HOME UNIVERSITY LIBRARY 

 
 UNIVERSITY OF CALIFORNIA 
 AT LOS ANGELES 
 
 GIFT OF 
 
 Dr. ERNEST C. MOORE
 
 HOME UNIVERSITY LIBRARY 
 OF MODERN KNOWLEDGE 
 
 No. 68 
 
 Editors : 
 
 HERBERT FISHER, M.A., F.B.A. 
 PROF. GILBERT MURRAY, LlTT.D., 
 
 LL.D., F.B.A. 
 
 PROF. J. ARTHUR THOMSON, M.A. 
 PROF. WILLIAM T. BREWSTER, M.A.
 
 THE HOME UNIVERSITY LIBRARY 
 OF MODERN KNOWLEDGE 
 
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 SCIENCE 
 Already Published 
 
 ANTHROPOLOGY By R. R. MARETT 
 
 AN INTRODUCTION TO 
 
 SCIENCE By J. ARTHUR THOMSON 
 
 EVOLUTION By J. ARTHUR THOMSON and 
 
 PATRICK GEDDES 
 
 THE ANIMAL WORLD By F. W. GAMBLE 
 
 INTRODUCTION TO MATHE- 
 MATICS By A. N. WHITEHEAD 
 
 ASTRONOMY By A. R. HINKS 
 
 PSYCHICAL RESEARCH . . . . By W. F. BARRETT 
 THE EVOLUTION OF PLANTS By D. H. SCOTT 
 CRIME AND INSANITY .... By C. A. MERCIER 
 MATTER AND ENERGY . . . , By F. SODDY 
 
 PSYCHOLOGY By W. McDoucALL 
 
 PRINCIPLES OF PHYSIOLOGY By J. G. MCKENDRICK 
 THE MAKING OF THE EARTH By J. W. GREGORY 
 
 ELECTRICITY By GISBERT KAPP 
 
 MAN: A HISTORY OF THE 
 
 HUMAN BODY By A.KBITH 
 
 THE ORIGIN AND NATURE OF LIFE 
 
 By BENJAMIN MOORE 
 
 Future Issues 
 
 THE CARE OF CHILDREN ... By R. A. BENSON, M.D. 
 
 DISEASE By W. T. COUNCILMAN 
 
 PLANT LIFE By J. B. FARMER 
 
 NERVES By D. F. HARRIS 
 
 A STUDY OF SEX By J. ARTHUR THOMSON 
 
 THE MINERAL WORLD .... By SIR T. H. HOLLAND
 
 DISEASE 
 AND ITS CAUSES 
 
 BY 
 
 W. T. COUNCILMAN, A.M., M.D., LL.D. 
 
 Professor of Pathology, Harvard University 
 
 NEW YORK 
 HENRY HOLT AND COMPANY 
 
 LONDON 
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 COPYRIGHT, 191}, 
 
 BY 
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 r 
 
 PREFACE 
 
 IN this little volume the author has endeavored 
 to portray disease as life under conditions which 
 differ from the usual. Life embraces much that is 
 unknown and in so far as disease is a condition of 
 living things it too presents many problems which 
 are insoluble with our present knowledge. Fifty 
 years ago the extent of the unknown, and at that 
 time insoluble questions of disease, was much 
 greater than at present, and the problems now are 
 in many ways different from those in the past. 
 No attempt has been made to simplify the subject 
 by the presentation of theories as facts. 
 
 The limitation as to space has prevented as full 
 a consideration of the subject as would be de- 
 sirable for clearness, but a fair division into the 
 general and concrete phases of disease has been 
 attempted. Necessarily most attention has been 
 given to the infectious diseases and their causes. 
 This not only because these diseases are the most 
 important but they are also the best known and 
 give the simplest illustrations. The space given 
 to the infectious diseases has allowed a merely 
 cursory description of the organic diseases and 
 
 215143
 
 iv PREFACE 
 
 such subjects as insanity and heredity. Of the 
 organic diseases most space has been devoted to 
 disease of the heart. There is slight consideration 
 of the environment and social conditions as causes 
 of disease. 
 
 Very few authors are mentioned in the text and 
 no bibliography is given. There is lack of litera- 
 ture dealing with the general aspects of disease ; 
 the book moreover is not written for physicians, 
 and the list of investigators from whose work the 
 knowledge of disease has been derived would be 
 too long to cite. 
 
 It has been assumed that the reader has some 
 familiarity with elementary anatomy and physi- 
 ology, and these subjects have been considered 
 only as much as is necessary to set the scene for 
 the drama. I am indebted to my friend, Mr. 
 W. R. Thayer, for patiently enduring the reading 
 of the manuscript and for many suggestions as to 
 phrasing.
 
 CONTENTS 
 
 CHAPTER I 
 
 PAGE 
 DEFINITION OF DISEASE. CHARACTERISTICS OF LIVING MATTER. 
 
 CELLS AS THE LIVING UNITS. AMCEBA AS TYPE OF A UNICELLULAR 
 ANIMAL. THE RELATION OF LIVING MATTER TO ENVIRONMENT. 
 CAPACITY OF ADAPTATION TO ENVIRONMENT SHOWN BY LIVING MATTER. 
 
 INDIVIDUALITY OF LIVING MATTER. THE CAUSES OF DISEASE 
 EXTRINSIC. THE RELATION OF THE HUMAN BODY TO THE ENVIRON- 
 MENT. THE SURFACES OF THE BODY. THE INCREASE OF SURFACE 
 BY GLAND FORMATION. THE REAL INTERIOR OF THE BODY REPRE- 
 SENTED BY THE VARIOUS STRUCTURES PLACED BETWEEN THE SURFACES. 
 
 THE FLUIDS OF THE BODY. THE NERVOUS SYSTEM. THE HEART 
 AND BLOOD-VESSELS. THE CELLS OF THE BLOOD. THE DUCTLESS 
 GLANDS 9 
 
 CHAPTER H 
 
 No SHARP LINE OF DEMAHKATION BETWEEN HEALTH AND DIS- 
 EASE. THE FUNCTIONAL NUTRITIVE AND FORMATIVE ACTIVITIES 
 OF CELLS. DESTRUCTION AND REPAIR CONSTANT PROCESSES IN 
 LIVING MATTER. INJURIES TO THE BODY. THE EFFECT OF HEAT. 
 
 THE ACTION OF POISONS. THE LESIONS OF DISEASE. REPAIR. 
 
 THE LAWS GOVERNING REPAIR. RELATION OF REPAIR TO COM- 
 PLEXITY OF STRUCTURE AND AGE. THE RESERVE FORCE OF THE 
 BODY. COMPENSATORY PROCESSES IN THE BODY. OLD AGE. 
 THE DIMINUTION OF RESISTANCE TO THE EFFECTS OF THE ENVIRON- 
 MENT A PROMINENT FACTOR IN OLD AGE. DEATH. How BROUGHT 
 ABOUT. CHANGES IN THE BODY AFTER DEATH. THE RECOGNITION 
 
 OF DEATH . .... 40 
 
 CHAPTER III 
 
 THE GROWTH OF THE BODY. GROWTH MORE RAPID IN EM- 
 BRYONIC PERIOD. THE COORDINATION AND REGULATION OF GROWTH. 
 TUMORS. - THE GROWTH OF TUMORS COMPARED WITH NORMAL 
 GROWTH. SIZE, SHAPE AND STRUCTURE OF TUMORS. THE 
 GROWTH CAPACITY OF TUMORS AS SHOWN BY THE INOCULATION OF 
 TUMORS OF MICE. BENIGN AND MALIGNANT TUMORS. EFFECT OF 
 INHERITANCE. ARE TUMORS BECOMING MORE FREQUENT? THE 
 EFFECT PRODUCED BY A TUMOR ON THE INDIVIDUAL WHO BEARS IT. 
 RELATION OF TUMORS TO AGE AND SEX. THEORIES AS TO THE
 
 vi CONTENTS 
 
 PAGE 
 
 CAUSE OF TUMORS. THE PARASITIC THEORY. THE TRAUMATIC 
 THEORY. THE EMBRYONIC THEORY. THE IMPORTANCE OF THE 
 EARLY RECOGNITION AND REMOVAL OF TUMORS 64 
 
 CHAPTER IV 
 
 THE REACTIONS OF THE TISSUES OF THE BODY TO INJURIES. 
 INFLAMMATION. THE CHANGES IN THE BLOOD IN THIS. THE EMI- 
 GRATION OF THE CORPUSCLES OF THE BLOOD. THE EVIDENT 
 CHANGES IN THE INJURED PART AND THE MANNER IN WHICH THESE 
 ARE PRODUCED. HEAT, REDNESS, SWELLING AND PAIN. THE PRO- 
 DUCTION OF BLISTERS BY SUNBURN. THE CHANGES IN THE CELLS 
 OF AN INJURED PART. THE CELLS WHICH MIGRATE FROM THE 
 BLOOD VESSELS ACT AS PHAGOCYTES. THE MACROPHAGES. THE 
 MICROPHAGES. CHEMOTROPISM. THE HEALING OF INFLAMMA- 
 TION. THE REMOVAL OF THE CAUSE. CELL REPAIR AND NEW 
 FORMATION. NEW FORMATION OF BLOOD VESSELS. ACUTE AND 
 CHRONIC INFLAMMATION. THE APPARENTLY PURPOSEFUL CHAR- 
 ACTER OF THE CHANGES IN INFLAMMATION 79 
 
 CHAPTER V 
 
 INFECTIOUS DISEASES. THE HISTORICAL IMPORTANCE OF EPI- 
 DEMICS OF DISEASE. THE LOSSES IN BATTLE CONTRASTED WITH 
 THE LOSSES IN ARMIES PRODUCED BY INFECTIOUS DISEASES. THE 
 DEVELOPMENT OF KNOWLEDGE OF EPIDEMICS. THE VIEWS OF 
 HIPPOCRATES AND ARISTOTLE. SPORADIC AND EPIDEMIC DISEASKS. 
 THE THEORY OF THE EPIDEMIC CONSTITUTION. THEORY THAT 
 THE CONTAGIOUS MATERIAL is LIVING. THE DISCOVERY OF BAC- 
 TERIA BY IXEWENHCECK IN 1675. THE RELATION OF CONTAGION 
 
 TO THE THEORY OF SPONTANEOUS GENERATION. NEEDHAM AND 
 SPALLANZANI. THE DISCOVERY OF THE COMPOUND MICROSCOPE IN 
 1605. THE PROOF THAT A LIVING ORGANISM is THE CAUSE or A 
 DISEASE. ANTHRAX. THE DISCOVERY OF THE ANTHRAX BACILLUS 
 IN 1851. THE CULTIVATION OF THE BACILLUS BY KOCH. THE 
 MODE OF INFECTION. THE WORK OF PASTEUR ON ANTHRAX. 
 THE IMPORTANCE OF THE DISEASE 97 
 
 CHAPTER VI 
 
 CLASSIFICATION OF THE ORGANISMS WHICH CAUSE DISEASE. 
 BACTERIA: SIZE, SHAPE, STRUCTURE, CAPACITY FOR GROWTH, MULTI- 
 PLICATION AND SPORE FORMATION. THE ARTinriAL CULTIVATION 
 OF BACTERIA. THE IMPORTANCE OF BACTERIA IN NATURE. VA- 
 RIATIONS IN BACTERIA. SAPROPHYTIC AND PARASITIC FORMS. 
 PROTOZOA. STRUCTURE MORE COMPLICATED THAN THAT OF BACTERIA. 
 DISTRIBUTION IN NATURE. GROWTH AND MULTIPLICATION. 
 CONJUGATION AND SEXUAL REPRODUCTION. SPORE FORMATION. 
 THE NECESSITY FOB A FLUID ENVIRONMENT. THE FOOD OF PROTO- 
 ZOA. PARASITISM. THE ULTRA-MICROSCOPIC OR FILTERABLE OR- 
 GANISMS. THE LIMITATION OF THE MICROSCOPIC. PORCELAIN 
 FILTERS TO SEPARATE ORGANISMS FROM A FLUID. FOOT AND 
 MOUTH DISEASE PRODUCED BY AN ULTRA-MICROSCOPIC ORGANISM. 
 OTHER DISEASES so PRODUCED. Do NEW DISEASES APPEAR? , . 116
 
 CONTENTS vii 
 
 CHAPTER VH 
 
 PAGE 
 
 THE NATURE OF INFECTION. THE INVASION OF THE BODY 
 FROM ITS SURFACES. THE PROTECTION OF THESE SURFACES. CAN 
 BACTERIA PASS THROUGH AN UNINJURED SURFACE ? INFECTION 
 FROM WOUNDS. THE WOUNDS IN MODERN WARFARE LESS PRONE TO 
 INFECTION. THE RELATION OF TETANUS TO WOUNDS CAUSED BY 
 THE TOY PISTOL. THE PRIMARY Focus OR ATRIUM OF INFECTION. 
 THE DISSEMINATION OF BACTERIA IN THE BODY. THE DIFFERENT 
 DEGREES OF RESISTANCE TO BACTERIA SHOWN BY THE VARIOUS 
 ORGANS. MODE OF ACTION OF BACTERIA. TOXIN PRODUCTION. 
 THE RESISTANCE OF THE BODY TO BACTERIA. CONFLICT BETWEEN 
 PARASITE AND HOST. ON BOTH SIDES MEANS OF OFFENSE AND 
 DEFENSE. PHAGOCYTOSIS. THE DESTRUCTION OF BACTERIA BY 
 THE BLOOD. THE Toxic BACTERIAL DISEASES. TOXIN AND ANTI- 
 TOXIN. IMMUNITY. THE THEORY OF EHRLICH 135 
 
 CHAPTER VIII 
 
 SECONDARY, TERMINAL AND MIXED INFECTIONS. THE EXTEN- 
 SION OF INFECTION IN THE INDIVIDUAL. TUBERCULOSIS. THE TU- 
 BERCLE BACILLUS. FREQUENCY OF THE DISEASE. THE PRIMARY 
 Foci. THE EXTENSION OF BACILLI. THE DISCHARGE OF BA- 
 CILLI FROM THE BODY. INFLUENCE OF THE SEAT OF DISEASE ON THE 
 DISCHARGE OF BACILLI. THE INTESTINAL DISEASES. MODES OF 
 INFECTION. INFECTION BY SPUTUM SPRAY. INFECTION OF WATER 
 SUPPLIES. EXTENSION OF INFECTION BY INSECTS. TRYPANASOME 
 DISEASES. SLEEPING SICKNESS. MALARIA. THE PART PLAYED 
 BY MOSQUITOES. PARASITISM IN THE MOSQUITO. INFECTION A8 
 
 INFLUENCED BY HABITS AND CUSTOMS. HOOKWORM DISEASE. 
 
 INTERRELATION BETWEEN HUMAN AND ANIMAL DISEASES. PLAGUE. 
 PART PLAYED BY RATS IN TRANSMISSION. THE PRESENT EPI- 
 DEMIC OF PLAQOB 159 
 
 CHAPTER IX 
 
 DISEASE CARRIERS. THE RELATION BETWEEN SPORADIC CASES 
 OF INFECTIOUS DISEASE AND EPIDEMICS. SMALLPOX. CEREBHO- 
 BPINAL MENINGITIS. POLYOMYELITIS. VARIATION IN THE SUSCEP- 
 TIBILITY OF INDIVIDUALS. CONDITIONS WHICH MAY INFLUENCE SUS- 
 CEPTIBILITY. RACIAL SUSCEPTIBILITY. INFLUENCE OF AGE AND 
 SEX. OCCUPATION AND ENVIRONMENT. THE AGE PERIOD OF 
 INFECTIOUS DISEASES 185 
 
 CHAPTER X 
 
 INHERITANCE AS A FACTOR IN DISEASE. THE PROCESS OF CELL 
 MULTIPLICATION. THE SEXUAL CELLS DIFFER FROM THE OTHER 
 CELLS OF THE BODY. INFECTION OF THE OVUM. INTRAUTEHINE 
 INFECTION. THE PLACENTA AS A BARRIER TO INFECTION. VARI- 
 ATIONS AND MUTATIONS. THE INHERITANCE OF SUSCEPTIBILITY TO 
 DISEASE. THE INFLUENCE OF ALCOHOLISM IN THE PARENTS ON THE 
 DESCENDANTS. THE HEREDITY OF NERVOUS DISEASES. TRANS- 
 MISSION OF DISEASE Br THE FEMALE ONLY. HEMOPHILIA. THE 
 INHERITANCE OF MALFORMATIONS. THE CAUSES OF MALFORMA- 
 TIONS. MATERNAL IMPRESSIONS HAVE NO INFLUENCE. EUGENICS. 197
 
 viii CONTENTS 
 
 CHAPTER XI 
 
 PAGE 
 CHRONIC DISEASES. DISEASE OF THE HEART AS AN EXAMPLE. 
 
 THE STRUCTURE AND FUNCTION OF THE HEART. THE ACTION OF 
 THE VALVES. THE PRODUCTION OF HEART DISEASE nr INFECTION. 
 
 THE CONDITIONS PRODUCED IN THE VALVES. THE MANNER IN 
 WHICH DISEASE OF THE VALVES INTERFERES WITH THEIR FUNCTION. 
 
 THE COMPENSATION OF INJURY BY INCREASED ACTION OF HEART. 
 THE ENLARGEMENT OF THE HEART. THE RESULT OF IMPER- 
 FECT WORK OF THE HEART. VENOUS CONGESTION. DROPSY. 
 CHRONIC DISEASE OF THE NERVOUS SYSTEM. INSANITY. RELA- 
 TION BETWEEN INSANITY AND CRIMINALITY. ALCOHOLISM AND 
 SYPHILIS FREQUENT CAUSES OF INSANITY. THE DIRECT AND IN- 
 DIRECT CAUSES OF NERVOUS DISEASES. THE RELATION BETWEEN 
 SOCIAL LIFE AND NERVOUS DISEASES. FUNCTIONAL AND ORGANIC 
 DISEASE. NEURASTHENIA 219 
 
 CHAPTER' xn 
 
 THE RAPID DEVELOPMENT OF MEDICINE IN THE LAST FIFTY YEARS. 
 THE INFLUENCE OF DARWIN. PREVENTIVE MEDICINE. THE 
 DISSEMINATION OF MEDICAL KNOWLEDGE. THE DEVELOPMENT OF 
 CONDITIONS IN RECENT YEARS WHICH ACT AS FACTORS OF DISEASE. 
 FACTORY LIFE. URBAN LIFE. THE INCREASE OF COMMUNICATION 
 BETWEEN PEOPLES. THE INTRODUCTION OF PLANT PARASITES. 
 THE INCREASE IN ASYLUM LIFE. INFANT MORTALITY. WEALTH 
 AND POVERTY AS FACTORS IN DISEASE 241 
 
 GLOSSARY 250 
 
 INDEX . 252
 
 DISEASE AND ITS CAUSES 
 
 CHAPTER I 
 
 DEFINITION OF DISEASE. CHARACTERISTICS OF LIVING 
 MATTER. CELLS AS THE LIVING UNITS. AMCEBA AS 
 TYPE OF A UNICELLULAR ANIMAL. THE RELATION OF LIV- 
 ING MATTER TO THE ENVIRONMENT. CAPACITY OF ADAP- 
 TATION TO THE ENVIRONMENT SHOWN BY LIVING MATTER. 
 INDIVIDUALITY OF LIVING MATTER. THE CAUSES OF 
 DISEASE. EXTRINSIC. THE RELATION OF THE HUMAN 
 BODY TO THE ENVIRONMENT. THE SURFACES OF THE 
 BODY. THE INCREASE OF SURFACE BY GLAND FORMA- 
 TION. THE REAL INTERIOR OF THE BODY REPRESENTED 
 BY THE VARIOUS STRUCTURES PLACED BETWEEN THE SUR- 
 FACES. THE FLUIDS OF THE BODY. THE NERVOUS 
 SYSTEM. THE HEART AND BLOOD-VESSELS. THE CELLS 
 OF THE BLOOD. THE DUCTLESS GLANDS. 
 
 THERE is great difficulty, in the case of a sub- 
 ject so large and complex as is disease, in giving 
 a definition which will be accurate and compre- 
 hensive. Disease may be defined as "A change 
 produced in living things in consequence of which 
 they are no longer in harmony with their environ- 
 ment." It is evident that this conception of dis- 
 ease is inseparable from the idea of life, since only 
 a living thing can become diseased. In any dead 
 body there has been a preexisting disease or in- 
 jury, and, in consequence of the change produced, 
 that particular form of activity which constitutes 
 9
 
 10 DISEASE AND ITS CAUSES 
 
 life has ceased. Changes such as putrefaction 
 take place in the dead body, but they are changes 
 which would take place in any mass similarly 
 constituted, and are not influenced by the fact 
 that the mass was once living. Disease may also 
 be thought of as the negation of the normal. 
 There is, however, in living things no definite 
 type for the normal. An ideal normal type may 
 be constructed by taking the average of a large 
 number of individuals; but any single individ- 
 ual of the group will, to a greater or less extent, 
 depart from it. No two individuals have been 
 found in whom all the Bertillon measurements 
 agree. Disease has reference to the individual; 
 conditions which in one individual would be re- 
 garded as disease need not be so regarded in 
 another. Comparisons between health and dis- 
 ease, the normal and the abnormal, must be made 
 not between the ideal normal and abnormal, but 
 between what constitutes the normal or usual and 
 the abnormal in a particular individual. 
 
 The conception of disease is so inseparably 
 associated with that of life that a brief review of 
 the structure and properties of living things is 
 necessary for the comprehension of the definition 
 which has been given. Living matter is subject 
 to the laws which govern matter, and like matter 
 of any other sort it is composed of atoms and 
 molecules. There is no force inherent in living 
 matter, no vital force independent of and differing
 
 DISEASE AND ITS CAUSES 11 
 
 from the cosmic forces; the energy which living 
 matter gives off is counterbalanced by the energy 
 which it receives. It undergoes constant change, 
 and there is constant interchange with the environ- 
 ment. The molecules which compose it are con- 
 stantly undergoing change in their number, kind 
 and arrangement. Atom groups as decomposition 
 products are constantly given off from it, and in 
 return it receives from without other atom groups 
 with which it regenerates its substance or in- 
 creases in amount. All definitions of life convey 
 this idea of activity. Herbert Spencer says, 
 "Life is the continuous adjustment of internal 
 relations to external conditions." The molecules 
 of the substances forming the living material are 
 large, complex and unstable, and as such they 
 constantly tend to pass from the complex to the 
 simple, from unstable to stable equilibrium. 
 The elementary substances which form living 
 material are known, but it has hitherto not been 
 found possible artificially so to combine these 
 substances that the resulting mass will exhibit 
 those activities which we call the phenomena of 
 life. The distinction between living and non- 
 living matter is manifest only when the sum of the 
 activities of the living matter is considered; any 
 single phenomenon of the living may appear also 
 in the non-living material. Probably the most 
 distinguishing criterion of living matter is found 
 in its individuality, which undoubtedly depends
 
 12 DISEASE AND ITS CAUSES 
 
 upon differences in structure, whether physical 
 or chemical, between the different units. 
 
 Certain conditions are essential for the con- 
 tinued existence of living matter. It must be sur- 
 rounded by a fluid or semi-fluid medium in order 
 that there may be easy interchange with the 
 environment. It must constantly receive from 
 the outside a supply of energy in the form of 
 food, and substances formed as the result of the 
 intracellular chemical activity must be removed. 
 In the case of many animals it seems as though 
 the necessity of a fluid environment for living 
 matter did not apply, for the superficial cells 
 of the skin have no fluid around them; these 
 cells, however, are dead, and serve merely a 
 mechanical or protective purpose. All the liv- 
 ing cells of the skin and all the cells beneath 
 this have fluid around them. 
 
 Living matter occurs always in the form of 
 small masses called "cells," which are the living 
 units. The cells vary in form, structure and size, 
 some being so large that they can be seen with 
 the naked eye, while others are so small that they 
 cannot be distinctly seen with the highest 
 power of the microscope. The living thing or 
 organism may be composed of a single cell or, 
 in the case of the higher animals and plants, 
 may be formed of great numbers of cells, those 
 of a similar character being combined in masses 
 to form organs such as the liver and brain.
 
 DISEASE AND ITS CAUSES 
 
 13 
 
 In each cell there is a differentiated area con- 
 stituting a special structure, the nucleus, which 
 contains a peculiar material called "chromatin." 
 The nucleus has chiefly to do with the multi- 
 plication of the cell and contains the factors 
 which determine heredity. The mass outside of 
 the nucleus is termed "cytoplasm," and this may 
 be homogeneous in appearance or may contain 
 granules. On the outside 
 there is a more or less 
 definite cell membrane. 
 It is generally believed 
 that the cell material has 
 a semi-fluid or gelatinous 
 consistency and is con- FIQ L _ DlAQBA1I OP 
 tained within an intra- CELL. 1. Cell membrane. 
 
 cellular meshwork. It is 2 ; Cel1 sub tai ? ce <>r <?yto- 
 
 plasm. 3. Nucleus. 4. Nu- 
 an extraordinarily com- clear membrane. 5. Nu- 
 
 plex mass, whether re- cleolus - 
 
 garded from a chemical or physical point of 
 
 view. (Fig. 1.) 
 
 A simple conception of health and disease can 
 be arrived at by the study of these conditions in 
 a unicellular animal directly under a microscope, 
 the animal being placed on a glass slide. For this 
 purpose a small organism called "Amoeba" 
 (Fig. 2), which is commonly present in fresh- 
 water ponds, may be used. This appears as a 
 small mass, seemingly of gelatinous consistency 
 with a clear outline, the exterior part homogeneous,
 
 14 DISEASE AND ITS CAUSES 
 
 the interior granular. The nucleus, which is seen 
 with difficulty, appears as a small vesicle in the 
 interior. Many amoebae show also in the in- 
 terior a small clear space, the contractile vesicle 
 which alternately contracts and expands, through 
 which action the movement of the intracellular 
 fluid is facilitated and waste products removed. 
 The interior granules often change their position, 
 
 FIG. 2. AMCEBA. 1. Nucleus. 2. Contractile vesicle. 
 3. Nutritive vacuole containing a bacillus. 
 
 showing that there is motion within the mass. 
 The amoeba slowly moves along the surface of 
 the glass by the extension of blunt processes 
 formed from the clear outer portion which ad- 
 here to the surface and into which the interior 
 granular mass flows. This movement does not 
 take place by chance, but in definite directions, 
 and may be influenced. The amoeba will move 
 towards certain substances which may be placed 
 in the fluid around it and away from others. In 
 the water in which the amoebae live there are
 
 DISEASE AND ITS CAUSES 15 
 
 usually other organisms, particularly bacteria, on 
 which they feed. When such a bacterium comes 
 in contact with an amoeba, it is taken into its 
 body by becoming enclosed in processes which 
 the amoeba sends out. The enclosed organism 
 then lies in a small clear space in the amoeba, sur- 
 rounded by fluid which has been shown to differ 
 in its chemical reaction from the general fluid of 
 the interior. This clear space, which may form 
 at any point in the body, corresponds to a stomach 
 in a higher animal and the fluid within it to the 
 digestive fluid or gastric juice. After a time the 
 enclosed organism disappears, it has undergone 
 solution and is assimilated; that is, the sub- 
 stances of which its body was composed have 
 been broken up, the molecules rearranged, and 
 a part has been converted into the substance of 
 the amoeba. If minute insoluble substances, such 
 as particles of carmine, are placed in the water, 
 these may also be taken up by the amoeba; but 
 they undergo no change, and after a time they are 
 cast out. Under the microscope only the gross 
 vital phenomena, motion of the mass, motion 
 within the mass, the reception and disintegration 
 of food particles, and the discharge of inert sub- 
 stances can be observed. The varied and active 
 chemical changes which are taking place cannot 
 be observed. 
 
 Up to the present it has been assumed that 
 the environment of the amoeba is that to which
 
 16 DISEASE AND ITS CAUSES 
 
 it has become adapted and which is favorable to 
 its existence. Under these conditions its struc- 
 ture conforms to the type of the species, as do 
 also the phenomena which it exhibits, and it can 
 assimilate food, grow and multiply. If, during 
 the observation, a small crystal of salt be placed 
 in the fluid, changes almost instantly take place. 
 Motion ceases, the amoebae appear to shrink 
 into smaller compass, and they become more 
 granular and opaque. If they remain a suffi- 
 ciently long time in this fluid, they do not regain 
 their usual condition when placed again in fresh 
 water. None of the phenomena which character- 
 ized the living amoebae appear: we say they are 
 dead. After a time they begin to disintegrate, 
 and the bacteria contained in the water and on 
 which the amoebae fed now invade their tissue 
 and assist in the disintegration. By varying the 
 duration of the exposure to the salt water or the 
 amount of salt added, a point can be reached 
 where some, but not all, of the amoebae are de- 
 stroyed. Whether few or many survive depends 
 upon the degree of injury produced. Much the 
 same phenomena can be produced by gradually 
 heating the water in which the amoebae are con- 
 tained. It is even possible gradually to accustom 
 such small organisms to an environment which 
 would destroy them if suddenly subjected to it, 
 but in the process of adaptation many individuals 
 will have perished.
 
 DISEASE AND ITS CAUSES 17 
 
 It is evident from such an experiment that 
 when a living organism is subject to an environ- 
 ment to which it has not become adapted and 
 which is unfavorable, such alterations in its struc- 
 ture may be produced that it is incapable of 
 living even when it is again returned to the con- 
 ditions natural to it. Such alterations of struc- 
 ture or injuries are called the lesions of disease. 
 We have seen that in certain individuals the 
 injury was sufficient to inhibit for a time only 
 the usual manifestations of life; these returned 
 when the organism was removed from the un- 
 favorable conditions, and with this or preceding 
 it the organisms, if visibly altered, regained the 
 usual form and structure. We may regard this as 
 disease and recovery. In the disease there is both 
 the injury or lesion and the derangement of vital 
 activity dependent upon this. The cause of the 
 disease acted on the organism from without, it 
 was external to it. Whether the injurious ex- 
 ternal conditions act as in this case by a change 
 in the surrounding osmotic pressure, or by the 
 destruction of ferments within the cell, or by the 
 introduction into the cell of substances which 
 form stable chemical union with certain of its 
 constituents, and thus prevent chemical processes 
 taking place which are necessary for life, the re- 
 sult is the same. 
 
 The experiments with the amoebae show also 
 two of the most striking characteristics of living
 
 18 DISEASE AND ITS CAUSES 
 
 matter. 1. It is adaptable. Under the influ- 
 ence of unusual conditions, alterations in 
 structure and possibly in substance, may take 
 place, in consequence of which the organisms under 
 such external conditions may still exhibit the 
 usual phenomena. The organism cannot adapt 
 itself to such changes without undergoing change 
 in structure, although there may be no evidence 
 of such changes visible. This alteration of struc- 
 ture does not constitute a disease, provided the 
 harmonious relation of the organism with the 
 environment be not impaired. An individual 
 without a liver should not be regarded as diseased, 
 provided there can be such an internal adjust- 
 ment that all of the vital phenomena could go on 
 in the usual manner without the aid of this use- 
 ful and frequently maligned organ. 2. It is in- 
 dividual. In the varying degrees of exposure to 
 unfavorable conditions of a more serious nature 
 some, but not all, of the organisms are destroyed ; 
 in the slight exposure, few; in the longer, many. 
 Unfavorable conditions which will destroy all in- 
 dividuals of a species exposed to them must be 
 extremely rare. 1 There is no such individuality in 
 non-living things. In a mass of sugar grams each 
 
 1 They do, however, take place, since within compara- 
 tively few years whole species have completely disappeared; 
 for example, the great auk and the passenger pigeon. In 
 these cases it is not known what part disease played in the 
 destruction.
 
 DISEASE AND ITS CAUSES 19 
 
 grain shows just the same characteristics and re- 
 acts, in exactly the same way as all the other grains 
 of the mass. Individuality, however expressed, 
 is due to structural variation. It is almost im- 
 possible to conceive in the enormous complexity 
 of living things that any two individuals, whether 
 they be single cells or whether they be formed of 
 cell masses, can be exactly the same. It is not 
 necessary to assume in such individual differences 
 that there be any variation in the amount and 
 character of the component elements, but the 
 individuality may be due to differences in the 
 atomic or molecular arrangements. There are 
 two forms of tartaric-acid crystals of precisely 
 the same chemical formula, one of which reflects 
 polarized light to the left, and the other to the 
 right. All the left-sided crystals and all the right- 
 sided are, however, precisely the same. The 
 number of possible variations in the chemical 
 structure of a substance so complex as is proto- 
 plasm is inconceivable. 
 
 In no way is the individuality of living matter 
 more strongly expressed than in the resistance 
 to disease. The variation in the degree of resist- 
 ance to an unfavorable environment is seen in 
 every tale of shipwreck and exposure. In the 
 most extensive epidemics certain individuals are 
 spared; but here care must be exercised in inter- 
 preting the immunity, for there must be differences 
 in the degree of exposure to the cause of the
 
 20 DISEASE AND ITS CAUSES 
 
 epidemic. It would not do to interpret the im- 
 munity to bullets in battle as due to any indi- 
 vidual peculiarity, save possibly a tendency in 
 certain individuals to remove the body from the 
 vicinity of the bullets; in battle and in epidemics 
 the factors of chance and of prudence enter. No 
 other living organism is so resistant to changes in 
 environment as is man, and to this resistance he 
 owes his supremacy. By means of his intelligence 
 he can change the environment. He is able to 
 resist the action of cold by means of houses, 
 fire and clothing; without such power of in- 
 telligent creation of the immediate environment 
 the climatic area in which man could live would 
 be very narrow. Just as disease can be acquired 
 by an unfavorable environment, man can so 
 adjust his environment to an injury that harmony 
 will result in spite of the injury. The environ- 
 ment which is necessary to compensate for an 
 injury may become very narrow. For an indi- 
 vidual with a badly working heart more and more 
 restriction of the free life is necessary, until finally 
 the only environment in which life is even toler- 
 ably harmonious is between blankets and within 
 the walls of a room. 
 
 The various conditions which may act on an 
 organism producing the changes which are neces- 
 sary for disease are manifold. Lack of resistance 
 to injury, incapacity for adaptation, whether it 
 be due to a congenital defect or to an acquired
 
 DISEASE AND ITS CAUSES 21 
 
 condition, is not in itself a disease, but the disease 
 is produced by the action on such an individual 
 of external conditions which may be nothing 
 
 FIG. 3. A SECTION OP THE SKIN. 1. A hair. Notice 
 there is a deep depression of the surface to form a small bulb 
 from which the hair grows. 2. The superficial or horny 
 layer of the skin; the cells here are joined to form a dense, 
 smooth, compact layer impervious to moisture. 3. The 
 lower layer of cells. In this layer new cells are continually 
 being formed to supply those which as thin scales are cast 
 off from the surface. 4. Section of a small vein. 9. Section 
 of an artery. 8. Section of a lymphatic. The magnification 
 is too low to show the smaller blood vessels. 5. One of the 
 glands alongside of the hair which furnishes an oily secretion. 
 6. A sweat gland. 7. The fat of the skin. Notice that hair, 
 hair glands and sweat glands are continuous with the surface 
 and represent a downward extension of this. All the tissue 
 below 2 and 3 is the corium from which leather is made. 
 
 more than those to which the individuals of the 
 species are constantly subject and which produce 
 no harm.
 
 22 DISEASE AND ITS CAUSES 
 
 All of the causes of disease act on the body from 
 without, and it is important to understand the 
 relations which the body of a highly developed 
 organism such as man has with the world external 
 to him. This relation is effected by means of the 
 various surfaces of the body. On the outside is 
 the skin (Fig. 3), which surface is many times 
 
 FIG. 4. DIAGRAMMATIC SECTION OP A SURFACE SHOWING 
 
 THE RELATION OF GLANDS TO THE SURFACE. (a) Simple Of 
 
 tubular gland; (6) compound or racemose gland. 
 
 increased by the existence of glands and such 
 appendages to the skin as the hair and nails. A 
 gland, however complicated its structure, is 
 nothing more than an extension of the surface 
 into the tissue beneath (Fig. 4). In the course 
 of embryonic development all glands are formed 
 by an ingrowth of the surface. The cells which 
 line the gland surface undergo a differentiation
 
 23 
 
 in structure which enables them to perform cer- 
 tain definite functions, to take up substances 
 from the same source of supply and transform 
 them. The largest gland on the external surface 
 of the body is the mammary gland (Fig. 5) in 
 
 FlG. 5. A SECTION OF THE MAMMARY GLAND. (a) The 
 
 ducts of the gland, by which the milk secreted by the cells 
 which line all the small openings, is conveyed to the nipple. 
 All these openings are continuous with the surface of the skin. 
 On each side of the large ducts is a vein filled with blood 
 corpuscles. 
 
 which milk is produced; there are two million 
 small, tubular glands, the sweat glands, which 
 produce a watery fluid which serves the purpose 
 of cooling the body by evaporation; there are 
 glands at the openings of the hairs which produce
 
 24 DISEASE AND ITS CAUSES 
 
 a fatty secretion which lubricates the hair and 
 prevents drying, and many others. 
 
 The external surface passes into the interior 
 
 FIG. 6. PHOTOGRAPH OF A SECTION OF THE LUNG OP A 
 MOUSE, x x are the air tubes or bronchi which communicate 
 with all of the small spaces. On the walls of the partitions 
 there is a close network of blood vessels which are separated 
 from the air in the spaces by a thin membrane. 
 
 of the body forming two surfaces, one of which, 
 the intestinal canal, communicates in two places, 
 at the mouth and anus, with the external surface;
 
 DISEASE AND ITS CAUSES 25 
 
 and the other, the genito-urinary surface, which 
 communicates with the external surface at one 
 place only. The surface of the intestinal canal 
 is much greater in extent than the surface on the 
 exterior, and finds enormous extensions in the 
 lungs and in the great glands such as the liver 
 and pancreas, which communicate with it by 
 means of their ducts. The extent of surface with- 
 in the lungs is estimated at ninety-eight square 
 yards, which is due to the extensive infoldings of 
 the surface (Fig. 6), just as a large surface of 
 thin cloth can, by folding, be compressed into a 
 small space. The intestinal canal from the mouth 
 to the anus is thirty feet long; the circumference 
 varies greatly, but an average circumference of 
 three inches may safely be assumed, which would 
 give between seven and eight square feet of sur- 
 face, this being many times multiplied by adding 
 the surfaces of the glands which are connected 
 with it. A diagram of the microscopic structure 
 of the intestinal wall shows how little appreci- 
 ation of \he extent of surface the examination 
 with the naked eye gives (Fig. 7). By means of 
 the intestinal canal food or substances necessary 
 to provide the energy which the living tissue 
 transforms are introduced. This food is liquefied 
 and so altered by the action of the various fluids 
 formed in the glands of the intestine and poured 
 out on the surface, that it can pass into the in- 
 terior of the body and become available for the
 
 26 DISEASE AND ITS CAUSES 
 
 living cells. Various food residues representing 
 either excess of material or material incapable 
 of digestion remain in the intestine, and after 
 undergoing various changes, putrefactive in 
 character, pass from the anus as feces. 
 
 By means of the lungs, which represent a part 
 of the surface, the oxygen of the air, which is 
 
 a. 
 
 FlG. 7. A SECTION OF THE SMALL INTESTINE TO SHOW THE 
 
 LARGE EXTENT OF SURFACE, (a) Internal surface. The 
 small finger-like projections are the villi, and between these 
 are small depressions forming tubular glands. 
 
 indispensable for the life of the cells, is taken 
 into the body and carbonic acid removed. The 
 interchange of gases is effected by the blood, 
 which, enclosed in innumerable, small, thin-walled 
 tubes, almost covers the surface, and comes in 
 contact with the air within the lungs, taking 
 from it oxygen and giving to it carbonic acid.
 
 J6 
 
 34
 
 37 
 
 FlG. 8. A LONGITUDINAL SECTION THROUGH THE MIDDLE 
 
 OF THE BODY SHOWING THE EXTERNAL AND INTERNAL SUR- 
 FACES AND THE ORGANS. 
 
 1. The skull. 
 
 2. The brain, showing the 
 convolutions of the gray ex- 
 terior in which the nerve cells 
 are most numerous. 
 
 3. The white matter in the 
 interior of the brain formed of 
 nerve fibres which connect the 
 various parts of this. 
 
 4. The small brain or cere- 
 bellum. 
 
 5. The interior of the nose. 
 Notice the nearness of the 
 upper part of this cavity to 
 the brain. 
 
 G. The hard or bony palate 
 forming the roof of the mouth. 
 
 7. The soft palate which 
 hangs as a curtain between the 
 mouth and the pharynx. 
 
 8. The mouth cavity. 
 
 9. The tongue. 
 
 10. The beginning of the 
 gullet or oesophagus. 
 
 11. The larynx. 
 
 12. The windpipe or trachea. 
 
 13. The resophagus. 
 
 14. The thyroid gland. 
 
 15. The thymus gland or 
 sweetbread. 
 
 16. The large vein, vena 
 cava, which conveys the blood 
 from the brain and upper body 
 into the heart. 
 
 17-25. Lymph nodes; 17, of 
 the neck; 25, of the abdomen. 
 
 18. Cross section of the arch 
 of the aorta or main artery of 
 the body after it leaves the 
 heart. 
 
 19. The sternum or breast 
 bone. 
 
 20. The cavity of the heart. 
 
 21. The liver. 
 
 22. The descending aorta at 
 the back of the abdominal 
 cavity. 
 
 23. The pancreas. 
 
 24. The stomach. 
 
 26. Cross section of the in- 
 testines. 
 
 27. The urinary bladder. 
 
 28. The entrance into this of 
 the ureter or canal from the 
 kidney. 
 
 29. Cross sections of the pu- 
 bic bone. 
 
 30. The canal of the urethra 
 leading into the bladder. 
 
 31. The penis. 
 
 32. The spinal cord. 
 
 33. The bones composing the 
 spinal column. 
 
 34. The sacrum. The space 
 between this and No. 29 is the 
 pelvis. 
 
 35. The coccyx or extremity 
 of the back bone. 
 
 36. The rectum. 
 
 37. The testicles.
 
 DISEASE AND ITS CAUSES 27 
 
 The genito-urinary surface is the smallest of 
 the surfaces. In the male (Fig. 8, 27, 28, 30) 
 this communicates with the general external 
 surface by the small opening at the extremity 
 of the penis, and in the female by the opening 
 into the vagina. In its entirety it consists in a 
 surface of wide extent, comprising in the male the 
 urethra, a long canal which opens into the bladder, 
 and is continuous with ducts that lead into the 
 genital glands or testicles. The internal surface 
 of the bladder is extended by means of two long 
 tubes, the ureters, into the kidneys, and receives 
 the fluid formed in these organs. In the female 
 (Fig. 9) there is a shallow external orifice which 
 is continued into the bladder by a short canal, 
 the urethra, the remaining urinary surface being 
 the same as in the male; the external opening 
 also is extended into the short, wide tube of 
 the vagina, which is continuous with the canal of 
 the uterus. This canal is continued on both 
 sides into the Fallopian tubes or oviducts. There 
 is thus in the female a more complete separation 
 of the urinary and the genital surfaces than in 
 the male. Practically all of the waste material 
 of the body which results from cell activity and 
 is passed from the cells into the fluid about them 
 is brought by the blood to the kidneys, and re- 
 moved by these from the blood, leaving the body 
 as urine. 
 
 Between these various surfaces is the real in-
 
 28 
 
 terior of the body, in which there are many sorts 
 of living tissues, 1 each, of which, in addition to 
 
 FlG. 9. A LONGITUDINAL SECTION THROUGH THE FEMALE 
 
 PELVIS. 
 
 1. The Fallopian tube which forms the connection be- 
 
 tween the ovary and the uterus. 
 
 2. The ovary. 
 
 3. The body of the uterus. 
 
 4. The uterine canal. 
 
 5. The urinary bladder represented as empty. 
 
 6. The entrance of the ureter. 
 
 7. The pubic bone. 
 
 8. The urethra. 
 
 9. The vagina. 
 
 10. The common external opening or vulva. 
 
 11. The rectum and anus. 
 
 1 A tissue represents an aggregate of similar cells with 
 the intercellular substances in relation with these as connec- 
 tive tissue, muscular tissue, etc. Where such cell aggregates
 
 DISEASE AND ITS CAUSES 29 
 
 maintaining itself, has some function necessary for 
 the maintenance of the body as a whole. Many of 
 these tissues have for their main purpose the ad- 
 
 Fio. 10. - THE LUNGS AND WINDPIPE. Parts of the lungs 
 have been removed to show the branching of the air tubes 
 or bronchi which pass into them. All the tubes and the 
 surfaces of the lungs communicate with the inner surface 
 of the body through the larynx. 
 
 justment and coordination of the activities of the 
 different organs to the needs of the organism as 
 a whole. The activity of certain of the organs is 
 
 are localized and where the cells are arranged in structures 
 having definite form and size and performing a definite func- 
 tion, it is customary to designate such structures as organs, 
 as the brain, liver, etc.
 
 30 DISEASE AND ITS CAUSES 
 
 essential for the maintenance of life; without 
 others life can exist for a time only; and others, 
 such as the genital glands, while essential for the 
 preservation of the life of the species, are not es- 
 sential for the individual. There is a large amount 
 of reciprocity among the tissues; in the case of 
 paired organs the loss of one can be made good by 
 increased activity of the remaining, and certain 
 of the organs are so nearly alike in function that 
 a loss can be compensated for by an increase or 
 modification of the function of a nearly related 
 organ. The various internal parts are connected 
 by means of a close meshwork of interlacing 
 fibrils, the connective tissue, support and strength 
 being given by the various bones. Everywhere 
 enclosing all living cells and penetrating into the 
 densest of the tissues there is fluid. We may 
 even consider the body between the surfaces as 
 a bag filled with fluid into which the various cells 
 and structures are packed. 
 
 The nervous system (Fig. 8) represents one 
 of the most important of the enclosed organs. 
 It serves an important function, not only in regu- 
 lating and coordinating all functions, but by 
 means of the special senses which are a part of 
 it, the relations of the organism as a whole with 
 the environment are adjusted. It consists of a 
 large central mass, the brain and spinal cord, 
 which is formed in the embryo by an infolding 
 of the external surface, much in the same way
 
 DISEASE AND ITS CAUSES 31 
 
 that a gland is formed; but the connection with 
 the surface is lost in further development and it 
 becomes completely enclosed. Connected with 
 the central nervous mass, forming really a part 
 of it and developing from it, are the nerves, which 
 appear as white fibrous cords and after divid- 
 ing and subdividing, are as extremely fine mi- 
 croscopic filaments distributed to all parts of 
 the body. By means of the nerves all impres- 
 sions are conveyed to the brain and spinal cord; 
 all impulses from this, whether conscious or 
 unconscious, are conveyed to the muscles and 
 other parts. The brain is the sole organ of 
 psychical life; by means of its activity the impres- 
 sions of the external world conveyed to it through 
 the sense organs are converted into consciousness. 
 Whatever consciousness is, and on this much has 
 been written, it proceeds from or is associated 
 with the activity of the brain cells just as truly 
 as the secretion of gastric juice is due to the 
 activity of the cells of the stomach. The activity 
 of the nervous system is essential for extra-uterine 
 life; life ceases by the cessation of circulation 
 and respiration when either the whole or certain 
 small areas of its tissue are destroyed. In intra- 
 uterine life, with the narrow and unchanging en- 
 vironment of the fluid within the uterine cavity 
 which encloses the foetus, life is compatible with 
 the absence or rudimentary development of the 
 nervous system. The foetus in this condition
 
 32 DISEASE AND ITS CAUSES 
 
 may be otherwise well developed, and it would be 
 not a misuse of words to say that it was healthy, 
 since it is adjusted to and in harmony with its 
 narrow environment, but it would not be normal. 
 The intra-uterine life of the unborn child, it must 
 be remembered, is carried out by the transmis- 
 sion of energy from the mother to the foetus by 
 means of the close relation between the maternal 
 and foetal circulation. It is only when the free 
 existence demands activities not necessary in 
 intra-uterine life that existence without a central 
 nervous system becomes impossible. 
 
 It is essential in so complicated a structure as 
 the body that some apparatus should exist' to pro- 
 vide for the interchange of material. The innu- 
 merable cell units of the body must have mate- 
 rial to provide energy, and useless material which 
 results from their activity must be removed. 
 A household might be almost as much embar- 
 rassed by the accumulation of garbage and ashes 
 as by the absence of food and coal. The food, 
 which is taken into the alimentary canal and 
 converted by the digestive fluids into material 
 more directly adapted to the uses of cells, must 
 be conveyed to them. A supply of oxygen is 
 essential for the life of the cells, and the supply 
 which is given by respiration must be carried 
 from the lungs to every cell of the body. All 
 this is effected by the circulation of the blood, 
 which takes place in the system of branching
 
 DISEASE AND ITS CAUSES 
 
 33 
 
 closed tubes in which the blood remains (Fig. 
 11). Certain of these tubes, the arteries, have 
 strong and elastic walls and serve to convey and 
 
 FlG. 11. A DIAGRAMMATIC VIEW OF THE BLOOD VESSELS. 
 
 An artery (a) opens into a system of capillaries, (c) and after 
 passing through these collects into a vein (6). Notice that 
 the capillaries connect with other vascular territories at 
 numerous points (d). If the artery (a) became closed the 
 capillaries which it supplies could be filled by blood coming 
 from other sources. 
 
 distribute the blood to the different organs and 
 tissues. From the ultimate branches of the ar- 
 teries the blood passes into a close network of 
 tubes, the capillaries, which in enormous numbers
 
 34 DISEASE AND ITS CAUSES 
 
 are distributed in the tissues and have walls so 
 thin that they allow fluid and gaseous interchange 
 between their contents and the fluid around them 
 to take place. The blood from the capillaries is 
 then collected into a series of tubes, the veins, by 
 which it is returned to the heart. This circulation 
 is maintained by means of a pumping organ or 
 heart, which receives the blood from the veins and 
 by the contraction of its powerful walls forces this 
 into the arteries, the direction of flow being deter- 
 mined as in a pump, by a system of valves. The 
 waste products of cell life pass from the cells into 
 the fluid about them, and are in part directly re- 
 turned into the blood, but for the greater part 
 pass into it indirectly through another set of ves- 
 sels, the lymphatics. These are thin-walled tubes 
 which originate in the tissues, and in which there 
 is a constant flow towards the heart, maintained 
 by the constant but varying pressure of the 
 tissue around them, the direction of flow being 
 maintained by numerous valves. The colorless 
 fluid within these vessels is termed "lymph." At 
 intervals along these tubes are small structures 
 termed the lymph nodes, which essentially are 
 filters, and strain out from the fluid substances 
 which might work great injury if they passed 
 into the blood. Between the capillary vessels 
 and the lymphatics is the tissue fluid, in which 
 all the exchange takes place. It is constantly 
 added to by the blood, and returns fluid to the
 
 DISEASE AND ITS CAUSES 35 
 
 blood and lymph; it gives material to the cells 
 and receives material from them. 
 
 In addition to the strength and elasticity of 
 the wall of the arteries, which enables them to 
 resist the pressure of the blood, they have the 
 power of varying their calibre by the contraction 
 or expansion of their muscular walls. Many of 
 the organs of the body function discontinuously, 
 periods of activity alternating with comparative 
 repose; during the period of activity a greater 
 blood supply is demanded, and is furnished by 
 relaxation of the muscle fibres which allows the 
 calibre to increase, and with this the blood flow 
 becomes greater in amount. Each part of the 
 body regulates its supply of blood, the regulation 
 being effected by means of nerves which control 
 the tension of the muscle fibres. The circulation 
 may be compared with an irrigation system in 
 which the water supply of each particular field 
 is regulated not by the engineer, but by an auto- 
 matic device connected with the growing crop 
 and responding to its demands. 
 
 The blood consists of a fluid, the blood plasma, 
 in which numerous cells are contained. The most 
 numerous of these are small cup-shaped cells 
 which contain a substance called haemoglobin, 
 to which the red color of the blood is due. There 
 are five million of these cells in a cubic millimeter 
 (a millimeter is .03937 of an inch), giving a total 
 number for the average adult of twenty-five
 
 36 
 
 DISEASE AND ITS CAUSES 
 
 trillion. The surface area of all these, each being 
 one thirty-three hundredth of an inch in diam- 
 eter, is about thirty-three hundred square yards. 
 The haemoglobin which they contain combines 
 in the lungs with the oxygen in the inspired air, 
 and they give up this indispensable substance to 
 the cells everywhere in the body. There are also 
 
 FIG. 12. THE VARIOUS CELLS IN THE BLOOD, (a) The 
 red blood cells, single and forming a roll by adhering to one 
 another; (6) different forms of the white blood cells; those 
 marked " 1 " are the most numerous and are phagocytic for 
 bacteria. 
 
 eight thousand leucocytes or colorless cells in a 
 cubic millimeter of blood, this giving a total 
 number of four billion in the average adult, and 
 these vary in character and in relative numbers 
 (Fig. 12). The most numerous of these are round 
 and slightly larger than the red cells; they have 
 a nucleus of peculiar shape and contain granules 
 of a definite character. These cells serve an im-
 
 DISEASE AND ITS CAUSES 37 
 
 portant part in infectious diseases in devouring 
 and destroying parasites. They have power of 
 active independent motion and somewhat re- 
 semble certain of the free living unicellular organ- 
 isms. The blood plasma, when taken from the 
 vessels, clots or passes from a fluid into a gelati- 
 nous or semi-solid condition, which is due to the 
 formation within it of a network of fine threads 
 termed fibrin. It is by means of the clotting of the 
 blood that the escape of blood from ruptured 
 vessels is arrested. 
 
 Several of the organs of the body, in addition 
 to the formation of secretions which are dis- 
 charged on the surfaces by means of then- ducts, 
 produce also substances which pass directly into 
 the blood or lymph, and have an influence in 
 stimulating or otherwise regulating the activity 
 of other organs. There are also certain organs 
 of glandular structure which are called the 
 ductless glands; these are not connected with 
 the surface and all their secretion passes into the 
 blood. It is a part of recent knowledge that the 
 substances produced in these glands are of great 
 importance for the body, some of them even 
 essential for the maintenance of life. In front 
 of the neck is such an organ, the thyroid gland 
 (Fig. 8, 14). Imperfect development or absence 
 of this organ, or an inactive condition of it, pro- 
 duces in the child arrested growth and deficient 
 mental development known as cretinism, and in 
 
 215143
 
 38 DISEASE AND ITS CAUSES 
 
 the adult the same condition gives rise to mental 
 deterioration, swelling of the skin, due to a greater 
 content of water, and loss of hair. This deficiency 
 in the production of thyroid secretion can be 
 made good and the symptoms removed by feed- 
 ing the patient with similar glands removed from 
 animals. The very complex disease known as 
 exophthalmic goitre, and shown by irregular 
 and rapid action of the heart, protruding eye- 
 balls and a variety of mental symptoms, is also 
 associated with this gland, and occasioned not 
 by a deficiency but by an excess or perversion of 
 its secretion. 
 
 Adjoining the thyroid there are four small 
 glands, the parathyroids, each about the size 
 of a split pea. The removal of these glands in 
 animals produces a condition resembling acute 
 poisoning accompanied by spasmodic contraction 
 of the muscles. A small glandular organ at the 
 base of the brain, the pituitary body, produces 
 a secretion, one of the most marked properties 
 of which is a .control of growth, particularly 
 that of the bones. Most cases of giantism, 
 combined as they are with imperfect mentality, 
 are due to disease of this gland. There are glands 
 near the kidney which regulate the pressure 
 of the blood in the arteries by causing contraction 
 of their muscular walls. The sexual character- 
 istics in the male and female are due to an inter- 
 nal secretion produced by the respective sexual
 
 DISEASE AND ITS CAUSES 39 
 
 glands which affects growth, body development 
 and mentality. 
 
 So is the body constituted. A series of surfaces, 
 all connected, of enormous size, which enclose 
 a large number of organs and tissues, the activities 
 of which differ, but all are coordinated to serve 
 the purposes of the organism as a whole. We 
 should think of the body not as an assemblage 
 of more or less independent entities, but as a single 
 organism in which all parts are firmly knit to- 
 gether both in structure and in function, as are 
 the components of at single cell.
 
 CHAPTER II 
 
 No SHAKP LINE OF DEMARKATION BETWEEN HEALTH AND 
 DISEASE. THE FUNCTIONAL NUTRITIVE AND FORMA- 
 TIVE ACTIVITIES OF CELLS. DESTRUCTION AND REPAIR 
 CONSTANT PROCESSES IN LIVING MATTER. INJURIES TO 
 THE BODY. THE EFFECT OF HEAT. THE ACTION OF 
 POISONS. THE LESIONS OF DISEASE. REPAIR. THE 
 LAWS GOVERNING REPAIR. RELATION OF REPAIR TO 
 COMPLEXITY OF STRUCTURE AND AGE. THE RESERVE 
 FORCE OF THE BODY. COMPENSATORY PROCESSES IN THE 
 BODY. OLD AGE. THE DIMINUTION OF RESISTANCE 
 TO THE EFFECT OF THE ENVIRONMENT A PROMINENT 
 FACTOR IN OLD AGE. DEATH. How BROUGHT ABOUT. 
 CHANGES IN THE BODY AFTER DEATH. THE RECOG- 
 NITION OF DEATH. 
 
 THERE is no sharp line separating health from 
 disease; changes in the tissues of the same nature, 
 or closely akin to those which are found in dis- 
 ease, are constantly occurring in a state of health. 
 The importance of parasites in causing disease 
 has led to the conception of disease as almost 
 synonymous with parasitism; but it must be 
 remembered that the presence of parasites living 
 at the expense of the body is perfectly consistent 
 with a state of health. Degeneration, decay and 
 parasitism only become disease factors when the 
 conditions produced by them interfere with the 
 life which is the normal or usual for the individual 
 
 concerned. 
 
 40
 
 DISEASE AND ITS CAUSES 41 
 
 All the changes which take place in the cells 
 are of great importance in conditions of both 
 health and disease, for life consists in coordinated 
 cell activity. The activities of the cells can be 
 divided into those which are nutritive, those 
 which are functional and those which are forma- 
 tive. In the functional activity the cell gives 
 off energy, this loss being made good by the 
 receipt of new energy in the form of nutritive 
 material with which the cell renews itself. In 
 certain cells an exact balance seems to be main- 
 tained, but in those cells whose activity is periodic 
 function takes place at the expense of the cell 
 substance, the loss being restored by nutrition 
 during the period of repose. This is shown 
 particularly well in the case of the nerve cells 
 (Fig. 13). Both the functional and nutritive 
 activity can be greatly stimulated, but they 
 must balance; otherwise the condition is that 
 of disease. 
 
 The formative activity of cells is also essential 
 to the normal state. Destruction of cells is con- 
 stantly taking place in the body, and more rapidly 
 in certain tissues than in others. Dried and dead 
 cells are constantly and in great numbers thrown 
 off from the surface of the skin: such epidermic 
 appendages as the hair and nails grow and are 
 removed, millions of cells are represented in the 
 beard which is daily removed. Cells are con- 
 stantly being destroyed on the intestinal surface
 
 42 DISEASE AND ITS CAUSES 
 
 and in the glands. There is an enormous de- 
 struction of the blood cells constantly taking 
 place, certain essential pigments, as that of the 
 bile, being formed from the haemoglobin which 
 the red blood corpuscles contain and which 
 becomes available on their destruction. All 
 such loss of cells must be made good by the 
 
 FIG. 13. NERVE CELLS OP AN ENGLISH SPARBOW. (a) 
 Cells after a day's full activity; (6) cells after a night's repose. 
 In (a) the cells and nuclei are shrunken and the smaller clear 
 spaces in the cells are smaller and less evident than in (6). 
 (Hodge.) 
 
 formation of new ones and, as in the case of the 
 nutritive and functional activity, the loss and 
 renewal must balance. The formative activity 
 of cells is of great importance, for it is by means 
 of this that wounds heal and diseases are recovered 
 from. This constant destruction and renewal 
 of the body is well known, and it is no doubt 
 this which has given rise to the belief, widely
 
 DISEASE AND ITS CAUSES 43 
 
 held, that the body renews itself in seven years and 
 that the changes impressed upon it by vaccina- 
 tion endure for this period only. The truth is 
 that the destruction and renewal of most tissues 
 in the body takes place in a much shorter interval, 
 and, as we shall see, this has nothing to do with 
 the changes concerned in vaccination. All these 
 activities of the cells vary in different individuals, 
 in different parts and at different ages. 
 
 The lesions or injuries of the body which form 
 so prominent a part of disease vary in kind, 
 degree and situation, depending upon the char- 
 acter of the injurious agent, the duration of its 
 action and the character of the tissue affected. 
 The most obvious injuries are those produced 
 by violence. By a cut, blood vessels are severed, 
 the relations of tissues disturbed, and at the 
 gaping edges of the wound the tissue usually 
 protected by the skin is exposed to the air, re- 
 sulting in destruction of the cells contained in 
 a thin layer of the surface. -The discoloration 
 and swelling of the skin following a blow is due 
 to rupture of vessels and escape of blood and 
 fluid, and further injury may result from the 
 interruption of the circulation. 
 
 By the application of heat the tissue may be 
 charred and the albumen of the blood and tissue 
 fluids coagulated. Living cells are very sus- 
 ceptible to the action of heat, a temperature 
 of 130 degrees being the thermal death point,
 
 44 DISEASE AND ITS CAUSES 
 
 and even lower temperatures are fatal when their 
 action is prolonged. The action of the heat may 
 produce definite coagulation of the fluid within 
 the cells in the same way that the white of an 
 egg is coagulated. Certain of the albumens 
 of the body coagulate at a much lower tem- 
 perature than the white of the egg (as the 
 myosin, one of the albumens of the muscle which 
 coagulates at 115 F., egg white coagulating at 
 158 F.), and in addition to such coagulation or 
 without it the ferments within the cell and to the 
 action of which cellular activity is due may be 
 destroyed. 
 
 In diseases due to parasites, the parasite pro- 
 duces a change in the tissue in its immediate 
 vicinity often so great as to result in the death 
 of the cells. The most general direct cause of 
 lesions is toxic or poisonous substances, either 
 introduced from without or formed in the body. 
 In the case of the parasitic diseases the mere 
 presence of the parasite in the body produces 
 little or no harm, the injury being caused by 
 poisons which it produces, and which act both 
 locally in the vicinity of the parasite and at a 
 distance, being absorbed and entering the blood 
 stream. How certain of the poisonous substances 
 act is easy to see. Strong caustics act by coagu- 
 lating the albumen, or by the withdrawal of 
 water from the cell. Other poisons act by form- 
 ing stable chemical compounds with certain
 
 DISEASE AND ITS CAUSES 45 
 
 of the cell constituents and thereby preventing 
 the usual chemical processes from taking place. 
 Death from the inhalation of illuminating gas 
 is due to the carbon monoxide contained in this, 
 forming a firm chemical union with the haemo- 
 globin of the red corpuscles so that the function 
 of these as oxygen carriers is stopped. 
 
 In order that most poisons may act, it is es- 
 sential that they enter into the cell, and they 
 cannot do this unless they are able to combine 
 chemically with certain of the cell constituents. 
 To this is due the selective action of many 
 poisons. Morphine, for example, acts chiefly 
 on the cells of the brain; strychnine acts on 
 the cells of the spinal cord which excite mo- 
 tion and thus causes the characteristic muscular 
 spasm. The poisonous substances produced by 
 bacteria, as in the case of diphtheria, act on cer- 
 tain of the organs only. Different animal species 
 owe their immunity to certain poisons to their 
 cells being so constituted that a poison cannot 
 gain entrance into them; pigeons, for example, 
 cannot be poisoned by morphia. Individual varia- 
 tions play an important part also; thus, shell- 
 fish are poisonous for certain individuals and not 
 so for others. Owing to the variability of living 
 structures a substance may be poisonous at one 
 time and not at another, as the following example 
 shows. A man, very fond of crab meat, was once 
 violently poisoned after eating crabs, being at that
 
 46 DISEASE AND ITS CAUSES 
 
 time seemingly in his usual state of health, and 
 no illness resulted in others who had partaken of 
 the same crabs. Two months later a hearty meal 
 of crabs produced no ill result. There are also 
 individuals so constituted that so simple a food as 
 the egg is for them an active poison. 
 
 The lesions produced by the action of injurious 
 conditions are usually so distinctive in situation 
 and character that by the examination of the 
 body after death the cause of death can be ascer- 
 tained. The lesions of diseases may be very 
 obvious to the naked eye, or in other cases 
 only the most careful microscopic examination 
 can detect even the presence of alterations. In 
 the case of poisons the capacity of the cell for 
 adaptation to unusual conditions is of great 
 importance. It is probable that certain changes 
 take place within the cells, owing to which the 
 function can be continued in spite of the unusual 
 conditions which the presence of the poison 
 brings about. It is in this way that the habitual 
 use of such poisons as morphine, alcohol and 
 tobacco, to speak only of those best known, is 
 tolerated. The cell life can become so accustomed 
 to the presence of poisons that the cell activities 
 may suffer in their absence. 
 
 Repair of the injuries which the body receives 
 is effected in a variety of ways. We do not know 
 how intracellular repair takes place, but most 
 probably the cells get rid of the injured areas
 
 DISEASE AND ITS CAUSES 47 
 
 either by ejecting them, or chemical changes are 
 produced in the altered cell substance breaking 
 up and recombining the molecules. When single 
 cells are destroyed, the loss is made good by new 
 formation of cells, the cell loss stimulating the 
 formative activity of the cells in the vicinity. 
 The body maintains a cell and tissue equilibrium, 
 and a loss is in most cases repaired. The blood 
 fluid lost in a haemorrhage is quickly restored 
 by a withdrawal of the fluid from the tissues into 
 the blood, but the cells lost are restored by new 
 formation of cells in the blood-forming organs. 
 The blood cells are all formed in bone marrow and 
 in the lymph nodes, and not from the cells which 
 circulate in the blood, and the stimulus to new 
 cell formation which the loss of blood brings 
 about affects this remote tissue. 
 
 In general, repair takes place most easily in 
 tissues of a simple character, and where there 
 is the least differentiation of cell structure for 
 the purposes of function. A high degree of func- 
 tion in which the cell produces material of a 
 complex character necessitates a complex chemi- 
 cal apparatus to carry this out, and a complicated 
 mechanism is formed less easily than a simple 
 one. In certain tissues the cells have become 
 so highly differentiated that all formative activity 
 is lost. Such is the case in the nerve cells of the 
 brain and spinal cord, a loss in which tissue is 
 never repaired by the formation of new cells;
 
 48 DISEASE AND ITS CAUSES 
 
 and in the muscles the same is true. The least 
 differentiation is seen in those cells which serve 
 the purpose of mechanical protection only, as 
 the cells of the skin, and in these the formative 
 activity is very great. Not only must the usual 
 loss be supplied, but we are all conscious of slight 
 injuries of the surface which are quickly repaired. 
 
 Repair, other things being equal, takes place 
 more easily in the young than in the old. New 
 formation of cells goes on with great rapidity 
 in intra-uterine life, the child, beginning its exist- 
 ence as a single cell one two hundred and fiftieth 
 of an inch in diameter, attains in nine months 
 a weight of seven pounds. The only similar 
 rapidity of cell formation is seen in certain tumors; 
 although the body may add a greater amount 
 of weight and in a shorter time, by deposit of fat, 
 this in but slight measure represents a new forma- 
 tion of tissue, but is merely a storage of food 
 material in cells. The remarkable repair and 
 even the new formation of entire parts of the 
 body in the tadpole will not take place in the 
 completely developed frog. 
 
 Repair will also take place the more readily 
 the less complicated is the architectural structure 
 of the part affected. When a series of tissues 
 variously and closely related to one another 
 enter into the structure of an organ, there may 
 be new formation of cells; but when the loss in- 
 volves more than this, the complicated architec-
 
 DISEASE AND ITS CAUSES 49 
 
 tural structure will not be completely replaced. 
 A brick which has been knocked out of a building 
 can be easily replaced, but the renewal of an area 
 of the wall is more difficult. In the kidney, for 
 example, the destruction of single cells is quickly 
 made good by new cell formation, but the loss 
 of an area of tissue is never restored. In the liver, 
 on the other hand, which is of much simpler 
 construction, large areas of tissue can be newly 
 formed. For the formation of new cells in a part 
 there must be a sufficient amount of formative 
 material; then the circulation of the blood be- 
 comes more active, more blood being brought to 
 the part by dilatation of the vessels supplying it. 
 Repair after a loss can be perfect or imperfect. 
 The tissue lost can be restored so perfectly that 
 no trace of an injury remains; but when the 
 loss has been extensive, and in a tissue of complex 
 structure, complete restoration does not take 
 place and a less perfect tissue is formed which 
 is called a scar. Examination of the skin in 
 almost anyone will show some such scars which 
 have resulted from wounds. They are also found 
 in the internal organs of the body as the result 
 of injuries which have healed. The scar repre- 
 sents a very imperfect repair. In the skin, for 
 example, the scar tissue never contains such 
 complicated apparatus as hair and sweat glands; 
 the white area is composed of an imperfectly 
 vascularized fibrous tissue which is covered with
 
 50 DISEASE AND ITS CAUSES 
 
 a modified epidermis. The scar is less resistant 
 than the normal tissue, injury takes place more 
 easily in it and heals with more difficulty. 
 
 Loss brought about by the injuries of disease 
 can be compensated for, even when the healing 
 is imperfect, by increased function of similar 
 tissue in the body. There always seems to be 
 in the body under the usual conditions a reserve 
 force, no tissue being worked to its full capa- 
 city. Meltzer has compared the reserve force of 
 the body to the factor of safety in mechanical 
 construction. A bridge is constructed to sus- 
 tain the weight of the usual traffic, but is in 
 addition given strength to meet unusual and 
 unforeseen demands. The stomach provides 
 secretion to meet the usual demands of digestion, 
 but can take care of an unusual amount of food. 
 The work of the heart may be doubled by severe 
 exertions, and it meets this demand by increased 
 force and rapidity of contraction; and the same 
 is true of the muscles attached to the skeleton. 
 The constant exercise of this reserve force breaks 
 down the adjustment. If the weight of the traffic 
 over the bridge be constantly all that it can 
 carry, there quickly comes a time when some 
 slight and unforeseen increase of weight brings 
 disaster. The conditions in the body are rather 
 better than in the case of the bridge, because 
 with the increased demand for activity the 
 heart, for example, becomes larger and stronger,
 
 DISEASE AND ITS CAUSES 51 
 
 and reserve force rises with the load to be carried, 
 but the ratio of reserve force is diminished. 
 
 This discussion of injury and repair leads to 
 the question of old age. Old age, as such, should 
 not be discussed in a book on disease, for it is 
 not a disease; it is just as natural to grow old 
 and to die as it is to be born. Disease, however, 
 differs in many respects in the old as compared 
 with the young and renders some discussion of 
 the condition necessary. Changes are constantly 
 taking place in the body with the advance of 
 years, and in the embryo with the advance of 
 days. In every period of life in the child, in the 
 adult, in the middle-aged and in the old we meet 
 with conditions which were not present at earlier 
 periods. There is no definite period at which 
 the changes which we are accustomed to regard 
 as those of old age begin. This is true of both 
 the external appearances of age and the internal 
 changes. One individual may be fully as old, 
 as far as is indicated by the changes of age, at 
 fifty as another at eighty. 
 
 With advancing age certain organs of the 
 body atrophy; they become diminished in size, 
 and the microscopic examination shows absence 
 or diminished numbers of the cells which are 
 peculiar to them. The most striking example 
 of this is seen in the sexual glands of females, 
 and, to a less degree, in those of the male. There 
 is a small mass or glandular tissue at the root
 
 52 DISEASE AND ITS CAUSES 
 
 of the neck, the thymus, which gradually grows 
 from birth and reaches its greatest size at the 
 age of fifteen, when it begins slowly to atrophy 
 and almost disappears at the age of forty. This 
 is the gland which in the calf is known as the 
 sweetbread and is a delicious and valued article 
 of food. The tonsils, which in the child may be 
 so large as to interfere with breathing and swallow- 
 ing, have almost disappeared in the adult; and 
 there are other such examples. 
 
 In age atrophy is a prominent change. It is 
 seen in the loss of the teeth, hi the whitening 
 and loss of the hair, in the thinning of the skin 
 so that it more easily wrinkles, in the thinning 
 and weakening of the muscles so that there is 
 not only diminished force of muscular contrac- 
 tion, but weakening of the muscles of support. 
 The back curves from the action of gravity, 
 the strength of the support of the muscles at the 
 back not counteracting the pull of the weight 
 of the abdominal viscera in front. The bones 
 become more porous and more brittle. 
 
 The effect of atrophy is also seen in the dim- 
 inution of all functions, and in loss of weight in 
 individual organs. That the brain shares in the 
 general atrophy is evident both anatomically and 
 in function. Mental activity is more sluggish, 
 impressions are received with more difficulty, 
 their accuracy may be impaired by accompany- 
 ing changes in the sense organs, and the con-
 
 DISEASE AND ITS CAUSES 53 
 
 cepts formed from the impressions may differ 
 from the usual. The slowness of mental action 
 and the diminution in the range of mental ac- 
 tivity excited by impressions, and the slowness 
 of expression, may give a false idea of the value 
 of the judgment expressed. The expression 
 changes, the face becomes more impassive 
 because the facial muscles no longer reflect 
 the constant and ever changing impressions 
 which the youthful sense organs convey 
 to a youthful and active brain. That the 
 young should ape the old, should seek to 
 acquire the gravity of demeanor, to restrain the 
 quick impulse, is not of advantage. Loss of 
 weight of the body as a whole is not so appar- 
 ent, there being a tendency to fat formation 
 owing to the non-use of fat or fat-forming material 
 which is taken into the body. One of the most 
 evident alterations is a general diminution in 
 the fluid of the tissues, to which is chiefly due the 
 lack of plumpness, the wrinkles of age. The 
 facial appearance of age is given to an infant 
 when, in consequence of a long-continued diar- 
 rhea, the tissues become drained of fluid. Every 
 market-man knows that an old animal is not so 
 available for food, the tissues are tougher, more 
 fibrous, not so easily disintegrated by chewing. 
 This is due to a relative increase in the connective 
 tissue which binds all parts together and is repre- 
 sented in the white fibres of meat.
 
 54 DISEASE AND ITS CAUSES 
 
 Senile atrophy is complex in its causes and 
 modes of production. The atrophy affects dif- 
 ferent organs in different degree and shows great 
 variation in situation, in degree and in progress. 
 Atrophic changes of the blood vessels are of 
 great importance, for this affects the circulation 
 on which the nutrition of all tissues depends. 
 While there is undoubted progressive wear of 
 all tissues, this becomes most evident in the case 
 of the blood vessels of the body. It is rare that 
 arteries which can be regarded as in all respects 
 normal are found in individuals over forty, and 
 these changes progress rapidly with advancing 
 age. So striking and constant are these vascular 
 changes that they seem' almost in themselves 
 sufficient to explain the senile changes, and this 
 has been frequently expressed in the remark that 
 age is determined not by years, but by the con- 
 dition of the arteries. Comparative studies show 
 the falsity of this view, for animals which are 
 but little or not at all subject to arterial disease 
 show senile changes of much the same character 
 as those found in man. 
 
 There is another condition which must be 
 considered in a study of causes of age. In the 
 ordinary course of life slight injuries are con- 
 stantly being received and more or less perfectly 
 repaired. An infection which may but slightly 
 affect the ordinary well-being of the individual 
 may produce a considerable damage. Excess
 
 DISEASE AND ITS CAUSES 55 
 
 or deficiency or improper food, occasional or 
 continued use of alcohol and other poisons may 
 lead to very definite lesions. Repair after injury 
 is rarely perfect, the repaired tissue is more sus- 
 ceptible to injury, and with advancing age there 
 is constant diminution in the ease and perfection 
 of repair. The effect of the sum of all these 
 changes becomes operative: a vicious circle is 
 established in which injury becomes progressively 
 easier to acquire and repair constantly less per- 
 fect. There is some adjustment, however, in 
 that the range of activities is diminished, the 
 environment becomes narrower and the organism 
 adapts its life to that environment which makes 
 the least demands upon it. 
 
 Whether there is, entirely apart from all con- 
 ditions affecting nutrition and the effect of 
 injuries which disturb the usual cell activities, 
 an actual senescence of the cells of the body is 
 uncertain. In the presence of the many factors 
 which influence the obvious diminution of cell ac- 
 tivity hi the old, it is impossible to say whether 
 the loss of cell activity is intrinsic or extrinsic. 
 The life of the plant cell seems to be immortal; 
 it does not grow old. Trees die owing to acci- 
 dents or because the tree acquires in the course of 
 its growth a mass of tissue in which there is little 
 or no life, and which becomes the prey of para- 
 sites. The growing tissue of a tree is comprised 
 in a thin layer below the bark, and the life of this
 
 56 DISEASE AND ITS CAUSES 
 
 may seemingly be indefinitely prolonged by 
 placing it in a situation in which it escapes the 
 action of accidental injuries and decay, as by 
 grafting on young trees. Where the nature of 
 the dead wood is such that it is immune from 
 parasites and decay, as in the case of the Se- 
 quoias, life seems to be indefinitely prolonged. 
 The growing branches of one of these trees, 
 whose age has been estimated with seeming 
 accuracy at six thousand years, are just as fresh 
 and the tree produces its flowers and fruit in 
 the same degree as a youthful brother of one 
 thousand years. Nor does old age supervene 
 in the unicellular organisms. An amoeba assim- 
 ilates, grows and multiplies just as long as the 
 environment is favorable. 
 
 Old age in itself is seldom a cause of death. In 
 rare cases in the very old a condition is found in 
 which no change is present to which death can 
 be attributed, all organs seem to share alike in 
 the senescence. Death is usually due to some 
 of the accidents of life, a slight infection to which 
 the less resistant body succumbs, or to the rup- 
 ture of a weakened blood vessel in the brain, or 
 to more advanced decay in some organ whose 
 function is indispensable. The causes and con- 
 ditions of age have been a fertile source for specu- 
 lation. Many of the hypotheses have been 
 interesting, that of Metschnikoff, for example, 
 who finds as a dominating influence in causing
 
 DISEASE AND ITS CAUSES 57 
 
 senescence the absorption of toxic substances 
 formed in the large intestine by certain bacteria. 
 He further finds that the cells of the body which 
 have phagocytic powers turn their activity 
 against cells and tissues which have become 
 weakened. There may be absorption of injurious 
 substances from the intestines which the body 
 in a vigorous condition is able to destroy or to 
 counteract their influence, and these may be 
 more operative in the weaker condition of the 
 body in the old. Phagocytes will remove cells 
 which are dead and often cells which are super- 
 fluous in a part, but there is no evidence that 
 this is ever other than a conservative process. 
 Since it is impossible to single out any one con- 
 dition to which old age is due, the hypothesis of 
 Metschnikoff should have no more regard given 
 it than the many other hypotheses which have 
 been presented. 
 
 Death of the body as a whole takes place from 
 the cessation of the action of the central nervous 
 system or of the respiratory system or of the 
 circulation. There are other organs of the body, 
 such as the intestine, kidney, liver, whose func- 
 tion is essential for life, but death does not take 
 place immediately on the cessation of their 
 function. The functions of the heart, the brain 
 and the lungs are intimately associated. Oxygen 
 is indispensable for the life of the tissues, and 
 its supply is dependent upon the integrity of the
 
 58 DISEASE AND ITS CAUSES 
 
 three organs mentioned, which have been called 
 the tripos of life. Respiration is brought about 
 by the stimulation of certain nerve cells in the 
 brain, the most effective stimulus to these cells 
 being a diminution of oxygen in the blood sup- 
 plying them. These cells send out impulses to 
 the muscles concerned in inspiration, the chest 
 expands, and air is taken into the lungs. Respi- 
 ration is then a more complicated process than 
 is the action of the heart, for its contraction, 
 which causes the blood to circulate, is not im- 
 mediately dependent upon extrinsic influences. 
 Death is usually more immediately due to failure 
 of respiration than to failure of circulation, for 
 the heart often continues beating for a time after 
 respiration has ceased. Thus, in cases of drown- 
 ing and suffocation, by means of artificial respi- 
 ration in which air is passively taken into and 
 expelled from the lungs, giving oxygen to the 
 blood, the heart may continue to beat and the 
 circulation continue for hours after all evident 
 signs of life and all sensation has ceased. 
 
 By this general death is meant the death of 
 the organism as a whole, but all parts of the body 
 do not die at the same time. The muscles and 
 nerves may react, the heart may be kept beating, 
 and organs of the body when removed and sup- 
 plied with blood will continue to function. Cer- 
 tain tissues die early, and the first to succumb 
 to the lack of * oxygenated blood are the nerve
 
 DISEASE AND ITS CAUSES 59 
 
 cells of the brain. If respiration and circulation 
 have ceased for as short a time as twelve minutes, 
 life ceases in certain of these cells and cannot be 
 restored. This is again an example of the greater 
 vulnerability of the more highly differentiated 
 structure in which all other forms of cell activity 
 are subordinated to function. There are, however, 
 pretty well authenticated cases of resuscitation 
 after immersion in water for a longer period than 
 twelve minutes, but these cases have not been 
 carefully timed, and time under such conditions 
 may seem longer than it actually is; and there 
 is, moreover, the possibility of a slight gaseous 
 interchange between the blood and the water 
 in the lungs, as in the case of the fish which uses 
 the water for an oxygen supply as the mammal 
 does the air. There are also examples of apparent 
 death or trances which have lasted longer, and 
 the cases of fakirs who have been buried for 
 prolonged periods and again restored to life. 
 In these conditions, however, all the activities 
 of the body are reduced to the utmost, and respi- 
 ration and circulation, so feeble as to be imper- 
 ceptible to ordinary observation, suffice to keep 
 the cells living. 
 
 With the cessation of life the body is subject 
 to the unmodified action of its physical environ- 
 ment. There is no further production of heat 
 and the body takes the temperature of the sur- 
 roundings. The only exceptions are rare cases
 
 60 DISEASE AND ITS CAUSES 
 
 in which such active chemical changes take place 
 in the dead body that heat is generated by chem- 
 ical action. At a varying interval after death, 
 usually within twelve hours, there is a general 
 contraction and hardening of the muscles due 
 to chemical changes, probably of the nature of 
 coagulation, in them. This begins in the muscles 
 of the head, extends to the extremities, and 
 usually disappears in twenty-four hours. It is 
 always most intense and most rapid in its onset 
 when death is preceded by active muscular exer- 
 tion. There have been cases of instantaneous 
 death in battle where the body has remained in 
 the position it held at the moment of death, this 
 being due to the instantaneous onset of muscular 
 rigidity. The blood remains fluid for a time after 
 death and settles in the more dependent parts of 
 the body, producing bluish red mottled discolor- 
 ations. Later the blood coagulates in the vessels. 
 The body loses moisture by evaporation. Drying 
 of the surface takes place where the epidermis 
 is thin, as over the transparent part of the eye 
 and over areas deprived of epidermis. Decompo- 
 sition and putrefaction of the body due to bac- 
 terial action takes place. The bacteria ever 
 present in the alimentary canal make their way 
 from this into the dead tissue. Certain of these 
 bacteria produce gas which accumulates in the 
 tissues and the body often swells enormously. 
 A greenish discoloration appears, which is due
 
 DISEASE AND ITS CAUSES 61 
 
 to the union of the products of decomposition 
 with the iron in the blood; this is more promi- 
 nent over the abdomen and appears in lines along 
 the course of the veins. The rapidity with which 
 decomposition takes place varies, and is dependent 
 upon many factors, such as the surrounding 
 temperature, the nutrition of the body at the 
 time of death, the cause of death. It is usually 
 not difficult to recognize that a body is dead. 
 In certain cases, however, the heart's action 
 may be so feeble that no pulse is felt at the wrist, 
 and the current of the expired air may not move 
 a feather held to the nostril or cloud the surface 
 of a mirror by the precipitation of moisture upon 
 it. This condition, combined with unconscious- 
 ness and paralysis of all the voluntary muscles, 
 may very closely simulate death. The only 
 absolute evidence of death is given by such 
 changes as loss of body heat, rigor mortis or 
 stiffening of the muscles, coagulation of the 
 blood and decomposition.
 
 CHAPTER III 
 
 THE GROWTH OF THE BODY. GROWTH MORE RAPID IN 
 EMBRYONIC PERIOD. THE COORDINATION AND REGULA- 
 TION OF GROWTH. TUMORS. THE GROWTH OF TUMORS 
 
 COMPARED WITH NORMAL GROWTH. SlZE, SHAPE AND 
 
 STRUCTURE OF TUMORS. THE GROWTH CAPACITY OF 
 TUMORS AS SHOWN BY THE INOCULATION OF TUMORS 
 OF MICE. BENIGN AND MALIGNANT TUMORS. EFFECT 
 OF INHERITANCE. ARE TUMORS BECOMING MORE FRE- 
 QUENT ? THE EFFECT PRODUCED BY A TUMOR ON THE 
 INDIVIDUAL WHO BEARS IT. RELATION OF TUMORS TO 
 AGE AND SEX. THEORIES AS TO THE CAUSE OF TUMORS. 
 
 THE PARASITIC THEORY. THE TRAUMATIC THEORY. 
 
 THE EMBRYONIC THEORY. THE IMPORTANCE OF THE 
 EARLY RECOGNITION AND REMOVAL OF TUMORS. 
 
 THE power of growth is possessed by every 
 living thing, but growth is not limited to the 
 living. Crystals also will grow, and the rapidity 
 and character of growth and the maximum size 
 of the crystal depends upon the character of the 
 substance which forms the crystal. From the 
 single cell or ovum formed by the union of the 
 male and female sexual cells, growth is continu- 
 ous until a size corresponding to the type of 
 the species is attained. From this time onward 
 growth is limited to the degree necessary to sup- 
 ply the constant loss of material which the body 
 undergoes. The rapidity of the growth of the 
 body and of its component parts differs at diflfer- 
 62
 
 63 
 
 ent ages, and becomes progressively less active 
 from its beginning in the ovum until the adult 
 type of the species is attained. As determined 
 by the volume, the embryo increases more than 
 ten thousand times in size during the first month 
 of intra-uterine life. At birth the average weight 
 is six and a half pounds; at the end of the first 
 year eighteen and a half pounds, a gain of twelve 
 pounds; at the end of the second year twenty- 
 three pounds, a gain of four and a half pounds. 
 The growth is coordinated, the size of the single 
 organs bearing a definite ratio, which varies 
 within slight limits, to the size of the body, a large 
 individual having organs of corresponding size. 
 Knowing that the capacity of growth is one of 
 the inherent properties of living matter, it is 
 much easier to understand the continuance of 
 growth than its cessation. It is impossible to 
 avoid the conclusion that there is some internal 
 mechanism of the body which controls and regu- 
 lates growth. In the first chapter reference was 
 made to organs producing substances which pass 
 directly into the circulation; these substances act 
 by control of the activities of other parts, stimu- 
 lating or depressing or altering their function. 
 Two of these glands, the thymus, lying hi front, 
 where the neck joins the body and which attains 
 its greatest size at puberty, and the pituitary 
 body, placed beneath the brain but forming no 
 part of it, have been shown by recent investiga-
 
 64 DISEASE AND ITS CAUSES 
 
 tions to have a very definite relation to growth, 
 especially the growth of the skeleton. The 
 growth energy chiefly resides in the skeleton, 
 and if the growing animal has a diet sufficient 
 only to maintain the body weight, the skeleton 
 will continue to grow at the expense of the other 
 tissues, literally living upon the rest of the body. 
 Disease of the glands mentioned leading to an 
 increase or diminution or alteration of their func- 
 tion may not only inhibit or unduly increase the 
 growth of the skeleton, but may also interfere 
 with the sexual development which accompanies 
 the skeleton growth. 
 
 The difficulties which arise in an endeavor to 
 comprehend normal growth are greater when the 
 growth of tumors is considered. A tumor is a 
 mass of newly formed tissue which in structure, 
 in growth, and the relations which it forms with 
 adjoining tissues departs to a greater or less 
 degree from the type of the tissue to which it 
 is related in structure or from which it originates. 
 It is an independent structure which, like a 
 parasite, grows at the expense of the body, 
 contributing nothing to it, and its capacity for 
 growth is unlimited. A tumor cannot be con- 
 sidered as an organ, its activities not being co- 
 ordinated with those of the body. A part of the 
 body it certainly is, but in the household economy 
 it is to be considered as a wild and lawless guest, 
 not influenced by or conforming with the regu-
 
 DISEASE AND ITS CAUSES 65 
 
 lations of the household. The rapidity of growth 
 varies; certain tumors for years increase but 
 little in size, while others may be seen to increase 
 from day to day. The growth is often intermit- 
 tent, periods of great activity of growth alter- 
 nating with periods of quiescence. The nutrition 
 and growth of a tumor is only slightly influenced 
 by the condition of nutrition of the bearer. Its 
 cells have a greater avidity for food than have 
 those of the body, and, like the growing bones 
 of an insufficiently fed animal, growth in some 
 cases seems to take place at the expense of the 
 body, the normal cells not obtaining sufficient 
 nutriment to repair their waste. 
 
 A tumor may be of any size: so small as to 
 be invisible to the naked eye, or its weight may 
 exceed that of the individual who bears it. The 
 limitations to its growth are extrinsic and not 
 intrinsic. There is no distinct color. Certain 
 tumors have color which depends upon the 
 presence of a dark brown or black pigment within 
 the cells. Haemorrhages within them are not 
 infrequent, and they may be colored by the blood 
 or by pigments formed from it. Usually they 
 have a gray color modified by their varying 
 vascularity, or the cut surface may be mottled 
 due to areas of cell degeneration. The consist- 
 ency varies; some tumors are so soft that they can 
 be pressed through a sieve, others are of stony 
 hardness. There is no distinct shape, this being
 
 66 DISEASE AND ITS CAUSES 
 
 influenced by the nature of the tumor, the man- 
 ner of growth and situation. When the tumor 
 grows on or near a surface, it may project from 
 this and be attached by a narrow band only; in 
 the interior of the body it may be irregular in 
 outline, round or lobular, the shape being influ- 
 enced by many factors. Tumors like the tissues 
 of the normal body are nourished by the blood 
 and contain blood vessels often in great numbers. 
 
 A tumor arises by the cells of a part of the 
 body beginning to grow and taking on the charac- 
 teristics of a tumor. Its growth is independent, 
 the cells of the adjoining tissue taking no part 
 in it. The tissue in the vicinity of the tumor is 
 partly pushed aside by the mass, or the tumor 
 grows into it and the tissue disappears as the 
 tumor advances. The destruction of the sur- 
 rounding tissue is brought about partly by the 
 pressure which the tumor exerts, partly by the 
 compression of the blood vessels or the blcod 
 supply of the organs is diverted to the tumor. 
 
 The characteristics of a tumor are due to the 
 cells which it contains (Fig. 14). These often 
 become separated from the main mass and are 
 carried by the blood into other parts of the body, 
 where they grow and form tumors similar in 
 character to the parent tumor. In the extraor- 
 dinary capacity for growth possessed by tumor 
 cells, they resemble vegetable rather than animal 
 cells. There is no limit to the growth of a tumor
 
 DISEASE AND ITS CAUSES 67 
 
 save by the death of the individual who bears 
 it, thus cutting off the supply of nutrition. The 
 cells of tumors peculiar to man show a narrow 
 range of adaptation. They will grow only in 
 
 FIG. 14. PHOTOGRAPH OF A MICROSCOPIC PREPARATION 
 FROM A CANCER OF THE UTERUS. A large mass of cells is 
 extending into the tissue of the uterus which is shown as 
 the fibrous structure. Such a cell mass penetrating into the 
 tissue represents the real cancer, the tissue about the cell 
 masses bear the blood vessels which nourish the tumor cells. 
 
 the body of the individual to whom the tumor 
 belongs, and die when grafted on another indi- 
 vidual. In the case of tumors which arise in 
 animals, pieces of the tumor when grafted on
 
 68 DISEASE AND ITS CAUSES 
 
 another animal of the same species will grow, and 
 in this way the growth capacity of the tumor 
 cells has been estimated. Thus, by transplanting 
 a small section of a mouse tumor into other mice, 
 the small transplanted fragments will in two 
 weeks grow to the size of filberts, and each of 
 these will furnish material to engraft upon ten 
 mice. These new tumors are similar in character 
 to the original tumor, and really represent parts 
 of it in the same way that all the Baldwin apples 
 in the world are parts of the original tree which 
 was found in Baldwinville many years ago, and 
 as all the Concord grape vines are really parts of 
 the original vine. It has been estimated that 
 if all the growth capacity of this mouse tumor 
 were availed of by the successive inoculation of 
 other mice, a mass of tumor several times the 
 diameter of the sun would grow in two years. 
 The condition of the individual seems to exert 
 no influence upon the growth of the tumor. 
 Growth may be as rapid when the bearer is in 
 a condition of extreme emaciation as it is when 
 the bearer is well nourished and robust. 
 
 Those tumors which grow rapidly and invade 
 and destroy the surrounding tissue are called 
 malignant tumors or cancers, but in a strict sense 
 no tumor can be regarded as benign, for none 
 can serve a useful purpose. A tumor after a 
 period of slow growth can begin to grow rapidly. 
 Tumors may arise in any part of the body, but
 
 DISEASE AND ITS CAUSES 69 
 
 there are certain places of preference particu- 
 larly for the more malignant tumors. These 
 are places where the cells naturally have a marked 
 power of growth, and especially where growth is 
 intermittent as in the uterus and mammary 
 gland. 
 
 Little is known in regard to the influence of 
 inheritance on the formation of tumors. Study 
 of the tumors of mice show a slightly greater 
 susceptibility to tumor formation in the progeny 
 of mice who have developed tumors. Studies 
 of human families seem to show that heredity 
 has a slight influence, but in the frequency of 
 tumors such statistical evidence is of little value. 
 The question of inheritance has much bearing 
 on the origin of tumors. If the tumor is accidental 
 and due entirely to extraneous causes, inheritance 
 is not probable; but if there is some predisposi- 
 tion to tumor formation in certain individuals due 
 to some peculiarity, then inheritance may exert 
 an influence. 
 
 The question as to whether tumors are an 
 increasing cause of disease is equally difficult 
 of solution. The mortality statistics, if taken 
 at their face value, show an enormous increase 
 in frequency; but there are many factors which 
 must be considered and which render the decision 
 difficult and doubtful. Tumors are largely a 
 prerogative of age, and the increased duration 
 of life which preventive medicine has brought
 
 70 DISEASE AND ITS CAUSES 
 
 about brings more people into the age when 
 tumors are more common. Owing to the greater 
 skill in the diagnosis of tumors, especially those 
 of the internal organs, they are now recognized 
 more frequently and more deaths are correctly 
 ascribed to them. Deaths from tumors were 
 formerly often purposely concealed and attributed 
 to some other cause. 
 
 No age is immune to tumors. They may be 
 present at birth or develop shortly afterwards. 
 The age from five to twenty years is the most 
 free from them, that from forty-five to sixty-five 
 the most susceptible, particularly to the more 
 malignant forms. 
 
 A tumor is a local disease. The growing tis- 
 sue of the tumor is the disease, and it is evident 
 that if the entire tumor were removed the disease 
 would be cured. This is the end sought by surgi- 
 cal interference, but notwithstanding seemingly 
 thorough removal, the tumor often reappears 
 after an interval of months or years. There are 
 many conditions which may render the complete 
 removal of a tumor difficult or impossible. It 
 is often impossible to ascertain just how far the 
 tumor cells have invaded the neighboring struc- 
 tures; the situation of the tumor may be such 
 that an extended removal would injure organs 
 which are essential for life, or at the time of 
 removal the tumor cells may have been conveyed 
 elsewhere by the blood or lymphatic vessels.
 
 DISEASE AND ITS CAUSES 71 
 
 Successful removal depends mainly upon the 
 length of time the tumor has been growing. At 
 an early stage even the most malignant tumoi; 
 may be successfully removed. It is evident 
 from this how disastrous may be the neglect 
 of proper surgical treatment of a tumor. The 
 time may be very short between the first evi- 
 dence of the presence of a tumor and the develop- 
 ment of a condition which would render complete 
 removal impossible. 
 
 The effect of a tumor upon its bearer depends 
 upon its character and situation. Pain is very 
 commonly present, and is due to the pressure 
 which the growing tumor exerts upon the sensory 
 nerves. Pain may, however, not be present or 
 appear only at the last. A condition of mal- 
 nutrition and emaciation often results due to 
 the passage into the blood of injurious substances 
 formed in the tumor, or to the destruction of 
 important organs by the growing tumor. The 
 growth of a tumor in the intestine may obstruct 
 or close the canal and thus interfere with nutrition. 
 
 The cause or causes of tumors are unknown. 
 We know that the tumor represents essentially 
 an abnormal growth, and that this growth is due 
 to new formation of cells. In certain cases the 
 tumor repeats the structure of the organ or tissue 
 in which it originates, in others it departs widely 
 from this; always, however, its structure re- 
 sembles structures found hi the body at some
 
 72 DISEASE AND ITS CAUSES 
 
 period of life. The tumor cells, like all other 
 cells of the body, grow by means of the nutriment 
 which the body supplies; they have no intrinsic 
 sources of energy. The great problem is what 
 starts the cells to grow and why the growth 
 differs from that of normal tissue, why it is 
 not regulated and coordinated as are other 
 forms of growth. When a small piece of the 
 skin, for instance, is cut out growth as rapid as 
 that in tumors takes place hi the adjoining cells, 
 but it ceases when the loss is restored. The same 
 is true when a piece of the liver is removed. 
 
 Various hypotheses have been formed to ex- 
 plain the tumor, all of them of interest, and they 
 have had great importance in that the attempt 
 to prove or disprove the hypothesis by continued 
 observation and experiment along definite lines 
 has produced new knowledge. The various 
 theories as to cause may be divided into three 
 heads. 
 
 The parasitic theory. This supposes that a 
 living parasite invades the body, and by its 
 presence excites the cells of certain tissues to 
 grow in tumor form. It is known that active 
 growth of the cells of the body can be excited in 
 a number of ways, by chemical substances such 
 as certain of the coal tar products, and that it 
 often takes place under the influence of bacteria. 
 It is further known that parasites can produce 
 tumor-like growths in plants. The large, rough
 
 DISEASE AND ITS CAUSES 73 
 
 excrescences on the oaks are produced by a fly 
 which lays its eggs in or beneath the bark, and 
 the larva which develops from the egg secretes 
 a substance which causes the cells about it to 
 multiply, and a huge mass is formed which serves 
 the developing insect for both food and pro- 
 tection. Large tumor-like masses are formed on 
 the roots and stalk of cabbages as the result of 
 the invasion of the cells by a minute organ- 
 ism: the tumors of olive trees are due to a bac- 
 terium; the peculiar growths on cedar trees, the 
 so-called "witches' brooms," are produced by a 
 fungus, and there are many other such examples. 
 These have many analogies with tumors in ani- 
 mals. Under the stimulus of the parasite the 
 cells seem to have unlimited growth capacity 
 and a greater nutritive avidity than have the 
 normal plant cells; the character of the mass 
 produced differs as does the tumor, to a greater 
 or less extent, from the normal growth; on the 
 cedar, for instance, the "witches' broom" con- 
 sists of a thick mass of foliage with small stems 
 less green than the usual foliage, the leaves wider 
 and not so closely applied to the stems. The 
 entire plant suffers in its nutrition and a condition 
 resembling tumor cachexia 1 is produced, and there 
 are no fundamental differences between the plant 
 
 1 By cachexia is understood a condition of malnutrition 
 and emaciation which is usually accompanied by a pale salkfw 
 color of the skin.
 
 74 DISEASE AND ITS CAUSES 
 
 and animal tumors. Support has also been 
 given to the parasitic theory by the discovery 
 within tumor cells of bodies which were supposed 
 to be a peculiar sort of parasite. If the truth 
 of the parasitic theory could be proved, there 
 would be justifiable expectation that the tumor 
 disease might be controlled as are many of the 
 parasitic diseases, but the hypothesis awaits the 
 demonstration of its correctness. Despite the 
 study of tumors which is being actively pursued 
 in many places and by the most skilled investi- 
 gators, no parasites have been found in animal 
 tumors; the objects previously described as par- 
 asites have been found not to be such. It is 
 difficult to bring in accord with the parasitic 
 theory the great variation in tumor struc- 
 ture, the relation of certain tumors, as the 
 malignant tumors of the breast and uterus, 
 with the age of the bearer, the congenital 
 tumors which develop in intra-uterine life, and 
 there are many other conditions which oppose 
 the theory. 
 
 The traumatic * theory. There is much in 
 favor of this. In a certain number of cases 
 tumors do develop at the site of injuries. The 
 coincidence of injury and tumor is apt to be 
 overestimated because of the strong tendency 
 to connect succeeding events. Tumors are not 
 most common on those parts of the body which 
 
 1 By trauma is understood a wound or injury of any sort.
 
 DISEASE AND ITS CAUSES 75 
 
 are most exposed to injury. They are rare, for 
 instance, on the hands and feet, and very rarely 
 do they appear at the site of wounds caused by 
 surgical operations. For those tumors which 
 develop in intra-uterine life it is difficult to as- 
 sign injury as a cause. There does, however, seem 
 to be a relation between tumors and injuries of 
 a certain character. The natives of Cashmere 
 use in winter for purposes of heat a small char- 
 coal stove which they bind on the front of the 
 body; burns often result and tumors not in- 
 frequently develop at the site of such burns. 
 Injuries of tissue which are produced by the 
 X-ray not infrequently result in tumor formation 
 and years may elapse between the receipt of the 
 injury and the development of the tumor. These 
 X-ray injuries are of a peculiar character, their 
 nature but imperfectly understood, and the in- 
 jured tissues seem to have lost the capacity for 
 perfect repair. 
 
 In regard to the possible action of both in- 
 juries and parasites in causing tumors, the possi- 
 bility that their effects on different individuals 
 may not be the same must be considered. In 
 addition to the trauma or the parasite which may 
 be considered as extrinsic factors, there may be 
 conditions of the body, intrinsic factors, which 
 favor their action in tumor development. The 
 peculiar tissue growth within the uterus called 
 decidua, which occurs normally in pregnancy
 
 76 DISEASE AND ITS CAUSES 
 
 and serves to fasten the developing ovum to 
 the inner lining of the uterus, may be pro- 
 duced experimentally. This growth depends 
 upon two factors, an internal secretion derived 
 from the ovary and the introduction into the 
 uterus of a foreign body of some sort; in 
 the case of pregnancy the developing embryo 
 acts as the foreign body. It is not impos- 
 sible that some variation in the complex rela- 
 tions which determine normal growth may be 
 one factor, possibly the most important, in tumor 
 formation. 
 
 Another theory is that the tumor is the result 
 of imperfect embryonic development. The devel- 
 opment of the child from the ovum is the result 
 of a continued formation and differentiation of 
 cells. A cell mass is first produced, and the cells 
 in this differentiate into three layers called 
 ectoderm, entoderm and mesoderm, from which 
 the external and internal surfaces and the en- 
 closed tissues respectively develop, and the 
 different organs are produced by growth of the 
 cells of certain areas of these layers. The em- 
 bryonic theory assumes that in the course of 
 embryonic development not all the cell material 
 destined for the formation of individual organs 
 is used up for this purpose, that certain of the 
 embryonic cells become enclosed in the develop- 
 ing organs, they retain the embryonic capacity 
 for growth and tumors arise from them. There
 
 DISEASE AND ITS CAUSES 77 
 
 is no doubt that something like this does take 
 place. There is a relation between malformations 
 due to imperfect development of the embryo and 
 tumors, the two conditions occurring together too 
 frequently to be regarded as mere coincidence. 
 Also tumors may occur in parts of the body in 
 which there is no tissue capable of forming struc- 
 tures which may be present in the tumors. The 
 theory, however, is not adequate, but it may 
 be among the factors. 
 
 The problems concerned in the nature and 
 cause of tumors are the most important in medi- 
 cine at the present time. No other form of 
 disease causes a similar amount of suffering and 
 anxiety, which often extends over years and makes 
 a terrible drain on the sympathy and resources 
 of the family. The only efficient treatment for 
 tumors at the present time is removal by surgical 
 operation, and the success of the operation is in 
 direct ratio to the age of the tumor, the time 
 which elapses from its beginning development. 
 It is of the utmost importance that this should 
 be generally recognized, and the facts relating 
 to tumors become general knowledge. Tumors 
 form one of the most common causes of death 
 (after the age of thirty-five one in every ten 
 individuals dies of tumor); medical and surgical 
 resources are, in many cases, powerless to afford 
 relief and the tumor stands as a bar to the at- 
 tainment of the utopia represented by a happy
 
 78 DISEASE AND ITS CAUSES 
 
 and comfortable old age, and a quiet passing. 
 Every possible resource should be placed at the 
 disposal of the scientific investigation of the 
 subject, for with knowledge will come power to 
 relieve.
 
 CHAPTER IV 
 
 THE REACTIONS OP THE TISSUES OP THE BODY TO IN- 
 jtmiES. INFLAMMATION. THE CHANGES IN THE BLOOD 
 IN THIS. THE EMIGRATION OF THE CORPUSCLES OF THE 
 BLOOD. THE EVIDENT CHANGES IN THE INJURED PART 
 AND THE MANNER IN WHICH THESE ARE PRODUCED. 
 HEAT, REDNESS, SWELLING AND PAIN. THE PRODUC- 
 TION OP BLISTERS BY SUNBURN. THE CHANGES IN THE 
 CELLS op AN INJURED PART. THE CELLS WHICH MI- 
 GRATE FROM THE BLOOD-VESSELS ACT AS PHAGOCYTES. 
 THE MACROPHAGES. THE MICROPHAGES. CHEMO- 
 TROPISM. THE HEALING OF INFLAMMATION. THE RE- 
 MOVAL OP THE CAUSE. CELL REPAIR AND NEW 
 FORMATION. NEW FORMATION OF BLOOD-VESSELS. 
 ACUTE AND CHRONIC INFLAMMATION. THE APPAR- 
 ENTLY PURPOSEFUL CHARACTER OP THE CHANGES m 
 INFLAMMATION. 
 
 INJURY and repair have already been briefly 
 considered in their relation to the normal body 
 and to old age; there are, however, certain 
 phenomena included under the term inflamma- 
 tion which follow the more extensive injuries and 
 demand a closer consideration than was given 
 in Chapter II. These phenomena differ in 
 degree and character; they are affected by the 
 nature of the injurious agent and the intensity 
 of its action, by the character of the tissue which 
 is affected and by variations in individual resist- 
 ance to injury. A blow which would have no 
 effect upon the general surface of the body may 
 produce serious results if it fall upon the eye, 
 
 79
 
 80 DISEASE AND ITS CAUSES 
 
 and less serious results for a robust than for 
 a weak individual. 
 
 Most of the changes which take place after an 
 injury and their sequence can be followed under 
 the microscope. If the thin membrane between 
 the toes of a living frog be placed under the 
 microscope the blood vessels and the circulating 
 blood can be distinctly seen in the thin tissue 
 between the transparent surfaces. The arteries, 
 the capillaries and veins can be distinguished, 
 the arteries by the changing rapidity of the 
 blood stream within them, there being a quicken- 
 ing of the flow corresponding with each contrac- 
 tion of the heart ; the veins appear as large 
 vessels in which the blood flows regularly (Fig. 
 11). Between the veins and arteries is a large 
 number of capillaries with thin transparent walls 
 and a diameter no greater than that of the single 
 blood corpuscles; they receive the blood from the 
 arteries and the flow in them is continuous. 
 The white and red blood corpuscles can be dis- 
 tinguished, the red appearing as oval discs and 
 the white as colorless spheres. In the arteries and 
 veins the red corpuscles remain in the centre 
 of the vessels appearing as a rapidly moving 
 red core, and between this core and the wall of 
 the vessels is a layer of clear fluid in which the 
 white corpuscles move more slowly, often turning 
 over and over as a ball rolls along the table. 
 
 If, now, the web be injured by pricking it or
 
 DISEASE AND ITS CAUSES 81 
 
 placing some irritating substance upon it, a 
 change takes place in the circulation. The 
 arteries and the veins become dilated and the 
 flow of blood more rapid, so rapid, indeed, that 
 it is difficult to distinguish the single corpuscles. 
 In a short while the rapidity of flow in the dilated 
 vessels diminishes, becoming slower than the 
 normal, and the separation between the red and 
 white corpuscles is not so evident. In the slowly 
 moving stream the white corpuscles move much 
 more slowly than do the red, and hence accumu- 
 late in the vessels lining the inner surface and 
 later become attached to this and cease to move 
 forward. The attached corpuscles then begin 
 to move as does an amoeba, sending out pro- 
 jections, some one of which penetrates the wall, 
 and following this the corpuscles creep through. 
 Red corpuscles also pass out of the vessels, this 
 taking place in the capillaries; the white cor- 
 puscles, on the other hand, pass through the 
 small veins. Not only do the white corpuscles 
 pass through the vessels, but the blood fluid 
 also passes out. The corpuscles which have 
 passed into the tissue around the vessels are 
 carried away by the outstreaming fluid, and the 
 web becomes swollen from the increased amount 
 of fluid which it contains. The injured area of 
 the web is more sensitive than a corresponding 
 uninjured area and the foot is more quickly 
 moved if it be touched. If the injury has been
 
 82 DISEASE AND ITS CAUSES 
 
 very slight, observation of the area on the fol- 
 lowing day will show no change beyond a slight 
 dilatation of the vessels and a great accumulation 
 of cells in the tissue. 
 
 Everyone has experienced the effect of such 
 changes as have been described in this simple 
 experiment. An inflamed part on the surface 
 of the body is redder than the normal, swollen, 
 hot and painful. The usual red tinge of the skin 
 is due to the red blood contained in the vessels, 
 and the color is intensified when, owing to the 
 dilatation, the vessels contain more blood. The 
 inflamed area feels hot, and if the temperature be 
 taken it may be two or three degrees warmer 
 than a corresponding area. The increased heat 
 is due to the richer circulation. Heat, is pro- 
 duced in the interior of the body chiefly in 
 the muscles and great glands, and the in- 
 creased afflux of blood brings more heat to 
 the surface. A certain degree of swelling of 
 the tissue is due to the dilatation of the vessels ; 
 but this is a negligible factor as compared with 
 the effect of the presence of the fluid and cells 
 of the exudate. 1 The fluid distends the tissue 
 spaces, and it may pass from the tissue and ac- 
 cumulate on surfaces or in the large cavities 
 within the body. The greatly increased dis- 
 
 1 The term exudation is used to designate the passing 
 of cells and fluid from the vessels in inflammation; the mate- 
 rial is the exudate.
 
 DISEASE AND ITS CAUSES 83 
 
 charge from the nose in a "cold in the head" is 
 due to the exudation formed in the acutely in- 
 flamed tissue, and which readily passes through 
 the thin epithelial covering. Various degrees of 
 inflammation of the skin may be produced by 
 the action of the sun, the injury being due not 
 to the heat but to the actinic rays. In a 
 mild degree of exposure only redness and a 
 strong sense of heat are produced, but in pro- 
 longed exposure an exudate is formed which 
 causes the skin to swell and blisters to form, 
 these being due to the exudate which passes 
 through the lower layers of the cells of the 
 epidermis and collects beneath the impervious 
 upper layer, detaching this from its connections. 
 If a small wad of cotton, soaked in strong am- 
 monia, be placed on the skin and covered with a 
 thimble and removed after two minutes, minute 
 blisters of exudate slowly form at the spot. 
 
 The pain in an inflamed part is due to a number 
 of factors, but chiefly to the increased pressure 
 upon the sensory nerves caused by the exudate. 
 The pain varies so greatly in degree and char- 
 acter that parts which ordinarily have little 
 sensation may become exquisitely painful when 
 inflamed. The pain is usually greater when the 
 affected part is dense and unyielding, as the 
 membranes around bones and teeth. The pain 
 is often intermittent, there being acute paroxysms 
 synchronous with the pulse, this being due to
 
 84 DISEASE AND ITS CAUSES 
 
 momentary increase of pressure when more blood 
 is forced into the part at each contraction of 
 the heart. The pain may also be due to the 
 direct action of an injurious substance upon the 
 sensory nerves, as in the case of the sting of an 
 insect where the pain is immediate and most 
 intense before the exudate has begun to appear. 
 
 When an inflamed area is examined, after 
 twenty-four hours, by hardening the tissue in 
 some of the fluids used for this purpose and cut- 
 ting it into very thin slices by means of an in- 
 strument called a microtome, the microscope 
 shows a series of changes which were not appar- 
 ent on naked eye examination. The texture is 
 looser, due to the exudate which has dilated all 
 the spaces in the tissue. Red and white corpuscles 
 in varying numbers and proportions infiltrate 
 the tissue; all the cells which belong to the part, 
 even those forming the walls of the vessels, are 
 swollen, the nuclei contain more chromatin, and 
 the changes in the nuclei which indicate that 
 the cells are multiplying appear. The blood 
 vessels are dilated, and the part in every way 
 gives the indication of a more active life within 
 it. There are also evidences of the tissue in- 
 jury which has called forth all the changes which 
 we have considered. (Fig. 15.) 
 
 The microscopic examination of any normal 
 tissue of the body shows within it a variable 
 number of cells which have no intimate associ-
 
 DISEASE AND ITS CAUSES 
 
 85 
 
 ation with the structure of the part and do not 
 seem to participate in its function. They are 
 found in situations which indicate that these 
 cells have power of active independent motion. 
 
 
 FlG. 15. A SECTION OF AN INFLAMED LUNG SHOWING THE 
 
 EXUDATE WITHIN THE AIR SPACES. Compare this with Fig. 6. 
 Fig. 15 is from the human lung, in which the air spaces are much 
 larger than in the mouse. 
 
 In the inflamed tissue a greatly increased num- 
 ber of these cells is found, but they do not ap- 
 pear until the height of the process has passed, 
 usually not before thirty-six or forty-eight hours 
 after the injury has been received. The num-
 
 86 
 
 DISEASE AND ITS CAUSES 
 
 bers present depend much upon the character of 
 the agent which has produced the injury, and 
 they may be more numerous than the ordinary 
 leucocytes which migrate from the blood vessels. 
 All these changes which an injured part under- 
 goes are found when closely analyzed to be 
 purposeful; that is, they are in accord with the 
 
 FIG. 16. PHAGOCYTOSIS, a, b, c are the microphages 
 or the bacterial phagocytes, (a) Contains a number of round 
 bacteria, and (6) similar bacteria arranged in chains, and (c) 
 a number of rod-shaped bacteria, (d) Is a cell phagocyte 
 or macrophage which contains five red blood corpuscles. 
 
 conditions under which the living matter acts, 
 and they seem to facilitate the operation of 
 these conditions. It has been said that the life 
 of the organism depends upon the coordinated 
 activity of the living units or cells of which it is 
 composed. The cells receive from the blood 
 material for the purpose of function, for cell 
 repair and renewal, and the products of waste
 
 DISEASE AND ITS CAUSES 87 
 
 must be removed. In the injury which has 
 been produced in the tissue all the cells have 
 suffered, some possibly displaced from their 
 connections, others may have been completely 
 destroyed, others have sustained varying degrees 
 of injury. If the injury be of an infectious char- 
 acter, that is, produced by bacteria, these may 
 be present in the part and continue to exert 
 injury by the poisonous substances which they 
 produce, or if the injury has been produced by 
 the action of some other sort of poison, this may 
 be present in concentrated form, or the injury 
 may have been the result of the presence of a 
 foreign body in the part. Under these conditions, 
 since the usual activities of the cells in the injured 
 part will not suffice to restore the integrity of 
 the tissue, repair and cell formation must be 
 more active than usual, any injurious substances 
 must be removed or such changes must take 
 place in the tissue that the cell life adapts itself 
 to new conditions. 
 
 All life in the tissues depends upon the circu- 
 lation of the blood. There is definite relation 
 between the activity of cells and the blood supply; 
 a part, for instance, which is in active function 
 receives a greater supply of blood by means of 
 dilatation of the arteries which supply it. If the 
 body be exactly balanced longitudinally on a 
 platform, reading or any exercise of the brain 
 causes the head end to sink owing to the relatively
 
 88 DISEASE AND ITS CAUSES 
 
 greater amount of blood which the brain re- 
 ceives when in active function. The regulation 
 of the blood supply is effected by means of 
 nerves which act upon the muscular walls of 
 the arteries causing, by the contraction or the 
 relaxation of the muscle, diminution or dilata- 
 tion of the calibre of the vessel. After injury 
 the dilatation of the vessels with the greater 
 afflux of blood to the part is the effect of the 
 greatly increased cell activity, and is a neces- 
 sity for this. In many forms of disease it has 
 been found that by increasing the blood flow 
 to a part and producing an active circulation 
 in it, that recovery more readily takes place and 
 many of the procedures which have been found 
 useful in inflammation, such as hot applications, 
 act by increasing the blood flow. So ultimate 
 is the association between cell activity, as shown 
 in repair and new formation of cells, and the 
 blood flow, that new blood vessels frequently 
 develop by means of which the capacity for 
 nutrition is still more increased. The cornea 
 or transparent part of the eye contains no blood 
 vessels, the cells which it contains being nour- 
 ished by the tissue fluid which comes from the 
 outside and circulates in small communicating 
 spaces. If the centre of the cornea be injured, 
 the cells of the blood vessels in the tissue 
 around the cornea multiply and form new ves- 
 sels which grow into the cornea and appear as a
 
 DISEASE AND ITS CAUSES 89 
 
 pink fringe around the periphery; when repair 
 has taken place the newly formed vessels dis- 
 appear. 
 
 The exudate from the blood vessels in various 
 ways assists in repair. An injurious substance 
 in the tissue may be so diluted by the fluid that 
 its action is minimized. A small crystal of salt 
 is irritating to the eye, but a much greater amount 
 of the same substance in dilute solution causes 
 no irritation. The poisonous substances produced 
 by bacteria are diluted and washed away from 
 the part by the exudate. Not only is there a 
 greater amount of tissue fluid in the inflamed 
 part, but the circulation of this is also in- 
 creased, as is shown by comparing the outflow 
 in the lymphatic vessels with the normal. The 
 fluid exudate which has come from the blood 
 and differs but slightly from the blood fluid 
 exerts not only the purely physical action of 
 removing and diluting injurious substances, but 
 in many cases has a remarkable power, exer- 
 cised particularly on bacterial poisons, of neu- 
 tralizing poisons or so changing their character 
 that they cease to be injurious. 
 
 We have learned, chiefly from the work of 
 Metschnikoff , that those white corpuscles or leu- 
 cocytes which migrate from the vessels in the 
 greatest numbers have marked phagocytic prop- 
 erties, that is, they can devour other living 
 things and thus destroy them just as do the
 
 90 DISEASE AND ITS CAUSES 
 
 amoebae. In inflammations produced by bacteria 
 there is a very active migration of these cells 
 from the vessels; they accumulate in the tissue 
 and devour the bacteria. They may be present 
 in such masses as to form a dense wall around 
 the bacteria, thus acting as a physical bar to 
 their further extension. The other form of 
 amoeboid cell, which Metschnikoff calls the 
 macrophage, has more feeble phagocytic action 
 towards bacteria, and these are rarely found 
 enclosed within them. It is chiefly by means 
 of their activity that other sorts of substances 
 are removed. They often contain dead cells or 
 cell fragments, and when haemorrhage takes 
 place in a tissue they enclose and remove the 
 granules of blood pigment which result. They 
 often join together, forming connected masses, 
 and surround such a foreign body as a hair, or a 
 thread which the surgeon places in a wound to 
 close it. They may destroy living cells, and do 
 this seemingly when certain cells are in too great 
 numbers and superfluous in a part, their action 
 tending to restore the cell equilibrium. The 
 foreign cells do even more than this : they them- 
 selves may be devoured by the growing cells of 
 the tissue, seemingly being actuated by the same 
 supreme idea of sacrifice which led Buddha to 
 give himself to the tigress. 
 
 The explanation of most of the changes which 
 take place in inflammation is obvious. It is a
 
 DISEASE AND ITS CAUSES 91 
 
 definite property of all living things that repair 
 takes place after injury, and certain of the changes 
 are only an accentuation of those which take 
 place in the usual life; but others, such as the 
 formation of the exudate, are unusual; not only 
 is the outpouring of fluid greatly increased, but 
 its character is changed. In the normal tran- 
 sudation 1 the substances on which the coagu- 
 lation of the blood depends pass through the 
 vessel wall to a very slight extent, but the exu- 
 date may contain the coagulable material in such 
 amounts that it easily clots. The interchange 
 between the fluid outside the vessels and the 
 blood fluid takes place by means of filtration and 
 osmosis. There is a greater pressure in the 
 vessels than in the fluid outside of them, and the 
 fluid filters through the wall as fluid filters through 
 a thin membrane outside of the body. Osmosis 
 takes place when two fluids of different osmotic 
 pressure are separated by animal membrane. 
 Difference in osmotic pressure is due to differ- 
 ences in molecular concentration, the greater 
 the number of molecules the greater is the pres- 
 sure, and the greater rapidity of flow is from the 
 fluid of less pressure to the fluid of greater pres- 
 sure. The molecular concentration of tissue and 
 blood fluid is constantly being equalized by the 
 process of osmosis. In the injured tissue the 
 
 1 By transudation is meant the constant interchange 
 between the blood and the tissue fluid.
 
 92 
 
 conditions are more favorable for the fluid of 
 the blood to pass from the vessels : by filtration, 
 because owing to the dilatation of the arteries there 
 is increased amount of blood and greater pressure 
 within the vessels, and the filtering membrane 
 is also thinner because the same amount of 
 membrane (here the wall of the vessel) must 
 cover the larger surface produced by the dilata- 
 tion. It is, moreover, very generally believed 
 that there are minute openings in the walls of the 
 capillaries, and these would become larger in 
 the dilated vessel just as openings in a sheet of 
 rubber become larger when this is stretched. 
 Osmosis towards the tissue is favored because, 
 owing to destructive processes the molecular 
 pressure in the injured area is increased; an in- 
 jured tissue has been shown to take up fluid 
 more readily outside of the body than a corre- 
 sponding uninjured tissue. The slowing of the 
 blood stream, in spite of the dilatation of the 
 vessels, is due to the greater friction of the sus- 
 pended corpuscles on the walls of the vessels. 
 This is due to the loss from the blood of the 
 outstreaming fluid and the relative increase in 
 the number of corpuscles, added to by the un- 
 evenness of surface which the attached corpuscles 
 produce. 
 
 The wonderful migration of the leucocytes, 
 which seems to show a conscious protective action 
 on their part, takes place under the action of
 
 DISEASE AND ITS CAUSES 93 
 
 conditions which influence the movement of cells. 
 When an actively moving amceba is observed it is 
 seen that the motion is not the result of chance, 
 for it is influenced by conditions external to the 
 organism; certain substances are found to at- 
 tract the amoebae towards them and other sub- 
 stances to repel them. These influences or forces 
 affecting the movements of organisms are known 
 as tropisms, and play a large part in nature; 
 the attraction of various organisms towards a 
 source of light is known as heliotropism, and 
 there are many other instances of such attrac- 
 tion. The leucocytes as free moving cells also 
 come under the influence of such tropisms. 
 When a small capillary tube having one end 
 sealed is partially filled with the bacteria which 
 produce abscess and placed beneath the skin 
 it quickly becomes filled with leucocytes, these 
 being attracted by the bacteria it contains. 
 Dead cells exert a similar attraction for the 
 large phagocytes. Such attraction is called 
 chemotropism and is supposed to be due in the 
 cases mentioned, to the action of chemical sub- 
 stances such as are given off by the bacteria or 
 the dead cells. The direction of motion is due to 
 stimulation of that part of the body of the leuco- 
 cyte which is towards the source of the stimulus. 
 The presence in the injured part of bacteria or 
 of injured and dead cells exerts an attraction 
 for the leucocytes within the vessels causing
 
 94 DISEASE AND ITS CAUSES 
 
 their migration. When the centre of the cornea 
 is injured, this tissue having no vessels, all the 
 vascular phenomena take place in the white 
 part of the eye immediately around the cornea, 
 this becoming red and congested. The migra- 
 tion of leucocytes from the vessels takes place 
 chiefly on the side towards the cornea, and the 
 migrated cells make their way along the devious 
 tracts of the communicating lymph spaces to the 
 area of injury. The objection may be raised that 
 it is difficult to think of a chemical substance pro- 
 duced in an injured area no larger than a milli- 
 meter, diffusing through the cornea and reaching 
 the vessels outside this in such quantity and 
 concentration as to affect their contents, nor 
 has there been any evidence presented that 
 definite chemical substances are produced in 
 injured tissues; but there is no difficulty in view 
 of the possibilities. It is not necessary to assume 
 that an actual substance so diffuses itself, but 
 the influence exerted may be thought of as a 
 force, possibly some form of molecular motion, 
 which is set in action at the area of injury and 
 extends from this. No actual substance passes 
 along a nerve when it conveys an impulse. 
 
 We have left the injured area with an increased 
 amount of fluid and cells within it, with the 
 blood vessels dilated and with both cells and 
 fluid streaming through their walls, and the cells 
 belonging to the area actively repairing damages
 
 DISEASE AND ITS CAUSES 95 
 
 and multiplying. The process will continue as 
 long as the cause which produces the injury con- 
 tinues to act, and will gradually cease with the 
 discontinuance of this action, and this may be 
 brought about in various ways. A foreign body 
 may be mechanically removed, as when a thorn 
 is plucked out; or bacteria may be destroyed by 
 the leucocytes; or a poison, such as the sting of 
 an insect, may be diluted by the exudate until 
 it be no longer injurious, or it may be neutralized. 
 Even without the removal of the cause the power 
 of adaptation will enable the life of the affected 
 part to go on, less perfectly perhaps, in the new 
 environment. The excess of fluid is removed 
 by the outflow exceeding the inflow, or it may 
 pass to some one of the surfaces of the body, 
 or in other cases an incision favors its escape. 
 The excess of cells is in part removed with the 
 fluid, in part they disappear by undergoing 
 solution and in part they are devoured by other 
 cells. With the diminishing cell activity the 
 blood vessels resume their usual calibre, and when 
 the newly formed vessels become redundant they 
 disappear by undergoing atrophy in the same 
 way as other tissues which have become useless. 
 
 When these changes take place rapidly the 
 inflammation is said to be acute, and chronic 
 when they take place slowly. Chronic inflamma- 
 tion is more complex than is the acute, and there 
 is more variation in the single conditions. The
 
 96 DISEASE AND ITS CAUSES 
 
 chronicity may be due to a number of condi- 
 tions, as the persistence of a cause, or to incom- 
 pleteness of repair which renders the part once 
 affected more vulnerable, to such a degree even 
 that the ordinary conditions to which it is sub- 
 jected become injurious. A chronic inflammation 
 may be little more than an almost continuous 
 series of acute inflammations, with repair continu- 
 ously less perfect. Chronic imflammations are a 
 prerogative of the old as compared with the 
 young, of the weak rather than the strong.
 
 CHAPTER V 
 
 INFECTIOUS DISEASES. THE HISTORICAL IMPORTANCE OP 
 EPIDEMICS OF DISEASE. THE LOSSES IN BATTLE CON- 
 TRASTED WITH THE LOSSES IN ARMIES PRODUCED BY 
 
 INFECTIOUS DISEASES. THE DEVELOPMENT OF KNOWL- 
 EDGE OF EPIDEMICS. THE VIEWS OF HIPPOCRATES 
 AND ARISTOTLE. SPORADIC AND EPIDEMIC DISEASES. 
 THE THEORY OF THE EPIDEMIC CONSTITUTION. THEORY 
 THAT THE CONTAGIOUS MATERIAL IS LlVING. THE 
 DISCOVERY OF BACTERIA BY IXEWENHOECK IN 1675. 
 THE RELATION OF CONTAGION TO THE THEORY OF 
 SPONTANEOUS GENERATION. NEEDHAM AND SPALLAN- 
 ZANI. THE DISCOVERY OF THE COMPOUND MICROSCOPE 
 IN 1605. THE PROOF THAT A LIVING ORGANISM is THE 
 CAUSE OF A DISEASE. ANTHRAX. THE DISCOVERY OF 
 THE ANTHRAX BACILLUS IN 1851. THE CULTIVATION 
 OF THE BACILLUS BY KOCH. THE MODE OF INFEC- 
 TION. THE WORK OF PASTEUR ON ANTHRAX. THE 
 IMPORTANCE OF THE DISEASE. 
 
 THESE are diseases which are caused by living 
 things which enter the tissues of the body and, 
 living at the expense of the body, produce injury. 
 Such diseases play an important part in the life 
 of man; the majority of deaths are caused di- 
 rectly or indirectly by infection. No other diseases 
 have been so much studied, and in no other 
 department of science has knowledge been cap- 
 able of such direct application in promoting the 
 health, the efficiency and the happiness of man. 
 This knowledge has added years to the average 
 length of life, it has rendered possible such great 
 engineering works as the Panama Canal, and 
 has contributed to the food supply by making 
 97
 
 98 DISEASE AND ITS CAUSES 
 
 habitation possible over large and productive 
 regions of the earth, formerly uninhabitable 
 owing to the prevalence of disease. It is not too 
 much to say that our modern civilization is 
 dependent upon this knowledge. The massing 
 of the people in large cities, the factory life, the 
 much greater social life, which are all prominent 
 features of modern civilization, would be difficult 
 or impossible without control of the infectious 
 diseases. The rapidity of communication and 
 the increased general movement of people, which 
 have developed in equal ratio with the massing, 
 would serve to extend widely every local out- 
 break of infection. The principles underlying 
 fermentation and putrefaction which have been 
 applied with great economic advantage to the 
 preservation of food were many of them developed 
 in the course of the study of the infectious dis- 
 eases. Whether the development of the present 
 civilization is for the ultimate advantage of man 
 may perhaps be disputed, but medicine has 
 made it possible. 
 
 The infectious diseases appearing in the form 
 of great epidemics have been important factors 
 in determining historical events, for they have 
 led to the defeat of armies, the fall of cities and 
 of nations. War is properly regarded as one of 
 the greatest evils that can afflict a nation, since 
 it destroys men in the bloom of youth, at the 
 age of greatest service, and brings sorrow and care
 
 DISEASE AND ITS CAUSES 99 
 
 and poverty to many. But the most potent 
 factor in the losses of war is not the deaths in 
 battle but the deaths from disease. If we desig- 
 nate the lives lost in battle, the killed and the 
 wounded who die, as 1, the loss of the German 
 army from disease in 1870-71 was 1.5, that of 
 the Russians in 1877-78 was 2.7, that of the 
 French in Mexico was 2.8, that of the French 
 in the Crimea 3.7, that of the English in Egypt 
 4.2. The total loss of the German army in 
 1870-71 from wounds and disease was 43,182 
 officers and men, and this seems a small number 
 compared with the 129,128 deaths from small- 
 pox in the same period in Prussia alone. In the 
 Spanish American war there were 20,178 cases 
 of typhoid fever with 1,580 deaths. In the South 
 African war there were in the British troops 
 31,118 cases of typhoid with 5,877 deaths, and 
 5,149 deaths from other diseases while the loss 
 in battle was 7,582. The Athenian plague which 
 prevailed during the Peloponnesian war, 431- 
 405 B. c., not only caused the death of Pericles, 
 but according to Thucydides a loss of 4,800 
 Athenian soldiers, and brought about the down- 
 fall of the Athenian hegemony in Greece. In 
 the Crimean war between 1853-56, 16,000 
 English, 80,000 French and 800,000 Russians 
 died of typhus fever. The plague contributed 
 as much as did the arms of the Turks to the 
 downfall of Constantinople and the Eastern
 
 100 DISEASE AND ITS CAUSES 
 
 Empire in 1453. It was the plague which in 
 1348 overthrew Siena from her proud position 
 as one of the first of the Italian cities and the 
 rival of Florence, and broke the city forever, 
 leaving it as a phantom of its former glory and 
 prosperity. The work on the great cathedral 
 which had progressed for ten years was suspended, 
 and when it was resumed it was upon a scale 
 adjusted to the diminished wealth of the city, 
 and the plan restricted to the present dimensions. 
 As a little relief to the darkness the same plague 
 saw the birth of the novel in the tales of Boccac- 
 cio, which were related to a delighted audience 
 of the women who had fled from the plague in 
 Florence to a rural retreat. 
 
 The knowledge which has come from the study 
 of infectious disease has served also to broaden 
 our conception of disease and has created pre- 
 ventive medicine; it has linked more closely to 
 medicine such sciences as zoology and botany; 
 it has given birth to the sciences of bacteriology 
 and protozoology and in a way has brought all 
 sciences more closely together. Above all it 
 has made medicine scientific, and never has 
 knowledge obtained been more quickening and 
 stimulating to its pursuit. 
 
 Although the dimensions of this book forbid 
 much reference to the historical development 
 of a subject, some mention must still be made 
 of the development of knowledge of the infec-
 
 DISEASE AND ITS CAUSES 101 
 
 tious diseases. It was early recognized that 
 there were diseases which differed in character 
 from those generally prevalent; large numbers 
 of people were affected in the same way; the 
 disease beginning with a few cases gradually 
 increased in intensity until an acme was reached 
 which prevailed for a time and the disease gradu- 
 ally disappeared. Such diseases were attributed 
 to changes in the air, to the influence of planets 
 or to the action of offended gods. The priests 
 and charlatans who sought to excuse their in- 
 ability to treat epidemics successfully were quick 
 to affirm supernatural causes. Hippocrates 
 (400 B. c.), with whom medicine may be said to 
 begin, thought such diseases, even then called 
 epidemics, were caused by the air; he says, 
 "When many individuals are attacked by a 
 disease at the same time, the cause must be 
 sought in some agent which is common to all, 
 something which everyone uses, and that is the 
 air which must contain at this time something 
 injurious." Aristotle recognized that disease 
 was often conveyed by contact, and Varro (116- 
 27 B. c.) advanced the idea that disease might be 
 caused by minute organisms. He says, " Certain 
 minute organisms develop which the eye cannot 
 see, and which being disseminated in the air 
 enter into the body by means of the mouth and 
 nostrils and give rise to serious ailments." In 
 spite of this hypothesis, which has proved to be
 
 102 DISEASE AND ITS CAUSES 
 
 correct, the belief became general that epidemics 
 were due to putrefaction of the air brought about 
 by decaying animal bodies, (this explaining the 
 frequent association of epidemics and wars,) by 
 emanations from swamps, by periods of unusual 
 heat, etc. 
 
 With the continued study of epidemics the 
 importance of contagion was recognized; it was 
 found that epidemics differed in character and 
 in the modes of extension. Some seemed to 
 extend by contact with the sick, and in others 
 this seemed to play no part; it was further found 
 impossible in many cases to show evidence of 
 air contamination, and contamination of the 
 air by putrefactive material did not always 
 produce disease. Most important was the recog- 
 nition that single cases of diseases which often 
 occurred in epidemic form might be present and 
 no further extension follow; this led to the as- 
 sumption in epidemics of the existence of some 
 condition in addition to the cause, and which 
 made the cause operative. In this way arose 
 the theory of the epidemic constitution, a sup- 
 posed peculiar condition of the body due to 
 changes in the character of the air, or to the 
 climate, or to changes in the interior of the earth 
 as shown by earthquakes, or to the movements 
 of planets; in consequence of this peculiar 
 constitution there was a greater susceptibility 
 to disease, but the direct cause might arise in
 
 DISEASE AND ITS CAUSES 103 
 
 the interior of the body or enter the body from 
 without. The character of the disease which 
 appeared in epidemic form, the "Genius epidem- 
 icus," was determined not by differences in the 
 intrinsic cause, but by the type of constitution 
 which prevailed at that time. The first epidemic 
 of cholera which visited Europe in 1830-37 was 
 for the most part referred to the existence of a 
 peculiar epidemic constitution for which various 
 causes were assigned. It was only when the 
 second epidemic of this disease appeared in 1840 
 that the existence of some special virus or poison 
 which entered the body was assumed. 
 
 Meanwhile, by the study of the material of 
 disease knowledge was being slowly acquired 
 which had much bearing on the causes. The 
 first observations which tended to show that the 
 causes were living were made by a learned Jesuit, 
 Athanasius, in 1659. He found in milk, cheese, 
 vinegar, decayed vegetables, and in the blood 
 and secretions of cases of plague bodies, which he 
 described as tiny worms and which he thought 
 were due to putrefaction. He studied these 
 objects with the simple lenses in use at that time, 
 and there is little doubt that he did see certain 
 of the larger organisms which are present in 
 vinegar, cheese and decaying vegetables, and it 
 is not impossible that he may have seen the 
 animal and vegetable cells. 
 
 The first description of bacteria with illustra-
 
 104 DISEASE AND ITS CAUSES 
 
 tions showing their forms was given by Los- 
 wenhoeck, a linen dealer in Amsterdam in 1675. 
 The fineness of the linen being determined by 
 the number of threads in a given area, it is 
 necessary to examine it with a magnifying lens, 
 and he succeeded in perfecting a simple lens 
 with which objects smaller than had been seen 
 up to that time became visible. It must 
 be added that he was probably endowed with 
 very unusual acuteness of vision. He found in 
 a drop of water, in the fluid in the intestines of 
 frogs and birds, and in his evacuations, objects 
 of great minuteness which differed from each 
 other in form and size and in the peculiar motion 
 which some of them possessed. In the year 1683 
 he presented to the Royal Society of London a 
 paper describing a certain minute organism 
 which he found in the tartar of his teeth. After 
 these observations of Loewenhoeck became known 
 to the world they quickly found application hi 
 disease, although the author had expressed him- 
 self very cautiously in this regard. The strongest 
 exponent of the view of a living contagion was 
 Plenciz, 1762, a physician of Vienna, basing 
 his belief not only on the demonstration of minute 
 organisms by IxEwenhoeck which he was able 
 to verify, but on certain shrewdly conceived 
 theoretical considerations. He was the first to 
 recognize the specificity of the epidemic diseases, 
 and argued from this that each disease must
 
 DISEASE AND ITS CAUSES 105 
 
 have a specific cause. "Just as a certain plant 
 comes from the seed of the same plant and not 
 from any plant at will, so each contagious disease 
 must be propagated from a similar disease and 
 cannot be the result of any other disease." Fur- 
 ther he says, "It is necessary to assume that 
 during the prevalence of an epidemic the con- 
 tagious material undergoes an enormous increase, 
 and this is compatible only with the assumption 
 that it is a living substance." But as is so often 
 the case, speculation ran far ahead of the obser- 
 vations on which it is based. There was a long 
 gap between the observations of Lo3wenhoeck 
 and the theories of Plenciz, justified as these 
 have been by present knowledge. In the spirit 
 of speculation which was dominant in Europe and 
 particularly in Germany in the latter half of the 
 eighteenth and the first half of the nineteenth 
 centuries, hypotheses did not stimulate research, 
 but led to further speculations. As late as 1820 
 Ozanam expressed himself as follows: "Many 
 authors have written concerning the animal 
 nature of the contagion of disease; many have 
 assumed it to be developed from animal sub- 
 stance, and that it is itself animal and possesses 
 the property of life. I shall not waste time in 
 refuting these absurd hypotheses." The theory 
 of a living contagion was too simple, and not 
 sufficiently related to the problems of the universe 
 to serve the medical philosophers.
 
 106 DISEASE AND ITS CAUSES 
 
 Knowledge of the minute organisms was slowly 
 accumulating. The first questions to be deter- 
 mined were as to their nature and origin. How 
 were they produced? Did they come from bodies 
 of the same sort according to the general laws 
 governing the production of living things, or did 
 they arise spontaneously? a question which 
 could not be solved by speculation but by experi- 
 ment. The first experiments, by Needham, 1745, 
 pointed to the spontaneous origin of the organ- 
 isms. He enclosed various substances in care- 
 fully sealed watch crystals from which the air 
 was excluded, and found that animalculi ap- 
 peared in the substance, and argued from this 
 that they developed spontaneously. In 1769, 
 Spallanzani, a skilled experimental physiologist, 
 in a brilliant series of experiments showed the 
 imperfect character of Needham's work and the 
 fallacy of his conclusions. Spallanzani placed 
 fluids, which easily became putrid, in glass tubes, 
 which he then hermetically sealed and boiled. 
 He found that the fluid remained clear and un- 
 changed ; if, however, he broke the sealed point 
 of such a tube and allowed the air to enter, 
 putrefaction, or in some cases fermentation, of 
 the contents took place. He concluded that 
 boiling the substances destroyed the living germs 
 which they contained, the sealed tubes prevented 
 the air from entering, and when putrefaction 
 or fermentation of the contents took place the
 
 DISEASE AND ITS CAUSES 107 
 
 organisms to which this was due, being con- 
 tained in the air, entered from without. Objec- 
 tion was made to the conclusions of Spallanzani 
 that heating the air in the closed tubes so changed 
 its character as to prevent development of organ- 
 isms in the contents. This objection was finally 
 set aside by Pasteur, who showed that it was not 
 necessary to seal the end of the tube before boiling, 
 but it could be closed by a plug of cotton wool, 
 which mechanically removed the organisms from 
 the air which entered the tube, or if the tube 
 were bent in the shape of a U and the end left 
 open, organisms from the air could not pass into 
 the tube against gravity when air movement 
 within the tube was prevented by bending. 
 The possibility of spontaneous generation can- 
 not be denied, but that it takes place is against 
 all human experience. 
 
 It was not possible to attain any considerable 
 knowledge of the Jbacteria discovered by Loe- 
 wenhoeck until more perfect instruments for 
 studying them were devised. Lenses for studying 
 objects were used in remote antiquity, but the 
 compound microscope in which the image made 
 by the lens is further magnified was not dis- 
 covered until 1605, and when first made was so 
 imperfect that the best simple lenses gave clearer 
 definition. With the betterment of the micro- 
 scope, increasing the magnifying power and the 
 sharpness of the image of the object seen, it became
 
 108 DISEASE AND ITS CAUSES 
 
 possible to classify the minute organisms accord- 
 ing to size and form and to study the separate 
 species. The microscope has now reached such 
 a degree of perfection that objects smaller than 
 one one hundred thousandth of an inch in diam- 
 eter can be clearly seen and photographed. 
 
 Great impetus was given to the biological 
 investigation of disease by the discoveries which 
 led to the formulation of the cell theory in 1840 
 and the brilliant work of Pasteur on fermentation, 1 
 but it was not until 1878 that it was definitely 
 
 1 The interesting analogy between fermentation and 
 infectious disease did not escape attention. A clear fluid 
 containing in solution sugar and other constituents necessary 
 for the life of the yeast cells will remain clear provided all 
 living things within it have been destroyed and those in the 
 air prevented from entering. If it be inoculated with a 
 minute fragment of yeast culture containing a few yeast 
 cells, for a time no change takes place; but gradually the 
 fluid becomes cloudy, bubbles of gas appear in it and its taste 
 changes. Finally it again becomes clear, a sediment forms 
 at the bottom, and on re-inoculating it with yeast culture 
 no fermentation takes place. The analogy is obvious; the 
 fluid in the first instance corresponds with an individual 
 susceptible to the disease, the inoculated yeast to the con- 
 tagion from a case of transmissible disease, the fermentation 
 to the illness with fever, etc., which constitutes the disease, 
 the returning clearness of the fluid to the recovery, and like 
 the fermenting fluid the individual is not susceptible to a new 
 attack of the disease. It will be observed that during the 
 process both the yeast and the material which produced the 
 disease have enormously increased. Fermentation of immense 
 quantities of fluid could be produced by the sediment of 
 yeast cells at the bottom of the vessel and a single case of 
 smallpox would be capable of infecting multitudes.
 
 DISEASE AND ITS CAUSES 109 
 
 proved that a disease of cattle called anthrax was 
 due to a species of bacteria. What should be 
 regarded as such proof had been formulated by 
 Henle in 1840. To prove that a certain sort of 
 organism when found associated with a disease 
 is the cause of the disease, three things are 
 necessary : 
 
 1. The organism must always be found in the 
 diseased animal and associated with the changes 
 produced by the disease. 
 
 2. The organism so found must be grown 
 outside of the body in what is termed pure cul- 
 tures, that is, not associated with any other 
 organisms, and for so long a time with constant 
 transfers or new seedings that there can be no 
 admixture of other products of the disease in 
 the material in which it is grown. 
 
 3. The disease must be produced by inocu- 
 lating a susceptible animal with a small portion 
 of such a culture, and the organism shown in 
 relation to the lesions so produced. 
 
 It is worth while to devote some attention to 
 the disease anthrax. This occupies a unique 
 position, in that it was the first of the infec- 
 tious diseases to be scientifically investigated. 
 In this investigation one fact after another was 
 discovered and confirmed; some of these facts 
 seemed to give clearer conceptions of the disease, 
 others served to make it more obscure; new 
 questions arose with each extension of knowledge;
 
 110 DISEASE AND ITS CAUSES 
 
 in the course of the work new methods of investi- 
 gation were discovered; the sides of the arch 
 were slowly and painfully erected by the work 
 of many men, and finally one man placed the 
 keystone and anthrax was for a long time the 
 best known of diseases. Men whose reputation 
 is now worldwide first became known by their 
 work in this disease. It was a favorable disease 
 for investigation, being a disease primarily of 
 cattle, but occasionally appearing in man, and 
 the susceptibility of laboratory animals made 
 possible experimental study. 
 
 Anthrax is a disease of domestic cattle affecting 
 particularly bovine cattle, horses and sheep, 
 swine more rarely. The disease exists in practi- 
 cally all countries and has caused great economic 
 losses. There are no characteristic symptoms 
 of the disease; the affected cattle have high fever, 
 refuse to eat, their pulse and respiration are 
 rapid, they become progressively weaker, un- 
 able to walk and finally fall. The disease lasts 
 a variable time; in the most acute cases animals 
 may die in less than twenty-four hours, or the 
 disease may last ten or fourteen days; recovery 
 from the disease is rare and treatment has no 
 effect. It does not appear in the form of epi- 
 demics, but single cases appear frequently or 
 rarely, and there is seemingly no extension from 
 case to case, animals in adjoining stalls to the sick 
 are not more prone to infection than others of
 
 DISEASE AND ITS CAUSES 111 
 
 the herd. On examination after death the blood 
 is dark and fluid, the spleen is greatly enlarged 
 (one of the names of the disease "splenic fever" 
 indicates the relation to the spleen) and there is 
 often bloody fluid in the tissues. 
 
 Where the disease is prevalent there are numbers 
 of human cases. Only those become infected who 
 come into close relations with cattle, the infection 
 most commonly taking place from small wounds 
 or scratches made in skinning dead cattle or in 
 handling hides. The wool of sheep who die of 
 the disease finds its way into commerce, and those 
 employed in handling the wool have a form of 
 anthrax known as wool-sorters' disease in which 
 lesions are found in the lungs, the organisms 
 being mingled with the wool dust and inspired. 
 In Boston occasional cases of anthrax appear in 
 teamsters who are employed in handling and 
 carrying hides. The disease in man is not so 
 fatal as in cattle, for it remains local for a time 
 at the site of infection, and this local disease can 
 be successfully treated. 
 
 The beginning of our knowledge of the cause 
 dates from 1851, when small rod-shaped bodies 
 (Fig. 17) were found in the blood of the affected 
 cattle, and by the work of a number of observers 
 it was established that these bodies were con- 
 stantly present. Nothing was known of their 
 nature; some held that they were living organ- 
 isms, others that they were formed in the
 
 112 DISEASE AND ITS CAUSES 
 
 body as a result of the disease. Next the 
 causal relation of these bodies with the disease 
 was shown and in several ways. The disease 
 could be caused in other cattle by injecting 
 blood containing the rods beneath the skin, 
 certainly no proof, for the blood might have 
 contained in addition to the rods something 
 which was the real cause of the disease. Next 
 it was shown that the blood of the unborn calf 
 of a cow who died of the disease did not con- 
 tain the rods, and the disease could not be 
 produced by inoculating with the calf's blood 
 although the blood of the mother was infectious. 
 This was a very strong indication that the rods 
 were the cause; the maternal and foetal blood 
 are separated by a membrane through which 
 fluids and substances hi solution pass; but 
 insoluble substances, even when very minutely 
 subdivided, do not pass the membrane. If the 
 cause were a poison hi solution, the foetal blood 
 would have been as toxic as the maternal. The 
 blood of infected cattle was filtered through 
 filters made of unbaked porcelain and having 
 very fine pores which allowed only the blood 
 fluid to pass, holding back both the blood cor- 
 puscles and the rods, and such filtered blood was 
 found to be innocuous. It was further shown 
 that the rods increased enormously in number in 
 the infected animal, for the blood contained them 
 in great numbers when but a fraction of a drop
 
 DISEASE AND ITS CAUSES 113 
 
 was used for inoculation. Attempts were also 
 made with a greater or less degree of success to 
 grow the rod shaped organisms or bacilli in vari- 
 ous fluids, and the characteristic disease was pro- 
 duced by inoculating animals with these cultures; 
 but it remained for Koch, 1878, who was at that 
 time an obscure young country physician, to show 
 the life history of the organism and to clear up 
 the obscurity of the disease. Up to that time, 
 although it had been shown that the rods or bacilli 
 contained in the blood were living organisms and 
 the cause of the disease, this did not explain the 
 mode of infection ; how the organisms contained in 
 the blood passed to another animal, why the 
 disease occurred on certain farms and the adjoin- 
 ing farms, particularly if they lay higher, were free. 
 Koch showed that in the cultures the organisms 
 grew out into long interlacing threads, and that 
 in these threads spores which were very difficult to 
 destroy developed at intervals; that the organ- 
 isms grew easily in bouillon, in milk, in blood, 
 and even in an infusion of hay made by soaking 
 this in water. This explained, what had been 
 an enigma before, how the fields became sources 
 of infection. The infection did not spread from 
 animal to animal by contact, but infection took 
 place from eating grass or hay which contained 
 either the bacilli or their spores. When a dead 
 animal was skinned on the field, the bacilli con- 
 tained in the blood escaped and became mingled
 
 114 DISEASE AND ITS CAUSES 
 
 with the various fluids which flowed from the 
 body and in which they grew and developed 
 spores. It was shown by Pasteur that even when 
 a carcass was buried the earthworms brought 
 spores developed in the body to the surface and 
 deposited them in their casts, and in this way also 
 the fields became infected. From such a spot of 
 infected earth the spores could be washed by the 
 rains over greater areas and would find opportunity 
 to develop further and form new spores in puddles 
 of water left on the fields, which became a culture 
 medium by the soaking of the dead grass. The 
 contamination of the fields was also brought about 
 by spreading over them the accumulations of 
 stable manure which contained the discharges of 
 the sick cattle. The tendency of the disease to 
 extend to lower-lying adjacent fields was due to 
 the spores being washed from the upper fields to 
 the lower by the spring freshets. Meanwhile 
 Pasteur had discovered that by growing the organ- 
 isms at higher temperatures than the animal body, 
 it was possible to attenuate the virulence of the 
 bacilli so that inoculations with these produced 
 a mild form of the disease which rendered the in- 
 oculated animals immune to the fatal disease. 
 The description of Pasteur's work on the disease 
 as given in the account of his life by his son-in- 
 law is fascinating. 
 
 Hides and wool taken from dead animals in- 
 variably contained the spores which could pass
 
 DISEASE AND ITS CAUSES 115 
 
 unharmed through some of the curing processes, 
 and were responsible for some of the cases in 
 man. Owing to the introduction of regulations 
 which were based on the knowledge of the cause 
 of the disease and the life history of the organism, 
 together with the prophylactic inoculation de- 
 vised by Pasteur, the incidence of the disease 
 has been very greatly lessened. Looking at the 
 matter from the lowest point of view, the money 
 which has been saved by the control of the disease, 
 as shown in its decline, has been many times the 
 cost of all the work of the investigations which 
 made the control possible. It is a greater satis- 
 faction to know that many human lives have been 
 saved, and that small farmers and shepherds have 
 been the chief sharers in the economic benefits. 
 The indirect benefits, however, which have re- 
 sulted from the application of the knowledge of 
 this disease, and the methods of investigation de- 
 veloped here, to the study of the infections more 
 peculiar to man, are very much greater.
 
 CHAPTER VI 
 
 CLASSIFICATION OP THE ORGANISMS WHICH CAUSE DISEASE. 
 BACTERIA : SIZE, SHAPE, STRUCTURE, CAPACITY FOR 
 GROWTH, MULTIPLICATION AND SPORE FORMATION. 
 THE ARTIFICIAL CULTIVATION OF BACTERIA. THE IM- 
 PORTANCE OF BACTERIA IN NATURE. VARIATIONS IN 
 BACTERIA. SAPROPHTTIC AND PARASITIC FORMS. 
 PROTOZOA. STRUCTURE MORE COMPLICATED THAN THAT 
 OF BACTERIA. DISTRIBUTION EN NATURE. GROWTH 
 AND MULTIPLICATION. CONJUGATION AND SEXUAL RE- 
 PRODUCTION. SPORE FORMATION. THE NECESSITY FOR 
 A FLUID ENVIRONMENT. THE FOOD OF PROTOZOA. 
 PARASITISM. THE ULTRA-MICROSCOPIC OR FILTERABLE 
 ORGANISMS. THE LIMITATION OF THE MICROSCOPE. 
 PORCELAIN FILTERS TO SEPARATE ORGANISMS FROM A 
 FLUID. FOOT AND MOUTH DISEASE PRODUCED BY AN 
 ULTRA-MICROSCOPIC ORGANISM. OTHER DISEASES so 
 PRODUCED. Do NEW DISEASES APPEAR? 
 
 THE living organisms which cause the infectious 
 diseases are classified under bacteria, protozoa, 
 yeasts, moulds, and ultra-microscopic organisms. 
 It is necessary to place in a separate class the 
 organisms whose existence is known, but which 
 are not visible under the highest powers of the 
 microscope, and have not been classified. The 
 yeasts and moulds play a minor part in the pro- 
 duction of disease and cannot be considered in 
 the necessary limitation of space. 
 
 The bacteria (Fig. 17) are unicellular organ- 
 isms and vary greatly in size, shape and capacity 
 
 116
 
 DISEASE AND ITS CAUSES 117 
 
 of growth. The smallest of the pathogenic or 
 disease-producing bacteria is the influenza ba- 
 cillus, 1/51000 of an inch in length and 1/102000 
 
 FIG. 17. VARIOUS FORMS OF BACTERIA, a, b, c, d, Round 
 bacteria or cocci: (a) Staphylococci, organisms which occur 
 in groups and a common cause of boils; (b) streptococci, 
 organisms which occur in chains and produce erysipelas 
 and more severe forms of inflammation; (c) diplococci, or 
 paired organisms with a capsule, which cause acute pneu- 
 monia; (d) gonococci, with the opposed surfaces flattened, 
 which cause gonorrhoaa. e, f, g, h. Rod-shaped bacteria or 
 bacilli: (e) diphtheria bacilli; (/) tubercle bacilli; (g) anthrax 
 bacilli; (h) the same bacilli in cultures and producing spores; 
 a small group of spores is shown, (i) Cholera spirillse. (j) 
 Typhoid bacilli, (k) Tetanus bacillus; i, j, k are actively 
 motile, motion being effected by the small attached threads. 
 (/) The screw-shaped spirochite which is the cause of syphilis. 
 
 of an inch in thickness; and among the largest 
 is a bacillus causing an animal disease which is 
 1, 2000 of an inch in length and 1/25000 of an
 
 118 DISEASE AND ITS CAUSES 
 
 inch in diameter. Among the free-living non- 
 pathogenic forms much larger examples are found. 
 In shape bacteria are round, or rod-shaped, or 
 spiral; the round forms are called micrococci, the 
 rod-shaped bacilli and the spiral forms are called 
 spirilli. A clearer idea of the size is possibly 
 given by the calculation that a drop of water 
 would contain one billion micrococci of the usual 
 size. Then* structure in a general way conforms 
 with that of other cells. On the outside is a cell 
 membrane which encloses cytoplasm and nucleus; 
 the latter, however, is not in a single mass, but 
 the nuclear material is distributed through the 
 cell. Many of the bacteria have the power of 
 motion, this being effected by small hair-like 
 appendages or flagella? which may be numerous, 
 projecting from all parts of the organisms or from 
 one or both ends, the movement being produced by 
 rapid lashing of these hairs. A bacterium grows 
 until it attains the size of the species, when it 
 divides by simple cleavage at right angles to the 
 long axis forming two individuals. In some of 
 the spherical forms division takes place alter- 
 nately in two planes, and not infrequently the 
 single individuals adhere, forming figures of long 
 threads or chains or double forms. The rate of 
 growth varies with the species and with the 
 environment, and under the best conditions may 
 be very rapid. A generation, that is, the interval 
 between divisions, has been seen to take place in
 
 DISEASE AND ITS CAUSES 119 
 
 twenty minutes. At this rate of growth from a 
 single cholera bacillus sixteen quadrillion might 
 arise in a single day. Such a rate of growth is 
 extremely improbable under either natural or 
 artificial conditions, both from lack of food and 
 from the accumulation in the fluid of waste pro- 
 ducts which check growth. Many species of 
 bacteria in addition to this simple mode of multi- 
 plication form spores which are in a way analogous 
 to the seeds of higher plants and are much more 
 resistant than the simple or vegetative forms; they 
 endure boiling water and even higher degrees of 
 dry heat for a considerable time before they are 
 destroyed. When these spores are placed in 
 conditions favorable for bacterial life, the bac- 
 terial cells grow out from them and the usual 
 mode of multiplication continues. This capacity 
 for spore formation is of great importance, and 
 until it was discovered by Cohn in 1876, many 
 of the conditions of disease and putrefaction 
 could not be explained. Spores, as the seeds of 
 plants, often seem to be produced when the 
 conditions are unfavorable; the bacterium 
 then changes into this form, which under natural 
 conditions is almost indestructible and awaits 
 better days. 
 
 The bacteria are divided into species, the classi- 
 fication being based on their forms, on the mode of 
 growth, the various substances which they produce 
 and their capacity for producing disease. The dif-
 
 120 DISEASE AND ITS CAUSES 
 
 ferentiation of species in bacteria is based chiefly 
 upon their properties, there being too little dif- 
 ference in form and size to distinguish species. 
 The introduction of methods of culture was fol- 
 lowed by an immediate advance of our knowledge 
 concerning them. This method consists in the 
 use of fluid and solid substances which contain the 
 necessary salts and other ingredients for their 
 food, and in or on which they are planted. The 
 use of a solid or gelatinous medium for growth has 
 greatly facilitated the separation of single species 
 from a mixture of bacteria; a culture fluid contain- 
 ing sufficient gelatine to render it solid when cooled 
 is sown with the bacteria to be tested by placing 
 in it while warm and fluid, a small portion of 
 material containing the bacteria, and after being 
 thoroughly mixed the fluid is poured on a glass 
 plate and allowed to cool. The bacteria are in 
 this way separated, and each by its growth forms 
 a single colony which can be further tested. It 
 is self-evident that all culture material must be 
 sterilized by heat before using, and in the manipu- 
 lations care must be exercised to avoid contami- 
 nation from the air. The refraction index of the 
 bacterial cell is so slight that the microscopic 
 study is facilitated or made possible by staining 
 them with various aniline dyes. Owing to 
 differences in the cell material the different 
 species of bacteria show differences in the facility 
 with which they take the color and the tenacity
 
 DISEASE AND ITS CAUSES 121 
 
 with which they retain it, and this also forms 
 a means of species differentiation. The inter- 
 relation of science is well shown in this, for it 
 was the discovery of the aniline dyes in the latter 
 half of the nineteenth century which made the 
 fruitful study of bacteria possible. 
 
 From the simplicity of structure it is not 
 improbable that the bacteria are among the 
 oldest forms of life, and all life has become adapted 
 to their presence. They are of universal distribu- 
 tion; they play such an important part in the 
 inter-relations of living things that it is probable 
 life could not continue without them, at least not 
 in the present way. They form important food 
 for other unicellular organisms which are im- 
 portant links in the chain; they are the agents of 
 decomposition, by which the complex substances 
 of living things are reduced to elementary sub- 
 stances and made available for use; without them 
 plant life would be impossible, for it is by their 
 instrumentality that material in the soil is so 
 changed as to be available as plant food; by 
 their action many of the important foods of man, 
 often those especially delectable, are produced; 
 they are constantly with us on all the surfaces 
 of the body ; masses live on the intestinal surfaces 
 and the excrement is largely composed of bac- 
 teria. It has been said that life would be impos- 
 sible without bacteria, for the accumulation of 
 the carcasses of all animals which have died would
 
 122 DISEASE AND ITS CAUSES 
 
 so encumber the earth as to prevent its use; but 
 the folly of such speculation is shown by the fact 
 that animals would not have been there without 
 bacteria. It has been shown, however, that the 
 presence of bacteria in the intestine of the higher 
 animals is not essential for life. The coldest 
 parts of the ocean are free from those forms 
 which live in the intestines, and fish and birds 
 inhabiting these regions have been found free 
 from bacteria; it has also been found possible 
 to remove small animals from their mother by 
 Caesarian section and to rear them for a few 
 weeks on sterilized food, showing that digestion 
 and nutrition may go on without bacteria. 
 
 Certain species of bacteria are aerobic, that is, 
 they need free oxygen for their growth ; others are 
 anaerobic and will not grow in the presence of 
 oxygen. Most of the bacteria which produce 
 disease are facultative, that is, they grow either 
 with or without oxygen; but certain of them, 
 as the bacillus of tetanus, are anaerobic. There 
 is, of course, abundance of oxygen in the blood 
 and tissues, but it is so combined as to be un- 
 available for the bacteria. Bacteria may further 
 be divided into those which are saprophytic or 
 which find favorable conditions for life outside 
 of the body, and the parasitic. Many are ex- 
 clusively parasitic or saprophytic, and many are 
 facultative, both conditions of living being pos- 
 sible. It has been found possible by varying in
 
 DISEASE AND ITS CAUSES 123 
 
 many ways the character of the culture medium 
 and temperature to grow under artificial con- 
 ditions outside of the body most, if not all, of 
 the bacteria which cause disease. Thus, such 
 bacteria as tubercle bacilli and the influenza 
 bacillus can be cultivated, but they certainly 
 would not find natural conditions which would 
 make saprophytic growth possible. 
 
 Bacteria may be very sensitive to the presence 
 of certain substances in the fluid in which they 
 are growing. Growth may be inhibited by the 
 smallest trace of some of the metallic salts, as 
 corrosive sublimate, although the bacteria them- 
 selves are not destroyed. If small pieces of gold 
 foil be placed on the surface of prepared jelly 
 on which bacteria have been planted, no growth 
 will take place in the vicinity of the gold foil. 
 
 Variations can easily be produced in bacteria, 
 but they do not tend to become established. 
 In certain of the bacterial species there are 
 strains which represent slight variations from 
 the type but which are not sufficient to consti- 
 tute new species. If the environment in which 
 bacteria are living be unusual and to a greater 
 or less degree unfavorable, those individuals in 
 the mass with the least power of adaptibility 
 will perish, those more resistant and with greater 
 adaptability will survive and propagate; and 
 the peculiarity being transmitted a new strain will 
 arise characterized by this adaptability. Bac-
 
 124 DISEASE AND ITS CAUSES 
 
 teria with slight adaptability to the environment 
 of the tissues and fluids of the animal body can, 
 by repeated inoculations, become so adapted to 
 the new environment as to be in a high degree 
 pathogenic. In such a process the organisms 
 with the least power of adaptation are destroyed 
 and new generations are formed from those of 
 greater power of adaptation. When bacteria are 
 caused to grow in a new environment they may 
 acquire new characteristics. The anthrax bacilli 
 find the optimum conditions for growth at the 
 temperature of the animal body, but they will 
 grow at temperatures both above and below this. 
 Pasteur found that by gradually increasing the 
 temperature they could be grown at one hundred 
 and ten degrees. When grown at this tem- 
 perature they were no longer so virulent and 
 produced in animals a mild non-fatal form of 
 anthrax wh^ch protected the animal when in- 
 oculated with the virulent strain. The well 
 known variations in the character of disease, 
 shown in differences in severity and ease of trans- 
 mission, seen in different years and hi different 
 epidemics, may be due to many conditions, but 
 probably variation in the infecting organisms is 
 the most important. 
 
 The protozoa, like the bacteria, are unicellular 
 organisms and contain a nucleus as do all cells. 
 They vary in size from forms seen with difficulty 
 under the highest power of the microscope to
 
 forms readily seen with the unaided eye. Their 
 structure in general is more complex than is the 
 structure of bacteria, and many show extreme 
 differentiation of parts of the single cells, as a 
 firm exterior surface or cuticle, an internal skele- 
 ton, organs of locomotion, mouth and digestive 
 organs and organs of excretion. They are more 
 widely distributed than are the bacteria, and 
 found from pole to pole in all oceans and in all 
 fresh water. There are many modes of multi- 
 plication, and these are often extremely compli- 
 cated. The most general mode and one which is 
 common to all is by simple division ; a modification 
 of this is by budding in which projections or buds 
 form on the body and after separation become 
 new organisms. In other cases spores form within 
 the cell which become free and develop further 
 into complete organisms. These simple modes 
 of multiplication often alternate in the same 
 organism with sexual differentiation and con- 
 jugation. There is never a permanent sexual 
 differentiation, but the sexual forms develop 
 from a simple and non-sexual organism. Usually 
 the sexual forms develop only in a special environ- 
 ment; thus the protozoon which in man is the 
 cause of malaria, multiplies in the human blood 
 by simple division, but in the body of the mos- 
 quito multiplication by sexual differentiation 
 takes place. Under no conditions is multiplica- 
 tion so rapid as with the bacteria, and in general
 
 126 DISEASE AND ITS CAUSES 
 
 the simpler the form of organism the more rapid 
 is the multiplication. It is common to all of 
 the protozoa to develop forms which have great 
 powers of resistance, this being due in some 
 cases to encystment, in which condition a resistant 
 membrane is formed on the outside, in others 
 to the production of spores. A fluid environment 
 is essential to the life of the protozoa, but the 
 resistant forms can endure long periods of dryness 
 or other unfavorable environmental conditions. 
 The universal distribution of the protozoa is 
 due to this; the spores or cysts can be carried 
 long distances by the wind and develop into 
 active forms when they reach an environment 
 which is favorable. Their distribution in water 
 depends upon the amount of organic material 
 this contains. In pure drinking water there 
 may be very few, but in stagnant water they are 
 very numerous, living not on the organic material 
 in solution in this, but on the bacteria which 
 find in such fluid favorable conditions for exist- 
 ence. The food of protozoa consists chiefly of 
 other organisms, particularly bacteria, and they 
 are classed with the animals. The protozoa are 
 the most widely distributed and the most uni- 
 versal of the parasites. The infectious diseases 
 which they produce in man, although among 
 the most serious are less in number than 
 those produced by bacteria. So marked is the 
 tendency to parasitism that they are often para-
 
 DISEASE AND ITS CAUSES 127 
 
 sitic for each other, smaller forms entering into 
 and living upon the larger. Variation does not 
 seem to be so marked in the protozoa as in the 
 bacteria, though this is possibly due to our greater 
 ignorance of them as a class. We are not able, 
 except in rare instances, to grow them in pure 
 culture, and study innumerable generations under 
 changes in the environment, as the bacteria have 
 been studied. 
 
 If we regard the living things on earth from 
 the narrow point of view as to whether they are 
 necessary or useless or hostile to man, the pro- 
 tozoa must be regarded as about the least useful 
 members of the biological society. It is very 
 possible that such a conclusion is due to ignorance; 
 so closely are all living things united, so dependent 
 is one form of cell activity upon other forms that 
 it is impossible to foretell the result of the 
 removal of a link. The protozoa do not seem 
 to be as necessary for the life of man as are the 
 bacteria; they produce many of the diseases of 
 man, many of the diseases of animals on which 
 man depends for food; they cause great destruc- 
 tion in plant life, and in the soil they feed upon 
 the useful bacteria. It is well to remember, 
 however, that fifty years ago several of the organs 
 of the body whose activity we now recognize 
 as furnishing substances necessary for life were 
 regarded as useless members and, since they 
 became the seat of tumors, as dangerous mem-
 
 128 DISEASE AND ITS CAUSES 
 
 bers of the body. The only organ which now 
 seems to come into such a class is the vermiform 
 appendix, and its lowly position among organs 
 is due merely to an unhappy accident of de- 
 velopment. 
 
 The class of organisms known as the filterable 
 viruses or the ultra-microscopic or the invisible 
 organisms have a special interest in many ways. 
 The limitation in the power of the microscope 
 for the study of minute objects is due not to a 
 defect in the instrument but to the length of 
 the wave of light. It is impossible to see clearly 
 under the microscope using white light, objects 
 which are smaller in diameter than the length 
 of the wave which gives a limit of 0.5 /*. or 1/125,- 
 000 of an inch. By using waves of shorter length, 
 as the ultra-violet light, objects of 0.1 p. or 
 1/250000 of an inch can be seen; but as these 
 methods depend upon photography for the 
 demonstration of the object the study is diffi- 
 cult. The presence of objects still smaller than 
 0.1 m. can be detected in a fluid by the use of 
 the dark field illumination and the ultra-micro- 
 scope, the principle of which is the direction of 
 a powerful oblique ray of light into the field of 
 the microscope. The objects are not visible as 
 such, but the dispersion of the light by their 
 presence is seen. 
 
 The demonstration that infectious diseases 
 were produced by organisms so small as to be
 
 DISEASE AND ITS CAUSES 129 
 
 beyond demonstration with the best micro- 
 scopes was made possible by showing, that some 
 fluid from a diseased animal was infectious; 
 and capable of producing the disease when in- 
 oculated into a susceptible animal. The fluid 
 was then filtered through porcelain filters 
 which were known to hold back all objects of 
 the size of the smallest bacteria and the disease 
 produced by inoculating with the clear filtrate. 
 There are a number of such filters of different 
 degrees of porosity manufactured, and they are 
 often used to procure pure water for drinking, 
 for which use they are more or less, generally 
 however, less efficacious. The filter has the form 
 of a hollow cylinder and the liquid to be filtered 
 is forced through it under pressure. For domestic 
 use the filter is attached by its open end to the 
 water tap and the pressure from the mains forces 
 the water through it. In laboratory uses, denser 
 filters of smaller diameters are used, and the 
 filter is surrounded by the fluid to be tested. 
 The open end of the filter passes into a vessel 
 from which the air is exhausted and filtration 
 takes place from without inward. The test of 
 the effectiveness of the filter is made by adding 
 to the filtering fluid some very minute and 
 easily recognizable bacteria and testing the 
 filtrate for their presence. These filters have 
 been studied microscopically by grinding very 
 thin sections and measuring the diameter of the
 
 130 DISEASE AND ITS CAUSES 
 
 spaces in the material. These are very numerous, 
 and from 1/25000 to 1/1000 of an inch in 
 diameter, spaces which would allow bacteria 
 to pass through, but they are held back by the 
 very fine openings between the spaces and by 
 the tortuosity of the intercommunications. When 
 the coarser of such filters have been long in 
 domestic service hi filtering drinking water, bac- 
 teria may grow in and through them giving 
 greater bacterial content to the supposed bac- 
 teria-free filtrate than hi the filtering water. 
 
 That an animal disease was due to such a 
 minute and filterable organism was first shown 
 by Loeffler in 1898 for the foot and mouth disease 
 of cattle. This is one of the most infectious and 
 easily communicable diseases. The lesions of 
 the disease take the form of blisters which form 
 on the lips and feet and in the mouths of cattle, 
 and inoculation with minute quantities of the 
 fluid in the blisters produces the disease. Loeffler 
 filtered the fluid through porcelain filters, hoping 
 to obtain a material which inoculated into other 
 cattle would render them immune, and to his 
 surprise found that the typical disease was pro- 
 duced by inoculating with the filtrate. Natu- 
 rally the first idea was that the disease was caused 
 by some soluble poison and not by a living organ- 
 ism, but this was disproved in a number of ways. 
 The most powerful poison known is obtained 
 from cultures of the tetanus bacillus of which
 
 DISEASE AND ITS CAUSES 131 
 
 0.000,000,1 of a gram (one gram is 15.43 grains) 
 kills a mouse, or one gram kills ten million 
 mice. Loeffler found that 1/30 gram of the con- 
 tents of the vesicles killed a calf of two hundred 
 kilograms weight, and assuming that the essen- 
 tial poison was present in the fluid in one part to 
 five hundred it would be several hundred times 
 more powerful than the tetanus poison. Further, 
 the disease produced by inoculation of the filtrate 
 was itself inoculable and could be transmitted 
 from animal to animal. It was also found that 
 when the virus was filtered several times it ceased 
 to be inoculable, showing that each time the 
 fluid was passed through the filter some of the 
 minute organisms contained in it were held back. 
 It is not known whether these organisms belong 
 to the bacteria or protozoa, and naturally noth- 
 ing is known as to their form, size and structure. 
 Up to the present about twenty diseases are 
 known to be due to a filterable virus, and among 
 these are some of the most important for animals 
 and for man. Among the human diseases, yellow 
 fever, poliomyelitis, and dengue are so produced; 
 of the animal diseases in addition to foot and 
 mouth disease, pleuropneumonia, cattle plague, 
 African horse sickness, several diseases of fowls 
 and the mosaic disease of the tobacco plant 
 have all been shown to be due to a filterable 
 virus. Of these organisms the largest is that 
 which produces pleuropneumonia in cattle, and
 
 132 DISEASE AND ITS CAUSES 
 
 this alone has been cultivated. It gives a slight 
 opacity to the culture fluids, and when magnified 
 two thousand diameters appears as a minute 
 spiral or round or stellate organism having a 
 variety of forms. Its size is such that it passes 
 the coarse, but is held back by the finer, filters 
 and it is possible that this does not belong to 
 the same class with the others. 1 The diseases pro- 
 duced by the filterable viruses taken as a class 
 show much similarity. They run an acute course, 
 are severe, and the immunity produced by the 
 attack endures for a long time. 
 
 Considered in its biological relations, infection 
 is the adaptation of an organism to the environ- 
 ment which the body of the host offers. It is 
 rather singular that variations in organisms 
 represented by such adaptation do not more 
 frequently arise, in which case new diseases 
 would frequently occur. It cannot be denied 
 that new diseases appear, but there is no certain 
 evidence that they do, and there is equally no 
 evidence that diseases disappear. From the 
 meagre descriptions of diseases, usually of the 
 epidemic type, which have come down to us from 
 the past, it is difficult to recognize many of the 
 diseases described. The single diseases are 
 recognized by comparing the causes, the lesions 
 
 1 Flexner has recently succeeded in isolating and cultivat- 
 ing the organism of poliomyelitis, but the organism is so 
 small that its classification is not possible.
 
 and the symptoms with those of other dis- 
 eases, and new diseases are constantly being 
 separated off from other diseases having more 
 or less common features. Many new diseases 
 have been recognized and named, but it is 
 always more than probable that previously they 
 were confounded with other diseases. Small- 
 pox is such a characteristic disease that one 
 would think it would have been recognized as 
 an entity from the beginning, but although 
 the description of some of the epidemics in 
 remote times conform more or less to the 
 disease as we know it, the first accurate descrip- 
 tion is in the eighth century by the Arabian 
 physician Rhazes. Cerebro-spinal meningitis 
 was not recognized as a separate disease until 
 1803, diphtheria not until 1826, and the sepa- 
 ration between typhoid and typhus fever was not 
 made before 1840. Nor is it sure that any dis- 
 eases have disappeared, although there seems to 
 have been a change in the character of many. 
 It is difficult to reconcile leprosy as it appears 
 now with the universal horror felt towards it, 
 due to the persistence of the old traditions. It 
 is possible, however, that the disease has not 
 changed its character, but that such diseases as 
 smallpox, syphilis, and certain forms of tuber- 
 culosis were formerly confounded with leprosy, 
 thus giving a false idea of its prevalence. 
 
 In certain cases the adaptation of the organism
 
 134 DISEASE AND ITS CAUSES 
 
 is for a narrow environment; for example, the 
 parasitism may extend to a simple species only, in 
 others the adaptation may extend to a number of 
 genera. In certain cases the adaptation is mutual, 
 extending to both parasite and host and resulting 
 in symbiosis, and this condition may be advanta- 
 geous for both. Certain of the protozoa harbor 
 within them cells of algse utilizing to then* own 
 advantage the green chlorophil of the algse in 
 obtaining energy from sunlight and in turn 
 giving sustenance to the algse. Although the 
 algse are useful guests, when they become too 
 numerous the protozoan devours them. It is 
 evident that symbiosis is the most favorable 
 condition for the existence of the parasite, and 
 an injurious action exerted by the parasite on 
 the host unfavorable. The death of the host 
 is an unfortunate incident from the parasite's 
 point of view in that it is deprived of habita- 
 tion and food supply, being placed in the same 
 unfortunate situation as may befall a social para- 
 site by the death of his host.
 
 CHAPTER VII 
 
 THE NATURE op INFECTION. THE INVASION OP THE BODY 
 FROM ITS SURFACES. THE PROTECTION OP THESE SUR- 
 FACES. CAN BACTERIA PASS THROUGH AN UNINJURED 
 SURFACE. INFECTION FROM WOUNDS. THE WOUNDS 
 IN MODERN WARFARE LESS PRONE TO INFECTION. THE 
 RELATION OP TETANUS TO WOUNDS CAUSED BY THE TOY 
 PISTOL. THE PRIMARY Focus OR ATRIUM OF INFEC- 
 TION. THE DISSEMINATION OF BACTERIA IN THE BODY. 
 THE DIFFERENT DEGREES OF RESISTANCE TO BACTERIA 
 
 SHOWN BY THE VARIOUS ORGANS. MODE OF ACTION OP 
 
 BACTERIA. TOXIN PRODUCTION. THE RESISTANCE OP 
 THE BODY TO BACTERIA. CONFLICT BETWEEN PARASITE 
 AND HOST. ON BOTH SIDES MEANS OF OFFENSE AND 
 DEFENSE. PHAGOCYTOSIS. THE DESTRUCTION OF BAC- 
 TERIA BY THE BLOOD. THE Toxic BACTERIAL DIS- 
 EASES. TOXIN AND ANTITOXIN. IMMUNITY. THE 
 THEORY OP EHRLICH. 
 
 As has been said, infection consists in the 
 injury of the body by living organisms which 
 enter it. The body is in relation to the external 
 world by its surfaces only, and organisms must 
 enter it by some one of these surfaces. It is true 
 that the bacteria in the intestine either those 
 normally present or unusual varieties may, 
 under certain circumstances, produce substances 
 which are injurious when absorbed; but this 
 is not infection, and is analogous to any other 
 sort of poisoning. Each surface of the body has 
 its own bacterial flora. Organisms live on the 
 135
 
 136 DISEASE AND ITS CAUSES 
 
 surface either on matter which is secreted by 
 the surface or they use up an inappreciable 
 amount of body material. Many of these bac- 
 teria are harmless, some are protective, producing 
 by their growth such changes in the surface fluids 
 that these become hostile to the existence of 
 other and pathogenic forms. The surfaces also 
 frequently harbor pathogenic organisms which 
 await some condition to arise which will permit 
 them to effect entrance into the tissues. 
 
 The surfaces of the body protect from invasion 
 to a greater or less degree. The skin protects 
 by the impervious horny layer on the outside, the 
 external cells of which are dead and constantly 
 being thrown off. Bacteria are always found on 
 and in this layer, but the conditions for growth 
 here are not very favorable and the surface is 
 constantly cleansed by desquamation. The new 
 cells to supply the loss are produced in the deepest 
 layer of the epidermis, and the movement of 
 cells and fluids takes place from within outwards. 
 The protection is less perfect about the hairs 
 and the sweat glands. Infection by the route 
 of the sweat glands is, however, uncommon, for 
 the sweat is a fluid unfavorable for bacterial 
 growth and the flow acts mechanically in 
 washing away organisms which may have entered 
 the ducts. Infection by the route of the hair 
 follicles is common. There is no mechanical 
 cleansing as by the sweat, the space around the
 
 DISEASE AND ITS CAUSES 137 
 
 hair is large and the accumulated secretion of 
 the hair glands and the desquamated cells furnish 
 a material in which bacteria may grow. Growing 
 as a mass in this situation, they may produce 
 sufficient toxic material to destroy adjacent 
 living cells and thus effect entrance. Infection 
 from the eye is not common, the surface, though 
 moist, is smooth; the eyelashes around the 
 margin of the lids give some mechanical pro- 
 tection from the entrance of bacteria contained 
 in dust, and the movements of the lids and the 
 constant and easily accelerated secretion of 
 tears act mechanically in removing foreign sub- 
 stances. It is possible that the mechanical 
 cleansing of the skin by the daily bath may have 
 some action in preventing infection. 
 
 The internal surfaces are much more exposed 
 to attack and the protection is not so efficient. 
 The moisture of these surfaces is both a pro- 
 tection and a source of danger. It protects by 
 favoring the lodgment near the orifices of organ- 
 isms which are in the inspired air, for when 
 bacteria touch a moist surface they cannot be 
 raised from this and carried further by air cur- 
 rents. The moisture is a source of danger in 
 that it favors the growth of bacteria which lodge 
 on the surface. The respiratory surface which 
 is most exposed to infection from the air is 
 further protected by the cilia, which are fine 
 hair-like processes covering the cells of the
 
 138 DISEASE AND ITS CAUSES 
 
 surface and which by their constant motion 
 sweep out fine particles of all sorts which lodge 
 upon them. The cavity of the mouth harbors 
 large numbers of organisms, many of them patho- 
 genic. It forms a depot from which bacteria may 
 pass to communicating surfaces and infection 
 from these may result. Food particles collect 
 hi the mouth and provide culture material, and 
 there are many crypts and irregularities of surface 
 which oppose mechanical cleaning. Infection 
 of the middle ear, the most common cause of 
 deafness, takes place by means of the Eustachian 
 tube which connects the cavity of the ear with 
 the mouth. Organisms from the mouth can 
 extend into the various large salivary glands 
 by means of the ducts and give rise to infections. 
 The tonsils, particularly in children, provide a 
 favorable surface for infection. The mucous 
 surface extends into these forming deep pockets 
 lined with very thin epithelium, and in these 
 debris of all sorts accumulates and provides 
 material favorable for bacterial growth. 
 
 The lungs at first sight seem to offer the most 
 favorable surface for infection. The surface, 
 ninety -seven square yards, is enormous; it is 
 moist, the epithelial covering is so thin as to 
 give practically no mechanical protection, large 
 amounts of air constantly pass in and out, and 
 the surface is in contact with this. They are 
 protected from infection in many ways. The
 
 DISEASE AND ITS CAUSES 139 
 
 tubes or bronchi by which the air passes into 
 and from the lungs are covered with cilia; the 
 surface area of these tubes constantly enlarges as 
 they branch, the sum of the diameters of the 
 small tubes being many times greater than that 
 of the windpipe, and this enlargement by re- 
 tarding the motion of the air favors the lodg- 
 ment of particles on the surface whence they 
 are removed by the action of the cilia. The 
 entering air is also brought closely in contact with 
 a moist surface at the narrow opening of the 
 larynx. That bacteria and other foreign sub- 
 stances can enter the lungs in spite of these 
 guards is shown not only by the infections which 
 take place here, but also by the large amount of 
 black carbon deposited in them from the soot 
 contained in the air. 
 
 Infection rarely takes place from the surface 
 of the gullet or oesophagus which leads from the 
 mouth to the stomach. This is due to the smooth- 
 ness of the surface and to the rapidity with which 
 food passes over it. Infection by the stomach 
 also is rare, for this contains a strong acid secre- 
 tion which destroys many of the bacteria which 
 are taken in with the food. It is found impos- 
 sidle to infect animals with cholera unless the 
 acidity of the stomach contents be neutralized 
 by an alkali. Many organisms, although their 
 growth in the stomach is inhibited, are not 
 destroyed there and pass into the intestines,
 
 140 DISEASE AND ITS CAUSES 
 
 where the conditions for infection are more 
 favorable. This large and very irregular surface 
 is bathed in fluid which is a good culture medium 
 and but a single layer of cells covers it. The 
 organisms which cause many of the infectious 
 diseases in both man and animals find entrance 
 by means of the alimentary canal, as cholera, 
 dysentery, typhoid fever, chicken cholera, hog 
 cholera. 
 
 Infection by the genito-urinary surface is 
 comparatively rare. The surface openings are 
 usually closed, and the discharge of urine has a 
 mechanical cleansing effect. The wide tube of 
 the vagina is further protected by a normal 
 bacterial flora which produces conditions hostile 
 to other and pathogenic bacteria. The most 
 common infections are the sexual diseases, which 
 are due to organisms which find favorable con- 
 ditions for growth in and on the surface and 
 which are conveyed from a similar surface by 
 sexual contact. 
 
 It remains a question whether bacteria can 
 penetrate an intact surface producing no injury 
 at the point of entrance and be carried by the 
 lymph or blood into internal organs where they 
 produce disease. Internal infections are often 
 found with seemingly intact body surfaces, but 
 it is impossible to exclude the presence of minute 
 or microscopic surface injuries by which the 
 organisms may have entered. It is also possible
 
 DISEASE AND ITS CAUSES 141 
 
 that . a slight injury at the point of entrance 
 may heal so completely as to leave no trace. 
 
 The chief danger from wounds is that their 
 surfaces may become infected. Death from 
 wounds is due more frequently to infection than 
 to the actual injury represented by the wounds. 
 Much depends upon the character of the wound. 
 Infection of clean wounds which are made by 
 a sharp cutting instrument and from which there 
 is abundant haemorrhage with sealing of the edges 
 of the wound by clotted blood, rarely happens. 
 Typical wounds of this sort are often made in 
 shaving, and infection of such wounds is extraor- 
 dinarily rare. If, with the wound, pathogenic 
 organisms are placed in the tissue, or foreign 
 substances such as bits of clothing are carried 
 in with a bullet, for example, or if the instrument 
 causing the wound be of such a character as to 
 produce extensive lacerations of tissue, infection 
 is more apt to occur. The less frequency of in- 
 fection in modern wars is in part due to the 
 simpler character of the wounds and in part to 
 the fact that modern fixed ammunition is practi- 
 cally free from germs. The old spear-head, the 
 arrow, the cross bow bolt, had little regard 
 for the probabilities of infection. Whether in- 
 fection follows a wound depends both upon 
 the entry of pathogenic organisms and upon 
 these finding in the tissues suitable opportunities 
 for growth. In wounds in which there is much
 
 142 DISEASE AND ITS CAUSES 
 
 laceration of tissue organisms find the most 
 favorable conditions for development. The very 
 slight wounds produced by the exploded cap 
 in the toy pistol give suitable conditions for 
 the development of the bacillus which produces 
 tetanus or lockjaw. The deaths of children 
 from lockjaw following a Fourth of July cele- 
 bration have often exceeded the total deaths 
 in a Central American revolution. The tetanus 
 bacillus is a widely distributed organism, whose 
 normal habitat is in the soil and which is usually 
 present on the dirty hands of little boys. The 
 toy-pistol wounds are made by small bits of paper 
 or metal being driven into the skin by the explo- 
 sion of the cap. The wound is of little moment, 
 the surface becomes closed, and a bit of foreign 
 substance, a few dead cells and the tetanus 
 bacilli from the surface remain enclosed and in 
 a few days the fatal disease develops. 
 
 Infection of the surfaces of old wounds such 
 as the surface of an ulcer takes place with dif- 
 ficulty. Large numbers of leucocytes which 
 give protection by phagocytosis are constantly 
 passing to the surface, and there is also a constant 
 stream of fluid towards the surface. On such 
 a surface there may be an abundant growth of 
 pathogenic organisms, but no infection results. 
 
 In most infections there is a focus where the 
 infectious organisms are localized; this may 
 correspond to the point of entrance on a surface
 
 DISEASE AND ITS CAUSES 143 
 
 or it may be in the interior of the body, the 
 organisms being deposited there after entrance. 
 At this primary localization, the atrium of in- 
 fection, 1 the organisms multiply and from this 
 point further invasion takes place. Many second- 
 ary foci may be formed in the organs by distri- 
 bution of the organisms, or there may be infection 
 of the blood and fluids of the body. The injuries 
 which are produced depend upon the nature of 
 the infecting organisms. The most common 
 lesion consists in the death of the tissue about 
 the infecting organisms. In most cases the sum 
 of the changes are so characteristic that from 
 them the nature of the infection is easily deter- 
 mined, and these changes often give names to 
 the disease; thus tuberculosis is a disease charac- 
 terized by the formation of tubercles or little 
 nodules in the body. The situation of the foci 
 of disease is determined by many conditions, 
 the most important being the varying resistance 
 of the different organs of the body to the growth 
 of bacteria. Certain organs, such as the central 
 nervous system, the muscles, the testicles and 
 the ovaries, have a high resistance to the growth of 
 bacteria. The disease may be localized in certain 
 organs because only in these do the bacteria find 
 favorable conditions for growth. In spite of a 
 high general resistance to infection the lesions in 
 
 1 The comparison here is with the atrium of a Pompeiian 
 house.
 
 144 DISEASE AND ITS CAUSES 
 
 chronic glanders are most marked in the muscles, 
 those of poliomyelitis in the spinal cord. There 
 are few bacterial diseases which are localized in 
 the blood, but many of the diseases caused by 
 protozoa have this localization. In every infec- 
 tion some organisms enter the blood, which acts 
 as a carrier and deposits them in the organs. 
 
 Bacteria cause disease by producing substances 
 called toxines which are poisonous to the cells, 
 and of which two sorts are distinguished. One 
 form of toxines is produced by the bacteria as a 
 sort of secretion, and is formed both in the body 
 and when the bacteria are growing in cultures. 
 Substances of this character, many of them 
 highly poisonous, are produced both by animals 
 and plants. They may serve the purpose both 
 of offence and defence, as in the case of the snake 
 venom, and in other cases they seem to benefit 
 their producers in no way whatever, and may 
 even be injurious to them. After the different 
 cereals have been grown for succeeding years in 
 the same place, growth finally diminishes not 
 from the exhaustion of the soil, but from the 
 accumulation! in it of substances produced by 
 the plants. Beneath certain trees, as the Norway 
 maple, grass will not grow, and it has been shown 
 that the tree produces substances which inhibit 
 the growth of grass. When bacteria are grown in 
 a culture flask, growth ceases long before the 
 nutritive material has been consumed, from the
 
 DISEASE AND ITS CAUSES 145 
 
 accumulation of waste products in the fluid. 
 The other class of toxic substances, called endo- 
 toxines, are not secretion products, but are con- 
 tained in the bacterial substance and become 
 active by the destruction and disintegration of 
 the bacteria. They can be artificially produced 
 by grinding up masses of bacteria, and in the 
 body the destruction and solution of bacteria 
 which is constantly taking place sets them free. 
 The toxines and the endotoxines are of an albu- 
 minous nature, and act only when they come in 
 contact with the living cells within the body. 
 When taken into the alimentary canal they are 
 either not absorbed or so changed by the digestive 
 fluids as to be innocuous. Many of the ordinary 
 food substances, even a material apparently so 
 simple as the white of an egg, are highly in- 
 jurious if they reach the tissues in an unchanged 
 form. 
 
 By means of these substances the bacteria 
 produce such changes in their environment 
 within the body that this becomes adapted to 
 their parasitic existence. In symbiosis the bac- 
 teria probably undergo changes by which they 
 become adapted to the environment, and in 
 parasitism the environment becomes adapted to 
 them. In the same way man can change his 
 immediate environment by means of clothing, 
 artificial heating, etc., and adapt it to his needs; 
 or by hardening his body he can adapt it to the
 
 146 DISEASE AND ITS CAUSES 
 
 environment. The pathogenic bacterium finds 
 the living tissue hostile, its cells devour him, the 
 tissue fluids destroy him, and by means of the 
 toxines he changes the environment from that 
 of living to dead tissue, or in other ways so alters it 
 that it is no longer hostile. The parasite has also 
 means of passive defence comparable to the armor 
 of the warrior in the past. It may form a protective 
 mantle called a capsule around itself, which serves 
 to protect it from the action of the body fluids. 
 Such capsule formation is a very common thing in 
 the pathogenic organisms, and they are found only 
 when these are growing in the body and do not 
 appear in cultures (Fig. 17 c). 
 
 It is evident that just as the parasite has his 
 weapons of offence and defence so has the host, 
 otherwise there would be no recovery from in- 
 fectious diseases. Although many of the infec- 
 tious diseases have a high mortality, which in 
 rare instances reaches one hundred per cent, the 
 majority do recover. In certain cases the recov- 
 ery is attended by immunity, the individual 
 being protected to a greater or less degree from 
 a recurrence of the same disease. The immunity 
 is never absolute; it may last for a number of 
 years only, and usually, if the disease be again 
 acquired, the second attack is milder than the 
 primary. Probably the most enduring immunity 
 is in smallpox, although cases are known of two 
 and even three attacks; the immunity is high
 
 DISEASE AND ITS CAUSES 147 
 
 in scarlet fever, measles, mumps and typhoid 
 fever. The immunity from diphtheria is short, 
 and in pneumonia, although there must be a 
 temporary immunity, future susceptibility to 
 the disease is probably increased. In certain 
 cases the immunity is only local; the focus of 
 disease heals because the tissue there has evolved 
 means of protection from the parasite, but if 
 any other part of the body be infected, the 
 disease pursues the usual course. A boil, for 
 example, is frequently followed by the appear- 
 ance of similar boils in the vicinity due to the 
 infection of the skin by the micrococci from the 
 first boil, which by dressings, etc., have become 
 spread over the surface. 
 
 The natural methods of defence of the host 
 against the parasites have formed the main 
 subject in the study of the infectious diseases 
 for the last twenty years. Speculation in this 
 territory has been rife and most of it fruitless, 
 but by patient study of disease in man and by 
 animal experimentation there has been gradually 
 evolved a sum of knowledge which has been ap- 
 plied in many cases to the treatment of infectious 
 diseases with immense benefit. Research was 
 naturally turned to this subject, for it was evident 
 that the processes by which the protection of 
 the body was brought about must be known 
 before there could be a really rational method 
 of treatment directed towards the artificial in-
 
 148 DISEASE AND ITS CAUSES 
 
 duction of such processes, or hastening and 
 strengthening those which were taking place. 
 Previous to knowledge of the bacteria, their 
 mode of life, their methods of infection and 
 knowledge of the defences of the body, most of 
 the methods of prevention and treatment of the 
 infectious diseases was based largely on con- 
 jecture, the one brilliant exception being the 
 discovery of vaccination by Jenner in 1798. 
 
 The host possesses the passive defences of 
 the surfaces which have already been considered. 
 The first theories advanced in explanation of 
 immunity were influenced by what was known 
 of fermentation. One, the exhaustion theory, 
 assumed that in the course of disease substances 
 contained in the body and necessary for the 
 growth of the bacteria became exhausted and 
 the bacteria died in consequence. Another, the 
 theory of addition, assumed that in the course 
 of the disease substances inimical to the bacteria 
 were formed. Both these theories were inade- 
 quate and not in accord with what was known 
 of the physiology of the body. The most general 
 mode of defence is by phagocytosis, the property 
 which many cells have of devouring and digest- 
 ing solid substances (Fig. 16 p). Although this 
 had been known to take place in the amoebae 
 and other unicellular organisms, the wide extent 
 of the process and its importance in immunity 
 was first recognized by Metschnikoff in 1884
 
 DISEASE AND ITS CAUSES 149 
 
 and the phagocytic theory of immunity advanced 
 and defended by a brilliant series of experi- 
 ments by Metschnikoff and his pupils conducted 
 in the Pasteur Institute. Metschnikoff s first 
 observations were made on the daphnea, a small 
 animalcule just visible to the naked eye which 
 lives in fresh water. The structure of the organ- 
 ism is simple, consisting of an external and in- 
 ternal surface between which there is a space, 
 the body cavity; daphnese are transparent and 
 can be studied under the microscope while living. 
 Metschnikoff observed that certain of them in 
 the aquarium gradually lost their transparency 
 and died, and examining these he found they 
 were attacked by a species of fungus having long, 
 thin spores. These spores were taken into the 
 intestine with other food; they penetrated the thin 
 wall of the intestine, passed into the body cavity, 
 multiplied there, and in consequence the animal 
 died. In many cases, however, those penetrating 
 became enclosed in cells which the body cavity 
 contains and which correspond with the leuco- 
 cytes of the blood; in these the spores were 
 digested and destroyed. The daphnese in which 
 this took place recovered from the infection. 
 Here was a case in which all the stages of 
 an infectious disease could be directly fol- 
 lowed under the microscope, and the whole 
 process was simple in comparison with infec- 
 tions hi the higher animals. The pathogenic
 
 150 DISEASE AND ITS CAUSES 
 
 organism was known, the manner and site of 
 invasion was clear, it was also evident that if 
 the multiplication of the parasite was un- 
 checked the animal died, but if the parasite 
 was opposed by the body cells and destroyed 
 the animal recovered. The studies were carried 
 further into the diseases of the higher animals, and 
 it was found the leucocytes in these played the 
 same part as did the cells in the body cavity of 
 the daphnea. The introduction of bacteria into 
 certain animals was followed by their destruc- 
 tion within cells and no disease resulted; if this 
 did not take place, the bacteria multiplied and 
 produced disease. Support also was given the 
 theory by the demonstration at about the same 
 time that in most of the infectious diseases the 
 leucocytes of the blood became increased in 
 number, that in pneumonia, for instance, in- 
 stead of the usual number of eight thousand in a 
 cubic millimeter of blood, there were often thirty 
 thousand or even fifty thousand. At about the 
 same time also chemotaxis, or the action of chemi- 
 cal substances hi attracting or repelling organ- 
 isms, excited attention, and all these facts together 
 became woven into the theory. It was soon 
 seen, however, that this theory, based as it was 
 on observation and supported by the facts ob- 
 served, was not, at least in its first crude form, 
 capable of general application. Many animals 
 have natural immunity to certain diseases; they
 
 DISEASE AND ITS CAUSES 151 
 
 do not have the disease under natural conditions, 
 nor do they acquire the disease when the organ- 
 isms causing it are artificially introduced into 
 their tissues by inoculation. Such natural im- 
 munity seemed to be unconnected with defence by 
 phagocytosis, for the leucocytes of the animal 
 might or might not have phagocytic reaction 
 to the particular organisms to which the animal 
 was immune. It was also seen that recovery 
 from infection in certain diseases was uncon- 
 nected with phagocytosis. It had also been 
 demonstrated, by German observers chiefly, 
 that the serum of the blood, the colorless fluid 
 in which the corpuscles float, was itself destruc- 
 tive, and that in an animal rendered immune to 
 a special bacterium the destructive action of 
 the serum on that organism was greatly increased. 
 In this hostile serum the bacteria often became 
 clumped together in masses, the bodies became 
 swollen, broken up, and finally disintegrated. 
 This property of the serum was described as 
 due to a substance in the serum called alexine, 
 which in the immune animal became greatly 
 increased in amount. It was even denied by 
 some that phagocytosis of living bacteria took 
 place, and that all those included in the cells were 
 dead, having been destroyed in the first instance 
 by the serum. The strife became a national 
 one between the French and Germans, on the 
 one side in France the phagocytic theory was
 
 152 DISEASE AND ITS CAUSES 
 
 defended, and in Germany, on the other, the 
 theory of serum immunity. The mass of ex- 
 perimental work which poured from the labora- 
 tories of the two countries in attack and defence 
 became so great that it could not easily be fol- 
 lowed. It had a good influence because, without 
 the stimulation of this national rivalry, the knowl- 
 edge which gradually arose from this work would 
 not have been so quickly acquired. It is inter- 
 esting that the mode of action of the serum in 
 destroying bacteria was demonstrated not by 
 a German but by Bordet, a French observer 
 and a pupil of Metschnikoff. He showed that 
 the serum contained two distinct substances, 
 each necessary for the destructive action. The 
 separate action of these substances can be studied 
 since one is thermolabile, or destroyed by heating 
 the serum to one hundred and thirty-three de- 
 grees; the other thermostabile, or capable of 
 withstanding a greater degree of heat. These 
 substances are known only by their effect, they 
 have never been separated from the serum. The 
 thermostabile substance, or amboceptor, as it 
 is generally called, has in itself no destructive 
 action on the bacteria; but in some way so alters 
 them that they can be acted on by the thermo- 
 labile substance called complement whose action 
 is destructive. The amount of amboceptor may 
 increase in the course of infection and its for- 
 mation stimulated, the amount of complement
 
 DISEASE AND ITS CAUSES 153 
 
 remains unchanged. The action of the ambo- 
 ceptor is specific, that is, directed against a single 
 species of bacterium only; the destructive power 
 of the blood may be very great against a single 
 bacterium species and have no effect on others. 
 There seem naturally to be many different 
 amboceptors in the blood, and the number may 
 be very greatly increased. It has been shown 
 as a result of the work of many investigators 
 that the shield has two faces, there is destruction 
 both by cells and fluids and there is interaction 
 by both. The amboceptors so necessary for the 
 destructive action of the serum are produced 
 by the body cells, particularly the leucocytes. 
 The serum assists in pagocytosis by the action 
 on bacteria of substances called opsonins which 
 are contained in it, and the formation of which 
 can be very greatly stimulated. Again, not all 
 inclusion of bacteria within leucocytes is indica- 
 tive of phagocytosis; in many cases the bacteria 
 seem to find the best conditions for existence 
 within the leucocytes, and these and not the 
 bacteria are destroyed. 
 
 So far it has been shown that the best defence 
 of the body is, as is the best defence in war, 
 by offensive measures, as illustrated by phago- 
 cytosis and destruction by the serum. Both 
 of these actions can be increased by their ex- 
 ercise just as the strength of muscular con- 
 traction can be increased by exercise, and the
 
 154 DISEASE AND ITS CAUSES 
 
 facility for doing everything increased by habit. 
 Certain of the infectious diseases are, as has been 
 said, essentially toxic in their nature, and in 
 cultures the organisms produce poisonous sub- 
 stances. By the injection into the tissues of 
 such substances the same disturbances are pro- 
 duced as when the bacteria are injected. Such 
 a disease is diphtheria. In this there is only a 
 superficial invasion of the tissues. The diph- 
 theria bacilli are located on the surface of the 
 tonsils or pharynx or windpipe, where, as a result 
 of their action, the membrane so characteristic 
 of the disease is produced. The membrane may 
 be the cause of death when it is so extensively 
 formed as to occlude the air passages, but the 
 prominent symptoms of the disease, the fever, 
 the weakness of the heart and the great prostra- 
 tion are due not to the presence of the membrane, 
 but to the action of toxic substances which are 
 formed by the bacteria growing in the super- 
 ficial lesions and absorbed. Tetanus, or lock- 
 jaw, is another example of these essentially toxic 
 diseases. The body must find some means of 
 counteracting or destroying these injurious toxic 
 substances. It does this by forming antagonis- 
 tic substances called antitoxines, which act not 
 by destroying the toxines, but by uniting with 
 them, the compound substance being harmless. 
 It has been found that the production of anti- 
 toxine can be so stimulated by the injection of
 
 DISEASE AND ITS CAUSES 155 
 
 toxine that the blood of the animal used for the 
 purpose contains large amounts of antitoxine. 
 The horse is used in this way to manufacture 
 antitoxine, and the serum injected into a patient 
 with diphtheria has a curative action, a greater 
 
 FIG. 18. DIAGRAM TO ILLUSTRATE EHRLICH'S THEORY OP 
 ANTITOXINE FORMATION. The surface of the cell (n) is cov- 
 ered with receptors some of which (6) fit the toxine molecule, 
 (a) allowing the toxine to act upon the cell. Under the stimu- 
 lus of this the cell produces these receptors in excess which 
 enter into the blood and there combine with the toxine as La 
 a 1 6 l , thus anchoring it and preventing it from acting upon the 
 cells. The receptors c and d do not fit the toxine molecule. 
 
 amount being thus introduced than the patient 
 can manufacture. 
 
 A very ingenious theory which well accords 
 with the facts has been given by Ehrlich in ex- 
 planation of the production of antitoxine and 
 of the reaction between toxine and antitoxine 
 (Fig. 18). This is based on the hypothesis, which
 
 156 DISEASE AND ITS CAUSES 
 
 is in accord with all facts and generally accepted, 
 that the molecules which enter into the structure 
 of any chemical substance have in each particular 
 substance a definite arrangement, and that in 
 a compound substance each elementary sub- 
 stance entering into the compound molecule has 
 chemical affinities, most of which may be satis- 
 fied by finding a suitable mate. Ehrlich assumes 
 that the very complex chemical substances which 
 form the living cells have many unsatisfied 
 chemical affinities, and that it is due to this that 
 molecules of substances adapted for food can 
 enter the cells and unite with them; but there 
 must be some coincidence of molecular structure 
 to enable the union to take place, the comparison 
 being made of the fitting of a key into a lock. 
 The toxines that produced by the diphtheria 
 bacillus being the best example are substances 
 whose molecular structure enables them to com- 
 bine with the cells of the body, the combination 
 being effected through certain chemical affinities 
 belonging to the cells termed receptors. Unless 
 the living cells have receptors which will enable 
 the combination with the toxine to take place, 
 no effect can be produced by the toxine and the 
 cells are not injured. This is the case in an 
 animal naturally immune to the action of the 
 diphtheria bacillus or its toxines. In the case 
 of the susceptible animal the receptors of the 
 cells of the different organs combine with the
 
 DISEASE AND ITS CAUSES 157 
 
 toxine to a greater or less extent, which explains 
 the fact that different degrees of injury are 
 produced in the different tissues; the toxine of 
 tetanus, or lockjaw, for example, combines by 
 preference with the nervous tissue, that of diph- 
 theria with the lymphatic tissue. It is known 
 that in accordance with the general law of in- 
 jury and repair, a loss in any part of the body 
 stimulates the tissue of the same kind to new 
 growth and the loss is thus repaired; it is as- 
 sumed that the cell receptors which combine 
 with the toxine are lost for the cell which then 
 produces them in excess. The receptors so 
 produced pass into the blood, where they com- 
 bine with the toxine which has been absorbed; 
 the combination is a stable one, and the toxine 
 is thus prevented from combining with the tissue 
 cells. The antitoxine which is formed during the 
 disease, and the production of which in the horse 
 can be enormously stimulated by the injection 
 of toxine, represents merely the excess of cell 
 receptors, and when the serum of the horse 
 containing them is injected in a case of diph- 
 theria the same combination takes place as in 
 the case of receptors provided by the patient. 
 In the case of the destruction of bacteria in 
 the blood by the action of amboceptor and 
 complement, the amboceptor must be able to 
 combine with both the bacterial cell and the 
 complement which brings about its destruction,
 
 158 DISEASE AND ITS CAUSES 
 
 and just as antitoxine is formed so new ambo- 
 ceptors may be formed. 
 
 Few hypotheses have been advanced in science 
 which are more ingenious, in better accord with 
 the facts, have had greater importance in en- 
 abling the student to grasp the intricacies of 
 an obscure problem, and which have had an 
 equal influence in stimulating research. The 
 immunity which results from disease in accord- 
 ance with this theory, is due not to condition^ 
 preventing the entrance of organisms into the 
 body, but to greater aptitude on the part of the 
 cells to produce these protective substances hav- 
 ing once learned to do so. An individual need 
 not practise for many years, having once learned 
 them, those combinations of muscular action used 
 in swimming; but the habit at once returns 
 when he falls into the water. 
 
 Infectious diseases and recovery are phases 
 of the struggle for existence between parasite 
 and host, and illustrate the power of adaptation 
 to environment which is so striking a character- 
 istic of living matter.
 
 CHAPTER VIII 
 
 SECONDARY, TERMINAL AND MIXED INFECTIONS. THE 
 EXTENSION OF INFECTION IN THE INDIVIDUAL. TUBER- 
 CULOSIS. THE TUBERCLE BACILLUS. FREQUENCY OF 
 THE DISEASE. THE PRIMARY Foci. THE EXTENSION 
 OF BACILLI. THE DISCHARGE OF BACILLI FROM THE 
 BODY. INFLUENCE OF THE SEAT OF DISEASE ON THE 
 DISCHARGE OF BACILLI. THE INTESTINAL DISEASES. 
 MODES OF INFECTION. INFECTION BY SPUTUM SPRAY. 
 INFECTION OF WATER SUPPLIES. EXTENSION OF IN- 
 FECTION BY INSECTS. TRYPANOSOME DISEASES. SLEEP- 
 ING SICKNESS. MALARIA. THE PART PLAYED BY MOS- 
 QUITOES. PARASITISM IN THE MOSQUITO. INFECTION 
 AS INFLUENCED BY HABITS AND CUSTOMS. HOOKWORM 
 DISEASE. INTER-RELATION BETWEEN HUMAN AND ANIMAL 
 DISEASES. PLAGUE. PART PLAYED BY RATS IN TRANS- 
 MISSION. THE PRESENT EPIDEMIC OF PLAGUE. 
 
 THE infectious diseases are often complicated 
 by secondary infections, some other organism 
 finding opportunity for invasion in the presence 
 of the injuries produced in the primary disease. 
 In many diseases, such as diphtheria, scarlet fever 
 and smallpox, death is frequently due to the 
 secondary infection. The secondary invaders 
 not only find local conditions favoring a suc- 
 cessful attack, but the activity of the tissue 
 cells on which the production of protective sub- 
 stances essentially depends has suffered by the 
 primary infection, or the cells are occupied in 
 meeting the exigencies of this. The body is in 
 the position of a state invaded by a second 
 
 159
 
 160 DISEASE AND ITS CAUSES 
 
 power where all its forces and resources are en- 
 gaged in repelling the first attack. 
 
 What are known as terminal infections occur 
 shortly before death. No matter what the 
 disease which causes death, in the last hours of 
 life the body usually becomes invaded by organ- 
 isms which find their opportunity in the then 
 defenceless tissues, and the end is often hastened 
 by this invasion. 
 
 There are also mixed infections in which two 
 different organisms unite in attack, each in some 
 way assisting hi the action of the other. The 
 best known example of this is in the highly in- 
 fectious disease of swine known as hog cholera. 
 It has been shown that in this disease two organ- 
 isms are associated, one an invisible and filtera- 
 ble organism, and the other a bacillus. It was first 
 supposed that the bacillus was the specific organ- 
 ism ; it was found in the lesions and certain, but 
 not all, the features of the disease were produced 
 by inoculating hogs with pure cultures. The 
 disease so produced is not contagious, and the 
 contagious element seems to be due to the filter- 
 able virus. 
 
 The modes of transmission of infectious diseases 
 are of great importance and are the foundation 
 of measures of public health. In the preceding 
 chapter we have seen that in the infected indi- 
 vidual the disease extends from one part of the 
 body to another. There is a primary focus of
 
 DISEASE AND ITS CAUSES 161 
 
 disease from which the extension takes place, 
 and the study of the modes of extension in the 
 individual throws some light on the much more 
 difficult subject of the transmission of disease 
 from one individual to another. There are four 
 ways by which extension in the individual may 
 take place. 
 
 1. By continuity of tissue, an adjoining tissue 
 or organ becoming infected by the extension 
 of a focus of infection. 
 
 2. By means of lymphatics. Organisms easily 
 enter these vessels which are in continuity with 
 the tissue spaces and receive the exudate from 
 the focus of infection. The organisms are 
 carried to the lymph nodes, which, acting as 
 filters, retain them and for a time prevent a 
 further extension. The following illustrates the 
 importance of the part the nodes may play in 
 mechanically holding back a flood of infection. 
 A physician examined after death the body of 
 a person who died from infection with a very 
 virulent micrococcus and in the course of the 
 examination slightly scratched a finger. One 
 of the organs of the body was removed, sent to 
 a laboratory and received by a laboratory worker, 
 a woman physician, who had slight abrasions 
 and fissures in the skin of the hands from contact 
 with irritating chemicals. In the course of a 
 few hours the wound on the finger of the man 
 became inflamed, intensely painful, and red
 
 162 DISEASE AND ITS CAUSES 
 
 lines extended up the arm in the course of the 
 lymphatic vessels, showing that the organisms 
 were in the lymphatics and causing inflammation 
 in their course. The lymph nodes in the arm- 
 pit into which these vessels empty became greatly 
 inflamed, swollen, and an abscess formed in them 
 which was opened. There was high fever, great 
 prostration, a serious illness from which the man 
 did not recover for several months. The woman 
 only handled the organ which was sent to the 
 laboratory in order to place it in a fluid for 
 preservation. She also had a focus of infection 
 of a finger with the same red lines on the arm, 
 showing extension by the lymphatics; but there 
 was no halt of the infection in the armpit, for 
 all the lymph nodes there had been removed 
 several years before in the course of an opera- 
 tion for a tumor of the breast. A general in- 
 fection of the blood took place, there was very 
 high fever, and death followed in a few days. 
 The halt of the infection is important in allow- 
 ing time for the Tjody to make ready its means 
 of defence. One cannot avoid comparing the 
 lymph node with a strong fortress thrown in 
 the path of a victorious invading army behind 
 which the defenders may gather and which 
 affords them time to renovate their strength. 
 
 3. By means of the blood. The blood vessels 
 are universally distributed, the smaller vessels 
 have thin walls easily ruptured and easily pene-
 
 DISEASE AND ITS CAUSES 163 
 
 trated. It is probable that in every infection 
 some organisms enter the blood which, under 
 usual conditions, is peculiarly hostile to bacteria. 
 These may, however, be carried by the blood to 
 other organs and start foci of infection in these. 
 
 4. By means of continuous surfaces. The 
 bacteria may either grow along such surfaces 
 forming a continuous or more or less broken 
 layer, or may be carried from place to place in 
 the fluids which bathe them. 
 
 All these modes of extension are well shown 
 in tuberculosis. This disease is caused by a 
 small bacillus which does not produce spores, has 
 no power of saphrophytic growth under natural 
 conditions, and is easily destroyed. Moisture 
 and darkness are favorable conditions for its 
 existence, sunlight and dryness the reverse. 
 There are three varieties or strains of the tubercle 
 bacilli which infect respectively man, cattle 
 and birds, and each class of animals shows con- 
 siderable resistance to the varieties of the bacil- 
 lus which are most infectious for the others. 
 
 The primary seat of the infection in man is 
 generally in the upper part of the lung. The 
 organisms settle on the surface here and cause 
 multiplication of the cells and an inflammatory 
 exudate in a small area. With the continuous 
 growth of the bacilli in the focus, adjoining areas 
 of the lung become affected, and there is further 
 extension in the immediate vicinity by means
 
 164 DISEASE AND ITS CAUSES 
 
 of the lymphatics. Small nodules are formed 
 and larger areas by their coalescence. Infection 
 with tuberculosis is so common that at least 
 three-fourths of all individuals over forty show 
 evidences of it. The examination of two hun- 
 dred and twenty-five children of the average age 
 of five years who had died of diphtheria showed 
 tuberculous infection in one-fifth of the cases and 
 the frequency of infection increases with age. 
 The defence on the part of the body is chiefly 
 by the formation of dense masses of cicatricial 
 tissue which walls off the affected area and in 
 which the bacilli do not find favorable condi- 
 tions for growth. This mode of defence, which 
 is probably combined with the production of 
 substances antagonistic to the toxines produced 
 by the bacilli, is so efficacious that in the great 
 majority of cases no further extension of the 
 process takes place. In certain cases, however, 
 the growth of the bacilli in the focus is un- 
 checked, the tissue about them is killed and 
 becomes converted into a soft semi-fluid mate- 
 rial; further extension then takes place. All 
 parts of the enormous surface of the lungs are 
 connected by means of the system of air tubes 
 or bronchi, and the bacilli have favorable oppor- 
 tunity for distribution, which is facilitated by 
 sudden movements of the air currents in the 
 lung produced by coughing. The defence of 
 the body can still keep pace with the attack,
 
 and even in an advanced stage the infection can 
 be checked in some cases permanently; in others 
 the check is but temporary, the process of soften- 
 ing continues, and large cavities are produced 
 by the destruction of the tissue. On the inner 
 surface of these cavities there may be a rapid 
 growth of bacilli. 
 
 From the lungs the bacilli are carried by the 
 lymphatics to the lymph nodes at the root of 
 the lungs, in which a similar process takes place; 
 this, on the whole, is favorable, because further 
 extension by this route is for a time blocked. 
 The extension by means of surfaces continues, 
 the abundant sputum which is formed in the 
 lungs and which contains large numbers of 
 bacilli, becomes the vehicle of transportation. 
 The windpipe and larynx may become infected, 
 the back parts of each are more closely in con- 
 tact with the sputum and are the parts most 
 generally infected. A large part of the sputum 
 is swallowed and infection of the intestine takes 
 place, the lesions taking the form of large ulcers. 
 From the intestinal ulcers there is further ex- 
 tension by means of the lymphatics, to the 
 large lymph nodes in the back of the abdominal 
 cavity (Fig. 8 25) ; the bacilli may also pass 
 from the ulcers into the abdominal cavity and 
 be distributed over the surface of the peritoneum 
 resulting in tuberculous peritonitis. When the 
 disease has reached an advanced stage, bacilli
 
 166 DISEASE AND ITS CAUSES 
 
 in small numbers continually pass into the blood 
 and are distributed by this over the body, pro- 
 ducing small nodules in many places. In rare 
 cases distribution by the blood is the principal 
 method of extension, and immense numbers of 
 small foci of disease are produced, the form of 
 disease being known as acute miliary tuber- 
 culosis. Although the bacilli are distributed 
 everywhere, certain organs, as the brain and 
 muscles, are usually exempt, because in these 
 the conditions are not favorable to further 
 growth of the bacilli. Tuberculosis, although 
 frequently a very acute disease, is usually one 
 of the best types of a chronic disease and may 
 last for many years. The chronic form is char- 
 acterized by periods of slow or rapid advance 
 when conditions arise in the body favorable for 
 the growth of the bacilli, and periods when the 
 disease is checked and quiescent, the defensive 
 forces of the body having gained the upper 
 hand. Often the intervention of some other 
 disease so weakens the defences of the body 
 that the bacilli again find their opportunity. 
 Thus typhoid fever, scarlet fever and other dis- 
 eases may be followed by a rapidly fatal ad- 
 vance of the tuberculosis, starting from some 
 old and quiescent focus of the disease. 
 
 Tuberculosis is also one of the best examples 
 of what is known as latent infection. In this 
 the infectious organisms enter the body and
 
 DISEASE AND ITS CAUSES 167 
 
 produce primary lesions in which the organisms 
 persist but do not extend owing to their being 
 enclosed in a dense and resistant tissue, or to 
 the production of a local immunity to their action. 
 Dr. Head has recently examined the children 
 of households in which there was open tuber- 
 culosis in some member of the household. By 
 open tuberculosis is understood a case from which 
 bacilli are being discharged. He found with 
 scarcely an exception that all the children in 
 such families showed evidences of infection. 
 The detection of slight degrees of tuberculous 
 infection is now made easy by certain skin re- 
 actions on inoculation of the skin with a substance 
 derived from the tubercle bacilli. Such latent 
 infections may never become active and in the 
 majority of cases do not. When, however, in 
 consequence of some intercurrent disease or 
 conditions of malnutrition the general defences 
 of the body become weakened extension follows. 
 Such latent infections explain the enormous 
 frequency of tuberculosis in prisons. Under the 
 general prison conditions infection in the prisons 
 probably does not take place to any extent, and 
 the disease is as common when the prisoners are 
 kept in individual cells as in common prisons. 
 It is probable that in these cases the prisoners 
 have latent tuberculosis when entering, and the 
 disease becomes active under the moral and 
 physical depression which prison life entails.
 
 168 DISEASE AND ITS CAUSES 
 
 For the extension of infection from one indi- 
 vidual to another the infecting organisms must 
 in some way be transferred. The most important 
 of the conditions influencing this are the localiza- 
 tion of the disease and the character of the in- 
 fectious organisms, particularly with regard to 
 their resistance to the conditions met with out- 
 side of the body. The seat of disease influences 
 the discharge of organisms; thus, if the disease 
 involve any of the surfaces the organisms become 
 mingled with the secretions of the surface and 
 are discharged with these. If the seat of disease 
 be in the lungs, the throat or the mouth, the 
 sputum forms the medium of extension, which 
 can take place in many ways. The sputum 
 may become dried, forms part of the dust and the 
 organisms enter with the inspired air. The 
 organisms which cause most of the diseases 
 in which the sputum becomes infectious are 
 quickly destroyed by conditions in the open, such 
 as the sunlight and drying; street dust does not 
 play so prominent a part in extension as is gen- 
 erally supposed. Organisms find much more 
 favorable conditions within houses. It is now gen- 
 erally recognized that infection with tuberculosis 
 does not take place in the open, but in houses 
 in which the bacilli on being discharged are not 
 destroyed. The hands, the clothing and surround- 
 ings even with the exercise of the greatest care 
 may become soiled with the saliva.
 
 DISEASE AND ITS CAUSES 169 
 
 It has been shown that in coughing and speak- 
 ing very fine particles of spray are formed by the 
 intermingling of air and saliva, which may be 
 projected a considerable distance and remain float- 
 ing in the air for some time. These particles are 
 so fine as to be invisible; they may be inspired, 
 and their presence in the air forms an area of in- 
 determinate extent around the infected person 
 within which such infection is possible. Such 
 spray formation is also an important means of the 
 extension of infection in the sick individual, for it 
 is continually formed and inspired. It is in this 
 way that the extreme prevalence of broncho-pneu- 
 monia in infants and young children is to be 
 explained. No matter what the essential disease, 
 an almost constant finding in young children 
 after death is small areas of inflammation in 
 the lungs in and around the terminations of the 
 air tubes. The situation renders it evident that 
 the organisms which caused the lesions entered 
 the lung by the air tubes. The mouth of the 
 child is unclean and harbors numbers of the 
 same sort of organisms as those causing the lung 
 inflammation; but in the absence of such a 
 mode of infection as is given by spray formation 
 it is difficult to see how the extension from the 
 mouth to the lungs could take place. The weak- 
 ened condition of the body in these cases favors 
 the secondary infection. 
 
 If the disease be located in the intestines,
 
 170 DISEASE AND ITS CAUSES 
 
 as in typhoid fever and dysentery, the organisms 
 are contained in the fecal discharges, and by 
 means of these the infection is extended. In 
 typhoid fever, dysentery and cholera massive in- 
 fections of the populace may take place from the 
 contamination of a water supply and the disease 
 be extended over an entire city. One of the most 
 striking instances of this mode of extension was in 
 the epidemic of cholera in Hamburg in 1892. 
 There were two sources of water supply, one of 
 which was infected, and the cases were distributed 
 in the city in the track of the infected supply. 
 Many such instances have been seen in typhoid 
 fever. Certain articles of food, particularly milk, 
 serve as sources of infection. This is more apt to 
 happen when the organism causing the infection 
 grows easily outside of the body. A few such or- 
 ganisms entering into the milk can multiply enor- 
 mously in a few hours and increase the amount of 
 infectious material. In all these cases the sick in- 
 dividual remains a source* of infection, for it is 
 almost impossible to avoid some contamination of 
 the body and the immediate surroundings with 
 the organisms contained in the discharges. 
 
 Transmission by air plays but little part in 
 the extension of infection. In such a disease as 
 smallpox, where the localization is on the surface 
 of the body, the organisms are contained in or 
 on the thin epithelial scales which are constantly 
 given off. These are light, and may remain
 
 DISEASE AND ITS CAUSES 171 
 
 floating in the air and carried by air currents 
 just as is the pollen of plants. There seem to 
 have been cases of smallpox where other modes 
 of more direct transmission could be excluded ^ 
 and in which the organisms were carried in the 
 air over a considerable space. All sorts of in- 
 termediate objects, both living and inanimate, 
 such as persons, domestic animals, toys, books, 
 money, etc., can serve as conveyors of infection. 
 
 Insects play a most important part in the 
 transmission of disease, and in certain cases, 
 as when a disease is localized in the blood, this 
 is the only means of transmission. There are 
 three ways in which the insect plays the role of 
 conveyor. 
 
 1. The insect may play a purely passive part 
 in that its exterior surface becomes contaminated 
 with the discharges of the sick person, and in this 
 way the organisms of disease may be conveyed 
 to articles of food, etc. The ordinary house fly 
 conveys in this way the organisms of typhoid and 
 dysentery. Flies seek the discharges not only for 
 food, but for the purpose of depositing their eggs, 
 and the hairy and irregular surface of their feet 
 facilitates contamination and conveyance. When 
 flies eat such discharges the organisms may pass 
 through the alimentary canal unchanged and be 
 deposited with their feces; they also often vomit 
 or regurgitate food, and in this way also con- 
 taminate objects. Flies very greedily devour the
 
 172 DISEASE AND ITS CAUSES 
 
 sputum of tuberculous patients, and the tubercle 
 bacilli contained in this pass through them un- 
 changed and are deposited in their feces. 
 
 2. Diseases which are localized in the blood 
 are transmitted by biting flies. The biting 
 apparatus becomes contaminated with the or- 
 
 FIG. 19. TRYPANOSOMES FROM BIRDS. All the trypano- 
 somes are very much alike. They contain a nucleus repre- 
 sented by the dark area in the centre and a fur-like membrane 
 terminating in a long whip-like flagellum. They have the 
 power of very active motion within the blood. 
 
 ganisms contained in the blood, and these are 
 directly inoculated into the blood of the next 
 victim. The trypanosome diseases form the 
 best example of this mode of transmission. The 
 trypanosomes are widely distributed, exclusively 
 parasitic, flagellated protozoa which live in the 
 blood of a large number of animals and birds 
 (Fig. 19). They may give rise to fatal diseases,
 
 DISEASE AND ITS CAUSES 173 
 
 but in most cases there is mutual adaptation 
 of host and parasite and they seem to do no 
 harm. One of the most dangerous diseases in 
 man, the African sleeping sickness, is caused 
 by a trypanosome, and the disease of domestic 
 cattle in Africa, nagana, or tsetse fly disease, 
 is also so produced. In certain regions of 
 Africa where a biting fly, the Glossina mor- 
 sitans, occurs in large numbers, it has long 
 been known that cattle bitten by these flies 
 sickened and died, and this prevented the set- 
 tling and use of the land. In the blood of 
 the sick cattle swarms of trypanosomes are 
 found. The source from which the tsetse fly 
 obtained the trypanosomes which it conveyed 
 to the cattle was unknown until it was dis- 
 covered that similar trypanosomes exist in the 
 blood of the wild animals which inhabit the 
 region, but these have acquired by long resi- 
 dence in the region immunity or adaptation 
 to the parasite and no disease is produced. 
 With the gradual extension of settlement of the 
 country and the accompanying destruction of 
 wild life the disease is diminishing. Some of the 
 inter-relations of infections are interesting. The 
 destruction of wild animals in South Africa which, 
 by removing the sources of nagana, rendered the 
 settlement of the country possible was due chiefly 
 to the introduction of another infectious disease, 
 rinderpest, which not only destroyed the wild
 
 174 DISEASE AND ITS CAUSES 
 
 animals but produced great destruction of the 
 domestic cattle as well. 
 
 The sleeping sickness has many features of 
 interest. In the old slavery days it was found 
 that the negroes from the Congo region in the 
 course of the voyage or after they were landed 
 sometimes were affected with a peculiar disease. 
 They were lethargic, took little notice of their 
 surroundings, slept easily and finally passed 
 into a condition of somnolence in which they 
 took no food and gradually died. There was no 
 extension of the disease and it was attributed 
 to extreme homesickness and depression. A 
 similar disease has been known for more than 
 one hundred years on the west coast of Africa, 
 and attracted a good deal of interest and curi- 
 osity on account of the peculiar lethargy which it 
 produced and from which it has received the name 
 of "sleeping sickness." Although apparently in- 
 fectious in its native haunts, it lost the power of 
 spreading from man upon removal to regions 
 where it did not prevail. At first confined to 
 a very small region on the Niger river, it gradu- 
 ally extended with the development of trade 
 routes and the general increase of communi- 
 cations which trade brings, until it prevails 
 in the entire Congo basin, in the British and 
 German possessions in East Africa, and is ex- 
 tending north and south of these regions. The 
 cause of the disease and its mode of conveyance
 
 DISEASE AND ITS CAUSES 175 
 
 was discovered in 1903. The fly Glossina pal- 
 palis which conveys the disease is a biting fly 
 about thelsize of the common house fly and lives 
 chiefly in the vicinity of water. When such a fly 
 bites an individual who has sleeping sickness its bite 
 can convey the disease to monkeys, on whom the 
 transmission experiments were made. After bit- 
 ing the fly is infectious for a period of two days. 
 After this it is harmless, unless it again obtains a 
 supply of living trypanosomes. There is quite a 
 period in which there are no symptoms of the dis- 
 ease, although trypanosomes are found in the blood 
 and in the lymph nodes, and the individual is a 
 source of infection. The peculiar lethargy which 
 has given the disease its name does not appear un- 
 til the nervous system is invaded by the parasites. 
 It is impossible to compute accurately the num- 
 bers of deaths from this disease in the region 
 of Victoria Nyanza alone the estimates extend to 
 hundreds of thousands. 
 
 3. In the third mode of insect conveyance 
 the insect does not play a merely passive role, 
 but becomes a part of the disease, itself under- 
 going infection, and a period in the life cycle of 
 the organism takes place within it. In all these 
 cases quite a period of time must elapse before 
 the insect is capable of transmitting the disease; 
 in malaria, which is the best type of such a disease, 
 this period is ten days. Malaria is due to a small 
 protozoan, the Plasmodium malaria, which was
 
 176 DISEASE AND ITS CAUSES 
 
 discovered by Lavaran, a French investigator, 
 in 1882. The organism lives within or on the 
 surface of the red blood corpuscles. It first 
 appears as a very minute colorless body with 
 active amoeboid movements, and increases in 
 size, attacks a succession of corpuscles, and finally 
 attains a size as large as or larger than a cor- 
 puscle. The corpuscles attacked become pale 
 by the destruction of haemoglobin, swell up and 
 disintegrate, the haemoglobin becoming converted 
 into granules of black pigment inside the para- 
 site. Having attained a definite size the organism 
 forms a rosette and divides into a number of 
 forms similar to the smallest seen inside the 
 corpuscles; these small forms enter other cor- 
 puscles and the cycle again begins. This cycle 
 of development takes place in forty -eight hours, 
 and segmentation is always accompanied by a 
 paroxysm of the disease shown in a chill followed 
 by fever and sweating which is due to the effect 
 of substances liberated by the organism at the 
 time of segmentation. A patient may have two 
 crops of the parasite developing independently 
 in the blood, and the two periods of segmentation 
 give a paroxysm for each, so that the paroxysms 
 may appear at intervals of twenty-four hours 
 instead of forty-eight (Fig. 20). This cycle of 
 development may continue for an indefinite time, 
 and there may be such a rapid increase in the 
 parasites as to bring about the death of the
 
 DISEASE AND ITS CAUSES 177 
 
 individual; but with him the parasite would also 
 perish, for there would be no way of extending 
 the infection and providing a new crop. The 
 disease has been transmitted by injecting the 
 infected blood into a normal individual. 
 
 FIG. 20. PART OF THE CYCLE OF DEVELOPMENT OF THE 
 ORGANISM OF MALARIA, a-g, Cycle of forty-eight hour develop- 
 ment, the period of chill eoinciding with the appearance of 
 / and g in the blood. The organisms g, which result from 
 segmentation, attack other corpuscles and a new cycle begins. 
 h, The male form or microgametocyte, with the protruding 
 and actively moving spermatozoa, one of which is shown 
 free, i and j are the macrogametes or female forms, k shows 
 one of these in the act of being fertilized by the entering sper- 
 matozoon. The differentiation into male and female forms 
 takes place in the blood, the further development of the 
 sexual cycle within the mosquito. 
 
 If a mosquito of the species anopheles bites the 
 affected person, it obtains a large amount of 
 blood which contains many parasites. Within 
 the mosquito the parasite undergoes a further 
 development into male and female sexual forms, 
 which may also form in the blood, termed respec- 
 tively microgametocyte and macrogamete. From
 
 178 DISEASE AND ITS CAUSES 
 
 the microgametocyte small flagellate bodies, 
 the male sexual elements microgametes or sper- 
 matozoa, develop and fertilize the macrogametes; 
 after fertilization this develops into a large body, 
 the oocyst which is attached to the wall of the 
 stomach of the mosquito. Within the oocyst, 
 innumerable small bodies, the sporozoites, develop, 
 make their way into the salivary glands and are 
 injected into the individual who becomes the prey 
 of the mosquito, and again the cycle of develop- 
 ment begins. The presence of the parasite within 
 the mosquito does not constitute a disease. So far 
 as can be determined, life goes on in the usual 
 way, and its duration in the insect is not 
 shortened. 
 
 The nature of the parasite which produces 
 yellow fever is unknown, for it belongs to the 
 filterable viruses; the infectious material, how- 
 ever, has been shown by inoculation to exist in 
 the blood, and the disease is transmitted by a 
 mosquito of another species, the stegomyia. 
 The development cycle within this takes a period 
 of twelve days, which time must elapse after the 
 mosquito has bitten before it can transmit the 
 disease. Here again the mutual interdependence 
 of knowledge is shown. Nothing could have 
 seemed less useful than the study of mosquitoes, 
 the differentiation of the different species, their 
 mode of life, etc., and yet without this knowledge 
 discoveries so beneficial and of such far-reaching
 
 DISEASE AND ITS CAUSES 179 
 
 importance to the whole human race as that 
 of the cause and mode of transmission of malaria 
 and yellow fever would have been impossible; 
 for it could easily have been shown that the 
 ordinary culex mosquito played no role. The 
 role which insects may play in the transmission 
 of disease was first shown by Theobald Smith 
 in this country, in the transmission by a tick 
 of the disease of cattle known as Texas fever. 
 The infecting organism pyrosoma bigenimum is 
 a tiny pear-shaped parasite of the red corpuscles. 
 Smith's investigations on the disease, published in 
 1893, is one of the classics in medicine, and one 
 of the few examples of an investigation which has 
 not been changed or added to by further work. 
 One of the most interesting methods of ex- 
 tension of infection, showing on what small 
 circumstances infection may depend, is seen in 
 the case of the hookworm disease, which causes 
 such devastation in the Southern States. The 
 organism which produces the disease, the Uncin- 
 aria, belongs to the more highly developed para- 
 sites, and is a small round worm one-third of an 
 inch long. The worms which inhabit the intes- 
 tines have a sharp biting mouth by which they 
 fasten themselves to the mucous membrane and 
 devour the blood. The most prominent symp- 
 tom of the disease is anaemia, or loss of blood, 
 due not only to the direct eating of the para- 
 site, but to bleeding from the small wounds
 
 180 DISEASE AND ITS CAUSES 
 
 caused by its bite. Large numbers of eggs are 
 produced by the parasite which are passed out 
 with the feces, which becomes the only infectious 
 material. In a city provided with water-closets 
 and a system of sewerage there would be no 
 means of extension of infection. The eggs in 
 the feces in conditions of warmth and moisture 
 develop into small crawling larvae which can 
 penetrate the skin, producing inflammation of 
 this, known in the region as the ground itch. 
 The larvae enter the circulation and are carried 
 to the lungs, where they perforate the capillaries 
 and reach the inner surface; from this they pass 
 along the windpipe, and then by way of the 
 gullet and stomach reach their habitat, the small 
 intestine. Unfortunately, the habits and poverty 
 of the people in every way facilitate the exten- 
 sion of the infection. There is no proper disposal 
 of the feces, few of the houses have even a privy 
 attached to them, and the feces are distributed 
 in the vicinity of the houses. This leads to con- 
 tamination of the soil over wide areas. Most 
 of the inhabitants of the country go barefoot 
 the greater part of the year, and this gives ready 
 means of contact with the larvae which crawl over 
 the surface of the ground. The disease is neces- 
 sarily associated with poverty and ignorance, the 
 amount of blood is reduced to a low point, and 
 industry, energy and ambition fall with the 
 blood reduction; the schools are few and in-
 
 DISEASE AND ITS CAUSES 181 
 
 efficient; the children are backward, for no 
 child can learn whose brain cells receive but a 
 small proportion of the necessary oxygen; and 
 a general condition of apathy and hopelessness 
 prevails in the effected communities. The con- 
 trol of the disease depends upon the disinfection 
 of the feces, or at least their disposal in some 
 hygienic method, the wearing of shoes, and the 
 better education of the people, all of which con- 
 ditions seem almost hopeless of attainment. The 
 infection is also extended by means of the negroes 
 who harbor the parasite, but who have acquired a 
 high degree of immunity to its effects and whose 
 hygienic habits are even worse than those of the 
 whites. The organism was probably imported 
 with the negroes from Africa and is one of the 
 legacies of slavery. 
 
 The diseases of animals are in many ways 
 closely linked with those of man. In the case 
 of the larger parasites, such as the tapeworms 
 and the trichina, there is a direct interchange of 
 disease with animals, certain phases of the life 
 cycle of the organisms are passed in man and 
 others in various of the domestic animals. A 
 small inconspicuous tapeworm inhabits the 
 intestine of dogs and seems to produce no 
 ill effects. The eggs are passed from the dog, 
 taken into man, and result in the formation 
 of large cystic tumors which not infrequently 
 cause death. Where the companionship be-
 
 182 DISEASE AND ITS CAUSES 
 
 tween dog and man is very close, as in Iceland, 
 the cases are numerous. 
 
 Most of the diseases in animals caused by 
 bacteria and protozoa are not transmitted to 
 man, but there is a conspicuous exception. 
 Plague is now recognized as essentially an animal 
 disease affecting rats and other small rodents, 
 and from these the disease from time to time 
 makes excursions to the human family with dire 
 results. The greatest epidemics of which we 
 have any knowledge are of plague. In the time 
 of Justinian, 542 B. c., a great epidemic of plague 
 extended over what was then regarded as the 
 inhabited earth. This pandemic lasted for fifty 
 years, the disease disappeared and appeared 
 again in many places and caused frightful de- 
 struction of life. Cities were depopulated, the 
 land in many places reverted to a wilderness, 
 and the works of man disappeared. The actual 
 mortality cannot be known, but has been esti- 
 mated at fifty millions. Plague played a large 
 part in the epidemics of the Middle Ages. An 
 epidemic started in 1346 and had as great an 
 extension as the Justinian plague, destroying 
 a fourth of the inhabitants of the places attacked ; 
 and during the fifteenth and sixteenth and seven- 
 teenth centuries the disease repeatedly raised 
 its head, producing smaller and greater epidemics, 
 the best known of which, from the wonderful 
 description of De Foe, is that of London in 1665,
 
 DISEASE AND ITS CAUSES 183 
 
 and called the Black Death. Little was heard 
 of the disease in the nineteenth century, although 
 its existence in Asia was known. In 1894 it 
 appeared in Hong Kong, extended to Canton, 
 thence to India, Japan, San Francisco, Mexico, 
 and, in fact, few parts of the tropics or temperate 
 regions of the earth have been free from it. Mor- 
 tality has varied greatly, being greatest in China 
 and in India; in the last the estimate since 1900 
 is seven million five hundred thousand deaths. 
 The disease is caused by a small bacillus dis- 
 covered in 1894 which forms no spores and is 
 easily destroyed by sunlight, but in the dark is 
 capable of living with undiminished virulence 
 for an indefinite time. The disease in man 
 appears in two forms, the most common known 
 as bubonic plague, from the great enlargement 
 of the lymph nodes, those of the groin being 
 most frequently affected. The more fatal form 
 is known as pneumonic plague, and in this the 
 lungs are the seat of the disease. 
 
 In the old descriptions of the disease it was 
 frequently mentioned that large numbers of dead 
 rats were found when it was prevalent, and the 
 most striking fact of the recent investigations is 
 the demonstration that the infection in man is due 
 to transference of the bacillus from infected rats. 
 There are endemic foci of the disease where it 
 exists in animals, the present epidemic having 
 started from such a focus in Northern China, in
 
 184 DISEASE AND ITS CAUSES 
 
 which region the Tarabagan, a small fur-bearing 
 animal of the squirrel species, was infected. Rats 
 are easily infected, the close social habits of the 
 animal, the vermin which they harbor, and the 
 habits of devouring their dead fellows favor the 
 extension of infection. The disease extends 
 from the rat to man chiefly by means of 
 the fleas which contain the bacilli, and in cases of 
 pneumonic plague from man to man by means 
 of sputum infection. The disease once estab- 
 lished in animals tends to remain, the virus being 
 kept alive by transmission from animal to 
 animal, and the persistence of the infection is 
 favored by mild and chronic cases.
 
 DISEASE CARRIERS. THE RELATION BETWEEN SPORADIC 
 CASES OF INFECTIOUS DISEASE AND EPIDEMICS. SMALL- 
 POX. CEREBRO-SPINAL MENINGITIS. POLYOMYELITIS. 
 VARIATION IN THE SUSCEPTIBILITY OF INDIVIDUALS. 
 CONDITIONS WHICH MAY INFLUENCE SUSCEPTIBILITY. 
 RACIAL SUSCEPTIBILITY. INFLUENCE OF AGE AND SEX. 
 OCCUPATION AND ENVIRONMENT. THE AGE PERIOD 
 OF INFECTIOUS DISEASES. 
 
 WE have seen that insects serve as carriers 
 of disease in two ways : in one, by becoming 
 contaminated with organisms they serve as 
 passive carriers, and in the other they undergo in- 
 fection and form a link in the disease. The more 
 recent investigations of modes of transmission 
 of infectious diseases have shown that man, in 
 addition to serving while sick as a source of 
 infection, may serve as a passive carrier in two 
 ways. For infection to take place not only 
 must the pathogenic organism be present, but it 
 must be able to overcome the passive and active 
 defences of the body and produce injury. Patho- 
 genic organisms may find conditions favorable 
 for growth on the surfaces of the body, and may 
 live there, but be unable to produce infection, 
 and the individual who simply harbors the 
 185
 
 186 DISEASE AND ITS CAUSES 
 
 organisms can transmit them to others. Such 
 an individual may be a greater source of infection 
 than one with the disease, because there is no 
 suspicion of danger. The organisms which thus 
 grow on the surfaces have in some cases been 
 shown to be of diminished virulence, but in 
 others have full pathogenic power. Such pas- 
 sive carriers of infection have been found for 
 a number of diseases, as cerebro-spinal menin- 
 gitis, diphtheria, poliomyelitis and cholera. In 
 all these cases the organisms are most frequently 
 found in those individuals who have been exposed 
 to infection as members of a family in which 
 there have been cases of disease. The other 
 sort of carrier has had and overcome the disease, 
 but mutual relations have been established with 
 the organism which continues to live in the 
 body cavity. Diphtheria bacilli usually linger 
 in the throat after convalescence is established, 
 and until they have disappeared the individual 
 is more dangerous than one actually sick with 
 the disease. Health officers have recognized this 
 in continuing the quarantine against the disease 
 until the organism disappears. In typhoid fever 
 bacilli may remain in the body for a long time 
 and be continually discharged, as in the well- 
 known case of "typhoid Mary." l 
 
 1 This was the case of a woman, by occupation a cook, 
 whose numerous exchanges of service were accompanied by 
 the appearance of cases of typhoid fever in the families.
 
 DISEASE AND ITS CAUSES 187 
 
 Single cases of certain infectious diseases may 
 appear in a community year after year, and 
 at intervals the cases become so numerous that 
 the disease is said to be epidemic. Such a disease 
 is smallpox. This is a highly infectious disease, 
 towards which all mankind is susceptible. Com- 
 plete protection against the disease can be con- 
 ferred by Jenner's discovery of vaccination. 
 The disease becomes modified when transferred 
 to cattle, producing what is known as cowpox, 
 in which vesicles similar to those of smallpox 
 appear on the skin. The inoculation of man 
 with the contents of such a vesicle produces a 
 mild form of disease known as vaccinia, which 
 protects the individual from smallpox. This 
 protection is fully as adequate as that produced 
 by an attack of smallpox, and we are war- 
 ranted in saying that if thorough vaccination, 
 or the inoculation with vaccinia, were carried 
 out smallpox would disappear. There are great 
 difficulties in the way of carrying out effective 
 vaccination of the whole population, which 
 are accentuated by the active opposition of 
 people who are ignorant and wilfully remain so. 
 There exists in every state a number of people 
 unprotected by vaccination, and among these 
 single cases of smallpox appear. The unpro- 
 
 This became so marked that an examination was made and 
 she was found to be a typhoid carrier and as such constantly 
 discharging typhoid bacilli. She is now isolated.
 
 188 DISEASE AND ITS CAUSES 
 
 tected individuals gradually increase in number, 
 forming an inflammable material awaiting the 
 spark or infection which produces a conflagration 
 in the one case and an epidemic in the other. 
 
 Cerebro-spinal meningitis is another example 
 of a disease which exists in sporadic and epidemic 
 form. This disease is caused by a small micro- 
 coccus, the organisms joined in pairs. The seat 
 of the disease is in the meninges or membranes 
 around the brain and spinal cord. The micro- 
 cocci enter the body from the throat and nose, 
 and either pass directly from here into the 
 meninges, or they enter into the blood and are 
 carried by this into the meninges. The organisms 
 are easily destroyed and cannot long survive the 
 conditions outside the body, so that for infection 
 to take place the transmission must be very direct. 
 Carriers who have the organisms in the throat, 
 but who do not have the disease, are the principal 
 agents in dissemination. The mortality is high, 
 and even in recovery permanent damage is often 
 done to the brain or to the organs of special 
 sense. Sporadic cases constantly occur in small 
 numbers, and it is difficult or impossible to trace 
 any connection between these cases. At varying 
 intervals, often twenty years intervening, an 
 epidemic appears which sometimes remains 
 local in a city or state, sometimes extends to 
 adjoining cities or states, and may even extend 
 over a very large area. In the epidemics the
 
 DISEASE AND ITS CAUSES 189 
 
 mortality is much higher than in the sporadic 
 cases. The same explanation given for small- 
 pox cannot apply here, for there is not a similar 
 accumulation of susceptible material. We know 
 there is a great deal of variation in the virulence 
 of the different pathogenic organisms, and the 
 virulence can be artificially increased and dimin- 
 ished. In epidemics of meningitis the virulence 
 of the organisms is increased, as is shown by the 
 greater mortality. It is highly probable that 
 such epidemics are due to changes which arise 
 in the organisms from causes we do not know and 
 which increase their capacity for harm. It is 
 possible that such a change would convert a 
 carrier into a case of disease, the organism ac- 
 quiring greater powers of invasion. Such a 
 strain of organisms arising in one place and 
 producing an epidemic could be transported 
 to another locality and exert the same action, 
 or similar changes in the organisms could arise 
 simultaneously in a number of places. Analogies 
 to such conditions are given in plants. In cer- 
 tain plants it has been shown that from unknown 
 causes there appears a tendency to the produc- 
 tion of variations. A very beautiful herbaceous 
 peony known as " Bridesmaid " after having 
 grown for a number of years in single form, in 
 one year wherever grown suddenly became 
 double. The peculiar thing with the lower 
 unicellular organisms is that the changes which
 
 190 DISEASE AND ITS CAUSES 
 
 so arise do not tend to become permanent, the 
 organism reverts to its usual character, the 
 disease to its sporadic type. 
 
 A very fatal form of poliomyelitis has for a 
 number of years prevailed in Sweden. In the 
 United States there have been continually a 
 number of single cases of the disease, and it is 
 not impossible that a more pathogenic strain of 
 the organism has developed in Sweden and has 
 been imported into this country, giving rise to 
 the much greater extension of the disease in a 
 number of places. 
 
 The most cursory study of the infectious 
 diseases shows that there is great variation hi 
 the susceptibility of individuals. Even in the 
 most severe epidemics all are not equally affected, 
 some escape the infection, others have the dis- 
 ease lightly, others severely, some die. Chance 
 enters into this, but plays a small part, for the 
 same varying individual susceptibility is shown 
 experimentally. If a given number of animals 
 of the same species, age and weight, even those 
 from the same litter, be inoculated with a given 
 number of bacteria shown to be pathogenic for 
 that species, the results differ. If the dose be 
 necessarily fatal, death will take place at inter- 
 vals; if a dose smaller than the fatal be used, 
 some animals will die, others will recover. The 
 defences of the organism being centred in the 
 activity of the living tissue, any condition which
 
 DISEASE AND ITS CAUSES 191 
 
 depresses cell activity may have an effect in 
 increasing susceptibility to infection. Animals 
 which ordinarily are not susceptible to infection 
 with a certain organism may be made so by 
 prolonged hunger, or fatigue, by the influence 
 of narcotics, by reduction of the body tempera- 
 ture, by loss of blood. In man prolonged fatigue, 
 cold, the use of alcohol to excess and even psychic 
 depression increases susceptibility. It has been 
 shown that such conditions are accompanied 
 by a diminution in the power of the blood to 
 destroy bacteria. 
 
 There is variation in the susceptibility to 
 infection in the different races of man. If a 
 race be confined to one habitat with close inter- 
 course between the people, such a race may 
 acquire a high degree of immunity to local dis- 
 eases by a gradual weeding out of the individuals 
 who are most susceptible. A degree of compara- 
 tive harmony may be gradually established 
 between host and parasite, as is the case in wild 
 animals. These have few diseases, the weak 
 die, the resistant breed; they harbor, it is true, 
 large numbers of parasites, but there is mutual 
 adjustment between parasite and host. Diseases 
 in animals greatly increase under the artificial 
 conditions of domestication. Certain highly 
 specialized breeds of cattle, as the Alderneys, 
 are much more susceptible to tuberculosis than 
 the less specialized. The high development of
 
 192 DISEASE AND ITS CAUSES 
 
 the variation which consists in a marked ability 
 to produce milk fat is probably combined with 
 other qualities, shown in diminished resistance to 
 disease, and under natural conditions the varia- 
 tion would not have persisted. The introduction 
 of a new disease into an isolated people has often 
 been attended with dire consequences. It is much 
 the same thing with the introduction of disease 
 of plants. In Europe the brown-tail moth and 
 the gypsy moth produce continuously a certain 
 amount of damage to the trees, but their para- 
 sitic enemies have developed with them and 
 check their increase. These pests were brought 
 to this country in which there were no condi- 
 tions retarding their increase and have produced 
 great damage. 
 
 It is very difficult to estimate the degree of 
 racial susceptibility. The negro race seems to 
 be more susceptible to certain diseases, such as 
 tuberculosis and smallpox, less so to others, as 
 yellow fever, malaria and uncinariasis. What 
 are apparently differences in susceptibility may 
 be explained by racial customs. A statistical in- 
 quiry into death in India from poisonous snakes 
 might be interpreted as showing a marked 
 resistance on the part of the white to the action 
 of the venom, but it is merely a question of the 
 boots of the whites and the naked feet and legs 
 of the natives. The relatively greater frequency 
 of smallpox in the blacks is due to the greater
 
 DISEASE AND ITS CAUSES 193 
 
 difficulties in carrying out vaccination measures 
 among them and the greater opportunity for 
 infection which results from their less hygienic 
 life. It has always been noted that when plague 
 prevails in Oriental cities, the natives are more 
 frequently attacked than are Europeans. This 
 does not depend upon differences in suscepti- 
 bility, but on the better hygienic conditions of 
 the whites which prevent the close relation 
 to rats and vermin by which infection is ex- 
 tended. There would be but little extension of 
 the hookworm disease in a community where 
 shoes were worn and the habits were cleanly. 
 
 It is by no means improbable that the forma- 
 tion of the habits of civilization was influenced by 
 infection. Most of these habits, such as personal 
 cleanliness, the avoidance of close contact, the 
 demand for individual utensils for eating and 
 drinking, are all of distinct advantage in opposing 
 infection. Certain habits, on the other hand, 
 such as kissing, which probably represents the 
 extension of a habit of sexual origin, are disad- 
 vantageous and infection is often transmitted 
 in this way. In syphilitic infection the mouth 
 forms one of the most common localizations 
 of the disease and may contain the causal organ- 
 isms in great numbers. This, the spirochceta 
 pallida, is an organism of great virulence, and 
 man is the most susceptible animal. The dis- 
 ease, like gonorrhoea, is essentially a sexual dis-
 
 194 DISEASE AND ITS CAUSES 
 
 ease, the primary location is in the sexual organs, 
 and it is transmitted chiefly by sexual contact. 
 Of all the infectious diseases, it is the one most 
 frequently transmitted to the unborn child; in 
 certain cases the disease is transmitted, in others 
 the developing foetus may be so injured by the 
 toxic products of the disease that various imperfec- 
 tions of development result, as is shown in deform- 
 ities, or in conditions which render the entire 
 organism or individual organs, particularly the 
 nervous system, more susceptible to injury. 
 Following the primary localization of the ac- 
 quired form of the disease, there is usually second- 
 ary localization in the mucous membrane of 
 the mouth, and the disease may be transmitted 
 by kissing or by the use of contaminated utensils. 
 The habit of indiscriminate kissing is one which 
 might with great benefit be given up. 
 
 There is definite relation between age and 
 the infectious diseases. In general, suscepti- 
 bility is increased in the young; young animals 
 can be successfully inoculated with diseases to 
 which the adults of the species are immune, and 
 certain human diseases, such as scarlet fever, 
 measles and whooping cough, seem to be the 
 prerogatives of the child. It must be remem- 
 bered, however, that one attack of these diseases 
 confers a strong and lasting immunity and 
 children represent a raw material unprotected 
 by previous disease. Where measles has been
 
 DISEASE AND ITS CAUSES 195 
 
 introduced into an island population for the 
 first time, all ages seem equally susceptible. 
 All ages are equally susceptible to smallpox, 
 and yet in the general prevalence of the disease in 
 the prevaccination period it was almost confined 
 to children, the adults being protected by a 
 previous attack. The habits and environment 
 at different ages have an influence on the oppor- 
 tunities for infection. There is comparatively 
 little opportunity for infection during the first 
 year, in which period the infant is nursed and 
 has a narrow environment within which infection 
 is easily controlled. With increasing years the 
 opportunities for infection increase. When the 
 child begins to move and crawl on hands and 
 knees the hands become contaminated, and 
 the habit of putting objects handled into the 
 mouth makes infection by this route possible. 
 Food also becomes more varied, milk forms an 
 important part of the diet, and we are now appre- 
 ciating the possibilities of raw milk in conveying 
 infection. With the enlarging environment, 
 with the school age bringing greater contact 
 of the child with others, there come greater 
 opportunities for infection which are partly offset 
 by the increase in cleanliness. The dangers of 
 infection in the school period are now greatly 
 lessened by medical inspection and care of the 
 school children. In the small epidemic of small- 
 pox which prevailed in Boston from 1881 to
 
 196 DISEASE AND ITS CAUSES 
 
 1883, there was a sharp decline in the incidence 
 of the disease in children as soon as the school 
 age was reached, this being due to the demand 
 of vaccination as a condition for entrance into 
 the schools. Many of the infectious diseases 
 are much milder in children than in adults. 
 This is the case in typhoid fever, malaria and 
 yellow fever. The comparative immunity of the 
 natives to yellow fever in regions where this 
 prevails seems to be due to their having acquired 
 the disease in infancy in so mild a form that it 
 was not recognized as such. 
 
 The infectious diseases are preeminently the 
 diseases of the first third of life. After the age 
 of forty man represents a select material. He 
 has acquired immunity to many infections by 
 having experienced them. Habits of life have 
 become fixed and there is a general adjustment to 
 environment. The only infectious disease which 
 shows no abatement in its incidence is pneumonia, 
 and the mortality in this increases with age. 
 Between thirty-five and fifty-five man stands 
 on a tolerably firm foundation regarding health; 
 after this the age atrophies begin, the effects of 
 previous damage begin to be apparent, and the 
 tumor incidence increases.
 
 CHAPTER X 
 
 INHERITANCE AS A FACTOR IN DISEASE. THE PROCESS OF 
 CELL MULTIPLICATION. THE SEXUAL CELLS DIFFER 
 
 FROM THE OTHER CELLS OF THE BODY. INFECTION OF 
 
 THE OVUM. INTRA-UTERINE INFECTION. THE PLACENTA 
 AS A BARRIER TO INFECTION. VARIATIONS AND MUTA- 
 TIONS. THF INHERITANCE OF SUSCEPTIBILITY TO DIS- 
 EASE. THE INFLUENCE OF ALCOHOLISM IN THE PARENTS 
 ON THE DESCENDANTS. THE HEREDITY OF NERVOUS DIS- 
 EASES. TRANSMISSION OF DISEASE BY THE FEMALE 
 ONLY. HEMOPHILIA. THE INHERITANCE OF MALFOR- 
 MATIONS. THE CAUSES OF MALFORMATIONS. MATER- 
 NAL IMPRESSIONS HAVE NO INFLUENCE. EUGENICS. 
 
 THE question of inheritance of disease is 
 closely associated with the study of infection, 
 and the general subject of heredity in its bearing 
 on disease can be considered here. By heredity 
 is understood the transference of similar charac- 
 teristics from one generation of organisms to 
 another. The formation of the sexual cells is a 
 much more complex process than that of the 
 formation of single differentiated cells, for the 
 properties of all the cells of the body are repre- 
 sented in the sexual cells, to the union of which 
 the heredity transmission of the qualities of the 
 parents is due. In the nucleus of all the cells 
 in the body there is a material called chromatin, 
 which in the process of cell division forms a 
 197
 
 198 DISEASE AND ITS CAUSES 
 
 convoluted thread ; this afterwards divides into a 
 number of loops called chromosomes, the number 
 of which are constant for each animal species. 
 In cell division these loops divide longitudinally, 
 one-half of each going to the two new cells which 
 result from the division; each new cell has one-half 
 of all the chromatin contained in the old and also 
 
 FIG. 21. DIAGRAM SHOWING THE RELATION OF THE 
 SEXUAL CELLS TO THE SOMATIC CELLS OR THOSE OF THE GEN- 
 ERAL BODY. The sexual cells are represented to the left of 
 the line at the bottom of diagram and are black. From the 
 fertilized ovum at the top there is a continuous cell develop- 
 ment, with differentiation represented in the cell groups of 
 the bottom row. It is seen that the sexual cells are formed 
 directly from the germ cell and contain no admixture from 
 the cells of the body. 
 
 one-half of the cytoplasm or the cell material 
 outside of the nucleus. The process of sexual 
 fertilization consists in the union of the male 
 and female sex cells and an equal blending of 
 the chromatin contained in each (Fig. 22). In 
 the process of formation of the sexual cells a 
 diminution of the number of chromosomes
 
 DISEASE AND ITS CAUSES 199 
 
 contained in them takes place, but this is pre- 
 ceded by such an intimate intermingling of the 
 chromatin that the sexual cells contain part of 
 all the chromosomes of the undifferentiated cells 
 from which they were formed. The new cell 
 which is formed by the union of the male and 
 female sexual cells and which constitutes a new 
 organism, contains the number of chromosomes 
 characteristic of the species and parts of all the 
 chromatin of the undifferentiated cells of male 
 and female ancestors. As a result of this the 
 most complicated mechanism in nature, it is evi- 
 dent that in a strict sense there can be no here- 
 dity of a disease because heredity in the mammal 
 is solely a matter of the chromosomes and these 
 could not convey a parasite. The new organism 
 can, however, quickly become diseased and, by 
 the transference of disease to it and by either 
 parent, there is the appearance of hereditary trans- 
 mission of disease, though in reality it is not such. 
 The ovum itself can become the site of infec- 
 tion; this, which was first discovered by Pasteur 
 in the eggs of silkworms, takes place not infre- 
 quently in the infection of insects with pro- 
 tozoa. In Texas fever the ticks which transmit 
 the disease, after filling with the infected blood, 
 drop off and lay eggs which contain the parasites, 
 and the disease is propagated by the young 
 ticks in whom the parasites have multiplied. 
 The same thing is true in regard to the African
 
 200 DISEASE AND ITS CAUSES 
 
 relapsing or tick fever, which is also transferred 
 by a tick. In the white diarrhoea of chickens 
 the eggs become infected before they are laid 
 and the young chick is infected before it emerges 
 from the shell. It is highly improbable, and 
 there is no certain evidence for it, that the 
 extremely small amount of material contributed 
 by the male can become infected and bring 
 infection to the new organism. In the cases 
 in which disease of the male parent is trans- 
 ferred to the offspring, it is either by an in- 
 fection of the female by the male, with 
 transference of the infection from her to the 
 developing organism, or with the male sexual 
 cells there may be a transference to the female 
 of the infectious material and the new organism 
 may be directly infected. No other disease in 
 man is so easily and directly transferred from 
 either parent to offspring as is syphilis, and the 
 disease is extremely malignant for the foetus, 
 usually causing death before the normal period 
 of intra-uterine development is reached. 
 
 The mother gives the protection of a narrow 
 and unchanging environment and food to the 
 new organism which develops within the uterus, 
 and there is always a membranous separation 
 between them. Disease of the mother may 
 affect the foetus in a number of ways. In most 
 cases the membrane of separation is an efficient 
 guard preventing pathogenic organisms reaching
 
 DISEASE AND ITS CAUSES 
 
 201 
 
 FIG. 22. DIAGRAMMATIC REPRESENTATION OF THE PRO- 
 CESS OP FERTILIZATION. (Boveri.) In the first cell (a) the 
 ovum is shown in process of fertilization by the entering sper- 
 matozoon or male sexual element. In the following cells there 
 is shown the increase in amount of the male material and the 
 final intimate commingling in g which precedes the first seg- 
 mentation, g represents a new organism formed by the union 
 of the male or female cell but differing from either of them.
 
 202 DISEASE AND ITS CAUSES 
 
 the foetus from the mother. In certain cases, 
 however, the guard can be passed. In small- 
 pox, not infrequently, the disease extends from 
 the mother to the foetus, and the child may die 
 of the infection or be born at term with the scars 
 resulting from the disease upon it. Syphilis 
 in the mother in an active stage is practically 
 always extended to the foetus. We have said 
 that in an infectious disease substances of an 
 injurious character are produced by bacteria, 
 and such substances being in solution in the 
 blood of the infected mother can pass through 
 the membranous barrier and may destroy the 
 foetus although the mother recovers from the 
 infection. 
 
 Living matter is always individual, and this 
 individuality is expressed in slight structural va- 
 riations from the type of the species as shown 
 in an average of measurements, and also in slight 
 variations in function or the reactions which 
 living tissue shows towards the conditions act- 
 ing upon it. The anatomical variations are more 
 striking because they can be demonstrated by 
 weight and measure, but the functional varia- 
 tions are equally numerous. Thus, no two 
 brains react in exactly the same way to the 
 impressions received by the sense organs; there 
 are differences in muscular action, differences 
 in digestion; these variations in function are 
 due to variations in the structure of living ma-
 
 DISEASE AND ITS CAUSES 203 
 
 terial which are too minute for our compara- 
 tively coarse methods of detection. In the 
 enormous complexity of living matter it is im- 
 possible that there should not be minute differ- 
 ences in molecular arrangement and to this such 
 functional variations may be due. Chemistry 
 gives us a number of examples of variations in the 
 reaction of substances which with the same com- 
 position differ in the molecular arrangement. 
 Even in so simple a mechanism as a watch 
 there are slight differences in structure which 
 gives to each watch certain individual charac- 
 teristics, but the type as an instrument con- 
 structed for recording time remains. In the 
 fusion of the chromosomes of the male and 
 female sexual cells, to which the hereditary 
 transmission of the ancestral qualities to the" 
 new offspring is due, there are differences" in 
 the qualities of each, for the individuality of the 
 parents is expressed in the germ cells, and 
 the varying way in which these may fuse gives to 
 the new cell qualities of its own in addition to 
 qualities which come from each ancestor, and 
 from remote ancestors through these. The 
 qualities with which the new organism starts 
 are those which it has received from its ancestors 
 plus its individuality. The fact that the sexual 
 cells are formed from the early formed cells of 
 the new organism which represent all of the qual- 
 ities of the fertilized ovum or primordial cell,
 
 204 DISEASE AND ITS CAUSES 
 
 renders it unlikely that the new offspring will 
 contain qualities which the parents have acquired. 
 The question of the inheritance of characteristics 
 which the parents have acquired as the result 
 of the action of environment upon them is one 
 which is still actively investigated by the students 
 of heredity, but the weight of evidence is op- 
 posed to this belief. 
 
 In the new organism the type of the species is 
 preserved and the variations from the mean to 
 which individuality is due are slight. We are 
 accustomed to regard as variations somewhat 
 greater departures from the species type than is 
 represented in individuality, but there is no 
 sharp dividing line between them. 
 
 Very much wider departures from the species 
 type are known as mutations. Such variations 
 and mutations, like individuality, may be ex- 
 pressed in qualities which can be weighed and 
 measured, or in function, and all these can be 
 inherited; certain of them known as dominant 
 characteristics more readily than others, which 
 are known as recessive. If these variations 
 from the type are advantageous, they may be 
 preserved and become the property of the species, 
 and it is in this way that the characteristics of 
 the different races have arisen. Certain of the 
 variations are unfavorable to the race. The 
 varying predisposition to infection which un- 
 doubtedly exists and may be inherited represents
 
 DISEASE AND ITS CAUSES 205 
 
 such a variation. Tuberculosis is an instance of 
 this; for, while the cause of the disease is the 
 tubercle bacillus, there is enormous difference in 
 the resistance of the body to its action in different 
 individuals. The disease is to a considerable 
 extent one of families, but while this is true 
 the degree of the influence exerted by heredity 
 can be greatly overestimated. The disease is 
 so common that in tracing the ancestry of tuber- 
 culous patients it is rare to find the disease not 
 represented in the ancestors. A further difficulty 
 is that the environment is also inherited. The 
 child of a tuberculous parent has much better 
 opportunity to acquire the infection than a child 
 without such an environment [page 167]. Other 
 diseases than the infectious seem to be inherited, 
 of which gout is an example. In gout there is 
 an unusual action of the cells of the body which 
 leads to the formation and the retention in the 
 body of substances which are injurious. Here it is 
 not the disease which is inherited, but the vari- 
 ation in structure to which the unusual and 
 injurious action of the cells is due. 
 
 While tuberculosis and gout represent in- 
 stances in which, although the disease itself 
 is not inherited yet the presence of the disease 
 in the ascendants so affects the germinal material 
 that the offspring is more susceptible to these 
 particular diseases, much more common are the 
 cases in which disease in the parents produces a
 
 206 DISEASE AND ITS CAUSES 
 
 defective offspring, the defect consisting in a 
 general loss of resistance manifested in a variety 
 of ways, but not necessarily repeating the dis- 
 eased condition of the parent. In these cases 
 the disease in the parents affects all the cells of 
 the body including the germinal cells, and the 
 defective qualities in the germ cells will affect 
 the cells of the offspring which are derived from 
 these. There is a tendency in these cases to the 
 repetition in the offspring of the disease of 
 the parents, because the particular form of the 
 parental disease may have been due to or in- 
 fluenced by variation of structure. One of the 
 best examples of affection of the offspring by 
 diseased conditions of the parents produced by 
 a toxic agent which directly or indirectly affects 
 all the cells of the body is afforded by alcohol 
 when used in excess. Since drunkenness has 
 become a medical rather than a moral question, a 
 great deal of reliable data has accumulated in 
 regard to it as a factor in the heredity of disease. 
 Grotjahn gives the following examples: Six 
 families were investigated in which there were 
 thirty-one children. In all these families the 
 father and grandfather on the father's side were 
 chronic alcoholics, and in certain of the families 
 drunkenness prevailed in the more remote 
 ancestors. The following was the fate of the 
 children : eight died shortly after birth of general 
 weakness, seven died of convulsions in the first
 
 DISEASE AND ITS CAUSES 207 
 
 month, three were malformed, three were idiotic, 
 three were feeble-minded, three were dwarfs, 
 three were epileptics, two were normal. In a 
 second group of three families there were twenty 
 children. The fathers were drunkards, but 
 their immediate ancestors were free : four children 
 died of general weakness, three of convulsions 
 in the first month, two were feeble-minded, one 
 was a dwarf, one was an epileptic, seven were nor- 
 mal. In a family in which both father and mother 
 and their ancestors were drunkards there were 
 six children: three died of convulsions within 
 six months, one was an idiot, one a dwarf, and one 
 an epileptic. For comparison there were taken 
 from the same station in life ten families in which 
 there was no drunkenness: three children died 
 from general weakness, three from intestinal 
 troubles, two of nervous affection, two were feeble- 
 minded, two were malformed, fifty were normal. 
 Legrain has studied on a larger scale the de- 
 scendants of two hundred and fifteen families 
 of drunkards in which there were eight hundred 
 and nineteen children. One hundred and forty- 
 five of these were insane, sixty -two were criminals, 
 and one hundred and ninety-seven drunkards. 
 Of course all this cannot be attributed to alcohol 
 alone. There is first to be considered a probable 
 variation in the nervous system which is ex- 
 pressed in the alcoholic habit; second, the en- 
 vironment consisting in poverty, bad associates,
 
 208 DISEASE AND ITS CAUSES 
 
 etc., which the alcoholic habit brings; third, 
 the alcohol alone. That defective inheritance 
 so frequently takes the form of alcoholism is 
 largely due to the environment. There has never 
 been the opportunity to study on a large scale 
 the effect of the complete deprivation of alcohol 
 from a people living in the environment of modern 
 civilization. There is a possibility, and even 
 probability, that the defective nervous organ- 
 ization which predisposes to alcoholism would 
 seek satisfaction hi the use of some other 
 sedative drug. So complex are all the inter- 
 relations of the social system that it would 
 be possible to regard alcohol as an agent use- 
 ful in removing the defective, were it not for 
 its long-enduring action and its effects on the 
 descendants, procreation not being affected by 
 its use. 
 
 Diseases of the nervous system are particu- 
 larly apt to affect the offspring, and often the 
 inherited condition repeats that of the parents. 
 This is due to the fact that most of the nervous 
 diseases depend both upon intrinsic factors which 
 consist in some defective condition of the nervous 
 system representing a variation, and extrinsic 
 factors due to environment or occupation which 
 make the basal condition operative. The definite 
 relation between alcoholism and insanity is due to 
 alcohol acting not as an intrinsic but an extrinsic 
 factor, bringing into effectiveness the hereditary
 
 DISEASE AND ITS CAUSES 209 
 
 weakness of the nervous system. The influence 
 of heredity in producing insanity is variously 
 estimated at from twenty-six per cent to sixty 
 per cent of all cases. This great difference in 
 the estimation of the hereditary influence is 
 due to the personal equation of the statistician, 
 and the care with which other factors are elimi- 
 nated. In the more severe form of the hereditary 
 degeneration the same pathological conditions 
 are repeated in the descendants. In certain 
 cases the severity of the condition increases 
 from generation to generation. According to 
 Morel there may be merely what is recognized 
 as a nervous temperament often associated 
 with moral depravity and various excesses in 
 the first generation; in the second, severe neu- 
 roses, a tendency to apoplexy and alcoholism; 
 in the third, psychic disturbances, suicidal 
 tendencies and intellectual incapacity; and in 
 the fourth, congenital idiocy, malformations 
 and arrests of development. There are some 
 very definite data with regard to inheritance 
 in the nervous disease known as epilepsy. The 
 essential condition in this consists in attacks 
 of unconsciousness, usually accompanied by a 
 discharge of nerve force shown in convulsions, 
 the attack being often preceded by peculiar 
 sensations of some sort known as the aura. In 
 the most marked forms of the affection heredity 
 plays but little part, owing to the early super-
 
 210 DISEASE AND ITS CAUSES 
 
 vention of imbecility and helplessness, and it 
 is a greater factor in the better classes of society 
 than in the proletariat. In the better classes, 
 owing to the greater care of the cases and the 
 avoidance of exciting causes of the attacks, the 
 disease is better controlled and rarely advances 
 to the extent that it does among the poor. The 
 association of epilepsy and alcoholism is espe- 
 cially dangerous, for a slight amount of alcohol 
 may greatly accentuate the disease. In five 
 hundred and thirty -five children in whose parent- 
 age there were sixty-two male and seventy-four 
 female epileptics, twenty-two were born dead, one 
 hundred and ninety-five died from convulsions 
 in infancy, twenty-seven died in infancy from 
 other causes, seventy-eight were epileptics, eleven 
 were insane, thirty-nine were paralyzed, forty- 
 five were hysterical, six had St. Vitus's dance, 
 one hundred and five were ordinarily healthy. 
 That variations in the nervous system which 
 produce more or less unusual mental peculiarities 
 and which do not take the form of nervous 
 disease are inherited, the most superficial con- 
 sideration shows. A child in its mental character- 
 istics is said to take after one or the other of its 
 parents, certain habits and mental traits are the 
 same, often even the handwriting of a child 
 resembles that of a parent. 
 
 In certain cases the inheritance is transmitted 
 by the female alone. This is the case hi the
 
 DISEASE AND ITS CAUSES 211 
 
 haemophilia, the unfortunate subjects of which are 
 known as bleeders. There is in this a marked 
 tendency to haemorrhage which depends upon an 
 alteration hi the character of 'the blood which 
 prevents clotting. This, the natural means of 
 stopping bleeding from small wounds, being in 
 abeyance, fatal haemorrhage may result from 
 pulling a tooth or from an insignificant wound. 
 There is a seeming injustice in the inheritance, 
 for the females do not suffer from the disease 
 although they transmit it, while the males who 
 have the disease cannot even create additional 
 sympathy by transmitting it. 
 
 The most obvious inheritance is seen in the 
 case of malformations. These represent wide 
 departures from the type of the species as repre- 
 sented in the form. There is no hard and fast 
 line separating the slight departures from the nor- 
 mal type known as variations and mutations, 
 from the malformations. Certain of the malforma- 
 tions known as monstrosities hardly represent the 
 human type. These are the cases in which the 
 foetus is represented in a formless mass of tissue, 
 or there is absence of development of important 
 parts such as the nervous system or there is more 
 or less extensive duplication of the body. There 
 has always been a great deal of popular interest 
 attached to the malformations owing to the part 
 which maternal impressions are supposed to 
 play in their production. In this, some striking
 
 212 DISEASE AND ITS CAUSES 
 
 impression made on the pregnant woman is 
 supposed to affect in a definite way the structure 
 of the child. The cases, for instance, in which 
 a woman sees an accident involving a wound 
 or a loss of an arm and the child at birth shows 
 a malformation involving the same part. There 
 is no association between maternal impressions 
 and malformations, although there have been 
 many striking coincidences. All malformations 
 arise during the first six weeks of pregnancy 
 known as the embryonic period, in which the 
 development of the form of the child is taking 
 place, and during which time there is little con- 
 sciousness of pregnancy. Maternal impressions 
 are usually received at a later period, when the 
 form of the child is complete and it is merely 
 growing. It must be remembered also that 
 there is neither nervous nor vascular connection 
 between the child in the uterus and the mother, 
 the child being from the period of conception 
 an independent entity to which the mother 
 gives nutriment merely. Of course, as has been 
 said, the mother may transmit to the child sub- 
 stances which are injurious, and in certain cases 
 parasites may pass from the mother to the foetus. 
 The same types of malformations which occur in 
 man are also seen in birds, and it would require 
 a more vigorous imagination than is usual to 
 believe that a brooding hen could transmit an 
 impression to an egg and that a headless chick
 
 DISEASE AND ITS CAUSES 213 
 
 could result from witnessing the sacrifice of an 
 associate. The idea of the importance of maternal 
 impressions in influencing the character of the 
 offspring is a very old one, a well-known instance 
 being the sharp practice of Jacob's using peeled 
 wands to influence the color of his cattle. In 
 regard to coincidences the great number of 
 cases in which strong impressions made on 
 the mind of the pregnant mother without result 
 on the offspring are forgotten. The belief has 
 been productive of great anxiety and even 
 unhappiness during a period which is necessarily 
 a trying one, and should be dismissed as being 
 both theoretically impossible and unsupported 
 by fact. 
 
 The malformations are divided anatomically 
 into those characterized, first, by excess forma- 
 tion, second, by deficient formation, third, by 
 abnormal displacement of parts. They are due 
 to intrinsic causes which are in the germ, and 
 which may be due to some unusual conditions 
 in either the male or female germ cell or an 
 imperfect commingling of the germinal material, 
 and to extrinsic causes which physically, as hi 
 the nature of a shock or chemically as by the ac- 
 tion of a poison, may affect the embryo through 
 the mother. Malformations are made more 
 numerous in chickens by shaking the eggs before 
 brooding. A number of malformations are pro- 
 duced by accidental conditions arising in the
 
 214 DISEASE AND ITS CAUSES 
 
 environment; for instance, the vascular cord con- 
 necting mother and child may become wound 
 around parts constricting them or even cutting 
 them off, and the membrane around the child 
 may become adherent to certain parts and pre- 
 vent the development of these. The extrinsic 
 causes are more operative the more unfavorable 
 is the environment of the mother. Malforma- 
 tions are more common in illegitimate children 
 than in legitimate and more common in alcoholic 
 mothers; there is an unfavorable environment of 
 poverty in both cases, added to in the latter and 
 usually in the former by the injurious action 
 of the alcohol. 
 
 The more extensive malformations have no 
 effect on heredity, because the subjects of them 
 are incapable of procreation. The malformations 
 which arise from the accidents of pregnancy 
 and which are compatible with a perfectly 
 normal germ are in the nature of acquired 
 characteristics and are not inherited. Those 
 malformations, however, which are due to 
 qualities in the germinal material itself are 
 inherited, and certain of them with remarkable 
 persistence. There are instances in which the 
 slight malformation consisting in an excess of 
 ringers or toes has persisted through many 
 generations. It may occasionally lapse in a 
 generation to reappear later. In certain cases, 
 notably in the bleeders, the inheritance is trans-
 
 DISEASE AND ITS CAUSES 215 
 
 milled by Ihe female alone, in olher cases by Ihe 
 sexes equally, bul Ihere are no cases of Irans- 
 mission by Ihe male line only. Il is evidenl 
 lhal when Ihe same malformalion affecls bolh 
 Ihe male and Ihe female line Ihe heredilary 
 influence is much slronger. A case has been 
 relaled lo me in which mosl of Ihe inhabilanls 
 in a remole mounlain valley in Virginia where 
 Ihere has been much intermarriage have one 
 of Ihe joinls of Ihe fingers missing. There is a 
 very prevalenl idea lhal in close intermarriage 
 in families varialions and malformalions oflen 
 unfortunate for Ihe individual are more common. 
 All experimenlal evidence oblained by inter- 
 breeding of animals shows lhal close interbreed- 
 ing is nol produclive of varialion, bul lhal 
 varialions exisling in Ihe breed become ac- 
 cenluated. Varialions eilher advanlageous or 
 disadvanlageous for Ihe race or individual may 
 eilher of Ihem become more prevalenl by 
 close intermarriage. Il seems, however, lo have 
 been shown by Ihe cusloms of Ihe human 
 race lhal very close intermarriage is disadvan- 
 lageous. 
 
 Eugenics, which signifies an allempl al Ihe 
 bellermenl of Ihe race by Ihe avoidance of bad 
 heredily, has wilhin recenl years attracted much 
 allenlion and is of importance. Some of ils 
 advocates have become so enlhusiaslic as lo 
 believe lhal il will be possible lo breed
 
 216 DISEASE AND ITS CAUSES 
 
 men as cattle and ultimately to produce a race 
 ideally perfect. It is true that by careful 
 selection and regulation of marriage certain 
 variations, whether relating to coarse bodily 
 form or to the less obvious changes denoted by 
 function, can be perpetuated and strengthened. 
 That the Semitic race excels in commerce is 
 probably due to the fact that the variation of 
 the brain which affected favorably the mental 
 action conducive to this form of activity, was 
 favorable for the race in the hostile environment 
 in which it was usually placed and transmitted 
 and strengthened by clbse intermarriage. It 
 is impossible, however, to form a conception of 
 what may be regarded as an ideal type of the 
 human species. The type which might be ideal 
 in a certain environment might not be ideal in 
 another, and environment is probably of equal 
 importance with the material. The eugenics 
 movement has enormously stimulated research 
 into heredity by the methods both of animal 
 experimentation and observation, and study of 
 heredity in man. As in all of the beginning 
 sciences there is not the close inter-relation of 
 observed facts and theory, but there is excess of 
 theory and dearth of facts. Certain considera- 
 tions, however, seem to be evident. It would 
 seem to be evident that individuals should be 
 healthy and enabled to maintain themselves in 
 the environment in which they are placed, but
 
 DISEASE AND ITS CAUSES 217 
 
 the qualities which may enable an individual 
 successfully to adapt himself to factory life, or 
 life in the crowds and strong competition of the 
 city, may not be, and probably are not, qualities 
 which are good for the race in general or for 
 his immediate descendants. At present our 
 attempts to influence heredity should be limited 
 to the heredity of disease only. We can cer- 
 tainly say that intermarriage between persons 
 who have tuberculosis or in whose families the 
 disease has prevailed is disadvantageous for the 
 offspring; the same holds true for insanity and 
 for nervous diseases of all sorts, for forms of 
 criminality, for alcoholism, and for those diseases 
 which are long enduring and transmitted by 
 sexual contact such as syphilis and gonorrhosa. 
 It is of importance that the facts bearing on 
 the hereditary transmission of disease should 
 become of general knowledge, in order that the 
 dangers may be known and voluntarily avoided. 
 No measures of preventive medicine are suc- 
 cessful which are not supported by a public 
 educated to appreciate their importance, and the 
 same holds true of eugenics. How successful 
 will be public measures leading to the prevention 
 of offspring in the obviously unfit by sterilization 
 of both males and females is uncertain. It is 
 doubtful whether public sentiment at the present 
 time will allow the measure to be thoroughly 
 carried out. Some results in preventing unfit
 
 218 DISEASE AND ITS CAUSES 
 
 heredity may be attained by the greater ex- 
 tension of asylum life, but the additional burden 
 of this upon the labor of the people would be 
 difficult to bear. At best such measures would 
 only be carried out in the lower class of society.
 
 CHAPTER XI 
 
 CHRONIC DISEASES. DISEASE OF THE HEART AS AN EX- 
 AMPLE. THE STRUCTURE AND FUNCTION OF THE HEART. 
 THE ACTION OF THE VALVES. THE PRODUCTION OF 
 HEART DISEASE BY INFECTION. THE CONDITIONS PRO- 
 DUCED IN THE VALVES. THE MANNER IN WHICH 
 DISEASE OF THE VALVES INTERFERES WITH THEIR FUNC- 
 TION. THE COMPENSATION OF INJURY BY INCREASED 
 ACTION OF HEART. THE ENLARGEMENT OF THE HEART. 
 THE RESULT OF IMPERFECT WORK OF THE HEART. 
 VENOUS CONGESTION. DROPSY. CHRONIC DISEASE OF 
 THE NERVOUS SYSTEM. INSANITY. RELATION BETWEEN 
 INSANITY AND CRIMINALITY. ALCOHOLISM AND SYPHILIS 
 FREQUENT CAUSES OF INSANITY. THE DlRECT AND IN- 
 DIRECT CAUSES OF NERVOUS DISEASES. THE RELATION 
 BETWEEN SOCIAL LIFE AND NERVOUS DISEASES. FUNC- 
 TIONAL AND ORGANIC DISEASE. NEURASTHENIA. 
 
 CHRONIC diseases are diseases of long duration 
 and which do not tend to result in complete 
 recovery; in certain cases a cause of disease 
 persists in the body producing constant damage, 
 or in the course of disease some organ or organs 
 of the body are damaged beyond the capacity 
 of repair, and the imperfect action of such dam- 
 aged organs interferes with the harmonious . 
 inter-relation of organs and the general well- 
 being of the body. The effect of damage in 
 producing chronic disease may not appear at 
 once, for the great power of adaptation of organs 
 219
 
 220 DISEASE AND ITS CAUSES 
 
 and the exercise of reserve force may for a time 
 render the damage imperceptible; when, however, 
 age or the supervention of further injury dimin- 
 ishes the power of adaptation the condition 
 of disease becomes evident. Chronic disease 
 may be caused by parasites when the relation 
 between host and parasite is not in high degree 
 inimical, as in tuberculosis, gonorrhoea, syphilis, 
 most of the trypanosome diseases and the dis- 
 eases produced by the higher parasites. In 
 certain cases the chronic disease represents 
 really a series of acute onsets; thus in the case 
 of the parasites there may be periods of complete 
 quiescence of infection but not recovery, the 
 parasites remaining in the body and attack- 
 ing when the defences of the body are in 
 some way weakened. In such cases there may 
 be temporary immunity produced by each ex- 
 cursion of the disease, but the immunity is not 
 permanent nor is the parasite destroyed. There 
 is a further connection between chronic dis- 
 ease and infection in that the damage to the 
 organs, which is the great factor underly- 
 ing chronic disease, is so often the result of an 
 . infection. 
 
 The infectious diseases are those of early life; 
 chronic disease, on the other hand, is most com- 
 mon in the latter third of life. This is due to 
 the fact that in consequence of the general wear 
 of the body this becomes less resistant, less
 
 DISEASE AND ITS CAUSES 
 
 capable of adaptation, and organic injury, which 
 in the younger individual would be in some way 
 compensated for, becomes operative. The terri- 
 tory of chronic disease is so vast that not even a 
 superficial review of the diseases coming under 
 this category can be attempted in the limits of 
 this book, and it will be best to give single 
 examples only, for the same general principles 
 apply to all. One of the best examples is given 
 in chronic disease of the heart. 
 
 The heart is a hollow organ forming a part of 
 the blood vascular system and serving to give 
 motion to the blood within the vessels by the 
 contraction of its strong muscular walls. It is 
 essentially a pump, and, as in a pump, the direc- 
 tion which the fluid takes when forced out of 
 its cavity by the contraction of the walls dimin- 
 ishing or closing the cavity space, is determined 
 by valves. The contraction of the heart, which 
 takes place seventy to eighty times in a minute, 
 is automatic and is due to the essential quality of 
 the muscle which composes it. The character, 
 frequency and force of contraction, however, can 
 be influenced by the nervous system and by the 
 direct action of substances upon the heart muscle. 
 The heart is divided by a longitudinal partition 
 into a right and left cavity, and these cavities 
 are divided by transverse septa, with openings 
 in them controlled by valves, each into two 
 chambers termed- auricle and ventricle. The
 
 222 DISEASE AND ITS CAUSES 
 
 auricle and ventricle on each side are completely 
 separated. 
 
 The circulation of the blood through the heart 
 is as follows: The blood, which in the veins of 
 the body is flowing towards the Tieart, passes 
 by two channels, which respectively receive the 
 blood from the upper and lower part of the 
 body, into the right auricle. When this becomes 
 distended it contracts, forcing the blood into 
 the right ventricle; the ventricle then contracts 
 and sends the blood into the arteries of the lungs, 
 the passage of blood into the auricle being pre- 
 vented by valves which close the opening between 
 auricle and ventricle when the latter contracts 
 upon its contents. When the ventricle empties 
 by its contraction the wall relaxes and the back 
 flow from the artery is prevented by crescentic- 
 shaped valves placed where the artery joins the 
 ventricle. A similar arrangement of valves is 
 on the left side of the heart. The pressure given 
 the blood by the contraction of the right ven- 
 tricle sends it through the lungs; from these, 
 after it has been oxygenated, it passes into the 
 left auricle, then into the left ventricle and from 
 this into the great artery of the body, the aorta, 
 which gives off branches supplying the capil- 
 laries of all parts of the body. Both of the 
 auricles and both of the ventricles contract at 
 the same time, the ventricular contraction fol- 
 lowing closely upon the contraction of the
 
 DISEASE AND ITS CAUSES 223 
 
 auricles. Contraction or systole is followed by 
 a pause or diastole during which the blood flows 
 from the veins into the auricles. The work 
 which the right ventricle accomplishes is very 
 much less than that of the left, and the right 
 ventricle has a correspondingly thinner wall. 
 The size of the heart is influenced by the size and 
 the occupation of the individual being larger 
 in the large individual than in the small, and 
 larger in the active and vigorous than in the 
 inactive. Generally speaking, the heart is about 
 as large as the closed fist of its possessor. 
 
 Imperfections of the heart which interfere 
 with its action may be the result of failure of 
 development or disease. An imperfect heart 
 which can, however, fully meet the limited 
 demands made upon it in intra-uterine life, 
 may be incapable of the work placed upon it 
 in extra-uterine life. Children with imperfectly 
 formed hearts may be otherwise perfect at birth, 
 but they have a bluish color due to the imper- 
 fect supply of the blood with oxygen, and are 
 known as blue babies. The condition becomes 
 progressively worse due to the progressive de- 
 mands made upon the heart, and death takes 
 place after some days or months or years, the time 
 depending upon the degree of the imperfection. 
 
 Much of the damage of the heart in later life 
 is due to infection. The valves of the heart 
 are a favorite place for attack by certain sorts
 
 224 DISEASE AND ITS CAUSES 
 
 of bacteria which get into the blood. This is 
 due to the prominent position of the valves 
 which brings them in contact with all the blood 
 in the body, the large extent and unevenness 
 of the surface and to the rubbing together and 
 contact of their edges when closed. At the site 
 of infection there is a slight destruction of tissue 
 and on this the blood clots producing rough 
 wart-like projections. The valves in some cases 
 are to a greater or less extent destroyed, they 
 may become greatly thickened and by the de- 
 posit of lime salts converted into hard, stony 
 masses. Essentially two conditions are produced. 
 In one the thickened, unyielding valves project 
 across the openings they should guard, and thus 
 by constricting the opening interfere with the 
 passage of blood either through the heart or 
 from it. In the other the valves are so damaged 
 that they cannot properly close the orifices 
 they guard, and on or after the contraction of 
 the cavities there is back flow or regurgitation 
 of the blood. If, for instance, the orifice of the 
 heart into the aorta is narrowed, then the left 
 ventricle can only accomplish its work of pro- 
 jecting into the aorta a given amount of blood 
 in a given time by contracting with greater 
 force and giving a greater rapidity to the stream 
 passing through the narrow orifice. This the 
 heart can do because, like all other organs of the 
 body, it has a large reserve force which enables
 
 DISEASE AND ITS CAUSES 225 
 
 it, even suddenly, to meet demands double the 
 usual, and like all other muscles of the body it 
 becomes larger and stronger by increased work. 
 The condition here is much simpler than when 
 the same valve is incapable of perfect closure, 
 or when both obstruction and imperfect closure, 
 are combined as they not infrequently are. In 
 such cases the ventricle must do more than in 
 the first case. It must force through the orifice, 
 which may be narrowed, the amount of blood 
 which is necessary to keep up the pressure within 
 the aorta and give to the circulation the neces- 
 sary rapidity of flow, and also the amount which 
 flows back into the heart through the imper- 
 fectly acting valve. This it can do by contracting 
 with greater force upon a larger amount of blood, 
 the cavity becoming enlarged to receive this. 
 Not only may such damage to the valves be pro- 
 duced, but the muscular tissue of the heart 
 may suffer from defective nutrition or from the 
 effect of poisons, whether these are formed in 
 the body as the effect of disease or introduced 
 from without ; or in consequence of disease 
 in the lungs the flow of blood through them 
 may be impeded, or disease elsewhere in the 
 body, as in the kidneys may, by increasing 
 the pressure of the blood within the arteries, 
 throw more than the usual amount of work 
 upon the heart. 
 
 The power of the heart in meeting these con-
 
 ditions, however various they are and however 
 variously they act, seems little short of marvel- 
 lous, and it goes on throwing three and one-third 
 ounces of blood seventy or eighty tunes a minute 
 into a tube against nine feet of water pressure, 
 working often perfectly under conditions which 
 would be fatal to a machine. As long as this 
 goes on the injury is said to be compensated for; 
 the increased work which the heart is able to 
 accomplish by the exercise of its reserve force 
 and by becoming larger and stronger enables it 
 to cope with the adverse conditions. With in- 
 creased demand for work there is a gradual 
 diminution of the reserve force. An individual 
 may be able to carry easily forty pounds up a 
 hill and by exerting all his force may carry 
 eighty pounds, but if he habitually carries the 
 eighty pounds, even though the muscles become 
 stronger by exercise the load cannot be again 
 doubled. The dilatation of the heart which is 
 so important in compensation is fraught with 
 danger, because any weakening of the muscle 
 increases the .dilatation, until a point is reached 
 when, owing to the dilatation of the orifices be- 
 tween auricles and ventricles, the valves become 
 incompetent to close them. 
 
 When the heart is not able to accomplish 
 its work, the effect of the condition becomes 
 apparent by the accumulation of blood within 
 the veins and a less active circulation. This
 
 DISEASE AND ITS CAUSES 227 
 
 affects the nutrition and the capacity for work 
 of all the organs of the body, and the im- 
 perfect function of the organs may in a variety 
 of ways make still greater demands upon an 
 already overloaded heart. Other conditions 
 supervene. The increased pressure within the 
 veins and capillaries due to the impossibility 
 of the blood in the usual amount passing through 
 or from the heart increases the amount of fluid 
 in the tissues. There is always an interchange 
 between the blood within the vessels and the 
 fluid outside of them; the passage of fluid from 
 the vessels is facilitated by the increased pressure 
 within them, just as pressure upon a filtering 
 fluid increases the rapidity of filtration, and the 
 increase of pressure within veins and capillaries 
 impedes passage of tissue fluid into them. The 
 fluid accumulates within the tissues leading to 
 dropsy, or the accumulation may take place in 
 some of the cavities of the body. The dimin- 
 ished flow of blood through the lungs prevents 
 its proper oxygenation; this may also be inter- 
 fered with by the accumulation of fluid within the 
 air spaces of the lungs. 
 
 Every additional burden thrown upon the heart 
 increases the evil. In women the additional 
 burden of pregnancy may suffice to overcome a 
 compensation which has been perfect, and the 
 same may result from an acute attack of disease. 
 Age, diminishing as it does the capacity for work in
 
 228 DISEASE AND ITS CAUSES 
 
 all organs, diminishes the compensation capacity 
 of the heart, and a heart which at the age of forty 
 acts perfectly may break down at the age of fifty. 
 Compensation may be gained in other ways, as 
 by reducing the demand made upon the heart 
 by changing the mode of life, by leading an 
 inactive rather than an active life, by avoiding 
 excitement or any condition which entails work 
 of the heart. Social conditions are of great 
 importance; it makes a great difference whether 
 the unfortunate possessor of such a heart be a 
 stevedore whose capital lies in the strength of 
 his muscles, or a more fortunately placed member 
 of society for whom the stevedore works and 
 whose occupation or lack of occupation does 
 not interfere with the adjustment of his external 
 relations to the condition of his heart. 
 
 Disease of the nervous system does not differ 
 from disease elsewhere. The system is complex 
 in structure and in function. It consists in 
 nerves which are composed of very fine fibrils 
 distributed in all parts of the body and serve 
 the purpose of conduction, and a central body 
 composed of the brain and spinal cord which 
 is largely cellular in character; it receives 
 impressions by means of the nerves and sends 
 out impulses which produce or affect action in 
 all parts. By means of the organs of special 
 sense, the brain receives impressions from the 
 outer world which it transforms into the con-
 
 DISEASE AND ITS CAUSES 229 
 
 cepts of consciousness. Many of the impressions 
 which the central nervous system receives from 
 nerves other than those of special sense and even 
 many of the impressions from these and the 
 impulses which it sends out do not affect con- 
 sciousness. The memory faculty is seated in 
 the brain and all parts of the brain are closely 
 connected by means of small nerve fibres. The 
 nervous system plays an important part in the 
 internal regulation and coordination of all parts 
 of the body, and it is by means of this that the 
 general adjustment of man with his environment 
 is effected. 
 
 Malformations of the brain, except very gross 
 conditions which are incompatible with extra- 
 uterine existence, are not very common. At 
 birth those parts of the brain which are the 
 seat of memory and what are understood as the 
 higher faculties are very imperfectly developed. 
 Variations in structure are extremely common, 
 there are differences in different individuals in 
 the nerves and in the number, size, form and 
 arrangement of the nerve cells, and so complex 
 is the structure that considerable variation can 
 exist without detection. The tissue of the 
 central nervous system has a considerable degree 
 of resistance to the action of bacteria, but is, 
 however, very susceptible to injury by means 
 of poisons. Serious injury or destruction of 
 tissue of the brain and spinal cord is never
 
 230 DISEASE AND ITS CAUSES 
 
 regenerated or repaired, but adjustment to such 
 conditions may be effected by reciprocity of 
 function, other cells taking up the functions of 
 those which were destroyed. 
 
 Certain parts of the brain are associated with 
 definite functions; thus, there are areas which 
 influence or control speech and motion of parts 
 as the arm or leg, and there are large areas known 
 as the silent areas whose function we do not 
 know. All activity of the central nervous system, 
 however expressed, is due to cell activity and is 
 associated with consumption of cell material 
 which is renewed in periods of repose and sleep. 
 Fig. 13 shows a nerve cell of a sparrow at the end 
 of a day's activity and the same after the repose 
 of a night. 
 
 Diseases of the nervous system have a special 
 interest in that they so often interfere with man 
 in his relations with his fellows. In diseases of 
 other organs the disturbances set up concern 
 the individual only. Thus, others need not be 
 disturbed save by the demands made on their 
 sympathies by an individual with a cold in the 
 head or a cancer of the stomach. Disease of 
 the nervous system is another affair, instead of 
 those reactions and expressions of activity to 
 which we are accustomed and to which society 
 is adjusted, the reactions and activities are 
 unusual and the individual in consequence does 
 not fit into the social state and is said to be
 
 DISEASE AND ITS CAUSES 231 
 
 anti-social. There are all possible grades of 
 this, from mere unpleasantness in the social 
 relations with such an individual, to states in 
 which he is dangerous to society and must be 
 isolated from it. Insanity is an extreme case. 
 There is no disease signified in the expression, 
 but it is merely a legal term to designate those 
 individuals whose actions are opposed to the 
 social state and who are not responsible for them. 
 In insanity there is falsity in impressions, in 
 conceptions, in judgment, a defective power of 
 will and an uncontrollable violence of emotion. 
 The individual is prevented from thinking the 
 thoughts or feeling the feelings and doing the 
 duties of the social body in the community in 
 which he lives. The insane are out of harmony 
 with their social environment, but not necessarily 
 in opposition to it. 
 
 There is no very sharp line between insanity 
 and criminality. The criminal is in direct an- 
 tagonism to the laws of social life. An insane 
 person may cause the same injury to society 
 as a criminal, but his actions are not voluntary, 
 whereas the criminal is one who can control his 
 actions, but does not. Mentally degenerated 
 persons, however, can be both insane and crim- 
 inal. Whatever the state of society, this repro- 
 bates the actions of one opposed to it; in a 
 society in which it were usual to appropriate the 
 possessions of others or to devour unpleasant or
 
 232 DISEASE AND ITS CAUSES 
 
 useless relatives, virtue and lack of appetite would 
 be reprobated as unsocial. 
 
 The symptoms of insanity or the manner in 
 which the defective action of the brain expresses 
 itself and the various underlying pathological 
 changes vary, and by combining these it has been 
 possible to subdivide insanity into a number 
 of distinct forms. There are both intrinsic and 
 extrinsic causes of insanity. The intrinsic are 
 the structural differences in the brain as com- 
 pared with the normal or usual, whether these 
 are due to imperfection in development or to 
 defective heredity or to the injury of disease; 
 the extrinsic causes are those which come from 
 without and bring the intrinsic into activity. 
 Syphilis is a frequent cause of insanity, and 
 probably the only cause of the condition known 
 as general paralysis of the insane, acting by 
 means of the injury which it produces in the cor- 
 tex of the brain. The abuse of alcohol is another 
 fertile cause, but the changes produced in this 
 are not so obvious as in the case of syphilis. 
 Tumors of the brain are not infrequently a cause, 
 and the same is true of infections, even those 
 not located in the brain. How susceptible the 
 brain is to the effects of the toxines of the in- 
 fectious diseases is shown in the frequency of 
 delirium in these diseases. There is an interesting 
 relation between this and alcoholism. Alcohol 
 abuse may produce injury, but not sufficient
 
 DISEASE AND ITS CAUSES 233 
 
 to manifest itself under ordinary conditions; 
 when, however, the action of toxic substance 
 is superadded to the effect of the alcohol the 
 delirium of fever is more marked. 
 
 Probably of greater importance than the ac- 
 quired pathological conditions of the brain in pro- 
 ducing insanity is a congenital condition in which 
 the nervous system is defective. The most fertile 
 cause of insanity lies in the inheritance; by this it 
 must not be understood that insane parents pro- 
 duce insane offsprings, but that conditions inher- 
 ited from immediate or remote ancestors appear 
 in a diminished resistance of the nervous system 
 which is sooner or later expressed as insanity. 
 Given such a defective nervous system, extrin- 
 sic conditions which would have no effect on 
 another individual or would be felt in different 
 ways may produce insanity. In these cases 
 occupation plays a great role. The excitement 
 and privations of war especially in the tropics 
 and the ennui of camps leads to insanity in 
 soldiers; occupations such as that of the baker 
 in which there is loss of sleep and the men- 
 tal strain of students can all act in the same 
 way. A woman who gives no sign of nervous 
 defect may become insane under the strain of 
 pregnancy. 
 
 Although insanity is determined by the social 
 relations of man, that part of the social organiza- 
 tion which is termed Society, and which has been
 
 234 DISEASE AND ITS CAUSES 
 
 developed by the idle as a diverting game, is a 
 fertile source of nervous disease and even of 
 insanity, affecting particularly females. The 
 strenuosity of the life, the nervous excitement 
 alternating with ennui, the lack and improper 
 times of sleep, the lack of rest and particularly 
 of restful occupation, the not infrequent use of 
 alcohol in injurious amounts, are all factors 
 calculated to make a defect operative. The 
 so-called "coming out" of young girls is an 
 important element in the game, and their head- 
 long plunge into such a life at a period under any 
 conditions full of danger to the nervous system is 
 especially to be reprobated. If we consider 
 the influence of the game in other respects as 
 conducing to lack of moral sense, to alcoholic 
 abuse (for without the seeming stimulation, 
 but which is really the blunting of impressions 
 which alcohol brings, the game would not be 
 possible), to discontent, to mental enfeeblement, 
 it is all bad. Curiously enough the game is one 
 which in all periods has been played by the idle, 
 but its evil influence is greater now than before 
 when it was the game of royalty chiefly, because 
 there are now more people living from the work 
 of others. 
 
 The unusual mental action of the insane not 
 infrequently expresses itself by suicide. The 
 analysis of three hundred deaths from suicide 
 showed pathological changes in the brain in
 
 DISEASE AND ITS CAUSES 235 
 
 forty-three per cent, and when we think that 
 mental disturbances are very often without 
 recognizable anatomical changes after death, 
 the percentage is very large. In another analysis 
 of one hundred and twenty-four suicides forty- 
 four of these were mentally affected to various 
 degrees. Five of the men and seven women were 
 epileptics, in ten of the families there was hys- 
 teria, twenty-four of the men and four of the 
 women were chronic alcoholics. 
 
 It is extremely difficult at the present time to 
 say whether insanity is increasing. Statistics 
 in all lands giving the numbers committed to 
 insane hospitals show on their face a great in- 
 crease, but so many factors enter into these 
 statistics that their value is uncertain. There 
 is now an ever-increasing provision for the care 
 of the insane. Owing to the recognition of 
 insanity as a part of nervous disease and its 
 separation from criminality there is no longer 
 the same attempt to conceal it as was formerly 
 the case, and hospitals for the insane are no longer 
 associated with ideas of Bedlam. It is generally 
 believed that modern conditions in the hurry 
 and excitement of life, and the extreme social 
 differences, the greater urban life, the greater 
 extension of factory life, all tend to an increase 
 in insanity, but there is no absolute proof that 
 this is true. We know very little about insanity 
 in the Middle Ages, but the conditions then were
 
 236 DISEASE AND ITS CAUSES 
 
 not conducive to a quiet life. There prevailed 
 then as now excess and want, luxury and poverty, 
 enjoyment and deprivations, balls and dinner par- 
 ties and other features of the social game. There 
 were factions in the cities, public executions, not 
 infrequent sieges, scenes of horror, epidemics, 
 famines, and all these combined with religious 
 superstition and the often unjust and cruel laws 
 should have been factors for insanity. There 
 were actual epidemics of insanity affecting masses 
 of the population, as shown in the children's cru- 
 sade, the Jewish massacres and the dancing 
 mania in the Rhine provinces. Where civilization 
 seems to be the highest, statistics show the 
 most insane, but this most probably depends up- 
 on better recognition of the condition and 
 better provision for asylum care. 
 
 The so-called functional diseases have a close 
 relation with diseases of the nervous system, for 
 they chiefly concern the reactions of nerve tissue. 
 Disease expressing itself in disturbance of function 
 only, does not seem to fit in with the conceptions 
 of disease which have been expressed, nor can 
 we imagine a disturbance of function which 
 does not depend upon a change of material. 
 Living matter does not differ intrinsically from 
 any other sort of matter; like other matter its 
 reactions depend upon its composition structure 1 
 
 1 By structure as used in this wide sense, there must be 
 understood not merely the anatomical structure, which is
 
 DISEASE AND ITS CAUSES 237 
 
 and the character of the action exerted upon it. 
 By functional disease there is expressed merely 
 that no anatomical or chemical change is discover- 
 able in the material which gives the unusual reac- 
 tion. The further our researches into the nature of 
 disease extend, particularly the researches into 
 the physiology and chemistry of disease, the 
 smaller is the area of functional disease. In 
 functional disease there may be either vague 
 discomfort or actual pain under conditions 
 when usually such would not be experienced, 
 and on examination no condition is found which 
 in the vast majority of cases would alone give 
 rise to that impression on the nervous system 
 which is interpreted as pain. In the production 
 of the sensations of disease there can be change 
 at any place along the line, in the sense organs, 
 in the conducting paths or in the central organ. 
 Thus there may be false visual impressions 
 which may be due to changes in the retina or 
 in the optic nerve or in the brain matter to 
 which the nerve is distributed. It is perfectly 
 possible that substances of an unusual character 
 or an excess or deficiency of usual substances in 
 the fluids around brain cells may so change them 
 that such unusual reactions appear. There may 
 be, of course, very marked individual susceptibil- 
 
 revealed by the dissecting knife and microscope, but molecu- 
 lar structure, or the manner in which elements are arranged 
 to form the molecule, as well.
 
 238 DISEASE AND ITS CAUSES 
 
 ity, which may be congenital or acquired. The 
 perception of every stimulus involves activity of 
 the nerve cells, and it is possible that the constant 
 repetition of stimuli of an ordinary character may 
 produce sufficient change to give rise to unusual 
 reactions, and this particularly when there is 
 lack of the restoration which repose and sleep 
 bring. We know into what a condition one's 
 nervous system may be thrown by the incessant 
 noise attending the erection of a building in the 
 vicinity of one's house or the pounding of a 
 plumber working within the house, this being 
 accentuated in the latter case by the thought 
 of impending financial disaster. Even the con- 
 fused and disagreeable sound due to the clatter 
 of high-pitched women's voices at teas and 
 receptions may, when frequently repeated, be 
 productive of changes in the nerve cells suffi- 
 ciently marked to give rise to the unusual reac- 
 tions which are evidence of disease. 
 
 In the condition known as neurasthenia, which 
 is often taken as a type of a functional disease, 
 the basal and intrinsic cause is activity of the 
 nervous system with the using up of material 
 which is not compensated for by the renewal 
 which comes in repose and sleep. Neurasthenia 
 is one of the common conditions of our civili- 
 zation, found among children and adults, the 
 poor and rich, the idle and the factory worker; 
 it is rife in the scholastic professions and among
 
 DISEASE AND ITS CAUSES 239 
 
 those who earn their living by brain work. It 
 seems to be more common in the upper classes 
 and particularly in the women, but this is because 
 these are more subject to medical care and the 
 condition is more in evidence. There are alj 
 sorts of symptoms attached to the condition, 
 for the unusual mental action can be variously 
 expressed. The cerebral form has been thus de- 
 scribed by a well-known medical writer: "One of 
 the most characteristic features of cerebral neuras- 
 thenia is a weary brain. The sensation is familiar 
 enough to any fagged man, especially if he fall 
 short of sleep. Impressions seem to go half into 
 one's head and there sink into a woolly bed and 
 die. Voices sound far off, the lines of a book 
 run into one another and the meaning of them 
 passes unperceived. Doors bang and windows 
 rattle as they never did before; if a shoestring 
 breaks, an imprecation is upon the lips. Business 
 matters are in a conspiracy to go wrong. Letters 
 are left unopened partly from want of will, 
 partly from a senseless dread lest they contain 
 bad news. At night the patient tosses on his bed 
 possessed by all the cares which blacken with 
 darkness. Headache is common, loss of memory 
 is distressing, and in severe cases it is wider and 
 deeper than mere inattention can explain. There 
 is often the torture of acute hearing, or an in- 
 ability to suppress attention; the hater of clocks 
 and crowing cocks is a neurasthenic." The
 
 240 DISEASE AND ITS CAUSES 
 
 disease is especially common in the women 
 players of the social game, and its unhappy 
 victims too often seek relief from the nervous 
 irritability which is a common early symptom 
 in still greater nervous excitement. It is a sad 
 commentary on our civilization that one of the 
 means of treatment for these persons which has 
 been found efficacious is to supply them with some 
 restful household occupation such as knitting 
 or plain sewing, and there are institutions which 
 combine refuge from social activities, often called 
 duties, with simple occupation.
 
 CHAPTER XII 
 
 THE RAPID DEVELOPMENT OF MEDICINE IN THE LAST FIFTY 
 YEAKS. THE INFLUENCE OF DARWIN. PREVENTIVE 
 MEDICINE. THE DISSEMINATION OF MEDICAL KNOWL- 
 EDGE. THE DEVELOPMENT OF CONDITIONS IN RECENT 
 YEARS WHICH ACT AS FACTORS OF DISEASE. FACTORY 
 LIFE. URBAN LIFE. THE INCREASE OF COMMUNICA- 
 TION BETWEEN PEOPLES. THE INTRODUCTION OF PLANT 
 PARASITES. THE INCREASE IN ASYLUM LIFE. INFANT 
 MORTALITY. WEALTH AND POVERTY AS FACTORS IN 
 DISEASE. 
 
 CERTAIN conditions have arisen in the past 
 fifty years which have profoundly affected the 
 thoughts, the beliefs and the activities of man. 
 Within this period what is generally known as 
 Darwinism, including under this evolution, has 
 developed. Unlike theories which came from 
 philosophical speculation only, the theory of 
 evolution was one which could be subjected to 
 observation and experiment. It freed man's mind 
 from dogmas, it stimulated the imagination, it 
 enlarged the territory in which it seemed possible 
 to extend knowledge by the methods of science, 
 and has resulted in an enormous increase of 
 knowledge. This has been more striking in medi- 
 cal science than elsewhere, and in this of more 
 far-reaching influence. Evolution coincided with 
 another important development. History shows 
 241
 
 242 DISEASE AND ITS CAUSES 
 
 that all great periods of civilization have at 
 their back sources of energy. In the civilizations 
 of the past such sources of energy have come 
 from the enslavement of conquered peoples or 
 from commerce, or more direct forms of robbery, 
 which have enabled a favored class to appropri- 
 ate for its purposes the results of the work of 
 others. While these sources have not been 
 absent in the development of our civilization, 
 the great source of energy has come from the 
 rapid, and usually wasteful and reckless, utiliza- 
 tion of the stored energy of the earth. The 
 almost incredible advance in medical and other 
 forms of scientific knowledge and the utilization 
 of this knowledge is largely due to the greater 
 forces which we have become possessed of. 
 
 Disease plays such a large part in the life of 
 man and is so closely related to all of his activ- 
 ities that the changes in this period must have 
 exerted an influence on disease. We have already 
 seen that within the period we have obtained 
 knowledge of the causes of disease and the con- 
 ditions under which these causes became oper- 
 ative. The mystery which formerly enveloped 
 disease is gone; disease is recognized as due to 
 conditions which for the most part are within 
 the control of man, and like gravity and chemical 
 attraction it follows the operation of definite 
 laws. There has been developed within the 
 period what is- known as preventive medicine,
 
 DISEASE AND ITS CAUSES 243 
 
 which aims rather at prevention than cure, and 
 the resources of prevention are capable of much 
 greater extension. 
 
 Have there been new conditions developed 
 within the period, or an increase of existing 
 conditions which can be regarded as disease fac- 
 tors and which counterbalance the results which 
 have come from the knowledge of prevention 
 and cure? There has been an increase of certain 
 factors of immense importance in the extension 
 of disease. These are: 
 
 1. The increase in industrialism, involving as 
 this does an increase in factory life. In many 
 ways this is a factor in disease, (a) By favoring 
 the extension of infection, particularly in such 
 diseases as tuberculosis. (6) The life indoors, and 
 frequently with the combination of insufficient air 
 and space, produces a condition of malnutrition 
 and deficient general resistance, (c) The family life 
 is interfered with by the mothers, whose primary 
 duty is the care of home and children, working in 
 factories, and the too frequent conversion of the 
 house into a factory, (d) The influence of factory 
 life is towards a loss of moral stamina rendering 
 more easy of operation the conditions of alco- 
 holism and general immorality. How great has 
 been this increase in industrialism, fostered as 
 it has been by conditions both natural and arti- 
 ficially created by unwise legislation, is shown 
 hi the figures from the last census. The number
 
 244 DISEASE AND ITS CAUSES 
 
 of factory operatives increased forty per cent 
 between 1899 and 1909 and the total population 
 of the country in the period between 1900 and 
 1910 increased twenty per cent. It is probable 
 that the future will see an extension rather than 
 a diminution of mass labor. 
 
 2. The increase in urban life is as conspicuous 
 as the increase in industrialism. In 1880, twenty- 
 nine and five-tenths per cent of the population 
 was urban and seventy and five-tenths per cent 
 was rural; in 1910, forty -six and three- tenths per 
 cent was urban and fifty -three and seven-tenths 
 was rural, the increase being most marked in cities 
 of over five hundred thousand inhabitants. Of the 
 total increase in population between 1900 and 1910, 
 seven-tenths per cent was in the cities and three- 
 tenths per cent in the country. City life in itself is 
 not necessarily unhealthy and there are many 
 advantages associated with it. The conditions 
 which have chiefly fostered it are the immigration 
 of people who are accustomed to community 
 life, the increase in factory life and the increased 
 number of people of wealth who seek the advan- 
 tages which the city gives them. The city has 
 always been the favored playground for the 
 social game. The unhealthy conditions of city 
 life are due to the crowding, the more uncertain 
 means of livelihood, the greater influence of vice 
 and alcoholism. Prostitution and the sexual 
 diseases are almost the prerogatives of the cities.
 
 DISEASE AND ITS CAUSES 245 
 
 3. All means of transportation have increased 
 and communication between peoples has become 
 more extended and more rapid. In the past 
 isolation was one of the safeguards of the people 
 against disease. With the increase and greater 
 rapidity of communication there is a tendency 
 not only to loss of individuality in nations as 
 expressed in dress, customs, traditions and be- 
 liefs, but many diseases are no longer so strictly 
 local as formerly pellagra, for example. Only 
 those diseases which are transmitted by insects 
 which have a strictly local habitat remain endemic, 
 although the region of endemic prevalence may 
 become greatly extended, as is seen in the dis- 
 tribution of sleeping sickness. Diseases of plants 
 and of animals have become disseminated. Any 
 plants desirable for economic use or for beauty 
 of foliage and flower become generally distributed, 
 their parasites are removed from the regions where 
 harmonious parasitic inter-relations have been 
 established, and in new regions the parasites 
 may not find the former restrictions to their 
 growth. There have been many examples of 
 this, such as the ravages of the brown-tail and 
 gypsy moths which were introduced into New 
 England and of the San Jose scale which was 
 introduced into California. There have been 
 many other examples of the almost incredible 
 power of multiplication of an animal or plant 
 when taken into a new environment, removed
 
 246 DISEASE AND ITS CAUSES 
 
 from conditions which held it in check, as 
 the introduction of the mongoose into Jamaica, 
 the rabbit into Australia, the thistle into New 
 South Wales and the water-plant chara into 
 England. 
 
 It is very difficult to say, but it seems as though 
 there is an increasing unevenness in the distribu- 
 tion of wealth, an increase in the number of 
 persons who live at the expense of the laboring 
 class. Mass labor, effective though it be, makes 
 it easier to divert the proceeds of labor from the 
 laborers. The evidence of this is seen in the in- 
 crease in number and the prosperity of those pur- 
 suits which purvey to luxury, as the automobile 
 industry and the florists' trade and the greatly 
 increased scope and activity of the social game. 
 On the other hand, there is an increase in the 
 number of people who are to a greater or less 
 extent dependent upon extraneous aid, evinced 
 among other ways by the increase in the asy- 
 lum populations. Both these conditions, wealth 
 and poverty, are important disease factors. 
 Tuberculosis is now a disease of the proletariat 
 chiefly. The measures both of prevention and 
 cure can be and are carried out by the well-to-do, 
 but the disease must remain where there are the 
 conditions of the slums. Of all the conditions 
 favoring infant mortality poverty comes first. 
 In Erfurt, a small city of Germany, of one thou- 
 sand infants born in each of the different classes,
 
 DISEASE AND ITS CAUSES 247 
 
 there died of the illegitimate children three hun- 
 dren and fifty -two; of those of the laboring 
 class, three hundred and five; of those in the 
 medium station (official class largely), one hun- 
 dred and seventy-three; of those in higher 
 station, eighty-nine. The same relation of infant 
 mortality to poverty becomes apparent when 
 estimated in other ways. In Berlin, with an 
 average infant mortality of one hundred and 
 ninety-six per thousand, the deaths in the best 
 districts of the city were fifty-two and in the 
 poorer quarters four hundred and twenty. The 
 effect of poverty is seen particularly in the bottle- 
 fed infants; with natural nursing the child of 
 poverty has almost as good a chance as the 
 child of wealth. From reasons which are almost 
 self-evident, the mortality in illegitimate infants 
 is almost double that of the legitimate. The 
 greater infant mortality in poverty is due to 
 the more numerous children preventing indi- 
 vidual care, the separation of the mother from 
 the nursing child in consequence of the demand 
 made upon her earning capacity, and the decline 
 in breast nursing. Wealth is on the whole more 
 advantageous from the narrow point of view of 
 disease than is poverty, but if we regard its 
 influence on the race its advantages are not so 
 evident. Nothing can be worse for a race than 
 that it should die out, and wealthy families have 
 never reproduced themselves. Conditions always
 
 248 DISEASE AND ITS CAUSES 
 
 tending to destruction are a necessary part of 
 the environment of poverty; wealth voluntarily 
 creates these conditions, and chiefly by the per- 
 nicious influence of its amusements on the young. 
 A new and in many respects a nobler concep- 
 tion of medicine has been developed. Formerly 
 medical practice was almost exclusively a per- 
 sonal service to the sick individual, and measures 
 looking toward the general relief of disease and 
 its prevention received scanty consideration. 
 The idea of a wider sendee to the city, to the 
 state, to the nation, to humanity rather than 
 the personal service to the individual, is becoming 
 dominant in medicine. This is seen in the estab- 
 lishment of laboratories by boards of health in 
 cities and states in which knowledge obtained by 
 exact investigations can be made of direct service 
 to the people; in the medical inspection of schools 
 and factories; in promulgating laws directed 
 against conditions which affect health, in the 
 extension of hospitals, and in divers other ways. 
 The idea of public service and of returning to 
 the people in an effective way some of the results 
 of their labor also underlies the large donations 
 which have been given for the creation of special 
 laboratories and institutes in which, through 
 research, greater knowledge of disease may be 
 obtained and made available. The researches 
 which have been made on the nutrition of man 
 and the nutritive value of different foods are of
 
 DISEASE AND ITS CAUSES 249 
 
 great importance, and this knowledge has not 
 yet begun to be applied as it should be. 
 
 There seems to be a balance maintained be- 
 tween the restriction of disease by prevention 
 and the increased influence of social conditions 
 which are in themselves factors of disease. 
 Preventive medicine seems to have made pos- 
 sible, by restricting their harmful influence, the 
 increase in industrialism, in urban life, and in 
 the intercommunications of peoples. The most 
 important aid in the future to the influence of 
 preventive medicine must be the education of 
 the people so that the conditions of disease, the 
 intrinsic and the extrinsic causes and the manner 
 in which these act, shall all become a part of 
 general knowledge, and the sympathy of the 
 people with health legislation and their active 
 assistance in carrying out measures of preven- 
 tion may be obtained. The effect of social con- 
 ditions on disease must become more generally 
 recognized.
 
 GLOSSARY 
 
 ATROPHY A condition of imperfect nutrition producing 
 diminution in size and loss of function of parts. 
 
 BERTILLON A French anthropologist who devised a 
 system of measurements of the human body for purposes of 
 identification. 
 
 BLOOD-PLASMA The fluid of the blood. 
 
 CELL The unit of living matter. Living things may be 
 unicellular or composed of a multitude of cells which are inter- 
 dependent. The general mass of material forming the cell is 
 termed cytoplasm. In this there is a differentiated area 
 termed nucleus which governs the multiplication of cells. In 
 the nucleus is a material termed chromatin which bears the 
 factors of heredity. 
 
 CHEMOTROPISM The influence of chemical substances in 
 directing the movement of organisms. 
 
 EXUDATE The material which passes from the blood into 
 an injured part and causes the swelling. 
 
 FIBRIN The gelatinous material formed in the blood 
 when it clots. 
 
 HAEMOGLOBIN A substance which gives the red color to 
 the blood; by means of its ready combination with the oxygen 
 of the air in the lungs this necessary element is carried to all 
 parts of the body. 
 
 INFLAMMATION Literally a "burning"; the changes which 
 take place in a part after injury. 
 
 LYMPH The fluid which is contained in the lymphatic 
 vessels nodes. Circumscribed masses of cells connected 
 with the lymphatic vessels. 
 
 OSMOSIS The process of diffusion between fluids of dif- 
 ferent molecular pressures. 
 
 SPORE FORMATION A mode of reproduction in lower forms 
 of life by which resistant bodies, spores, are formed. These 
 have many analogies with the seed of higher plants. 
 
 SYMBIOSIS A mutual adaptation between parasite and 
 host. 
 
 TRANSUDATION The normal interchange of fluid between 
 250
 
 GLOSSARY 251 
 
 the blood and the tissue fluids. The material interchanged 
 is the transudate. 
 
 TROPISM The influence of forces which direct the move- 
 ment of cells. 
 
 ULTRA-MICROSCOPE A form of microscope which by means 
 of oblique illumination renders visible objects so small as to 
 be invisible with the ordinary microscope. 
 
 VIRUS A substance either living or formed by living 
 things which may cause disease.
 
 INDEX 
 
 Amoeba, 13 
 Anthrax, 109 
 Antitoxin, 154 
 
 Bacteria, 116 
 
 adaptation in, 123 
 
 aerobic, 122 
 
 anaerobic, 122 
 
 artificial cultivation of, 119 
 
 distribution in nature, 121 
 
 growth and reproduction, 
 118 
 
 mode of action in disease, 
 144 
 
 size, 117 
 
 spore formation, 118 
 
 substances affecting growth 
 of, 123 
 
 toxin production by, 144 
 
 variations in, 123 
 Blood, 35 
 
 circulation of, 33, 80 
 
 vessels, 32 
 Body, 22 
 
 defenses of, 146 
 
 organs of, 28 
 
 reserve force of, 50 
 
 surfaces of, 22 
 Brain, 31 
 
 Cerebro-spinal meningitis, 
 
 188 
 
 Chemotropism, 93 
 Cretinism, 37 
 
 Darwinism, 240 
 Death, 57 
 
 decomposition after, 51 
 rigor after, 60 
 signs of, 59 
 Disease, 1 
 
 action of poisons, 44 
 acute and chronic, 219 
 industrialism as factor in, 
 
 243 
 lesions of, 46 
 
 superstitions concerning, 
 
 10 
 
 urban life as factor in, 244 
 wealth and poverty as fac- 
 tors in, 246 
 Ductless glands, 37 
 
 Embryo, 77 
 Epilepsy, 209 
 Eugenics, 215 
 
 Foetus, 32 
 
 infection of, 200 
 Foot and Mouth Disease, 129 
 
 252
 
 INDEX 
 
 253 
 
 Glands, 22 
 Growth, 62 
 
 Heart, 33, 221 
 
 disease of, 223 
 Heliotropism, 93 
 Heredity, 197 
 
 influence of alcohol, 206 
 
 of insanity, 209 
 
 variations and imitations, 
 
 204 
 Hookworm disease, 179 
 
 Immunity, 148 
 
 theories of, 149 
 
 natural, 150 
 Infection, 135 
 
 from external surface, 
 136 
 
 from genito-urinary sur- 
 face, 137 
 
 from lungs, 138 
 
 from mouth, 138 
 
 from stomach and intes- 
 tines, 139 
 
 from wounds, 141 
 
 in children, 195 
 
 in wild animals, 191 
 
 latent, 166 
 
 mixed, 160 
 
 racial susceptibility to, 
 191 
 
 resistance to, 143 
 
 by air, 170 
 
 by insects, 171 
 
 Infectious diseases, 97 
 
 carriers of, 185 
 
 comparison with fermenta- 
 tion, 108 
 
 epidemics of, 98 
 
 endemic, epidemic and 
 sporadic forms, 188 
 
 modes of transmission, 161 
 Inflammation, 80 
 
 acute and chronic, 95 
 Injury, 54-74 
 Insanity, 231 
 
 causes of, 232 
 
 question of increase, 235 
 
 Lesion, 17 
 Leucocytes, 36 
 
 migration of, 92 
 Living matter, 10 
 
 Malaria, 175 
 
 r6le of mosquito in trans- 
 mitting, 178 
 
 Malformations, 211 
 heredity of, 215 
 
 Maternal impressions, 212 
 
 Nervous system, 228 
 
 disease of, 230 
 
 effect of social life on, 233 
 Neurasthenia, 238 
 
 Old age, 51 
 atrophy in, 51 
 blood vessels in, 54
 
 254 
 
 INDEX 
 
 Old age continued. 
 causes of death in, 56 
 in animals and plants, 55 
 mental activity in, 53 
 
 Osmosis, 91 
 
 Opsonius, 153 
 
 Ovum, 201 
 
 fertilization of, 198 
 infection of, 199 
 
 Phagocytosis, 86 
 
 Plague, 182 
 
 transmission by animals, 
 183 
 
 Plasmodium Malariae, 175 
 
 Preventive medicine, 242 
 
 Protozoa, 124 
 
 distribution in nature, 125 
 mode of growth, 125 
 sexual differentiation, 125 
 spore formation, 125 
 
 Polyomyelitis, 190 
 
 Repair, 46 
 
 conditions influencing, 47 
 
 Scar, 49 
 
 Skin, 21 
 
 Sleeping sickness, 173 
 
 Smallpox, 187 
 
 Spontaneous generation, 106 
 
 Sunburn, 83 
 Syphilis, 193 
 
 Tetanus, 142 
 Thymus, 52 
 Thyroid, 37 
 Tonsils, 52 
 Toxins, 144 
 Tropisms, 93 
 Trypanosomes, 172 
 Tuberculosis, 163 
 
 infection by sputum, 169 
 
 modes of extension, 163 
 Tumors, 64 
 
 benign and malignant, 69 
 
 cells of, 66 
 
 color, size and shape, 65 
 
 growth of, 65 
 
 importance of, 77 
 
 origin of, 66 
 
 question of increase, 69 
 
 theories of cause, 71 
 
 treatment of, 77 
 Typhoid fever, 170 
 
 Ultra-microscopic organisms, 
 128 
 
 Virus, 128 
 Yellow fever, 178
 
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