypophysectomized
animals has shown that there is a marked depression in the respiratory
exchange of oxygen and carbon dioxide, and that the ability to metabo-
lize carbohydrate becomes heightened; that is to say, the animal with-
out developing glycosuria can tolerate a larger quantity of sugar than
the normal animal. This effect on carbohydrate metabolism may how-
ever be associated not so much with the function of the anterior lobe as
with that of the posterior, for, as we shall see later, Cushing and his
pupils have found that extract of the posterior lobe has a marked effect
on the assimilation limit of carbohydrate.
Attempts have been made to graft the pituitary, especially the anterior
lobe, into various parts of the body. It has been found, however, that
within a few days the grafts atrophy and disappear unless there has
been complete removal of the pituitary itself, in which case the graft
may remain for a month or so and the otherwise fatal outcome of hypophy-
sectomy be warded off. Sometimes, where the graft has remained for a
longer time, it is said that a temporary increase in the growth of the
animal has been noticed.
Other observers have investigated the effects in normal animals of
continuous oral administration of pituitary substance or of subcutaneous
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766 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
injection of extract. The earlier results were indefinite and confusing,
but recently Brailsford Robertson®* has sueceeded in isolating from the
anterior lobe a substance called tethelin, which accelerates growth in
young animals and is thought to have a possible value in hastening the
healing process in wounds.
Tethelin is precipitated by dry ether from an aleoholie extract of the
carefully isolated anterior lobes. It contains 1.4 per cent of phosphorus
and nitrogen in the proportion of four atoms for every atom of phos-
phorus, two of the nitrogen atoms being present as amino groups and
one in an imino group. The effects on growth of mice are in every par-
ticular like those of the administration of anterior lobes, and consist in
retardation of the first portion of the third growth’ eycle,* followed by
acceleration of the latter portion of this cycle. When fully grown,
_tethelin-fed mice also differ from normal animals in being smaller in
size but of greater weight, with a distinct difference in the condition of
the coat. Normal animals at fourteen months of age have ‘‘shaggy,’
staring and discolored coats,’’ whereas in tethelin-fed animals they have
the glossy and silky appearance of young animals. During growth, nor-
mal animals display a greater variability in weight than tethelin-fed
animals.
Extraordinary effects have been observed by Clark®* to be produced
by feeding laying hens with pituitary gland. Thus, by giving to one-
year-old hens, in addition to their usual food, 20 milligrams of fresh
pituitary substance for four days, it was found that the average daily
number of eggs laid by a batch of 655 hens was raised from 273 during
the four days preceding the pituitary feeding to 352 during the four
days of the administration, these results being obtained at a time of
year when the natural egg-production of the hens was diminishing. It
was further observed that not only is the output of eggs greatly increased
as a result of the pituitary feeding, but likewise their fertility, for in
another experiment in which 35 hens were kept along with two cockerels
of the same breed, not only was the output of eggs increased (from 18 up
to 33), but the fertility of the eggs was greatly enhanced.
Functions of the Posterior Lobe or Pars Nervosa.—aAs already men-
tioned, excision of this part of the pituitary can be tolerably well with-
stood by the animal, so much so indeed that from its behavior after the
operation we can conclude little as to the function of the lobe. On the
other hand, extracts of the posterior lobe injected into normal animals
produce effects that are very striking, indicating that the main function
*Robertson has contributed valuable and very extensive data on the normal curve of growth of
white mice kept under carefully controlled conditions. Three growth cycles are present: the first
attains its maximum velocity between seven and fourteen davs after birth; the second, between
tyenty one enc tyenty cig days; and the third about = weeks, ee which the velocity decreases
rogressively, until further growth. ceases hetween,the fifti and sixtieth weeks s di birth.
BiGifB8 BY MichoSS ri Se we
THE PITUITARY BODY 767
of this lobe is production of an autocoid. The extracts have more or less an
epinephrine-like action. Such extracts, rendered protein-free and steril-
ized, are obtainable on the market under the various names of pituitrin,
hypophysin, ete. From them a erystallizable material has been obtained,
but this is probably a mixture of various substances. In discussing the
functions of these various extracts, it must be remembered that the inter-
mediary part (pars intermedia) is included with the posterior lobe in
their preparation.
Although the effect of pituitary extract on plain muscle fiber (and on
glandular tissue) appears, on first sight, to be very like that produced
by epinephrine, it has been found on closer examination that the two
substances really act in different ways. The rise in blood pressure pro-
duced by pituitary autacoid is likely to be more prolonged than that
produced by epinephrine. It stimulates increased cardiac activity, but
after the vagi have been cut or sufficient atropine administered to para-
lyze them, the pituitary autacoid continues to stimulate the strength of
the heartbeat without producing the acceleration noted with epinephrine.
Whereas epinephrine has little or no action on the coronary vessels or
on those of the lungs, pituitary autacoid usually produces constriction of
both types of vessel; and on the renal arteries the actions of the two
autacoids are entirely different, for epinephrine has a marked constric-
ing effect, while the pituitary autacoid produces dilatation.
Another striking difference in the extracts from the two glands is re-
vealed by repeating the injection after the-effect of a previous one has
completely passed off. With epinephrine the original effect is repro-
duced; with pituitrin, on the other hand, the effect of the second injec-
tion is very often. the reverse of that of the first; that is to say, the blood
pressure, instead of rising, may fall, or the rise be very much less
marked. Whether this effect of the second dose is caused by the action
of an autacoid having a chalonic rather than a hormonic influence, or
whether it is due to a reversed effect of the same hormone, it is impos-
sible at present to say. The chaloniec effect in any case is much more
evanescent than the hormonic, and it is not caused by cholin, as some
have suggested. The effect of epinephrine, it will be remembered, is
abolished by ergotoxin and apocodeine. These drugs, on the other hand,
have no influence on the action of pituitrin. The difference in action
between the two autacoids is usually explained by assuming that the
epinephrine acts on the receptor substance associated in some way with
terminations of the sympathetic nerve fibers in involuntary muscle,
whereas pituitrin acts directly on the involuntary muscle fibers themselves.
Other types of involuntary fiber are also acted on by pituitrin. The
uterine contractions for example are stimulated (Fig. 197) ; so are those of
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768 THE ENDOCRINE .ORGANS, OR DUCTLESS GLANDS
the intestine (in contrast to the inhibiting effect of epinephrine), and of the
bladder-ureter musculature. Dilatation of the pupil of the excised frog
Bras
1
a eC ANC
Mbt. |-S:
Dog: SL ee LANES ROO TTP
fe
yp
Anata AACOONCAnaTOVATRUCArAndua tvtvadeny
Fig. 197.—Tracing showing the action of pituitrin on the uterine contractions and blood pressure
in a dog. Made by Barbour’s method. (From Jackson.)
eye is produced. The effect of pituitrin on the muscle of the bronchioles
is shown in Fig. 198.
The glands on which the pitnitrin has the.most pronounced action are
THE PITUITARY BODY 769
the mammary glands and the kidneys. The effect on the kidney is evi-
denced by the remarkable increase in the urinary flow following injection
of the pituitrin. This diuresis might of course be due merely to the
vasodilatation that we have seen such extracts produce—a vasodilatation
which is all the more marked because the vessels elsewhere in the body
undergo constriction. But pituitrin continues to cause increased urinary
outflow in the absence of aiy demonstrable vascular change; it also acts
after the administration of atropine, so that it is considered by most
observers to act on the excretory epithelium of the convoluted tubules
aE IB TT AR
i
POT WY ae, On Poe oe
ee oe ia scale aia
TMU TNTOOOTTUMTOTT UT UCTU An Tt MAA aan INT TTTTAAE
Fig. 198—Tracing showing the constricting action of pituitrin on the bronchioles and its effect
on blood pressure in a spinal dog. (From Jackson.)
in much the same way as certain diuretics, like diuretin. This renal
hormonie action of pituitrin would appear to be analogous with that of
secretin on the epithelium of the pancreas. Another reason for believ-
ing that the secretory hormone is independent of that producing vaso-
dilatation of the renal vessels is the fact that a repeated dose of pituitrin,
although, as we have seen, it usually has a depressor action on the blood
vessels, still produces a stimulating effect on the excretion of urine.
The value of pituitrin-as a diuretie in clinical practice is now well
recognized. :
The effect on milk secretion is best demonstrated by placing a cannula
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770 - THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
in the mammary ducts so that the milk may freely flow out. By observ-
ing the rate of outflow during the injection of pituitrin, it will be found
that a remarkable increase occurs. After this increased secretion has
ceased, however, the injection of more pituitrin has no further effect,
indicating that the influence of the first injection must have been, not so
much to stimulate the secretion of milk, as to accelerate the outflow of
that which previously had-been secreted and ‘had collected in the alveoli
and ducts. This effect explains why the pituitary galactagogue should
have very little if any effect on the total production of milk or on the
total amount. of fat and other constituents contained in it. Histologic
examination of sections of a resting mammary gland and of the same
gland after administration of the pituitrin, bears out the above interpre-
tation of the action. Alveoli in the resting state will be found largely
distended with milk and the epithelium flattened against the basal mem-
brane, whereas alveoli from the gland after pituitary activity show small °
shriveled-up alveoli, containing little milk, and with epithelium that is
well marked and stands out prominently from the basal membrane.
These facts taken together indicate that pituitrin stimulates the mus-
cular fibers of the ducts of the mammary glands, thus squeezing out the
milk contained in them. Muscular fibers have been described as existing
between the basal membrane and epithelial cells, much in the same way
as they do in the case of the sweat glands. At least Schafer has suc-
ceeded in demonstrating in this position rod-shaped nuclei which prob-
ably belong to muscular fibers.°° By their contraction, the milk in the
alveoli is expelled into the ducts. It has also been found that pituitrin
stimulates the secretion of cerebrospinal fluid, and that this stimulation
is independent of a rise in blood pressure.
Pituitrin has a distinct effect on carbohydrate metabolism. After its
intravenous or subcutaneous injection, a marked lowering in the toler-
ance for sugar is observed (page 652), usually to such an extent that
glycosuria becomes established. Cushing and his pupils have concluded
that the posterior lobe contributes an autacoid which stimulates the utili-
zation of sugar in the body. Confirmatory evidence for this view is fur-
nished by the observation that mechanical stimulation of the posterior
lobe, such as is produced by puncturing it with a needle, is followed by
a temporary glycosuria, which is said to be as pronounced as that fol-
lowing puncture of the diabetic center (page 672), provided glycogen is
present in the liver. The production of this carbohydrate autacoid would
appear to be under the control of the sympathetic nervous system, for it
has been found by Cushing and others that stimulation of the superior
cervical ganglion, which has been known for many years to be fre-
quently followed by elycasunia, hast is effect only provided the posterior
THE PITUITARY BODY V7
lobe of the pituitary is intact. Even surgical manipulation of the pitui-
tary may excite a hypersecretion of pituitrin, which would account for
the glycosuria often observed after experimental excision or partial
destruction of the pituitary. A similar irritation may be set up in disease
of the gland. —
The glycosuria which is usually observed after partial hypophysectomy
soon passes off, to be followed by a permanent condition of increased
tolerance for sugar, because now less pituitrin is being produced. It is
said that during the stage of increased tolerance diabetes can not be pro-
duced even by excision of the pancreas. The glycosuria produced by
irritation of the posterior lobe is accompanied by a marked polyuria (dia-
betes insipidus), which may outlast the glycosuria.
Fig. 199.—A, To show the appearance before the onset of acromegalic symptoms; B, The ap-
pearance after seventeen ycars of the disease. (After Cainpbell Geddes.)
Clinical Characteristics
Because of their importance from a physiologic standpoint, we shall
now proceed to review briefly some of the more important facts that have
so far been brought to light by clinical observations. The pathologic
condition most frequently observed affecting the pituitary is an adenom-
atous growth particularly located in the anterior lobe. Besides pro-
ducing general symptoms of pressure, such as diminution of the visual
field and, perhaps, headache, a shadow can usually be observed when the
patient is examined by means of the x-rays. General symptoms, com-
monly ascribed to a hypersecretion of the autacoid of the anterior lobe of
the pituitary—hyperpituitarism—begin sooner or later to show them-
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772 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
selves. These symptoms are almost exactly opposite in character to those
observed in animals after removal of this portion of the gland. Thus,
the bones of the extremities and of the face become stimulated to in-
creased growth, so that if the patient is young, and the epiphyses there-
fore not ossified, remarkable elongation of the long bones occurs, pro-
ducing the condition known as gigantism. On the other hand, if the dis-
ease does not develop until after ossification is complete, its effects be-
come most marked in the bones of the face, the lower jaw becoming
Fig. 200.—Hand of a person affected with acromegaly.
enormously hypertrophied and the supraorbital ridges very prominent.
The long bones also become enlarged at their extremities, and there may
be some increase in length of the vertebral column, although the stature
does not increase because of kyphosis (curvature of the spine). The
condition is called acromegaly. Nutritive disturbances of the skin and
hairs also become marked, causing the skin to become dry and yellowish,
and the hairs to undergo’ abnormal increase over the body. An early
symptom of the condition is a failure of the sexual power (Figs. 199
and 200.)
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THE PITUITARY BODY 7113
After a time the disease begins to affect the pars intermedia et nervosa,
and disturbances in carbohydrate metabolism come to be observed, con-
sisting usually in a diminished tolerance accompanied by glycosuria, in
the early stages of the disease, followed’ by increased tolerance in the
later stages. The glycosuria is usually accompanied by marked polyuria.
It should be observed that sometimes tumor of the pituitary has been
found to exist postmortem though none of the above symptoms had been
recorded during life. In these cases it is probable that the disease from
the start had been of such a nature as to produce a tendency to hypo-
pituitarism rather than hyperpituitarism, for the symptoms are very like
those observed in animals after partial or complete removal of the gland.
If the condition commences before adolescence, the body fails to grow,
_ although the child may continue to increase in weight because of the
remarkable deposition of fat in the tissues. Sexual development is strik-
ingly interfered with, and the secondary sexual characteristics fail to
show themselves. In boys, for example, the pubic hairs fail to extend up
to the umbilicus; and the hairs on the chin do not develop, whereas the
hair of the scalp grows profusely. The bones remain of the female type,
and a broad pelvis, rounded limbs, small feet and hands are often ob-
served. In these cases there is usually excessive tolerance for carbohy-
drates, which may explain the adiposity, sugar being converted into fat.
In the light of the experimental results, the effect on carbohydrate
metabolism may be explained as due to involvement of the posterior
lobe. Mental development is retarded, and psychic derangements are
sometimes observed.
Where the hypopituitarism does not develop until after adolescence,
some of the above symptoms will of course be missed, but many will be
observed, such as dryness of the skin, loss of hair, and the tendency in
the male to adopt certain of the female characteristics, particularly with
regard to the growth of hair. Obesity and increased tolerance for sugar
are also evident, and pigmentation of the skin, something like that of
Addison’s disease, is said often to be a prominent feature. Operative
interference in the early stages in many of these cases is of undoubted
benefit, as is shown by the brilliant work of Harvey Cushing, to which
the reader is referred for further information.
The Relationship of the Pituitary Gland with Other Endocrine
Organs
The relationship of the pituitary gland with other endocrine organs
seems to be an intimate one.
1. With the Thyroid and Parathyroid Glands.—That enlargement of
the pituitary oceurs after thyroidectomy in man has been known for a
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174 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
considerable number of years. The enlargement affects more particu-
larly the pars anterior, although changes are also described in the pars
intermedia et nervosa. Accompanying the enlargement of the anterior
lobe, vesicles containing colloid-like material often become developed in
it, but even after the hypertrophy has proceeded to a considerable de-
gree, this colloid does not contain iodine, nor does an extract have the
same physiologic effect as one of the thyroid gland. It can not replace
thyroid extract in the treatment of patients with goiter or myxedema,
or ameliorate the symptoms produced in animals by the removal of the
thyroid gland. Deposition of colloid-like material in the pars anterior
also occurs in myxedema. Histologic changes in the pars intermedia et
nervosa, although less pronounced than in the pars anterior, are never-
theless said to be perfectly distinct following thyroidectomy, and to con-
sist in an increase in the hyaline and granular masses which have already
been described as present to a certain extent in the normal gland.
Less direct evidence of an association in function between the pituitary
and the thyroid is furnished by the similarity of the effects produced on
the sexual functions and on the general development of young animals
by the removal of either gland. In both cases the animals fail to grow
properly; the sexual organs remain undeveloped; and the mental func-
tions are infantile in type. In hypophysial deficiency, however, extreme
adiposity is likely to be more marked than is the case in cretinism.
-2. With the Sexual Organs.—That the pituitary gland has much to do.
with the development of the sexual organs has already been shown. Fur-
ther evidence of a relationship between the sexual glands and the pitui-
tary is furnished by the following observations. After castration en-
largement occurs in the pituitary, and on histologic examination the
gland is found to contain a large number of oxyphile cells, particularly
in the pars anterior. This influence of the sexual glands on the pituitary
is believed to depend on the interstitial cells present in them, for it has
been found that if the ovary or testis is transplanted into other parts of
the body after the castration, the changes in the pituitary do not occur,
although, as we shall see, the transplanted gland becomes entirely
atrophied except for the interstitial cells. The enlargement of the pitui-
tary during pregnancy—an enlargement which often brings it to two or
three times its normal weight—is further evidence of its Association
with the ovary.
3. With the Suprarenals.—Association of function is suggested in this
case by the fact that extracts of suprarenal and pituitary have very much
the same effects on involuntary muscular fiber and glandular structures,
and it is said that the two extracts mutually facilitate each other’s
action in this regard. It should be remembered, howe hae wii
Digitized by Microson® ee pituitr
THE PITUITARY BODY 175
and epinephrine do not appear to act on exactly the same peripheral
mechanism (see page 767).
4, With the Isles of Langerhans.—Since pituitrin affects carbohydrate
metabolism, which is thought to be primarily controlled by the Isles of
Langerhans, it is claimed by some observers that a relationship also
exists between the pituitary and these structures. Injections of duodenal
extracts are also said to cause a hypersecretion of pituitrin into the
cerebrospinal fluid.
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CHAPTER LXXXV
THE PINEAL GLAND AND THE GONADS
THE PINEAL GLAND
This peculiar structure lies between the anterior corpora quadrigem-
ina, and weighs about two-tenths of a gram. It is largest in the early
years of life, and undergoes retrogressive changes after puberty. Micro-
scopically it consists of epithelial cells arranged loosely in trabeculae,
with large sinus-like capillaries between them; neuroglia and sometimes
muscle-fiber cells are also present. Curious globules of caleareous mat-
ter (brain-sand) are also found, especially in the pineal gland of man.
The gland is developed from an evagination of the third ventricle, and
it is homologous with the so-called median eye of reptiles.
The functions of the pineal gland are obscure. In cases where its
extirpation has been successfully accomplished (in the fowl), it has been
found that the body growth is stimulated and the sexual characteristics
developed more quickly. This result would seem to indicate that the
clinical observation that tumors of the pineal gland are associated in
young boys with abnormal growth of the skeleton and with the early
development of the secondary sexual characteristics, depends on the
fact that a condition of hypopinealism is produced by the growth of a
tumor. The immediate effects of the injection of extract of pineal gland
are not characteristic, consisting merely of a fall in blood pressure, which
is, however, obtainable when an extract of practically any cellular organ
is Injected. Prolonged administration of an extract to growing animals
is said to accelerate the growth and to bring about a precocious develop-
ment of the sexual organs; but this result is somewhat difficult to inter-
pret, for, as we have just seen, similar changes occur after experimental
removal of the gland.
THE GONADS OR THE GENERATIVE ORGANS
The Generative Glands of the Male
The structures which are responsible for the well-known influence of
the testicles on the development of the male sexual characteristics are
the so-called interstitial cells of Leydig, which consist of polygonal-
shaped epithelial-like cells, with well-marked nuclei and nucleoli. Lipoid
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THE PINEAL QLAND AND THE GONADS TT7
granules, staining black with osmic acid, are also present in the cyto-
plasm. The degree of development of the interstitial cells varies in dif-
ferent animals, being marked in the cat and man and ill-marked in the
rat and rabbit. In animals which show seasonal changes in sexual activ-
ity, the cells are most: prominent between the periods of sexual activity,
when the semeniferous epithelium is less evident. They also become
prominent in cases where the semeniferous epithelium is atrophied,
either as a result of disease or following ligation of the vas deferens done
in such a way that the artery and nerves to the testicles are not included
in the ligature. When the testicle or a portion of it is grafted into
another part of the body, the semeniferous epithelium degenerates, but
the interstitial cells remain alive and become quite prominent. It is
believed that the interstitial cells are responsible for the production of
an autacoid that has to do with the development of. accessory sexual
characteristics.
The effects of castration are not significant in animals below the verte-
brata. In all of these, however, they are very pronounced. The cas-
trated male frog fails to show development of the thumb pad, but this
development immediately ensues if portions of testis from another frog
be placed in the dorsal lymph sac. In birds the results are more pro-
nounced; in the castrated male chick the comb, spurs, wattles, etc., fail to
develop, but will usually do so if some testis from another bird is trans-
planted into its tissues. In mammals the effects are most striking in
animals that develop marked male characteristics, such as the growth
of antlers in stags. These fail to develop properly and are prematurely
shed after castration. In man also, as is well-known from a study of
eunuchs, castration has a very profound effect. Hair fails to grow on the
face; the larynx remains undeveloped; the epiphyses are a long time in
ossifying, so that the stature may become great, but at the same time
the limb bones may be more delicate than usual; the sutures of the skull
are slow in closing ; and the whole architecture of a castrated male comes
to be very like that of the female. Confirmatory evidence of the influ-
ence of the testicles on the development of secondary sexual character-
istics is afforded by the observation that malignant tumors of the testes
in boys are associated with the premature development of the secondary
sexual characteristics, and that these may recede after the removal of
the tumor.
As a result of castration, interesting changes have also been observed
in other ductless glands. Thus, the suprarenal cortex and the thymus
become enlarged, whereas the thyroid and pituitary become atrophied.
The metabolic functions also become tardy, as is evidenced by a tendency
to the deposition of fat.’
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7178 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
When the castration is performed on an adult man, the above changes
in the sexual characteristics are of course not so evident, although the
prostate, ete., atrophy. The effect on the metabolic functions is, how-
ever, very marked, there being a striking tendency to increased forma-
tion of fat. It is interesting that accompanying this there should usually
occur a lowering of the assimilation limit for carbohydrate, so that glyco-
suria is very readily induced. We can not assume, therefore, as Cush-
ing has done in the case of hypopituitarism, that the fat deposition is
attendant upon an improper combustion of carbohydrate.
These remarkable effects of castration have naturally prompted ob-
servers to study the influence of injection of testicular extract on the
development of sexual characteristics in different animals, but the re-
sults have in general been considered to be negative in character.
The Female Generative Organs
It is well known that, besides their function in producing ova, the
ovaries also produce autacoids that have to do not only with the fixa-
tion of the embryo in utero, but also with the changes that occur during
pregnancy in the maternal organism. It is however at present uncertain
as to where these autacoids are produced in the ovary. The two most
likely sources are the stroma cells and the corpus luteum. In the stroma
of the ovary of certain animals, groups of cells have been described
having a different appearance from those of ordinary stroma cells.
They have been called the interstitial cells of the ovary, and are believed
to be analogous with the similar structures found in the testicle. It is
possible, however, that these interstitial cells are nothing more than
cells derived from previous corpora lutea. The latter are formed by
proliferation of the follicular epithelium which remains after extrusion
of the ovum, and by the ingrowing into the follicle of the so-called theca
cells and blood vessels. The fully developed corpus Juteum in most
animals consists of cells arranged in trabecule converging toward the
scar which formed at the place where the follicle had burst. The luteal
cells, as they are called, are characterized by containing considerable
quantities of lipoid material.
That the ovary produces some autacoid is evidenced by both clinical
and experimental observations. Thus, if both ovaries are removed in a
young animal (odphorectomy or spaying), it is well known that not
only does the uterus fail to develop properly, but the external changes
characteristic of puberty in the female fail to materialize, although act-
ually the general effects are not so pronounced as they are in the male
after castration. Menstruation does not set in; the mammary glands fail
to develop; and there is a tendency for the hair to grow as in the male,
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THE PINEAL GLAND AND THE GONADS 779
When the operation is performed in adult life, the changes are not very
pronounced, except that menstruation ceases and the uterus and mam-
mary glands atrophy. Metabolism also becomes altered, causing a
tendency to the deposition of fat, and in the case of the human animal at
least, there is frequently evidence of mental disturbance.
Attempts to acquire more definite information regarding the physio-
logic effects of the ovarian autacoid have recently been made by Schafer
and Itagaki.*° Extracts were prepared from the corpus luteum or Graafian
follicles or from the hilum ovariae, and observations were made on the
effect produced on the behavior of the chief forms of unstriated muscle
by adding the extracts to isolated preparations of uterus or intestine
or by injecting the extracts into animals. Applied to the isolated prepa-
rations, extract of follicular tissue or of liquor folliculi was found to
increase the force and rate of the rhythmic contractions of the uterus as
well as its tone, whereas inhibition was produced when extract of the
hilum was used. Extract of corpus luteum, when injected into the
veins, was found to cause the uterus to increase its contraction or if
‘quiescent to begin contracting. It was further noted that extracts of
hilum eaused a fall in arterial blood pressure, whereas those of corpus
luteum had little or no effect. It would appear from these observations
that the extracts contain two different autacoids, one having a hormonic
and the other a chalonic action on plain muscular fiber.
Extract of corpus luteum when intravenously injected also stimulates
the outpouring of the milk from the mammary glands, although not so
markedly so as extract of pituitary gland. This pituitary-like action is
not obtained with extracts of ovary that do not contain corpora lutea.
Besides being concerned in the outpouring of milk, corpus luteum has
also been shown to be related in some way to the development of the
mammary gland during pregnancy. These glands become developed in
young virgin rabbits after the continuous administration for a month
or so of extract of corpus luteum, and they also develop in unimpreg-
nated animals when the corpus luteum is made to develop by artificial
means such as puncturing the Graafian follicle. Furthermore, destruc-
tion of the corpora lutea in a pregnant rabbit arrests development of
the mammary glands. The corpus luteum has also an important fune-
tion in connection with the formation of the uterine decidua and the
fixation of the embryo. Thus, after destruction of the corpus luteum at
an early period in pregnancy, the embryo fails to become adherent to
the uterus.
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780 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS
DUCTLESS GLANDS REFERENCES*
(Monographs)
58Vincent, Swale: Internal Secretions and the Ductless Glands, Ed. Arnold, London.
59Biedl: The Internal Secretory Organs, Wm. Wood & Co., 1913.
60Schéfer, Sir HE. A.: The Endocrine Organs, Longmans, Green’ & Co., New York and
London, 1916.
(Original Papers)
61Fulk, M. E., and Macleod, J. J. R.: Am. Jour. Physiol., 1916, xl, 21.
62Folin, O., Cannon, W. B., and Denis, W.: Jour. Biol. Chem., 1913, xiii, 447.
8sCannon, W. B., and Gray, H.: Am. Jour. Physiol., 1914, xxxiv, 232; also with Men-
denhall, W. L.: Ibid., 243 and 251.
6¢Hartman, T. H., and others: Am. Jour. Physiol., 1915, xxxviii, 433; ibid., 1917, xliii,
311; ibid., xliv, 353; ibid., 1918, xlv.
ssHoskins, R. G.: Am. Jour. Physiol., 1912, xxix, 363; Jour. Pharm. and Exp. Therap.,
1911, ili, 93; Am, Jour. Physiol., 1915, xxxvii, 471; ibid., 1916, xli, 513.
seStewart, G. N., and Rogoff, J. M.: Jour. Lab. and Clin. Med., 1918, iii, 209. See full
bibliography by Rogoff in this paper.
stElliott, T. R.: Jour. Physiol., 1912, xliv, 374.
S8Stewart, G. N.: Jour. Exp. Med., 1911, xiv, 377; ibid., 1912, xv, 547; ibid., xvi, 502.
s9Stewart, - N., Rogoff, J. M., and Gibson: Jour. Pharm. and Exper. Therap., 1916,
viii, 205.
70Meltzer, 8. J.: Deutsch. med. Wehnschr., 1909, xiii.
T1Stewart, G.N.: Jour. Exper. Med., 1912, xv, 547.
72Cannon, W. B., et al: Am. Jour. Physiol., 1911, xxviii, 64; ibid., 1914, xxxiii, 356;
also Bodily Changes in Hunger, Fear, and Rage, Appleton, 1915.
73Cannon, W. B., and Cattell, McKeen: Am. Jour. Physiol., 1916, xli, 74.
v4Macleod, J. J. R., and Pearce, R. G.: Am. Jour. Physiol. 1912, xxix, 419.
T5Marine, D.: Personal communication.
7éMarine, D.: Jour. Exper. Med., 1914, xix, 89.
77Marine, D., and Lenhart, ©. H.: Jour. Exper. Med., 1910, xii, 311; ibid., 1911, xiii,
455; also Bull. Johns Hopkins Hosp., 1910, xxi, 95.
78Marine, D., and Kimball, O. P.: Jour. Lab. and Clin. Med., 1917, iii, 41. /
79Kendall, E. C.: Boston Med. and Surg. Jour., 1916, 175, 557; also Proc. Am. Physiol.
Soc., Am. Jour. Physiol., 1918, xliv.
soPaton, Noel and Finlay: Quart. Jour. Exp. Physiol., 1917, x, 203. Paton, Noel,
Finlay and Watson, A.: Ibid., 233, 243, 315, and 377. i
8iMacCallum, W. G., ete.: Jour. Exper. Med., 1909, xi, 118; ibid., 1913, xviii, 646;
Jour. Pharm. and Exper. Therap., 1911, ii, 421.
s2Cushing, Harvey: The Pituitary Body and Its Disorders, J. B. Lippincott Co., 1912.
s8Horsley, V.: Brit. Med. Jour., 1885, i, 111.
84Robertson, Brailsford, and Ray, L. A.: Jour. Biol. Chem., 1916, xxiv, 347, 363, 385,
397, 409.
s5Clark, L. N.: Jour. Biol. Chem., 1915, xxii, 485.
*The numbering is in continuation with, that for metabolism.
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PART, IX
THE CENTRAL NERVOUS SYSTEM
CHAPTER LXXXVI
THE EVOLUTION OF THE NERVOUS SYSTEM
The nervous system of the higher animals consists of the nerve cen-
ters, and the nerves with their various interconnecting tracts. The
nerve tract and centers are located mainly in the spinal cord and brain,
where, by their interlacement, they form an extremely complex struc-
ture. The exact position of the centers and the course and connections
of the tracts with the centers are problems which, under the title of
neurology, have during recent years been contributed to more particu-
larly by the anatomist and the pathologist. The information thus
gathered tells us the possible tract or tracts of nerve fibers through which
the various centers may communicate either with one another or with
the structures outside the central nervous system upon which they
act. Since each of these centers may, however, be played upon by in-
fluences coming from different regions of the body, it is evident that there
must remain, as an equally important aspect of the subject, the investi-
gation of the means by which the various available centers and tracts are
brought into communication and action at the proper time. In other
words, we must investigate the functional uses of the available paths.
We may compare the central nervous system with a telephone system,
the exchanges representing the nerve centers, and the wires the nerve
trunks. Any incoming wire may be connected by the operator with
any outgoing wire, but a knowledge of how each wire runs does not tell
us under what conditions the various wires will be connected for trans-
mission of messages. It is the same with the nervous system; the neurolo-
gist can tell us how the tracts and centers run, but not the conditions
under which they may act together. This it is the duty of the physiologist
to ascertain.
Since it is the degree of development of the central nervous system
which determines an animal’s position in the evolutionary scale, much
information concerning the relative importance of the various parts of
781
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782 THE CENTRAL NERVOUS SYSTEM
it can be gleaned from a survey of the conditions under which the
nervous system makes its appearance in the lowest forms of animal
life. In the case of unicellular organisms, such as the ameba, the ap-
plication of a stimulus to the surface causes a movement, because the
protoplasm of the organism possesses, among its other properties, those
of excitability, conductivity and contractility. In the case of multicel-
lular organisms, on the other hand, some cells are set aside and spe-
cialized for the assimilation of food, others for movement, others to
receive stimuli from the outside, and yet others to compose the tougher
tissues which protect the surface of the animal from injury. This loca-
tion of specifie function in specialized groups of cells makes it necessary,
for the welfare of the organism as a whole, that some means of com-
munication should be provided between the distant parts of the animal,
for otherwise the cells which are occupied in absorbing food would be
unable to move away or be protected from harm when some destructive
agency approached them, and. indeed the moving (muscle) cells could
never know when the welfare of the organism as a whole demanded that
they should become active.
It is probable that, in some of the lower organisms, the messages trans-
mitted from one group of cells to the others are carried by chemical
substances present in the circulating fluid—hormones, as they are called
(page 729). For the quick adaptation that is necessary in the struggle
for existence, however, such hormones are usually too slow in bringing
about the response, and very early in the evolutionary scale we find that cer-
tain cells become differentiated for this special purpose. The cells thus
specialized constitute the nervous system, their differentiation, as would
be expected, being, however, antedated by that of the cells that form the
muscular tissues. In the sponges, for example, muscle cells become
developed from ameboid epithelium and from a layer underneath the
external epithelium. These muscle cells contract slowly so as to. cause
opening and closing of the small mouths, or oscula, on the surface of
the sponge in response to movements in the sea water. They are in-
dependent of any nervous structures.
In certain Celenterates the muscle cells respond a little more quickly
than in the sponges, and this greater efficiency is found to be dependent
upon the appearance of a localized, very primitive nervous system’.
This: nervous system consists of specially modified epithelial cells, or
receptors, sending branches from their inner ends, which either come in con-
tact with the muscle cells, or effectors, or become interlaced so as to form a
network. In the region between the receptors and the effectors the net-
work at first serves merely as a structure whereby the entire muscula-
ture of the animal can Pe ghroneht, yt harmonious action from a single
THE EVOLUTION OF THE NERVOUS SYSTEM 783
point on the surface, as, for example, in the case of the sea anemone
(No. 2 of Fig. 201). In the jellyfish, which in contrast to the sea anemone
is a free moving animal, we find that the receptors are more highly special-
ize and, therefore, much more sensitive, and that the impulses which they
receive are transmitted to a more definite nerve network, capable not only
of conveying the excitatory process from one part of the animal to another,
Spenge
Sea anemone
Simple form in
earthworm
association neurons
in earthworm
Fig. 201.—Diagram to show gradual evolution of nervous system from an epithelial cell (e)
and muscle fiber (m) in the sponge (1) to a specialized epithelial cell or receptor (r) and muscle
cell in the sea anemone (2); then to a receptor and motor neuron joining in a ganglion (Gang.),
in simple form seen in the earthworm (3). Most of the ganglia in this and other segmented
inverlebrates show also the internuncial or association neurons as indicated in 4.
but also of imprinting on the impulse a characteristic rhythmie activity
which brings about the contraction of the bell and the swimming movement
of the animal. The network now assumes the function of an adjuster
as well as a transmitter of impulses. j
So far the adjuster is an extremely simple structure, and it is possible
that the effector and receptor organs are directly connected by fibers
running through it. When we come to the segmented invertebrates
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784 THE CENTRAL NERVOUS SYSTEM
(such as the earthworm, crayfish, lobster, ete.) much more definite spe-
cialization of the adjuster occurs, for now this intermediate nervous tis-
sue becomes collected into so-called ganglia, a pair existing for each
segment and the various pairs being connected by definite
nerve structures, constituting the ganglion chain. It is in
this group of animals that we have, for the first time, def-
inite evidence of the existence of the neuron, which may be
considered as the elementary unit of which the nervous sys-
tem of all the higher animals is built. A neuron may be
either sensory or motor, and in both cases it consists of
a cell with a nucleus, one long process, called the azon,
and several short branching processes, called the den-
drites. The axon in its course may give off a branch,
or more, at right angles,—these are sometimes called
collaterals,—and at its end it may break up into very fine
branches called a synapsis. In a sensory neuron the im-
pulse is transmitted from the end of the axon to the
nerve cell, whereas in a motor neuron it is transmitted
in the opposite direction from the cell to the end of the
axon (Fig. 203).
The simplest arrangement of sensory and motor neu-
rons to constitute the nervous system is seen in the
earthworm, in which it forms the simplest type of reflex
arc (Fig. 201, No. 3). The sensory neuron has its cell
body in the skin, and its axon proceeds to one of the
segmental ganglia, in which are large nerve cells whose
thick axons pass out from the ganglion as motor fibers
to the muscles of the body wall. The dendrites of the
motor neuron and the branching of the termination of
the sensory neuron cause a very fine interlacement of
nerve fibers in the ganglia, forming a network known
as the neuropile. The sensory impulse, on reaching the
ganglion, is transmitted by the synapsis to the den-
drites, probably without the fibers actually joining to-
gether; that is, the nerve impulses pass from the one
to the other set of branches by contact rather than by
transmission through continuous tissue.
By such an arrangement it is evident that the nervous
apparatus in each segment could cause a contraction of
Fig. 202.—Dia-
gram of nervous
system of seg-
mented _inverte-
brate; a, supra-
esophageal gan-
glion; 6, subeso-
phageal ganglion;
oe, esophagus or
gullet.
the muscles of its own neighborhood, but that a stimulus applied to one re-
ceptor would be incapable of calling forth a contraction of the muscles of a
far distant segment, much less a coordinated contraction of the musculature
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THE EVOLUTION OF THE NERVOUS SYSTEM 785
of the whole animal such as would be required for locomotion. To render
this possible it is necessary that some means of communication become es-
tablished between the different segmental ganglia. This is effected by
association neurons, each of which, as the name implies, consists of a nerve
cell with its dendrites located in one ganglion and of an axon, which passes
to the next or even to some more distant ganglion, where it ends by
synapsis. The important point to note is that these association neurons
do not leave the central nervous system; they merely connect -various
ganglia.
So far the ganglia of each segment are of equal importance, but if
we examine further we shall find that at the head end of the animal
several of the ganglia become fused together to form a larger ganglion,
c
Fig. 203.—Schema of simple reflex arc; r, receptor in an epithelial membrane; a, afferent fiber; s,
synapsis; c, nerve cell of center; e, efferent fiber; m, effector organ.
a
which lies just behind the gullet, and from which fibers proceed around
the gullet to unite in front of it in another large ganglion, which usually
shows three lobes. These larger ganglia receive afferent nerve fibers
from the closely adjacent primitive sense organs for sight, sound and
smell, from structures, that is, that are really highly specialized recep-
tors. The cells of the retina and ear have been made capable of reacting
to impulses of light or sound instead of those of pain, touch or tempera-
ture, to which the receptors of the integument are especially sensitized.
They are distance receptors (projicient receptors), and it is evident that
the nerve reflexes with which they are concerned are of a higher order
than those located in the segmental ganglia themselves.
Some of the neurons of the head ganglia are merely motor and act on
the muscles of the head end of the animal, but others are purely associa-
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786 THE CENTRAL NERVOUS SYSTEM
tion neurons and proceed down the ganglion chain to terminate by
synapses in one or other of the segmental ganglia. These association
neurons exercise a dominating influence over the activities of the seg-
mental ganglia, so that they may determine the response of the animal
when its safety is threatened by some approaching enemy. When, for
example, the stimulus produced by some sight or sound of an approach-
ing enemy is received by the head ganglia, these will transmit impulses
down the ganglion chain which so influence the various nerve cells of
this chain as to produce in all of them a coordinated action for the pur-
pose. of removing the animal from danger. Even should some local
stimulant be acting on one or more of the segments, the response may be
inhibited on account of stimuli meanwhile transmitted by way of asso-
ciation neurons from the large head ganglia; in other words, the part
controlled by the segmental ganglia becomes subservient to the whole
through the dominating control of the head ganglia.
This illustrates the beginnings of the integration of the nervous system;
and as we pass to the study of the higher animals, we shall see that this
integration becomes more and more complicated, and that, as it does so,
the nerve centers acquire the power of storing away the impressions they
receive, which they may afterwards apply to regulate the refiex response.
Thus memory and volition come to find their place in the nervous inte-
gration of the animal. The afferent stimulus arriving, let us suppose,
at nerve cells controlling the movement of the leg, may fail to cause
a response of the corresponding muscles because of impulses meanwhile
transmitted by association neurons from higher memory centers, for
the animal may have learned by experience that such a movement as the
local stimulus would in itself call forth is opposed to its own best in-
terests. This experience will have been stored away in memory nerve
centers, so that, whenever the local stimulus is repeated, impulses are
discharged from the memory centers to the local nerve centers, and
the reflex response does not occur, or is much modified in nature. For
storing away these memories and for related psychologic processes of
volition, ete., the anterior portions of the nervous system in higher ani-
mals become very highly developed so as to constitute the brain, and
the simple chain of ganglia of the invertebrates is replaced by the
spinal cord. ;
As we ascend the scale of the vertebrates, the brain becomes more
and more developed, until in the higher mammalia, such as man, very
few reflex actions can occur independently of the higher centers which
are located in it. The reflex are now involves, not one nerve center,
but several, and of these the most important are located in the -brain.
There is thus no essential difference in the general nature of integra-
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THE EVOLUTION OF THE NERVOUS SYSTEM 187
tion in the nervous system of the lower as compared with the higher
animals, but there is a very distinct morphologic difference: in the lower
or invertebrate animals the ganglion nerve chain is ventral to the alimen-
tary canal, whereas in the higher or vertebrate, the spinal cord, which
takes the place of the ganglia, is dorsal to the alimentary eanal. In both
groups the head ganglia are dorsal to the alimentary canal, but in the
vertebrates these become much more definite in structure, and constitute
the brain. This morphologic difference between vertebrates and inverte-
brates is probably not so fundamental as at first sight it may appear to
be, for, as Gaskell has shown, it is possible that the alimentary eanal of
the invertebrates is really homologous with the central canal of the
spinal cord and the ventricles of the brain of the vertebrates. Accord-
ing to this observer, what has really happened in the latter group of
animals is that the ganglia have grown up so as to surround the alimen-
tary canal and so constitute a continuous structure, a new alimentary
canal being meanwhile provided by the enclosure of a space as a result
of ventral downgrowth of the body walls. Although this view has not
been generally accepted by biologists, there is no inherent reason why it
should not be accepted. It is no more to be wondered at than the well-
known fact that a new respiratory system becomes developed in the
passage from aquatic to land amphibians.
The fibers of the sensory neurons in vertebrates are collected together
to form the posterior roots of the spinal cord, and the cell bodies of these
neurons are located not on the surface, as in invertebrates, but in the
posterior root ganglia, the cells being connected to the fibers by T-shaped
junctions. The olfactory nerve is the only one in the higher vertebrates
which retains its primitive condition.
In the vertebrate animals the spinal member in the integration of the
central nervous system is the motor neuron, the fibers being collected in
the anterior roots. Toward the cell of this neuron impulses are transmitted,
not only from the segment in which it is itself located, but by way of as-
sociation neurons from other segments or from far distant parts of the
central nervous system. In other words, this motor neuron may transmit
impulses which cause the muscles to perform local independent move-
ments, which are coordinated with those of adjacent segments and which
may be of widely varying types. The motor neuron has therefore very
appropriately been called the final common path, and it will be one of our
main objects later to show the conditions under which several different
competing influences may obtain possession of this path.
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CHAPTER LXXXVII
THE PROPERTIES OF EACH PART OF THE REFLEX ARC
Having briefly traced the physiologic development of the nervous sys-
tem, we are prepared to consider in greater detail the peculiar function
of each of the parts which enter into the formation of the reflex are.
THE RECEPTOR
With the advance in animal organization is associated the development
of the ability to appreciate and discriminate between external phe-
nomena, special organs called receptors being evolved to receive the
stimuli which these occasion. Those receptors which are distributed
over the skin of the animal are called external or exteroceptors, and are
especially adapted to react to such stimuli as temperature, pressure,
and pain, but at the fore end of the animal certain receptors become more
highly specialized so as to react to stimuli coming from a distance—
that is, to stimuli that are not produced by. contact of external objects
with the surface of the animal. These specialized receptors—sometimes
ealled projicient—include the eye, the ear, and the olfactory epithelium.
Receptors are also provided in the interior of the organism for the pur-
pose of receiving stimuli dependent upon the activities of the organism
itself. They may be called internal receptors, and we may further dis-
tinguish two groups of them—namely, those which come from the sur-
faces of the mucous membranes and those which come from the sub-
stance of the various organs and tissues themselves, as, for example,
from the substance of muscle or tendon.
A receptor may be defined in a general way as a mechanism in which
some particular kind of stimulus produces changes that result in the
excitation of the nerve fiber with which the receptor is connected, al-
though the stimulus in itself is incapable of exciting the nerve fiber. In
other words, as Sherrington puts it, the receptor has the threshold of
its excitability raised to every kind of stimulus save one, toward which
it is lowered. A nerve fiber, for instance, responds to every kind of
stimulus approximately equally; a receptor will also respond to these
same stimuli, but with great inequality, since each receptor is specialized
to react to one kind of stimulus and to others only when these are very
strong.
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THE PROPERTIES OF EACH PART OF THE REFLEX ARC 789
It is often a difficult matter to determine just exactly what it is in
the nature of the stimulus that makes it capable of affecting one receptor
and not another; for example, it is often merely a question of the rate
of vibration of the stimulus. Light and heat rays are both due to
vibration of the ether which fills space. When these vibrations are
slow, they stimulate receptors that have been specialized for apprecia-
tion of temperature, but when they are rapid and exist as rays of light,
they no longer affect the temperature receptors but only the highly spe-
cialized receptors of the retina. Similar vibrations of the air in place
of the ether cause sound and stimulate the auditory receptors. It is
quite likely that the receptors in different groups of animals are attuned
to react to different rates of vibration. For example, a cat can hear
higher pitched notes than man, and it is possible that the retinas of
some animals respond to rays vibrating with a different frequency from
those to which the retina of man is adapted. In this connection it is of in-
terest to note that the touch receptors of the skin respond so promptly
to stimulation that one hundred vibrations of a tuning fork per second
can be felt as separate stimuli, whereas to the ear at this frequency the
fork emits a continuous note. The receptors of touch are therefore more
prompt in their response than the receptors of the auditory nerve.
When once the receptor has been stimulated, the impulse passes and
is transmitted to the nerve centers, where it is translated into a par-
ticular sensation. The conditions are really not unlike those which ob-
tain in the case of the various physical instruments used to receive and
convert into the electric current stimuli of heat, light, chemical energy,
ete. The receiver required to bring about this transformation must be
especially constructed in each case, that for light being the actinometer,
that for motion the dynamo, that for heat the thermopile, and that for
chemical energy the concentration cell. Each of these physical instru-
ments may be considered as a specialized receptor for the purpose of
producing an electric current out of other forms of energy.
In accepting the above analogy we must not fail to bear in mind
that very feeble stimuli are often able to set in operation nerve impulses
that are as potent as those produced by much stronger stimuli. Here
again, we have a physical analogue in the case of relay currents, in
which a feeble electric current may operate to complete the circuit from
independent sources of electric discharge and thus set in motion a much
larger amount of energy.
These general considerations of the nature of a receptor naturally
lead us to the law of the specific properties of nerve, which is to the
effect that, however excited, each nerve of special sense gives rise to
its own peculiar sensation: Thus, in whatever way the chorda tympani
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790 THE CENTRAL NERVOUS SYSTEM
nerve is stimulated (chemically, mechanieally or electrically) during its
passage across the tympanum, the sensation evoked is that of taste.
And so with the receptor; whatever the means by which it is excited,
whether by the particular kind of stimulus for which it is adapted or by
excessive intensities of other stimuli, excitation always evokes the same
sensation. If the optic nerve or retina is mechanically stimulated, as
by pressure against the outer canthus of the eye or by an electric cur-
rent, the sensation is that of light. Applying these facts to less well-
known receptors, such as those of heat and cold, it is interesting to note
that stimulation of a ‘‘cold spot’? by extreme heat or by mechanical
or electrical stimuli brings out the sensation of cold.
Properties of Epicritic and Protopathic Receptors
A valuable grouping of receptors of the skin has been demonstrated by
Head and his pupils by experiments on himself. Head found after sec-
tion of the skin nerves—of the radial nerve, for example—that deep
pressure and pain were still present in the area supplied by the nerve,
indicating that these deep sensations are carried by the sensory fibers
present in the muscular nerves. In such a paralyzed sensory region the
power of general localization is fairly good, although light, touch, tem-
perature and superficial pain are entirely absent in the overlying skin.
In the case of the fingers the nerves of deep sensibility run in the ten-
dons of the finger muscles, so that after severance of the cutaneous
nerves and tendons of the hand, all sensibility is gone.
During the regeneration of the cut nerve the cutaneous sensations re-
appear at two periods: one group, called the protopathic, begins to ap-
pear in from seven to twenty-six weeks, whereas the other, called epicritic,
does not fully appear for one or two years.?, The protophatic sensations
are of a distinctly lower order than the epicritic. When they alone are
present, there is the sensation. of pain, but not that of fine touch; tem-
perature sensations are felt when extreme degrees of heat or cold—above
38° C. or below 20° C.—are applied to the skin, but not for slight de-
grees; the power of discriminating between two points is almost entirely
absent; and the sense of localization is very imperfect. For example, the
person will often refer the point that has actually been stimulated to a
neighboring normal portion of skin. Protopathie sensibility is more or
less distributed in spots, and it is strongly ‘‘affective’’ in character, caus:
ing an intense subjective sensation. A stimulus that causes only moderate
pain under normal conditions produces in a ‘‘protopathic area’’ a pain
that may be intense.
The epicritic sensation as will be inferred from the fi 5
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THE PROPERTIES OF EACH PART OF THE REFLEX ARC 791
to finer grades of stimulation. By it we can feel the lightest touch and
can discriminate the finest grades of temperature between 26° and 37° C.
The power of localization of the stimulus_and the ability to discriminate
between two points also return with epicritic regeneration.
In the spinal cord the nerve fibers carrying one kind of sensation
are grouped together, in the sense that pain sensations, whether deep or
protopathic, run in the same column in the cord. Likewise temperature
sensations, whether protopathic or epicritic, run together.
The Peculiarities of Each of the Separate Sensations
Temperature.—The receptors for temperature are arranged in groups,
some being sensitized for heat, others for cold. These groups of receptors
are called heat and cold spots. They can be very easily detected on an
Fig. 204.—Thermoesthesiometer.
area of skin by means of a pointed hollow vessel, through which water is
made to flow at a temperature a little below or a little above that of the
skin. The instrument is called a thermo-esthesiometer. On a part of the
skin where there are no heat and cold spots, the thermo-esthesiometer will
elicit no sensation either of heat or of cold. This is charted on an outline
drawing of the part as a neutral spot. At other places it will call forth
a sensation of heat, indicating the presence of heat spots, or at others a °°
sensation of cold, indicating the presence of cold spots. It will be noted
that certain of the spots are much more reactive than others, and that those
of cold are much the more numerous (see Fig. 205). Both heat and cold
spots are most frequent at the nipples; then, in order, come the chest, the
nose, the anterior surface of the arm, and the abdomen. They are least
marked on the exposed surface of the skin, such as the face, and they are
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792 ; THE CENTRAL NERVOUS SYSTEM
also very infrequent in the scalp. They are almost absent from the mucous
membranes, which explains why one is able to swallow a liquid that is too
hot for the hand.
The acuteness of the temperature sensation, as with all the other cu-
taneous sensations, depends very much on the condition of the skin,
being most sensitive when this is at the ordinary temperature, but very
imperfect when it is either very hot or very cold. There is also very
marked adaptation of the sense. This can be very well shown by the simple
experiment of taking three vessels of water, one at a moderate tempera-
ture, one very hot and one very cold. If a finger of one hand is placed
in the hot water and a finger of the other in the cold, and they are left
there for a short time, until the skin has assumed the same temperature
as the water, and then transferred to the lukewarm water, the finger
Fig. 205.—Cold spots (A) and heat spots (B) of an area of skin of the right hand. In each
case the most intense sensations were experienced in the black areas, less intense in the lined,
and least in the dotted. The blank areas represent parts where no special sensation of either
kind was experienced. (From Goldseheider.)
transferred from the cold water will feel hot, and that transferred from
the hot water will feel cold. Temperature sensation also produces a
marked positive after-effect. Thus, if a cold coin is placed on the fore-
head and then removed, the cold sensation will persist for some time in
the area of skin on which the coin was laid.
That the receptors for heat and cold respond only to one kind of
stimulus, or if to others, only when these are excessive, can be well il-
lustrated by the experiment of touching a cold spot with a very hot ob-
ject: the sensation will be that of cold. The hot object has so pronounced
a power of stimulation that it has overstepped the threshold for heat
of the cold-adapted receptors. The sensation of cold is elicited more
promptly than that of warmth. The distinction between a warm and a
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THE PROPERTIES OF EACH PART OF THE REFLEX ARC 793
hot bath may really depend on the fact that in the latter the cold spots
are stimulated as well as those of heat. It is at least interesting to note
that the physiologic reflexes stimulated by either a cold or a very hot
bath are the same; thus, a rise of blood pressure and a contraction of the
muscles of the skin occur in both cases.
The Touch Sense.—In order to investigate the touch sense accurately,
von Frey has devised a method of using hairs of different thickness each
mounted on a different handle. The hair which produces a sensation
of touch when pressed on the skin so that it just bends is then similarly
pressed on one scale pan of a balance, and the weight required in the
other scale pan to hold the beam horizontal when the hair just bends, is
ascertained. From the diameter of the hair one can then calculate how
many grams per square millimeter are necessary to elicit the sensation
of touch. The following quantitative results have been obtained by ap-
plying von Frey’s method to different parts of the body:
Gm. per sq. mm..
Tongue and nose ......... cere cece cece tees 2
LADY tsderacacsiisie enor dpapae ana east nee agen ee Seeneadtcarans 2.5
Finger tip and forehead.........-.---+eseeeee 3
Back of finger........cccsscenn nese econeeees 5
PANG oc ohn os CRKS DEES REARS GE REEVES 7
FPOKGATIN: i yeisawitys 2g spect tad dasguleacds gs. tyeree SH eee 8
Back: of ‘hand soc sccccancwidicaa neat eawawalen 12
Calf, shoulders pax deesioncrgwacia teamed sig 16
Abdomen: -i:sxiecsoscccanaeesd sanewesyneesee es 26
Outside of thigh............ ccc cece cece eee 26
Shit Bid SOle: w.2ey-sadianda wonders oe Seawria ays Os 28
Back of PoreaxMycc esis vres caceaoraGin ees ex 33
LOIN bcvax wwe tes view ted (sneaks pe meeeea tee 48
That the sense of touch is located in spots—touch spots—can best be
demonstrated on the ealf of the leg. If this is shaved and then carefully
explored with a fairly stiff hair, it will be found that there are only
some twelve to fifteen spots in an area of a square centimeter at which
the hair can be felt. Between these spots there is no sensation of touch.
That these spots are composed of specialized receptors can be very clearly
shown by pressing a fine needle into one of them, when no pain will be
experienced but only a peculiar shotty sense of pressure.
Careful examination of the position of the touch spots will further
show that they are grouped around hair follicles, particularly on the side
from which the hair extends—the windward side, we may call it. This
fact explains the well-known experience that an object may be felt more
acutely on a hairy surface than after that surface has been shaved. The
hairs bend slightly when the object comes in contact with them, thus °
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194 THE CENTRAL NERVOUS SYSTEM
causing pressure to be exerted on the hair follicles, so that the touch
corpuscles in the neighborhood of the follicles, or perhaps the fine nerve
plexus which surrounds them, becomes excited. The influence of hairs
in increasing the touch sensation can be demonstrated by the von Frey
method; for example, in one experiment over an area of 9 square mil-
limeters of skin with hairs present, 2 milligrams were found to produce the
sensation, whereas after the hairs had been removed, it required 36 milli-
grams.
The frequency of touch corpuscles differs very much in different parts
of the body. They are most plentiful on the fingers, relatively infrequent
over the skin of the back, and very scarce in the skin; directly over bony
surfaces. They are entirely absent from the cornea, the conjunctiva
of the upper lid, and the glans penis. The adequate stimulus for touch
is evidently deformation of the surface. Pressure exerted over all the
touch corpuscles of a portion of skin is not felt. This can be demon-
strated by dipping the finger into mercury. The pressure of the mereury
is felt on the surface but not in the submerged portion of the finger.
Touch is the most responsive of all the sensations. Thus, as has already
been noted, a tuning fork ean be felt vibrating by the finger when to
the ear its note is a continuous one, and the stimuli produced by a re-
volving serrated wheel can be felt by the fingers as separate even up
to a rate of five or six hundred stimuli per second. Adaptation is also
a marked feature of the touch sense, as is the experience of every one
who has worn flannel underclothing or a plate of false teeth.
Closely related to the tactile sense is the power of discrimination be-
tween two points. This is tested by finding at what distance the two
points of a pair of calipers stand in order to be distinguished as separate.
The result in any given part of the body varies a little according to
whether the points rest on touch corpuscles and according to the rela-
tionship of the calipers to the hair follicles. On an average, however,
we may take the following distances in millimeters as being those at
which the two points can be distinguished over different areas of the
body:
mm,
Tip Of tongues. si sae%c0 veees sews ssawh sees 11
Volar surface of finger tip............000008 2.3
Dorsal of first phalanx................00008 6.8
Palm Of hand iss edad oacsaaes eeueww One aka 11.3
Back of: Wands sis cice-¢ ve whee sweated acd aiwredee eee 31.6
Back of Nk i... .iesccaeace evens svewe saws 64.0
Middle of back, upper arm and side.......... 67.1
It is clear from this list that the power of discrimination tends to
diminish in proportion to, the lessening mobility of the part. It is greatest
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 795
at the tip of the tongue and the tip of the fingers; it is least on the
relatively immobile skin of the back. These distances are much less when
the points rest on two touch corpuscles. Under these conditions, for in-
stance, the distance for the volar side of the finger tip or even for the
palm of the hand may be only one-tenth of a millimeter; and for the
arm and back it may become reduced to half a millimeter.
Localization of touch is a very accurate process, at least in the most
sensitive parts of the skin, but nevertheless it is very probably a mat-
ter of education. An evidence of this is the fact that in the much more
highly specialized retina the power of localization of objects in the visual
field is a process of education and experience. For this reason a person
from whom a congenital cataract has been removed, can not locate the
objects which he sees until after he has learned by his experience of touch,
taste, ete., to associate the portion of the retina stimulated with a certain
part of the visual field. If this is true for the retina, it is also probably
true for touch. The famous experiment of Aristotle is explicable on the
same basis. If the fingers are crossed and a marble placed between the
crossed fingers, it will be felt as double, since now it touches two skin
surfaces which have not been accustomed to touch the same object, but
educated to feel different objects. Experience associates those two skin
areas with different objects, not with the same object.
The Pain Sense.—It was at one time thought that the sensation of pain
was due to overstimulation of any kind of receptor, but it is now known that
for this, as for other skin sensations, there exist special receptors. Thus,
it is found that in certain parts of the body, such as the cornea, and to a
certain extent in the glans penis, pain receptors alone are present, and
in disease the sense of pain may be entirely abolished, whereas that of
touch remains, this condition being called analgesia. Overstimulation of
a touch spot does not, as we have seen, cause pain but only a sense of
pressure. Although pain is appreciated by special receptors, the charac--
ter of the pain is dependent on the other sense receptors simultaneously
excited; for example, a throbbing pain is due to the simultaneous pres-
sure produced by dilated blood vessels, ete. A sensation of pain accom-
panies certain reflexes of a protective nature (nociceptive reflexes, page
825), and when the reflex is absent the part is likely to suffer damage. On
this account the pain nerves may be regarded as trophic nerves. The
sense of pain may also occur in structures which are devoid of ordinary
sensibility, such as the intestine and the ureter.
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CHAPTER LXXXVIII
THE PROPERTIES OF EACH PART OF THE REFLEX
ARC (Cont’d)
THE NERVE NETWORK
In all animals above the Celenterates, no direct protoplasmic continuity
exists between the various neurons, the transmission of the nerve impulse
depending on contiguity rather than continuity of the elements that con-
stitute the reflex are. This transmission may be effected through a syn-
apsis coming in contact either with dendrites or with nerve cells. It is
extremely difficult to know whether there is really any anatomic con-
tinuity between the various fibers which form the network in the gray
matter of the central nervous system. We shall not attempt to discuss
this vexed question here, but in order that we may learn something of the
possible functions of a nerve network, we may consider that present in
the walls of the intestine (plexus of Auerbach and Meissner.) This plexus
seems to have an important function to perform in connection with the
myenteric reflex (see page 466). At least it has been shown by Meek? that
after transsection of the intestine the muscular and epithelial structures be-
come regenerated considerably earlier than the nervous plexus, but that
the myenteric reflex, which, it will be remembered, is characterized by a
wave of inhibition preceding one of contraction does not occur until after
the plexus has been regenerated.
ee
NETWORK ON SKIN NERVES
A very important type of nerve network, from the medical viewpoint,
is that which is produced close to their receptor endings by the branch-
ing of the afferent fibers of the skin. Through these branches the vas-
cular reactions following the application of an irritant to the sensory
surface take place without the intervention of any nerve cells. It used
to be thought that such reflex vasodilatation depended upon the trans-
mission of an impulse along an afferent neuron to an efferent vaso-
dilator neuron, a view strictly in consonance with the neuron hypothesis.
That such is not the case, however, is shown by the fact observed by
Ninian Bruce* that irritants such as mustard oil applied to the skin
or cornea continue to PERHHSE FBP RMRSBeL REpetion for some time after
man
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 197
section of the posterior roots of the spinal cord, but fail to do so if
the nerve fibers are cut and allowed to degenerate, or if the stimuli are
blocked by applying cocaine to the skin. What actually happens is
evidently that the impulse set up by the irritant as it travels up the
afferent fiber passes on to one of the branches above referred to, along
which it then proceeds to the blood vessels, which it causes to dilate..
That such vasodilator impulses may be transmitted down the fibers of
an afferent nerve has been confirmed by Bayliss, who found that vaso-
dilatation occurred in the hind limb when the posterior spinal roots
were stimulated (see page 234).
Post. root
gang:
ATK SS
Ploy Teletelefele [ofelefelolelsfeTe]
Fig. 206.—Diagram to show axon reflex of sensory nerve fiber of skin. A stimulus applied to
the skin is transmitted by the sensory fiber (N), part of it going to the spinal cord (SC), and
part of it passing by the collateral (C) to the arteriole (4), which it causes to dilate.
In this peripheral branching of the afferent fibers of the skin, we
have therefore a sort of neuropile which, like that of certain forms of
Celenterates (see page 782), is capable. of serving as a pathway for the
transmission of a sensory impulse to an effector organ without the in-
tervention of nerve cells. Such a reflex is known as an axon reflex, and
it is evident that it may occur through any fiber which gives off branches,
one traveling to a sensory surface, the other to some effector organ, as
oceurs in the hypogastric nerves to the bladder (see page 883).
THE SYNAPSIS
At the point of contact between a branch of one neuron and a nerve
cell of the next, we have seen that there exists a structure known as
the synapsis. Although this is described by histologists as a tuft-like
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798 THE CENTRAL NERVOUS SYSTEM
branching of the end of the axon (Fig. 207), it may really consist of a
sort of membrane—the synaptic membrane. It permits the nerve im-
pulse to pass in one direction only, from synapsis to cell. Of what this
membrane may be composed, we do not know, but there can be no
doubt as to its great functional importance in connection with the in-
tegration of the central nervous system; for example, failure of an im-
pulse to pass between two neurons may be due to retraction of the
synaptic membrane from the cell, or to alteration in its permeability to-
wards the nerve impulse, perhaps as a consequence of changes in surface
Fig. 207.—Arborization of collaterals from the posterior root fibers around the cells of the
posterior horn. A, ascending fiber in posterior columns; B, collaterals; C, cells of posterior horn;
£, synapsis. (From Ramon y Cajal.)
tension. Similar changes may also be brought about by the action of
electrolytes or by chloroform, strychnine, and other drugs. As we shall
see when we come to study the reflexes of the higher animals, there can
be little doubt that it is in the synaptic membrane that many of the
peculiarities reside which characterize conduction in a reflex are as
compared with that in a nerve trunk. The phenomena of summation,
of reciprocal inhibition, of facilitation, ete, are undoubtedly depend-
ent upon such alterations. The synapsis is also almost certainly the
seat of fatigue in the central nervous system, and it is possibly the
structure whose physiologic, acti tty posomes upset in surgical shock.
THE PROPERTIES OF EACH PART OF THE REFLEX ARC 799
THE NERVE CELL
Aside from being a meeting place of fibers coming from various
sources, the nerve cell may have other functions, such as that of rein-
forcing impulses, just as a relay may reinforce an electric current. It
is also responsible for maintaining the nutrition of the axon with which
it is connected. In the case of the posterior root fibers of higher ani-
Fig. 208—Normal cell from the anterior horn, stained to show Nissl’s granules. a, the axon.
(From Howell.)
mals, this function is probably the most important which the cell per-
forms, for it has been found by separating the ganglia from their blood
supply in the frog that, although the cells degenerate in about two
weeks, sensory impulses continue to be transmitted through the gan-
glia. Similar observations have been made in the case of the crab, in
which the cell bodies of the neurons lie on the surface of the ganglion
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800 THE CENTRAL NERVOUS SYSTEM
mass, from which they can be separated, leaving merely the neuropile,
through which, however, the reflex continues to be conveyed. After a
time, of course in this case also the reflex disappears, because
an axon can not live indefinitely after it has been separated from its
nerve cell,
These facts regarding the general function of the nerve cell arouse
our curiosity as to its morphologic structure. When nerve cells are
fixed and stained in various ways they show certain elements in the
Fig. 209.—Part of an anterior cornual cell from the calf’s spinal cord, stained to show neurofibrils.
ax, axon; a, b, c, dendrites. (From Bethe.)
cytoplasm—namely, (1) large granules or masses, which stain deeply
with basic dyes and are called Nissl bodies (Fig. 208), and (2) a fine
network of fibrils passing through the cell substance from. one process or
dendrite to another—neurofibrils (Fig. 209). These appearances in fixed
and stained preparations are possibly, however, entirely artificial; for when
nerve cells are preserved in a living state—by being suspended in some of:
the animal’s own lymph or blood serum—it is found, when they are ex-
amined by the aid of the ultramicroscope (see page 52), that the cytoplasm
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THE PROPERTIES OF EACH PART OF THE REFLEX ARC 801
is composed of a viscous fluid full of extremely minute granules, each of
which apparently consists of a colloidal solution surrounded by a lipoid
envelope (Fig. 210). When the temperature is raised, the granules dis-
appear, and when the cells are deprived of oxygen, the cytoplasm and
nucleus become swollen. ~ »Y
. uw
» Y]
yw
Motor leg area Visuosensory Visuopsychic
Fig. 220.—Three sections through different parts of the cerebral cortex.
For description see
content. (Redrawn from Starling.)
the cerebral cortex leaves yet uncharted those vast areas lying between
them which do not respond in any definite way to artificial stimulation,
and the ablation of which results only in more or less indefinite symp-
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structurally composed.
CEREBRAL LOCALIZATION 853
toms. In order to discover the exact function of these areas, it has been
necessary to employ an entirely different method—that of histologic
and embryologie examination. When the patterns of the gray cortex
are compared with the habits of the animals, in different groups of
animals (phylogenetic study), or even in different individuals of the
INTERMEDIATE
POSTCENTRAL
3
p
a
Dp
°
z
4
>
Fs
Fig. 221.—The location of the chief motor and sensory areas on the outer (A) and mesial (B)
aspects of the human brain, as determined by the microscopic structure of the cortex. These
maps are only approximately accurate, but they indicate in a general way how the cortex is
(From Starling after Campbell.)
same group (ontogenetic study), much useful knowledge concerning
cerebral localization can also be gained. In the human animal much
progress is being made by comparing the structural pattern of the cor-
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854 THE CENTRAL NERVOUS SYSTEM
tex in different parts of the normal brain with that found in the bra
of psychopathic individuals whose mental symptoms have been cat
fully studied before death.*
For the purpose of this work it is necessary to recognize sever
lamine or layers of nerve cells and nerve fibers composing the corte
The most practical division is represented in Fig. 220, and is as follow
(1) a narrow superficial layer of nerve fibers, with a few seattered cells
the outer fiber lamina or molecular layer; (2) a much wider layer of sma
medium and large pyramidal cells—the outer or pyramidal cell lamin
(3) a layer of granules—the middle cell lamina; (4) an inner lay.
of nerve fibers, sometimes containing large solitary cells (Betz cells)-
the inner fiber lamina; (5) a layer of polymorphic cells—the inner ce
lamina. This five-layer type undergoes structural modifications in +]
different regions of the cortex, and these modifications possess a di
tinct functional significance. The only part of the brain in which the
can not be recognized is the hippocampus and the pyriform lobe. Base
on the relative thickness of these layers maps of the cerebral corte
have been produced. The most important are those of Brodmann an
Campbell, of which the latter is reproduced in Fig. 221. Two r
gions can be very definitely located; namely, the precentral or Bet
cell area, and the visual or visuosensory area of Campbell. Surroundir
the visuosensory area is a definite visuopsychic area, and similarl
bordering on the precentral is the so-called intermediate precentr.
area. At the very front of the frontal lobe is the prefrontal area. Pos
central and intermediate postcentral areas are indicated, but the r
mainder of the center is undefined.
Reasoning from phylogenetic and ontogenetic data, we can assign -
each of these lamine a functional significance, which according to Bc
ton is as follows: The outer cell lamina (pyramidal cell lamina) prob
bly constitutes the ‘‘higher level’’ basis for the carrying on of the high
or psychic cerebral functions. It is a prominent feature of the hume
cortex, the last cell layer to be evolved, and the one which undergo
retrogression most readily. During development it rapidly attains m
turity in the visuosensory area, next in the visuopsychic, and only lat:
in the prefrontal region. In the visuopsychice area it is practically «
the same depth as in the visuosensory, whereas in the prefrontal region
develops according to the mental capacity of the animal. In patients e
hibiting symptoms of dementia this layer of cells is distinctly deficien
These facts indicate that the outer or pyramidal cell lamina ‘‘subserv
the psychic or associational functions of the cerebrum’’—(Bolton).
*An excellent account of the physiologic basis for such work is given by Bolton in Leonard Hil
Further Advances in Physiology. We have made free use of this article in the present review a
would strongly recommend its perusal by any who may desire further information.”
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2
CEREBRAL LOCALIZATION 855
The middle cell lamina is much hypertrophied in the so-called projec-
tion areas of the cerebrum—for example, in the visuosensory area (see
Fig. 20), where it is so thick that it is usually described as being divided
into two parts by a narrow fiber band (the line of Gennari). Diminution
in the layer occurs in the visuosensory area in long-standing cases of
atrophy. ‘‘It seems therefote primarily to subserve the function of re-
ceiving afferent impressions whether these arrive directly from the lower
sensory neurons or indirectly through other regions of the cerebrum.’’
The fifth or inner cell lamina is the first to become differentiated, and
it is of extremely constant depth in the adult.: It is not much affected in
amentia, unless when this is very severe, as in patients who are unable
to carry on the ordinary animal functions. In short, ‘‘it subserves the lower
voluntary and instinctive activities of the animal economy’’—(Bolton).
Taking the results as a whole, it appears that the region ‘of the cortex
behind the Rolandic fissure consists of: sensory areas and association
areas which may be immediately connected with them (visuopsychie and
intermediate posteentral) or more removed (in parietal lobe). The por-
tion in front of the Rolandic fissure, on the other hand, contains the
efferent areas, of which the precentral may be regarded as of lowest
grade. The motor discharges from it are conditioned upon impulses
coming partly from the adjacent intermediate precentral area, in which
again are elaborated those received from the sensory areas, and partly
from those coming from the prefrontal region, which is the most com-
plex zone of association. This last is indeed the supreme dominating
area. It coordinates or integrates the activities of the other association
areas and may be considered as the seat of the intellect. The evidence
for this high evolution of the prefrontal area is very strong. It is the
last portion of the cortex to be evolved and the first to undergo retro-
gression. In idiots and imbeciles the thickness of the pyramidal cell
layer in this region is directly proportional to the mental power, and
in dementia degrees of retrogression occur that vary directly with the
existing grade of dementia. In normal brains this layer is the very
one which varies in depth in different individuals. Along with its high
development in the brain of man, as compared with that of other ani-
mals, goes hand in hand a great increase in the other association areas.
Thought is the product of integration between these various associa-
tion areas, and articulate and written language the outward manifesta-
tion of the process. It is owing to the relatively great extent and com-
plexity.and constant development of the prefrontal lobe that man so far
excels even the highest apes in his intellectual activity, and it is owing
to the relative functional development of this lobe that individuals dif-
fer from one another in their mental powers.
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CHAPTER XCVII
CONDITIONED AND UNCONDITIONED REFLEXES
In studying the reflexes in the spinal animal, we have seen that t
effect of a given stimulus or of different stimuli is predictable wi
absolute certainty. There is a fatality in the responses. When t
higher centers are included in the reflex are, the reflexes become mo
fied or inhibited by events occurring in other parts of the central nm
vous system and the results come to be more and more unpredictab
The reflex comes to be a conditioned reflex (Pavlov). Studies of t
circumstances affecting these conditioned reflexes really constitute
study of the function of the higher centers in the brain. Since su
experiments must be performed. on the lower animals, we are limited
the investigation to motor responses, for we have no way whatever
studying the subjective sensations produced. The motor phenomena |
which the animal may express its sensations can be interpreted by
only in terms of psychologic ideas that in large part are derived frc
our own experiences. Obviously the conclusions that can be drav
are subject to very great sources of error, and it must be considered
one of the greatest advances of modern physiology that Pavlov ai
others should have succeeded in evolving methods by which we may :
rive at conclusions regarding the nature of certain of the integratio
that occur in the higher centers of the nervous system preceding t
motor manifestations by which the animal expresses its sensations.
The methods employed for the study of these higher integrations
the central nervous system all depend on the reactions of the anin
that are associated with the taking of food. When the food is aci
ally placed in the mouth, it excites a secretion of saliva, whatever t
circumstances may be. This is an unconditioned reflex. Suppose, ho
ever, that every time food is given a particular sound is made; aft
some time it will be found that the occurrence of the sound alone
sufficient to cause a secretion of saliva. In other words, a condition
reflex has been formed. Similarly, sight or smell or any other type
sensation may be made the excitant for the conditioned refiex. T
secretion now becomes psychic instead of merely physiologic. To quc
Bayliss: ‘‘Any phenomenon of the outer world for which the animal
question possesses appropriate receptors can be drawn into tempora
856
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CONDITIONED AND UNCONDITIONED REFLEXES 857
association with salivary secretion, so that it becomes an exciter of se-
eretion if only it has been frequently presented at the same time with
the unconditioned reflex stimulus, food in the mouth.”’
Work along lines similar to that devised by Pavlov has more recently
been undertaken by students of animal behavior, who have utilized the
acquired habits of an animal in searching for its food in order to study the
influence of conditioning circumstances on its procedure. The advantage
of this method depends mainly on the fact that it can be applied to all
groups of animals. In carrying out such an observation, the animal is
placed in one compartment of a cage, from which it is then released to
a second compartment, the end of which is divided into two passage-
ways, one leading to food, the other leading to some compartment in
which the animal is punished for its mistake as by receiving an electric
shock. Objects such as colored lights are placed in the different pas-
sageways, and the animal by repeated trial comes ultimately to learn
which particular colored light signifies the passage along which he
will receive food.