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 A TEXT-BOOK 
 
 OF 
 
 DENTAL HISTOLOGY 
 
 AND 
 
 EMBRYOLOGY 
 
 INCLUDING 
 
 LABORATORY DIRECTIONS 
 
 BY 
 
 FREDERICK BOGUE NOYES, B.A., D.D.S. 
 
 PROFESSOR OF HISTOLOGY, NORTHWESTERN UNIVERSITY DENTAL SCHOOL, 1896-1914 
 
 PROFESSOR OF HISTOLOGY AND ORTHODONTIA, COLLEGE OF DENTISTRY 
 
 UNIVERSITY OF ILLINOIS, 1914 
 
 THIRD EDITION, THOROUGHLY REVISED 
 
 WITH A CHAPTER ON THE ABSORPTION OF THE ROOTS OF TEETH 
 By NEWTON GEORGE THOMAS, M.A., D.D.S. 
 
 PROFESSOR OF HISTOLOGY, NORTHWESTERN UNIVERSITY DENTAL SCHOOL, 1917-1919 
 
 SECRETARY AND PROFESSOP OF HISTOLOGY, COLLEGE OF DENTISTRY, UNIVERSITY 
 
 OF ILLINOIS, 1919 
 
 WITH 343 ILLUSTRATIONS AND 21 PLATES 
 
 LEA & FEBIGER 
 
 PHILADELPHIA AND NEW YORK
 
 COPYRIGHT 
 
 LEA & FEBIGER 
 
 1921 
 
 PRINTED IN U. S. A.
 
 Go mg Ifatbcr 
 
 Dr. 
 
 TUHbose long ano active professional career bas been DevoteD, 
 
 witbout personal ambition or selfisb advancement, to 
 
 tbe QOO& of tbe Dental Profession, and wbose 
 
 unselftsbness anO sacrifice bave 
 
 possible all tbat 1 bave Done 
 
 or mas accomplish. 
 
 550717 
 ^
 
 PREFACE TO THE THIRD EDITION. 
 
 THE exhaustion of the second edition of this work has afforded 
 another opportunity for careful revision of the text and illustra- 
 tions, and the addition of some important material, which has 
 been developed since the appearance of the second edition. Many 
 new drawings and a number of new micrographs have been pre- 
 pared. The chapters on the lymphatics of the dental region and 
 the absorption of the roots of teeth have been added, and the 
 chapters on embryology, greatly enlarged. 
 
 The conditions at the present time, and especially the interest 
 of the medical profession in the mouth as a source of systemic 
 infection have put new emphasis on the teaching of histology, and 
 have greatly changed the attitude of the dental profession. The 
 need for a thorough knowledge of tissue structure and function is 
 realized as it never has been before, and the demand for thorough 
 training in the fundamental biological sciences has greatly increased. 
 
 The present interest and emphasis of the profession on the relation 
 of the pulpless tooth to systemic diseases has somewhat changed 
 the relative distribution of the text. The pages devoted to the 
 enamel have been reduced, those devoted to the dentin, cementum 
 and supporting tissues increased, and the chapter on the lym- 
 phatics added. 
 
 The work is primarily intended as an elementary text-book for 
 dental students, rather than an exhaustive treatise on dental 
 histology. For this reason, discussion of disputed ideas, presentation 
 of various opinions, and reference to the work which has developed 
 the subject have been largely and purposely avoided. It is the 
 author's opinion that it is better for the student to get a clear idea 
 of structure that he can use as a basis for thinking, rather than to 
 be left with a hazy impression of differences of opinion. 
 
 In the preparation of this (the third) edition the author is 
 specially indebted to Dr. Newton G. Thomas, who has prepared 
 and written the chapter on the Absorption of the Roots of Teeth, 
 and to Mrs. N. M. Frain, the artist for the department, who has 
 made the illustrations. F. B. N. 
 
 CHICAGO, 1921. 
 
 (V)
 
 CONTENTS. 
 
 INTRODUCTION 17 
 
 CHAPTER I 
 
 HOMOLOGIES 19 
 
 CHAPTER II 
 THE DENTAL TISSUES 28 
 
 CHAPTER III 
 THE ENAMEL 37 
 
 CHAPTER IV 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 41 
 
 CHAPTER V 
 CHARACTERISTICS OP THE ENAMEL TISSUE 63 
 
 CHAPTER VI 
 
 THE DIRECTION OF THE ENAMEL RODS IN THE TOOTH CROWN . . 77 
 
 CHAPTER VII 
 THE RELATION OF THE STRUCTURE TO THE CUTTING OF THE ENAMEL 84 
 
 CHAPTER VIII 
 
 THE STRUCTURAL REQUIREMENTS FOR STRONG ENAMEL WALLS . . 90 
 
 CHAPTER IX 
 STRUCTURAL DEFECTS IN THE ENAMEL 113 
 
 CHAPTER X 
 
 SPECIAL AREAS OF WEAKNESS FOR ENAMEL MARGINS 125 
 
 CHAPTER XI 
 THE DENTIN 135 
 
 CHAPTER XII 
 THE CEMENTUM 153 
 
 (vi)
 
 CONTENTS vii 
 
 CHAPTER XIII 
 DENTAL PULP 164 
 
 CHAPTER XIV 
 THE LYMPHATICS OF THE DENTAL REGION 181 
 
 CHAPTER XV 
 INTERCELLULAR SUBSTANCES 200 
 
 CHAPTER XVI 
 BONE 209 
 
 CHAPTER XVII 
 BONE FORMATION AND GROWTH 216 
 
 CHAPTER XVIII 
 PERIOSTEUM 222 
 
 CHAPTER XIX 
 THE ATTACHMENT OF THE TEETH 230 
 
 CHAPTER XX 
 THE PERIDENTAL MEMBRANE 237 
 
 CHAPTER XXI 
 THE CELLULAR ELEMENTS OF THE PERIDENTAL MEMBRANE . . . 250 
 
 CHAPTER XXII 
 ABSORPTION OF TEETH 275 
 
 CHAPTER XXII I 
 THE MOUTH CAVITY 288 
 
 CHAPTER XXIV 
 
 BIOLOGICAL CONSIDERATIONS FUNDAMENTAL TO EMBRYOLOGY . . . 298 
 
 CHAPTER XXV 
 EARLY STAGES OF EMBRYOLOGY 302 
 
 CHAPTER XXVI 
 
 THE DEVELOPMENT OF THE TOOTH GERM 321 
 
 CHAPTER XXVII 
 
 THE RELATION OF THE TEETH TO THE DEVELOPMENT OF THE FACE . 334
 
 viii CONTENTS 
 
 PART II. 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 (TWENTY-FIVE PERIODS IN THE LABORATORY) 
 
 PRELIMINARY 377 
 
 PERIOD I 382 
 
 PERIOD II 382 
 
 PERIOD III 384 
 
 PERIOD IV 387 
 
 PERIOD V 387 
 
 PERIOD VI 389 
 
 PERIOD VII 390 
 
 PERIOD VIII 391 
 
 PERIOD IX 391 
 
 PERIOD X 392 
 
 PERIOD XI 393 
 
 PERIOD XII 393 
 
 PERIOD XIII 394 
 
 PERIOD XIV 394 
 
 PERIOD XV 395 
 
 PERIOD XVI 395 
 
 PERIOD XVII 396 
 
 PERIOD XVIII 396 
 
 PERIOD XIX 397 
 
 PERIOD XX 398 
 
 PERIOD XXI 399 
 
 PERIOD XXII 400 
 
 PERIOD XXIII 401 
 
 PERIOD XXIV 401 
 
 APPENDIX. 
 
 CHAPTER I 
 THE GRINDING OF MICROSCOPIC SPECIMENS, USING THE GRINDING 
 
 MACHINE 403 
 
 CHAPTER II 
 THE THEORY OF HISTOLOGICAL TECHNIQUE 424 
 
 CHAPTER III 
 GENERAL HISTOLOGICAL METHODS 430 
 
 CHAPTER IV 
 FIXING AGENTS AND STAINING SOLUTIONS , 439 
 
 INDEX . .... 447
 
 DENTAL HISTOLOGY. 
 
 INTRODUCTION. 
 
 THE development in knowledge of the cell has had a most pro- 
 found effect upon the entire practice of medicine; in fact, the 
 progress of modern medicine has dated from the studies of cell 
 biology, the germ theory of disease being only one of the phases 
 of this development. In terms of the cell theory the functions 
 of the body are but the manifest expression of the activities of 
 thousands or millions of more or less independent but correlated 
 centers of activity. If these centers or cells perform their func- 
 tions correctly, the functions of the body are normal, but if they 
 fail to perform their office or work abnormally, the functions of 
 the body are perverted. In the last analysis, then, all physiology 
 is cell physiology, all pathology cell pathology. To modern medi- 
 cine, histology, or the cell structure of the organs and tissues of 
 the body, together with cell physiology, is the rational foundation 
 of all practice. This is as true for the dentist as for the physician 
 in regard to the soft tissues of the mouth and teeth that he is called 
 upon to handle. With caries of the teeth, the disease which most 
 demands the attention of the dentist, the case is somewhat different. 
 Caries of the teeth is an active destruction, by outside agencies, of 
 a formed material which is the result of cell activity, the teeth 
 themselves being passive. The cellular activities of organs and 
 tissues of the body may have an influence, but this is only in pro- 
 ducing those conditions of environment which render the activities 
 of the destructive agent efficient in their action upon the tooth 
 tissues. Though the dental tissues are passive, the phenomena 
 of caries can only be understood when the structure of the tissues is 
 understood, and not only must the treatment be based upon knowl- 
 edge of the structure of the tissues, but the mechanical execution 
 of the treatment is facilitated by that knowledge of structure. 
 
 In the preparation of cavities, the arrangement of the enamel 
 wall is determined by the knowledge of the direction of the enamel 
 2 (17)
 
 18 DENTAL HISTOLOGY 
 
 prisms in that locality, and to a certain extent the position of 
 cavity margins must be governed by the knowledge of the struct- 
 ure of the enamel. In the execution of the work a minute knowl- 
 edge of the direction of enamel rods becomes the most important 
 element in rapidity and success of operation. The longer the 
 author studies and teaches the structure of the enamel in its rela- 
 tion to the structure and preparation of enamel walls, the more 
 he finds himself using this knowledge at the chair in daily opera- 
 tions. He believes that nothing will do more to increase facility, 
 rapidity, and success of operation than a close study of the enamel 
 structure. 
 
 All tissues are made up of two structural elements cells and 
 intercellular substances. The cells give the vital characteristics, 
 the intercellular substances the physical character. The cells 
 are the active living elements, the intercellular substances are 
 formed materials produced by the activity of the cells, and more 
 or less dependent upon them to maintain their quality, but they 
 possess no vital properties. They surround and support the cells, 
 and the physical characteristics are given by them. An under- 
 standing of the relation of cells and intercellular substances in the 
 structure and function of tissues is absolutely fundamental to the 
 study of dental histology, and should be acquired in a thorough 
 study of general histology before the subject is undertaken. 
 
 At the time the first edition of this work was prepared the relation 
 of histologic structure of the enamel to the mechanical operation 
 of dentistry was receiving special attention because of the study of 
 cavity form for the prevention of the recurrence of caries, and the 
 changes necessitated in cavity preparation because of this study. 
 This phase of dental histology is just as important as ever, but it has 
 been so generally accepted and so clearly grasped that now most of 
 the applications in practice are taught, where they properly belong, 
 in Operative Dentistry and the Technique of Cavity Preparation. 
 In this edition, therefore, the space devoted to this subject is greatly 
 reduced. 
 
 The problem of the pulpless tooth which now occupies the fore- 
 most place in the attention of the dental and medical professions 
 emphasi/es the importance of the histology of the dentin and 
 cementum, and places new importance on the relation of cellular and 
 intercellular substances in the tissue. For the dental student this 
 subject should be given more careful consideration than is usual 
 in elementary courses of general histology.
 
 CHAPTER I. 
 HOMOLOGIES. 
 
 Exoskeleton. In studying the organization of animal forms they 
 are found, very early in the evolutionary stages, to develop some 
 sort of a framework, or skeleton, to support and protect the crea- 
 ture. In the lower and earlier forms this framework is formed 
 entirely of some sort of shell upon the outside of the creature, and 
 consequently is called an exoskeleton. This may be either horny 
 or chitinous in nature, as in the insects, crabs, etc., or it may be 
 calcified, as in the shell-fish, or it may be both. The exoskeleton 
 serves not only as a supporting framework, but also as a protection. 
 
 Endoskeleton. In the higher forms an internal framework, or 
 endoskeleton, is developed, which forms the scaffolding to support 
 the creature, but does not act as a protection. In the first place, 
 this is of cartilage, but may be changed into bone. 
 
 The exoskeleton is a product of the skin and may be of either 
 epithelial or connective-tissue origin, or from both. The skin is made 
 up of two parts: the epithelial covering or epidermis, and the sup- 
 porting connective-tissue layer, or derma. Both layers take part 
 in the formation of most exoskeletal structures. In the hair, the 
 shaft is of epithelium, the bulb of connective tissue. In the tooth, 
 the enamel is from the epithelium, the dentin, from connective tissue. 
 In all bony structures belonging to the exoskeleton the bone is 
 formed in fibrous tissue and is never preceded by cartilage. Bony 
 structures belonging to the endoskeleton are formed from cartilage. 
 In lower forms of animals they remain always cartilage. In man the 
 cartilage is partly converted into bone, all of the bones of the endo- 
 skeleton being preceded by cartilage. 
 
 The first trace of the endoskeleton is found in the lowest form 
 of vertebrate, the Amphioxus or Lancit, the lowest form of fish, 
 and appears as a rod or notochord in the dorsal region. There is 
 also an important difference in the nervous organization (Figs. 1 
 and 2). In the invertebrate the nervous system is represented 
 by a larger or smaller ganglion in the anterior or head end, corre- 
 sponding to the brain; this is dorsal to the alimentary canal. From 
 this a ring passes around the anterior end of the alimentary canal 
 
 (19)
 
 20 
 
 HOMOLOGIES
 
 ENDOSKELETON 
 
 21 
 
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 22 HOMOLOGIES 
 
 and unites with a chain of ganglia ventral to it. The nervous 
 system of the invertebrate then is, with the exception of the brain 
 ganglia, ventral to the alimentary canal, and corresponds to the 
 sympathetic system in higher animals. It will be noted that this 
 arrangement puts the nervous system, which controls the activity 
 of the individual, in the most protected position. The invertebrate 
 crawling upon the ground is subject to attack or injury from above, 
 but it may be cut almost in two before the nervous system is reached. 
 
 In the vertebrate the central nervous system appears as a chain 
 of ganglia dorsal to the alimentary canal and notochord (Fig. 2). 
 This difference is significant, and may be expressed roughly in 
 this way: The invertebrate framework is an outside protecting 
 shell, upon which the creature depends for protection. The verte- 
 brate framework is an internal structure to facilitate motion and 
 give support, and is accompanied by a development of the nervous 
 organization, so that the creature protects itself by more rapid 
 motion. In the invertebrate the digestive system is above or 
 dorsal to the nervous system, in the vertebrate the nervous system 
 is in the upper position, both structurally and functionally. 
 
 In ascending in the scale of organization the endoskeleton 
 increases in importance and development, while the exoskeleton 
 decreases in importance and development. 
 
 From the standpoint of comparative anatomy the teeth are 
 not a part of the osseous system, but appendages of the skin, and 
 are to be compared with such structures in the body as the hair 
 and the nails. The teeth are a part of the exoskeleton, and their 
 relation to the bones is entirely secondary for the purpose of 
 strength, the bone growing up around the tooth to support it. 
 
 Placoid Scales. In the skin of such fishes as the shark and the 
 dog-fish small calcified scales are found, which are made up of a 
 conical cap of calcified tissue like enamel, resting on a cone of 
 dentin which contains a vascular core or pulp. These are sur- 
 rounded by a basal plate of tissue like cementum into which the 
 fibers of the derma are imbedded. Only the tips of these scales 
 project through the skin. These are the structures from which 
 the teeth have been derived in evolution. 
 
 From the standpoint of development the mouth cavity is to 
 be regarded as a part of the outside surface of the body which has 
 been enclosed by the development of neighboring parts, and the 
 dermal scales, or rudimentary teeth, which are found in the skin 
 covering the arches forming the jaws, have undergone special
 
 UOMOLOGY AND ANALOGY 23 
 
 development for the purpose of seizing and masticating the animal's 
 food. In the simplest forms there is only a development in size 
 and shape of these scales, and they are supported only by the con- 
 nective tissue which underlies the skin. These teeth are easily 
 torn off in the attempt to hold a resisting prey, and in the shark 
 (Fig. 3) they are continually being replaced by new ones. In the 
 more highly developed forms the bone forming the jaw grows 
 upward around the bases of these scale-like teeth to support them 
 more firmly and render them more useful. 
 
 FIG. 3. Shark's skul! (Lamna cornubica), showing succession of teeth. 
 
 Homology and Analogy. In biology structures that are similar 
 in formation and origin are called homologous. Structures that 
 are similar in function are called analogous. A structure or organ 
 may be both homologous and analogous to another, but not neces- 
 sarily so. For instance, the wing of a fly is analogous to the wing 
 of a bird, because they are used for the same purpose, but they are 
 not homologous. The wing of a bat and the wing of a bird are 
 both analogous and homologous, being used for the same purpose, 
 and having similar structure and origin. The arm of man is homo- 
 logous to the wing of a bird, but not analogous to it. The jaws of 
 a crab or beetle are analogous to the jaws of man, but they are not 
 homologous structures, as the jaws of the crabs and insects are 
 modified legs. The teeth are said to be homologous to the dermal
 
 24 
 
 HOMOLOGIES 
 
 scales of certain fishes, and to the appendages of the skin, such as 
 the hair and nails, because they are similar in structure and origin 
 (Plate I). 
 
 Comparison of Structure. If the tooth is compared with the 
 hair in this way this will be better understood. The hair may be 
 considered as a horny structure composed of epithelial cells resting 
 upon a papilla of connective tissue. The tooth may be considered 
 a calcified structure, formed by epithelial cells, resting upon a papilla 
 of connective tissue, which is also partially calcified. 
 
 
 
 C 'i<f'c.?'ol>' > i -o' r ^' "^ ~tf\ 
 
 FIG. 4. Development of the hair: Sc, stratum corneum; SM, stratum malpighii, 
 C, derma; Dr, sebaceous gland; F, follicles; CZ, central, PZ, peripheral zone of 
 hair germ; HK, hair knob; P, beginning the formation of the hair papilla; P', 
 same in a later stage of development when it has become vascular. (Wiedersheim, 
 Comparative Anatomy of Vertebrates.) 
 
 Comparison of Origin. From a study of the development of the 
 tooth and the hair, the similarity of their origin and structure 
 becomes more apparent. 
 
 The first step in the development of the hair is a thickening of 
 the epithelium at a point, the epithelial cells multiplying and grow- 
 ing down into the connective tissue below, so as to make a two-
 
 PLATE I 
 
 Comparison of Structure of Tooth and Hair.
 
 COMPARISON OF ORIGIN 
 
 25 
 
 layered bag or cap, the connective tissue growing up in the form 
 of a cone-shaped papilla into the cavity of the cap (Fig. 4). The 
 epithelial cells of the inner layer, next to the connective tissue, 
 multiply rapidly and develop horny material and are pushed out 
 from the surface of the skin as the shaft of the hair. 
 
 In the development of the tooth there is at first a thickening of 
 the epithelium, and a mass of epithelial cells like that forming the 
 hair, but larger, grows dow r n into the connective tissue (Fig. 5). 
 This becomes bulbous, then invaginated, forming a two-layered 
 
 FIG. 5. Diagram to illustrate development of a tooth; A, inner layer of enamel 
 germ; B, outer layer; C, remains of intermediate cells; D, den tin; DL, dental 
 lamina; E, epithelium; E.G, enamel germ; En, enamel; F, dental furrow; 
 L.D, labiodental furrow; M, connective-tissue cells; O, odontoblasts; P, dentin 
 papilla; R. G, reserve germ; V, bloodvessel. (Cunningham's Anatomy.) 
 
 cap. The two layers are at first perfect and are farther from the 
 surface than the epithelial structure which develops the hair. A 
 cone-shaped papilla of connective tissue, the dental papilla, grows 
 up into the cavity of the epithelial organ corresponding to the bulb 
 of the hair. 
 
 The inner layer of epithelial cells produce the enamel, the outer 
 layer of connective-tissue cells, covering the connective-tissue 
 papilla, develop the dentin, leaving the pulp inside as the remains 
 of the dental papilla.
 
 26 
 
 HOMOLOGIES 
 
 Phylogeny is the history of the development or evolution of the 
 species. Ontogeny is the development of the individual. In homol- 
 ogous structure we may trace the similarity in their origin, both in 
 
 FIG. 6. Changes in the mandible with age; buccal and lingual view. 
 
 ontogeny, or the development of the individual, and in phylogeny, 
 or the development of the species. 
 
 Relation to the Bone. The relation of the bones of the jaws to 
 the teeth is entirely secondary and transient. The bone grows
 
 RELATION TO THE BONE 27 
 
 up around the roots of the teeth to support them, and is destroyed 
 and removed with the loss of the teeth or the cessation of their 
 function. In this way the development of the alveolar process 
 appears around the roots of the temporary teeth. All this bone 
 surrounding their roots is absorbed and removed with the loss of 
 the temporary dentition, and a new alveolar process grows up 
 around the roots of the permanent teeth as they are formed. This 
 development of bone around the roots of the teeth leads to the 
 changes in the shape of the body of the lower jaw, increasing the 
 thickness from the mental foramen and the inferior dental canal 
 upward (Fig. 6).. When the teeth are finally lost this bone is again 
 removed and the body of the jaw is reduced in thickness from 
 above downward. These phenomena have an important bearing 
 upon the causes and treatment of diseased conditions of the teeth, 
 particularly those which involve the supporting tissues. 
 
 From the dental standpoint it is important to note that the 
 teeth are formed first and the bone is developed to support them. 
 The use of the teeth through occlusion reacts upon the formation 
 of bone. The study of anatomy, as well as direct experiment, has 
 shown that muscular function, acting through occlusion, affects 
 the development, not only of the bone of the alveolar process, jaws 
 and face, but of the entire skull. It is most important for the 
 student to realize that the teeth are moving with reference to the 
 skull as a whole, through the entire period of development, and, in 
 fact, throughout life.
 
 CHAPTER II. 
 THE DENTAL TISSUES. 
 
 STUDY of the structure of the teeth shows that all teeth, 1 from 
 the simplest to the most complex, are composed of but four tissues 
 enamel, dentin, cementum, and the pulp, or formative tissue of 
 the dentin. All teeth are maintained in position and rendered func- 
 tionally useful by certain supporting tissues. 
 
 Even the simplest placoid scales, as found in the skin of the 
 shark and dog-fish, contain these four tissues. In many of the 
 specialized forms of teeth some of these tissues may be absent. For 
 instance, in the bony fishes the teeth are fastened to the bone by 
 an interlocking of bone and dentin, forming an ankylosed attach- 
 ment, and the cementum is absent; but in some of these there is 
 also a slight formation of cementum. In the tusks of elephants 
 during the functional period the dentin is not covered by enamel, 
 but when the tusk first erupted there was a slight enamel cap, 
 which was at once broken or worn off. In many instances the 
 enamel seems to be entirely absent, and for that reason it has 
 sometimes been called the most inconstant of the dental tissues, 
 but in every case in which the development of the tooth has been 
 studied an enamel organ has been found. It is probably much 
 more nearly correct to consider that in all cases enamel is formed, 
 but that it may be so thin and transparent as to be very difficult 
 to recognize, and very soon may be entirely lost. 
 
 FUNCTIONS OF THE DENTAL TISSUES. 
 
 The Enamel. The enamel forms a hard protecting surface or 
 cap especially adapted to resist abrasion. It is the hardest animal 
 tissue, but brittle and inelastic, and dependent upon the support 
 of the elastic dentin for strength. Its function is to resist the 
 abrasion of friction. Its arrangement in many instances is found 
 specially modified for this purpose. 
 
 1 The formation of a satisfactory definition of a tooth is by no means an easy 
 matter. The word here is used to mean teeth that are derived in the phylogenetic 
 series from the placoid scale, as the starting-point of evolution. 
 (28)
 
 FUNCTIONS OF THE DENTAL TISSUES 29 
 
 The Dentin. The dentin is the strong elastic tissue forming 
 the great mass of the tooth, and gives to it its strength. Teeth 
 that are subjected to stress and force are often made up of dentin 
 without enamel. If, for instance, the tusks of the elephant, used 
 for such purposes as tearing down branches, spading up the ground, 
 and so on, were made up entirely of enamel, they would break off 
 the first time they were locked in the branches or driven into the 
 ground, but the elastic dentin gives and bends and will stand great 
 stress. The teeth of many animals which use their tusks in fighting 
 are constructed on the same plan. Such tusks usually have an 
 enamel cap when they first erupt, and in every case an enamel 
 organ is present in the tooth germ. 
 
 The Cementum. The cementum furnishes attachment for the 
 connective-tissue fibers which fasten the tooth to the bone or 
 surrounding tissues. It is formed on the enamel and dentin both 
 before and after the eruption of the teeth but only on portions 
 embedded in the tissues at the time of formation. The formation 
 of the cementum on the surface of the root fastens the surrounding 
 connective-tissue fibers to the tooth. The fibers are calcified along 
 with the matrix of the cementum which is built up around them. 
 These fibers in man and the higher animals extend to the bone and 
 the surrounding tissues and support the teeth against the forces 
 of mastication and hold the surrounding tissues in proper relation 
 to the teeth. The function of the cementum is therefore to attach 
 the connective-tissue fibers to the surface of the root. 
 
 The Pulp. The pulp is the remains of the formative organ of 
 the dentin. In teeth of continuous growth it remains actively 
 functional throughout the life of the tooth, but in teeth of limited 
 growth, after the typical development of dentin, it becomes func- 
 tional again only in response to irritations which, however, may 
 be local or reflex. The pulp performs two functions a vital func- 
 tion, the formation of dentin, and a sensory function, the response 
 to thermal change. 
 
 Summary. The dental tissues, i. e., enamel, dentin, cementum, 
 and pulp, are so called not simply because they are found in the 
 human teeth, but because all teeth are composed of these four 
 tissues. 
 
 It is true that in comparative dental histology considerable 
 difference exists in the microscopic structure of these tissues from 
 the teeth of different animals, but certain characteristics are very 
 persistent and quite characteristic of each.
 
 30 THE DENTAL TISSUES 
 
 DISTRIBUTION OF THE DENTAL TISSUES. 
 
 The arrangement and distribution of the dental tissues in the 
 structure of the human teeth is best studied in ground sections 
 cut longitudinally through the entire tooth (Plate II), and series 
 of transverse sections cut through the roots. For this purpose 
 the sections should not be too thin (from 10 to 20 microns). For 
 the study of the arrangement of the cementum and dentin in the 
 roots at least three transverse sections should be ground from each 
 root, one from the gingival, one from the middle, and one from the 
 apical third. 
 
 The Enamel. The enamel forms a cap over the exposed portion 
 of the tooth. Its function is to resist the abrasions of mastication. 
 It gives the detail of crown form to the tooth. It extends to the 
 gingival line, and, except in old age, is covered in the gingival 
 portions by the epithelium of the gingivae which lies in contact 
 with it but is not attached to it. It is thin in the gingival portion 
 and is normally overlapped slightly by the cementum at the gingival 
 line. It extends farther apically on the labial and lingual, and buccal 
 and lingual, than upon the proximal surfaces, especially on the 
 incisors, cuspids, and bicuspids. It is thickest in ,the occlusal 
 third of the axial surfaces, and on the occlusal surfaces of the 
 molars and bicuspids, especially over the cusps. In the incisors 
 and cuspids it is thickest in the occlusal third on the labial and over 
 the marginal ridges on the lingual. The dento-enamel junction, 
 though not parallel with the surface of the enamel is usually curved 
 in the same direction except near the cusps in molars and bicuspids, 
 where the curve is sometimes reversed, apparently to give greater 
 thickness of enamel where resistance to wear is most needed. 
 
 In the molars and bicuspids the dento-enamel junction in the 
 occlusal thirds on the buccal and lingual is usually curved in the 
 opposite direction. That is, while the surface of the enamel is 
 convex, the surface of the dentin is concave. It will be seen that 
 this not only gives a greater thickness to the enamel in the region 
 which will resist abrasion, but also gives it a firmer seat upon the 
 dentin. (Study illustrations in Chapter IX.) The dento-enamel 
 junction is seldom a smooth, even surface, but will appear scalloped 
 in sections, projections of dentin extending between projections 
 of enamel (Fig. 7). In three dimensions this means that rounded 
 projections of the enamel rest in rounded depressions of the dentin 
 surface, and pointed projections of the dentin extend between the
 
 PLATE II 
 
 Cm. 
 
 Ground Section of a Canine. 
 
 E, enamel; Cm, eementum; D, dentin; PC, pulp chamber; De, dento- 
 enamel junction; Eil, enamel defect; 6', junction of enamel and eementum 
 at the gingival line; Gt, granular layer of Tomes. (Reduced from a photo- 
 micrograph made in three sections.)
 
 DISTRIBUTION OF THE DENTAL TISSUES 
 
 31 
 
 rounded projections of the enamel. This is similar but much less 
 marked than the interlocking of the papilla of connective tissue 
 with the projections of the Malpighian layer of stratified squamous 
 epithelium of the skin and mucous membrane. In some cases these 
 
 FIG. 7. Dento-enamel junction. 
 
 projections of dentin into the enamel may be quite marked. This 
 scalloping of the dento-enamel junction gives a stronger attach- 
 ment of the enamel to the dentin, and accounts, partially at least, 
 for the difference that is observed in the ease with which enamel
 
 32 THE DENTAL TISSUES 
 
 can be removed from the dentin in the preparation of roots for 
 crowns. Where the two tissues join with smooth surfaces the 
 enamel can be comparatively easily cleaved away; where the 
 scalloping is marked it is removed with much greater difficulty. 
 
 The Dentin. The dentin gives the strength to the tooth. This 
 should never be lost sight of in operations, and sound dentin should 
 always be conserved to the greatest possible extent in the prepara- 
 tion of cavities. That the function of the dentin is to give strength 
 will be seen more clearly from a comparative study of teeth modified 
 for special functions. The dentin forms the greatest mass of the 
 tooth, the type form being determined by it. The cusps and ridges, 
 although different in form, are still represented in the dentin as 
 well as the number and shape of the roots, while the detail of the 
 form of the roots is modified by the addition of the cementum on 
 the surface. 
 
 The dentin forms a layer of comparatively even thickness sur- 
 rounding the central cavity or pulp chamber, which is occupied 
 by the formative organ. From this cavity a great number of-' 
 small tubules extend through the calcified dentin matrix to the 
 surface under the enamel and cementum. In the crown portion 
 the course of these tubules is characteristically curved like the 
 letter S or /, so that the tubules tend to enter the pulp chamber 
 at right angles to the surface and to end under the enamel at right 
 angles to the dento-enamel junction (Plate II). On closer study 
 these tubule directions will be found to be more complicated, but 
 in studying the distribution of dentin they should be noted. In 
 the root portion the tubules are usually comparatively straight, 
 that is, without the double curve, and are at about right angles 
 to the axis of the canal. 
 
 The outer layer of dentin under low magnification presents a 
 peculiar granular appearance, which is specially apparent under 
 the cementum. This is known as the granular layer of Tomes, 
 and is caused by irregular spaces in the dentin matrix which com- 
 municate with the dentinal tubules. 
 
 The Cementum. The cementum covers the dentin in the root 
 portion, and in most cases slightly overlaps the enamel at the 
 gingival line. This is not always true, for in some cases it just 
 meets the enamel, and in others there is a space where the dentin 
 is uncovered between the enamel and the cementum (Fig. 8). It 
 has not been positively determined whether this can ever be con- 
 sidered a normal condition, and the author has some reason to
 
 DISTRIBUTION OF THE DENTAL TISSUES 
 
 33 
 
 suppose that the sections showing this condition were from teeth 
 from which the gums had receded and the cementum was destroyed. 
 The sensitiveness which is so marked in some cases, where the 
 gums have receded beyond the gingival line, is probably due to 
 the loss of cementum and the uncovering of the granular layer of 
 Tomes. 
 
 The cementum is thin and structureless in appearance in the 
 gingival portion when viewed with low powers, but becomes thicker 
 
 FIG. 8. Gingival line, showing the relation of enamel and cementum. 
 
 in the apical third. In the thicker portions irregular spaces (lacuna?) 
 with radiating canals (canaliculi) are seen. In life these spaces 
 contain living cells (the cement corpuscles), which correspond 
 to the bone corpuscles found in the lacunae of bone. Upon the 
 convex surfaces of the root the cementum is thin; upon the con- 
 cave surfaces it is thicker. This increases with age, and so the 
 continuous formation of cementum tends to round the outlines 
 of the roots and to unite them where they approach each other. 
 The fibers which are built in the cementum are often imperfectly 
 3
 
 34 THE DENTAL TISSUES 
 
 calcified, especially where the layers are thick, so that in the ground 
 sections they may often be easily mistaken for canals, because the 
 imperfectly calcified fiber has shrunk in the preparation. 
 
 ADAPTATION IN THE DISTRIBUTION OF DENTAL TISSUES. 
 
 If the teeth of mammals are studied in a comparative way 
 many modifications will be found in the relative amount and dis- 
 tribution of the dental tissues, adapting the tooth to perform special 
 functions. A study of these modified or specialized teeth will 
 give a better understanding of the functions of the tissues in the 
 tooth. The human tooth may be taken as a type of omnivorous 
 tooth, and the arrangement and distribution of its tissues has 
 already been described. 
 
 Teeth of Continuous Growth. In many animals the teeth or 
 some special teeth are developed as weapons, or as implements 
 to aid in securing food. It is usually the cuspid teeth that show 
 this modification, as in the tusks of the boar and many species 
 of the carnivora, the tusks of the walrus, and other examples. 
 In the case of the elephant the incisors have been developed in the 
 same way. Whenever the teeth have been developed in size for 
 uses which require strength and the ability to withstand stress 
 and strain, the increase in size is by development of the mass of 
 dentin., the enamel often being entirely lost during the functional 
 period. If these teeth were composed chiefly of enamel they would 
 be too brittle. These tusks, which, as in the case of the elephant, 
 sometimes reach a weight of many hundreds of pounds, are usually 
 deeply embedded in the bone, and the concealed portion is covered 
 with a layer of cementum which attaches the fibers, holding them 
 to the bone, but they retain a conical pulp in a cone-shaped pulp 
 chamber at the base of the tooth, which continues to form dentin. 
 The tooth is pushed out of the socket, as the shaft of the hair is 
 pushed out, by the multiplication of cells covering the bulb. In 
 this way the size of the tooth is maintained as the exposed and 
 functional part is worn off. Strength and elasticity are required, 
 therefore the dentin is developed. The cementum which is formed 
 on the embedded portion for attachment of fibers is worn off as 
 soon as it is exposed to friction. 
 
 Chisel Teeth. The incisors of the rodents, as rats, mice, squirrels, 
 and beavers, present an interesting modification for a special 
 function. These teeth are used as chisels for cutting hard sub-
 
 ADAPTATION IN DISTRIBUTION OF DENTAL TISSUES 35 
 
 stances, as wood, shells of nuts, etc. Here strength and hardness 
 are required. The dentin is increased by the continual function 
 of a conical persistent pulp which continues to form dentin, and 
 the enamel organ is carried down into the socket, to the base of 
 the dental papilla, on the labial, instead of stopping at the gingival 
 line, as in the human incisors. In this position it continues to build 
 enamel on the labial side of the dentin. The enamel rods, instead 
 of being straight, are twisted about each other in a complicated 
 fashion, giving the maximum of hardness. As the incisors work 
 against each other by the movements of the jaw, the dentin is 
 worn off on the lingual side and the enamel kept in the form of a 
 chisel edge. There is also a modification of the temporomandibular 
 articulation, allowing the lower jaw to move forward and back as 
 well as up and down, but not laterally, so that the lower incisors 
 can be closed either lingually or labially to the upper, and in this 
 way both the upper and the lower incisors are made to sharpen 
 each other in use. In this case there is need for both strength 
 and hardness, and both dentin and enamel are continuously being 
 formed at the base of the tooth embedded in the socket, and the 
 cementum is formed over the embedded portions as the medium 
 of attachment. 
 
 Grinding Teeth. In a grinding tooth, as in the molar of the 
 horse and cow, and in a much more complicated form in the elephant, 
 the three tissues enamel, cementum, and dentin are arranged 
 so as to form, by the different rapidity of abrasion, corrugated 
 grinding surfaces like millstones. The conditions can be under- 
 stood if it is remembered that the cusps in the dentin are very 
 high, and are covered by a comparatively thin layer of enamel. 
 After the enamel is formed, and while the tooth is embedded in its 
 crypt in the bone, cementum is formed, covering the surface and 
 filling up the hollows between the cusps, so that the crown when 
 it first erupts is rounded, with enamel showing only at the tips of 
 the cusps. As soon as the tooth wears, the tip of the enamel is 
 worn through, so that the circumference of the crown shows first 
 cementum, then enamel, then dentin, then enamel, then cementum, 
 then enamel, and so on. The foldings of the enamel often become 
 very complicated, but the most complicated forms can be under- 
 stood in this way. 
 
 Descriptive Terms. In describing the structure of the teeth and 
 the arrangement of the structural elements of the tissues, direc- 
 tions are described with reference to three planes: The mesio-
 
 36 THE DENTAL TISSUES 
 
 disto-axial plane passing through the center of the crown from 
 mesial to distal and parallel with the long axis of the tooth. 
 
 The bucco-linguo-axial plane, a plane passing through the 
 center of the crown from buccal to lingual and parallel with the 
 long axis of the tooth. 
 
 The horizontal plane at right angles to the axial planes.
 
 CHAPTER III. 
 THE ENAMEL. 
 
 ENAMEL may be defined as the hard, glistening tissue covering 
 the crowns of the teeth in man and most mammals. It is the 
 hardest animal substance and contains less organic matter than any 
 other tissue of the body. 
 
 Histogenesis. The enamel is formed by the epithelial cells of the 
 inner tunic of the enamel organ. After the tissue is formed the cells 
 which produced it are destroyed and the tissue is left as a formed 
 material covering the dentin. 
 
 Structural Elements. -The enamel is composed of two structural 
 elements: (1) The enamel rods, or prisms. (2) A calcified sub- 
 stance which unites the rods into a continuous structure called the 
 cementing, or interprismatic substance. 
 
 The enamel differs from all other calcified tissues: 
 
 1. In origin. 
 
 2. In degree of calcification. 
 
 3. In relation to its formative organ. 
 
 4. In the form of the structural elements of the tissue. 
 
 It is well to emphasize these points of difference, for throughout 
 dental and medical writing, reasoning by analogy from bone con- 
 ditions to tooth conditions, and especially to changes in the enamel, 
 is often found. For instance, the argument has been made that 
 because there may be changes in the bones in pregnancy, " softening" 
 of the teeth would be expected. Many similar, though less crude, 
 arguments would not be made if it were remembered that histo- 
 logically, histogenetically, physiologically, and morphologically the 
 enamel stands alone. 
 
 Origin. The enamel is the only calcified tissue derived from the 
 epithelium. All other calcified tissues are connective tissues. 
 Histogenetically, then, the enamel is ultimately derived from the 
 epiblastic germ layer, while all other calcified tissues arise from the 
 mesoblast. Thus, even at the first step in the differentiation of the 
 cells, enamel is different and independent from bone, cementum, 
 or dentin. It is natural, therefore, to find the enamel differing 
 from bone in every other respect. On the other hand, the relation 
 
 (37)
 
 38 THE ENAMEL 
 
 of the enamel to the epithelium becomes more and more apparent. 
 For instance, imperfections in the structure of the enamel during 
 its formation are most likely to be produced by systemic conditions 
 which affect the epithelium. The eruptive fevers occurring during 
 enamel formation often produce imperfections of structure. Scarlet 
 fever is most pronounced in its epithelial effect, causing loss of skin, 
 loss of living epithelium of the alimentary tract, and often loss of 
 hair, and is likewise most likely to produce pitted teeth or hypoplasia 
 of the enamel. In other words, the same poison which is produced 
 by the germ of scarlet fever causes the death of epithelial cells, 
 of the skin, of the hair bulb, of the mucous membrane, and of the 
 enamel organ. 
 
 The most recent work of Dr. Black shows the brown and mottled 
 enamel of certain localities to be found associated with greatly 
 freckled skin. Enamel therefore must be considered as epithelial 
 in origin and ultimately from the epiblast, while all other calcified 
 tissues are connective tissue and ultimately of mesoblastic origin. 
 
 Degree of Calcification. The enamel is by far the hardest animal 
 tissue. Chemically it is composed of water, calcium phosphate, 
 carbonate, and a small amount of fluoride, magnesium phosphate, 
 and a trace of other salts. Normally it should contain no organic 
 matter. Von Bibra gives the following analysis: 
 
 Calcium phosphate and fluoride 89.82 
 
 Calcium carbonate 4.37 
 
 Magnesium phosphate 1.34 
 
 Other salts 0.88 
 
 Cartilage 3.39 
 
 Fat 0.20 
 
 It is very difficult to obtain enamel for chemical analysis entirely 
 free from dentin, and small portions of dentin clinging to it are 
 probably responsible for some of the organic matter given in the 
 above analysis. 
 
 In all the older analyses the enamel was said to contain 95 to 
 97 per cent, of inorganic matter and 3 to 5 per cent, of organic 
 matter, while the percentage in dentin was given as 72 per cent, 
 of inorganic and 28 per cent, of organic, and in bone as 68 per cent, 
 inorganic and 32 per cent, organic (dry compact bone). This in 
 itself shows an enormous difference in the degree of calcification 
 between enamel and the other hard tissues, but the results of more 
 recent work are still more remarkable. In most of the original 
 studies of the chemical composition, the enamel was broken into
 
 DEGREE OF CALCIFICATION 39 
 
 small pieces and dried for some time at a temperature above the 
 boiling-point of water, to drive off all the moisture. The dry 
 enamel was weighed and then ignited, and the loss in weight taken 
 as the amount of organic matter. In 1896 Mr. Charles Tomes, 1 
 of London, published the results of his chemical analysis of enamel 
 in which he showed that a large part of the loss of weight in ignition 
 was due to the loss of water. He carried out ignition in tubes to 
 collect the products of combustion, and found that between red 
 and white heat from 2 to 3 per cent, of water was given off. This 
 occurred suddenly and with almost explosive violence, blowing 
 large pieces to fragments. While this did not account entirely 
 for all of the matter previously considered organic, the character 
 of the product of combustion and the observation of the material 
 during ignition led him to conclude that the remaining portion 
 was due to the dentin adhering to the enamel, and that the enamel 
 contained not more than a trace of organic matter. 
 
 Dr. Leon ^^ T illiams attacked the problem from the microscopic 
 and microchemical side, and was forced to the conclusion that 
 normal enamel contains no organic matter. No trace of organic 
 matter can be found in sections of enamel by staining. And if 
 the enamel is dissolved by acid and the progress observed, not a 
 trace of organic matrix can be found. The conclusion is therefore 
 imperative that enamel is composed entirely of inorganic matter, 
 which has been deposited and calcined in the form of the tissue by 
 the formative cells. In other words, enamel is formed material 
 produced by cells and laid down in a definite structure, but it con- 
 tains no organic matrix, while all other calcified tissues are composed 
 of an organic matrix of ultimate fibrous and gelatin-yielding char- 
 acter, in which inorganic salts are deposited in a weak chemical 
 combination, and living cells are retained in spaces of the formed 
 material. 
 
 If bone or dentin is subjected to the action of acid, the com- 
 bination between the organic and inorganic matter is broken up 
 and the inorganic matter dissolved, leaving the organic portion, 
 which yields gelatin when boiled in water, in the form of the original 
 tissue. If enamel is treated with acid the cementing substance 
 between the rods is first attacked and is dissolved more rapidly, 
 then the rods are attacked from their sides, and finally the tissue 
 is entirely destroyed, leaving no trace of structure. Apparently 
 the greater the dilution of the acid the greater will be the extent 
 
 1 Journal of Physiology.
 
 40 THE ENAMEL 
 
 of the solution of the cementing substance before the rods are 
 destroyed. 
 
 If bone or dentin is burned or ignited, the organic matter will 
 be driven off and the inorganic portion will be left in the form of 
 the tissue, still showing its structure. If enamel is ignited, water 
 of combination and whatever foreign matter has clung to the pieces 
 is given off, but the form of the tissue is unchanged. To illustrate 
 the difference by a crude comparison : Bone matrix may be likened 
 to a piece of cloth into which inorganic salts have been deposited 
 until it has become stiff and rigid, but the web of the cloth is still 
 seen. The salts may be dissolved out and the cloth left, or the cloth 
 may be burned out and the salts left. The enamel may be compared 
 to a fossil in which, by molecular change, the organic matter has 
 been removed and inorganic matter substituted, so that no organic 
 matter remains, but the structure is preserved. If the inorganic 
 salts were dissolved, no trace of structure would remain. On the 
 other hand, by ignition, nothing but w r ater can be driven off. 
 
 Relation to the Formative Tissue. The enamel is produced by 
 epithelial cells, which are lost and destroyed after the tissue is 
 completed. Any such thing, therefore, as a vital change in the 
 tissue is biologically unthinkable. After the enamel is formed it 
 can be changed only by chemical and physical action of its envi- 
 ronment. 
 
 All other calcified tissues are formed by connective tissue, and 
 remain in vital relation with connective tissue of undifferentiated 
 character. Bone and dentin matrix are therefore simply calcified 
 intercellular substances containing living cells in the spaces of the 
 matrix which maintain its chemical quality. A change in the 
 character or amount of the matrix might possibly, therefore, be 
 brought about by the vital activity of these cells. Moreover, 
 the formed matrix is always in vital relation with undifferentiated 
 connective tissue, which may at any time undergo specialization 
 for the purpose of construction or destruction. There is therefore 
 no basis for comparison between pathologic conditions of bone and 
 enamel. 
 
 The Form of the Structural Elements. The enamel is made up 
 of prismatic rods of inorganic matter, held together by an inorganic 
 cementing substance. All other calcified tissues are made up of 
 fibrous intercellular substance, containing inorganic salts and 
 usually arranged in layers. 
 
 The structure of the enamel differs so greatly from all other 
 calcified tissues that it is difficult to compare them briefly.
 
 PLATE III 
 
 From .1. Howard Mummery's 
 "Mirrosropic AtKilomy of the Teeth."
 
 CHAPTER IV. 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL. 
 
 THE enamel is composed of two structural elements: 
 
 1. The enamel rods or prisms, sometimes called enamel fibers. 
 
 2. The interprismatic, or cementing substance. 
 
 Enamel Rods. The enamel rods are long, slender, prismatic 
 rods irregularly five or six-sided 1 and alternately expanded and con- 
 stricted throughout their length (Plate III and Fig. 9). They are 
 from three and four-tenths to four and five-tenths microns in diam- 
 eter, and many of them extend from the dento-enamel junction to 
 the surface of the enamel. They are of the same diameter at their 
 outer and inner ends. This last statement is emphasized, as the 
 direct opposite is stated in some standard text-books of histology. 
 In the 'formation of the tissue they are arranged so that the expan- 
 sions in adjoining rods come opposite to each other, and do not 
 
 1 This statement of the shape of the enamel prisms must be taken as a general 
 statement, just as columnar epithelial cells are described as five-sided in cross-section. 
 In the enamel prism, as in the epithelial cell, the form is the result of mutual pressure, 
 the outlines are never regular, and unevenness in the distribution of the pressure, or 
 lack of balance in direction will modify the form of the prisms. For further study 
 of the form and relation of the enamel rods the student is referred to The Microscopic 
 Anatomy of the Teeth, by J. Howard Mummery, Chapter II. 
 
 DESCRIPTION OF PLATE III. 
 (From J. Howard Mummery's "Microscopic Anatomy.") 
 
 Drawings from teased preparations of enamel from elephant, except Figs. 5, 6, 7 and 
 8, which are from sections. 
 
 FIG. 1. Double-grooved prisms (elephant), r, ridges; g, grooves. The ridges are 
 often seen projecting beyond the extremities of fragments. 
 
 FIG. 2. Single-grooved prisms (elephant), r, ridge; g, grooves. 
 
 FIG. 3. Two double-grooved prisms, transverse above (elephant). 
 
 FIG. 4. Fragments of prisms in transverse fracture (elephant). 
 
 FIG. 5. Four prisms from a section (elephant), showing surface marking and 
 prominence of the ridge at r. 
 
 FIG. 6. From elephant: bridges in transverse section. The interprismatic sub- 
 stance appeared dark and the bridges are very conspicuous as white lines. 
 
 FIG. 7. Elephant. From a section, showing a wing process in the enamel. 
 
 FIG. 8. Elephant. From a section, showing ridges and grooves, r, ridges; g, 
 grooves. 
 
 FIG. 9. Two prisms from elephant, showing needle-splitting (n) and intercolumnar 
 bridges (6). 
 
 FIG. 10. Fragment of elephant enamel in transverse section. Two entire double, 
 concave prisms are seen projecting, with feather edges and intercolumnar bridges (b) . 
 
 FIG. 11. Fragments of prisms seen obliquely (elephant). 
 
 (41)
 
 42 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 interlock with the constrictions, so that there is alternately a greater 
 and a less amount of cementing substance between them. 
 
 It is evident that the outer surface of the enamel is much greater 
 than the surface of the dentin at the dento-enamel junction. This 
 greater area is obtained in two ways: 
 
 1. The rods are at right angles to the dentin at the dento-enamel 
 junction, but are seldom at right angles to the outer surface. This 
 may be illustrated by bending the leaves of a book, or cutting a 
 stack of paper obliquely. The sheets of paper are of the same 
 thickness, but when cut at right angles to the sheets the area of 
 the cut surface is not so great as when the leaves are cut diagonally. 
 
 FIG. 9. Enamel rods isolated by scraping. (About 800 X) 
 
 2. Many of the enamel rods undoubtedly extend from the dento- 
 enamel junction to the surface of the enamel, though it is difficult 
 to follow individual rods through this distance, but there are also 
 short rods which extend from the surface part way to the dentin. 
 These short rods end in tapering points between converging rods 
 that extend the entire distance. The short rods are specially 
 numerous in the most convex portion of the surface, as over the tips 
 of the cusps, occlusal edges, and marginal ridges. These areas 
 therefore become of special importance in connection with the 
 formation of enamel walls, as will be considered in detail later on 
 (Fig. 105).
 
 43 
 
 Differences between Enamel Rods and Cementing Substance. 
 While the cementing substance and the substance of the rods are 
 both entirely inorganic, or, more correctly, are composed entirely 
 of inorganic salts, they differ in physical and chemical properties 
 as follows: 
 
 1. The cementing substance is not as strong as the prismatic 
 substance. 
 
 2. The cementing substance is more readily soluble in dilute 
 acids than the rod substance. 
 
 FIG. 10. Enamel rods in thin etched section. (About 800 X) 
 
 3. The cementing substance is of slightly different (greater) 
 refracting index than the substance of the rod. The author wishes 
 to emphasize these statements, as the exact opposite is found in 
 some of the standard texts, at least concerning the first and second 
 statements. The facts are, however, so easily demonstrable that 
 anyone may satisfy himself without difficulty. 
 
 Relative Strength of the Enamel Rods and the Cementing Substance. 
 The cementing substance is not as strong as the substance of 
 the rods. The most striking characteristics of the enamel, and the 
 first to attract the attention of the student and the operator, are its 
 hardness and its tendency to split or cleave in certain directions.
 
 44 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 On examination it is found that this is determined by the direction 
 of the rods, and is caused by the difference in strength between the 
 two substances. Sections ground at right angles to the rod direc- 
 tion are very difficult to prepare because of the tendency of the 
 section to break to pieces. 
 
 If a section that is beginning to crack (Fig. 11) is studied, the 
 crack is found to follow the line of the cementing substance running 
 around the rods. In some places a rod may be split through its 
 center, but most of the rods remain perfect, and the cementing sub- 
 stance breaks. In the same way a section cut in the direction of the 
 rods shows the crack following the lines of the cementing substance 
 (Fig. 12), here and there breaking across a few rods, and then fol- 
 
 FIG. 11. Transverse section of enamel rods. (About 80 X) 
 
 lowing the direction again; but the rods separate on the line of 
 union, not at the centers of the rods. This fact becomes fundamental 
 in the cutting of enamel and in the preparation of strong enamel 
 walls. 
 
 Relative Solubility of Enamel Rods and Cementing Substance. 
 If a thin section of enamel cut parallel with the direction of the 
 enamel rods is mounted in water and hydrochloric acid (2 per cent.) 
 is allowed to run under the cover-glass and the action observed, 
 it will be seen to attack the cementing substance more rapidly, 
 dissolving it out from between the enamel rods and attacking their 
 sides. If the action is stopped the ends of the rods will be seen pro- 
 jecting like the pickets of a fence, as shown in the photograph
 
 RELATIVE SOLUBILITY OF ENAMEL RODS 
 
 45 
 
 (Fig. 13). The more dilute the acid the greater will be the distance 
 to which the cementing substance is removed before the rods are 
 destroyed. 
 
 FIG. 12. Enamel showing direction of cleavage. (About 70 X) 
 
 Etching. If a section of enamel is ground at right angles to the 
 direction of the rods, mounted in glycerin and photographed, the 
 outline of the rods will be seen with difficulty (Fig. 14). The refract- 
 ing index of the two substances is so nearly the same that the section 
 seems of almost uniform transparency. The thinner the section, 
 
 FIG. 13. The effect of acid on a section of enamel. 
 
 the greater will be the difficulty of recognizing the rods. Oblique 
 illumination and the use of a small diaphragm will, however, resolve 
 them. If the section is washed and treated with 2 per cent, hydro-
 
 46 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 chloric acid for a few seconds, washed, and remounted in glycerin, 
 the rods are distinctly outlined (Fig. 15). The acid attacks the 
 cementing substance and the surface of the section is etched as if 
 an engraving tool had been run around the rods. The fine grooves 
 on the surface refract the light and outline the rods. The difference 
 in appearance in longitudinal sections, that is, sections parallel with 
 the direction of enamel rods, is quite as striking. For the study 
 
 FIG. 14. Enamel ground at right angles to the rods. Not treated with acid. 
 
 (About 500 X) 
 
 of enamel rod directions this etching is of the greatest importance. 
 Only one side of the section should be acted upon by the acid, and 
 the section should be mounted etched side up. If etched upon both 
 surfaces, the grooves in the lower surface cannot be in focus at the 
 same time as those of the upper surface and will blur the definition. 
 The difference in the solubility of the rods and cementing sub- 
 stance is beautifully illustrated in the effect of caries on the structure 
 of the enamel and caries of the enamel cannot be understood unless 
 these fundamental facts are remembered. The question, "What 
 causes the difference in solubility between the enamel rods and
 
 RELATIVE SOLUBILITY OF ENAMEL RODS 
 
 47 
 
 the cementing substance?" cannot be satisfactorily answered at 
 the present time. While both the rods and the cementing substance 
 are normally composed entirely of inorganic salts, there may be 
 different salts in the two substances, or the salts may be in different 
 physical condition. There is great need for careful work in this 
 field. Recent work has strongly emphasized the distinctness of 
 the two structural elements of the enamel. 
 
 FIG. 15. The same section as Fig. 14 after treatment with acid. (About 500 X) 
 
 First, the study of the beginnings of caries of the enamel, and 
 the effect of caries upon the structure of the enamel, brought out 
 the difference in solubility in acids and showed the extent of tissue 
 injury before a cavity is formed. Later, the study of hypoplasia 
 developed the fact that certain pathologic or abnormal conditions 
 may hinder or entirely prevent the formation of the rods while the 
 cementing substance is formed, and still more recently the investi- 
 gation of dystrophies of the enamel occurring in certain prescribed 
 localities, showed perfect rod formation and entire absence of the 
 cementing substance. These facts suggest the hypothesis that the
 
 48 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 enamel rods and the cementing substance have a different origin, 
 or are formed by different cells, and that pathological conditions 
 may prevent the formation of one and not the other. In view of 
 these factors it is very necessary that a new investigation of the 
 process of enamel formation be undertaken, as present knowledge 
 of the process does not explain such conditions. 
 
 Difference in Refracting Index between the Rods and the Cementing 
 Substance. The cementing substance is of slightly greater refract- 
 ing index than the substance of the rods. If it were not for this it 
 would be impossible to see the rods in unetched sections, either 
 longitudinal or transverse. The appearance of striation seen in 
 longitudinal sections is also dependent upon this difference in action 
 on transmitted light. 
 
 THE EFFECT OF CARIES ON THE STRUCTURE OF THE ENAMEL. 
 
 At this point the effect of caries on the structure of the enamel 
 should be studied as a demonstration of the difference in solubility 
 between the enamel rods and the interprismatic substance. 
 
 During the last ten years of his life the work of the late Dr. G. V. 
 Black was largely devoted to the study of the beginning of caries of 
 the enamel and the extent of tissue injury before an actual cavity is 
 produced. This has placed a tremendous emphasis upon the value for 
 the preservation of the teeth, of the treatment of caries in its early 
 rather than in its later stages. It is safe to say that if caries progresses 
 until a patient is aware of a cavity, the tooth has been injured more 
 than is necessary in the most radical treatment of the same cavity 
 in its beginning stages. One who has not studied carefully the 
 effect of caries on the structure of the enamel, so as to recognize 
 the extent of injury to the structure of the tissue by its appearance 
 to the naked eye, can never be considered fit to prepare cavities 
 as a treatment for the disease. The beginnings of caries must be 
 divided into two classes: (1) Those occurring in natural defects 
 of structure; (2) those beginning upon smooth surfaces. 
 
 Caries Beginning in Natural Defects of Structure. These are the 
 positions in which caries first appears and in which it presents the 
 greatest intensity, because they offer ideal conditions. Such open 
 grooves and imperfectly closed pits in the enamel as are illus- 
 trated in Chapter IX become filled with food debris, which 
 furnish ideal culture media for acid-forming bacteria. At the 
 opening of the defect the acid is washed away by the saliva as fast
 
 THE EFFECT OF CARIES ON THE ENAMEL 49 
 
 as it is formed, but at the bottom of the groove it is confined and 
 acts upon the enamel, dissolving out the cementing substance 
 from between the rods and following the rod direction toward the 
 dento-enamel junction. The form of the disintegrated tissue in 
 such positions is always that of a cone or wedge, with the apex 
 at the opening of the pit or groove and the base toward the dento- 
 enamel junction. The formation of acid in these positions is often 
 so rapid and the confinement so perfect that the carious process 
 here manifests its greatest intensity, the action often dissolving 
 the rods as well as the cementing substance and progressing across 
 the rods. But even when the action follows the rod direction, the 
 form will be broader toward the dentin, as the rods are inclined 
 
 FIG. 16. A split tooth, showing caries beginning in an occlusal groove. 
 
 toward the defect. Figs. 16 and 17 show split teeth illustrating 
 the disintegration of the enamel around occlusal defects. The 
 disintegration area appears white by reflected light because the 
 cementing substance has been removed from between the rods and 
 the resulting air spaces refract the light. As soon as this disinte- 
 gration reaches the dento-enamel junction, the acid formed passes 
 through the now porous enamel and acts much more rapidly upon 
 the dentin. Because of the branching of the dentinal tubules at 
 the dento-enamel junction, the action upon the dentin spreads 
 rapidly along this line. Soon some of the loosened rods between 
 the bottom of the defect and the dentin are either entirely dissolved 
 or displaced or dislodged, and the microorganisms are admitted 
 to the dentin. The decalcified dentin matrix becomes food material 
 4
 
 50 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 for the bacteria, and the space produced by the destruction of 
 tissue furnishes greater space for decomposing foodstuffs. The 
 acids formed attack the enamel from within outward, producing 
 
 FIG. 17. A split tooth, showing caries progressing in an occlusal groove. 
 
 what has been called backward or secondary decay of enamel. 
 At the mouth of the defect the acid is still washed away, and there 
 is little action upon the tissue. The condition progresses, there- 
 fore, until, as in Fig. 18, the entire occlusal enamel has been under- 
 mined, and all of the undermined area has been greatly weakened 
 
 FIG. 18. A split tooth, showing the undermining of the occlusal enamel by caries 
 spreading at the dento-enamel junction. 
 
 by the solution of the cementing substance from between the rods. 
 In ground sections of such areas as shown in Fig. 21 the disinte- 
 grated area appears dark by transmitted light. Fig. 19 shows
 
 THE EFFECT OF CARIES ON THE ENAMEL 
 
 51 
 
 the progress of secondary decay from an occlusal cavity. In this 
 way it often happens that the entire occlusal enamel is destroyed 
 before the original defect is noticeably enlarged. 
 
 The general form of the disintegrated area in caries beginning 
 in natural defects may be described diagrammatically, as in the 
 enamel a cone or wedge with the apex toward the mouth of the 
 defect and the base toward the dento-enamel junction, and in the 
 
 FIG. 19. A section showing the undermining of the enamel and secondary or 
 backward decay at 1. 
 
 dentin a cone or wedge with the base at the dento-enamel junction 
 and the apex toward the pulp. 
 
 Caries Beginning on Smooth Surfaces. Caries upon smooth sur- 
 faces of the enamel is always due to the growth of a colony of 
 bacteria which becomes attached to the surface by the formation 
 of material, causing them to adhere to the surface and at the same 
 time confining their acid products in contact with the enamel
 
 52 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 preventing its dissipation in the saliva and allowing it to combine 
 with the inorganic salts of the tissue elements. This is not the 
 place to consider the bacteriology of caries, but the effect upon 
 the structure of the enamel cannot be understood without a clear 
 conception of the microbic plaques. A growth of masses of micro- 
 organisms upon the surface of a tooth does not constitute a plaque. 
 Many very filthy mouths are found where most of the surfaces of 
 the teeth are covered by thick, furry masses, and where there is 
 little or no attack of the enamel. Either acid is not formed or it 
 is at once lost by solution in the saliva. Caries shows the greatest 
 intensity in comparatively clean mouths, in which something in 
 the nature of the saliva causes the bacteria to produce a tough 
 zooglea, which attaches them to the tooth surface and confines 
 the products of their activity. This zooglea presents some of the 
 phenomena of a dialyzing membrane. Through it the micro- 
 organisms receive their food materials, and their products are 
 neutralized by chemical action on the surface upon which the 
 colony is growing. Colonies lodge in the most favorable spots 
 and extend from these points into areas that are less liable to main- 
 tain their attachment. The more perfect the confinement of the 
 acid, and the more rapid the rate of its formation, the greater will 
 be the intensity of the destructive process. The more easily the 
 colony is able to maintain itself in its position and extend upon 
 the surface, the greater is the liability. As the colony becomes 
 thickest at the point of beginning, it is evident that the most acid 
 is formed here, and it is therefore the point of greatest intensity. 
 It is also the point at which the growth began, and therefore 
 the spot where the action on the tissue has been longest in opera- 
 tion. It is also apparent that there may be great intensity with 
 limited liability, and great liability with very low intensity, and 
 the effect upon the tissue will be different in the two cases. 
 
 The appearance of the tissue becomes an index for estimating 
 the intensity and liability in a given case. The character of the 
 effect of the disease on the appearance of the enamel, as well as the 
 direction of the extension upon the surface of the tooth, become 
 most important factors in the diagnosis of any case, and the diag- 
 nosis is the basis for the treatment required. The increased appre- 
 ciation of the extent of disintegration of the enamel before an 
 actual cavity is apparent in a tooth has been one of the most 
 important results of Dr. Black's study of caries of the enamel. 
 The author has been intimately associated with this work, and has
 
 THE EFFECT OF CARIES ON THE ENAMEL 
 
 53 
 
 been amazed at the extent and character of the effect of caries 
 upon the structure of the enamel in what may be called the early 
 stages of the disease. 
 
 Progress of Caries. A colony of bacteria becomes attached 
 to the proximal surface of an incisor just to the gingival of the 
 contact point, and remains there some time. If the surface of 
 the tooth can then be examined, a white spot will be seen at Fig. 
 21; the area appears white because the cementing substance has 
 been removed from between the enamel rods, as will be seen later, 
 and the air that occupies the spaces diffuses the light. If a tooth 
 is split through such a spot and viewed from the surface, the appear- 
 ance will be as shown in Fig. 20. If a section were ground through 
 the spot and the tissue preserved, the ends of the enamel rods 
 
 FIG. 20 
 
 FIG. 21 
 
 21. 
 
 FIG. 20. A split tooth cut through 
 a white spot as is shown in Fig. 
 
 FIG. 21. A superior central incisor, 
 showing a white spot just to the gingival 
 of the contact point. 
 
 would be seen pointed and projecting like the pickets of a fence, 
 giving the same appearance as that produced by the action of 
 acid upon a ground section, as illustrated in Fig. 15. 
 
 The surface of the enamel is therefore no longer smooth, but 
 roughened. The roughness may often be felt by passing a very 
 fine-pointed steel explorer over the surface. If the colony be 
 dislodged at this stage it is evident that it is much easier for a new 
 one to become attached. These whitened areas are often invisible 
 unless the tissue is dried, because the saliva fills the spaces. If the 
 surface is dried the refraction of the light by the air whitens the 
 affected area. 
 
 A good comparison is furnished in a very familiar phenomenon. 
 Snow is white because the air and the microscopic ice crystals
 
 54 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 are of different refracting index, and the light is diffused by passing 
 from air and ice crystals. If a snowball is saturated with water 
 it loses its whiteness and becomes translucent, because the water, 
 which is nearly of the same refracting index as ice, fills the spaces 
 between the ice crystals, and the light is not diffused. If the white 
 area of such a tooth is split through the center with an aluminum 
 disk charged with emery powder, the enamel rods will be found 
 
 FIG. 22. A thin section of carious enamel ground on the cover-glass with balsam: 
 E, sound enamel; X, carious enamel in which the cementing substance had been 
 dissolved from between the rods. 
 
 entirely separated by the solution of the cementing substance, 
 and the cross-striation will be much more apparent because the 
 unevenness in the diameter of the rods has been increased by the 
 action of the acid. 
 
 Formerly it was impossible to grind a section through such a 
 spot and preserve the tissue. Until methods were devised by Dr. 
 Black, it was impossible to preserve the tissue and examine its
 
 THE EFFECT OF CARIES ON THE ENAMEL 
 
 55 
 
 condition. These methods demonstrate definitely that in the dis- 
 integrated area the cementing substance is dissolved in large areas 
 before any of the rods are dissolved or destroyed. The first sections 
 
 FIG. 23. Carious enamel ground on the cover-glass by the shellac method. In 
 the region X the cementing substance dissolved from between the rods has been 
 replaced by shellac. 
 
 of such areas were obtained by polishing the surfaces and cementing 
 the split tooth to the cover-glass with balsam, completing the grind- 
 ing and mounting without loosening the section. In this way the
 
 56 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 spaces between the rods were filled with balsam and so were held in 
 place. Fig. 22 shows a photograph of a section made in this way, and 
 the spaces between the rods and the distinct cross-striation are seen. 
 Later it was found that by dehydrating and immersing in a solu- 
 tion of brown shellac, the shellac could be made to take the place 
 of the lost cementing substance, then the polished surface of the 
 sawed-out section could be fastened to the cover-glass with shellac, 
 and the specimen handled more easily. Fig. 23 shows a photo- 
 graph of carious enamel made in this way. The rods are preserved 
 in place and the dark shellac marks the disintegrated area very 
 clearly. 
 
 Stages in the Progress of Caries. The progress of caries on smooth 
 surfaces of the enamel may be divided into three periods, according 
 to its effect upon the structure of the tissue. 
 
 1. From the lodgment of the colony until the action reaches 
 the dento-enamel junction. 
 
 2. From the reaching of the dento-enamel junction until the 
 rods begin to fall out. 
 
 3. After a cavity is produced. 
 
 First Period. The form of the disintegrated tissue in the first 
 period is always that of an irregular cone. Its base is on the sur- 
 face of the enamel, its outline is the boundary of the colony, and 
 the apex is toward the dentin in the direction of the enamel rods 
 from the starting-point of the colony. The inner boundary of the 
 area is never even, but shows flame-like extensions toward the 
 dentin in the direction of the rods. This is more marked in some 
 cases than in others, and sometimes suggests that the presence of 
 a colony on the surface has been intermittent (Plates IV, V, VI). 
 
 The boundary between the perfect and the disintegrated area 
 is usually marked by a darker area, the significance of which is 
 not now understood. If the disease progresses continuously the 
 affected tissue always appears white by reflected light, but if the 
 progress has been intermittent, especially if there have been con- 
 siderable periods in which no colony has been attached to the sur- 
 face, the area darkens, becoming brownish or almost black. This 
 is produced by organic materials filling the space between the 
 enamel rods and decomposing, with the probable formation of sul- 
 phides of dark color in the spaces. If immunity to caries is attained 
 before the effect upon the tissue has penetrated to the dento-enamel 
 junction, this will occur, and the spot changes from a white to a 
 brownish or black color. Such spots will be found in some places
 
 PLATE IV 
 
 A Section through a Carious Spot in the First Period. 
 
 Showing extension of the attack on the surface toward the gimnval.
 
 PLATE VI 
 
 A Section through a Carious Spot in the Second Period. 
 
 X, disintegrated enamel at the point of first lodgment of the colony; 
 Z, disintegrated enamel as the result of the extension of the colony on the 
 surface toward the occlusal; E, sound enamel; D, clentin.
 
 THE EFFECT OF CARIES ON THE ENAMEL 
 
 57 
 
 on most teeth extracted from immune persons. Work of Dr. Miller 
 has indicated that such spots are more resistant to the progress 
 
 FIG. 24. A section through a white spot in the first period of attack: X, disinte- 
 grated enamel; E sound enamel; D, dentin.
 
 58 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 of caries than perfect enamel surfaces. At any time during the 
 first period, therefore, the destruction may be arrested by the com- 
 ing of immunity, which prevents the attachment of colonies to 
 the tooth surface by the formation of plaques. 
 
 FIG. 25. A section through a carious spot in the first period. The attack has 
 apparently been slow and intermittent: X, disintegrated enamel; E, sound enamel; 
 D, dentin. 
 
 Second Period. This period extends from the time when the 
 action of the acid reaches the dento-enamel junction until the 
 rods are destroyed or fall out. As soon as the solution of the 
 cementing substance reaches the dento-enamel junction at the 
 point of the advancing cone, the solution of the inorganic salts
 
 THE EFFECT OF CARIES ON THE ENAMEL 59 
 
 from the dentin matrix begins. It must be remembered that 
 the acid is formed by the microorganisms on the surface of the 
 
 FIG. 26. A section through a carious spot in the first period, showing the flame- 
 like projections toward the dentin: A', disintegrated enamel; E, sound enamel; 
 D, dentin.
 
 60 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 enamel, and filters through the spaces between the enamel rods. 
 The decalcification of the dentin may be considerable, while the 
 surface of the enamel is still preserved. In this period the swelling 
 of the surface is always noticeable. This results in increasing the 
 area of the contact and therefore allowing the colony to extend 
 its limits, increasing the extent of the surface attack. This is 
 especially noticeable toward the gingival, and is shown in Plate IV, 
 which is, however, shown in the first period of caries. In the 
 disintegrated area in this stage, as w T ell as in the first stage, the 
 diameter of the enamel rods is always considerably reduced and the 
 striation rendered more apparent. In caries of great intensity 
 
 FIG. 27. A tooth split through a spot, FIG. 28. A tooth split through 
 
 showing great intensity but low lia- spots, showing low intensity but 
 bility. great liability. 
 
 but low liability the reduction in the diameter of the enamel rods 
 is rapid, and they are soon destroyed, while the area of the surface 
 attacked is small (Fig. 27). 
 
 In caries of low intensity but great liability the diameter of the 
 rods is slowly reduced, while the area of surface attacked, and 
 consequently the area of disintegration, is large (Fig. 28). These 
 conditions should be studied in the macroscopic appearance of 
 caries at the chair. 
 
 The decalcified dentin matrix shrinks and more or less of a 
 space is formed under the enamel.
 
 THE EFFECT OF CARIES ON THE ENAMEL 
 
 61 
 
 The action of the acid follows the tubules of the dentin toward 
 the pulp, and spreads through their branches laterally near the 
 dento-enamel junction so that the form of the disintegrated dentin 
 is always that of a cone, with the base at the dento-enamel junction 
 and the apex toward the pulp chamber. It is important, however, 
 to remember that in this stage no microorganisms have entered 
 the tissue, and the effect upon it is the result of the action of sub- 
 stances formed upon the surface. The extent of enamel disintegra- 
 tion and decalcification of dentin, in this stage, is much greater 
 than anyone supposed before such specimens as the present illus- 
 trations were made. 
 
 FIG. 29. A drawing showing the microorganisms of caries growing through the 
 dentinal tubules. (G. V. Black.) 
 
 Third Period. This embraces the period after the enamel rods 
 have begun to fall out and an actual cavity is apparent. As soon 
 as this occurs the surface of the tooth at the point where the forma- 
 tion of the colony began is destroyed and the protected point is 
 lost, and the extension of surface attack ceases. The microorgan- 
 isms are admitted to the dentin, where they grow through the 
 dentinal tubules, spreading rapidly at the dento-enamel junction 
 (Fig. 29). The dentin is always decalcified in advance of the 
 penetration of the microorganisms. The acid formed within the 
 cavity attacks the cementing substance between the enamel rods,
 
 62 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 and proceeds from the dento-enamel junction outward. This 
 is called secondary or backward decay of the enamel, and as a 
 result of it, large areas are disintegrated until they are sufficiently 
 weakened to break into the cavity. This condition is shown in 
 Fig. 19, in which the area indicated by 1 has had the cementing 
 substance entirely removed from between the rods, and is in the 
 same structural condition as the disintegrated areas in the first 
 and second stage. It is safe to say that in the past few cavities 
 have been filled until the enamel has caved in. It is equally certain 
 that in a large proportion of cases, by the time this has happened, 
 the removal of all disintegrated tissue will require a greater loss 
 of tooth substance than would be required for the prevention of 
 a new surface attack, at the margin of the filling, if the case had 
 been treated as a beginning instead of a burrowing decay.
 
 CHAPTER V. 
 CHARACTERISTICS OF THE ENAMEL TISSUE. 
 
 FROM what has been said of the structural elements of the tissue, 
 their physical and chemical properties, and their arrangement 
 in the tissue, it is apparent that the striking characteristics of the 
 enamel are the result of these factors; and that it can be intelli- 
 gently dealt with only by thinking of it always in these terms. 
 
 FIG. 30. Enamel showing cleavage. 
 
 The enamel may be crudely compared to a pavement made 
 up of tall columns closely cemented together by an inorganic 
 cement. The wear comes on the ends of the columns, and they 
 furnish great resistance to the abrasion of friction. When sup- 
 ported upon a good and elastic foundation it is very difficult to 
 break it down, but when an opening has been made in it, and the 
 foundation removed from underneath, the columns are com- 
 paratively easily split off and tumbled into the opening (Fig. 30). 
 This figure is crude, but it is a very helpful one in learning to think 
 of the enamel in terms of its structural elements. 
 
 (63)
 
 64 
 
 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 Straight Enamel. Upon the axial surfaces of the teeth the rods 
 are usually straight and parallel with each other, and most of 
 split extend from the dentin to the surface. Such enamel will 
 split or cleave in the direction of the rods with comparative ease, 
 and breaks down very readily when the dentin is removed from 
 
 under it. It will usually cleave through 
 its entire thickness and break away from 
 sound dentin when properly attacked 
 with sharp hand instruments. Such 
 enamel is called straight enamel, as 
 contrasted with gnarled enamel. It is 
 best illustrated by cutting sections 
 labiolingually through the incisors, 
 though there is considerable variation 
 in different teeth (Figs. 12 and 31). 
 
 Gnarled Enamel. Upon the occlusal 
 surfaces of the molars and bicuspids, 
 and especially over the tips of cusps 
 and marginal ridges, the rods are seldom 
 straight and parallel through the thick- 
 ness of the enamel, but are wound and 
 twisted about each other, especially in 
 the deeper half toward the dento-enamel 
 junction. This is known as gnarled 
 enamel, and its appearance is in marked 
 contrast with straight enamel. 
 
 Toward the surface the rods are 
 usually straight and parallel for a 
 longer or shorter distance, but as the 
 dento-enamel junction is approached 
 they become twisted. This is true of 
 most of the occlusal surfaces of molars 
 and bicuspids, but the gnarled condition 
 extends farther toward the surface 
 over the tips of the cusps, or the point 
 at which the rods were first completed 
 
 in the growth of the crown. As cleavage is caused by the difference 
 in the strength of the rods and cementing substance, it is easy to see 
 that gnarled enamel will not split or cleave easily when resting upon 
 sound dentin. This is often encountered in extending occlusal 
 cavities. The straight portion will split, but where the rods begin to 
 
 FIG. 31. Straight enamel 
 rods.
 
 EFFECT OF STRUCTURE ON THE CUTTING OF ENAMEL 65 
 
 twist they break off, leaving a portion resting on the dentin which 
 will resist the attack of any cutting instrument from the surface 
 (Figs. 32, 33, and 34). 
 
 FIG. 32. Gnarled enamel. (About 80 X) 
 
 The Effect of Structure on the Cutting of Enamel. The two kinds 
 of enamel may be compared to straight-grained pine wood and a 
 5
 
 GG 
 
 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 pine knot. The first will split easily in the direction of the fiber, 
 the latter will split only in an irregular way and with the greatest 
 difficulty. This difference 
 in the arrangement of the 
 structural elements leads to 
 the difference in the feeling 
 of various teeth to cutting 
 instruments, and is the 
 basis for the clinical ex- 
 perience of hard and soft 
 teeth. It is not a matter of 
 degree of calcification, but 
 
 FIG. 33. Gnarled enamel. 
 
 FIG. 34. Gnarled enamel from etched section 
 (About 100 X) 
 
 the arrangement of the structural elements, and gnarled enamel 
 will break down as rapidly under the effect of caries as will 
 straight enamel.
 
 APPEARANCES CHARACTERISTIC OF ENAMEL 67 
 
 From a study of the positions in which the rods are usually 
 twisted about each other, and those in which they are usually 
 straight, it seems probable that the twisting is due to movements 
 in the dental papilla and the enamel organ during the formation 
 of the tissue. These movements may be produced by variations 
 in the blood-pressure which cause oscillations, or shiftings of the 
 tissues on each other. These differences in the arrangement of 
 the structural elements of the enamel must be constantly kept in 
 mind, and will be referred to many times in connection with the 
 use of cutting instruments on the enamel and the preparation of 
 cavity w y alls. 
 
 APPEARANCES CHARACTERISTIC OF ENAMEL. 
 
 Striation. Striation is the appearance of fine light and dark 
 markings occurring alternately in the length of the enamel rods. 
 This is not unlike the striation of voluntary muscle fibers, and has 
 a similar cause. It is seen both in thin sections cut in the direction 
 of the rods, and in isolated enamel rods. It is caused by the alter- 
 nate expansions and constrictions of the rods and the difference 
 in the refracting index between the rods and the cementing sub- 
 stance. 
 
 If isolated rods (Fig. 35) are observed with a | or yV objective, 
 they will be seen to be marked by alternate light and dark areas 
 across the rods; on changing the focus up and down, the light 
 and dark areas will change places, just as in looking at a red blood 
 corpuscle the center may appear dark and the rim light, or the 
 center light and the rim dark, depending upon the exactness of 
 focus. This is caused by the refraction of the light as it passes 
 through the convex and concave portions of the rod. If the 
 cementing substance were of exactly the same refracting index 
 as the rods, when the rods were fastened together in the tissue 
 there would be no appearance of striation, but as it is not, refrac- 
 tion of light occurs in passing from rod substance to cementing 
 substance, and the striation is apparent in sections. There is con- 
 siderable difference in the distinctness of striation in different 
 sections of enamel. This is probably due to the fact that the 
 cementing substance has more nearly the same refracting index 
 as the rods in some specimens. "When the formation of enamel 
 has been studied it will be^found^that^the enamel rods have been 
 formed by globules which are deposited one on top of the other
 
 68 
 
 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 to form the rods, and the cementing substance fills up the space. 
 The globules in the adjacent rods come opposite each other, so 
 that there is alternately a greater and a less amount of cementing 
 substance between the rods. Each cross-mark therefore repre- 
 sents a globule deposited in the formation of the rod, and striation 
 may be said to be a record of the growth of the individual rods 
 (Figs. 36 and 37). 
 
 Imperfections in the cementing substance render the striation 
 more apparent because they increase the difference in refraction 
 
 FIG. 35. Isolated enamel rods. (About 1000 X) 
 
 between the two substances. The action of acid either upon 
 isolated rods or upon sections renders striation more apparent 
 because it attacks the cementing substance faster than the globules 
 forming the rods, and therefore increases the refraction. Von 
 Beber has claimed that the appearance of striation was caused 
 by the action of acid on the section, and that even in mounting 
 in balsam the acidity of the balsam affected the tissue. It is true 
 that any action of acid increases the distinctness of the cross- 
 striation, but it is not the cause of it. 
 
 Stratification, or the Bands of Retzius. If longitudinal sections 
 of moderate thickness are observed with the low power, brownish 
 bands are seen running through the enamel, which suggests the
 
 APPEARANCES CHARACTERISTIC OF ENAMEL 69 
 
 appearance of stratification in rocks. These were first described 
 by Retzius and were named after him the brown bands or striae 
 of Retzius. A better name would be incremental lines. 
 
 FIG. 36. Enamel showing both striation and stratification. (About 80 X) 
 
 FIG. 37. Enamel showing striation. (About 1000 X)
 
 70 
 
 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 The bands of Retzius, or incremental lines, are caused by actual 
 coloring matter which is deposited with the inorganic salts in the 
 
 FIG. 38. -Tip of an incisor. (About 50 X) 
 
 formation of the tissue. They are therefore best seen with low 
 powers and in sections that are not too thin. In sections that are 
 thinner than the diameter of a single rod, or less than four microns,
 
 APPEARANCES CHARACTERISTIC OF ENAMEL 
 
 71 
 
 they become almost invisible. For the study of the bands of Retzius 
 sections should be ground labiolingually through the incisors, 
 buccolingually through the bicuspids and molars, striking the 
 center of the cusps. They may be studied also in mesiodistal 
 sections, but the sections should be in such a direction as to be at 
 
 FIG. 39. Incisor tip showing stratification or incremental lines. Rods at A were 
 fully formed at the time the rods at B were beginning to form. (About 50 X) 
 
 right angles to the zones. Fig. 38 shows the tip of an incisor in 
 which the bands are very well marked. They are seen to begin at 
 the dento-enamel junction on the incisal edge, and sweep in larger 
 and larger zones around this point. Each band represents what 
 was at one time the surface of the enamel already formed, and the 
 line upon which formation was progressing. They are therefore
 
 72 
 
 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 truly incremental lines. The zones reach the surface of the enamel 
 first at the point over the center of beginning calcification, and 
 the succeeding bands extend from the surface of the enamel, near 
 the occlusal, to the dento-enamel junction much farther apically, 
 
 FIG. 40. Stratification of enamel; the cusp of a bicuspid: De, dento-enamel 
 junction; Ed, enamel defect showing in the heavy stratification band; Ig, inter- 
 globular spaces in the dentin. (About 40 X) 
 
 and corresponding lines are seen on opposite sides of the section. 
 In Fig. 39, the band which is at the surface at A and A' reaches the 
 dento-enamel junction at B and B'. This means that when the 
 enamel rods which form the surface at A were completed, the rods
 
 APPEARANCES CHARACTERISTIC OF ENAMEL 
 
 73 
 
 at B were just beginning to be formed at the dento-enamel junc- 
 tion. A layer of functioning ameloblasts occupied this position. 
 The bands of Retzius are always curved and usually pass obliquely 
 across the enamel rods, but are parallel neither with the dento- 
 enamel junction or the surface of the enamel. As they pass toward 
 the gingival the angle which they form with the axis of the tooth 
 becomes greater. Any disturbance of nutrition which affects the 
 formation of enamel is always shown in the increased distinctness 
 of the bands (Fig. 40). 
 
 The bands of Retzius therefore form a record of the formation 
 of the tissue, and by their study the points of beginning calcifi- 
 
 FIG. 41. Lines of Schreger. (About 5 X) 
 
 cation and the manner of the development of the tooth crown may 
 be followed. This will be considered again in connection with the 
 grooves, pits, and natural defects of enamel. 
 
 Lines of Schreger. These are lines appearing in the enamel 
 extending from the dento-enamel junction to or toward the sur- 
 face. They are caused by the direction in w r hich the enamel rods 
 are cut. They may be seen in sections, but are best shown by 
 photographing the cut surface of the enamel by reflected light and 
 with very low magnification. The rods are twisting about each 
 other, and in one streak they are cut longitudinally, in the next 
 obliquely, and the alternations of these directions cause the appear- 
 ance of the lines (Fig. 41).
 
 74 
 
 Nasmyth's Membrane (The Enamel Cuticle.) There has been a 
 vast amount of writing and fruitless speculation in regard to this 
 structure. The facts which have led to all this speculation can be 
 simply stated. If a freshly extracted tooth, that has not been 
 exposed to wear, is decalcified or treated with dilute nitric acid 
 
 (other acids may be used) a membrane can be floated from its 
 surface which is found to be made up of two layers. (1) A clear 
 structureless layer which was in contact with the surface of the 
 enamel and bears the imprints of the ends of the enamel rods on its 
 surface. (2) An outer cellular layer made up of a layer or layers of 
 epithelial cells. Unfortunately the study of Nasmyth's membrane 
 seems to have been made from extracted teeth and not from sections 
 which retained the teeth and all of the supporting tissues in relation. 
 Two distinct explanations have been given to this structure:
 
 75 
 
 (1) Owen and Tomes considered it as not epithelial but a deposit 
 of coronal cementum on the surface of the enamel before the erup- 
 tion of the tooth as occurs in the teeth of ungulates. (2) Huxley, 
 Lent, Kolliker, Waldyer, Paul, Mummery and others have recog- 
 nized its epithelial origin and described its structure in detail. All 
 
 FIG. 43 
 
 have considered it as in some way related in origin to the enamel 
 organ but as to the way in which it is formed or the nature of the 
 relationship there is no agreement. 
 
 In the opinion of the writer, coronal cementum occurs on the 
 enamel surface and in the grooves of the crowns of many human
 
 76 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 teeth but is in no way related to the structure described as Nas- 
 myth's Membrane. On the other hand, while the structure is 
 undoubtedly of epithelial character, he does not believe that it is 
 related to the enamel organ or the formative epithelium of the 
 enamel in origin. 
 
 On the eruption of the tooth the epithelium of the gingival fold, 
 at least on the deeper portions is held firmly against the surface of 
 the enamel by the pressure of the surrounding tissues, and the 
 surface cells are quite firmly adherent to the surface of the enamel. 
 The multiplication of epithelial cells in the deep portion of the 
 gingival fold causes the epithelium to be pushed outward along the 
 surface of the enamel and the layer separated from the surface of 
 the enamel by the action of acid is this layer which has been sepa- 
 rated from the epithelium lining the gingival space. In this con- 
 nection decalcified sections with all of the tissues in relation should 
 be studied and attention is called to the comparison of the structure 
 of the gingival fold of the tooth and the nail fold of the finger nail. 
 
 Nasmyth's membrane undoubtedly has some important relations 
 to normal and pathologic conditions, especially those beginning 
 in the gingival space. 
 
 Enamel Spindles. 1 Especially in the region of the cusps and the 
 points where enamel formation begins in the calcification of the 
 tooth peculiar spindle-like spaces are seen extending from the dento- 
 enamel junction into the enamel. These have often been described, 
 and much has been written in regard to them, but there is no agree- 
 ment among investigators as to their cause or significance. They 
 are apparently spaces in the interprismatic substance and between 
 the enamel rods. They appear to communicate with dentinal 
 tubules. In some cases at least, they appear to be filled with granu- 
 lar material. They are easily demonstrated, but not so easily 
 explained. 
 
 1 For further discussion of these structures the student is referred to Microscopic 
 Anatomy of the Teeth by Mummery, p. 78 el seq.
 
 CHAPTER VI. 
 
 THE DIRECTION OF THE ENAMEL RODS IN 
 TOOTH CROWN. 
 
 THE 
 
 IN describing the direction of the enamel rods and their arrange- 
 ment in what may be called the architecture of the tooth crown, 
 they are always considered as extending from the dento-enamel 
 junction outward. This is not only convenient, but logical, as 
 they are formed in that way, beginning at the dento-enamel junc- 
 tion and being completed at the surface. Enamel is formed from 
 within outward, the cells which produce it lying outside of the 
 tissue already formed, and there are many things about the arrange- 
 ment of the rods and their relation to each other that are understood 
 only when this is borne in mind. 
 
 The direction of the enamel rods is described by referring them 
 to the horizontal and axial planes, which have been previously 
 defined (page 35). The centigrade scale, that is, the division of 
 the circle into one hundred equal arcs, is used because those familiar 
 with instrument nomenclature are already familiar with these 
 angles, and readily picture them. 1 When a rod is said to be inclined 
 12 centigrades occlusally from the horizontal plane, it means that 
 if a plane at right angles to the long axis of the tooth is passed 
 through the end of the rod at the dento-enamel junction, the rod 
 
 1 In the centigrade division the 
 circle is divided into one hundred 
 parts, each called a centigrade. One 
 centigrade is equal to 3.6 degrees of 
 the astronomical circle, 25 centi- 
 grades to 90 degrees, 12i centigrades 
 to 45 degrees. The cut gives a com- 
 parison of the two systems of measur- 
 ing angles. 
 
 270 
 
 180 
 Centigrade division. 
 
 (77)
 
 78 DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 will lie to the occlusal of it and form an angle of 12 centigrades 
 with it. In the same way, if a rod is said to be inclined 12 centi- 
 grades buccally from the mesiodistal plane, it means that if a plane 
 parallel with the axis of the tooth, and extending from mesio to 
 distal, is passed through the end of a rod at the dento-enamel 
 junction, the rod will lie to the buccal of it, and form an angle of 
 12 centigrades with it. By a little practice with these terms the 
 direction of the enamel rods can be very easily and clearly pictured 
 to the mind. 
 
 The General Direction of Enamel Rods. The general direction of 
 the enamel rods has been variously described by different authors, 
 but all of these general statements are very imperfect and often 
 misleading. For instance, they are sometimes said to radiate from 
 the center of the crown or the pulp chamber, but it will be seen 
 that this does not apply to the rods which form the lingual slopes 
 of the buccal cusps, or the buccal slopes of the lingual cusps of 
 bicuspids and molars. 
 
 Again, they have been said to be, in general, perpendicular to 
 the surface, but it will be found from the study of sections that 
 there are very few places upon the surface where this is true, and 
 that in many places they are far from perpendicular to the surface. 
 From a study of sections it will be seen that the general arrange- 
 ment of enamel rods, in the architecture of the tooth crown is such 
 as to give the greatest strength to the perfect tissue, and to furnish 
 the greatest resistance to abrasion in the use of the teeth for mas- 
 tication. In a buccolingual section through a bicuspid (Fig. 44), 
 beginning at the gingival line, the enamel is normally slightly over- 
 lapped by the cementum, and in the gingival third the rods are 
 inclined more or less apically from the horizontal plane. The 
 degree of inclination varies considerably. It may be as much 
 as 12 centigrades, but is usually not more than 6. In general, 
 the more convex the surface, the greater will be the inclination. 
 At some point between the junction of the gingival and middle 
 thirds and the middle of the middle third of the surface they are 
 in the horizontal plane and at right angles to the axis of the tooth, 
 and at this point they are usually very nearly perpendicular to 
 the surface. Passing occlusally from this point, they incline more 
 and more occlusally until in the occlusal third they reach an inclina- 
 tion of ] 8 to 20 centigrades occlusally from the horizontal. 
 
 The rods which form the tip of the buccal cusps do not reach 
 the tip of the dentin cusp, but the buccal slope of the dentin. This
 
 THE GENERAL DIRECTION OF ENAMEL RODS 79 
 
 FIG. 44. Diagram of enamel rod directions, from a photograph of a buccolingunl 
 section of an upper bicuspid. 
 
 FIG. 45. Diagram of enamel rod directions, drawn from a mesiodistal section of a 
 
 bicuspid.
 
 80 
 
 DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 becomes important, as will be seen later. Over the tip of the 
 dentin cusp the rods are in the axial plane, but in this position they 
 are usually very much twisted. Passing down the lingual slope, 
 they become more and more inclined lingual ly from the mesio- 
 
 FIG. 46. Disturbance of enamel rod directions on labial surface of a cuspid. 
 
 (About 80 X)
 
 THE GENERAL DIRECTION OF ENAMEL RODS 81 
 
 distal axial plane, and the degree of inclination is related to the 
 height of the cusp the taller the cusp, the greater the inclination. 
 At the developmental groove or pit they meet the rods of the 
 lingual cups, which are inclined in the opposite direction. 
 
 FIG. 47 Disturbance of enamel rod directions on lingual surface of same tooth as 
 Fig. 48. (About 80 X) 
 
 6
 
 82 DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 In a mesiodistal section (Fig. 45) the plan of arrangement will 
 be seen to be the same, the tip of the marginal ridge corresponding 
 to the tip of the cusp. In an incisor the arrangement is similar, 
 the lingual marginal ridge corresponding to a rudimentary cusp. 
 This general plan should be studied in several sections of the various 
 classes of teeth before the rod direction is studied more minutely. 
 
 Effect of Hypoplasia. Whenever a hypoplasia groove appears 
 upon the surface, the rod directions will be found to be more or 
 less disturbed. Fig. 46 show r s a position on the labial surface of a 
 cuspid. In this position the disturbance of the enamel rod direc- 
 tion is very marked. The rods tend to be in whorls and the struct- 
 ure is more or less deficient. On the lingual side of the same sec- 
 tion (Fig. 47) the disturbance in structure is so great that it is 
 difficult to make out the rod direction. Many such areas will be 
 found in sections. Some condition which has affected the nutri- 
 tion of the enamel-forming cells results in a local disturbance of 
 the structural elements. 
 
 SPECIAL AREAS. 
 
 The Gingival Third. There is much variation in enamel rod 
 direction in different teeth as the gingival line is approached. 
 The inclination apically from the horizontal may be very great, 
 as much as 12 to 15 centigrades in some instances, as in Fig. 48, 
 but this is exceptional. It may be very slight, or the rods may 
 be almost in the horizontal plane. The direction of the rods in 
 these areas become very important in the preparation of the gin- 
 gival wall of proximal cavities, and cavities in the gingival third 
 of buccal and labial surfaces. 
 
 The Tips of the Cusps. In studying the rod directions in the 
 region of the cusps and marginal ridges, it must be borne in mind 
 that the formation of enamel begins at the dento-enamel junction, 
 at separate points, and that the growth is recorded in the tissue 
 by the bands of Retzius, each band having been at one time the 
 surface of the enamel cap then formed. In a buccolingual section 
 the formation of the buccal and lingual cusps will be shown 
 (Chapter IX). While the little caps are growing they are being 
 carried apart by the growth of the dental papilla and enamel organ, 
 until the calcifications unite at the dento-enamel junction. When 
 this occurs the dental papilla has reached its maximum mesiodistal 
 diameter. The enamel organ, however, will continue to grow, and
 
 SPECIAL AREAS 
 
 83 
 
 as the rods are completed first just over the tip of the dentin cusp, 
 the continued growth causes an increase in the inclination of the 
 
 FIG 48. Direction of enamel rods in the gingival third. 
 
 rods in their outer portion. This often leads to a curving of the 
 rods at their outer portion.
 
 CHAPTER VII. 
 
 THE RELATION OF THE STRUCTURE TO THE 
 CUTTING OF THE ENAMEL. 
 
 THERE are two methods of cutting enamel to chop or cleave it, 
 and to shave or plane it. 
 
 Cleaving or Chopping Enamel. In the cleavage of the enamel 
 the action of the instrument more nearly resembles that of splitting 
 ice than that of splitting wood. The ax for splitting wood is strongly 
 wedge-shaped, and the wedge pries the fibers apart. In splitting 
 ice a small nick is made on the surface and then a sharp blow cracks 
 the ice in the direction of the cleavage. In a similar way the chisel 
 applied to the surface of the enamel makes a slight scratch or 
 bearing on the surface, and the force applied at a slight angle to 
 the direction of the rods cracks the tissue through in the rod direc- 
 tion. The bevel of the instrument is designed to give strength 
 and keenness of edge, not to act as a wedge. In order to cleave 
 the enamel it is always necessary that there be a break or opening 
 in the tissue, and usually that the dentin be removed from under it. 
 Only a small portion can be split off at a time. The edge of the 
 chisel should be placed on the enamel a quarter or half a milli- 
 meter from the opening, rarely more, and so piece after piece is 
 split into the cavity. Fig. 49 shows a section of enamel. The 
 edge of the chisel is placed at 1, with the shaft in the relation to 
 enamel rod direction indicated; a tap of a steel mallet will split 
 off a piece, and the chisel is moved back to position 2 and a second 
 piece is split off. L T ndermined enamel will split easily in this way. 
 As soon as a point is reached where the enamel rests on sound 
 dentin, it is recognized by the resistance. Straight enamel can be 
 split off from sound dentin without difficulty if attacked in the 
 proper way, but if the inner portion is gnarled and twisted, it can 
 only be cleaved by removing the dentin from under it. Such enamel, 
 if resting on dentin, will split as far as the rods are straight; but 
 where they begin to twist they will break off, leaving a portion 
 which is very difficult to remove by attacking it from the surface. 
 If the dentin is removed from under gnarled enamel, it will crack 
 (84)
 
 CLEAVING OR CHOPPING ENAMEL 
 
 85 
 
 through in an irregular way, following the general direction of the 
 rods. 
 
 In preparing teeth for crowns it is often necessary to remove 
 a large amount of enamel. This is always more efficiently accom- 
 plished by the intelligent use of sharp instruments than by force 
 alone. The enamel on axial surfaces, especially in the gingival 
 
 FIG. 49. Position of chisel in cleaving enamel. 
 
 half of the crown, is usually straight, and if a cleavage line can 
 once be established, the enamel can be more easily and rapidly 
 removed by splitting it off piece after piece than in any other way. 
 In doing this a straight or contra-angled chisel is often the most 
 efficient instrument, and it must be remembered that the "root 
 trimmers" are more properly called " enamel cleavers," and that
 
 86 RELATION OF STRUCTURE TO CUTTING OF ENAMEL 
 
 they are used to cleave the enamel, not to scrape or hoe it off, their 
 form being adapted to give a strong palm grasp of the instrument. 
 
 Fig. 50 illustrates the use of the enamel cleaver for the removal 
 of gingival enamel from an axial surface. The line of cleavage 
 being established, the edge of the instrument is placed on the 
 
 FIG. 50. The use of enamel cleaver in removing enamel. 
 
 enamel half a millimeter from the broken edge, and the force 
 which should be strong, quick, and sharp, is applied in the direc- 
 tion indicated, and piece after piece is split off, progressing from 
 the occlusal toward the gingival. In preparing the wall of a cavity 
 the outline form should be attained by cleavage, and this is the 
 first step in the preparation of the cavity. 
 
 After the enamel has been removed by cleavage to the point
 
 SHARP INSTRUMENTS 
 
 87 
 
 where the margin is to be laid, the wall must be completed by 
 cutting the enamel in an entirely different way. 
 
 Planing or Shaving Enamel. In this manner of cutting enamel 
 the tissue is removed without reference to the rod direction, and 
 without injury to its structure (Figs. 51, 52, and 53). The chisel 
 is used like the blade of a plane. The cutting edge is placed against 
 the surface with the shaft of the instrument almost parallel to it, 
 and the tissue is shaved away. In this way the rods that have 
 been cracked apart by the cleavage are removed, and the walls 
 arranged in terms of its structural elements so as to gain the 
 required strength of margin. 
 
 Sharp Instruments. Chisels and hatchets for use in cleaving or 
 planing enamel must be keenly sharp. If a dull edge is placed on 
 
 FIG. 51 
 
 FIG. 52 
 
 FIG. 53 
 
 FIGS. 51, 52, and 53. The use of the chisel in planing or shaving enamel. (Black.) 
 
 the surface of the enamel it will rest across the ends of many rods, 
 and force applied will only crumble them, but will not split the 
 tissue. The edge must be keen (Fig. 54), so as to engage between 
 the rods and so start the cleavage. Cutting instruments as fur- 
 nished by dental supply houses are not tempered hard enough to 
 hold an edge. There is no fault to be found with the supply houses 
 for this, for they make them as the dentist wants them, and any 
 dealer will furnish hard-tempered instruments if they are ordered. 
 To use hand instruments successfully in cutting enamel, the stock 
 instruments must either be retempered or they must be ordered 
 hard tempered. The cutting edge of the blade of an enamel instru- 
 ment should be straw-colored when tempered. 
 
 The chisel and hatchets are the instruments for removing enamel.
 
 88 RELATION OF STRUCTURE TO CUTTING OF ENAMEL 
 
 The burr is the instrument for removing hard dentin. When the 
 burr is used on enamel it should be remembered that it is used as 
 a revolving chisel. It is by the thoughtful use of hand instruments 
 
 FIG. 54. The relation of the edge of a sharp and a dull chisel. 
 FIG. 55 FIG. 56 FIG. 57 
 
 FIGS. 55, 56, and 57. The use of the chisel in cleaving enamel. Opening an occlusal 
 
 cavity. (Black.)
 
 SHARP INSTRUMENTS 89 
 
 that knowledge of enamel rod direction is gained, and only by 
 the use of them can the enamel walls be prepared in terms of their 
 structural elements. In cleaving undermined enamel the edge 
 may be used either with a pulling or a pushing motion. For 
 instance, in opening a cavity in the occlusal surface of a bicuspid, 
 the buccal portion of undermined enamel is split off by placing 
 the instrument as shown in Figs. 55 and 56. The bevel of the 
 blade is held toward the cavity and the shaft of the instrument at 
 a slight angle to the rod direction, and the force is applied in the 
 direction of the shaft. The lingual portion may be removed by 
 placing the instrument as indicated in Fig. 57, the bevel of the 
 blade away from the cavity and the force applied in the direction 
 of the bevel by a pulling force in the direction of the shaft. This 
 is the way in which force is applied on enamel cleavers. The pitch 
 of the bevel in an enamel cleaver and its relation to the shaft of 
 the instrument is extremely important, and the efficiency of an 
 instrument may easily be ruined by careless honing. Every time 
 a cutting instrument is applied to the enamel it must be done with 
 a knowledge of the relation of the cutting edge and the force to 
 the direction of the enamel rods, until it becomes entirely auto- 
 matic. The author emphatically believes that the acquirement of 
 this knowledge and skill will do more to increase facility and suc- 
 cess in the preparation of cavity walls than any other manipulative 
 factor. The preparation of enamel walls requires the continual 
 application of the knowledge of enamel structure. Enamel is a 
 very hard tissue, but it is composed of structural elements, and 
 walls prepared without reference to them will prove their own 
 weakness.
 
 CHAPTER VIII. 
 
 THE STRUCTURAL REQUIREMENTS FOR STRONG 
 ENAMEL WALLS. 
 
 FROM the consideration of the physical character of the enamel, 
 its structural elements and their properties, it is evident that the 
 strength of any enamel wall is dependent upon the arrangement 
 of the rods in the tissue which makes up the walls and their relation 
 to the dentin. Certain requirements for strength can be clearly 
 stated, and these are applicable to all enamel walls. They cannot 
 always be secured with equal facility or perfection, but in propor- 
 tion as these principles are observed and attained the wall will be 
 strong; as they are imperfectly attained or ignored the wall will 
 be weak and unreliable. When these conditions are understood 
 very many failures can be clearly seen to have been the result of 
 their neglect. 
 
 Structural Requirements. 1. The enamel must rest upon sound 
 dentin. 
 
 2. The rods which form the cavosurface angle must have their 
 inner ends resting upon sound dentin. 
 
 3. The rods which form the cavosurface angle must be supported 
 by a portion of enamel in which the inner ends of the rods rest on 
 sound dentin and the outer ends are covered by the filling material. 
 
 4. The cavosurface angle 1 must be trimmed or bevelled so that 
 the margin will not be liable to injury in condensing the filling 
 material against it (Fig. 58). 
 
 These requirements should be considered one by one. 
 
 The Enamel Must Rest upon Sound Dentin. That is, the enamel 
 plate must have the support of sound dentin, and all portions which 
 are undermined by the removal of dentin must be cut away. When 
 the inner ends of the rods which form the enamel plate rest upon 
 sound dentin, the elasticity of the dentin gives to the enamel a 
 certain degree of elasticity, but the enamel itself without this support 
 
 1 The cavosurface angle is defined as the angle formed by the surface of the tooth 
 and the wall of the cavity. 
 
 (90)
 
 ENAMEL MUST REST UPON SOUND DEN TIN 
 
 91 
 
 is extremely brittle. A force that causes it to give will crack it 
 through its entire thickness. No filling material or substitute for 
 the lost dentin can restore the original conditions. Figs. 58 and 59 
 illustrate these requirements. The enamel plate a, b, c, d rests upon 
 sound dentin. The rods which form the cavosurface angle at b 
 run uninterruptedly to the dentin, and their inner ends rest on it 
 
 PP 
 
 - 
 
 FIG. 58. The structural requirements for a strong enamel wall. 
 
 at e. The rods, b, e are also supported by a portion of enamel, 
 a, b, e, made up of rods whose inner ends rest upon the dentin and 
 whose outer ends are covered in by the filling material, altogether 
 supporting the marginal rods like a buttress. And the cavosurface 
 angle is bevelled, including from | to -1- of the enamel wall, so as to 
 remove the sharp corner which would be in danger of crumbling 
 under an instrument. An enamel wall should be considered no
 
 92 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 stronger after the filling is inserted than it was before. Moreover, 
 when the dentin has been decalcified or destroyed by the action 
 of caries, the acid which has decalcified the dentin has also acted 
 upon the enamel, dissolving the cementing substance from between 
 the rods, from within outward, often to a great extent, and the 
 structure is very imperfect. Enamel that has been so weakened 
 
 vJ^^^^il^^-^'vV'afKv^-^/'^^jS, 
 
 
 FIG. 59. -The structural requirements for a strong enamel wall: a, b, the level 
 of the cavosurface angle. The rods forming the margin of the cavity at b reach the 
 dentin at e, and are supported by the portion a, b, e. 
 
 will not withstand the force of mastication, and sooner or later will 
 crack or break away from the filling material. It should be removed 
 and the wall formed in tissue whose structure is perfect. Occasion- 
 ally cases arise where an operator decides to leave some unsupported 
 enamel, but its weakness and the possibility of restoring it if it
 
 ENAMEL MUST REST UPON SOUND DENTIN 
 
 93 
 
 breaks away without destroying the original operation must always 
 be considered. It is sometimes supposed that it is only necessary 
 to have sound enamel resting on sound dentin, but by looking at 
 Figs. 60 and 61 it will be seen that the first requirement may be 
 present, but not the second. In these illustrations the enamel 
 plate is resting on sound dentin, but the tissue has been cut in such 
 
 
 i mitmmk 
 
 FIG 60. Improperly prepared enamel wall. The portion a, b. c has the inner ends 
 of the rods cut off and they do not reach the dentin. 
 
 a way that the inner ends of the rods have been cut off. The rods 
 that form the cavosurface angle do not extend to the dentin, but 
 run out on the cavity wall at d, and the portion a, b, c is held to- 
 gether only by the cementing substance. This is not strong enough 
 to sustain the force necessary to condense the filling material or 
 the forces received upon the surface of the tooth after the filling 
 is completed. It will crack on the line of the cementing substance
 
 94 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 and chip out. The inclination of the entire wall must be increased 
 to a little more than to reach the rod direction. Such a wall as 
 this may easily be made, in preparing a cavity wall, with a stone 
 or a burr, but would not be liable to be formed with hand instru- 
 ments. Such walls as this account for the chipping of many margins 
 
 Mt/ KfL'-i'.trv. hTviyi *- 
 
 FIG. 61. Improperly prepared enamel wall. The portion a, b, c is not supported 
 
 by dentin. 
 
 and the failure of fillings along the gingival wall. The tissue is 
 cracked to pieces in inserting the filling material, and the pieces 
 fall out later. This occurs often in the gingival walls of compound 
 cavities. 
 
 The Rods Forming the Cavosurface Angle Must be Supported. 
 This is the key to strong enamel walls. The more perfect the sup- 
 port the stronger the wall. If an enamel wall is cut exactly in the 
 direction of the rods, as in Fig. 62, the rods forming the margin
 
 THE RODS FORMING THE CAVOSURFACE ANGLE 
 
 95 
 
 are held together only by cementing substance, and a compara- 
 tively slight force on the surface in the direction toward the cavity 
 will break them off. If the same wall is trimmed, as indicated by 
 the line, the same force would do no damage, as the rods which 
 receive it are supported by the portion which is covered by the filling 
 
 FIG. 62. Enamel wall cut in the direction of the rods. The marginal rods are not 
 supported. It should be trimmed in the line indicated. 
 
 material. It is interesting to note that in the wearing down of the 
 enamel by use, nature provides the same support for the rods 
 which form the angle of the worn and tooth surfaces. Fig. 63 
 shows the tip of a worn incisor. The rods at A reach the dentin 
 at C and are supported by the portion A, B, C. When caries occurs 
 on an abraded surface it starts by the rods at the dento-enamel
 
 96 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 junction, chipping out and forming a protected niche for the lodg- 
 ment of a colony. 
 
 Bevel the Cavosurface Angle. It is not always necessary to bevel 
 the cavosurface angle where the rods are inclined toward the 
 cavity. In such places the rods forming the margin are well sup- 
 ported and the angle need not be bevelled unless it is so sharp that 
 it would be in danger of being injured. 
 
 There are two reasons for bevelling the cavosurface angle: (1) 
 To protect a sharp angle from injury; (2) to gain support for the 
 
 FIG. 63. The tip of a worn incisor. The rods forming the angle at A reach the 
 dentin at C, and are supported by the piece A, B, C. 
 
 marginal rods. The first occurs where the enamel rods are inclined 
 toward the cavity, the second where they are inclined away from 
 the cavity. 
 
 Classes of Cavities. From a consideration of the direction of 
 the enamel rods in the tooth crown, and the positions where caries 
 begins on the enamel, enamel walls may be divided, according to 
 their structural type, into two classes (Fig. 64): 
 
 1. Those in which the enamel rods are inclined toward the 
 cavity, characteristic of cavities on occlusal surfaces and cavities 
 beginning in fissures and pits. 
 
 2. Those in which the enamel rods are inclined away from the 
 cavity, characteristic of cavities on smooth surfaces.
 
 CLASSES OF CAVITIES 
 
 97 
 
 In the first class it is comparatively easy to obtain a strong 
 margin, and this is fortunate, for when the filling is completed the 
 margin will be subjected to the full force of mastication. In the 
 second it is comparatively difficult to obtain a strong margin, 
 but only sufficient strength is required to withstand the force of 
 condensing the filling material, as after the filling is completed it 
 will be obliged to withstand little force from mastication. 
 
 From a careful observation of the failures of fillings (his own 
 and those of other operators), the author believes a very large 
 
 FIG. 64. The two classes of cavities. Those with the rods inclined toward the cavity, 
 and those with the rods inclined away from the cavity. 
 
 number are due to structurally imperfect enamel walls. A study 
 of enamel structure as related to cavity preparation will do more 
 to improve the quality of the operation and to increase the facility 
 of its execution than any one factor. This study is a clinical study 
 guided by examination of the microscopic structure of the tissue. 
 In operating at the chair the detail of enamel rod direction as it is 
 applied to cavity preparation is learned, but to do so hand instru- 
 ments must be used and a sufficient knowledge of the tissue must 
 have been acquired to think of it always in their use in terms of 
 its structural elements.
 
 98 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 The steps in the preparation of an enamel wall are: 
 
 1. The cleavage of the enamel until the outline form of the 
 cavity is reached. 
 
 2. The trimming of the enamel walls. 
 
 3. The preparation of the margins. 
 
 FIG. 65. Occlusal fissure in an upper bicuspid, showing direction of rods. 
 
 (About 80 X) 
 
 Every enamel wall should be prepared according to these steps. 
 The first not only removes the tissue more or less disintegrated 
 and weakened by caries, but also places the margin of the filling 
 in a position where it is not likely to be covered by the growth of
 
 CLASSES OF CAVITIES 
 
 99 
 
 a colony of bacteria. It also determines the direction of the enamel 
 rods so that the walls can be completed in terms of its structural 
 elements. 
 
 FIG. 66. The same section as Fig. 65, showing the position of the chisel in cleaving 
 the enamel to open the cavity. 
 
 The second step is accomplished by the shaving or planing pro- 
 cess, and should always increase the inclination of the entire enamel 
 wall slightly, so as to extend a little beyond the rod directions, 
 and remove the portions that have been cracked or splintered by
 
 100 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 the cleavage. After cleavage the enamel wall will usually have 
 a more or less whitish look. This is caused by the cracking of the 
 cementing substance between the rods. The light is refracted by 
 
 FIG. 67. Preparation of enamel walls in occlusal fissure cavities (the same as Figs. 
 
 65 and 66). 
 
 the air in these microscopic spaces and imparts this whitish or 
 snowy look to the tissue. These portions are removed by planing 
 or shaving, and the tissue obtains its bluish, translucent appearance.
 
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 0! 
 
 T 
 fl 
 
 in 
 
 ID 
 
 'S 
 
 a 
 
 
 a" 
 
 (D 
 
 0) 
 
 b 
 
 - 
 
 
 *
 
 CLASSES OF CAVITIES 
 
 101 
 
 The third step is also accomplished by the planing process, and 
 should be carried out with two objects in mind: (1) To so form 
 the cavosurface angle that the tissue will not be liable to injury 
 in the condensation of the filling material against it, and (2) to 
 leave rods whose outer ends will be covered by the filling material 
 to support those which form the actual margin of the cavity. 
 
 The steps in the preparation of enamel walls may be made more 
 clear by photomicrographs. Plate VII shows a portion of enamel 
 close to a carious cavity which is to be extended to the left. The 
 chisel is placed close to the margin and the portion is split off. The 
 
 FIG. 68. The relation of the cavity to the crown (the same as Figs. 66 and 67). 
 
 wall then appears whitish, for, as is seen, the cementing substance 
 has cracked in several places, disturbing the structure, and in 
 several places rods have been broken across. The wall must now 
 be planed so as to increase the inclination of the entire wall slightly, 
 and finally the cavosurface angle must be bevelled, involving from 
 \ to \ of the thickness of the enamel wall to give support to the 
 rods forming the margins. In this case the rods are straight and 
 parallel, but in Plate VIII they are twisted. If the dentin is removed 
 from under this enamel and the chisel placed as indicated, the por- 
 tion will be split out, but not only has the tissue been splintered,
 
 102 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 but a considerable portion is left in which the rods have been 
 broken across. By feeling of the margin with the chisel this can 
 easily be determined, and the angle of the wall must be increased 
 by planing so as to leave the wall in the position shown in Plate 
 VIII, 3, and finally the cavosurface angle must be bevelled. 
 
 FIG. 09. -The trimming of the walls instead of lapping the filling material on the 
 
 slope of the cusps. 
 
 Preparation of Simple Occlusal Cavities. Caries often begins 
 in the mesial and distal pits of the upper bicuspids, and in pre- 
 paring the cavities for filling they must be united. Fig. 65 is a 
 buccolingual section through a first superior bicuspid. Suppose 
 caries has reached the dento-enamel junction in both the mesial 
 and distal pits, and they are to be united along the groove. A 
 small spear drill is carried into the mesial pit until the dento-enamel 
 junction is reached, then a small inverted cone burr is carried into 
 the dentin just under the enamel and drawn from the dentin to
 
 PREPARATION OF SIMPLE OCCLUSAL CAVITIES 
 
 103 
 
 the surface of the enamel. When a narrow cut has been made 
 from the mesial to the distal pit, a chisel placed at the edge of the 
 opening will split out the enamel as indicated in Fig. 71. Now the 
 walls must be planed so as to bring the buccal and lingual walls 
 into the axial plane, and the structural requirements will have 
 
 FIG. 70. Caries beginning in an occlusal defect of a molar. (About 80 X) 
 
 been completed (Fig. 67). Fig. 68 shows the relation of the cavity 
 to the crown. 
 
 It has often been advised to allow the filling to extend on to 
 the natural slopes of the cusps, as indicated in Fig. 69. It will 
 be seen, however, that a stronger enamel wall and a stronger edge
 
 104 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 of filling material will be obtained if the enamel wall is bevelled 
 to the point where the margin of the filling is desired and the filling 
 finished to this position. 
 
 Fig. 70 shows a buccolingual section through a molar with a 
 small cavity in a mesial pit. Caries has undermined the enamel 
 
 FIG. 71. The preparation of the enamel walls of the cavity shown in Fig. 70. 
 
 slightly toward the buccal, but has attacked the enamel on the 
 surface, extending toward the lingual farther than the enamel 
 has been undermined at the dento-enamel junction. Applying 
 the chisel to the surface, the undermined enamel is split away, 
 as is indicated in Fig. 71. The buccal wall is planed until it is in
 
 PREPARATION OF SIMPLE OCCLUSAL CAVITIES 105 
 
 the axial plane, and the cavosurface angle bevelled. It is not 
 necessary to extend the cavity to the lingual beyond the point 
 where sound dentin is reached, but the disintegrated enamel on 
 
 Fio. 72. The relation of the cavity to the crown (the same section was shown in 
 
 Figs. 70 and 71). 
 
 H j'^illiiliihlHiiiii'l'M 
 
 FIG. 73. A larger cavity in the occlusal surface of a molar. The position of the 
 chisel in opening the cavity.
 
 106 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 the surface must be removed. The enamel wall is therefore 
 inclined about 6 centigrades lingually from the axial plane, and it 
 is not necessary to bevel the cavosurface angle. The rods are 
 inclined toward the cavity, the rods forming the margins are 
 
 FIG. 74. A gingival third cavity in a bicuspid, showing the cleavage of the occlusal 
 and gingival walls as cleaved.
 
 PREPARATION OF SIMPLE OCCLUSAL CAVITIES 107 
 
 well supported, and the cavosurface angle is not so sharp as to be 
 endangered in condensing filling material. Fig. 72 shows the rela- 
 tion of the cavity to the crown. 
 
 FIG. 75. The preparation of the cavity shown in Fig. 74. 
 
 All occlusal defects should be filled as soon as the decay has 
 reached the dento-enamel junction, as all progress of the disease
 
 108 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 beyond that point requires sacrifice of tissue which otherwise 
 would be saved, and the enamel wall becomes less and less strong. 
 Fig. 73 shows a much more extensive occlusal cavity, one that 
 has been neglected until the enamel has been broken in, and as a 
 result there was much unnecessary loss of tooth structure. The 
 chisel is applied to the surface as indicated, and the undermined 
 enamel broken down until the sound dentin is reached. On the 
 buccal, the enamel wall is cut to the axial plane, and the cavosur- 
 face angle bevelled. If the decay in the dentin had reached the 
 tip of the dentin cusp, it would be necessary to remove the tip of 
 the enamel cusp and incline the wall about 8 centigrades buccally 
 
 FIG. 76. A gingival third cavity in a molar. 
 
 from the axial plane, in order to obtain a strong wall, and then 
 the cusp would be replaced by filling material. On the lingual 
 the undermined enamel is removed, and the wall inclined slightly 
 lingually from the axia 1 plane and the cavosurface angle bevelled 
 a little. Fig. 73 shows the relation of the cavity to the crown. 
 
 Gingival Third Cavities. Fig. 74 is a buccolingual section of a 
 superior bicuspid, showing a break in the enamel in the position 
 of a gingival third cavity. The occlusal wall is cleaved to find the 
 enamel rod direction, then planed to increase the inclination slightly, 
 leaving it inclined about 8 centigrades occlusally from the hori- 
 zontal plane, and the cavosurface angle bevelled to obtain support
 
 GINGIVAL THIRD CAVITIES 
 
 FIG. 77 
 
 109 
 
 1. Wall as cleaved. 
 
 FIG. 78 
 
 2. Wall as trimmed. 
 FIGS, 77 and 78. Preparation of occlusal wall of Fig. 76. (About 70 X).
 
 110 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 for the marginal rods. The gingival wall is prepared in the same 
 way, inclined gingivally about 6 centigrades from the horizontal 
 plane, and the cavosurface angle bevelled. Fig. 75 shows the walls 
 prepared. 
 
 FIG. 79. A cavity in the lingual pit of a lateral incisor. The position of the chisel 
 
 in opening the cavity. 
 
 Fig. 76 is a similar section from a molar. After chopping away 
 the occlusal wall until the cavity has been extended to the point 
 of greatest convexity of the surface, the wall is seen to be in the
 
 FIG. 80. The preparation of the gingival wall of the cavity shown in Fig. 79. 
 
 \ 
 
 FTG. 81. The preparation of the cavity shown in Fig. 79,
 
 112 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 condition shown in Fig. 77. Near the surface the enamel has broken 
 across the rods and near the dento-enamel junction the same thing has 
 happened, but in the rest of the distance the cleavage has followed 
 the enamel rod direction. The inclination of the wall is increased 
 by planing until this roughness has been removed, and then the 
 cavosurface angle is bevelled to support the marginal rods, and 
 preparation is complete, as shown in Fig. 78. 
 
 Fig. 79 shows a cavity in the lingual pit of a superior lateral 
 incisor. Caries has undermined the enamel to a considerable 
 extent, and the cavity will have to be larger than would otherwise 
 have been necessary. Placing the chisel close to the occlusal 
 margin, as indicated, the enamel is chipped away in that direction 
 and around the circumference. On the lingual wall the chisel 
 may be reversed and used with a pulling motion, like a hoe. In 
 this way the undermined enamel is chipped away and the tip of 
 the marginal ridge removed. The wall is then planed into the 
 horizontal plane and the cavosurface angle bevelled. Fig. 80 
 shows the structure of the gingival wall, and Fig. 81 the relation 
 to the crown.
 
 CHAPTER IX. 
 
 STRUCTURAL DEFECTS IN THE ENAMEL. 
 
 THE formation of enamel begins at the dento-enamel junction, 
 and the tissue is laid down from within outward, so that the enamel 
 in contact with the dentin is formed first and the surface of the 
 crown last. Enamel formation begins at several points for each 
 crown, the exact number and position of which has been the sub- 
 ject of much investigation. When enamel formation begins, 
 these points are close together, but they are carried farther apart 
 by the growth of the dental papilla, and are not united for some 
 time. The separate enamel caplets unite first at the dento-enamel 
 junction, and as the formation of the thickness of the enamel 
 progresses at these lines of union, there is always more or less 
 disturbance in structure. Even \vhere the union seems perfect, 
 sections will show more or less disturbance of enamel-rod direction, 
 arrangement of the rods, and relation to the cementing substance. 
 
 Every operator and student of dental anatomy is familiar with 
 the developmental lines. On the occlusal surfaces they are usually 
 marked by well-defined grooves, but upon the axial surfaces the 
 grooves may be very slight, scarcely more than slight depressions 
 of the surface, and consequently the}' are not thought of. It will 
 be found, however, that on these lines there is less perfect enamel 
 structure, and consequently the tissue is not as strong, and these 
 lines must be avoided in the preparation of enamel walls. The 
 cause of disturbance of structure will be better understood after 
 study of the development of the tooth germ and the formation 
 of enamel in the chapter on Dental Embryology, but some details 
 of the cause should be touched upon here. The study of the dia- 
 grams of the growth of the tooth crown will illustrate the conditions 
 (see Chapter XXVI, Fig. 278), and shows a buccolingual section 
 through the tooth germ of a bicuspid just before the formation 
 of the dentin and the enamel begins. The odontoblasts (dentin- 
 forming cells) and the ameloblasts (enamel-forming cells) are in 
 contact at what will be the dento-enamel junction. The odonto- 
 blasts form dentin on their outer surface, beginning at the tip of 
 8 (113)
 
 114 
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 the dentin cusp, and progress from without inward and extend 
 down the slopes of the cusps. The ameloblasts form enamel on 
 their inner surface and progress from within outward and down 
 
 FIG. 82 
 
 FIG. 83 
 
 FIG. 84 
 
 FIG. 85 
 
 FIG. 86 
 
 FIG. 87 
 
 FIGS. 82 to 87. Diagrams showing the growth of the crown of a bicuspid. 
 
 the slopes of the cusps. In this way little caplets of dentin covered 
 by enamel are formed over the horns of the dental papilla; the 
 caps are, of course, thickest where formation has been going on
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 115 
 
 longest. While these caps are forming, the dental papilla is increas- 
 ing in size, and so they are carried farther and farther apart (Figs. 
 
 FIG. 88. The section from which Figs. 82 to 87 were drawn: A, tip of den tin cusp; 
 B, lines showing little caps of enamel formed before calcifications from separate 
 centres united; C, lines showing amount of enamel formed when calcifications 
 united. 
 
 FIG. 89.- Occlusal defects from an old tooth. 
 
 82 to 87). As soon as the calcifications reach each other at the 
 dento-enamel junction and unite, the increase in the diameter of
 
 116 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 the dental papilla ceases. The layer of ameloblasts, which are 
 tall columnar cells, now cover the surface of the enamel and receive 
 
 FIG. 90. A deep, open groove 
 
 FIG. 91. A shallow groove.
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 111 
 
 FIG. 92. A very deep groove, showing the effect of caries at the bottom. 
 
 FIG. 93. The pit in a lateral incisor filled with coronal cementum. Interglobular 
 spaces are seen in the den tin.
 
 118 
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 their nourishment and the materials for the formation of enamel 
 from the blood supply through the stratum intermedium. As 
 the blood supply comes from above, it is evident that the cells 
 high up along the slopes of the cusps will receive most, while those 
 
 FIG 94 
 
 FIG. 95 
 
 FIG. 94. Occlusal surface of the lower third molar, showing the grooves. 
 FIG. 95. The same tooth sliced for sectioning: 1, the piece from which the section 
 shown in Figs. 96 and 97 was ground. 
 
 at the bottom of the groove get what is left. The formation is 
 therefore more rapid along the slopes and less rapid at the point 
 of union. As growth continues, this difference in supply increases, 
 and accordingly formation at the bottom of the groove is first 
 
 FIG. 9fi. The section ground from 1, Fig. 95, showing the depth of the fissure. 
 
 slowed and finally stopped, and the result is a defect. The taller 
 the cusps the greater will be the interference and the deeper the
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 119 
 
 defective groove. In studying sections (Figs. 88 to 92) it is very 
 noticeable that teeth with long pointed cusps have more open 
 grooves, and the defect often extends almost or quite to the dento- 
 enamel junction. 
 
 FIG. 97. Higher magnification of the fissure shown in Fig. 96. (About GO X) 
 
 The bands of Retzius, which are the incremental lines of the 
 enamel about these grooves, should be studied. It will be seen 
 that they always dip down around the groove, and that more 
 enamel has been formed between one band (Figs. 98 and 99) and 
 the next on the slope of the cusps than at the bottom of the groove. 
 In teeth with very flat, low cusps the closure of the grooves may be 
 very perfect, leaving only a slight depression (Fig. 91).
 
 120 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 The importance of these defects as positions of beginning caries 
 cannot be overestimated, as they furnish ideal conditions in areas 
 that would otherwise be immune, and they are the positions in 
 which the attacks of caries are first manifested. These occlusal 
 grooves appear in great variety. Some are simply shallow, open 
 grooves, in which the surface of the enamel is perfect (Fig. 88) ; 
 some are very deep and entirely empty (Figs. 89, 90, and 92); 
 others are apparently filled with a granular, more or less structure- 
 less calcified material which appears to have been deposited in 
 the groove after the enamel was completed (Figs. 93, 98, and 99). 
 This is probably of the nature of cementum. It was formed after 
 
 FIG. 98. An occlusal defect in a worn tooth. The fissure is filled with coronal 
 
 cementum. 
 
 the enamel was completed, but while the tooth was enclosed in 
 its follicle in the crypt in the bone. It is to be compared with the 
 coronal cementum that is characteristic of the complex grinding 
 teeth of the ungulates and other herbivorous animals. A study 
 of these defects furnishes the basis for the operative rule that "all 
 grooves must be cut out to the point where the margin will be on 
 a smooth surface." For if they are not, a defect will be left at the 
 margin of the cavity which offers ideal conditions for the beginning 
 of a new decay. When caries begins in such a defect at the margin 
 of a filling, it progresses at the bottom of the defect until the dento- 
 enamel junction is reached, and then extends in the dentin and 
 may destroy the entire crown without showing upon the surface
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 121 
 
 (page 50). The extent of these defects is much greater than 
 would be supposed from the observation of the teeth in the 
 mouth. Fig. 94 shows the occlusal view of a lower third molar, 
 
 FIG. 99. Higher magnification of Fig. 98. The fissure filled with granular calcified 
 material. Notice the direction of the bands of Ret/ius around the fissure. 
 
 extracted because of disease of the peridental membrane, from a 
 man aged about forty years. Examining these grooves with a 
 fine-pointed explorer, it would not stick any place. No operator
 
 122 
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 would think of cutting them out and filling them. The crown was 
 sawed through from buccal to lingual, as shown in Fig. 95, and the 
 piece marked 1 is shown in Figs. 96 and 97. The grooves are open 
 
 E 
 
 1 V&X&S& 
 
 "".. ?<:^3ss:?^??r-' ' 
 
 FIG. 100. Structural defects in developmenta grooves on axia surfaces (Black.) 
 
 two-thirds of the distance to the dento-enamel junction, and show 
 slight action of caries. Suppose caries had started in the central 
 pit, and a small round filling had been made, open defects would 
 
 FIG. 101. Structural defects in developmental grooves on axial surfaces. (Black.) 
 
 be left at the margin where every groove radiated from the central 
 cavity, and these would be just as liable to recurrent decay as 
 they were originally, and, if caries occurred, it would progress at
 
 STRUCTURAL DEFECTS IN THE ENAMEL 123 
 
 the depth of the groove, reach the dento-enamel junction, and 
 progress in the dentin, until the occlusal enamel was so undermined 
 that it would break in under the force of mastication. On the 
 other hand, if the grooves are cut out to a point where the cavity 
 margin will be on a smooth surface, there is no possibility of recur- 
 rent caries if the filling material is properly inserted. This one 
 illustration, which might be duplicated a thousand times, there- 
 fore is the rational basis for the rule, "All grooves must be cut out 
 to their ends." 
 
 Caries does not occur in all open grooves. Fig. 90 shows an 
 open groove in a section from a tooth in which the wear indicates 
 
 FIG. 102. Defects on the axial surface in the enamel. 
 
 that it was not from a young person, but most of the grooves that 
 escape are not open, but more or less entirely filled with structure- 
 less calcified matter or coronal cementum. Figs. 93, 98, and 99 
 are very good illustrations of this class of grooves. 
 
 The condition in pits from which grooves extend, as the lingual 
 pits of incisors and the buccal pits of molars, show the same con- 
 dition as the grooves, except that the defect is both broader and 
 deeper. But pits that are sometimes found on the tips of cusps 
 and on smooth surfaces show an entirely different structural con- 
 dition, and are pathologic in character. 
 
 In places where the union of the enamel plates seems perfect,
 
 124 
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 as, for instance, on the labial surface of the incisors or the buccal 
 surface of the bicuspids, and the line of union is marked only by 
 a slight depression of the surface, the section will show disturbance 
 of structure. Fig. 101, a drawing made by Dr. Black a good many 
 years ago, shows such a position. At the surface the rods and 
 their arrangement seem very perfect, but from a point about one- 
 third the distance to the dento-enamel junction there are no rods 
 at all, but apparently a number of calcospherites in a granular 
 
 FIG. 103. A section through such a defect as that shown in Fig. 102. (About SO X) 
 
 calcific substance. In Fig. 101, another of Dr. Black's illustrations, 
 the rods are very irregular, and are separated by large areas of 
 structureless calcified material. Grooves are often found in unusual 
 or atypical positions. Fig. 102 shows a groove running over the 
 mesial marginal ridge and down on the mesial surface. Fig. 103 
 shows a section through such a defect. Notice the folding of the 
 enamel into the dentin and the disturbance of the rods about 
 the groove and between its base and the dentin.
 
 CHAPTER X. 
 
 SPECIAL AREAS OF WEAKNESS FOR ENAMEL 
 MARGINS. 
 
 THERE are certain positions which in the perfect crown are 
 areas of great strength, but which, because of the peculiar structure 
 of the tissue in these places, become areas of weakness when cavity 
 
 FIG. 104 Buccolingual section of upper bicuspid. Enamel is broken from grinding. 
 A to B, area of weakness for enamel margin. (About 20 X) 
 
 (125)
 
 126 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 margins are made in them. The treatment of beginning caries 
 would lead to no failures in these positions, for cavity margins 
 would never be extended into them, except in the treatment of 
 
 FIG. 105. Enamel over tip of dentin cusp: D, dentin cusp. (About 80 X). From 
 same section as Fig. 104
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 127 
 
 burrowing caries and neglected cases. The extension of caries 
 at the dento-enamel junction often requires the extension of the 
 margin into the area of danger. In considering these areas and in 
 the preparation of cavities, as well as the areas of imperfect struct- 
 ure considered in Chapter IX, it is important to place as much 
 emphasis on the necessity of not extending cavity margins into the 
 areas of weakness, as on cutting away the dangerous area and leaving 
 the margin in a safe position, when the area cannot be avoided. 
 
 In considering the relation of the enamel and dentin, and in 
 studying the arrangement of the enamel-rod direction in the 
 "architecture" of the tooth crown, 
 it has been pointed out that the 
 dentin cusps and the dentinal 
 marginal ridges are not directly 
 under the corresponding points on 
 the surface of the enamel, but are 
 nearer to the axis of the tooth. 
 The areas on the surface of the 
 enamel, from the point directly 
 Over the tip of the dentin cusp 
 or ridge to the tip of the enamel 
 cusps or ridges, become areas of 
 weakness when a cavity is ex- 
 tended into them. 
 
 Fig. 104 is a photomicrograph 
 of a .buccolingual section of a 
 superior bicuspid, and Fig. 105 is 
 a higher magnification of the same, 
 made to illustrate the condition. 
 It will be seen that if decay has 
 
 extended at the dento-enamel junction to the tip of the dentin 
 cusp, and the enamel walls were left in the axial plane, the rods 
 which form the surface of the enamel from the margin of the cavity 
 to the tip of the cusp "are not supported by dentin," and would be 
 likely to be broken and fall away, leaving a defect at the margin 
 of the filling. If decay beginning in the groove or pit has extended 
 only to point C, Fig. 104, the wall may be trimmed in the axial 
 plane and an ideal wall produced; but if it has reached point D, 
 Fig. 104, it must be inclined buccally so as to remove the tip of the 
 cusp, as indicated in the dotted line, and the cusp restored by the 
 filling material. The region of the surface indicated by A-B, 
 
 FIG. 106. A bicuspid cut for sec- 
 tioning. Sections were ground from 
 the positions marked by the lines 
 1, 2, 3, 4, and 4 is also shown in 
 Figs. 107, and 108.
 
 128 
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 while an area of strength in the perfect tissue, becomes a position 
 of weakness when cavity margins are extended into it. A careful 
 
 FIG. 107. Section ground from Fig. 106, through mesial part and marginal ridge. 
 If caries has extended at the dento-enamel junction to A, the wall may be in the 
 axial plane; if it has reached B, the wall must be inclined as indicated by the dotted 
 line. (About 30 X) 
 
 observer will find many failures that are the result of bad enamel- 
 wall preparation in these areas. The same conditions exist in the
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 129 
 
 region of the marginal ridges. Figs. 107 and 108 show the mesial 
 marginal ridge of a superior bicuspid. If this is filled before the 
 destruction of dentin has extended beyond the point A, the mesial 
 wall may be cut in the axial plane as indicated; but if it has reached 
 the tip of the dentin ridge at point B, it must be inclined mesially, 
 so as to reach the tip of the enamel ridge. Figs. 109 and '110 show 
 
 FIG. 108. A higher magnification of Fig. 107, showing enamel-rod directions in the 
 region of the marginal ridge. 
 
 the distal marginal ridge in a second molar. Notice the inclination 
 of the rods from the tip of the dentin ridge. If decay has reached 
 this point the wall must be inclined distally, so as to reach the rod 
 direction, or a frail margin will be left and one which will not sus- 
 tain the force of mastication. Neglected caries in the lingual pits 
 of incisors often present the same conditions as found in the mar- 
 
 9
 
 130 
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 ginal ridges of the occlusal surface of molars and bicuspids. The 
 same conditions are also often encountered in the preparation of 
 simple cavities in the mesial or distal surfaces of incisors, when 
 caries has followed the dento-enamel junc- 
 tion toward the lingual. Fig. Ill shows a 
 superior central incisor from which sections 
 ' were cut as indicated. Suppose caries to 
 have begun in the region of the contact 
 point and to have extended to the point 
 a. If the lingual enamel wall were pre- 
 FIG. 109. An upper pared at the line A, Fig. 112, a very frail 
 molar showing the posi- wa n wou ld resu lt. Force coming upon this 
 
 tion of the section shown . . 
 
 in Fig. no. wall from the lingual by the occlusion of the 
 
 lower incisors, would be likely to break out 
 
 or crack a triangular piece of enamel, and the filling would fail 
 along the lingual wall. If, however, the wall be laid in the line at 
 B, a strong wall is produced, against which gold can be properly 
 condensed without danger, and which will withstand the force of 
 occlusion. 
 
 FIG. 110. The section ground from Fig. 109. 
 
 Dentists are often tempted to prepare simple cavities in the 
 mesial surfaces of first and second bicuspids and occasionally in 
 the molars. If this is ever done, it must be with the full knowledge
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 131 
 
 both of the liability of recurrence of caries and the structure of 
 the enamel, for experience shows that such operations usually fail, 
 either by recurrence of caries at the bucco- 
 gingival or linguogingival angles, or by the 
 breaking out of the enamel of the marginal 
 Bridge. Fig. 115 shows the mesial surface of 
 a superior bicuspid. There was a white spot 
 on the contact point, but no actual cavity, 
 as the enamel rods had not fallen out. A 
 section was ground through this point, and 
 Fig. 116 shows a photomicrograph of it. 
 The enamel rods have fallen out of the 
 disintegrated area, and the decalcification 
 in the dentin is shown. If this had been 
 treated as a simple cavity the occlusal wall 
 would have required an inclination of 18 centigrades occlusally 
 from the horizontal plane to reach the enamel-rod direction. 
 There is very little support offered by the dentin for the enamel 
 of the marginal ridge, and the portion over to the occlusal groove 
 would be likely to be broken off by the force of mastication. The 
 conditions of the occlusal wall are better shown in Fig. 117. 
 
 FIG. 1 1 1. A superior 
 central incisor, showing 
 the position of sections 
 in Figs. 112, 113 and 114. 
 
 FIG. 112. Section 1, Fig. Ill, showing the enamel worn from the marginal ridges. 
 
 Any number of illustrations of these conditions might be made, 
 but the subject may be summed up by saying: The surface of the 
 enamel from the point directly over the dentin cusp or ridge to the 
 tip of the enamel cusp or ridge, which is an area of great strength 
 in the perfect crown, is a region of weakness for an enamel wall. 
 It is fully as important not to extend into this area unnecessarily
 
 132 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 FIG. 113. Section 2, Fig. Ill, showing position of weak and strong lingual walls. 
 
 FIG. 114. A higher magnification of the mesial marginal ridge, shown in Fjg. 113. 
 
 (About 60 X)
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 133 
 
 as to form the wall proper when caries has extended so as to 
 involve it. When caries of a smooth surface approaches a mar- 
 ginal ridge which receives the force of occlusion, the wall must 
 
 FIG. 115. Occlusal and mesial views of a superior bicuspid, showing position of 
 section. A beginning caries could be seen on the surface, but it does not show well 
 in the picture. The section from the buccal piece is shown in the following illustra- 
 tions. 
 
 FIG. 116. The section ground from the buccal piece, Fig. 115.
 
 134 
 
 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 be extended so that the enamel receives full support from sound 
 dentin. 
 
 FIG. 117. The enamel over the mesial marginal ridge to the oblique groove, show- 
 ing a region of weakness for the occlusal wall of a simple proximal cavity.
 
 CHAPTER XI. 
 THE DENTIN. 
 
 THE dentin may be defined as a connective tissue whose inter- 
 cellular substance is calcified. It is apparently homogeneous in 
 structure, but penetrated by minute canals, which contain proto- 
 plasmic projections from cells lying within a cavity enclosed by 
 the tissue. 
 
 The Function of the Dentin. The dentin makes up the mass of 
 the tooth, giving to it its general form, each cusp and root being 
 indicated in it. It gives to the tooth its elastic strength, and the 
 enamel, being hard and very resistant to abrasion but extremely 
 brittle, is dependent upon the elastic support of the dentin. This 
 has been elaborated to a considerable extent in the chapter on the 
 Dental Tissues. The fact that the dentin gives the strength to 
 the tooth should never be lost sight of in operating, and sound 
 dentin should never be sacrificed unnecessarily in the preparation 
 of cavities. 
 
 Structural Elements of the Dentin. The structural elements of 
 the dentin may be stated as : 
 
 1. The dentin matrix. 
 
 2. The sheaths of Newman and the dentinal tubules. 
 
 3. The contents of the dentinal tubules or the dentinal fibrils. 
 While these are the elements of which the tissue is composed, 
 
 there are other characteristic appearances found in the dentin, 
 caused by special conditions or arrangement of these elements 
 which must be studied. These are the granular layer of Tomes, 
 the interglobular spaces, the lines of Schreger, and secondary 
 dentin. 
 
 Origin of the Tissue (Histogenesis). The dentin, like all of the 
 other calcified tissues except the enamel, is a connective tissue, and 
 is formed by the dental papilla, which is a conical papilla of con- 
 nective tissue rich in bloodvessels and covered on its surface by 
 the layer of dentin-forming cells, the odontoblasts. The dentin 
 is formed from without inward, leaving the remains of the dental 
 papilla in the cavity of the formed dentin as the dental pulp. Before 
 
 (135)
 
 136 THE DENT IN 
 
 the tooth is erupted, and up to the time that the full length of the 
 root is formed, a characteristic thickness of dentin is formed, which 
 is called the primary dentin. After this time dentin is formed by 
 the pulp only intermittently, in response to irritations and trophic 
 impulses, producing secondary dentin. Secondary dentin is always 
 more irregular in the arrangement of the tubules, and more imper- 
 fect in structure than the primary dentin. The boundary line 
 between two periods of dentin formation can always be picked out 
 by changes in the direction or character of the dentinal tubules. 
 
 The Dentin Matrix. The dentin matrix is a solid, apparently 
 homogeneous, and very elastic substance, through which the den- 
 tinal tubules extend. It is translucent in appearance and slightly 
 yellowish in color. In broken or split sections to the unaided eye 
 it has a yellowish color by reflected light, and a characteristic 
 luster due to the refraction of light by the tubules. In ground 
 sections, by transmitted light, under the microscope, it is very 
 translucent and shows no indication of structure. 
 
 The matrix consists of an organic basis of ultimately fibrous 
 character, yielding gelatin on boiling, with which the inorganic 
 salts are chemically combined. The relation of organic and inor- 
 ganic matter in the dentin matrix is similar to the condition in 
 the bone matrix and that of all calcified connective tissues. Appar- 
 ently the organic basis is first formed, and then the inorganic 
 salts are combined with it in a weak chemical union. If the dentin 
 is treated with dilute acid, the inorganic matter is dissolved and 
 the organic basis is left retaining the form of the tissue. If the 
 organic matter is burned out, it leaves the inorganic matter in the 
 characteristic form. 
 
 Von Bibra gives the following analysis of perfectly dry dentin : 
 
 Organic matter 27.61 
 
 Fat 0.40 
 
 Calcium phosphate and fluoride 66.72 
 
 Calcium carbonate 3.36 
 
 Magnesium phosphate 1 . 08 
 
 Other salts 0.83 
 
 Mr. Charles Tomes pointed out that such analyses as this failed 
 to take account of about 8 per cent, of water which is held as water 
 of combination, and which is driven off at about red heat. 
 
 It is evident that the organic matter in the dentin is of two kinds 
 the organic basis of the matrix, which is of gelatin-yielding 
 character, and the protoplasmic contents of the dentinal tubules. 
 Variations, therefore, in the proportion of organic and inorganic
 
 THE SHEATHS OF NEWMAN 137 
 
 matter in the dentin might be caused by differences in the propor- 
 tions of organic and inorganic constituents of the matrix, or by 
 variations in the size of the tubules and the amount of material 
 contained in them. 
 
 If dentin changes in its degree of calcification with age, this 
 might be brought about by the reduction in the size of the tubules, 
 or by the adding of inorganic constituents to the matrix. 
 
 The amount of material contained in the dentinal tubules is 
 much greater than is generally realized. If 2^ is considered the 
 average diameter of the dentinal tubules, and they are separated 
 by an average of SM of dentin matrix. Some idea of the relative 
 volume of the dentin matrix and the contents of the tubules can be 
 obtained, but this is greatly increased by the very numerous side 
 branches which connect the neighboring tubules. This matter can 
 be visualized by taking a lump of soft clay and boring it full of holes, 
 making the holes two inches in diameter, and separated by eight 
 inches of clay. 
 
 The ultimately fibrous character of the dentin matrix can be 
 made out only in various stages of decalcification and decomposi- 
 tion. In the original condition no trace of the fibrous character 
 can be seen. By maceration with acids and alkalies the intertubular 
 material assumes a fibrous appearance, as if bundles of white con- 
 nective-tissue fibers had been fused together. There is apparently 
 no definite arrangement of these fibers and there is no indication 
 of the arrangement of the substance in layers. 
 
 The Sheaths of Newman. There has been much discussion as 
 to the character of these structures, which were first discovered 
 in 1863 by Newman. Some investigators have denied their exist- 
 ence entirely, explaining the appearance in some other way. These 
 structures are in no sense a sheath surrounding the dentinal fibril 
 and lying in the dentinal tubule, but are that portion of the matrix 
 which forms the immediate wall of the tubule. That this material 
 differs from that which occupies the rest of the space between the 
 tubules is certain, and is shown by the examination of ground sec- 
 tions, the action of stains upon ground sections, and the action of 
 the matrix when boiled with strong acids and alkalies. In Fig. 
 118, a photograph of a ground section, there is evidently a dif- 
 ference in the refracting index of the portion of the matrix imme- 
 diately surrounding the tubules. Apparently the sheaths of New- 
 man are composed of a material similar to that forming elastic 
 connective-tissue fibers, and known as elastin. This substance
 
 138 THE DEN TIN 
 
 is very resistant to the action of acids and alkalies. After the 
 remainder of the intertubular material has been destroyed by 
 boiling with strong acid, the sheaths remain like hollow elastic 
 fibers, having the appearance of pipe-stems, which resist long-con- 
 tinued action of the boiling acid. Some authors have suggested 
 that the great elasticity of the dentin was largely due to the presence 
 of this substance. 1 
 
 Fid. 118. Dentin showing tubules in cross-section: Dt, dentinal tubules; D, dentin 
 matrix; S, shadow of sheaths of Newman. (About 1150 X) 
 
 The Dentinal Tubules. The dentin matrix is penetrated every- 
 where by minute branching tubules, which radiate from the central 
 cavity or pulp chamber and extend to the outer surface of the 
 dentin at the dento-enamel junction or the dento-cemental junction, 
 where they end blindly or in irregular enlargements. These tubules 
 are from 1.1 to 3 microns in diameter. One hundred measurements 2 
 made at random from ground sections gave the extreme measure- 
 ment: 3, largest; 1.5, smallest; and average, 2.95. Fifty measure- 
 ments from one longitudinal section of tubules at their pulpal 
 extremity gave an average of 2.6; largest, 3; smallest, 1.5; and 
 50 measurements at the dento-enamel junction of the same section 
 
 1 Hawazawa, Tokyo: A Study of the Minute Structure of Human Dentin, Trans. 
 Panama Pacific Dental Congress, 1915, p. 80, and Dental Cosmos, February and 
 March, 1917, vol. ix. 
 
 2 Kolliker gives 5 microns, also Schafer; Owen, 2.5 microns.
 
 DIRECTION OF TUBULES IN CROWN PORTION 
 
 139 
 
 gave the following: Average, 1.2; largest, 1.5; smallest, 0.75. 
 These measurements were made with an eye-piece micrometer, 
 using yg- oil-immersion objective and No. 3 ocular. 
 
 At the present time there is a fertile field for investigation offered 
 in regard to the size of dentinal tubules. Many statements have 
 been made that have not been supported by tabulated measure- 
 ments, and no definite statement can be made as to the variations 
 and size of the dentinal tubules in different teeth, the teeth of 
 different animals, or in the human teeth at different ages. 
 
 FIG. 119. A section showing the primary curvatures of the dentinal tubules in the 
 crown portion. (About 20 X) 
 
 Direction of Tubules in Crown Portion. In the crown portion and 
 the gingival portion of the dentin the tubules pass from the pulp 
 chamber to the dento-enamel junction, or the dento-cemental 
 junction, in sweeping curves, which were called by Tomes the 
 primary curvatures. These have been described as /- or ^-shaped 
 (Fig. 119). The tubule tends to enter the pulp chamber at right 
 angles to the surface, and to end at the dento-enamel junction at 
 right angles to that surface. In the dentin forming the axial walls 
 of the pulp chamber the tubules make two bends in passing from 
 the pulp chamber to the surface of the dentin. In the first the con- 
 vexity is directed apically, in the second it is directed occlusally.
 
 140 
 
 THE DEN TIN 
 
 The outer extremity of the tubule is therefore considerably farther 
 to the occlusal than the point at which it opens into the pulp cham- 
 
 FIG. 120. A section showing the primary curvature of the dentinal tubules in the 
 gingival portion. (About 20 X) 
 
 FIG. 121. A section showing compound curves near the dento-enamel junction. 
 
 (About 80 X)
 
 DIRECTION OF TUBULES IN CROWN PORTION 
 
 141 
 
 her (Fig. 120). The outer part of this double curve is often complex 
 instead of simple (Fig. 121). The course of the dentinal tubules 
 is not a direct one, but that of an open spiral. This may easily 
 be demonstrated by changing the focus up and down in examining 
 sections cut at right angles to the direction of the tubules. When 
 examined in longitudinal sections this spiral course gives to the 
 
 FIG. 122. Dentin at dento-enamel junction, showing tubules cut longitudinally 
 
 (About 760 X) 
 
 tubule the appearance of having little wavy curves throughout 
 its length. These have often been called the secondary curvatures. 
 Each wave represents a turn in the spiral. As many as two hun- 
 dred have been counted in the length of a single tubule, or about 
 one hundred in a millimeter of length. 
 
 The dentinal tubules give off minute lateral branches, which 
 extend from one tubule to another. These are very minute, and
 
 142 
 
 THE DENTIN 
 
 in the crown portion of the dentin are not at all conspicuous, but 
 in the region of the dento-enamel junction the tubules branch 
 
 FIG. 123. Dentin from the root, showing tubules cut longitudinally and the fine 
 connecting branches. (About 700 X)
 
 DENTINAL TUBULES IN THE ROOT PORTION 
 
 143 
 
 dichotomously, each fork having about the same diameter as the 
 original tubule (Fig. 122). These forkings of the tubules resemble 
 the appearance of the delta of a river on the map. The branches 
 anastomose with each other very freely. This anastomosis of the 
 tubules at the dento-enamel junction is very important in deter- 
 mining the spreading of caries in this area. It probably also explains 
 the sensitiveness of this area noticed in the preparation of cavities, 
 which will be noted again in considering the sensitiveness of the 
 dentin. 
 
 FIG. 124. Granular layer of Tomes: L, lacunae of cementum; GT, granular layer of 
 Tomes; Ig, interglobular spaces. (About 200 X) 
 
 The Dentinal Tubules in the Root Portion. In the root portion 
 of the dentin the tubules ordinarily show only the secondary curves, 
 their general direction being at right angles to the axis of the pulp 
 canal. Throughout their course they give off an enormous number 
 of very fine branches extending from tubule to tubule. These are
 
 144 THE DENTIN 
 
 so numerous that in suitably prepared sections they may be said 
 to look like the interlacing twigs of a thicket or the rootlets of 
 plants in the soil. Fig. 123 gives a very good idea of the appear- 
 ance. 
 
 At the dento-cemental junction the tubules end in irregular 
 anastomosing spaces, which cause the appearance of the granular 
 layer of Tomes (Fig. 124). 
 
 From a consideration of the preceding it will be seen that it is 
 usually not difficult to determine whether a field of dentin seen 
 under the microscope was taken from the crown or the root of a 
 tooth. The structural characteristics of the two regions may be 
 summarized as follows: In the crown, the tubules show both the 
 primary and the secondary curves. In the root, the tubules show 
 only the secondary curves. In the crown, the lateral branches are 
 few and inconspicuous and the tubules branch in a characteristic 
 way at the dento-enamel junction. In the root, the lateral branches 
 are very numerous throughout the length of the tubule, and they 
 end in the characteristic spaces of the granular layer of Tomes. 
 
 The Dentinal Fibrils. In life the dentinal tubules are occupied 
 by protoplasmic projections of the odontoblasts known as the 
 dentinal fibrils, or fibers of Tomes. As the dentin matrix is formed 
 and calcified under the influence of the odontoblasts, a portion of 
 their protoplasm is left in the tubules of the matrix as the dentinal 
 fibril. These structures were first described by John Tomes, who 
 recognized their true character. They may be demonstrated in 
 decalcified sections, and they will be seen projecting from the 
 odontoblasts, when the pulp is removed from a freshly extracted 
 tooth, by cracking it and picking the pulp out. In this way a 
 portion of the fibril is pulled out of the tubules. The fibrils will be 
 considered more especially in connection with the pulp, to which 
 they properly belong. 
 
 In the author's opinion very little is positively known about the 
 contents of the dentinal tubules. While it is very apparent that in 
 young forming dentin the tubules are filled by cytoplasmic projec- 
 tions of the odontoblasts, it is by no means certain that all of the 
 tubules of the dentin, in an old tooth are still occupied by living 
 cytoplasm. What the fate of the cytoplasmic contents of the tubules 
 is when secondary dentin is formed is not known. Several things 
 are theoretically possible but there is little or no direct evidence 
 on the matter.
 
 THE GRANULAR LAYER OF TOMES 145 
 
 The Granular Layer of Tomes. The granular layer of Tomes is 
 the outer layer of the dentin next to the cementum. The granular 
 appearance is caused by irregular spaces in the dentin matrix which 
 connect with the ends of the dentinal tubules, and which are filled 
 with protoplasm continuous with that of the fibrils. Tomes first 
 called attention to this layer, and for this reason it bears his name. 
 
 With magnifications of from 50 to 100 diameter it is easily seen 
 in ground sections, either longitudinal or transverse, and appears 
 as a layer filled with little dark spots or granules, the spaces which 
 have been filled with the debris of grinding. It is separated from 
 the cementum by a thin, clear layer, apparently of structureless 
 dentin matrix, which is more apparent in higher magnifications. 
 The granular layer is sometimes seen in the crown portion just 
 under the enamel, but it is never as well marked in this position. 
 
 The layer is seen in sections ground from freshly extracted teeth 
 as well as from old dry teeth, showing that these are true spaces 
 and are not produced by the shrinkage of partially calcified dentin 
 matrix. Tomes called the spaces in the granular layer "inter- 
 globular spaces," but this term should not be used, as the structures 
 generally known as the interglobular spaces are different in location 
 and character, and will be considered later. 
 
 The granular layer is not seen in decalcified sections. So far as 
 the author is aware, no one has called attention to this fact before. 
 In decalcified sections stained with hematoxylin and eosin the 
 position of the granular layer is always occupied by a clear layer 
 which takes the stain in an entirely different w r ay from the rest of 
 the dentin matrix, and in which no indication of spaces can be 
 seen. \Yhile the fibrils in the tubules through most of the dentin 
 take the hematoxylin stain and can be easily seen, they cannot be 
 followed into this clear layer, and no indication of protoplasmic 
 contents of irregular spaces can be seen. 1 
 
 Dr. Skillen has worked out a method of demonstrating the 
 granular layer of Tomes in decalcified sections which is reported in 
 an article by Dr. Newton G. Thomas in the Dental Cosmos for 
 June, 1920. 
 
 Most authorities state that the spaces of the granular layer 
 communicate with the canaliculi of the cementum, as well as with 
 the tubules of the dentin. This the author has been unable to 
 
 1 The appearance of the tissue in decalcified sections had led to some doubt in the 
 writer's mind as to the interpretation of the character of the layer by authors who 
 have described it. 
 
 10
 
 146 THE DENTIN 
 
 confirm. On the other hand, the granular layer seems to be sepa- 
 rated from the cementum by a thin layer of dentin which is clear 
 and apparently structureless. This is separated from the cementum 
 by a dark line, and the first layer of cementum usually does not 
 contain any lacunae or canaliculi. This is supported by some 
 of the experiments that have been made with extracted teeth. In 
 experimenting on the diffusion of drugs through dentin, it was 
 found that liquids sealed in the pulp chambers of extracted teeth 
 could not be detected in the liquids in which the teeth were placed 
 unless the cementum was removed from them. In the recent 
 experiments of Dr. Southwell, of Milwaukee, in which air was 
 forced through the dentin from the pulp chamber to test the sealing 
 of cavities by filling materials, the air did not escape from the 
 cementum, which would be the case if dentinal tubules connected 
 with the canaliculi of the cementum. 
 
 If the spaces of the granular layer are filled with the protoplasmic 
 enlargements of the ends of the dentinal fibrils, this would give 
 a very reasonable explanation of the sensitiveness of slight caries 
 and erosion at the gingival line, as the anastomosis through the 
 granular layer would affect the fibrils of the entire tooth. 
 
 The Interglobular Spaces. There has been considerable mis- 
 understanding in dental histology in regard to these spaces, owing 
 to the confusing of two entirely different things. Tomes called 
 the spaces of the granular layer, \vhich have already been described, 
 interglobular spaces. As has been seen, they are true spaces in 
 the dentin matrix which connect with the dentinal tubules and 
 are filled with protoplasm. 
 
 In 1850 J. Czermak 1 described areas of imperfectly calcified 
 dentin matrix, which appear as spaces in dried dentin, and called 
 them interglobular spaces. These have been so called by most 
 writers since. It seems important to the author that the term be 
 restricted to these and some other used to indicate the spaces 
 of the granular layer, which are of entirely different character. 
 
 The interglobular spaces of Czermak are caused by some dis- 
 turbance in the calcification of the organic matrix of the dentin 
 They occur in zones (Fig. 125) which correspond to the dentin 
 matrix, being calcified at a given time, and there is usually seen 
 a corresponding disturbance in the calcification of the enamel, 
 which was being formed at the same time and manifested as a 
 more or less strongly marked band of Ritzius. 
 
 1 Beitrag zur Mikro-Anatoruie rlf>r Menschlichen-Zahne.
 
 THE INTERGLOBULAR SPACES 
 
 147 
 
 In the calcification of the dentin matrix the inorganic salts are 
 combined with the organic matrix in spherical areas which become 
 united. The boundaries of these areas of uncalcified matrix are 
 
 FIG. 125. A drawing showing a zone of interglobular spaces in the dentin. 
 
 (Black.) 
 
 , 
 
 FIG. 126. Interglobular spaces in dentin. (About 60 X) 
 
 therefore very irregular, and made up of concave facets where 
 they join the spherical surfaces of the fully calcified matrix (Figs. 
 126 and 127). A study of the illustrations and the appearance
 
 148 
 
 THE DENTIN 
 
 of the layer of forming dentin next to the dental papilla of a devel- 
 oping tooth will make this intelligible. 
 
 If the dentin is dried the organic matrix in these areas gives up 
 
 ! 
 
 J 
 
 FIG. 127. Interglobular spaces in dentin. Some empty, some filled with debris. 
 
 (About 80 X) 
 
 FIG. 128. Intci globular spaces in dentin: Ig, first line of interglobular spaces; 
 Jij', second line of interglobular spaces. (About 30 X)
 
 THE LINES OF SCHREGER 
 
 149 
 
 water and shrinks, and the interglobular spaces become true 
 spaces, partially filled with the shrunken matrix. In this condition 
 they can be filled with colored collodion or any other material 
 If, however, they are studied in sections of teeth which have never 
 been allowed to dry, no space appears, and the dentinal tubules 
 continue without change of course or diameter through the area. 
 While they are, therefore, not empty spaces, they are areas of the 
 organic basis of the dentin which are bounded by globular surfaces 
 of the fully calcified matrix, and their name is properly significant. 
 Zones of interglobular spaces may occur at any portion of the 
 dentin, either in the crown or root, but they are more common in 
 the crown and near the enamel. Often more than one zone can be 
 seen, as in Fig. 128, which shows two disturbances in calcification, 
 and disturbances in the structure of the enamel will be seen at 
 corresponding positions. 
 
 FIG. 129. A root broken on a line of interglobular spaces. This tooth was ex- 
 tracted by Dr. G. V. Black, and was pulled apart in extraction, a shows the form 
 of the root and a, b the separation on the line of growth. (Black.) 
 
 The zones of interglobular spaces appear in all grades, from a 
 complete band of uncalcified matrix to widely scattered patches. 
 Fig. 129 shows a tooth in Dr. Black's collection which was broken 
 in extraction, because of the presence of such a zone in the root. 
 
 The interglobular spaces are of great importance in modifying 
 the direction of the progress of caries in the dentin. 
 
 The Lines of Schreger. As in the case of the interglobular spaces, 
 there seems to be considerable misunderstanding in the literature
 
 150 THE DENTIN 
 
 and certain structures which have very different meanings have 
 been called the "lines of Schreger." 
 
 An arrest in the formation of dentin often occurs before the 
 crown is completed. When the activity has begun again the 
 dentinal tubules follow a slightly different direction. In a longi- 
 tudinal section this change in the direction of the tubules produces 
 a line. Several such lines may be seen in a single section, though 
 they are by no means to be found in all longitudinal sections. 
 
 Schreger's lines have been most often confused with zones of 
 interglobular spaces, and they seem to be identical with the incre- 
 mental lines in the dentin described by Owen. It is unfortunate 
 that these names should have been used, for a thoughtful study of 
 the tissue makes their interpretation perfectly evident, and they 
 are of no great significance. 
 
 Secondary Dentin. It is by no means easy to define secondary 
 dentin or to pick out any particular piece of dentin in a section and 
 to say whether it is primary or secondary. In general, the tubules 
 are smaller, fewer, and less regularly arranged in secondary than 
 in primary dentin. In general, it seems that the smaller the remain- 
 der of the dental papillae becomes, the more imperfect dentin it 
 forms, until finally it simply throws down granular calcified material. 
 
 The formation of dentin begins at the dento-enamel junction, 
 at a number of points in each tooth, and progresses from without 
 inward (strange to say, exactly the opposite statement has been 
 made several times in papers by very prominent men). This 
 matter will be taken up more in detail in the Chapters on Dental 
 Embryology and Dentition. It is enough to say here that in 
 studying all sections of dentin, whether cut longitudinally or trans- 
 versely, the formation of dentin began at the dento-enamel junc- 
 tion and the dento-cemental junction, and progressed toward the 
 pulp chamber. 
 
 From the study of longitudinal and transverse sections it is 
 apparent that a certain typical amount of dentin is formed before 
 the tooth is erupted and while it is coming into full occlusion. 
 This is primary dentin. In it the tubules are very regular in size 
 and arrangement. From this time on the formation of dentin is 
 intermittent, and apparently is the response to some outside con- 
 dition. These conditions may arise in the tooth in which the forma- 
 tion occurs, or the irritation of one tooth may cause tissue formation 
 in all or part of the others. It has not been determined whether 
 such reflex trophic stimuli are confined to the same lateral half or
 
 SECONDARY DENTIN 
 
 151 
 
 the same nerve distribution. Apparently the formation of dentin 
 proceeds again, after a pause, in all teeth. It will seem, therefore, 
 that the mere exposure of the entire crown to conditions of thermo- 
 change produces sufficient stimulus to the pulp tissue to cause a 
 renewal of dentin formation. After the first period of rest the 
 dentin formed in the second period is so nearly identical, and the 
 direction of the tubules so nearly the same, that it is usually impos- 
 sible to recognize the junction except at a few points in the circum- 
 
 FIG. 130. Secondary dentin: A, margin of primary dentin, showing a few of the 
 tubules continuing into secondary dentin; P, pulp chamber. (About 80 X) 
 
 ference of a transverse section. After each period of rest, however, 
 the difference in structure between the succeeding portions becomes 
 more marked. Fig. 130 shows an area from a longitudinal section 
 when the line A was the pulpal wall of the dentin. There was 
 probably a considerable period of rest, when for some reason a 
 new formation of dentin was begun. But apparently only some 
 of the odontoblasts took part in the new formation of dentin matrix, 
 for not nearly all of the tubules are continued, and those that do 
 continue show a sharp change in their direction and a difference 
 in diameter and character (Figs. 131 and 132).
 
 152 
 
 THE DENTIN 
 
 These characteristic changes in the structure of the dentin that 
 is formed as the pulp becomes smaller seem to the author of great 
 practical importance. 
 
 FIG. 131. A transverse section of a root, showing the reduction in the size of the 
 pulp and formation of secondary dentin: A, A, points at which the changes in the 
 direction of the tubules show dentin formed at different periods; C, cementum 
 thickened and each lamella thicker in the concavity of the dentin; also, the number 
 of lacunae greater. 
 
 Fiu. 132. A transverse section of a root, showing changes in the form of the pulp 
 canal by the formation of secondary dentin.
 
 CHAPTER XII. 
 THE CEMENTUM. 
 
 THE cementum may be defined as a connective tissue whose 
 intercellular substance is calcified and arranged in layers (lamellae) 
 around the circumference of a tooth root, the cells being found 
 in spaces (lacunae) irregularly placed in or between the layers. 
 
 Structurally the cementum is more closely related to the sub- 
 periosteal bone than any other tissue, the only differences being 
 that in general the lacunae in bone are much more uniform in size, 
 shape, arrangement of the canaliculi, and their position \vith refer- 
 ence-to the lamellae than those in cementum. In bone the lacunae are 
 usually found between the lamellae. In cementum the lacunae may 
 be between the lamellae, but they are more often enclosed within 
 their substance and they are found most often where the lamellae 
 are thick. 
 
 Some writers have described Haversian canals in the cementum, 
 but the author has never seen anything that could properly be called 
 an Haversian canal in the cementum from human teeth. Canals 
 containing bloodvessels are not uncommon, but in these the lamellae 
 are never arranged concentrically around the canal, as they are in 
 Haversian systems. For the last fifteen years the author has had 
 under personal observation each year, in the course of class work, 
 not less than 200 longitudinal sections, and 300 transverse sec- 
 tions of the root, ground from human teeth, and in that time 
 he has never seen what could be called an Haversian canal. In 
 the same time he has examined many hundreds of sections cut 
 through the decalcified jaws of various mammals, including the 
 sheep, pig, cat, and dog, with the same negative result. 
 
 Function. The function of the cementum is to attach to the 
 tooth the connective-tissue fibers which hold it in position and 
 support the surrounding tissues. 
 
 The formation of cementum begins as soon as the tooth begins 
 to erupt, and continues, at least intermittently, as long as the 
 tooth remains in place, whether it contains a live pulp or not. 
 
 The function of the cementum cannot be too strongly empha- 
 
 (153)
 
 154 THE CEMENTUM 
 
 sized, and must be continually borne in mind. If, for any reason, 
 the tissues are detached from the surface of the root, they can only 
 be reattached by the formation of a new layer of cementum on the 
 surface of the root, which will embed the surrounding connective- 
 tissue fibers. In order to accomplish this the tissues must lie in 
 physiologic contact with the surface of the root, and the conective- 
 tissue cells must be actively functional. 
 
 That the tissues may be reattached to the surface of a root is 
 both theoretically possible and clinically demonstrable, but for 
 it to occur, biological laws must be observed and the conditions are 
 very difficult to control, especially with the old methods involving 
 the excessive use of strong antiseptics. It is well to remember 
 "that a dentist can never cure a suppurating pocket along the side 
 of a tooth root," but if the conditions can be controlled the cells 
 of the tissue may form a new layer of cementum, reattaching the 
 tissues and so close the pocket. It is a biological problem, not a 
 matter of drugs, except as they are a means of producing cellular 
 reaction. 
 
 In view of its function, therefore, the cementum becomes not the 
 least but the most important of the dental tissues, for no matter 
 how perfect the crown may be, without firm attachment the tooth 
 becomes useless and is soon lost. 
 
 Histogenesis. The cementum is formed by connective-tissue 
 cells lying between the fibers of the tissue which clothes the surface 
 of the root and which becomes specialized for this function. Their 
 origin is undoubtedly similar to that of the osteoblasts, but they 
 are not osteoblasts, either morphologically or functionally, as will 
 be seen later in the study of the peridental membrane, where the 
 cementoblasts and the formation of cementum will be considered. 
 
 Structural Elements. The structural elements of the cementum 
 are: 
 
 1. The lamellae. 
 
 2. The lacuna? and canaliculi. 
 
 3. The cement corpuscles. 
 
 4. The ejnbedded fibers of the peridental membrane. 
 
 The Lamellae of the Cementum and Their Arrangement. The 
 lamellae of the cementum resemble those of bone, but they are very 
 much more irregular both in thickness and appearance. They 
 may be extremely thin and almost transparent, or they may be 
 quite thick and coarsely granular. They arc not nearly as easily 
 observed as those of bone, for in bone the lamellae are marked off
 
 THE LAMELLA OF THE CEMENTUM 155 
 
 by the lacunae which lie between them, while in cementum the 
 lacunae may be entirely absent, and when present are irregularly 
 placed. 
 
 ' 
 
 * 
 
 '''- ^.'- 
 
 cc 
 
 ^mmmmmimmm 
 
 FIG. 133. Hypertrophy of the cementum on the side of the root of a lower molar 
 near the neck of the tooth. From a lengthwise section: human, a, dentin; 6, cementum; 
 c, fibers of peridental membrane. From b to c the cementum is normal and the 
 incremental lines fairly regular, but at d one of the lamellae is greatly thickened. 
 At e this lamella is seen to be about equal in thickness with the others. The next 
 two lamellae are thin over the greatest prominence, but one is much thickened at g, 
 and both at h. These latter seem to partially fill the valleys which were occasioned 
 by the first irregular growth. (1 in. obj.) 
 
 In the gingival portion of the root the lamellse are always thin 
 and very transparent, and lacunae are seldom seen. The entire 
 thickness of the tissue is transparent, and the appearance of the 
 
 FIG. 134. Hypertrophy from root of cuspid: human, in which the irregularity is 
 confined to the first lamella: o, dentin; b, thickened first lamella; c, subsequent 
 lamella?, which are seen to be fairly regular. (1 in. obj ) 
 
 lamellae can be seen only by using a very small diaphragm or oblique 
 illumination. In this position the tissue is largely made up of 
 embedded connective-tissue fibers, which are, however, so perfectly
 
 156 
 
 THE CEMENTUM 
 
 calcified that they cannot be demonstrated in ground sections. 
 In decalcified sections they are very easily seen. 
 
 The cementum becomes gradually thicker in the middle third 
 of the root, and is thickest in the apical third. It will be seen that 
 this increase in thickness is caused chiefly by the greater thickness 
 of each individual lamella. In longitudinal sections the cementum 
 
 . 
 
 fj^. />:'V-V ."'' ";"./: f-r.r'^^h 
 
 r*s& &f^-r--&'l.'t:&i^a3H2?5&&m ^t 
 
 FIG. 135.- -Apex of root of an upper bicuspid tooth with irregularly developed 
 cementum: a, a, dentin; b, b, pulp canals. The lamella? of cementum are marked 
 1, 2, 3, 4, 5, 6, 7, 8, 9; rf, d, d, absorption areas that have been refilled with cemen- 
 tum. It will be seen that the apices of the roots were originally separate, but became 
 fused with the deposit of the second lamella of cementum, and that in this the irreg- 
 ular growth began and was most pronounced. It has continued through the subse- 
 quent lamella?, but in less degree. It will also be noticed that the absorption areas, 
 d, d, d, have proceeded from certain lamellae. That between the roots has broken 
 through the first lamella and penetrated the dentin, and has been filled with the 
 deposit of a second lamella. Other of the absorptions have proceeded from lamellae 
 which can be readily made out. The small points, e, seem to have been filled with 
 the deposit of the last layer of the cementum, while others have one, two, or more 
 layers covering them. (2 in. obj.) 
 
 is often found becoming suddenly thicker at a certain point, and if 
 examined closely, it will be seen that each layer is continued apically, 
 but with greater thickness. Fig. 135 illustrates this condition near 
 the apex of the root. From a study of the lamellte, therefore, it 
 is apparent that the entire root is clothed with successive layers, 
 and that these layers are formed intermittently, but continue to 
 be formed as long as the tooth is in position. In a general way the
 
 THE LAMELLAE OF THE CEMENTUM 157 
 
 number of layers is an index to the age of the person at the time 
 the tooth was extracted (Figs. 136 and 137). The rate of formation 
 is not uniform; for instance, a number of layers may be formed 
 within a short time, and again, a considerable time may elapse 
 between the formation of one layer and the next. The time, however, 
 does not seem to determine the thickness of the layer. 
 
 If a considerable number of teeth of persons of twenty years of 
 age were sectioned, the lamellae counted, and this number compared 
 with the number found in teeth extracted from persons of forty, 
 
 FIG. 136. A transverse section of a root extracted from a young person. The 
 cementum is thin, but is thicker in the grooves on the proximal sides. 
 
 a fairly regular increase in the number of layers will be noticed, and 
 so on, for fifty, sixty, seventy, or eighty years. 
 
 It is important to remember in connection with this formation 
 of cementum that the teeth move, more or less, under the influence 
 of natural forces throughout life, and that every slight change 
 in position must be accomplished by the formation of a new layer 
 of cementum, to reattach connective-tissue fibers in new positions 
 or adjust them to new directions of strain. . 
 
 The first layer of cementum is formed while the tooth is still in 
 its crypt, but apparently no connective-tissue fibers are calcified 
 into it. This forms the first apparently clear and structureless
 
 158 
 
 THE CEMENTUM 
 
 layer which lies next to the granular layer of Tomes (Fig. 138). 
 Even in the teeth the entire length of whose roots are formed 
 before they begin to erupt, there is no attachment until some stress 
 comes upon the crown. The tooth is lying loose in its crypt and can 
 be picked out with very little force. Bicuspids are often accident- 
 ally extracted in the extraction of temporary molars. As soon as 
 
 FIG. 137. A transverse section of a root from an old person. This root had carried 
 a crown for many years. The section was cracked and one edge broken. 
 
 the tooth comes through the gum a new layer of cementum is formed 
 over the entire root, attaching the fibers to its surface, and as the 
 tooth moves occlusally, layer after layer is formed. This will be 
 considered again in connection with the peridental membrane. 
 
 The Lacunae and Canaliculi.- The lacunae of the cementum cor- 
 respond with the lacunae of bone. They differ from those of bone.
 
 THE LACUNA AND CANALICULI 
 
 159 
 
 however, in that they are more irregular in shape, size, position, 
 and relation to the lamellae, and in the number and direction of the 
 canaliculi radiating from them. In bone the lacunae are fairly 
 regular in shape, the long diameter exceeding the short diameter 
 by about one-third. Sections cut through their long axis give an 
 oval outline, the length of which is about three times as great as 
 the width. Sections cut through their short axis give an oval 
 outline, the long diameter being about twice that of the short. 
 The spaces are therefore flattened between the lamellae. In 
 
 FIG. 138. Cementum near the apex of the root: GT, granular layer of Tomes; 
 L, lacunae; B, point at which fibers were cut off and reattached. (About 54 X) 
 
 cementum there is no regularity whatever, either in size or in shape. 
 Some are a little larger than the lacunae in bone, some are very much 
 smaller. They may be almost exactly the shape of typical bone 
 lacunae or they may be distorted into almost any form, sometimes 
 being almost stellate, often pear-shaped, sometimes round, and 
 occasionally pyramidal. The lacunae of bone are fairly uniformly 
 placed, and lie between one lamella and the next. 1 There is no 
 
 This is not absolutely correct, there being much more irregularity in the arrange- 
 ment of the lacunse in thick subperiosteal bone than in either cancellous or Haversian 
 system bone. To be strictly accurate, the above statement must be limited to 
 Haversian system bone (Plate XIII).
 
 160 THE CEMENTUM 
 
 regularity in the relation of the lacunae of the cementum to the 
 lamellae. They sometimes lie between one lamella and the next, 
 but they are more often entirely in the substance of one. They 
 occur only where the lamellae are thick, and there may be large 
 areas with considerable aggregate thickness of cementum in which 
 there are no lacunae at all. 
 
 The number and direction of the canaliculi w r hich radiate from 
 the lacunae of cementum is extremely irregular, but in general 
 there are more extending from the lacunae toward the surface than 
 toward the dentin. 
 
 The Cement Corpuscles. The cement corpuscles correspond 
 exactly to bone corpuscles. They are the cells found in the lacunae. 
 These are simply embedded cementoblasts and are typical connec- 
 tive-tissue cells. They are made up of granular cytoplasm and 
 contain a faintly staining nucleus. Extensions of the protoplasm 
 undoubtedly extend into the canaliculi. These cells bear the same 
 relation to the matrix of the cementum that bone corpuscles do to 
 that of bone. What this is, is not known in any definite way, but 
 it is known that when bone corpuscles are killed or die, the matrix 
 becomes a foreign body, and is either absorbed or cut off from the 
 portion in which the corpuscles are living, to be absorbed or cast 
 out as a sequestrum. The same conditions are true of cementum. 
 For instance, there are many cement corpuscles in the lacunae in the 
 region of the apex of the root. If this portion be bathed in pus for 
 a long time, the cement corpuscles are killed, and the tissue becomes 
 saturated with poisonous materials, so that tissue cells cannot lie 
 in contact with it and live. In order to restore a healthy condition, 
 the necrosed cementum must be removed mechanically until tissue 
 is reached with which cells may lie in physiological contact without 
 injury. Conditions which can only be understood through a knowl- 
 edge of the structure of the tissue often arise in connection with the 
 treatment of alveolar abscess. It should always be remembered 
 that the treatment of an abscess is a biological problem. 
 
 The Embedded Fibers of the Peridental Membrane. The embedded 
 fibers of the peridental membrane are in the strictest sense com- 
 parable with the fibers of Sharpe in bone. They are, however, in 
 many places much more perfectly calcified. To appreciate the 
 relation of the embedded fibers to the matrix, the tissue must be 
 studied both in ground and decalcified sections. For instance, in 
 the gingival portion, from the study of ground sections, the presence 
 of embedded fibers would never be suspected, but if decalcified
 
 EMBEDDED FIBERS OF PERIDENTAL MEMBRANE 161 
 
 sections are studied it will be found to be almost entirely composed 
 of calcified fibers. In the middle and apical thirds of the root, 
 where the lamellae are thicker, the calcification of these fibers is 
 often not as perfect as that of the rest of the matrix. In the prepara- 
 tion of ground sections, therefore, the imperfectly calcified fibers 
 
 FIG. 139. Two fields of cementum showing penetrating fibers: GT, granular layer of 
 Tomes; C, cementum not showing fibers; F, penetrating fibers. (About 54 X) 
 
 shrink and consequently appear as canals in the cementum. In 
 fact, they have often been mistaken for canals. They are usually 
 not seen unless the section happens to cut in their direction. These 
 will be seen in many of the illustrations of cementum. In Fig. 138 
 several layers are seen next to the dentin, in which no fibers appear, 
 11
 
 162 
 
 THE CEMENTUM 
 
 then in several layers the fibers are plainly seen, and finally, the 
 surface layers show no fibers. This probably means that before 
 and after these layers were formed there was a change in the position 
 
 D 
 
 FIG. 140. Record in the calcified tissue of an absorption repaired: D, dentin; 
 Cm, cementum filling absorption cavity. (About 40 X) 
 
 of the tooth and the fibers were all cut off in this area and reattached 
 in a new direction, adapting them to the new directions of strain. 
 
 FIG. 141. Thick lamellae of cementum with many lacuna;, filling an absorption in 
 dentin: L, lacunre; H, Howship's lacunce filled; D, dentin. (About 250 X) 
 
 It is often necessary to study ground sections very closely to deter- 
 mine whether certain appearances are embedded fibers or canaliculi 
 radiating from the lacuna?. The appearance of these fibers should
 
 ABSORPTION AND REPAIR OF CEMENT UM 163 
 
 be studied in Fig. 139. It should be noted that wherever special 
 stress is "exerted upon a bundle of fibers the cementum is thick 
 around them. This may be seen in decalcified sections in Figs. 204, 
 231 and Plate XVII and in ground sections in Figs. 138 and 139. 
 When the next layer is formed, if the fibers are cut off, the additional 
 thickness of the last layer is removed. The unequal thickness of 
 the last formed layer is not usually seen in the layers beneath it 
 to as great an extent. 
 
 Absorption and Repair of the Cementum. From what has already 
 been said about the cementum, it will be understood that this tissue 
 is continually undergoing changes, that new layers are being added, 
 and that often before an addition is made there is absorption 
 enough of it at least to cut off the fibers. When an absorption 
 occurs on the side of a root which cuts into the dentin, the excava- 
 tion in the dentin may be filled by the cementum subsequently 
 formed (Figs. 140 and 141). From a study of ground sections in 
 class work such absorptions are not uncommon. They probably 
 occur when the cusps first come into occlusion in eruption.
 
 CHAPTER XIII. 
 DENTAL PULP. 
 
 Definition. The dental pulp may be defined as the connective 
 tissue occupying the central cavity of the dentin. 
 
 It is composed of embryonal connective tissue which is more 
 closely related to the tissue occupying the spaces of cancellous 
 bone than to any other. 
 
 Functions. The functions of the dental pulp are: 
 
 1. A vital function, the formation of dentin. 
 
 2. A sensory function responding to thermal and chemical change 
 and traumatic irritation. 
 
 Vital Function. The vital function is the formation of dentin 
 and is performed by the layer of odontoblasts. These cells also, 
 by means of their dentinal fibrils, maintain the same relation to the 
 dentin matrix that the bone and cement corpuscles bear to the matrix 
 of bone and cementum. When the pulp is removed from a tooth its 
 dentin becomes dead dentin in the same sense that bone in which 
 the bone corpuscles have been killed is necrosed bone. That there 
 is a constant reaction between the protoplasm of the odontoblasts 
 and the substance of the dentin matrix, or that the presence of the 
 living protoplasm is necessary to prevent degeneration of the matrix, 
 is evidenced by the changes in the physical properties of the dentin 
 after the pulp has been lost. That the tooth remains functional 
 after the loss of the pulp is due to the fact that, except at the minute 
 foramina, the dentin is not in physiologic contact with any tissue 
 excepting enamel and cementum, and that the cementum attaches 
 the tooth to the surrounding tissues, receiving its nourishment 
 from the surface and not from the dentin. 
 
 When the pulp is removed and its place filled by a non-irritating 
 material, the dentin becomes entirely encased in cementum, the 
 foramina probably being covered over as the subsequent lamellse 
 are formed. The author wishes to emphasize, however, the vital 
 relations of the pulp to the dentin matrix. Dead dentin is never 
 as good as living dentin, consequently a tooth from which the pulp 
 has been removed can never be considered just as good as one with 
 the living pulp. 
 (164)
 
 VITAL FUNCTION 165 
 
 The production of the dentin matrix is, of course, the principal 
 part of the vital function of the pulp. It is begun in the develop- 
 ment of the tooth before the dental papilla is converted into the 
 dental pulp, by being enclosed in the dentin formed. After the 
 tooth is fully formed the pulp retains its ability to build dentin 
 matrix as long as it retains vitality, but this function is exercised 
 only in response to conditions of environment which are probably 
 excited through the intervention of its sensory function responding 
 to thermal change and chemical irritation. The sensory function 
 causes a trophic impulse which is manifested by the production of 
 another portion of dentin matrix reducing the size of the pulp 
 chamber. That this is a. reflex and not purely a local matter is 
 indicated by the fact that formations of dentin occur in one tooth 
 when the irritation is in another, and apparently the irritation of 
 one tooth will excite dentin formation in all of the teeth on that 
 side, at least in some instances. On the other hand, purely local 
 responses are found where a few odontoblasts respond to the irrita- 
 tion of their fibrils by the formation of dentin. 
 
 This matter has been referred to under the heading of Secondary 
 Dentin, and it is best studied by the record it leaves in the formed 
 tissue. 
 
 The Sensory Function. In regard to sensation, the pulp resembles 
 an internal organ, as in its normal condition it is always enclosed 
 in the cavity of the dentin. It has no sense of touch or localization, 
 and responds to stimuli only by sensations of pain. The pain is 
 usually located correctly with reference to the median line, but 
 apart from that it is located only as it is referred to some known 
 lesion. If several pulps were exposed on the same side of the mouth, 
 and in teeth of both the upper and lower arches, so that they could 
 be irritated without impressions reaching the peridental membrane, 
 if the patient were blindfolded it would be impossible for him to 
 tell which of the pulps was touched. This characteristic becomes 
 extremely important in diagnosis. 
 
 The pain originating from a tooth pulp may be referred to the 
 wrong tooth or to almost any point on the same side supplied by 
 the fifth cranial nerve. 
 
 The dental pulp is especially sensitive to changes in temperature, 
 amounting almost to a temperature sense, having no exact parallel 
 in any other tissue of the body. This does not amount to a recogni- 
 tion of heat or cold as such, but a special resentment to sudden 
 changes. For instance, if a tooth is isolated and so protected by
 
 166 DENTAL PULP 
 
 non-conductors that the soft tissues cannot be stimulated, and a 
 jet of hot and then cold water be thrown upon its crown, it will 
 respond to each with a sharp sensation of pain, but the patient 
 cannot tell which is hot and which is cold. It is the sudden change 
 that produces the reaction. This is the basis of very important 
 differential diagnosis, for, as is true with most organs, in pathologic 
 conditions its sensory function is exaggerated. 
 
 Histogenesis. The dental pulp is the remains of the dental 
 papilla; the dental papillae for the temporary teeth appearing in 
 the mesodermal tissue of the jaw arches very early in fetal life. 
 The cellular elements are at first very closely placed and large, 
 but they grow smaller and take on the typical form of pulp cells 
 as the intercellular substance is increased. By the sixteenth week 
 the dental papillae for the temporary teeth are covered by a layer 
 of tall columnar cells, which will begin the formation of dentin about 
 that time. After the beginning of dentin formation the transi- 
 tion from the dental papillae to the dental pulp is very gradual, and 
 it would be impossible to draw any sharp line of demarcation 
 between them. 
 
 Structural Elements. The structural elements of the dental 
 pulp are: 
 
 1. Odontoblasts. 
 
 2. Connective-tissue cells. 
 
 3. Intercellular substance. 
 
 4. Bloodvessels. 
 
 5. Nerves. 
 
 6. Lymphatic vessels. 
 
 The Odontoblasts. -The odontoblasts are tall columnar cells 
 which form the outer layer of the pulp adjacent to the dentin, and 
 from which cytoplasmic fibrils extend into the dentinal tubules. 
 
 The character of the odontoblasts changes very greatly with the 
 age of the tissue, and the activity of dentin formation. While the 
 primary dentin is being formed they are tall columnar cells, each 
 containing a large oval nucleus, rich in chromatin and located in 
 the pulpal third of the cell. From the dentinal end of the cell cyto- 
 plasm is continued, without any line of demarcation, into the den- 
 tinal tubule as the dentinal fibril. In some instances two fibrils 
 may be sent from a single odontoblast. The character of the 
 odontoblast is beautifully seen in Fig. 142, a photograph by Professor 
 Rose. 
 
 After the tooth is erupted, but while the formation of dentin 
 is actively going on, the odontoblasts, while somewhat smaller,
 
 THE ODONTOBLASTS 
 
 167 
 
 FIG. 142. Odontoblasts and forming dentin: E, forming enamel; D, forming dentin; 
 O, odontoblasts; Dp, body of dental papilla. (From photomicrograph by Rose.) 
 
 *S ^^SfffinSrfU 
 
 FIG. 143. Odontoblasts. The section cuts obliquely through the odontoblasts; 
 F, fibrils; A*, nuclei of odontoblasts; X', nuclei of connective-tissue cells; W, layer 
 of Weil, not well shown. (About 80 X)
 
 168 
 
 DENTAL PULP 
 
 retain the same typical appearance. They may be easily demon- 
 strated either in decalcified sections or by removing pulps from the 
 pulp chamber of freshly extracted teeth. Professor Salter has 
 described two sets of processes besides (Fig. 
 144) the dentinal fibril process. As a result of 
 teasing the fresh pulps, he considered that fine 
 projections of the cytoplasm extended from 
 the sides of the cells, uniting them to the ad- 
 joining odontoblasts (Fig. 144). These he 
 called the lateral processes. He also described 
 cytoplasmic projections from the pulpal end of 
 the odontoblasts into the layer of Weil. It is 
 probable that these appearances were the result 
 of teasing, and are not true structural charac- 
 teristics, as the work of other investigators has 
 not confirmed their presence. It is easy to 
 understand how teasing the cells apart might 
 produce appearances which might be interpreted 
 as processes, but careful work upon sections 
 does not show their presence. 
 
 In old pulps \vhere the formation of dentin 
 has been intermittent and very infrequent for 
 a long time, the odontoblasts are smaller, lose 
 their columnar form more or less, and become 
 pear-shaped or globular. 
 
 As dentin is one of the most highly specialized 
 connective tissues, the odontoblasts are among 
 the most highly differentiated connective-tissue 
 cells. They are the only connective-tissue cells 
 of columnar form. Morphologically, they are 
 very similar to columnar epithelium, but epi- 
 thelial cells never have such processes as the 
 dentinal fibrils. Occasionally, in young and 
 actively growing bone, osteoblasts are found 
 which are distinctly columnar in form, but they 
 are never as tall as the odontoblasts, and the 
 nucleus is more nearly in the center of the cell. 
 In the case of the osteoblast the cytoplasmic 
 
 FIG. 144. Dia- , . , , . , ,. ,. 
 
 gram of odonto- processes which extend into the canaliculi cor- 
 blasts and dentinal respond to the dentinai fibril process of the 
 
 fibrils. (C. H. Sto- i.ii r P , i i u 
 
 we ii) odontoblast. Ihe homologies between the 
 
 V
 
 THE MEM BRAN A E BORIS 169 
 
 osteoblasts and the odontoblasts have often been lost sight of 
 in the discussions over the character of the latter and their relation 
 to the formation and sensitiveness of the dentin. 
 
 The Membrana Eboris. The odontoblasts form a single layer of 
 cells on the surface of the pulp in contact with the dentin. This 
 layer was very early recognized to be related to the formation of 
 the dentin, and was called the membrana eboris, or the membrane 
 of the ivory. The name has no importance now except as it is found 
 in the literature. 
 
 Size of the Odontoblasts. From what has been said it will be 
 recognized that the size and shape of the odontoblasts vary greatly 
 in different sections. This is true not only of pulps from different 
 animals, and pulps at different periods of development, but of 
 different parts of the same .pulp. In the coronal portion of a pulp 
 from a fully developed tooth, but one in which the formation of 
 dentin is still going on, the average measurements would be about 
 5ju in diameter and 25 to 30/x in height. During early stages of 
 dentin formation, before the crown is fully formed, they are con- 
 siderably larger and taller, and in the pulps of a calf they are much 
 larger than in smaller animals and man. In a constricted pulp, 
 as, for instance, in the mesial root of a lower first molar, the odonto- 
 blasts on the constricted sides will be shorter and relatively thicker 
 than on the buccal and lingual, where the long axis of the cell is 
 in the direction of the long diameter of the pulp, but this simply 
 means that the formation of dentin on the constricted side is rela- 
 tively farther advanced than on the buccal and lingual, and the 
 cells show older phases. It is evident that the supply of nourish- 
 ment to the cells in the constricted portions is more imperfect, 
 and that the ones farthest from the main vessels are most affected, 
 so that dentin formation is slowed and made more imperfect here, 
 while it still continues in full vigor around the expanded portions 
 of the pulp. This has been spoken of in connection with the study 
 of the dentin (see Figs. 131 and 132). 
 
 Origin of the Odontoblasts. The odontoblasts are specialized 
 connective-tissue cells. It is therefore to be expected that they 
 should be formed from undifferentiated connective-tissue cells, as 
 osteoblasts are formed from similar cells of the inner layer of the 
 periosteum and embryonal cells of the tissue filling the cancellous 
 and marrow spaces. The odontoblasts are therefore developed 
 from embryonal cells deeper in the pulp which take their place 
 in the odontoblastic layer. This probably explains the appearance
 
 170 DENTAL PULP 
 
 of some sections, and also, the author believes, the views of some 
 men in regard to the odontoblasts and the dentinal fibrils. In some 
 sections from old pulps the odontoblasts seem to be in an incomplete 
 layer, and their form is more like that of typical connective-tissue 
 cells. 
 
 In considering the origin of the odontoblasts it should be noted: 
 That in the first differentiation of these cells in the embryo. They 
 appear first where epidermal cells (inner tunic of the enamel organ) 
 are in contact with mesodermal cells (the outer layer of the dental 
 papilla). This is true in the formation of the entire length of the 
 root the enamel organ extending on down the dental papilla 
 beyond the point where enamel formation stops. (See Chapter 
 XXVI). The author believes that the meaning and importance of 
 this relationship has not yet been grasped. 
 
 Connective-tissue Cells., The cells in the dental pulp, aside from 
 the odontoblasts, are typical connective-tissue cells such as are 
 found in embryonal tissue. They are of three forms round, 
 spindle-shaped, and stellate. In the crown or bulbous portion the 
 cells are mostly stellate, while in the root portion they are largely 
 spindle-shaped, with the axis of the spindle parallel with the canal. 
 It seems difficult for students to get an idea of their arrangement, 
 and the nucleus is often mistaken for the entire cell. The cells do 
 not lie in contact in a compact tissue, but are widely scattered in 
 the intercellular substance. There is a small ovoid nucleus, which 
 takes the stain deeply, surrounded by a mass of granular cytoplasm 
 stretching away into very fine threads. In the spindle-shaped cells 
 the cytoplasm is stretched out in only two directions. In the stellate 
 cells there may be three, four, or more, stretching away in any direc- 
 tion. Plate IX was very carefully drawn with the camera lucida 
 so as to represent accurately the number, size, and position of the 
 cells in that field as seen with the y 1 ^ oil immersion. It is very 
 difficult in a drawing to represent the third dimension of space, 
 and to show that some of the processes are extending in a plane 
 at right angles to the paper. An idea of this can only be obtained 
 by the very careful use of the fine adjustment while studying the 
 cells with the high power. 
 
 The round cells are probably white blood corpuscles or undiffer- 
 entiated connective-tissue cells which may develop either into stel- 
 late or spindle-shaped. 
 
 The Arrangement of the Cells. Immediately beneath the layer 
 of odontoblasts, for a space about one-half or two-thirds as wide
 
 PLATE IX 
 
 A Field from the Coronal Portion of the Pulp from 
 a Human Molar. 
 
 In the corner the stage micrometer shows i, 1 ,,, of a millimeter drawn 
 with the same lens. The field shows the branching of a bloodvessel and 
 the connective-tissue cells of the pulp. Drawn from ,'_. oil-immersion 
 lens with caniera lueida. (About 12OO X-)
 
 THE INTERCELLULAR SUBSTANCE 171 
 
 as the odontoblastic layer, the cells are very scarce, making a clear 
 line in many sections. This is known as the layer of Weil, and 
 contains many fine nerve fibers which are not stained by ordinary 
 methods. Beyond the layer of Weil for a space perhaps twice as 
 wide as the height of the odontoblasts, the cells are very closely 
 placed. Through the remainder of the pulp they are much more 
 widely but comparatively evenly scattered. 
 
 The Intercellular Substance. Very little is really known about 
 the character of the intercellular substance of the pulp. It contains 
 few fibers, and these in no way resemble bundles of white or elastic 
 connective tissue. The appearance in the section is more as if a 
 structureless gelatinous material had been coagulated by the reagents. 
 
 There are, of course, connective-tissue fibers in connection with 
 the walls of the larger bloodvessels and nerves, and to a certain 
 extent in the gelatinous material. In studying the intercellular 
 substance in the sections it is necessary to remember that it is 
 filled with the protoplasmic projections from the cells, and these 
 are stained, appearing like fibers in the matrix. There is need for 
 further investigation of the character of the intercellular substance. 
 
 The Bloodvessels. The dental pulp is an extremely vascular tissue, 
 and the arrangement of the vessels, the structure of their walls, 
 and the nature of the intercellular substance through which they 
 run render the tissue especially susceptible to the pathological 
 conditions which are associated with alterations in the circulation. 
 
 Usually several arterial vessels enter the pulp through foramina 
 in the region of the apex. These vessels have their origin in the rich 
 vascular network of the cancellous bone (chapter on Peridental 
 Membrane). The arteries follow the central portion of the pulp, 
 giving off many branches as they pass occlusally, and finally form 
 a very rich plexus of capillaries near the surface of the pulp. From 
 these capillaries the blood is collected into the veins, which follow 
 courses parallel to the arteries, leaving the pulp through the same 
 foramina in the region of the apex. It is important to notice that 
 an artery is entering and a vein leaving the tissue through very 
 minute canals in the calcified dentin (Fig. 145). Dr. Stowell has 
 made a very beautiful diagram of the arrangement of the blood- 
 vessels in a single-rooted tooth, which is shown in Plate X. Prep- 
 arations such as would reproduce this diagram can be made by 
 injecting the bloodvessels with an inert material and destroying 
 the soft tissues by artificial digestion.
 
 172 
 
 DENTAL PULP 
 
 Toward the periphery of the pulp very delicate vessels pass 
 outward terminating in loops just beneath the odontoblasts. 
 These are shown in Fig. 146. 
 
 FIG. 145. A section through the apex of a root showing three foraminse, A, B, 
 and C. (Talbot.) 
 
 Structure. The delicacy of the walls of the bloodvessels is one 
 of the most striking histologic characteristics of the dental pulp.
 
 PLATE X 
 
 Bloodvessels of the Dental Pulp. (After Stowell ) 
 
 A well-injected pulp studied under a binocular microscope make^ a 
 very beautiful object which no Hat picture can represent. The larger 
 bloodvessels lying at the centre branch and divide, forming a network 
 which becomes very fine at the surface.
 
 THE BLOODVESSELS 
 
 173 
 
 The largest arteries show only a few muscle fibers in the media 
 and a very slight condensation of fibrous tissue for an adventitia. 
 There is no distinct boundary between the capillaries and the veins, 
 and the vessels continue to have only a wall of endothelial cells 
 after they have reached a size much greater than that of capillaries. 
 Because of this peculiarity of structure the statement is to be found 
 in many text-books of histology that the largest capillaries in the 
 body are found in the dental pulp. These vessels should probably 
 not be considered as capillaries, but as veins whose walls have the 
 structure of capillaries. Even in the largest veins the media is very 
 
 FIG. 146. Dental pulp showing bloodvessel loops extending to the periphery, close 
 to the layer of odontoblasts. 
 
 imperfect, and there is only a slight condensation of fibrous tissue 
 to represent the adventitia. This peculiarity of the bloodvessel 
 walls in the pulp renders the tissue peculiarly susceptible to hyper- 
 emia and inflammation. 
 
 Fig. 147 is a photograph of a bloodvessel whose size can be 
 estimated from the number of red corpuscles seen in it, and the 
 wall is made up of a single layer of endothelial cells. There is no 
 indication of either media or adventitia. The intercellular substance 
 of the pulp being of gelatinous, semifluid character, gives no support 
 to these delicate walls,
 
 174 
 
 DENTAL PULP 
 
 In Plate IX the author has drawn very carefully, with the 
 camera lucida, using a yV immersion lens, a field showing the branch- 
 ing of a small bloodvessel. The size of the endothelial cells, position 
 of their nuclei in the wall of the vessel, and the size, position, and 
 shape of the connective-tissue cells, are represented as accurately 
 as possible. The field is from the coronal portion of the pulp of a 
 human molar. The caliber of such a vessel as this would depend 
 
 FIG. 147. A pulp bloodvessel, showing the thin wall: C, blood corpuscles in the 
 vessel; Bl, bloodvessel wall showing nuclei of endothelial cells; N, nuclei of con- 
 nective-tissue cells in the body of the pulp; /, intercellular substance, showing a few 
 fibers. (About 200 X) 
 
 almost entirely upon the blood-pressure. The endothelial cells 
 will stretch to a very considerable extent under increased pressure, 
 becoming very thin at all points except around the nucleus. When 
 the pressure is decreased the contractility of the cytoplasm pulls 
 the cells together, making it thicker and less in diameter. It is 
 very important to remember these facts in connection with hyper- 
 emia of the dental pulp. It is difficult in such an illustration 
 to give any representation of the third dimension of space, which
 
 PLATE XI 
 
 A Field from the Pulp of an Unerupted Tooth of a Sheep. 
 
 The bloodvessels are cut transversely. 'About 1OOOX.)
 
 LYMPHATICS OF THE DENTAL PULP 175 
 
 is essential to a real understanding of the connective-tissue cells 
 of the pulp. These are bits of cytoplasm with a nucleus forming 
 a small, irregular central mass, from which the cytoplasm is stretched 
 away in all directions through the intercellular substance, ending 
 in very fine threads. 
 
 Plate XI is drawn in the same way from a transverse section of 
 the pulp of an unerupted tooth of a sheep. The vessels are all 
 cut transversely and are seen crowded with red blood corpuscles. 
 They are not distended, and some show slight condensation of fibrous 
 tissue around them. 
 
 In a normal pulp there are many capillaries so small that a single 
 corpuscle passes them with difficulty, but in pathologic conditions 
 they become distended to many times their normal diameter. 
 
 Lymphatics of the Dental Pulp. It was for a long time believed 
 that the dental pulp contained no lymphatic vessels. In 1907-1909, 
 Schweitzer succeeded in injecting lymphatic vessels in the pulp. 1 
 In 1916-1917 Dr. K. Dewey and the author repeated the work of 
 Schweitzer in the Histological Laboratory of the College of Dentistry 
 University of Illinois, and also succeeded in injecting the lymphatic 
 glands of the neck in dogs by injections in the dental pulp. 2 
 
 FIG. 148 
 
 Fig. 148 shows a portion of the pulp of a young dog. The blood- 
 vessels are injected with gelatin carmin, the lymphatics with Berlin 
 blue. Very fine vessels were found close to the surface of the 
 dentin (Fig. 149). From these capillaries vessels pass through the 
 
 1 Schweitzer: Ueler die lymphgefasse des Zahnfleisches und der Zahne beim 
 Menschen und bei Saiigethieren, Archiv. f. Micr. Anat., 1907, p. 807, 1909, p. 27. 
 
 2 A Study of the Lymphatic Vessels of the Dental Pulp, Dental Cosmos, vol. lix, 
 1917, pp. 436-44; Journal of the American Medical Association. Oct. 12-1918, vol. 
 
 ii, pp 1179-1184. '
 
 176 
 
 DENTAL PULP 
 
 central portions of the tissue and pass through the apical foramina 
 where they anastomose with the vessels of the peridental mem- 
 
 
 FIG. 149. Diagrammatic drawing of a section of a tooth, showing injected 
 lymphatic vessels in the pulp. 
 
 FIG. 150 
 
 brane (Fig. 164). For their course from this point see p. 195. In 
 the body of the pulp independent lymph vessels are found and peri- 
 vascular lymph sheath surrounding bloodvessels (Fig. 150).
 
 THE NERVES OF THE DENTAL PULP 177 
 
 The Nerves of the Dental Pulp. Few subjects in connection with 
 dental histology have received more attention than the distribu- 
 tion of the nerves of the dental pulp, especially in relation to the 
 sensitiveness of the dentin. 1 
 
 For fifteen years or more Dr. Howard Mummery has been doing 
 work on the distribution of the nerves of the dental pulps. He has 
 described nerve-end-cells lying between the odontoblasts at their 
 pulpal end, from which neuro-fibrils extend through the layer of 
 odontoblasts and enter the dentinal tubules with the fibers of Tomes. 
 According to his description these cells form true sensory neurons the 
 axon of which extend throughout the dentin in the dentinal tubules, 
 their dendrons connecting with the terminal fibrils of the axons 
 entering the pulp through the apical foramina He considers the 
 odontoblasts as the builders of the dentin matrix, or at least the 
 calcification of it, and the nerve-end-cells to perform the sensory 
 functions formerly ascribed to the odontoblasts and their fibrils. 
 
 Support for almost any idea can be found in the literature, 
 but many of the conditions described have been shown to be 
 errors in microscopic interpretation, and many others have failed 
 to receive support by reinvestigation. The most recent work in this 
 country upon this subject was done fifteen or twenty years ago 
 by Prof. Carl Huber, of Ann Arbor. The author has repeated 
 some of his work, and has never seen any specimen that was con- 
 tradictory to his statements. Usually three or four nerve trunks 
 enter the dental pulp through the foramina. .These contain from 
 eight or ten to thirty or forty medullated nerve fibers. They pass 
 occlusally through the central portion of the pulp, but almost 
 immediately begin to give off branches, which pass toward the 
 periphery, branching and anastomosing in their course. Most of 
 the fibers lose their medullary sheath very soon after leaving the 
 nerve trunk, proceeding as beaded fibers, made up of an axis 
 cylinder with nuclei scattered along it. A bundle of such fibers, 
 breaking up to be distributed to one horn of the pulp, is shown in 
 Fig. 151. Other fibers retain their medullary sheath, following an 
 independent course through the pulp tissue, until they reach the 
 layer of Weil, where the sheath is lost and they join the plexus 
 of beaded fibers lying in this position (Fig. 151). From the plexus 
 
 1 Several investigators have described nerve fibers entering the dentinal tubules. 
 The most complete and elaborate work is that of Howard Mummery. For which the 
 student is referred to, Microscopic Anatomy of the Teeth. J. Howard Mummery, 
 p. 211. 
 
 12
 
 178 DENTAL PULP 
 
 in the layer of Weil beaded fibers are given off, passing between 
 and around the odontoblasts, forming a network around each cell, 
 and even passing over on to the end of the cell between it and the 
 dentin, but they have never been followed into the dentinal tubules. 
 In no instance and by no method that he has employed, has 
 Dr. Huber been able to demonstrate nerve fibers in the dentinal 
 tubules. 
 
 The sensitiveness of the dentin, in view of these observations, 
 is due to the presence of living fibrils, connected with living odonto- 
 blasts which are in physiologic connection with nerve fibers. It 
 
 FIG. 151. -Nerve fibers in pulp from a human molar. (About 500 X) 
 
 is interesting to note that this is the only instance in which a con- 
 nective-tissue cell is intermediate between the outside world and 
 the nerve fiber. In all other instances an epithelial cell is inter- 
 mediate between the environment and the nervous system. The 
 sensitiveness of the dentin is therefore due to the irritability of the 
 cytoplasm of the fibril, transmitted through the continuity of cyto- 
 plasm to the odontoblasts and their reaction upon the surrounding 
 nerve fibers. The irritation to the fibril may be either traumatic, 
 chemical, or thermal. For instance, salt is sprinkled on exposed 
 living dentin, and a sharp sensation of pain is the result. It may be 
 supposed that chemical changes are set up in the cytoplasm of the 
 fibril which excite changes in the cytoplasm of the odontoblasts.
 
 THE NERVES OF THE DENTAL PULP 
 
 179 
 
 These react upon the cytoplasm of the nerve fiber, and so are trans- 
 mitted to the nerve center, being recognized, in consciousness, as a 
 sensation of pain. In the same way traumatic irritation caused, 
 for instance, by the cutting of dentin with a steel instrument sets 
 up changes in the fibril in the same fashion. It is impossible to 
 conceive of any vital activity of cytoplasm otherwise than as a form 
 of chemical action or molecular or atomic movement of its substance. 
 Certain clinical facts are well explained by these structural 
 facts. It is often noted in the preparation of cavities that the 
 dentin is most sensitive at the dento-enamel junction. This would 
 be expected when it is recalled that at the dento-enamel junction 
 the dentinal tubules fork and the fibrils anastomose, so that an 
 
 FIG. 152. Rose's diagram of nerves and bloodvessels of the pulp. 
 
 irritation to a few fibrils is not simply transmitted to their odonto- 
 blasts and the nerve endings in contact with them, but to all the 
 fibrils, and so to the nerves in contact with all of the odontoblasts. 
 The presence of dilute acids render the cytoplasm of the fibrils 
 much more irritable. The dentin in a carious condition is therefore 
 much more sensitive than that in a sound or normal area. The 
 sensitiveness of extremely hypersensitive dentin can often be 
 greatly reduced, if not entirely overcome, by cleansing the cavity 
 thoroughly, washing with tepid water, followed by a dilute alkali, 
 drying and sealing for a few days, when it will be usually found 
 that excavation can be carried out without excessive pain. The 
 sealing must be perfect. If it is leaky the cavity will be more sensi- 
 tive than ever at the end of the delay.
 
 180 DENTAL PULP 
 
 Teeth in which the size of the pulp chamber has been reduced 
 by the formation of secondary dentin are usually much less sensitive. 
 By this formation, as has been seen in the chapter on dentin, many 
 of the tubules are cut off and many of the fibrils reach the pulp 
 only by anastomosing with a few in the later formed dentin. The 
 transmission to the nerves of the pulp is thus made more difficult 
 and imperfect. 
 
 In all considerations of the sensitiveness of dentin, the purely 
 subjective and hysterical symptoms must be carefully watched for. 
 In many cases slight sensations are so magnified by fear and expec- 
 tation as to be considered intolerable. In such cases the diversion 
 of attention and the skilful use of suggestions are of more value 
 when coupled with delicacy of manipulation and operative skill 
 than any means of obtunding. In such cases, although the operator 
 is positive that the sensations are slight, it will never do any good 
 to tell the patient so, or to argue that what is being done cannot 
 hurt. They must be made to believe fully that something has been 
 done to destroy the sensitiveness, and then the attention must be 
 concentrated upon something, while the excavation is lightly and 
 skilfully performed. It makes very little difference what is done, 
 but it must attract the attention in order to plant the belief that the 
 sensitiveness has been removed, and then the attention must be 
 diverted until the manipulation is completed. 
 
 The nerves of the pulp not only respond with sensations of pain 
 from the irritation of the fibrils in the dentinal tubules, but because 
 of their confinement in a calcified chamber and the semifluid nature 
 of the tissue, they are very sensitive to pressure, either increased 
 or decreased. The normal response to changes of temperature, as 
 well as most of the pain in pathologic conditions of the pulp, 
 are probably caused by changes of pressure, through disturbance 
 of the blood circulation of the tissue. The nerves of the pulp con- 
 trol the walls of the arteries through the vasomotor reflexes, and 
 also by trophic fibers control the functional activity of the odonto- 
 blasts in the formation of the dentin. 
 
 In a single tooth the irritation resulting from a carious cavity 
 is found to cause the formation of dentin not simply in the region 
 reached by the irritated fibrils, but upon the entire wall of the pulp 
 chamber and apparently also in other teeth. It has seemed possible 
 to the author that in some instances osmotic conditions might be a 
 factor in the production of pain in the pulp, especially in the early 
 stages of caries.
 
 CHAPTER XIV. 
 THE LYMPHATICS OF THE DENTAL REGION. 
 
 GENERAL CONCEPTION OF THE LYMPHATIC CIRCULATION. 
 
 THE student generally finds difficulty in getting any clear idea of 
 the lymphatic circulation. It seems best, therefore, to make a 
 most simple and elementary statement of this most important 
 circulatory system as a basis for a study of the lymphatic vessels. 
 Life at present can be understood only in terms of a single cell. 
 Every living cell must be bathed in fluid from which the cytoplasm 
 receives the material for its constructive processes and to which it 
 gives up its waste products or the results of catabolism. Just as 
 the single-celled protozoan floating in a pond of water, so each cell 
 of every tissue of the body can be considered as bathed in a fluid 
 the lymph. The epithelium of all external and internal surfaces 
 makes a bounding layer which prevents the loss of the fluid. If a 
 slight cut or abrasion is made on the skin, removing the outer layer 
 of dried cells and not breaking the blood capillaries, there will appear 
 the exudation of a drop of yellowish fluid on the surface. This 
 fluid immediately coagulates and prevents further loss until the 
 continuity of the surface is restored. In this simple way we may 
 demonstrate the presence of the intercellular fluid or lymph. 
 
 For the health and nourishment of the cells this fluid must be in 
 circulation or the cells would be poisoned by their own products of 
 catabolism. In a very general way the blood circulatory system 
 may be said to be the means of bringing oxygen to the tissues and 
 the lymph circulatory system the means of supplying the material 
 for metabolism. 
 
 The fluid of the blood passes through the cells of the capillary 
 walls into the intercellular and tissue spaces, and in that sense may 
 be considered the source of the lymph. The passage of the blood 
 plasma through the capillary walls is not simply a matter of trans- 
 fusion or osmosis, but is a vital function of the cells of the capillary 
 walls. The intercellular lymph is not the same as the plasma of the 
 blood in the bloodvessels, for from it the cytoplasm of the tissue cells 
 have taken up material and to it they have given products of 
 metabolism. 
 
 (181)
 
 182 LYMPHATICS OF THE DENTAL REGION 
 
 The fluid from the intercellular and tissue spaces is collected by a 
 system of vessels, the lymphatic vessels, and returned to the blood 
 circulation through the thoracic duct emptying into the left sub- 
 clavian vein. On the right a very short, lymphatic duct, not more 
 than 10 to 12 mm. in length, empties into the right subclavian vein. 
 Very frequently no right lymphatic duct exists, the jugular and sub- 
 clavian trunks opening independently into the right subclavian vein. 
 
 Formerly it was supposed that the smallest of the lymph vessels 
 or lymph capillaries opened directly into the intercellular and tissue 
 spaces, but it has become more and more evident that this is not 
 correct but that the lymphatic vessels form a closed system opening 
 only into the subclavian veins. The intercellular fluid passes into 
 the lymph capillaries through their wall by a vital process. A 
 diagram of the lymphatic vessels and their relation to the blood 
 circulation is shown in Plate XII. 
 
 It is undoubtedly true that the blood capillaries also may take up 
 fluid from the tissue as well as give up fluid to it and it is certain that 
 they take up products of metabolism from the tissue cells. But as 
 a beginning and elementary idea the statement may be made that 
 the plasma of the blood passes out of the capillaries, bathes the cells, 
 giving up material to them and receiving products from them, 
 and is returned to the blood circulation through the lymphatic 
 vessels. 
 
 In comparing the two systems in Plate XII several things can be 
 noted: (1) The blood passes from the heart, through the arteries 
 to the capillaries and back to the heart in the veins; and is a closed 
 system all the way. The lymph is collected from the tissue spaces 
 by the lymphatic capillaries, passes through collecting trunks to the 
 glands, where it passes through the capillaries again and on to the 
 blood circulation through the subclavian vein. (2) The blood cir- 
 culation is the oxygen carrier, the lymphatic circulation the food 
 and waste carrier. (3) The blood circulation is rapid, the lymph 
 circulation slow. 
 
 Lymphatic Nodes or Glands. Along the course of the lymphatic 
 vessels are placed structures, lymphatic nodes or glands in which 
 the fluid must come in contact with masses of active cells for the 
 purpose of preventing infection carried in the current from reaching 
 the blood circulation and so the entire body. For the structure of 
 the lymph nodes and their relation to the lymphatic vessels the 
 student is referred to text-books of histology and anatomy.
 
 General Scheme of the Lymphatic System
 
 PARTS OF THE LYMPHATIC SYSTEM 183 
 
 PARTS OF THE LYMPHATIC SYSTEM. 
 
 To have a conception of this system, the fluid that circulates, the 
 cells it carries, the vessels through which it goes, and the tissue or 
 special structures through which it passes in its course, must be 
 studied in their relation to each other. 
 
 1. Lymph. 
 
 2. Leukocytes (cells found in the lymph). 
 
 3. Lymph vessels. 
 
 4. Lymphatic glands (lymph nodes). 
 
 Lymph. The lymph is a slightly viscous liquid, sometimes with 
 slightly yellowish color, no or very slight odor, slightly alkaline 
 reaction, and specific gravity of 1.012 to 1.022. Krause states that 
 the entire quantity of lymph is equal to one-third of the body weight. 
 Five and one-half liters have been collected from the thoracis duct 
 from man in twenty-four hours. The quantity is dependent upon 
 tissue activity. 
 
 From the most fundamental conception of it the lymph must be 
 slightly different from the plasma of the blood. And its chemical 
 composition must be variable. It is slightly less alkaline and 
 contains less fibrin than the blood plasma. 
 
 Leukocytes. The term leukocytes includes cells that are found 
 in the blood, lymph, and connective tissues, and is synonymous 
 with white blood corpuscles. 
 
 The leukocytes are soft cytoplasmic masses with no cell wall, 
 nearly colorless, extensible, and of varying refraction. They are 
 heavier than lymph or plasma and lighter than red corpuscles. They 
 are viscous, adhering to a glass slide and sticking to the walls of 
 vessels, resisting the current which carries them along, so that they 
 accumulate when the current slackens. 
 
 They possess all the biological properties of primitive cells, 
 mobility, sensibility, absorption, secretion and reproduction. 
 Such important functions as the absorption of foreign matter and 
 bacteria are dependent upon these primitive functions. 
 
 Leukocytes have been classified by their form, size, the character 
 of the nucleus and the granules found in the cytoplasm. 
 
 Lymphatic Vessels. Lymphatic vessels were discovered by the 
 ancient Greeks and were known by Aristotle (384-322 B.C.), but 
 the knowledge was lost and they were rediscovered by Nicholas 
 Massa in 1532 A.D. In 1563 Eustachius discovered the thoracic 
 duct.
 
 184 
 
 It was formerly believed that the lymph in the intercellular 
 spaces drained into the interfibrous spaces in the connective tissues, 
 that these became lined with endothelial cells and that the lymph 
 capillaries opened into them. It has been more and more apparent 
 that the lymphatic vessels present a system closed at the periphery, 
 and opening into the subclavian vein at the opposite extremity 
 This does not in any way change the action of the system. The 
 
 taking up of the lymph from the 
 tissue spaces cannot be thought of 
 as a simple process of filtration but 
 as a vital function of the cells form- 
 ing the closed ends of this term- 
 inal or collecting plexus of the lym- 
 phatic capillaries. The entire 
 system of the lymph vessels may 
 be more clearly understood if it 
 is thought of as made up of the 
 
 If- 
 
 FIG. 153 FIG. 154 
 
 FIGS. 153 and 154. Lymphatics in involution. Fig. 153, lymphatic vesicle 
 continuity with neighboring trunk; Fig. 154, isolated vesicle. (After Ranvier.) 
 
 following parts: (1) The network of origin or terminal plexus 
 of the lymphatic capillaries which take up the lymph from the 
 tissues and organs. (2) A few vessels collecting trunks drain a com- 
 paratively large area of the collecting capillary network and carry 
 the lymph from the network to the first lymphatic gland. (3) In 
 the gland or node it again breaks up into capillaries, but leaves the 
 gland through one vessel, the efferent vessel. (4) Larger and less 
 numerous efferent ducts which carry the lymph from one node to 
 another or from the last node to the venous system.
 
 PARTS OF THE LYMPHATIC SYSTEM 185 
 
 The structure of the vessels is different in the different parts but 
 may be described in general by saying that the capillaries and small 
 collecting vessels are lined by a single layer of exceedingly delicate 
 endothelial cells and the larger trunks show three layers similar 
 to the walls of the veins but more delicate in structure (Figs. 153 
 and 154). 
 
 Fia. 155 
 
 As a general statement the network of origin is in the subepithe- 
 lial connective tissue. The collecting and transporting trunks are 
 found in the connective tissue and are either superficial or deep, as 
 they are above or below the fascia. The superficial vessels are 
 usually more highly developed. 
 
 The total capacity of the network of origin is very great, being 
 equal to or greater than that of the veins, but the capacity is greatly 
 reduced in the collecting and efferent ducts, so that the entire system
 
 186 
 
 is representative of a cone, with the base in the network of origin and 
 the apex in the opening into the subclavian veins. 
 
 There are two entirely independent systems of the lymphatic 
 vessels, one emptying into the right subclavian vein through the 
 right lymphatic duct, draining the right side only as far as the level 
 of the diaphragm, and the other into the left subclavian vein through 
 the thoracic duct, draining all of the rest of the body. The area of 
 the body drained by each system is represented in the diagram in 
 Fig. 155. 
 
 The Network of Origin. The delicate vessels which form the net- 
 work of origin are often called the lymphatic capillaries. They 
 resemble the blood capillaries only in that their walls are formed 
 by a single layer of endothelial cells. They are of extremely variable 
 form, depending upon the character of the tissue in which they are 
 found. They form a very rich anastomosing network of very deli- 
 cate vessels, some idea of the structure of which can be had from 
 Figs. 156 and 157. A few very delicate vessels collect the lymph 
 from this network and carry it to the collecting trunks. The capil- 
 laries are without valves but the collecting vessels are abundantly 
 supplied with them (Fig. 154), which causes their characteristic 
 beaded appearance. Stained \vith silver nitrate the cells are more 
 easily outlined than those of the blood capillaries, showing cells 
 30 to 40 microns long. Their edges are wavy, forming lines like the 
 sutures of the skull. Their nuclei are oval and project into the 
 cavity of the vessel, especially when they are not distended. The 
 diameter of these vessels may be from 30 to 60 microns, which is 
 much greater than that of the blood capillaries. 
 
 The Collecting Trunks. The walls of the collecting vessels are 
 made up of three layers: (1) The endothelium. (2) A layer of 
 involuntary muscle. (3) An adventitious layer of white elastic 
 connective tissue. They are like the walls of the veins, but more 
 delicate, less destructible and more resilient to pressure. 
 
 Lymphatic Glands or Lymph Nodes. For the structure of the 
 lymph nodes the student is referred to text-books of histology. 
 They are by no means constant either in number, size or position. 
 In order to understand the lymphatics of the dental region it is 
 necessary to make a brief statement of the principal groups of the 
 head and neck and the regions which they drain. 
 
 The Lymphatics of the Head and Neck. The lymphatic glands of 
 the head and neck may be described as arranged in six groups, 
 forming a grandular collar or circle at the junction of the head and
 
 LYMPHATICS OF THE HEAD AND NECK 
 
 187 
 
 neck from which two vertical chains extend under the sterno- 
 mastoid muscle and along the large bloodvessels and nerves extend- 
 ing to where the neck joins the thorax. These main vertical chains 
 are flanked by lesser auxiliary chains (Fig. 158). 
 
 .-/.>." ^; : -'. ;: ':/ ; vi::;-'-:--' 
 
 FIG. 156 
 
 FIG. 157 
 
 The glandular collar is composed of (1) the suboccipital group; 
 (2) the mastoid group; (3) the parotid and subparotid group; (4) the
 
 188 
 
 LYMPHATICS OF THE DENTAL REGION 
 
 submaxillary group; (5) the submental group; (6) the retropharyn- 
 geal group. 
 
 1. The suboccipital group usually contains two glands. They 
 receive efferents from the occipital portion of the scalp. Their 
 efferents terminate in the highest glands of the substernomastoid 
 group of the vertical chain. 
 
 .. 
 
 FIG. 158 
 
 2. The Mastoid Group. There are usually two, one behind the 
 other, and are united by two or three trunks. They lie on the mas- 
 toid insertion of the mastoid muscle. They receive afferents from 
 the temporary portion of the scalp, from the external surface of the
 
 LYMPHATICS OF THE HEAD AND NECK 189 
 
 auricle, except the lobule, and the posterior surface of the external 
 auditory meatus. Their efferents empty into the superior glands of 
 the submastoid group after traversing the superior insertion of that 
 muscle. 
 
 3. The Parotid Group. This group is made up of (1) the sub- 
 cutaneous glands, which are often absent; (2) the glands contained 
 in the parotid space; (3) the subparotid glands. 
 
 The glands of the parotid space are situated on the external 
 surface of the gland or in its external substance. The superficial 
 ones are usually two or three in number. The deep ones are scattered 
 through the entire substance of the gland and are usually grouped 
 along the external jugular vein and the external carotid artery. 
 One constantly occupies the lower part of the space and is attached 
 close to the angle of the jaw in contact with the cervical fascia. 
 They receive afferents from the external surface of the auricle and 
 external auditory meatus, from the tympanum, from the skin of 
 the templar and frontal region, the eyelid and root of the nose. 
 They perhaps also receive vessels from the nasal fossa and the pos- 
 terior part of the alveolar border of the superior maxilla. Their 
 efferents empty into the substernomastoid group. 
 
 The subparotid glands are placed between the parotid and the 
 pharynx in the lateropharyngeal and posterior subglandular space. 
 They are in contact with the internal carotid and the internal 
 jugular. They are the starting-point of the lateropharyngeal 
 abscess (Qtiaine). They receive afferents from the nasal fossa, 
 nasal pharynx and Eustachian tube. Their efferents pass to the 
 glands of the deep cervical chain. 
 
 4. Submaxillary Glands. These glands, three to six in number, 
 are the most important from the dental standpoint. They form a 
 chain stretching along the inferior border of the mandible from the 
 insertion of the anterior belly of the digastric to the angle of the 
 jaw. They are found in the junction of the cutaneous and bony 
 surface of the submaxillary gland on which they rest. The largest 
 and most constant of the chain is found at the point where the 
 facial artery crosses the border of the mandible. They receive 
 afferents from the nose, the cheek, the upper lip and external part 
 of the lower lip, the anterior third of the lateral border of the tongue 
 and almost the whole of the gums, alveolar process and teeth of 
 both upper and lower arch. Their efferents descend on the cutaneous 
 surface of the submaxillary gland, across the hyoid bone and ter- 
 minate in the glands of the deep cervical chain, over the bifurcation
 
 190 LYMPHATICS OF THE DENTAL REGION 
 
 'of the carotid artery or much deeper, where the omohyoid crosses 
 the internal jugular vein. 
 
 5. The Submental Glands. These glands are extremely variable 
 in number and position. Usually one to four in number they are 
 found in the triangle between the anterior bellies of the digastric 
 muscle and the hyoid bone. They receive afferents from the chin, 
 the central portion of the lower lip, the tip of the tongue and the 
 anterior portion of the alveolar process and the lower incisor teeth. 
 The latter is probably not constant. 
 
 6. The Retropharyngeal Group. These glands are placed behind 
 the pharynx at the junction of the posterior and lateral surfaces, at 
 the apex of the lateral masses of the atlas. Usually two in number 
 they are in relation with the posterior wall of the pharynx and the 
 anterior surface of the rectus capitis anticus major and externally 
 with the constrictors of the pharynx. They are about two centi- 
 meters from the median line. They receive afferent vessels from 
 the mucous membrane of the nasal fossae and the cavities connected 
 with it, the nasal pharynx, Eustachian tube and perhaps the tym- 
 panum. Their efferent vessels empty into the superior glands of 
 the internal jugular chain. 
 
 Descending Cervical Chains. These extend from the glandular 
 collar through the neck to the thorax. The most important chain 
 is the deep cervical chain, one on each side, under the sternomastoid 
 muscle and in the subclavian triangle. The smaller are the external 
 jugular chain, the two anterior cervical chains, superficial and deep, 
 and the recurrent chain. 
 
 The deep cervical chain (Fig. 166) is one of the largest and most 
 important relays in the body. It contains fifteen to thirty glands. 
 It is made up of two groups: (1) the upper or sub sternomastoid 
 group, and (2) the lower or subclavian triangular group. Only the 
 first group will be considered. 
 
 Substernomastoid Glands. 1. External Glands: Behind and 
 external to the internal jugular vein. Afferent vessels are received 
 from the occipital and mastoid glands and from cutaneous lym- 
 phatics from the posterior part of the head and neck. 
 
 2. Internal Glands: Rest on the internal jugular or along its 
 external border. At different points in the chain, glands of special 
 importance are found; for instance: (a) Beneath the posterior belly 
 of the digastric, the principal terminus for lymphatics from the 
 tongue and gum about the lower teeth on the lingual. (&) "Where 
 the omohyoid crosses the internal jugular. Afferent vessels : These
 
 LYMPHATICS OF THE HEAD AND NECK 
 
 191 
 
 glands form the second relay for lymphs from the (a) retropharyn- 
 geal and (&) parotid and subparotid. 
 
 3. Submaxillary. 
 
 4. Submental glands. 
 
 5. The superficial and deep anterior cervical chain and the recur- 
 rent chain. They receive direct afferents from: (a) the majority 
 of the vessels from the tongue; (6) part of the nasal pharynx and 
 larynx; (c) the vault of the palate and soft palate; (d) the cervical 
 portion of the esophagus; (e) the nasal fossae; (/) the larynx and 
 trachea; (g) the thyroid body. 
 
 FIG. 159 
 
 The Network of Origin in the Dental Region. The lymphatic 
 network of origin is absolutely continuous over the whole of the 
 face, eyelids, conjunctiva, lips and the mucous membrane of the 
 lips, cheeks, gums and gingiva. Every papilla of the connective 
 tissue under the epithelium contains such networks of vessels as 
 are shown in Fig. 159 from papilla 1 of the hand. Exactly such 
 structures can be shown from the mucous membrane of the gum 
 and gingiva 1 . These capillaries empty into an exceedingly rich net- 
 work of very delicate vessels in the subcutaneous and submucous 
 layer, which is illustrated in Eig. 100. It is difficult for the element-
 
 192 
 
 LYMPHATICS OF THE DENTAL REGION 
 
 ary student to get any conception of the fineness, delicacy and inter- 
 communicating anastomosis of this network. From this network 
 a few collecting vessels lead to the afferent trunks going to the first 
 glands. There is therefore a more or less definite drainage for a 
 given area, though the network of origin is continuous. 
 
 Lymphatics of the Lips. In the lips there are two networks: one 
 in the subcutaneous layer of the outer surface and one in the sub- 
 mucous layer of the internal surface. These communicate freely 
 at the border of the lips. Each network is drained by a few collect- 
 ing trunks, which receive lymphatic vessels from the muscular 
 
 FIG. 160. Lymphatic vessels of the collecting network. (Sappey.) 
 
 layers that are less developed. The subcutaneous collecting vessels 
 of the lower lip, two to four in number on each side, frequently 
 cross and anastomose at the median line. Those from the middle 
 portion pass to the submental glands. Those from the region of the 
 commissure reach the most anterior of the submaxillary glands 
 (Fig. 161). The submucous collecting vessels, two or three on each 
 side, pass obliquely downward and outward to the region of the 
 facial artery and end in the submaxillary glands. They do not cross 
 or anastomose at the median line. There are two submucous and 
 two or three subcutaneous collecting vessels in the upper lip. They 
 all pass obliquely downward and outward, usually to the middle
 
 LYMPHATICS OF THE MOUTH AND GUMS 
 
 193 
 
 gland of the submaxillary chain. One of these may enter the most 
 external of the collecting trunk from the lower lip. 
 
 Lymphatics of the Mucous Membrane of the Mouth and Gums.- In 
 the mucous membrane of the mouth and gums the network of origin 
 forms an exceedingly close network. 
 
 From the outer surface of the mandible the collecting vessels form a 
 wreath of interlacing vessels at the reflection of the mucous mem- 
 brane from the bone to the cheek. The vessels increase in size as 
 they pass distally and finally penetrate the cheek and end in the 
 submaxillary glands, especially the last one. 
 
 FIG. 161 
 
 From the inner surface of the mandible a similar wreath of collecting 
 vessels is formed at the reflection of the tissue from the bone to the 
 floor of the mouth and tongue. From the anterior part, lingual to 
 the incisors, the vessels pass, with those from the tip of the tongue 
 to the submental glands. From the lateral portion they unite with 
 lymphatics from the anterior part of the lateral surface of the tongue 
 and pass to the glands of the submaxillary chain. From the region 
 of the second and third molars they probably join the lymphatics 
 from lateral portions of the base of the tongue in the region of the 
 tonsil and pass to the large gland of the deep cervical chain, placed 
 under the posterior belly of the digastric. 
 
 Outer Surface of the Maxilla. From the outer surface of the 
 upper arch the collecting vessels pass to a wreath of large vessels 
 at the reflection from the bone to the cheek. These increase in size 
 13
 
 194 
 
 LYMPHATICS OF THE DENTAL REGION 
 
 as they extend distally. At the level of the molars they pierce the 
 cheek, join the facial artery and terminate in the posterior glands 
 of the submaxillary chain (Fig. 162). On the lingual the collecting 
 vessels first pass obliquely backward and toward the median line of 
 
 FIG. 162. Lymphatic vessels of the palate. (After Sappey.) 
 
 the palate, then backward and upward at the junction of the hard 
 and soft palates. They pass in front of the anterior pillar of the 
 fauces, pierce the superior constrictor of the pharynx and end in the 
 large gland of the deep cervical chain under the posterior belly of the 
 digastric.
 
 LYMPHATICS OF THE PERIDENTAL MEMBRANE 195 
 
 Lymphatics of the Peridental Membrane. The lymphatic capillaries 
 in the papillae under the epithelium on the labial or buccal and 
 lingual surfaces of the gingivee pass to the collecting network in the 
 submucous connective tissue outside the periosteum on the surface 
 of the alveolar process (Fig. 162). The lymphatic capillaries from 
 the papillae under the epithelium lining the gingival space are col- 
 
 FIG. 163. Unstained section, showing lymph capillaries of the tooth side of the 
 gingiviB and their drainage through the ligamentum circulare to the peridental 
 membrane. 
 
 lected in very fine vessels which pierce the ligamentum circulars 
 very close to the surface of the cementum and extend in the inter- 
 fibrous tissue of the peridental membrane accompanying the blood- 
 vessels (Fig. 163). At the level of the apex of the root they receive 
 lymphatics coming from the dental pulp (Fig. 164) and pass through 
 the cancellous spaces of the bone to the inferior dental canal in the
 
 196 
 
 LYMPHATICS OF THE DENTAL REGION 
 
 
 
 s 
 
 FIG. 164. Transverse section just at the apex of the root, showing injected 
 lymphatic vessels in the peridental membrane and in the canals passing to the 
 pulp (oc., 2; obj., 16 mm.; reduced about one-third). 
 
 FIG. 10."), DDK'S head, showing lymphatic glands injected from dental pulp.
 
 LYMPHATICS OF THE TONGUE 197 
 
 lower and the infraorbital canal in the upper. They emerge on 
 the surface of the bone at the mental foramin, or the infraorbital 
 foramen and end in the posterior or middle glands of the submaxil- 
 lary chain, following the course of the facial artery (Fig. 165). A 
 great amount of work remains to be done on the drainage of the 
 teeth in different regions. Little or nothing is known of the course 
 of the vessels from the upper incisors, lower incisors and second and 
 third molars. Lymphatics from the lower incisors may pass to the 
 submental glands. Those from the upper incisors probably reach 
 the surface of the bone below the level of the floor of the nose and 
 join the vessel coming from the infraorbital canal, though it is 
 possible that some of them join vessels in the floor of the nose. It 
 is quite probable that lymphatics from the second and third molars 
 pass to the glands of the parotid group. 
 
 Lymphatics of the Dental Pulp. For many years the dental pulp 
 was said to be devoid of lymphatics and all attempts to inject vessels 
 in the dental pulp failed. In 1909 Schweitzer reported successful 
 injections of the dental pulp, and in 1914 Dr. Kaethe Dewey and 
 the author repeated Schweitzer's results and succeeded in injecting 
 lymph capillaries of the submaxillary lymph glands in the dog by 
 injections into the dental pulp and followed the course of the vessels 
 continuously from the pulp to the glands (Fig. 165). There is much 
 work to be done in this field before our knowledge will be at all 
 complete regarding both the perivascular lymph sheath and the 
 independent lymph vessels. The vessels begin at the surface of the 
 pulp and follow the course of the bloodvessels to the apical foramina, 
 where they join the lymphatics of the peridental membrane. Their 
 course from this point has already been followed. 
 
 Lymphatics of the Tongue. 1 The lymphatics of the tongue are 
 very highly developed and have been thoroughly studied. There 
 are two networks of origin: one superficial in the mucous membrane 
 and one deep in the muscular body of the tongue. Their efferent 
 vessels unite in the submucosa. 
 
 The collecting trunks are divided into four groups: (1) Anterior 
 apical. (2) Lateral marginal. (3) Posterior or basal. (4) Median 
 or central. 
 
 1. Anterior Apical Trunks. These vessels, two on each side, run 
 along the frenum to the posterior surface of the mandible. Here 
 they separate (Fig. 166) : (1) One runs downward and backward 
 
 1 See page 270, The Lymphatics by G. Delarnere, P. Poirer and B, Cuneo. 
 Edited by Cecil H. Leaf.
 
 198 
 
 LYMPHATICS OF THE DENTAL REGION 
 
 between the geniohyoglossus and the mylohyoid crosses the great 
 cornu of the hyoid bone behind the anterior belly of the digastric 
 and along the external border of the omohyoid to the gland of the 
 deep cervical chain where this muscle crosses the internal jugular 
 vein. (The general statement is that the more anterior the origin 
 in the tongue the lower the gland in the deep cervical chain to which 
 it goes.) (2) The second trunk passes to the submental gland. 
 
 FIG. 166 
 
 2. The Marginal Trunks. These vessels collect from all the 
 mucous membrane from the tip of the tongue to the V-shaped 
 groove on the dorsal surface. They are eight to twelve in number: 
 (1) One group, the external (three or four), pierce the mylohyoid 
 and pass around the inferior border of the mandible to the glands 
 of the submaxillary chain. (2) The internal (five or six). These 
 vessels run downward and backward on the muscles of the tongue 
 and all end in glands of the deep cervical chain. 
 
 3. Basal Trunks. These vessels (seven or eight) arise from 
 the region of the circurnvallate papillae and are the largest and
 
 LYMPHATICS OF THE TONGUE 199 
 
 most important vessels of the tongue. They form a medial and 
 lateral group and all terminate in the large gland of the deep cervical 
 under the posterior belly of the digastric. 
 
 4. The Central Trunks. These vessels arise from the middle part 
 of the dorsal network of the body of the tongue. Instead of running 
 outward they descend in the middle line between the two genio- 
 hyoglossi and end in the glands of the deep cervical chain.
 
 CHAPTER XV. 
 INTERCELLULAR SUBSTANCES. 
 
 DURING the last hundred years, knowledge of living things and 
 all thought of their structure and function has entirely changed. 
 The cell theory has abundantly established that the cell is the 
 structural and functional unit of all living objects, both plant and 
 animal, and that all manifestations of life are accomplished by the 
 chemical activity of the substance of the cell, which Huxley long 
 ago designated as "the physical basis of life." From a considera- 
 tion of the physical properties of cytoplasm, nothing is more appar- 
 ent than that the production of a highly organized body out of 
 it alone would be impossible. If the human body were composed 
 entirely of cytoplasm it would be a shapeless lump of jelly. It 
 is only by the production of material which has physical properties 
 of strength and rigidity through the activity of the cytoplasm that 
 the shape and function of a highly organized creature is possible. 
 This is accomplished through the metabolism of the cytoplasm 
 more or less analogous to the building up of a secretion by the cells 
 of a gland, though there is no intention to suggest any direct com- 
 parison between the two. In other words, all tissues are made up 
 of cells and intercellular substance, and the vital characteristics 
 are given to the tissue by the cells, the physical characteristics by 
 the intercellular substance. These intercellular or extracellular 
 materials possess none of the vital manifestations, and are entirely 
 dependent upon the cells for their formation and maintenance. 
 There is apparently a constant reaction between the cell and the 
 formed material which constitutes the intercellular substance, for 
 even the most highly specialized of intercellular substances repre- 
 sented by the dentin matrix changes in its properties if the cells 
 are removed. If the cells in the bone matrix are killed, that portion 
 of the tissue becomes necrosed bone and is as much a piece of 
 foreign matter as if a piece of bone toothbrush handle had been 
 shot into the body. The fibers of fibrous tissue have no ability to 
 grow, to attach themselves to any surface, or even to maintain 
 their present form without the presence of living cells or fibroblasts. 
 ( 200 )
 
 INTERCELLULAR SUBSTANCES 201 
 
 There has been a great deal of discussion as to the method of forma- 
 tion of intercellular substances by the cells, and the nature of the 
 reaction occurring between the cell and the formed material after 
 it has been produced. In several intercellular substances the 
 material passes through changes both of physical and of chemical 
 character, but these are carried out by reaction with materials 
 formed by the metabolism of the cell, for if the cells are removed 
 the formed material will not go through any such changes. The 
 intercellular substances, therefore, while they are chemically 
 extremely complex, belong to the simplest classes of protein mole- 
 cules, and have no such complexity of atomic movement, producing 
 conditions of recurrent unsatisfied affinity, without which no idea 
 of the metabolism of living cytoplasm can be obtained. Chemically, 
 living cytoplasm may be roughly viewed as constantly undergoing 
 chemical changes which are almost infinitely complex, and by means 
 of which simpler substances are acted upon and built into its own 
 molecule. Complex combinations are thrown off as products of 
 its metabolism, and simpler substances are formed as decomposi- 
 tion products, or waste materials. Dr. Brooks often used to say 
 in his lectures that the most striking characteristic of living things 
 was their ability to react upon their environment in such a way 
 as to become better and better suited to it. When living cytoplasm, 
 which is soft and without the physical properties of strength and 
 rigidity, requires protection from physical influences, substances 
 possessing these qualities are produced by it. Intercellular sub- 
 stances therefore were apparently formed by the cytoplasm in 
 response to physical conditions of its environment, and are one of 
 the phases of adaptation. 
 
 In the higher forms of animal life the class of tissues which 
 have produced these formed materials, for the purpose of support, 
 rigidity, and connection, are called the connective or supporting 
 tissue. The formed materials are of two classes those which are 
 to connect associated and dependent parts, and those which give 
 rigidity and protection. The fibrous tissues are of the first class, 
 and are made up of materials possessing strength and elasticity. 
 The bone and cartilage belong to the second class, and give strength 
 and rigidity. The first sustain pulling stress, the latter shearing 
 or bending stress, though both possess a certain amount of each. 
 
 Adaptability and the greatest range of variation are most strik- 
 ing characteristics of connective tissue which develop and change 
 to meet all kinds of requirements of both mechanical and physical
 
 202 
 
 INTERCELLULAR SUBSTANCES 
 
 environment to which they are subjected. These variations are 
 produced by the production of increased amount of the intercellular 
 material, its destruction, or the change of its character, under the 
 influence of the cells of the tissue. No tissue responds more quickly 
 to the demands made upon it by development or environment. 
 When the muscles grow larger and stronger by development, the 
 tendons and the bones to which they are attached change as quickly 
 and in proportion. From the appearance of the skeleton the 
 experienced anatomist can picture very accurately the muscular 
 development of the individual to whom it belonged. 
 
 The cell wall of plants may be used as one of the simplest examples 
 of supporting tissue. In this case each cell, in addition to its other 
 
 FIG. 167. Cells from the growing tip of a chestnut seedling. (Dahlgren and 
 
 Hepner.) 
 
 functions, produces its own supporting substance. These may be 
 observed in the cells of a growing root tip. Plant an onion, by 
 selecting one larger than a small glass, fill the glass with water, 
 and place the bulb on it. If this is placed in a sunny window, in 
 a few hours little rootlets will be seen stretching down into the 
 water. The rootlets of a sprouting chestnut also make very good 
 material (Fig. 167). If these are embedded in paraffin, the develop- 
 ment of the cells and the formation of their supporting walls can be 
 observed. The young cells near the tip will be found to be a mass 
 of granular cytoplasm, with a large nucleus in the center, and a 
 thin wall of cellulose which is the cell organ of support. As the 
 cell increases in size, vacuoles appear in the cytoplasm which become
 
 INTERCELLULAR SUBSTANCES 203 
 
 larger and larger. These vacuoles are filled with watery fluid which 
 is not a part of the cytoplasm. If the cell remained a solid mass 
 of cytoplasm, an enormous amount of food material would be 
 required, which would be out of all proportion to the work which 
 the cell is to perform. The vacuoles increase in size with the 
 growth of the cell until there is a rim of cytoplasm in contact with 
 the cell wall, and a central mass of cytoplasm surrounding the 
 nucleus and connected with that at the periphery by fine threads. 
 In still further growth these threads are broken, the nucleus is 
 pushed to one side, and the whole central portion becomes one 
 huge vacuole. There is now a cell wall, with a layer of cytoplasm 
 covering its inner surface, which is kept in reaction with the nucleus 
 by streaming around and around. This flowing of the cytoplasm in 
 plant cells may be easily observed in the delicate stamen hairs of 
 the ordinary Spiderwort, or in the cells of the water plants Chara 
 or Nitella, which are easily found in most ponds. In this example 
 it is seen that the cytoplasm remains in contact with the formed 
 material which it produces for support, and that it is only sufficient 
 in amount to form and maintain this material. 
 
 In general histology it has already been noted that the cells of 
 connective tissue are very similar, and that the tissues differ chiefly 
 in the character and arrangement of the intercellular substances. 
 It has also been emphasized that the connective tissues all originate 
 from a common form of embryonal connective tissue, or mesen- 
 chyme, and change from one form to another in development. 
 These mutations of the connective tissues are their most striking 
 characteristic, and must be clearly grasped if the bone, as an organ 
 of support, is to be understood. For instance, embryonal connec- 
 tive tissue is transformed into fibrous tissue; fibrous tissue becomes 
 arranged in a definite membrane, and is transformed into cartilage, 
 which is again removed and transformed into bone. All these 
 changes take place to meet the requirement of mechanical conditions 
 and environment. 
 
 If the subcutaneous tissue of an embryo be examined in sections 
 (Figs. 168 to 183) the cells will be found to be irregular masses of 
 cytoplasm with a nucleus in the central portion, and fine projec- 
 tions stretching out in all directions through an almost structureless 
 intercellular substance. The fine projections of the cytoplasm meet 
 those of the adjoining cells and form a network holding everything 
 together. Because of the nature of cytoplasm, however, these 
 possess very little strength, and very soon fine thread-like fibers
 
 204 
 
 INTERCELLULAR SUBSTANCES 
 
 are found appearing in the intercellular substance in contact with 
 cells. These unite with each other, forming continuous fibers, 
 
 FIG. 168. Embryonal connective FIG. 169. The same, a little more 
 
 tissue in an early stage of development, developed, showing the cellular elements 
 
 showing the cellular elements embedded lengthening in a common direction, 
 
 in the ground substance. (Black.) (Black.) 
 
 -^;^^;r^.^-^^^3 
 
 \ ^-^Trr^T^^T^TTTc^T^i^- 
 
 W " ^&^-:J'^-^^ 
 
 FIG. 170. The cells developed in spindle forms, fibroblasts with long filaments 
 extending from either end. (Black.) 
 
 FIG. 171. The developed white fibrous tissue. (Black.) 
 
 FIG. 172. Older white fibrous tissue, in which the cells are no longer seen, and 
 showing the wave-like course of the fibers. (Black.) 
 
 and very soon a strong network is produced which is entirely depen- 
 dent upon the cytoplasm of the cell which has formed and main-
 
 INTERCELLULAR SUBSTANCES 
 
 205 
 
 tains it. If this tissue is now subjected to pressure and strain, 
 the cells become flattened out and squeezed between the bundles 
 of fibers, which take on parallel directions, and so a tendon is 
 
 FIG. 173. Coarse white fibers, made up of bundles of the fine fibers, and showing the 
 mode of division by splitting off of a portion of the fibers of the bundle. (Black.) 
 
 FIG. 174. Coarse fiber breaking up into fine fibers. (Black.) 
 
 FIG. 175. Cross-sections of coarse fibers, showing some of their various forms. 
 
 (Black.) 
 
 FIG. 177. Cross-sec- 
 FIG. 176. Reticular or elastic fibers, showing the tions of the reticular 
 mode of division and the multipolar, or irregular, star fibers, showing some of 
 forms of the cells at the divisions. (Black.) their forms. (Black,)
 
 206 
 
 INTERCELLULAR SUBSTANCES 
 
 formed. A tendon must be considered as a highly specialized 
 form of connective tissue, arranged to supply tensile strength. 
 The degree of specialization of the tissue is judged by the extent to 
 
 FIG. 178. Connective-tissue cells from which reticular fibers are developed. 
 
 (Black.) 
 
 FIG. 179. Network of elastic fibers FIG. 180. Network of elastic fibers 
 
 from the point of reflection of the mu- teased out from elastic tendon, and 
 
 cous membrane of the lip from the gums. showing the usual mode of division. 
 
 (Black.) (Black.) 
 
 which its characteristic features are developed, either in quantity 
 or quality. In the tendon the fine, strong fibers have been gathered 
 into bundles; a round nucleus would occupy too much space.
 
 INTERCELLULAR SUBSTANCES 
 
 207 
 
 It has therefore become elongated and more or less rod-shaped, 
 and the cytoplasm has been squeezed out into thin leaf-like projec- 
 tions between the bundles. Each cell is in contact with several 
 fibers, and each fiber in contact with the cytoplasm of cells which 
 have produced them. 
 
 FIG. 181. Elastic fibers, showing their disposition to curl up when cut or broken. 
 
 (Black.) 
 
 It must be supposed that there is a constant reaction between 
 the substance of the formed material and materials produced by the 
 metabolism of the cytoplasm. In pathologic conditions the metab- 
 olism of the cytoplasm is disturbed, and there is a consequent 
 change in the quality of the fibers. So in some pathologic condi- 
 tions a relaxation and loss of tone is found in tendons and ligaments. 
 In inflammations of the gingivse the fibers become relaxed and 
 stretched, so that the gingivse are everted, but return to their 
 normal condition when the pathologic condition has subsided, and 
 the cells regain their normal metabolism. 
 
 FIG. 182. Cross-sections of elastic 
 fibers, showing their forms as seen in 
 a group passing between coarse white 
 fibers. (Black.) 
 
 FIG. 183. Tissue of the dental pulp, in 
 which the development of the cells is not 
 followed by any considerable formation of 
 fibers. (Black.) 
 
 To sum up what has been said, it is apparent that both phylo- 
 genetically and ontogenetically, intercellular substances have 
 been produced and are maintained by cells in response to mechanical 
 influences and to meet mechanical conditions. In all higher animals 
 certain tissues, the connective tissues, have been set apart for this 
 purpose, and the cells have been specialized to respond to mechan-
 
 208 INTERCELLULAR SUBSTANCES 
 
 ical stimuli and develop an intercellular substance adapted to the 
 condition. This makes the supposition necessary that an embryonal 
 connective-tissue cell may develop into any specialized form and 
 that the kind of cell into which it develops will be determined by 
 the character of mechanical stimuli which it receives. Just as the 
 epithelial cells have been specialized to respond to the environments 
 of light stimuli, vibration of the air, pressure, and chemical action 
 which connect the organism with its environment, connective- 
 tissue cells have been specialized to respond to mechanical stimuli,, 
 by the production of formed materials adapted to the mechanical 
 conditions. These conceptions are fundamental to an understand- 
 ing of bone structure and growth, and the mutations of connective 
 tissue in general. 
 
 In no branch of histology is a clear conception of intercellular 
 substances and the relation of cells to them as important as in the 
 study of the teeth and their associated structures. Caries cannot 
 be understood unless these fundamental ideas have been appreciated, 
 and many statements in dental literature would never have appeared 
 if the nature of intercellular substance and the relation of cytoplasm 
 so it had been understood.
 
 CHAPTER XVI. 
 BONE. 
 
 Definition. Bone may be defined as a connective tissue whose 
 intercellular substance is calcified and arranged in layers around 
 nutrient canals or spaces. The cells are placed in cavities, lacunae, 
 between the layers, and receive their nourishment through very 
 minute channels, canaliculi, which radiate from them and penetrate 
 the layers. 
 
 STRUCTURAL ELEMENTS. 
 
 The structural elements of bone are: 
 
 1. Bone matrix, or intercellular substance, which is always 
 arranged in layers or lamellae. 
 
 FIG. 184. From a section through the bone of a roebuck. The lacunae are seen 
 from above, and are filled with coloring matter. In places small dots are visible, 
 which represent the cross-sections of bone canaliculi. (850 X) (Szymonowicz.) 
 14 (209)
 
 210 
 
 BONE 
 
 2. The bone cells or bone corpuscles which are embedded in the 
 matrix between its layers. 
 
 3. Lacunae, or the spaces in which the cells are found. 
 
 4. Canaliculi, or the channels through the matrix by which 
 the embedded cells receive nourishment. 
 
 Bone Matrix. The bone matrix is composed of a dense organic 
 basis of ultimately fibrous character which yields gelatin upon 
 
 FIG. 185. From a section through the bone of a roebuck. The lacuna; are seen from 
 the side. (850 X) (Szymonowicz.) 
 
 boiling with water. With this inorganic salts are combined in a 
 weak chemical union, forming the hard substance of bone. By 
 treatment with acids the inorganic salts can be removed, leaving 
 the organic basis which retains the form of the tissue. In this 
 condition the rigidity of the bone is destroyed. On the other hand, 
 by calcining at red heat the organic basis can be removed, leaving 
 the inorganic substances which retain the form of the tissue. In 
 formation the organic basis is apparently formed first, and then
 
 HAVE RSI AN SYSTEM BONE 211 
 
 the salts of lime are combined with it, through the agency of the 
 formative cells or osteoblasts. . 
 
 Bone Corpuscles. Bone corpuscles are the cells lying in the 
 lacunae. Each cell contains a single well-defined nucleus, lying in 
 the centre of a granular cytoplasm. The cell apparently completely 
 occupies the lacunae, and from the central mass fine projections of 
 cytoplasm extend through the canaliculi, which bring the bone 
 corpuscles in intimate relation with certain areas of bone matrix. 
 The processes of one cell anastomose with those of its neighbors 
 through the canaliculi, so that there is a continuous network of 
 living cytoplasm throughout the matrix. 
 
 Lacunse. The lacunas are flat, oval spaces about 20 microns 
 long, 10 microns wide, and 5 or 6 microns thick. Their shape, 
 therefore, in sections depends upon the way in which they are cut, 
 as illustrated in Figs. 184 and 185. When cut lengthwise they would 
 appear as about 20 microns long and 6 wide in profile, or as about 
 20 microns long and 10 wide when seen from above. 
 
 Canaliculi. These radiate from the lacunse in all directions, 
 opening into them by larger channels which branch and divide, 
 becoming smaller as they pass farther into the matrix. They 
 anastomose freely with those from adjoining lacunae. 
 
 THE VARIETIES OF BONE. 
 
 There are three varieties of bone differing in the arrangement 
 of these structural elements. These are subperiosteal, Haversian 
 system, and cancellous bone. 
 
 Subperiosteal Bone. This form of bone must be regarded as 
 primarily a formative arrangement and more or less transitory, 
 in which the layers are arranged parallel with the surface, and 
 under a formative membrane. It contains canals (Volkmann's 
 canals) with bloodvessels (Fig. 186), connective tissue, etc. These 
 penetrate the layers which are never arranged concentrically around 
 them. It is always thin, that is, composed of comparatively few 
 layers, and when a considerable thickness is formed it is cut out 
 from within by absorptions beginning in the canals, and bone is 
 rebuilt with layers arranged concentrically around the channels 
 formed. In this way subperiosteal bone is converted into the 
 second form. 
 
 Haversian System Bone. In this variety the lamellae are arranged 
 concentrically around canals which contain bloodvessels, nerves,
 
 212 
 
 BONE 
 
 and embryonal connective tissue, and from which the cells in the 
 lacunae are nourished (Fig. 187). These canals are, in general, 
 parallel with the surface or the long axis of the bone and anastomose 
 with each other. A canal with the layers arranged around it con- 
 stitute a Haversian system. Between the Haversian systems are 
 remains of the subperiosteal layers (interstitial lamellae) that were 
 left by the absorption, and for that reason have been called fun- 
 damental lamellae. They have also been called ground lamellae. 
 Haversian system bone is often called compact bone, and makes up 
 
 FIG. 186. Subperiosteal bone, 
 showing Volkmann's canals. 
 
 FIG. 187. Haversian system bone: 
 Haversian canals. 
 
 the greater part of the shafts of the long bone, and the plates of 
 the flat ones. It is never allowed to become greater in thickness 
 than is necessary for strength, and when sufficient thickness has 
 been formed, the deeper part is cut out by absorptions in the 
 Haversian canals, converting them into large irregular spaces. 
 The formation of a few layers around these spaces transforms the 
 second type into the third or cancellous bone. 
 
 Cancellous Bone. In this variety the lamellae are arranged in 
 delicate plates surrounding large, irregular nutrient or marrow 
 spaces. These are filled by embryonal connective tissue and con-
 
 COMPACT BONE 213 
 
 tain bloodvessels and nerves. The plates of cancellous bone are 
 not arranged at haphazard, as might be supposed from a casual 
 observer of sections, but are disposed in definite arrangement, 
 which is determined by the directions of stress on the compact 
 bone which they support. (See illustrations in Chapter XXVII.) 
 They are not permanent and unchanging, but are continually being 
 rebuilt in new directions, in response to the mechanical conditions 
 to which the bone as a supporting organ is subjected. 
 
 THE ARRANGEMENT OF BONE. 
 
 Compact Bone. A knowledge of the structural elements of bone 
 can best be obtained by the study of sections ground from the 
 shaft of a long bone. An old dry bone should be sawed across, 
 near the middle of the shaft, in two places, so as to cut out a ring 
 about a quarter of an inch thick. Then saw the ring through in 
 two places with an arc of about a quarter of an inch on the outer 
 surface. From this tw r o slices should be sawed out, one transverse 
 to the long axis of the bone, the other parallel with it. These 
 are ground to not more than 8 or 10 microns in thickness and 
 mounted in hard balsam. From a study of these two the arrange- 
 ment of the lamellae, and the shape and character of the lacunse 
 can be made out. Upon the outer surface of the transverse section 
 will be found a larger or a smaller number of layers of subperiosteal 
 bone which encircle the shaft, and consequently are called the 
 circumferential lamellae. The number of these layers will depend 
 upon the position from which the section is taken, and the age of 
 the bone. If the bone is increasing in circumference at the point 
 from which the section is cut, there will be a considerable number 
 of layers, and they will be easily seen. If the bone has been growing 
 smaller in circumference at the point, there will be very little of 
 subperiosteal bone, and it will be comparatively hard to recognize. 
 The greatest part of the section will be made up of Haversian sys- 
 tems, in which from two or three to five or six layers are arranged 
 around an Haversian canal. The lacunas appear as irregularly oval 
 spaces about 5 or 6 microns across and 15 to 20 microns in length. 
 From them a great many minute canals radiate through the matrix, 
 both toward the Haversian canal and away from it. The character 
 of these canaliculi can only be appreciated by seeing them. They are 
 filled in life by projections of the protoplasm of the bone corpuscles. 
 They are suggestive of the rootlets of plants running through the
 
 214 BONE 
 
 soil, and as in that case the rootlets are absorbing material from 
 the soil and reacting with it, in this case the protoplasmic contents 
 of the canaliculi are reacting with the matrix, maintaining its quality. 
 The portion of matrix through which the canaliculi from one lacunae 
 extend belongs to the bone corpuscles which occupies the lacunae, 
 as will be seen later. These cells have been enclosed in the matrix 
 which they have formed. Between the Haversian systems will 
 be found a few layers of interstitial or fundamental lamellae. They 
 are the remains of layers which were formed under the periosteum 
 and were not entirely destroyed when it was replaced by Haversian 
 systems (Plate XIII). The amount of interstitial lamellae varies 
 greatly in different specimens, as will be seen by comparing figures. 
 
 The Haversian canals anastomose with each other; this will be 
 seen in many specimens. Many Haversian systems will be found 
 imperfect in form, as, for instance, those shown in Plate XIII. This 
 means that after these systems were completed, absorptions occurred 
 in a neighboring canal which attacked the layers of the system, and 
 later a new system was formed in this space by the deposit of 
 concentric lamellae. While bone is thought of as a hard and fixed 
 tissue, it is continually being built and rebuilt in this way. It is 
 only by the understanding of these possibilities that we get the 
 ideas that bone, while hard and rigid, is a plastic tissue and is con- 
 tinually being moulded by mechanical conditions to which it is 
 subjected. 
 
 It will be seen also that the arrangement of the lamellae becomes 
 a record of the changes that have occurred in the formation of the 
 tissue. The inner boundary of the section next to the marrow 
 cavity will show a few layers parallel with the surface. These are 
 known as the inner circumferential lamellae. It is a mistake, how- 
 ever, to think of them as surrounding the marrow cavity in the 
 same sense as the outer circumferential lamellae surround the bone. 
 If the section has been cut at a little distance from the center of 
 the shaft, it will have been noted that the marrow cavity is pene- 
 trated by very delicate spicules, and that in fact the marrow cavity 
 is produced by the spaces of cancellous bone, becoming larger and 
 larger until they become one continuous space. The inner circum- 
 ferential lamellae are therefore the layers which have been formed 
 around an enlarged nutrient or marrow space. 
 
 Cancellous Bone. The cancellous bone can best be studied in 
 decalcified sections. A field from the central portion of a flat 
 bone will show its typical arrangement. It is made up of delicate
 
 PLATE XIII 
 
 V 1 ' -^+''"L ~ - - *- * ' * < -v^ 
 " " -". _ * ^ - - 
 
 *V\* ,V -:: : 3* :& ^^^ ' - f- ^\^v^ - v 
 
 Kv^^?^^^ :;x ^- 
 
 sXd 
 
 ^ k ' ' i * " *"?- ' ' i ' ' * t ^F ' * ' 
 
 ' l /X '.t : -- U -^ ' , ' ^ '' - * ^ ^ 
 
 i j ' ' ^^ t * ' ; r ^ x 5. / M , v . * ^ **' ^ 4 * 
 
 ^ . " ^'T',! "*,' >' "" ". : ^ .. '* 'v y " ^''-!^'','-^" ; :i-'-^ 
 
 , x ' / / 
 
 '" * J* 
 
 -^% 
 
 'f.7 
 ' /. I 
 
 From a Ground Cross-section of the Diaphysis of the 
 Human Metatarsus. (Szymonowicz.) 
 
 <7, outer ground lamellae; h. inner groi\ncl lamellae; c, Haversian lamellae; 
 <l. interstitial lamellae. All canals and bone cavities are filled with coloring 
 matter and appear black. (9O X-)
 
 CANCELLOUS BONE 215 
 
 flattened spicules surrounding larger or smaller irregular spaces 
 which connect with each other very freely. Each spicule is com- 
 posed of a few lamellae which are arranged around the space. The 
 structure of the spicules often becomes complicated by absorp- 
 tions and rebuildings which have occurred to change their direc- 
 tion. The tissue which fills the spaces is a delicate, embryonal 
 connective tissue in which osteoblasts and osteoclasts appear in 
 response to mechanical conditions. It is richly supplied with 
 bloodvessels, nerves, and lymphatics. The lacunas and canaliculi 
 are in no respect different from those of the Haversian system and 
 subperiosteal bone.
 
 CHAPTER XVII. 
 BONE FORMATION AND GROWTH. 
 
 BONE is one of the latest tissues to be formed, and is always 
 developed from an antecedent connective tissue of less specialized 
 character. According to the character of the antecedent tissue 
 bone formation is of two varieties the formation from cartilage, 
 or endochondral bone formation, and that from fibrous connective 
 tissue, without the intervention of cartilage, or endomembranous 
 bone formation. 
 
 Endochondral Bone Formation. All of the bones of the endo- 
 skeleton are preformed in cartilage. The transformation of car- 
 tilage into bone is rather a substitution than a transformation, 
 for the original tissue is destroyed in the process, and a new and 
 more highly specialized one substituted for it. 
 
 Before ossification begins the cartilage has taken on the general 
 form of the bone and is covered by a definite perichondrium. Ossi- 
 fication begins at separate centers and progresses through the carti- 
 lage, but the separate centers do not unite until the bone is about 
 fully formed. In the long bone there are usually three centers 
 one near the center of the shaft, forming the hypophysis, and one 
 near either end, forming the epiphyses. These remain separated 
 by a layer of cartilage until the length of the bone has been fully 
 formed. 
 
 The first indication of the transformation of cartilage into bone 
 is an increase in the size of the lacunae and in the amount of cartilage 
 matrix, which also shows changes in character, having lime salts 
 deposited in it. The cartilage cells enlarge and show signs of 
 degeneration, the lacunae become arranged in row 7 s, and as they 
 increase in size, more in the direction parallel w r ith the axis of the 
 cartilage, the amount of matrix separating them is reduced. By 
 this time the perichondrium, on the surface of the cartilage opposite 
 to the center, has developed osteoblasts which begin the formation 
 of subperiosteal lamellae upon the surface of the cartilage, and the 
 perichondrium is transformed into periosteum. Opposite the 
 center osteoclasts appear, cutting into the cartilage, followed by 
 (216)
 
 ENDOCHONDRAL BONE FORMATION 
 
 217 
 
 buds of embryonal tissue. The osteoclasts dissolve away the 
 remains of the cartilage matrix, opening up the spaces between the 
 lacunae and converting the rows of lacunas into irregular channels 
 or primary marrow spaces. Upon the spicules of calcified cartilage 
 matrix, osteoblasts arrange themselves and begin to lay down 
 lamellae of bone. These changes progress from the center in both 
 
 Hyaline 
 
 cartilage 
 
 Area of 
 calcification 
 
 Capsules containing 
 "many cartilage cells 
 
 FIG. 188. From a longitudinal section of a finger of a three-and-a-half-months 
 human embryo. Two-thirds of the second phalanx is represented. At X a periosteal 
 bud is to be seen. (85 X) (Szymonowicz.) 
 
 directions, and all stages, from the typical hyaline cartilage to the 
 formation of bone, may be seen in one section. These stages are 
 illustrated by Figs. 188, 189, and 190. 
 
 From now on the bone grows by progressive transformation of 
 cartilage and by the growth of bone under the periosteum, which 
 will be considered under Bone Growth.
 
 218 
 
 BONE FORMATION AND GROWTH 
 
 Endomembranous Bone Formation. The bones which are not 
 preformed in cartilage are formed directly from fibrous tissue. 
 This is well illustrated in the mandible. In the region of Meckel's 
 cartilage and between it and the developing tooth germs the mesen- 
 chyme begins to show signs of specialization. Delicate fibers appear 
 in the intercellular substance. Along these the connective-tissue 
 cells arrange themselves, and, taking on the form of osteoblasts, 
 begin to lay down bone lamellae (Fig. 191). These stretch out 
 
 Cartilage cell^. | 
 
 Periosteum m 
 
 PericJiondral.. 
 
 bone 
 
 FIG. 189. The place marked X in the preceding figure with stronger magnification. 
 (185 X) (Szymonowicz.) 
 
 through the mesenchyme, forming a network of delicate spicules, 
 until they surround Meckel's cartilage, and grow up to the buccal 
 and the lingual of the tooth germs. As soon as this network of 
 bone lamellae, containing embryonal connective tissue in its pri- 
 mary marrow spaces, begins to take on definite form, there is a 
 specialization of the mesenchyme surrounding it, developing into 
 fibrous tissue which becomes a periosteum. From this time onward 
 the formation of bone progresses, as will be described under the 
 growth of bone.
 
 BONE GROWTH 
 
 219 
 
 Bone Growth. If sections are cut transversely through the shaft 
 of a long bone from a fetus, the surface will be found to be covered 
 by a well-formed periosteum, which is actively laying down layers 
 of subperiosteal bone. The central portion of the bone is made 
 up of a network of spicules surrounding primary marrow spaces, 
 
 Periosteum 
 
 Enlarged 
 cartilage 
 
 -elh 
 
 Endochondral 
 bone 
 
 Periuxteal hud 
 
 Blood-vessels 
 filled with 
 
 
 I Perichoudral 
 bone 
 
 Calcified 
 ca rtilage 
 
 FIG. 190.- From longitudinal section of a finger of a four months embryo. Only the 
 diaphysis of the second phalanx is represented. (85 X) (Szymonowicz.) 
 
 there being no true marrow cavity. The formation of the subperios- 
 teal layers does not progress at a uniform rate at all points on the 
 circumference, but they are piled up at certain points forming 
 longitudinal ridges with grooves between them. These grooves 
 become arched across, enclosing part of the connective tissue of 
 the inner layer of the periosteum, and contain bloodvessels and
 
 220 
 
 BONE FORMATION AND GROWTH 
 
 nerves. Soon after these spaces are enclosed absorptions begin in 
 their walls, destroying a large part of the subperiosteal lamellse 
 and forming primary marrow spaces. As soon as these spaces 
 have reached a certain size the absorptions stop, and osteoblasts 
 appear upon the wall of the space and begin to lay down lamellae 
 
 Osteo L 
 
 V-1 * 
 
 FIG. 191. Section through the lower jaw of an embryo sheep (decalcified with 
 picric acid). At a and immediately below are seen the fibers of a primitive marrow 
 cavity lying close together and engaged in the formation of the ground substance 
 of the bone, while the cells of the marrow cavity, with their processes, arrange 
 themselves on either side of the newly formed lamellae and functionate as osteoblasts. 
 (Bohm, Davidoff, Huber.) (300 X) 
 
 upon its circumference, until an Haversian system has been produced 
 with an Haversian canal at its center. In this way the bone increases 
 in diameter, and this process continues until a considerable thickness 
 of Ilaversian system bone is formed. In all bone growth there is 
 the alternation of formation, destruction, and rebuilding, and it 
 must be remembered that this continues as long as the bone func- 
 tions as an organ of support. As the shaft becomes larger the
 
 GROWTH OF MEMBRANE BONES 221 
 
 primary marrow spaces at the center are enlarged by the absorp- 
 tion, and a few lamellae are laid down again upon their walls, until 
 finally in the central portion of the shaft the true marrow cavity 
 is formed. As the thickness of Haversian system bone becomes 
 greater, absorptions occur in the Haversian canals, cutting out 
 large, irregular channels, around which a few lamellae are laid down, 
 and so the Haversian system bone becomes converted into cancel- 
 lous bone and is opened into the marrow cavity as it grows larger. 
 
 Growth of Membrane Bones. The growth of the membrane bone 
 progresses in a very similar way. As soon as the periosteum is 
 formed subperiosteal bone is laid down and converted into Haver- 
 sian system bone, forming the compact plate of the surface, leav- 
 ing the cancellous portion first formed at the center. When a 
 certain thickness of compact bone has been formed, absorptions 
 occur in the Haversian canals, converting the deeper portions into 
 cancellous bone. This process may be reversed. Absorptions may 
 occur under the periosteum, cutting deeply into the Haversian 
 system bone, and then a few subperiosteal layers laid down 
 upon it. When this occurs lamellae are laid down around the 
 marrow spaces, converting the cancellous bone into Haversian 
 system bone to maintain the required strength. In this way the 
 bones are moulded into shape, adapting them to the mechanical 
 conditions to which they are subjected. There is an oscillation 
 between formation and destruction, by which the balance adapted 
 to the mechanical conditions is maintained. It has often been noted 
 that bones are never allowed to become more bulky than is neces- 
 sary to perform their function.
 
 CHAPTER XVIII. 
 PERIOSTEUM. 1 
 
 Definition. The periosteum is the formative and protective 
 membrane which covers the outer surface of the bone. All perios- 
 teum has certain structural characteristics in common, but because 
 of structural differences two classes are recognized attached and 
 unattached each of which may be simple or complex. Perios- 
 teum may thus be classified as follows : 
 
 1. Unattached simple. 
 
 2. Unattached complex. 
 
 3. Attached simple. 
 
 4. Attached complex. 
 
 Function of Periosteum. The importance to the dentist of a 
 knowledge of the structure and function of the periosteum can 
 scarcely be exaggerated. It has been the knowledge of this tissue 
 and its function that has led to all the advancement in bone surgery 
 of modern time. Repair and regeneration of bone is largely accom- 
 plished through its agency. 
 
 The periosteum forms the immediate covering of all the bones 
 and is continuous over their entire surface except the portion covered 
 by cartilage. Each bone therefore has a periosteum of its own 
 which does not continue around the articulation to the bones with 
 which it joins. Bones that are united by suture are, however, 
 covered by a common periosteum. If the flesh and overlying 
 tissues are carefully removed from a long bone, the periosteum will 
 be seen as a smooth white, lustrous membrane, having much the 
 same appearance of a tendon on most of its surface. But at some 
 places which correspond to the positions where muscles or fascia 
 
 1 In the presentation of this chapter it is impossible adequately to express my 
 indebtedness to Dr. G. V. Black. Almost all of the illustrations are taken from The 
 Periosteum and Peridental Membrane, published by him in 1887. I have always 
 felt that this book had never received the attention it deserves. Only one thousand 
 copies of it were printed, and they were not sold until the orthodontists exhausted 
 the edition. The book is now entirely out of print and is very difficult to obtain. I 
 have studied this book for years and have repeated almost all of the work described 
 in it, but I have felt that it was impossible for text-book purposes to improve upon 
 the illustrations. 
 (222)
 
 FUNCTION OF PERIOSTEUM 223 
 
 were attached it appears ragged and dull, for the tissues had to 
 be cut to separate them from the outer layer of the periosteum, 
 to which they were firmly adherent. In all other places the tissues 
 separate easily in dissection ; in fact, are not attached at all, except 
 by the lightest of areolar tissue, which is very easily broken, and 
 the tissues may be separated from the surface of the membrane 
 with the finger or the handle of a scalpel. Now, if the periosteum 
 is slit along a smooth surface with the scalpel and the handle 
 inserted between the bone and the membrane, it will be found 
 to separate readily from the bone over most of its surface. If the 
 process is watched closely, little strings will be seen apparently 
 running from the periosteum to the bone, and being broken as they 
 are separated. These are mostly small bloodvessels which are 
 running into canals in the bone. In this process the periosteum 
 seems like a closely adapted sac or elastic glove, clothing the sur- 
 face of the bone, as if surrounding it in a fibrous bag. If the sepa- 
 ration of the periosteum from the bone is continued, it will be 
 found that it does not separate as easily in all places. As the 
 articular ends are approached it becomes suddenly fastened to the 
 underlying bone, and the blade of the knife must be used. The 
 periosteum now appears as a very thin, tough, and inelastic mem- 
 brane, that is torn with difficulty, but it is so thin that it is difficult 
 now to separate it from the bone without cutting it through. When 
 this point of attachment is reached it seems that the periosteum is 
 sinking into the substance of the bone, and from the examination 
 of its structure it is found that this is practically what has happened. 
 
 Comparing the periosteum to a sac surrounding the bone, it is 
 found sewed firmly down at the margin of the cartilage around 
 the articular ends. Besides the attachment around the cartilage, 
 the periosteum will be found adherent in the following positions: 
 ^Yhere muscles or fascia are attached to the outer layer of the 
 periosteum; where it approaches the insertion of tendons or liga- 
 ments; and where the skin or mucous membrane seem attached to 
 the underlying bone, as around the auditory meatus, the gums, 
 mucous membrane of the nose, etc. In all such positions the 
 periosteum is firmly attached to the bone in fact, becomes a part 
 of it and through this medium the connections between muscles, 
 fascia, etc., and the framework of the skeleton is accomplished. 
 
 This feature of the anatomy of the periosteum has never been 
 studied in the detail it deserves, especially by the dentist. It is 
 of the greatest importance in the management of the diseases of
 
 224 PERIOSTEUM 
 
 bone, especially those involving the formation of pus, for these 
 lines of attachment determine the direction in which the pus will 
 proceed along the surface of the bone. When pus generated within 
 the bone reaches the surface, it will lift an unattached periosteum 
 and run along the surface until it reaches a line of attachment. 
 Here it can penetrate the periosteum more easily than it can sepa- 
 rate it from the bone. When a line of attachment is reached, there- 
 fore, the direction of the burrowing is determined by the attached 
 areas. The pus penetrates the periosteum more easily than it 
 separates its attachments from the bone, but it lifts the unattached 
 periosteum so easily that it will often run along a line of attach- 
 ment for a long distance. 
 
 These factors often become of great importance in determining 
 the position in which alveolar abscesses will point. For instance, 
 if an abscess from a bicuspid root, or the mesial root of a molar, 
 reaches the surface of the bone above the attachment of the buc- 
 cinator, it cannot penetrate its attachment and pass downward 
 to open on the gum, but may run out over the surface of the muscle 
 and open on the cheek, producing the crow's foot scar so often seen. 
 An abscess from an upper cuspid may reach the surface of the bone 
 in the canine fossa between the attachments of the nasalis and 
 caninus, and lift the periosteum extending upward, and open at 
 the inner canthus of the eye between the orbicularis and the angular 
 head of the quadratus labii superioris. If these abscesses had been 
 reached with a lance, through the mucous membrane, at the proper 
 time, a disfiguring scar \vould have been avoided. Accurate knowl- 
 edge of the attached layers of the periosteum would have made it 
 certain that they could never point in the mouth cavity without 
 assistance. 
 
 Layers of the Periosteum. Periosteum is always composed of 
 two distinct layers: 
 
 1. An outer or fibrous layer, which is essentially protective and 
 to which muscles and fascite are attached. This may be either 
 simple or complex. 
 
 2. An inner or osteogenetic layer which is essentially the vital 
 functioning layer, and is, as its name indicates, concerned with 
 the formation of bone. This may be either simple or complex. 
 
 The Structural Elements. The periosteum is composed of the 
 following structural elements: 
 
 1. White fibers in coarse bundles (in the outer layer). 
 
 2. White fibers in very fine bundles (in the inner layer).
 
 SIMPLE UNATTACHED PERIOSTEUM 
 
 225 
 
 3. Elastic fibers. 
 
 4. The penetrating fibers, or white fibers of the periosteum, that 
 in the growth of bone are included in its substance. 
 
 5. Embryonal connective-tissue cells. 
 
 6. Osteoblasts or bone-forming cells. 
 
 7. Osteoclasts or bone-absorbing cells. 
 
 Unattached Periosteum. In the unattached periosteum the inner 
 layer is always simple, and the outer layer may be either simple 
 
 ' 
 
 FIG. 192. Non-attached periosteum from the shaft of the femur of the kitten: 
 B, bone; O, layer of osteoblasts. In the central portion of the figure they have been 
 pulled slightly away from the bone, displaying the processes to advantage. It will 
 be observed that the fibers of the periosteum do not enter the bone, a, inner layer 
 of fine white fibrous tissue (osteogenetic layer) showing the nuclei of the fibroblasts 
 and a number of developing connective-tissue cells, which probably become osteo- 
 blasts; c, outer layer, or coarse fibrous layer, in which fusiform fibroblasts are also 
 rendered apparent by double staining with hematoxylin and carmine; d, some 
 remains of the reticular tissue connecting the superimposed tissue with the periosteum, 
 (y'o immersion.) (Black.) 
 
 or complex, depending apparently upon the requirements of pro- 
 tection. In general, the more exposed the position the thicker is 
 the layer, and the larger and stronger the bundles of fibers of which 
 it is composed. 
 
 Simple Unattached Periosteum. Where the periosteum is covered 
 by a thick layer of muscles which are not attached to it, as in the 
 thigh, the thinnest and simplest form of periosteum is found. An 
 illustration, drawn by Dr. Black, of the periosteum from the femur 
 of a kitten will illustrate its structure (Fig. 192). The outer layer 
 15
 
 226 PERIOSTEUM 
 
 is composed chiefly of bundles of white fibers, most of which run 
 in a direction parallel with the long axis of the bone. The bundles 
 are comparatively small and much flattened, so as to be quite 
 ribbon-like. The inner layer contains a much greater number of 
 cells lying among extremely delicate fibers. In its outer portion 
 many of the cells are embryonal in character. In contact with 
 the surface of the bone is a continuous layer of osteoblasts which 
 are building subperiosteal bone in the young animal, processes of 
 
 FIG. 193. Periosteum from the shaft of the tibia of the pig, lengthwise section, 
 showing the complex arrangement of fibers in the coarse or outer fibrous layer that 
 sometimes occurs under muscles that perform sliding movements upon it: B, bone; 
 O, layer of osteoblasts. The tissue has been pulled slightly away from the bone in 
 mounting the section, and part of the osteoblasts have clung to the bone, some 
 have clung to the tissues, while others are suspended midway, their processes cling- 
 ing to each, a, layer of fine fibers; inner or osteogenetic layer of the periosteum; 
 b, first lamina of the coarse or outer fibrous layer, the fibers of which are, in this case, 
 circumferential, exposing the cut ends. It will be observed that there are ten lamina 
 in the make up of the outer layer, the lengthwise and circumferential fibers alternating. 
 The ones marked/ and i are very delicate ribbon-like forms, which have shifted from 
 their normal position in the mounting of the section, so as to present their sides to 
 view instead of their ends, thus displaying their structure to advantage. The illus- 
 tration shows how readily separable these lamina are. I, reticular tissue, (j., 
 immersion.) (Black.) 
 
 their cytoplasm extending into the canaliculi of the matrix which 
 they have formed. At one point in the illustration the osteoblasts 
 are pulled off from the surface of the bone and show these processes 
 stretched out of the canaliculi. 
 
 Complex Unattached Periosteum. In some places, especially 
 where muscles or tendons perform sliding movements over an 
 unattached periosteum, the outer layer, instead of being simple, 
 may be very complex. This is illustrated in Dr. Black's drawing
 
 SIMPLE ATTACHED PERIOSTEUM 
 
 227 
 
 (Fig. 193), from the periosteum of the tibia of a young pig. In 
 this instance the outer layer is composed of very much flattened 
 bundles of white fibers, arranged alternately longitudinally and 
 circularly. Ten layers may be counted in the section. The inner 
 layer is of the same character as in a simple specimen. 
 
 Attached Periosteum. The attached periosteum differs from the 
 unattached by having the fibers of the inner layer arranged in 
 bundles, around which the bone matrix is deposited by the osteo- 
 blasts, embedding them in the substance of the matrix and calcify- 
 
 FIG. 194. Simple attached periosteum: a, bone; b, osteo blasts; c, fibers of the 
 inner layer; D, bloodvessels of the inner layer; E, outer layer; F, muscle fibers at- 
 tached to outer layer. (Black.) 
 
 ing them with it. These fibers constitute the penetrating fibers. 
 They were first described by Sharpey, and have been called Shar- 
 pey's fibers. He, however, apparently did not understand their 
 importance or manner of formation. The fibers of the inner layer 
 are built into the substance of the bone in this way wherever tissues 
 are attached to the outer layer of the periosteum. 
 
 Simple Attached Periosteum. Where the pull of tissues attached 
 to the outer layer of the periosteum is in one direction, the fibers 
 of the inner layer are inclined in the same direction (Figs. 194 and
 
 228 
 
 PERIOSTEUM 
 
 195). As the surface of the bone is approached the fibers are 
 gathered into strong bundles to be inserted in the bone, the osteo- 
 
 FIG. 195. A photomicrograph of an attached periosteum similar to Fig. 194. From 
 the alveolar process of a sheep. (About 80 X) 
 
 FIG. 196. Attached periosteum from beneath the attachment of the muscles of 
 the lower lip of the sheep: A, bone; B, osteoblasts, with the fibers emerging from 
 the bone between them; C, inner layer with fibers decussating and joining the 
 inner side of the coarse fibrous layer in opposite directions (this is rather an unusual 
 form of this layer of the periosteum); D, coarse, fibrous layer; E, attachment of 
 muscular fibers. (Black.) 
 
 blasts covering the surface of the bone everywhere between the 
 fibers. The outer and inner layers are united by the interlacing
 
 COMPLEX ATTACHED PERIOSTEUM 229 
 
 of their fibers. At the junction of the outer and the inner layers 
 many bloodvessels are seen. 
 
 Complex Attached Periosteum. Where the pull upon the outer 
 layer is in many directions, the fibers of the inner layer, after emerg- 
 ing from the bone, break up into smaller bundles and anastomose 
 in all directions, arching around to interlace with the fibers of the 
 outer layer, and in this way they sustain force in all directions 
 (Fig. 196). This is illustrated in Dr. Black's drawing of a section 
 of attached periosteum from beneath the attachment of the muscles 
 of the lower lip of a sheep.
 
 CHAPTER XIX. 
 THE ATTACHMENT OF THE TEETH. 
 
 THAT the teeth are not a part of the osseous system, but are 
 appendages of the skin, supported in man by a special development 
 of bone forming the alveolar ridges of the maxillary bones, is as 
 well established as any fact concerning human dentition. The 
 work of Oscar Hertwig, published in 1874, established very clearly 
 the homology existing between the teeth and the dermal or placoid 
 scales of the ganoid, silurioid, and dipnoan fishes, both as to simi- 
 larity of structure and development. 
 
 Much has been written descriptive of the teeth of various animals, 
 their modifications of form, and attachment to adapt them to 
 modifications of function, and various classifications of the means 
 of attachment have been made. Of these, perhaps the best and 
 most logical is given by Charles Tomes in his Dental Anatomy, 
 describing four forms of attachment: (1) By fibrous membrane; 
 (2) by hinge-joint; (3) by ankylosis; (4) by insertion in a socket 
 or gomphosis. 
 
 These various forms of attachment will be taken up, and, if 
 possible, the comparison between them and the evolution of the 
 more complicated forms from the simpler will be shown. The study 
 must begin with an examination of the structure and attachment 
 of the placoid scales and the simplest form of tooth, as illustrated 
 in the shark. 
 
 Structure of Dermal Scales. The dermal scales are composed 
 of a conical cap of calcified tissue developed from within outward, 
 by an epithelial organ, and corresponding in structure to the 
 enamel. This cap is supported upon a conical papilla of calcified 
 tissue formed from without inward, and corresponding to dentin. 
 In the outer layer the arrangement of the fine tubules through the 
 calcified matrix correspond very closely to human dentin, but in 
 the inner portions it is to be understood only by considering the 
 formation of the dentin as progressing irregularly over the surface 
 of the pulp and so dividing the pulp tissue into portions enclosed 
 in large canals, from which the fine tubules radiate. The base of 
 (230)
 
 ATTACHMENT BY HINGE JOINT 
 
 231 
 
 this partially calcified papilla has a calcified connective tissue 
 built on to it by the derma or connective-tissue layer of the skin, 
 which corresponds to cementum forming the basal plate, spreading 
 out more or less in the connective-tissue layer of the skin, and 
 into which the fibers of this layer are built, so attaching the denticle 
 or dermal scale to the deep layer of the coreum. This tissue very 
 exactly resembles cementum. It is formed on the dentin as the 
 cementum of a human tooth is, and shows the connective-tissue 
 fibers embedded in it. In the ganoids the basal plates of adjoining 
 scales unite, forming the armor plates of such fish as the sturgeon 
 and gar-pike, and the dentical remains projecting from the surface 
 of the plates. 
 
 FIG. 197. Showing additions of bone of attachment to the bone of the jaw. 
 
 (Tomes.) 
 
 Attachment by Fibrous Membrane. In the simplest teeth, as of 
 the shark (Lamna cornubica, Fig. 3), which are typical dermal 
 scales, there is an exactly similar method of attachment, which 
 may be taken as the simplest and most rudimentary, or attachment 
 in a fibrous membrane. That is, there is no development or modi- 
 fication of the arch of the jaw, and the teeth have no direct attach- 
 ment to the bone; in fact (Fig. 197), the jaws themselves are chiefly 
 cartilage. 
 
 Attachment by Hinge Joint. The formation of the hinge attach- 
 ment as illustrated in many of the fishes (Fig. 198), may 
 be understood as a modification of the attachment in a fibrous
 
 232 
 
 THE ATTACHMENT OF THE TEETH 
 
 membrane in a more highly specialized creature. These hinged 
 teeth are found in many fishes and in the poison fangs of snakes. 
 The jaws are calcified, and the basal plate or cementum may be 
 considered as confined to, or specially developed on, one side of 
 the dentin papilla, which is also more highly developed, especially 
 in snakes. This cementum is built and calcified around the fibers 
 of the fibrous tissue which pass directly to the bone of the jaw 
 
 FIG. 198. Attachment by hinge joint. Tooth of a hake: a, vasodentin; b, pulp; 
 c, elastic hinge; d, buttress to receive /, formed out of bone of attachment; e, bone 
 of jaw; /, thickened base of tooth; g, enamel tip. (Tomes.) 
 
 at that point. This bone is to be regarded as an addition to the 
 jaw specially developed for each tooth. Thus, there is not only a 
 modification in the arrangement of the cementum, but a develop- 
 ment of bone for attachment of the tooth. The bloodvessels pass 
 through the fibers of the hinge to the pulp, and are not affected by 
 the motion of the tooth on the hinge; in fact, the pulp seems to be 
 attached to the hinge. There are many complications of this
 
 ATTACHMENT BY ANKYLOSIS 233 
 
 method of attachment, but this may be taken as the type and the 
 manner of its modification from the rudimentary conditions. The 
 distinction, in this form of attachment, from the dermal scale con- 
 sists in a modification of the arrangement of the cementum of the 
 basal plate and a development of bone from the jaw to attach 
 fibers which pass directly from cementum to bone. It should also 
 be said that there are developments in the hinge teeth related to 
 the third form of attachment, namely, ankylosis, which cannot 
 be understood until this form is studied. 
 
 Attachment by Ankylosis. The third form of attachment, anky- 
 losis (Fig. 199), or direct calcified union with the bone of the jaw, 
 cannot be understood without a careful study of the nature and 
 formation of the dentin in these rudimentary teeth. It is evident, 
 from a study of the dentin of the dermal scales, that compared 
 with human dentin, the tissue is rudimentary and not differentiated 
 from other similar connective tissues. The tubules are compara- 
 tively very irregular, and resemble strikingly the tubules found in 
 the secondary dentin formed by a degenerating pulp. The odonto- 
 blasts, or dentin-forming cells, are not like the highly specialized 
 cells which form the primary human dentin, but resemble very 
 closely simple spindle-shaped connective-tissue cells. The nucleus 
 is larger and oval in form, and the protoplasm stretches off from 
 it in one direction into a fibril instead of in two directions into a 
 spindle. The cells are much smaller than human odontoblasts and 
 nearer the size of ordinary spindle cells of the human pulp. In 
 fact, they look more like specially developed spindle cells than 
 odontoblasts. The formation of dentin begins on the surface, at 
 the apex of a cone-shaped papilla of connective tissue, and proceeds 
 inward. If the formation continues uniformly over the surface 
 of the papilla, a solid layer of fine-tubuled dentin results; but it 
 often proceeds irregularly, apparently having special reference 
 to the neighborhood of bloodvessels, so that irregular projections 
 of dentin are found on its inner surface, dividing the pulp more or 
 less into portions enclosed in larger channels or tubes. These may 
 be very regular in arrangement and form around bloodvessels 
 loops embedding the bloodvessel in the calcified tissue, producing 
 what has been called vaso or vascular dentin; but the formation 
 is still from the surface of the pulp until it is obliterated, except 
 for what remains in the larger canals. As distinguished from this 
 formation of dentin we find in the body of the dental papilla of 
 many fishes the formation of spicules of calcified tissue, which
 
 234 THE ATTACHMENT OF THE TEETH 
 
 resemble neither dentin nor typical bone, shooting down through 
 the substance of the pulp. They are more to be compared with 
 the first formation of bone in membranes, or in the embryonal 
 connective tissue of the body of the human jaw, which is afterward 
 
 FIG. 199. Tooth of scarus, showing attachment by ankylosis: 1, vertical section 
 of five pharyngeal teeth of Scarus muricatus; 2, section of a single tooth magnified: 
 a, osteodentin; b, dentin; c, enamel; d, cementum; 3, termination of a single 
 dentinal tubule. (Owen.) 
 
 removed by absorption and replaced by true Haversian system 
 bone. These calcifications contain lacunae, and have tubules or 
 canaliculi running through them, and so, as Tomes says, are inter-
 
 ATTACHMENT BY IMPLANTATION IN SOCKET 
 
 mediate between dentin and bone. They divide the pulp into 
 irregular spaces, and interdigitate, or perhaps actually join, the 
 formation of dentin which has been progressing from the surface 
 of the pulp. These spicules run down into the bone of the jaw, 
 forming an actual calcified attachment for the tooth with the 
 jaw; but in this view of it, it is to be regarded as a calcification or 
 rather a formation of bone in the pulp papilla interlocking with 
 the dentin. In some of the fishes, as in Scarus, there is at the same 
 time the remains of the cementum of the basal plate formed on the 
 outside of the dentin around the base of the cone. Ankylosis is 
 confined to the teeth of many fishes, and may be stated as a modi- 
 fication from the dermal scale, resulting in the reduction or loss of 
 
 FIG. 200. A, diagrams of tranverse sections through the jaws of reptiles showing 
 pleurodont (a), acrodont (b), and thecodont (c) dentitions. B. a, lower jaw of Zootoca 
 vivipara; b, of anguis fragilis. (After Leydig.) Weidersheim, Comparative 
 Anatomy of Vertebrates.) 
 
 the basal plate and an ossification of the pulp continuing through 
 the connective tissue at the base of the pulp to the body of the jaw. 
 Attachment by Implantation in Socket. The development of the 
 fourth form of attachment, by implantation in a socket, seems to 
 be an evolution starting from the same point but proceeding in 
 a different direction (Fig. 200). It is associated with the very 
 great increase in the size of the teeth and consequent necessity for 
 a stronger attachment. The evolution of this is illustrated in the 
 teeth of reptiles. Weidersheim classifies the teeth of reptiles as 
 (1) resting upon a ledge on the lingual side of the jaw pleurodont 
 dentition; (2) resting on a slight ridge around them acrodont 
 dentition; (3) lodged in permanent alveoli, as in the crocodile 
 thecodont dentition. These three classes illustrate three stages in 
 the development of the socket method of attachment.
 
 236 THE ATTACHMENT OF THE TEETH 
 
 In the simplest form there is a cone-shaped tooth, attached 
 to the bone around its base, by the fibers being built into the 
 cementum and bone. There is little modification of the rudimentary 
 form, and little development of bone for its attachment. In a 
 higher form the tooth has become long or peg-shaped, and the bone 
 has grown up around a portion of it to support it; but it is attached 
 to the bone by connective-tissue fibers, being built into the cemen- 
 tum on the surface of the tooth and into the bone of attachment on 
 the jaw. The development of the form of the tooth to the peg from 
 the cone may be understood as a continuing of the development of 
 odontoblasts, and the formation of dentin (which always begins 
 at the apex of the cone) farther and farther down the sides of the 
 dental papillae. Then the formation of the cementum, which begins 
 around the base of the cone and continues down on the outside of 
 the calcified dentin, covering its outer surface, and building the 
 connective-tissue fibers into the tooth. The development of bone 
 accompanies, or rather follows that of the tooth, building the 
 other ends of these fibers into the bone which is developed to 
 support the tooth. 
 
 Summary. To review the subject matter of this chapter, all 
 teeth have been evolved from the simple placoid scale. In the 
 simplest forms, as in the teeth of the shark, there is no relation 
 to the bone whatever, but the fibers of the subcutaneous tissue 
 are built into the basal plate of cementum. As the tooth becomes 
 larger and demands more support, there is added to the bone of 
 the jaw that which Tomes has called bone of attachment. The 
 osteoblasts build up additions to the jaw which surround and 
 embed the fibers, so that the fibers which were originally in the 
 subcutaneous tissue are fastened to the bone at one end and to 
 the cementum at the other. The evolutions of attachment by 
 hinge joint and by gomphosis are therefore direct evolutions from 
 the simple attachment in membrane. The form of ankylosis is 
 also evolved from the simplest type, but in this case the bone of 
 attachment is associated with the pulp, and the formation of bone 
 and dentin become interlocked and united.
 
 CHAPTER XX. 
 THE PERIDENTAL MEMBRANE. 
 
 IN one sense the peridental membrane may be considered as the 
 most important tissue to the dentist, for upon it the usefulness 
 of the teeth and their comfort to the individual is dependent. It 
 makes no difference how perfect a crown may be, or how perfectly 
 any damage which may have occurred to it may have been restored, 
 unless the peridental membrane is in a healthy and fairly normal 
 condition, the tooth will be useless, and the individual would be 
 much more comfortable without it. 
 
 Definition. The peridental membrane may be defined as that 
 tissue which fills the space between the surface of the root and the 
 bony wall of its alveolus, surrounds the root occlusally from the 
 border of the alveolus, and supports the gingivse. It is necessary 
 to emphasize the three parts of the definition. The peridental 
 membrane does not stop at the border of the bone, but continues 
 to surround the root as far as the tissues are attached to it. In 
 general, the dental profession has thought of the peridental mem- 
 brane as only that tissue which occupies the space between the 
 root and the wall of its alveolus. As will be seen from a study of 
 sections later (Figs. 203 and 204), the structure of the tissue sur- 
 rounding the root between the gingival line and the border of the 
 process is essentially the same as that in the alveolus, and quite 
 different from the much coarser fibrous mat forming the submucous 
 layer of the gum tissue. The peridental membrane also extends 
 into the free margin of the gum and is the means of its support, 
 holding the gingivse close to the surface of the tooth and supporting 
 them in the interproximal spaces. The importance of this portion 
 of the peridental membrane and the functions which it performs 
 have been strongly emphasized in the last few years, in their rela- 
 tion to the extensions of caries and the beginnings of pyorrhea. 
 Most of the diseases of the peridental membrane which result in 
 the final loss of the teeth have their beginnings in this portion. 
 
 Nomenclature. The peridental membrane belongs to the class 
 of fibrous membranes which form the covering of organs, the cap- 
 
 (237)
 
 238 
 
 THE PERIDENTAL MEMBRANE 
 
 sules of glands, and especially those membranes which cover the 
 organs of support. Its closest relative is the periosteum in the 
 attached portions, with which it has many points of structure 
 in common, but it differs from the periosteum in any position in 
 important respects. It has often been called the alveodental 
 periosteum, but this name implies that the periosteum is folded 
 down into the alveolus and back upon the surface of the root, 
 which is an entirely erroneous conception of the membrane. This 
 idea would imply that it was a double membrane having one layer 
 covering the bone and another covering the root, the two uniting 
 in the middle portions. But instead, the periosteum must be 
 
 FIG. 201. Drawing to show the arrangement of the fibers in a labiolingual section 
 through an incisor of a kitten. (Black.) 
 
 considered as stopping at the border of the alveolus, 1 and being 
 united with the peridental membrane around its circumference. 
 Many writers use the word pericementum in place of peridental 
 membrane. The author prefers, and in this book will use, the term 
 peridental membrane, though the two are synonymous. 
 
 Divisions. Purely for convenience in description, the peridental 
 membrane is divided into three portions: The gingival portion, 
 that portion of the membrane which surrounds the root occlusally 
 
 1 The student must be reminded that the word alveolus means a hole, and the 
 alveolar process, the portion of the bone which contains the holes. In dental writing 
 the word alveolus has often been incorrectly used in place of process or alveolar 
 process.
 
 PLATE XIV 
 
 Longitudinal Section of Peridental Membrane. 
 
 Stained \vith hematoxylin and eosin. Showing border of alveolar process.
 
 DIVISIONS 
 
 239 
 
 from the border of the alveolar process and supports the gingivse; 
 the alveolar portion, the portion of the membrane from the border 
 
 G ' 
 
 FIG. 202. Diagram of the fibers of the peridental membrane: G, gingival portion; 
 AL, alveolar portion; Ap., apical portion. (From a photograph of a section from 
 incisor of sheep.)
 
 240 THE PER1DENTAL MEMBRANE 
 
 of the process to the region of the apex of the root; and the apical 
 portion, which surrounds the apex of the root and fills the apical 
 space. These are illustrated in the diagram (Figs. 201 and 202). 
 
 The Structural Elements. These are: (1) White connective- 
 tissue fibers; (2) fibroblasts; (3) cementoblasts; (4) osteoblasts; (5) 
 osteoclasts; (6) epithelial structures which have sometimes been 
 called the glands of the peridental membrane; (7) bloodvessels; 
 (8) nerves; (9) lymphatic vessels. 
 
 Functions. The peridental membrane performs three functions: 
 (1) A physical function it maintains the tooth in relation to the 
 adjacent hard and soft tissues. (2) A vital function the formation 
 of bone on the alveolar wall and of cementum on the surface of 
 the root. (3) A sensory function the sensation of touch for the 
 tooth being exclusively in this membrane. 
 
 It is necessary to emphasize the two parts of the physical func- 
 tion; the peridental membrane not only supports the teeth in their 
 relation to the bones which carry them, and sustains them against 
 the forces of occlusion and mastication, but it also sustains the 
 soft tissues in their proper relation to the teeth. The second part 
 of the physical function is fully as important as the first, and the 
 study of the structure of the tissue related to it and the adaptation 
 of the form of the gingivse to the anatomic form of the teeth and 
 alveolar process, are important considerations which should never 
 be lost sight of in the making of artificial crowns. 
 
 Classes of Fibrous Tissue. The fibrous tissue of the peridental 
 membrane is entirely of the white variety, but may be divided 
 into two classes. The principal fibers and the indifferent or inter- 
 stitial tissue. The former perform the physical function of the 
 membrane, the latter simply fill in spaces between the bundles of 
 fibers and surround *and accompany the bloodvessels and the 
 nerves. 
 
 The Principal Fibers of the Peridental Membrane. These may 
 be defined as the fibers which, springing from the cementum, are 
 attached at their other extremities to the connective tissue support- 
 ing the epithelium, the fibrous mat of the gum tissue, the cementum 
 of the approximating tooth, the outer layer of the periosteum at 
 the border of the alveolar process, or the bone of the alveolar wall. 
 
 Arrangement. The principal fibers literally spring from the 
 cementum, the cementoblasts building up the matrix around them 
 and then calcifying both the matrix and the fibers, in this way 
 attaching them to the surface of the root. In most places the fibers
 
 ARRANGEMENT 241 
 
 as they spring from the cementum appear as good-sized bundles. 
 A short distance from the surface of the root they may break up 
 into smaller bundles which anastomose and interlace, passing 
 around bloodvessels and other fibers in their course and being 
 again united into large bundles for attachment at their other 
 extremity. 
 
 To arrive at an understanding of the arrangement of the fibers 
 of the peridental membrane, sections must be cut longitudinally, 
 both from buccal to lingual and from mesial to distal, and trans- 
 versely through all portions of the membrane. It therefore requires 
 the study of many sections to work out a complete conception. 
 After studying them out completely in this way one is impressed 
 with the beautiful adaptation of their arrangement to sustain the 
 tooth against all the forces to which it is subjected, and to support 
 the free margin of the gum, so that it will lie closely against the 
 gingival portion of the enamel. It is necessary, however, to remind 
 the student that connective tissues are formed in response to 
 mechanical conditions and stimuli, and therefore this arrangement 
 must be considered, not as having been designed to sustain the 
 forces, but as being the result of the forces to be sustained, and 
 therefore beautifully adapted to them. 
 
 The principal fibers of the peridental membrane are naturally 
 divided into a number of groups which differ in their arrangement 
 and function. In his latest book, Special Dental Pathology, Dr. 
 Black has given descriptive names to these groups. Passing from 
 the gingival line toward the apex of the root these groups are: 
 (1) The free gingival group, the fibers of which pass from the cemen- 
 tum occlusally into the gingiva to support it; (2) the trans-septal 
 group, passing from tooth to tooth, and supporting the inter- 
 proximal gingivse; (3) the alveolar crest group passing from the 
 cementum to the outer layer of the periosteum on the labial and 
 lingual and to the crest of the alveolar process on the mesial and 
 distal; (4) the horizontal group in the occlusal third of the alveolar 
 portion and passing at right angles to the axis of the tooth from 
 the cementum to the bone; (5) the oblique group in the apical 
 two-thirds of the alveolar portion and inclined occlusally as they 
 pass from cementum to bone; (6) the apical group, the group of 
 fibers radiating from the apex of the root to the bone around the 
 apical space. 
 
 Beginning at the gingival line, the fibers springing from the 
 cementum pass out at a short distance at right angles to its surface 
 16
 
 242 THE PERIDENTAL MEMBRANE 
 
 and then bend sharply to the occlusal, passing up into the gingivse 
 and uniting with the fibrous mat which supports the epithelium. 
 These are much more strongly marked on the lingual than on the 
 labial gingivse, because in mastication the lingual gingivse receives 
 more pressure of food, which would tend to crush it down (the free 
 gingival group). A little deeper the fibers springing from the 
 cementum on the labial and lingual pass out at right angles to 
 the cementum and are lost in the coarser fibrous mat of the gum 
 tissue. The distance which they extend before being lost in the 
 coarser fibers is always greater on the lingual than on the labial. 
 On the proximal sides the fibers springing from the cementum at 
 the same level, branch and interlace, passing across the inter- 
 proximal space, to be attached to the cementum of the approxi- 
 mating tooth. These fibers are of the greatest importance, a? 
 they produce the basket work which forms the supporting frame- 
 work for the interproximal gingivse (the trans-septal group). A 
 little farther apically the fibers as they come from the cementum 
 are inclined apically. A short distance from the cementum they 
 unite into very large and strong bundles which join with the fibers 
 of the outer layer of the periosteum, extending over the labial 
 and lingual border of the alveolar process (the alveolar crest group). 
 On the proximal sides the fibers at this level are attached to the 
 cementum of the adjoining tooth, or are inclined apically, to be 
 inserted in the bone of the septum (the alveolar crest group) . These 
 large bundles form a distinct layer, which has been called the dental 
 ligament, because they bind the teeth together across the septum 
 and attach them to the outer layer of the periosteum on the 
 labial and lingual borders of the alveolar process. They are the 
 only fibers which hold the teeth down in its socket. At the border 
 of the alveolar process, and in the occlusal third of the alveolar 
 portion, the fibers pass directly from the cementum to the bone 
 at right angles to the axis of the tooth (the horizontal group). In 
 this position the fibers are larger and stronger, and show less ten- 
 dency to break up into smaller bundles in their course than in 
 any other portion of the membrane. In the middle and apical 
 thirds of the alveolar portion the fibers are inclined occlusally as 
 they pass from the cementum to the bone. They spring from the 
 cementum in compact bundles, and show a strong tendency to 
 break up into fan-shaped fasciculi, spreading out as they approach 
 the bone, to be attached over a larger area of the alveolar wall. 
 These fibers literally swing the tooth in its socket and support it
 
 PLATE XV 
 
 ' 
 
 Longitudinal Section of Peridental Membrane.
 
 ARRANGEMENT 
 
 243 
 
 against the forces of mastication (the oblique group). In the apical 
 region fibers springing from the cementum pass out in all direc- 
 tions, spreading out in the same way, to be inserted into the bone 
 forming the wall of the apicals pace (the apical group) . 
 
 If force is exerted against the lingual surface of an incisor, the 
 fibers on the lingual side of the root in the occlusal third will sustain 
 part of the strain, preventing the crown from moving labially, 
 
 FIG. 203. Longitudinal section of the peridental membrane in the gingival portion 
 from a lamb (the labial gingivus). 
 
 and at the same time the fibers on the labial side of the root in the 
 apical space will also be under strain, preventing the apex of the 
 root from moving lingually. The general plan of arrangement 
 which has been described is illustrated in Dr. Black's diagram made 
 from a labiolingual section of an incisor of a young kitten (Figs. 
 201 and 202). 
 
 With this general plane of arrangement in mind individual 
 sections may be studied, examining the arrangement and appear-
 
 244 
 
 THE PERIDENTAL MEMBRANE 
 
 ance of the fibers in detail. Figs. 203 and 204 show the labial 
 and lingual gingivre from an incisor of a sheep. Notice that the 
 labial gingiva is taller and thinner, and the fibers passing up into 
 it are not as strongly marked. Notice also the distance to which 
 the fine fibers of the peridental membrane can be followed before 
 they are lost in the coarser mat of gum tissue. The lingual gin- 
 giva is broader and flatter, and the fibers passing up into it form a 
 strong and well-defined band. Under higher magnification, fibers 
 
 1'iG. 204. Longitudinal section of the peridental membrane in the gingival 
 portion (the lingual gingivus) : D, dentin; N, Nasmyth's membrane; C, cementum; 
 F, fibers supporting the gingivus; F l , fibers attached to the outer layer of the perios- 
 teum over the alveolar process; f 2 , fibers attached to the bone at the rim of the 
 alveolus; D, bone. (About 30 X) 
 
 would be seen cut transversely, which pass around the tooth in the 
 gingiva, helping to hold it closely against the enamel. In Fig. 
 205 the fibers uniting with the outer layers of the periosteum 
 are very well shown. Taking transverse sections in the gingival 
 portion and remembering that they are cut at right angles to these 
 through the same area, the distribution of the tissues will be better 
 understood. Fig. 206 shows a section cut close to the gingival 
 line. At A the epithelium on the labial surface of the gingiva 
 is seen, and at B the epithelium lining the gingival space. On the
 
 ARRANGEMENT 
 
 245 
 
 B P" ' ^- 
 
 FIG. 205. Longitudinal section of peridental membrane of young sheep, showing 
 fibers penetrating the cementum: D, dentin; C, cementum, showing embedded 
 fibers; F, fibers running to the outer layer of the periosteum, covering the alveolar 
 process; F l , fibers running to the bone at the border of the process; B, bone. (About 
 80 X)
 
 246 
 
 THE PERIDENTAL MEMBRANE 
 
 proximal sides of the roots the fibers will be seen passing from the 
 cementum of one tooth to that of the next. Fig. 207 is a little 
 deeper and shows the fibers attached around the entire circum- 
 ference of the root. Beginning at the middle of the labial surface, 
 the fibers will be found springing from the cementum and passing 
 out at right angles to it, to be lost in the fibrous mat supporting 
 the epithelium. The fine fibers of the peridental membrane can 
 be followed for about half the -distance to the epithelium before 
 they are lost in the coarser mat of gum tissue, and a fairly definite 
 boundary will be seen between what should be considered peri- 
 
 FIG. 206. Transverse section of the peridental membrane in the gingival portion, 
 from young sheep. The roots of two temporary incisors are cut across. The epithe- 
 lium lining the gingival space is shown part way around one. A, epithelium on 
 labial surface of gingivse; B, epithelium lining the gingival space. (About 60 X) 
 
 dental membrane and the gum tissue. As the distolabial angle 
 of the root is approached, the fibers passing from the cementum 
 tend to swing around distally, and pass to the mesiolabial angle 
 of the adjoining tooth. Along the proximal surface the network 
 which supports the interproximal gingivus is well shown. The 
 fibers springing from the cementum interlace and pass around 
 bloodvessels and fibers which are passing up into the gingiva, and 
 finally are inserted into the cementum of the next tooth. In this 
 way it will be seen that the teeth in the entire arch are firmly bound 
 together by the fibers in the gingival portion. This explains the
 
 PLATE XVI 
 
 E5^**r**T^-~~^C ' 
 
 :: * w^ 
 
 Transverse Section of Pendental Membrane. 
 
 Stained with hematoxylin and eosin. Alveolar portion.
 
 ARRANGEMENT 
 
 247 
 
 way in which the positions of all the teeth are affected by the loss 
 of a single one in the arch, and the way in which the movement of 
 one tooth will draw its neighbors after it. It also explains the 
 separation of the central incisors when the frenum labium passes 
 through between the teeth, and is inserted on the lingual surface 
 of the alveolar process. If these incisors are to be held together 
 permanently, normal attachment of fibers extending from the 
 
 FIG. 207. Transverse section of the peridental membrane in the gingival portion 
 (from sheep): E, epithelium; F, fibrous tissue of gum; B, point where peridental 
 membrane fibers are lost in fibrous mat of the gum; P, pulp; F', fibers extending 
 from tooth to tooth. (About 30 X) 
 
 cementum of one tooth to that of the other must be secured. The 
 fibers in this area are also well shown in Fig. 208, and it can be 
 understood how they form foundation upon which the interproximal 
 gingiva rests. The first step in the sagging of the interproximal 
 gum tissue is the cutting off of the fibers from the cementum, where 
 it bends occlusally, following the curve of the gingival line on the 
 proximal surface.
 
 248 
 
 THE PERIDENTAL MEMBRANE 
 
 FIG. 208. A portion of the peridental membrane between two incisors of a young 
 sheep, showing the fibers extending from tooth to tooth. 
 
 FIG. 209. Fillers at the border of the alveolar process (from sheep): D, dentin; 
 C, cemontum; F, filters extending from cementum to bone; Bl, bloodvessel; B, 
 bone. (About 80 X)
 
 PLATE XVII 
 
 Cm - 
 
 Al 
 
 - \ 
 
 Transverse Section of the Pendental Membrane in the 
 Occlusal Third of the Alveolar Portion (from Sheep). 
 
 V muscle fibers; I'.r, periosteum; Al, bone of the alveolar process, Pd. peri- 
 dental membrane fibers; P, pulp; D, clentin; Cm, cementum.
 
 PLATE XVIII 
 
 Diagram of Peridental Membrane. 
 
 .V, muscle fibers; Per, periosteum; D. clentin; P. pulp; Cm, cementum; P<1, 
 peridental membrane fibers; Al, bone of the alveolar process.
 
 ARRANGEMENT 249 
 
 i 
 
 Plate XVII shows a transverse section in the occlusal third of the 
 alveolar portion from the incisor of a sheep. Upon the labial a 
 few muscle fibers are seen and the periosteum covering the labial 
 surface of the process. Notice the medullary spaces in the bone 
 and the canals opening into the peridental membrane and perios- 
 teum. The light line forming the outer boundary of the dentin is 
 characteristic. Two layers of cementum are seen, and notice 
 the thickening of the layer where strong bundles are attached. 
 At the middle of the labial surface the fibers pass at right angles 
 to the cementum and are attached to the bone, but as the distolabial 
 angle of the root is approached the bundles swing distally to be 
 attached in the bone. In Plate XVIII, which was drawn very care- 
 fully from this section, the arrangement of the fibers is shown dia- 
 grammatically. Notice the way in which they pass over and 
 under each other and around the bloodvessels which wind through 
 them. This relation to the bloodvessels is important, and will be 
 considered again later in connection with the blood supply of the 
 membrane. The tangential fibers at the angle of the root hold 
 the tooth against the forces which tend to rotate it in its socket. 
 They are important in connection with all rotating movements in 
 orthodontia. It has long been noted that rotations were the hardest 
 movements to retain, especially if the tooth were moved in no 
 other direction. In this case, if the tooth were turned mesially 
 the fibers at the distolabial angle would spring the thin plate of the 
 alveolar process as a bow is bent, leaving a condition of stress in 
 the tissue which will tend to spring back into its old position and 
 drag the tooth with it. Notice the greater thickness of the mem- 
 brane on the lingual as compared with the labial. Figs. 205 and 
 209 show longitudinal sections at the border of the alveolar process. 
 Notice that the fibers can be seen running through the entire thick- 
 ness of the cementum. They are large, strong fibers and branch 
 very little in their course. Note the bloodvessel that is shown in 
 several of these sections, and the way in which it gives off branches 
 passing over the border of the processes and toward the cementum.
 
 CHAPTER XXI. 
 
 THE CELLULAR ELEMENTS OF THE PERIDENTAL 
 MEMBRANE. 
 
 Fibroblasts. The fibroblasts are found everywhere between the 
 fibers which they have formed and to which they belong. They 
 are spindle-shaped or stellate connective-tissue cells, having a 
 more or less flattened nucleus and a body of granular cystoplasm, 
 
 FIG. 210. Fibers and fibroblasts from transverse section of membrane: F, fibers 
 cut transversely; F 1 , fibers cut longitudinally, showing fibroblasts. (About 80 X) 
 
 which is squeezed out into thin projections between the fibers. 
 In sections stained with hematoxylin the cells take the stain strongly 
 and the fibers remain clear (Fig. 210). In this way the fibers are 
 marked out by the cells which lie between them. The number of 
 the fibroblasts in the membrane decreases with age. They are 
 large and numerous in the membrane of a newly erupted tooth and 
 are comparatively small and few in the membrane around an old 
 tooth. This is, however, characteristic of fibroblasts in connective 
 tissue generally. Fig. 210 shows a small field taken from the gingival 
 portion of the membrane between the teeth. The magnification 
 is low, the photograph being made with a f objective. The cells 
 (250)
 
 251 
 
 are seen as little dark dots lying between the fibers, which are 
 clear. Where the fibers are cut longitudinally they appear spindle- 
 shaped, but where the fibers are cut across they appear star-shaped. 
 They will be seen better in photographs made with higher magnifi- 
 cation, but an adequate idea of their form can only be obtained 
 by studying sections very carefully with a | or y 1 ^ objective and 
 using the fine adjustment to gain an idea of the third dimension 
 of space. They are shown in many of the illustrations of the 
 epithelial structures. 
 
 Cementoblasts. The cementoblasts are the cells which form 
 cementum. They cover the surface of the root everywhere between 
 the fibers which are embedded in the tissue. While these cells 
 perform the same function for the cementum as the osteoblasts 
 do for bone, they are quite different in form. They are always 
 
 FIG. 211. -Isolated cementoblasts, 
 showing the form of the cell as it fits 
 around the fibers springing from the 
 cementum. (Black.) 
 
 FIG. 212. Cementoblasts as seen in 
 a section at a tangent to the root and 
 just missing the cementum. The fibers 
 are left white, the cells are shaded. 
 (Black.) 
 
 flattened cells, sometimes almost scale-like, and when seen from 
 above, very irregular in outline. This irregularity in outline is 
 due to the projections of the cytoplasm around the fibers as they 
 spring from the cementum, the edges of the cell being notched and 
 scalloped to fit about them. There is a central mass of granular 
 cytoplasm which contains an oval and more or less flattened 
 nucleus, from which the cytoplasm extends in projections passing 
 partly around the fibers. Isolated cementoblasts are shown in 
 Fig. 211, drawn by Dr. Black. In order to obtain an idea of the 
 form of the cementoblasts, sections must be cut at a tangent to 
 the surface of the root, and just missing the surface of the cemen- 
 tum. In this way the fibers are cut across andi the cementoblasts
 
 252 THE PERI DENTAL MEMBRANE 
 
 are shown covering the entire surface between the fibers. These 
 are shown in Fig. 212, in which the fibers are left perfectly clear 
 in order to outline the cells more distinctly. In sections cut at 
 right angles to the surface of the roots (Figs. 223, 224, and 225) 
 the cementoblasts are shown as more or less flattened, but no 
 idea of the way in which they fit about the fibers can be 
 obtained. 
 
 Cytoplasmic processes extend from the body of the cemento- 
 blasts into the matrix of the cementum. These correspond to the 
 process of the osteoblasts which occupy the canaliculi of bone. 
 They, however, are not nearly as numerous or as regular in their 
 arrangement as the osteoblasts. Processes extending from these 
 cells in a direction from the cementum out into the tissue of the 
 membrane have not been demonstrated. 
 
 Cement Corpuscles. Occasionally a cementoblast becomes fast- 
 ened down to the surface and enclosed in the matrix that is formed. 
 They then lie in a lacuna and show processes radiating from them 
 into the canaliculi. These correspond to bone corpuscles, but 
 there is no such regularity of their disposition or arrangement with 
 reference to the lamellae, as is shown in the case of bone. In man 
 the cementum in the gingival half of the root is usually without 
 cement corpuscles. They often lie entirely within a single lamella 
 instead of between two, as is the case in bone. In general they are 
 found where the layers are thick and the embedded fibers are 
 not specially numerous. They are very often seen where absorp- 
 tions have been refilled by the formation of subsequent layers 
 (Figs. 140 and 141). 
 
 It is by the activity of the cementoblasts producing a new layer 
 of cementum that the fibers are attached to the surface of the 
 root. In studying many sections, places are found w r here the 
 fibers, though lying in contact with the surface are not attached 
 to the cementum. In some places it can be seen that they have 
 been cut off by absorptions. From a study of these layers it is 
 evident that there is a constant readjustment in the attachment of 
 the fibers to the root during the function of the tooth, which prob- 
 ably adapt it to slight changes of position resulting from wear and 
 other conditions. It is important to remember that whenever 
 the fibers have been stripped from the surface of the cementum, 
 they can be reattached to it only by the formation of a new layer 
 of cementum, building the fibers into it. This is certainly possible 
 if the conditions are properly controlled, but the cells of the tissue
 
 CEMENT CORPUSCLES 
 
 253 
 
 must be in a normal and vitally active condition, and the surface 
 of the root must be such that they can lie in physiological contact 
 
 Pd.M 
 
 Ms 
 
 Pd.B 
 
 H.B 
 
 P'iG. 213. Penetrating fibers in bone. A field from Plate XIX: Pd.M, pcridental 
 membrane; Ob 1 , osteoblasts of peridental membrane; Ol 2 , osteoblasts of medullary 
 space; Pd.B, solid subperidental and subperiosteal bone with embedded fibers; 
 Ms, medullary space formed by absorption of the solid subperidental bone with 
 embedded fibers; H.B, Haversian system bone without fibers built around the 
 medullary space. (About 200 X) 
 
 with it. The cure of a pyorrhea case therefore becomes a biological 
 problem. In this connection it is important to remember that a 
 surface of cementum which has long been bathed in pus may be so
 
 254 THE PERIDENTAL MEMBRANE 
 
 filled with poison that no cell can lie in contact with it and perform 
 its functions. 
 
 Osteoblasts. The osteoblasts of the peridental membrane are 
 exactly like osteoblasts in other positions. They cover the surface 
 of the bone of the alveolar wall lying between the fibers which 
 are embedded in it. Even in the young subject they are not found 
 in every position, while in an adjoining area the surface of the bone 
 may be covered with them. In the old subject they are generally 
 absent or have been reduced to flattened scales, which are very 
 difficult to demonstrate; but even in these cases areas will be 
 found in which osteoblasts are present. These are areas of active 
 bone formation. The osteoblasts lay down bone exactly as occurs 
 in attached portions of the periosteum, but after a little thickness 
 of this solid peridental bone has been formed it is perforated by 
 penetrating canals, on the walls of which absorptions occur, form- 
 ing spaces about which new Haversian system bone is formed. 
 This is illustrated in Plate XIX. In this way only sufficient sub- 
 peridental bone is left to furnish an attachment for the fibers. 
 
 Fig. 213 shows a higher magnification of a small area. The 
 osteoblasts are seen between the fibers on the surface of the alveolus, 
 and the fibers can be followed through the subperidental bone. 
 A large absorption area has been formed which has been partly 
 rebuilt, and the new-formed bone without embedded fibers is 
 lighter in color. An understanding of this building and rebuilding 
 of bone through the agency of the peridental membrane is neces- 
 sary to understand the development of the face and everything in 
 connection with tooth movement, whether physiological or artificial. 
 
 Osteoclasts. The osteoclasts of the peridental membrane are 
 not constant elements. They appear and disappear in response 
 to the same conditions which lead to their appearance and disap- 
 pearance in bone. They are always large, multinuclear cells, having 
 from three or four to thirty of forty nuclei (Fig. 214). They may 
 appear upon the surface of the cementum, upon the surface of the 
 alveolar wall, or within the medullary spaces of the bone. They 
 are formed from embryonal cells in the tissue in response to mechan- 
 ical stimuli. Morphologically they are in no respect different from 
 the osteoclasts in bone. 
 
 The osteoclasts are tissue destroyers and are the active agents 
 in the removal of any hard tissue. There is no difference in them, 
 whether they are destroying the fibrous tissue, bone, cementum, 
 or dentin (Fig. 215). In order for them to act, their cytoplasm
 
 PLATE XIX 
 
 PdB 
 
 Border of Growing Process. 
 
 Cm, cementum; P<1, peridental membrane; PdB, solid subpericlental and 
 subperiosteal bone with embedded fibers; .1/8, medullary space formed by 
 absorption of the solid bone; // B, Haversian system bone without fibers; 
 Per, periosteum. (About SO X-)
 
 OSTEOCLASTS 
 
 255 
 
 must lie in actual contact with the surface to be attacked. They 
 do not first decalcify and then remove, but apparently by applying 
 their cytoplasm to its surface the cells destroy the intercellular 
 substance, forming hollows in the surface, into which the cells 
 sink. These hollows have been called Howship's lacunae. The 
 cells usually appear in groups and spread out over the bone or 
 cementum to be attacked, but sometimes only two or three will 
 
 FIG. 214. Osteoclast absorption of bone over permanent tooth: Oc, osteoclasts; B , 
 bone of crypt wall; F, fibrous tissue of follicle wall; A, ameloblasts. (About 62 X) 
 
 be found at a point on the surface of the bone, and these will burrow 
 into the substance, forming a penetrating canal running through 
 the bone (Figs. 216 and 217). In these positions the osteoclasts 
 are usually comparatively small. As fast as the canal is formed 
 the embryonal cells of the membrane multiply and grow into the 
 space and at any point where absorption is going on the portion 
 destroyed is immediately replaced by embryonal connective tissue. 
 This will be noted in all the illustrations showing absorptions.
 
 256 
 
 Whenever absorption is going on formation is also going on in an 
 adjoining area. In this way the function of the tissue is maintained 
 until the last remnants of it are destroyed. The general statement 
 may be made that bone formation is always accompanied by bone 
 destruction, and bone destruction by rebuilding. The result depends 
 
 FIG. 215. Ostcoclasts in cancellous bone near the peridental membrane; in some 
 portions of the field osteoblasts are seen. As bone is removed note how embryonal 
 connective tissue replaces it. 
 
 upon which side the balance swings. The alternation of formation 
 and absorption in the removal of hard tissues is well illustrated 
 in the absorption of the roots of the temporary teeth. The absorp- 
 tion does not begin at one point and spread continuously over the 
 entire surface of the root. If it did so, all of the fibers would be
 
 OSTEOCLASTS 
 
 257 
 
 &m /.*:; 
 
 ' ^IlKfc. 
 
 FIG. 216. Osteoclast absorption forming penetrators of canal: a, bone matrix; 
 6, bloodvessel; c, embryonal connective tissue; d, new bone formation; e, osteo- 
 biasts; /, osteoclasts. (Black.) 
 
 FIG. 217. A longitudinal section through the remains of the alveolar process 
 around the root of a temporary tooth about to be shed (sheep) : C, the cementum on 
 the remains of the tooth; B, penetrating canals cut through the labial plate of bone. 
 17
 
 258 
 
 THE PERI DENTAL MEMBRANE 
 
 cut out and the tooth would drop off with at least a considerable 
 portion of the root. The process progresses in something of this 
 
 Cm 1 
 
 FIG. 218. Root of a temporary incisor, showing absorption and rebuilding of 
 rementum (from sheep): G, gingivus; D, den tin; Cm, cementum; Ab, absorption 
 cavity, showing Howship's lacunae; Cm 1 , new-formed cementum. (About 50 X) 
 
 fashion: At a point on the side of the root near the apex, where 
 the growth of the erupting tooth produces pressure, osteoclasts
 
 OSTEOCLASTS 259 
 
 appear in the membrane, cutting off the fibers, displacing the 
 cementoblasts, and arranging themselves in groups on the surface 
 of the root. These dissolve away the cementum and sink into 
 the tissue, perhaps cutting into the dentin for a short distance. 
 By this excavation the pressure is relieved, the osteoclasts disap- 
 pear, cementoblasts are formed in the embryonal connective tissue, 
 
 FIG. 219. A transverse section through an incisor from the same jaw as Fig. 218, 
 and at the level of Cm 1 , showing the refilling of the absorption cavity by new layers 
 of cementum. 
 
 and the deposit of cementum begins in the excavation, reattaching 
 the fibers in this area. As the rebuilding progresses, at a point a 
 little farther occlusally, osteoclasts appear and begin a new excava- 
 tion. In this manner the process continues. ^Yhen the absorption 
 stops in the second point, it begins again at the first, cutting much 
 deeper into the dentin, and, oscillating back and forth, it progresses 
 until all of the dentin may be destroyed, leaving the hollow cap of
 
 260 THE PERIDENTAL MEMBRANE 
 
 enamel, and even then new-forming cementum to maintain the 
 attachment will be found around the circumference. In this way 
 it will be seen that the function of the tooth is maintained until 
 its successor is ready to take its place in a very short time. The 
 importance of this arrangement will be more fully appreciated 
 after a study of the relation of the teeth to the development of 
 the face. Fig. 218 shows a longitudinal section through a temporary 
 incisor of a sheep. At Ab an absorption has just been completed, 
 for the osteoclasts have disappeared. The excavation is seen 
 filled with embryonal tissue and rebuilding is about to begin. At 
 Cm an older and much larger absorption space is seen which has 
 been partially replaced by a formation of new cementum reattach- 
 ing fibers. In Fig. 219 a transverse section of the root is seen which 
 is from the same jaw cut at the level of Cm, and shows the absorp- 
 tion refilled. This patchwork performance goes on in the same 
 way in the bone of the alveolar process, and its study is one of the 
 most interesting phases of the relation of the teeth to the develop- 
 ment of the face. Without a clear idea of this it is impossible to 
 understand how the teeth, after their roots are fully formed, can 
 move through three dimensions of space and retain their function 
 all the time. 
 
 Epithelial Structures. The epithelial structures of the peridental 
 membrane were first described by Dr. Black in his volume Perios- 
 teum and Peridental Membrane, published in 1887. At this time 
 Dr. Black considered them to be of lymphatic character and named 
 them endolymphatics. His conception of them was that they were 
 lymphatic channels crowded with adenoid cells. Since then the 
 form and appearance of the cells and the character of their reaction 
 with staining agents has shown the cells to be of epithelial character. 
 In the same year that Dr. Black's book was published, von Brunn 1 
 described the same structures. He considered them as epithelial 
 remains of the outer layer of the enamel organ, growing down 
 around the root beyond the gingival line where the formation of 
 enamel stops. It has seemed probable to the writer that this 
 was correct, but their histogenesis has not been sufficiently well 
 followed, and it presents an attractive field for research. These 
 structures undoubtedly originate from the epithelium of the enamel 
 organ, probably both of the outer and inner layers. In the authors' 
 opinion they have an important relation to the formation of ceinen- 
 
 1 Archiv f. Anatomic, 1887.
 
 DISTRIBUTION 
 
 261 
 
 turn which accounts for their persistence in the membrane. While 
 they are derived from an embryonal structure, the enamel organ, 
 it does not seem proper to regard them as embryonal remains, for 
 while, like all the cellular elements, they are more numerous in 
 young people than in old, they are persistent throughout life. They 
 have been shown in the membrane from a man aged seventy years, 
 and it does not seem logical to suppose that embryonal debris that 
 was useless to the organism would persist through life. Up to the 
 present time, however, nothing has been discovered about these 
 structures to throw any light upon their function. The author has 
 long been of the opinion that they were related to the formation of 
 cementum but this is not sufficiently established to be more than 
 suggested here. Specimens have strongly indicated that they were 
 important in some pathologic conditions. Their cells have been 
 found dead and degenerating in pathologic material beyond the 
 point showing any pathologic condition in other cells. These struc- 
 tures have been observed in sections from man, sheep, cat, dog, and 
 monkey. The best material for their study is a young sheep or pig. 
 
 FIG. 220. Diagram of glands of peridental membrane. (Black.) 
 
 Distribution. These structures are composed of cords or rows 
 of epithelial cells, surrounded by an extremely delicate basement 
 membrane (Fig. 220). In some cases there is a slight indication 
 of a circular arrangement of connective tissue around them. The 
 cords lie very close to the surface of the cementum, winding in 
 and out among the fibers (Fig. 221). They anastomose and join
 
 262 
 
 THE PERIDENTAL MEMBRANE 
 
 with each other, forming a network the meshes of which are com- 
 paratively close in the gingival portion (Fig. 222), and compara- 
 tively wide in the apical portion, the cords becoming scarcer as the 
 
 FIG. 221. A section cutting diagonally through the root, showing the network of 
 epithelial cords, A ; dentin, D ; cementum, Cm 
 
 apex of the root is approached, but the author has seen them in 
 sections from the apical third. 
 
 A binocular microscope was used to obtain a true conception
 
 THE ARRANGEMENT OF THE CELLS 
 
 263 
 
 of the way in which these cords wind in and out among the bundles 
 of fibers. The cords show a marked tendency to run out into the 
 membrane and loop back (Fig. 223), coming very close to the 
 surface of the cementum. 
 
 The ends of the loops toward the cementum often show enlarge- 
 ments which in some cases apparently lie directly in contact with 
 the cementum (Figs. 224 and 225). These enlargements next to 
 the cementum are shown in Fig. 223. 
 
 FIG. 222. Transverse section of the peridental membrane in the gingival portion, 
 showing the position of the epithelial cords. At 1 the loop shown in higher mag- 
 nification in Fig. 224 is seen. 
 
 The Arrangement of the Cells. There is no definite arrangement 
 of the cells in these cords. In some places there will be a ring of 
 irregular polyhedral or rounded cells which almost exactly resemble 
 a simple tubular gland. In other places there is a pretty definite 
 outer ring of cells and a central mass enclosed by them. The cells 
 are made up of granular cytoplasm, each containing an ovoid 
 nucleus that is rich in chromatin. The author has spent much 
 time attempting to work out the relation of these cords to the 
 epithelium lining the gingival space, thinking that possibly they 
 open into it. In a few places structures appearing very much like
 
 264 
 
 THE PERIDENTAL MEMBRANE 
 
 a duct have been seen, as shown in Fig. 227, but they are appar- 
 ently only unusually large cords. There is no regularity in places 
 where they are found, and no connection with the gingival space 
 has ever been discovered. Toward the gingival, as the gingival 
 line is approached, the cords seem to swing out away from the 
 cementum, especially on the proximal side, and to pass up into the 
 gingivse, where they are lost among the projections of the epithe- 
 lium. 
 
 EC 
 
 FIG. 223. Epithelial structures of the peridental membrane (from sheep) : Fb, 
 fihroblasts; .Ec, epithelial structures; Cb, cementoblasts; Cm, cementum; D, dentin. 
 (About 468 X) 
 
 Gland of Serres. Salter, in his Dental Pathology and Surgery, 
 quotes Serres, who assigns the function of a gland to the epithelium 
 lining the gingival space. This, the writer believes, is the first 
 reference to an appearance in the tissues that has been called the 
 gland of Serres. It has long been noted that the epithelium lining
 
 GLAND OF SERRES 
 
 265 
 
 the gingival space was lighter in structure, composed of larger 
 cells, and had no horny layer on its surface, as is true of the epithe- 
 
 FIG. 224. Epithelial structures: EC, epithelial cord, apparently showing a lumen; 
 Cb, cementoblasts; Cm, cementum; D, dentin. This loop is seen in Fig. 209. 
 
 
 FIG. 225. Transverse section, showing the cellular elements. (About 900 X)
 
 266 
 
 THE PERIDENTAL MEMBRANE 
 
 Hum on the outer surface of the gingivus. Upon the proximal 
 surfaces the projections of the epithelium which extend down 
 between the papillae of connective tissue, which constitute the 
 stratum papillaris, are specially long, and in the connective tissue 
 between them collections of small round cells are often found. It 
 is between these projections of epithelium that the cords of epithe- 
 
 FIG. 226. Epithelial structures (from sheep): Fb, fibroblasts; EC, epithelial struct- 
 ures; Cb, cemen to blasts; Cm, cementum; D, dentin. (About 700 X) 
 
 Hal cells which have been described are lost, and to this portion 
 of the tissue Dr. Black has again called attention, as the gland 
 of Series. Sufficient work has not yet been done upon this subject 
 to know whether this is a constant arrangement, or whether it is 
 found only in certain animals, or even whether it may not possibly 
 be pathologic. The appearance is shown in Figs. 228 and 229. 
 The work of the last five years has convinced the writer, that 
 this appearance is the reaction of the tissue to infection. One
 
 BLOODVESSELS 267 
 
 of the important functions of the supporting tissues about the 
 necks of the teeth is to resist and remove infection, and all the 
 structural elements of the tissue are arranged for that function. 
 The epithelium, the connective-tissue fibers, the capillary blood- 
 vessels, the lymphatics, and the cellular elements of the connective 
 tissue, are so arranged as to immediately respond to an invasion of 
 infecting organisms in such a way as to destroy and remove them if 
 possible. 
 
 FIG. 227. A very large cord which was at first mistaken for a duct. 
 
 Bloodvessels. The peridental membrane possesses a very rich 
 blood supply. A number of vessels enter the membrane in the 
 apical portion from the medullary spaces in the bone. Some of 
 these, passing through canals in the apex of the root, supply the 
 dental pulp, others pass up through the membrane. As they 
 extend occlusally they give off and receive branches which enter 
 the membrane from the bone of the alveolar w r all. In this way 
 the caliber of the principal vessels is maintained throughout their 
 course in the membrane. As they reach the border of the alveolar 
 process they give off branches which anastomose with the vessels 
 of the periosteum and gum tissue. 
 
 In the young membrane these vessels occupy a position closer to 
 the bone than the cementum, and as the membrane becomes thinner 
 they often come to lie in grooves in the bone. Vessels of any size
 
 268 
 
 THE PERJDENTAL MEMBRANE 
 
 are rarely seen close to the cementum, and the capillaries in the 
 membrane are rather scarce, though they are more numerous than 
 
 FIG. 228. Longitudinal section, cut mesiodistally: D, dentin; Cm, cementum 
 which has separated from the dentin; Gs, gingival space; Ep, epithelial projection 
 from the lining of the gingival space; EC, epithelial cords; Rs, small round cells in 
 the connective tissue. 
 
 in most connective tissues of as compact a character. The anas- 
 tomosis of the vessels in the membrane is quite rich. It is impor- 
 tant to remember that the cancellous bone of the process is richly
 
 BLOODVESSELS 
 
 269 
 
 supplied with bloodvessels, and the anastomosis with the vessels 
 of the membrane, from the alveolar wall and over the border of 
 the process, is important in the consideration of pathologic condi- 
 
 FIG. 229. A longitudinal section cut mesiodistally: E, epithelium of the gingivus; 
 Gs, gingival space; Cm, cementum which has separated from the den tin; EC, epithe- 
 lial cords. 
 
 tions. Iii alveolar abscess the vessels entering through the apical 
 space may be entirely cut off, but this does not disturb the blood 
 supply of the rest of the membrane. The removal of the pulp has
 
 270 THE PERIDENTAL MEMBRANE 
 
 often been advocated in the treatment of pathologic conditions 
 of the membrane, on the ground that the vessels entering the pulp 
 rob the membrane of blood supply, and that their removal made 
 recovery more certain. No one having a knowledge of the blood 
 supply of the membrane could advise this for that reason. 
 
 In their course through the membrane the vessels wind between 
 the principal fibers in a way that can only be appreciated by study- 
 ing sections with a binocular instrument, and when this condition 
 is realized it can be understood how some inflammations in the 
 membrane are set up. For instance, when force is applied to a 
 tooth, the principal fibers are stretched. This causes them to close 
 some spaces and open others. The vessels in the closed spaces are 
 constricted and the flow of blood through them partly shut off. 
 The vessels in the enlarged spaces dilate to compensate. If the 
 force is removed, the dilated vessels are again constricted, and the 
 constricted ones enlarged, and the result is a literal sawing upon 
 the walls of the bloodvessels which in a very short time will set up 
 an acute inflammation. This is extremely important in the applica- 
 tion of force in orthodontia, and often also in the use of the mallet 
 in condensing gold, especially for young patients. 
 
 Lymphatic Vessels. The lymphatics of the peridental membrane 
 have been described in Chapter XIV. The writer was first convinced 
 of their presence in the membrane by a study of the manner of 
 extension of destructive inflammations of the peridental membrane 
 and they were afterward demonstrated by injection. The collecting 
 vessels from the labial, buccal and lingual surfaces of the gums and 
 gingivse pass outside of the periosteum of the alveolar process to 
 the wreath of collecting trunks at the reflection of the tissues from 
 the surface of the bone to the lips or cheeks on the outside and to 
 the collecting trunks of the floor of the mouth and palate on the 
 inside. The collecting vessels from the papillae lining the gingival 
 spaces penetrate the ligamentum circulare very close to the cementum 
 and extend in the interfibrous tissue accompanying the bloodvessels 
 and nerves, through the peridental membrane as far as the apex of 
 the root, where they anastomose with the efferent vessels from the 
 dental pulp. They have been followed through the bone to the infra- 
 orbital canal and the inferior dental canal emerging from the corre- 
 sponding foramina and passing to the lymph nodes of the submaxil- 
 lary group. Injected vessels in the peridental membrane are shown 
 in Fig. 230.
 
 NERVES 
 
 271 
 
 Nerves. The nerves of the peridental membrane enter the 
 peridental membrane in company with the bloodvessels. Their 
 source is the same as that of the bloodvessels. The trunks entering 
 in the apical space contain from eight or ten to fifteen or twenty 
 medullated fibers. Some of these enter the dental pulp, others 
 extend up through the membrane, winding in and out among the 
 fibers, generally following the course of the bloodvessels. Many 
 trunks containing eight or ten fibers enter through the alveolar 
 
 FIG. 230. Transverse section of the peridental membrane; showing injected 
 lymphatic vessels (oc., 3; obj., 16 mm.; reduced about one-tenth). 
 
 wall. In this way a fairly rich plexus is formed, from which fibers 
 are continually given off to be lost in the tissue. They probably 
 terminate in beaded free endings. No special nerve endings have 
 been demonstrated. A few Pacinian corpuscles have been seen 
 near the gingival border. These are not generally found, however. 
 The nerves of the membrane give to it the sense of touch, which 
 is the only sensory function of the membrane. As has been noted 
 in connection with the dental pulp, the hard tissues and the pulp 
 have no sense of touch. The contact of any substance with the 
 surface of the tooth is reported to consciousness through the medium
 
 272 
 
 THE PERIDENTAL MEMBRANE 
 
 of the peridental membrane. For instance, the slightest touch of 
 a delicate instrument produces a slight movement of the tooth 
 which affects the nerves between the fibers. The delicacy of this 
 mechanism can be demonstrated by the following experiment. 
 Lightly touch the surface of the enamel and the patient will tell 
 at once not only which tooth is touched, but whether a steel instru- 
 ment or a wooden point or some soft material was used. If, how- 
 ever, the finger is placed upon a surface of the tooth and firm pressure 
 made in one direction, the contact of the point will not be recognized. 
 
 FIG. 231. -Young membranes (from sheep): D, dentin; Cm, cementum; Cm 1 
 thickening of cementum to attach fibers at the corner; Pd, peridental membrane; 
 B, bone forming the wall of the alveolus. (About 80 X) 
 
 The Changes in the Peridental Membrane with Age. The teeth 
 are formed in crypts in the bone, and when they begin to erupt 
 the roof of the crypt is removed by absorption, making an opening 
 large enough for the crown to pass. As the root is formed, the 
 tooth moves occlusally and the alveolus grows up around it, begin- 
 ning at the margins of the crypt. When the tooth first erupts, there- 
 fore, the alveolus is much larger than the root, and the fibers of 
 the peridental membrane are very long. The size of the alveolus 
 is reduced by the formation of bone, by the osteoblasts on its wall, 
 and the size of the root is increased by the formation, layer after
 
 CHANGES IN PERIDENTAL MEMBRANE WITH AGE 273 
 
 layer, on its surface. In this way the thickness of the membrane 
 is reduced. Figs. 231 and 232 were made to illustrate this change. 
 They were photographed with as nearly the same magnifications 
 as possible, so as to compare the thickness of the layers. In the 
 first, there are but two layers of cementum; notice the thickening 
 of the last-formed layer, to attach the strong bundles of fibers at 
 the angle of the root. The second is from a temporary tooth which 
 has been in position and function for a long time; notice the thick- 
 ness of the cementum and that the formation of bone and cementum 
 
 Pd 
 
 FIG. 232. Old membranes (from sheep): D, dentin; Cm, cementum; Pd, peri- 
 dental membrane; B, bone forming the wall of the alveolus; P, pulp. (About 
 80 X) 
 
 has reduced the thickness of the membrane to not more than one- 
 third of its original amount. Notice also that the surface of both 
 bone and cementum are not even, but scalloped, and that where 
 the cementum projects toward the alveolar wall there! is a depres- 
 sion in the bone, and where the bone projects toward the cementum 
 there is a depression in the cementum. There is therefore a dis- 
 tinct tendency for the two tissues to interlock but remain separated 
 by a layer of fibrous tissue. The author has never seen a specimen 
 showing a union between calcified substances of bone and cemen- 
 18
 
 274 THE PERIDENTAL MEMBRANE 
 
 turn. Two surfaces of cementum may become united by direct 
 calcification and the teeth fused together. This is illustrated in 
 many freak specimens to be found in any dental museum. It is 
 often stated that a tooth had become ankylosed to the bone, but 
 to the author's knowledge no specimen has ever been shown in 
 which the separating layer of fibrous tissue was not present. 
 
 Practical Consideration. These structural facts are of the greatest 
 practical importance, especially in the making of gold fillings for 
 young persons. Every operator has noticed the greatest difference 
 in the feeling of the instrument under the mallet upon different 
 teeth. In one instance it will ring under the steel mallet as if the 
 tooth were resting on an anvil; in another case it feels as if the 
 tooth were resting on a cushion. In the first case all of the force 
 of the blow is expended in the condensation of the gold. In the 
 second, a large proportion is lost in the movement of the tooth. 
 If the membrane is thin and the cementum and bone are inter- 
 locked, the tooth is firmly supported. If the membrane is thick 
 and the fibers long, as in the first illustration, the blow is dissipated 
 in the sag of the fibers. The tooth is jumping up and down in its 
 socket. The force used is dissipated, the gold is not condensed, 
 and in a very short time an acute inflammation is set up and the 
 tooth becomes very sore to the blows. This the author believes is 
 the explanation of the idea that gold w r ill not preserve teeth for 
 young children. It has often been said that children's teeth are 
 too soft for gold fillings. The difficulty is not with the enamel and 
 dentin, but because of the thickness of their membranes. The 
 gold is not sufficiently condensed to exclude moisture, and the 
 fillings fail. Serious damage also may be done to the membrane. 
 The Museum of the Northwestern University Dental School con- 
 tains an object lesson on this point. It consists in a bicuspid with 
 a beautifully condensed and finished gold filling in a mesial occlusal 
 cavity. The history accompanying it is somewhat as follows: 
 The operation was undertaken for a patient aged about fourteen 
 years. The tooth became exceedingly sore under the mallet; the 
 filling was, however, completed and polished, but a few days later 
 the tooth was picked out with the fingers. The peridental membrane 
 had been literally hammered to death. Stated in scientific terms, 
 the fibers had sawed upon the bloodvessels, exciting an acute inflam- 
 mation, resulting in complete stasis and the death of the tissue. 
 In all operations where gold is to be condensed in teeth with thick 
 membranes, they must be firmly supported so as to be held rigidly 
 against the blow.
 
 CHAPTER XXII. 
 
 ABSORPTION OF TEETH 
 
 BY NEWTON G. THOMAS, M.A., D.D.S. 
 
 THE absorption of teeth implies a phenomenon which is known 
 to occur in both dentitions. To the primary dentition, absorption 
 is a part of normal history; with the permanent dentition, it is 
 associated only with unrecognizable and inexplicable conditions or 
 strictly pathological agencies. Hence our first classification of 
 absorption is physiological and pathological. Under the former 
 comes the removal of temporary tooth roots and the roots of 
 implanted teeth, while under pathological comes the removal of 
 permanent tooth roots, either wholly or in part. These occurrences 
 form the ground work of this discussion. 
 
 FIG. 233. Showing normal absorption of a temporary molar without pressure 
 from a permanent successor. 
 
 Various causes of the absorption of the roots of temporary teeth 
 have been presented. The principal ones ascribed are pressure, the 
 ectodermic origin of enamel, blood-pressure, the gubernaculum 
 dentis, and the deposition of bone impelling the permanent teeth 
 occlusally. It is urged that the pressure of the erupting permanent 
 teeth instigates the process whether one or all of the causes of erup- 
 tion mentioned begins or maintains the movement. It is a common 
 observation that the pressure of permanent teeth sometimes fails to 
 stimulate absorption and again it is seen that temporary teeth 
 often absorb when their permanent successors are absent (Fig. 233). 
 
 (275)
 
 276 ABSORPTION OF TEETH 
 
 The absorption of the roots of the temporary teeth must be con- 
 sidered as Tomes considered it, a physiological or vital process, 
 which all of the factors named may abet but do not explain. 
 
 The agent of absorption presents an interesting but unfinished 
 study. Kolliker designated the multinucleated giant cell, the osteo- 
 clast, almost always seen in areas where osseous tissue is in process 
 of formation, the agent of tissue removal, because bone building is 
 always a composite of construction and destruction, a condition 
 not seen in tooth tissues because changes similar to those in bone 
 do not take place. Once teeth are formed they do not change form : 
 areas of their surfaces may be removed but it is never done for the 
 purpose of reconstruction as is the case when subperiosteal bone is 
 removed to give place to Haversian system bone. Where, however, 
 teeth are being physiologically removed osteoclasts are found with- 
 out fail. Until quite recently this cell maintained its designa- 
 tion as a tissue destroyer undisputed, but at the present time 
 its function is held in question. It is stated that there is no positive 
 evidence that the osteoclast is active in destroying bone. The 
 explanations given are that they are degenerate osteoblasts that 
 have become confluent, 1 or that have been affected by the agency 
 that is affecting the bone, and, therefore, are in a condition of 
 degeneracy. 2 Also destruction of bone by halisteresis as in osteo- 
 malacia is mentioned to prove that cells are not necessary to its 
 accomplishment. 3 Both of these hypotheses are unsatisfactory. 
 Sections can be readily produced in which osteoclasts show no 
 evidence of disintegration and in which there is no evidence that 
 they are the products of osteoblastic fusion. In fact cementoblasts, 
 the analogue of osteoblasts, are noticeably absent from the surfaces 
 of teeth on which absorption is in progress. 
 
 The origin of osteoclasts is controverted as much as their function. 
 Prentiss, Jackson and Dautschakoff state that they are derived from 
 the reticular cells of marrow. Kolliker, Bredichin, and Howell 
 suggest that they are osteoblastic in origin, the osteoblast in response 
 to some unknown stimulus fusing with its fellows to assume a new 
 function. \Yegener, Schaffer, 4 Fischer, 5 and Mallory 6 trace them to 
 
 1 Arey: The Origin, Fate and Significance of Osteoclasts, Tr. Chicago Path. Soc. , 
 pp. 231-234. 
 
 2 Stohr: Text-book of Histology, pp. 68 and 202. 
 
 3 Loc. cit. 
 
 4 Prentiss: Origin and Fate of Osteoclasts, Surg., Gynec. and Obstet., 1915, xx, 
 678. 
 
 * Anatomische Hefte, 1909. 
 
 6 Principles of Pathology, 1912, p. 52.
 
 ABSORPTION OF TEETH 277 
 
 endothelial cells, while Ranvier, Duval, and Bohm 1 assert that they 
 arise from lymphoid marrow cells. In the consideration of the 
 internal absorptions seen in dentin, Causch 2 avers that they are the 
 products of odontoblasts, as does Salter also. To this Causch adds 
 that tissue destruction does not depend upon them, as phagocytic 
 leukocytes may produce the same results. In this it will be observed 
 he harmonizes closely with Mallory. The specimens from which 
 the accompanying figures are made also testify to the endothelial 
 origin of the osteoclast. Fischer's assumption that the endothelium 
 of capillaries may destroy tissue is also closely related to Mallory's 
 statement. 3 Bland-Sutton, 4 in 1887, described giant cells formed by 
 the fusion of phagocytes, and adds: "The large multinuclear osteo- 
 clasts seen in places where vertebrate bone and teeth are under 
 absorption must also be placed in the same category.'' In accord 
 with the foregoing, Mallory says : " When an endothelial leukocyte 
 finds difficulty in dissolving a substance, as, for instance, lime or 
 certain fat products or the blastomyces, it frequently fuses with 
 other endothelial leukocytes to form a multinucleated mass of cyto- 
 plasm commonly termed a foreign body giant cell. If the foreign 
 body is too large for one leukocyte to incorporate (cholesterin crystals, 
 hairs) one or more giant cells are formed which surround it or plaster 
 themselves to its surface." 5 With this Delafield and Pruden agree. 6 
 It is quite certain that the osteoclast is not the product of mitosis, 
 as Kolliker thought, without cytoplasmic division as mitotic figures 
 are never seen in it. 
 
 In the sections which formed the basis of this study irregular 
 foveolse are seen in which are groups of cells apparently of leuko- 
 cytic origin, which to all appearances are active in the removal of 
 calcified tissue, while on the surface continuous with that on which 
 they work, are multinucleated cells in great numbers filling smoothly 
 formed spaces. This condition may be explained by the interpreta- 
 tion of Button, Causch and Mallory. The phagocytic endothelial 
 cells introduce the process of tissue removal and later fuse, according 
 to Mallory's hypothesis, continuing the destruction of tissues after 
 their fusion, thus forming the smooth bay-like excavations noted. 
 This also explains the low number of giant cells found in the early 
 
 1 Loc. cit. 
 
 2 Transactions of World's Columbian Dental Congress, p. 114. 
 
 3 Mallory: Principles of Pathology, 1912, p. 52. 
 
 4 Introduction to General Pathology, 1887, p. 124. 
 & Mallory: Principles of Pathology, 1912, p. 52. 
 6 Text-book of Pathology, p. 119.
 
 278 
 
 ABSORPTION OF TEETH 
 
 stages of endochondral bone formation, which point is mentioned 
 by Stohr 1 and emphasized by others. Phagocytic endothelial cells 
 or other means of calcified tissue removal may be employed. To 
 the writer it seems conclusive that Mallory's assumption is correct. 
 The procedure of tissue removal is difficult of explanation. To the 
 present time it has not been determined that osteoclasts or phago- 
 cytic leukocytes produce acid for this purpose. Also the process is 
 more than decalcification. In decalcification we know that certain 
 tissues resist acid for varying periods of time. In the process under 
 discussion we have complete tissue removal, the connective-tissue 
 matrix of the calcified structures and the dense peridental membrane 
 
 _ -' *f^t* s ~/!' : 
 
 **'& "* ^ : ^ v -.\ v r 
 
 ~ '-A-ftJS*- --_A^,_'_V-V->. , - _ - C . 
 
 FIG. 234. Section showing absorption of the tooth of a sheep: a, cementum ; fo, 
 osteoclasts in cementum and den tin; c, osteoclast in the peridental membrane. 
 
 as well (see 234, c). In connection with the endochondral bone 
 formation it has been suggested that reduction of blood supply 
 causes autolysis of cells in the cartilaginous matrix and a consequent 
 dissolution of the calcified cartilage spicules by the enzymes set 
 free. In the light of the foregoing the last hypothesis seems unneces- 
 sary. It is commonly accepted that osteoblasts may become osteo- 
 clasts because it is known that cells long inactive may change their 
 function, or that connective-tissue cells, under changed conditions, 
 may develop specializations, or cells long inactive may resume func- 
 tional activities of a different character from that carried on in 
 
 1 Text-book of Pathology, pp. 68 and 202.
 
 ABSORPTION OF TEETH 279 
 
 their earlier histories. Thus liberated cartilage or bone corpuscles 
 may become cartilage or bone builders. By injecting lamp-black 
 or bacteria into the subarachnoid space Weed 1 found that connective- 
 tissue cells became phagocytic and ameboid. Hassin 2 found that glia 
 cells did similarly, devouring myelin and migrating to the vessels 
 of the area as did Nissl and Altzheimer. 3 Similar phenomena have 
 been observed by various workers on other tissues and organs. 
 
 At birth the jaw contains all the deciduous teeth and likewise the 
 germs of the permanent teeth except the second and third molars. 
 Three to five years are required for the completion of the roots after 
 which they remain complete for a similar length of time. During 
 this period the permanent teeth have been developing in their 
 crypts after which they begin their occlusal movement. The first 
 observation of importance is the appearance of osteoclasts on the 
 roof of the crypt. Penetrating the crypt roof the permanent tooth 
 approaches the lingual surface of the temporary tooth if it is an 
 incisor or cuspid, and immediately between the roots if it be a pos- 
 terior tooth. Incisors of dogs have a tendency to point directly to 
 the apices of their temporary predecessors (see Fig. 235) while 
 those of sheep simulate those in the human mouth, approaching 
 the lingual surface. The difference presents interesting features for 
 our notice. The removal of the tissue in the path of the advancing 
 tooth is more rapid than the advance of that tooth with the result 
 that the way cleared is filled with young fibrous connective tissue 
 rich in budding capillaries (see Fig. 234, d). In the wake of the 
 tooth, bone spicules are developed supportive to the crypt for it will 
 be observed that at first the crypt moves with the structures it 
 contains, thus affording an important mechanical factor in the 
 development of the jaw. 
 
 Coincident with the approach of the permanent tooth germ to the 
 root of the temporary tooth osteoclasts appear on the approached 
 surface of the deciduous root. Also capillary loops develop extending 
 toward them in a manner strikingly similar to that seen when calcifi- 
 cation is in progress for these activities always call for a copious 
 blood supply (see Figs. 235, a, and 237, 6). The work of the osteoclast 
 is never long confined to the area mentioned. The stimulus afforded 
 
 1 The Establishment of the Circulation of the Cerebrospinal Fluid, Anat. Record, 
 x, 256-158. 
 
 2 Histopathological Changes in a Case of Amyotrophic Lateral Sclerosis, Med. 
 Rec., February 10, 1917. 
 
 3 Histologische und Histopathologische, Arbeiten von Nissl und Antzheimer, 
 1912.
 
 280 
 
 ABSORPTION OF TEETH 
 
 to the peridental membrane soon permeates it with the result that 
 osteoclasts appear on any surface, anywhere from the apex to the 
 gingival line (see Fig. 235, 6). Occasionally the spaces excavated 
 are filled with cementum and a new attachment made, but that is 
 far from consistent. Whereas the approaching permanent tooth 
 apparently is the original stimulus to the destructive process the 
 
 ' 
 
 FIG. 235. Section of a dog's tooth, showing internal and external absorption 
 A, capillary loops to absorption areas; B, absorption area at the cervix of the tooth; 
 C, foveolse on pulpal surface of root; D, cellular layer surrounding the pulp. 
 
 ragged edges made by the absorption doubtless afford a secondary 
 stimulus and the area where no new cementum is deposited has the 
 effect of a foreign body, all of which tends to speed the absorption. 
 The beginning of absorption and its continuance are in no way 
 affected seemingly by the fact of pulp extirpation provided that the
 
 ABSORPTION OF TEETH 281 
 
 root is aseptic. Instances are plentiful showing perfect removal of 
 the root leaving a cone of filling material in the tissue. A curious 
 and interesting phenomenon may be observed in the museum of the 
 Northwestern University Dental School. A series of teeth are 
 shown on which is a small tube of dentin around the canal which 
 was preserved around the pulp apparently in resistance to the absorb- 
 ing agents. Normally the process continues until the root is wholly 
 gone and it is often seen that the dentin of the crown has been 
 entirely removed and sometimes the enamel has been reduced to the 
 thinness of tissue paper. 
 
 Apparently no changes appear in the pulp due to the external 
 absorption of the tooth root. The writer has never observed any 
 effects upon the pulp due to changes in progress on the exterior of 
 the tooth root; it shows no reaction until it is invaded. The embry- 
 onic character of the tissue naturally undergoes immediate alteration 
 when its environment is changed. When the absorption has been 
 greatest at the apex and a large area of pulp is uncovered the effect 
 upon the pulp is widespread. Around the periphery new connective- 
 tissue elements appear extending farther and farther occlusally 
 around the pulpal walls until all the odontoblasts are lost and in 
 their places is a dense cellular zone containing a preponderance of 
 undifferentiated connective-tissue cells (see Fig. 235, d). Upon 
 the outer surface of this cellular layer osteoclasts appear and inter- 
 nal absorption accompanies that which progresses on the exterior 
 root surface (see Fig. 235, c). Some fibroblasts are seen and an 
 abundance of capillary loops extends radially from much enlarged 
 central vessels to the absorbing cells (see Fig. 235, a). Hence, the 
 pulp has been metamorphosed into a scrap of typical granulation 
 tissue. Should the opening into the pulp chamber elsewhere be 
 small, similar changes occur in the immediate vicinity of the penetra- 
 tion. The more distant parts of the pulp, be they coronal or apical, 
 remain practically normal until the point of invasion has become 
 large enough to affect the entire'structure or numerous penetrations 
 are made. 
 
 During this process it will be noted that although pressure may 
 be assigned as the stimulus to absorption that stimulus is never 
 retroactive. No osteoclasts ever appear inside the follicle of the 
 erupting tooth which causes the pressure. Also, should acid be 
 produced by the cells for the purpose of decalcification it never 
 affects the permanent tooth. The follicle seems to be a sufficient 
 protection against such emergencies, and it persists until the tooth
 
 282 
 
 ABSORPTION OF TEETH 
 
 reaches the surface of the gum. It may likewise be inferred that no 
 tissue can be referred to. as an absorbent organ, as we have seen that 
 absorption extends over the surfaces of the tooth externally as well 
 as internally. Absorption of the roots of the teeth of different 
 species is observed to follow a routine which is a modification of 
 the one described, the general principle being the same. 
 
 Under the head of physiological absorptions must be considered 
 the removal of implanted teeth. It has been long observed that 
 implanted teeth are of brief service in the mouth and that when they 
 are removed their root surfaces are pitted and rough or entirely 
 absorbed (Figs 236 and 241). Although to my knowledge no 
 sections of implanted teeth have ever been made with the sur- 
 rounding supporting tissue the explanation of both their short period 
 of serviceability and the pitted surfaces seems obvious. The 
 
 FIG. 236. A bicuspid tooth which was implanted and remained in the alveolus 
 about three years. (Fig. 119 in Special Dental Pathology, Black.) 
 
 inserted tooth is placed in an artificially created alveolus which 
 nature attempts to close. To do so agents for the removal of the 
 foreign body attack its surface and bone formation follows in the 
 wake filling the indentations with its extensions. The attack in this 
 case is uniform upon the surface of the root unless there are patho- 
 logical interferences. A great surgeon is accredited with saying 
 "The more perfect the operation of placing the tooth the more rapid 
 is the removal" (Gilmer). It is the projections of bone into the 
 foveolse made by the osteoclasts that give the tooth its firmness. 
 An a:-ray of such a tooth shows no clear periphery as is the case with 
 teeth normally attached but rather a confused picture due to the 
 bridges of bone extending into the tooth root. 
 
 Under pathological absorptions, first come those found in the 
 walls of the pulp chamber. Causch 1 mentions excavations in the 
 
 1 Tr. of World's Columbian Dental Congress, p. 114.
 
 ABSORPTION OF TEETH 
 
 283 
 
 pulpal walls as does Salter and describes the same as filled with 
 bone. 1 It will be remembered that the older histologists and some 
 modern ones call every tooth tissue bone, if it is not definite in 
 structure. It was his findings in these studies that led him to con- 
 sider odontoblasts as contributing to the formation of osteoclasts. 
 Absorptions in the dentin surrounding the pulp chamber and canals 
 
 FIG. 237. Section of a dog's tooth, showing blood supply to enamel forming cells. 
 A, ameloblasts; B, vessels. 
 
 are very common and not infrequently contain filling of calcified 
 material varying in structure from an irregularly arranged dentin 
 to a clear structureless deposit (Figs. 239 and 240). No one has 
 observed osteoclasts in the pulp-chamber of a tooth that has not 
 been invaded. But there is no reason for doubting that they may 
 appear there and other phagocytes as before mentioned may 
 
 Black: Special Dental Pathology, p. 265.
 
 284 
 
 ABSORPTION OF TEETH 
 
 accomplish the results observed. The observations are there and 
 frequently enough penetrate to the outside. Hess in a series of 
 studies on multiple foramina reports that canals are often formed 
 from within out to compensate for canals closed by secondary 
 deposits of cementum. 1 Such fillings of canals are common obser- 
 vations in ground sections (see Fig. 131). 
 
 Absorptions on permanent teeth are very common. They are 
 associated with impactions and are noticed on the apices of roots 
 about which are abscesses as well as around the cervices of teeth. 
 Sometimes the abscess is given as the possible cause of the tissue 
 
 FIG. 238. Photograph of section from which Fig. 237 was made. 
 
 destruction. It does not seem probable, however, that the acid con- 
 tent of pus destroys the tooth root and it is very certain that no cell, 
 osteoblast or osteoclast, ever approaches a root which has been 
 bathed in pus as it does under physiological conditions. Believing 
 that the tissue destruction is accomplished by cells and not by 
 acids, the excavations must be made before the pus reaches the 
 cementum, the cells being stimulated to activity possibly by the 
 
 1 Hess: The Development and Structure of the Tooth Apex and Features Per- 
 taining Thereto, Zahnheilkunde, 1917, xxxvi.
 
 ABSORPTION OF TEETH 
 
 285 
 
 inflammation. Explaining the other absorptions mentioned Inglis 
 suggests that such causes as protruding root canal fillings, broaches, 
 pericemental deposits and salivary calculus may instigate cellular 
 activity. 1 It is true that the absorptions occur most commonly 
 
 FIG. 239. Section of human tooth, showing an internal absorption area which 
 has been almost completely filled with structureless calcified material. A, primary 
 dentin; B, foveolse; C, structureless calcified material; D, root canal. 
 
 on the cervical and apical areas where inflammations are the com- 
 monest (see Fig. 241). 
 
 More interesting than the foregoing is the entire removal of the 
 roots of permanent teeth, sometimes limited to a single tooth, or, as 
 
 1 Burchard and Inglis: Dental Pathology and Therapeutics, 1912, p. 622.
 
 286 
 
 ABSORPTION OF TEETH 
 
 has been reported by Black, 1 of all the teeth, in exactly the same way 
 as deciduous teeth are removed. Where such removals have taken 
 
 FIG. 240. Showing absorption of pulpal walls and newly deposited, structureless, 
 calcined tissue. A, dentin; B, foveolae; C, new calcified tissue; D, canal. 
 
 FIG. 241. Showing absorption of a tooth implanted by Dr. Thomas L Gilmer. 
 When this radiograph was taken the tooth had been in the alveolus nearly three 
 years. (Fig. 118, Special Dental Pathology, Black). 
 
 Special Dental Pathology, p. 33.
 
 ABSORPTION OF TEETH 287 
 
 place the patient has never reported any accompanying symptoms. 
 The process has been painless. No etiology of such conditions is 
 forthcoming. That question being laid aside there is no reason for 
 doubting that the agents employed are the same as for deciduous 
 teeth. Could sections be made of these teeth in situ doubtless we 
 should find upon their surf aces osteoclasts accomplishing the purpose. 
 To summarize it seems strongly evident that osteoclasts or their 
 endothelial predecessors are the active agents of absorption, although 
 the method by which they accomplish it is unknown. Such is their 
 distribution on both the internal and external surfaces of the tooth 
 that neither the pulp nor the peridental membrane can logically be 
 termed an absorbent organ. These cells destroy soft and hard 
 tissues alike, outlines of them being visible in the dense peridental 
 membrane surrounding the wasting tooth (see Fig. 234, c). Especial 
 emphasis is laid upon the connective-tissue changes that take place, 
 changes in both the hard and soft tissues as well as changes in the 
 blood supply. What seems so evident in the study of the removal 
 of the temporary teeth and in bone seems a well justified explanation 
 of the removal of the structures mentioned where exact data is 
 so difficult to acquire.
 
 CHAPTER XXIII. 
 THE MOUTH CAVITY. 
 
 Mucous Membrane. The mucous membrane lining the mouth 
 cavity is composed of a layer of stratified squamous epithelium 
 supported upon a tunica propria, which is usually described as 
 composed of two parts the papillary layer and the reticular layer. 
 The epithelium and the tunica propria make up the mucous mem- 
 brane proper, which is supported upon a submucous layer com- 
 posed of a coarse network of white and elastic fibers, containing 
 the larger bloodvessels. 
 
 The Epithelium. The stratified squamous epithelium is provided 
 with a horny or corneous layer only in the portions covering the 
 alveolar process and the hard palate, or, in other words, where the 
 submucosa is firmly attached to the periosteum (Fig. 242). In 
 these positions the horny layer consists of dead cells which have 
 lost their nuclei and whose cytoplasm has been converted into 
 keratin or horny material. 
 
 These scale-like cell remains are closely packed into a protec- 
 tive layer. There is no distinct stratum lucidum separating the 
 dead from the living cells, as there is in the skin. In the deeper 
 portions the cells possess oval or rounded nuclei and become larger 
 and more polyhedral as the basemerft membrane is approached. 
 The cells of the deepest layer next to the basement membrane are 
 tall and approach the columnar form, but are never much greater 
 in height than width. The deep layer is often called stratum Mal- 
 pighii. The epithelium lining the gingival space and that covering 
 unattached portions is without the horny layer, and the cells are 
 larger and more loosely placed. The polyhedral cells in the middle 
 portion of the layer show distinct intercellular spaces across which 
 the cytoplasm extends in intercellular bridges. 
 
 Isolated cells from this region show the broken bridges project- 
 in'g from their surface, and for this reason have been called " pickle 
 or prickle cells." In these positions the thickness of the epithelial 
 layer is usually greater than in the attached portions of the mem- 
 brane (Fig. 243). 
 (288)
 
 SUBMUCOSA 
 
 289 
 
 Tunica Propria. The connective-tissue layer of the mucous 
 membrane interlocks with the epithelial layer by means of the 
 tunica papillaris, which is composed of very delicate white and 
 elastic connective-tissue fibers. They are usually about half as 
 tall as the thickness of the epithelium, and about one-third as wide 
 as they are tall. The height and character of the papillae varies 
 greatly, however, in different position. In the red border of the 
 lip and in the epithelium lining the gingival space they are very 
 tall and narrow, and approach very close to the surface of the 
 epithelium. Over the gums and the palate they are much shorter 
 
 FIG. 242. Stratified squamous epithelium covering the alveolar process: C, cor- 
 neous layer; P, papilla of connective tissue. (About 400 X) 
 
 and wider and do not extend more than half-way through the 
 epithelium. These papillae contain loops of capillary bloodvessels 
 and in some special nerve endings are found. 
 
 Reticular Layer. The reticular layer joins the papillary layer 
 without any line of demarcation, and is composed of the same kind 
 of tissue, the fibers being arranged in a delicate network. Every- 
 where in the tunica propria are found ducts from mucous gland 
 which lie in the deeper layers. 
 
 Submucosa. The submucosa is composed of firm connective 
 tissue in which the white fibers are in large, strong bundles, and 
 19
 
 290 
 
 THE MOUTH CAVITY 
 
 elastic fibers are scarce. It contains two plexuses of bloodvessels, 
 both more or less parallel with the surface. The outer is composed 
 of small vessels forming a small-meshed network, the deeper of 
 large vessels more widely separated. Lymphatic vessels everywhere 
 follow the course of the bloodvessels. 
 
 Glands of the Submucosa. The submucosa contains a great many 
 small tubular glands. These are distributed widely over the tongue 
 and membrane of the cheek and lip (Fig. 244). They are branched 
 tubular glands, sometimes simple and sometimes compound. 
 The body of the gland is always in the submucosa, though it may 
 extend into the underlying muscle. Some are serous and others 
 
 FIG. 243. Stratified squamous epithelium from unattached mucous membrane of 
 the mouth. The corneous layer is absent. (About 200 X) 
 
 mucous, while many of the larger ones contain cells of both types. 
 The secretion of these glands is probably much more important 
 than has been supposed. 
 
 Nerve Endings in the Mucosa. Sensory nerve endings of two 
 kinds are found in the mucous membrane. Krause's end bulbs 
 are found in many of the papillae, and other nerves terminate in 
 free endings lying between the epithelial cells. 
 
 The Tongue. The tongue is composed of a mass of voluntary 
 muscle fibers arranged in complicated interlacing bundles, covered 
 by the mucous membrane. The most striking characteristics of 
 the mucous membrane of the tongue (Fig. 245) are: (1) The 
 thinness of the submucosa, which holds it closelv to the mass of
 
 THE MUSCLES 
 
 291 
 
 muscle and allows very little movement of it; (2) the submucosa 
 in the dorsal surface contains no glands, though there are glands 
 among the muscle fibers whose ducts pass through the submucosa; 
 (3) the presence of the epithelial papilte upon its dorsal surface. 
 
 Mncous 
 gland' 
 
 Epithelium 
 
 of mucous 
 membrane 
 
 \Hair 
 follicles 
 
 Epidermis 
 
 Epithelium 
 
 of 
 membrane 
 
 'ross sections 
 
 Longitudinal 
 
 sections 
 of muscle 
 fibres 
 
 MUCOUS 
 
 membrane 
 
 with hii/h 
 
 papillie. 
 
 FIG. 244. SectioD through the upper lip of a two-and-a-half-y ear-old child. 
 (14 X) (Szymonowicz.) 
 
 The tongue is imperfectly divided vertically on the median line 
 by a band of connective tissue forming the median raphe or septum, 
 which causes the depression at the central line of the dorsal surface. 
 The Muscles. The muscles of the tongue include two groups 
 the extrinsic and the intrinsic. The extrinsic muscles comprise
 
 292 
 
 THE MOUTH CAVITY 
 
 the genioglossus, the hyoglossus, the styloglossus, and the palato- 
 glossus. These are all paired and extend from the skull or the hyoid 
 bone into the tongue. The intrinsic muscles comprise the principal 
 muscles of the tongue, the lingualis. A transverse section through 
 the body of the tongue in the central portion shows a complicated 
 network of muscle fibers running in three directions longitudinally, 
 transversely, and vertically. The longitudinal fibers are arranged 
 around the outer portion, forming a cortical layer about 5 mm. 
 thick. These constitute the chief bulk of the lingualis, supple- 
 
 FIG. 245. A section from the side of the tongue: E, epithelium; Sm, submucosa; 
 Bv, bloodvessels; M, muscle fibers; G, mucous glands. 
 
 mented by fibers from the styloglossus. The vertical fibers are 
 mostly deeply placed in the central portion on either side of the 
 raphe. They are chiefly derived from the genioglossus and radiate 
 toward the dorsal surface. The transverse fibers are entirely from 
 the lingualis except for a few from the palatoglossus. They arise 
 from the septum and interlace with the longitudinal and vertical 
 fibers. They break up into strands running between the longitu- 
 dinal fibers of the cortical portion, and spread out to a submucous 
 insertion.
 
 THE PAPILLA 
 
 293 
 
 The complicated movements of the tongue are accomplished 
 by the contractions of these sets of muscles. When the longitudinal 
 fibers are relaxed and the transverse fibers contracted the tongue 
 is rolled and extended. When the transverse fibers are relaxed 
 and the vertical fibers contracted the tongue is flattened. The 
 division of the tongue on the median line by the septum allows 
 each half to work independently, so that when the longitudinal 
 fibers are contracted on one side and relaxed on the other the tip 
 of the tongue is moved sidewise. 
 
 Fio. 246. Mucous membrane from the dorsal surface of the tongue of a kitten, 
 showing filiform and f ungiform papillae. 
 
 The Papillae. The roughness of the dorsal surface of the tongue 
 is caused by projections of the epithelium resting upon the tunica 
 propria, forming the papillae of the tongue. These projections are 
 not to be confused with the connective-tissue papillae in the tunica 
 propria of the mucous membrane. They are of three kinds the 
 filiform and fungiform papillae, which are found over the entire 
 dorsal surface, and the circumvallate papillae, which are limited 
 in number and confined to the posterior portion. The filiform are 
 much the more numerous, especially near the tip of the tongue. 
 They are from 0.5 to 2.5 mm. in height, and often end in brush-like 
 strands of epithelial cells.
 
 294 
 
 THE MOUTH CAVITY 
 
 The fungiform papillae form the red points on the surface of the 
 tongue, especially near the edges, because of the thinness of their 
 
 FIG. 247. Mucous membrane from the tongue of a rabbit, showing circumvallate 
 papillae, with taste-buds on their sides. 
 
 epithelium. They are low and rounded in form, from 0.5 to 1.5 mm. 
 in height, and are named from their mushroom-like appearance. 
 Fig. 246, a section from the tongue of a kitten, shows the form of 
 both of these papilhe. The circumvallate papillae usually number 
 
 FIG. 248. A section of a taste-bud: p, pore; g, gustatory cells; ep, epithelial 
 cells; s, sustentacular cells; h, bristles of the gustatory cells. (Schaefer.) 
 
 nine or ten, and are arranged in a V-shaped form near the base of 
 the tongue, with the apex extending backward. They are from 1
 
 THE TASTE-BUDS 
 
 295 
 
 to 1.5 mm. in height and from 2 to 3.5 mm. in width. They are 
 surrounded by a depression, so that the upper surface of the papillae 
 is not much above the general level of the membrane. 
 
 The Taste-buds. These are found chiefly on the sides of the 
 circumvallate papillae (Fig. 247), though they are occasionally 
 found in the epithelium of the fungiform papilla? and the soft palate, 
 and on the posterior surface of the epiglottis. They are always 
 entirely embedded in the epithelium and extend through its entire 
 thickness. The structures are ovoid in form, with the rounded 
 end toward the connective tissue and the pointed end at the sur- 
 
 Epithe- 
 lium 
 
 Tunica 
 propria 
 Lymph 
 nodule- 
 
 Oblique 
 
 sfdion 
 
 of duct 
 
 of mucous 
 
 qland 
 
 Muscle 
 
 fibres 
 cut 
 
 trans- 
 versely 
 
 FIG. 249. Section through a lingual follicle in man: x, crypt. (50 X) 
 (Szymonowicz.) 
 
 face, where a small opening, the taste-pore, communicates with 
 the mouth cavity (Fig. 248). Most of the cells are elongated and 
 spindle-shaped, and arranged like the leaves of an onion. Four 
 varieties may be recognized. The outer sustentacular cells form 
 the outer layer and are in contact with the epithelial cells. They 
 are elongated, with an oval nucleus near the center. The inner 
 sustentacular are rod-shaped cells, more slender in form, with a 
 nucleus at the base. The neuro-epithelial cells are elongated, 
 spindle-shaped cells at the center of the taste-bud. The nucleus 
 is at the base of the cell, and from the opposite end a stiff bristle- 
 like process extends through the taste-pore.
 
 296 
 
 THE MOUTH CAVITY 
 
 The basal cells are irregular in form with large oval nuclei; they 
 communicate with each other and the sustentacular cells by cyto- 
 plasmic bridges. They form the base of the taste-bud. The func- 
 
 Epithelium 
 of pharynx~~-gf 
 
 Mucous glands* 1 " 
 
 Blood vessel 
 
 Connective-tissue 
 capsule 
 
 FIG. 250. Section through a dog's tonsil. At x, x there are seen leukocytes 
 which have wandered out from the follicles. (15 X) (Szymonowicz.) 
 
 tion of the taste-buds is probably related to the function of degluti- 
 tion rather than the sensation of taste. 
 
 The Tonsil.- In the posterior part of the tongue and the wall 
 of the pharynx is found adenoid tissue in the form of solitary follicles
 
 THE TONSIL 297 
 
 lying in the tunica propria and invading the epithelium. This 
 adenoid tissue forms an organ which Waldeyer has called the lym- 
 phatic pharyngeal ring. This tissue is divided into three main 
 masses that lying in the base of the tongue forming the lingual 
 tonsil, that associated with the palate and lying between the pillars 
 of the pharynx and forming the palatine tonsil, and that situated 
 in the pharynx or pharyngeal tonsil. 
 
 The Lingual Tonsils These are situated in the base of the 
 tongue between the circumvallate papillae and the epiglottis. They 
 are rounded masses of adenoid tissue composed of solitary follicles 
 lying mostly in the tunica propria, and causing projections of the 
 surface that are easily seen. In the center of each mass is a deep 
 depression forming a blind pouch, known as the crypt (Fig. 249). 
 This is lined with stratified squamous epithelium like that of the 
 adjoining mucous membrane except that at various places the 
 lymphocytes have pushed their way through the epithelial cells, 
 and escape on the surface. 
 
 The Palatine Tonsils. These lie at the base of the tongue between 
 the anterior and the posterior pillars of the pharynx. They are 
 much larger than the lingual tonsils and are composed of from ten 
 to twenty follicles and a number of crypts. The epithelium cover- 
 ing them is pierced in many places by encroachments of the adenoid 
 tissue. The crypts always contain many lymphocytes (Fig. 250). 
 These are what are ordinarily called the tonsils, the infection of 
 which produces tonsillitis. 
 
 The Pharyngeal Tonsils. These lie on the posterior wall of the 
 nasal pharynx above the level of the palate. Their structure is 
 similar to that of the palatine tonsil. The crypts are five to six 
 in number and are often clothed with ciliated epithelium. Into 
 them open the ducts of mixed glands which form a distinct layer 
 under the follicle. Here also there is a migration of lymphocytes 
 through the epithelium. It is the hypertrophy of these which form 
 the adenoids so often found in children.
 
 CHAPTER XXIV. 
 
 BIOLOGICAL CONSIDERATIONS FUNDAMENTAL TO 
 EMBRYOLOGY. 
 
 History. Before beginning the study of embryology some topics 
 in general histology must be reviewed, and some general biologic 
 ideas considered. No real conception of the complicated process 
 of individual development can be obtained without laying a founda- 
 tion in the study of the cell as the units of life and the mechanism 
 through which the phenomena of life are manifested. 
 
 In embryology it is found that the individual in his physical 
 development passes through stages which correspond to the develop- 
 ment of the race or species to which he belongs, and a like compari- 
 son might be drawn in mental development and the acquirement 
 of knowledge. This is specially true of the subject of embryology. 
 
 Apparently the first ideas to occupy the speculative thought 
 of man when he became conscious of himself as an independent 
 being were the questions of his origin and the relation to his environ- 
 ment and destiny. These have become the basis for the development 
 of all religious thought. 
 
 Up to the beginning of the nineteenth century all considerations 
 of these subjects were purely speculative. The old question of 
 "What is life?" received endless discussion. In the nineteenth 
 century this question has been dropped into the background, and 
 the question, "What is the mechanism of life?" has been substituted 
 for it. The consideration of the latter question has resulted not 
 only in the marvellous advancement of medical knowledge and 
 surgical skill, but in the great development of deeper fundamental 
 thoughts. It must not be forgotten, however, that the develop- 
 ment of knowledge resulting from the consideration of the latter 
 question has not and does not promise to answer the old question, 
 "What is life?" any more than the laws of electricity and their 
 application to its use answer the question, "What is electricity?" 
 
 The discovery of the cell hypothesis and the propounding of 
 the theory of organic evolution have been the greatest factors 
 in the unification of knowledge and the stimulation of thought in 
 these fields. It is interesting to notice that these two theories, 
 closely related as they have become, had entirely independent 
 (298)
 
 RELATION OF NUCLEUS TO PROTOPLASM 299 
 
 origins and were long followed out without any immediate con- 
 nection. The theory of evolution was based upon consideration 
 of the forms of living things, their distribution and adaptation to 
 environment. 
 
 The Cell Theory. The cell theory had its origin in the study of 
 minute forms. Its beginnings were made possible by the develop- 
 ment of the compound microscope, which revealed their structure 
 and showed them to be small bodies made up of apparently a struct- 
 ureless, granular material which was called protoplasm, or the 
 ultimate substance of life. This material, as its name indicates 
 was originally supposed to be simple in structure and composition 
 and to be the life substance. Huxley's characterization of it as 
 the "physical basis of life" was the beginning of the study which 
 has revealed it to be very far from a simple substance, but rather 
 extremely complex both in structural arrangement and chemical 
 composition. In more recent biology, therefore, the word proto- 
 plasm is being dropped and the word cytoplasm or cell substance 
 substituted for it- 
 
 The early history of the cell theory was obstructed in its develop- 
 ment by the remains of the old Greek idea that living things could 
 originate from non-living matter, that the swamp breeds disease, 
 and the decomposing body of an animal, maggots. It required 
 fifty years of work on the cell theory for Virchow, in 1850, to pro- 
 pound his thesis that all living cells are derived from a preexisting 
 cell, and so establish the continuity of life, which has flowed on 
 from the beginning in an uninterrupted stream, each individual 
 being only a period. 
 
 When Schwann and Schleiden showed that the bodies of both 
 plants and animals, instead of being made up of homogeneous tissue, 
 were composed of millions of structural elements which they called 
 cells, the consideration of both plants and animals were for the 
 first time put upon a common basis. Naturally enough the first 
 thing to attract attention was the study of the form and arrange- 
 mentof these structural elements in the tissuesof animals andplants. 
 
 In following out this study it became more and more evident 
 that, while infinitely varied in the detail of their form and structure, 
 all cells had a common plan of organization and possessed struct- 
 ural characteristics common to all, at least in some stages of their 
 history. 
 
 Relation of the Nucleus to the Protoplasm. The first point to be 
 discovered in the internal organization of the cell was the nucleus,
 
 300 BIOLOGICAL CONSIDERATIONS OF EMBRYOLOGY 
 
 the meaning of which and its relation to the cytoplasm at once 
 attracted attention. As the result of a vast amount of work, it 
 was gradually established that the nucleus "exerts a controlling 
 and directing influence over the activity of the cytoplasm;" that 
 a cell deprived of its nucleus would continue to live for a longer or 
 shorter time, but that it would not grow and would not reproduce 
 another cell; that the phenomena of life manifested by destructive 
 metabolism would continue until the identity of the cytoplasm 
 was destroyed, but there would be no constructive metabolism. 
 The work of the cytoplasm is therefore dependent upon the 
 character of the nuclear material. 
 
 Cell Division. As first observed, cell division was supposed to 
 be an irregular cutting of the cytoplasm and the nucleus in two, 
 forming two individual cells. The cytoplasm by its constructive 
 changes does not continue to increase indefinitely, but as soon as 
 a certain size is reached it divides, a portion of the nucleus going 
 to each of the parts, which immediately begin to increase in size. 
 It was soon found that cell division was not always so simple, and 
 that in some cases changes in the nucleus preceded the division of 
 the cytoplasm. Two forms of cell division are therefore described, 
 the simple or direct, and indirect or karyokinetic cell division. 
 The simple is now known to be comparatively rare. 
 
 Indirect Cell Division. Indirect cell division must be considered 
 as a means by which the chromatic material of the nucleus is equally 
 and systematically distributed to the resulting cells. The nucleus, 
 in cell division, contains a beautiful structural mechanism, by which 
 the material which is to control the development of the resulting 
 cells and their activity is definitely distributed to them. In this 
 process there is no irregularity in the kind or amount of material 
 given to the two cells. 
 
 In this process the chromatin of the original nucleus is divided 
 into a definite number of pieces which are split in two, and half 
 of each sent to each new nucleus, where they form its chromatin 
 network. 
 
 The Vehicle of Transmission. It was discovered that the number 
 of chromosomes was constant in every cell division for all the cells 
 of all the tissues of the given species, and was, therefore, a charac- 
 teristic of the species; and that in all the cells of the body it was 
 always an even number, and that in the germ cells of the species 
 the number of chromosomes was exactly half that in the cells of 
 the body. This led to the immediate recognition of the chromatic
 
 CHEMICAL IDEAS 301 
 
 material as the vehicle of transmission. When in the study of fer- 
 tilization it was found that fertilization consists in the union of 
 two cells, each contributing both cytoplasm and nucleus, and that 
 the amount of chromatic material was equal from each, and exactly 
 half that found in the cells of the parent body, the equality of the 
 sexes in transmission was firmly established upon a cytologic basis. 
 It is interesting to note that this equality had previously been 
 claimed by the disciples of the evolutionary theory, and it was in 
 this field that the evolutionary theory and the cell theory first 
 met on common grounds (about 1875). 
 
 All the advancement in modern thought concerning heredity 
 and transmission has resulted from these discoveries. The practical 
 results are perhaps still more important in the artificial breeding 
 of plant's and animals, adapting them to their environment. The 
 work of such men as Burbank may be said to be the application 
 of the knowledge of the mechanism of cell division and inheritance 
 to horticulture and agriculture. 
 
 Chemical Ideas. At the present time the structural mechanism 
 of life, while inviting many fields for research, may be said to have 
 nearly reached the limit of possibilities of observation, and at the 
 present time the chemical phase is attracting the greatest attention. 
 Such questions as, "How does the nucleus influence the activity 
 of the cytoplasm?" are being eagerly investigated. Cytoplasm 
 while enormously complex in chemical composition, must, never- 
 theless, always be thought of as performing its vital functions by 
 chemical activity. It is constantly building simpler molecules into 
 its own, and so increasing in amount. For this its surface must be 
 bathed in materials with which it can react. It is evident that if 
 the mass increased indefinitely the volume would increase much 
 more rapidly than the surface, and this puts a limit upon the 
 growth. 
 
 The constructive metabolism of the cytoplasm is dependent 
 upon the presence of the chromatin in the nucleus. In the process 
 of metabolism, therefore, there must be interaction between the 
 chemical substances of the chromatin, cytoplasm, and food material. 
 The development of physiologic chemistry is rapidly affecting the 
 ideas of the cause and treatment of disease, and especially the 
 production of immunity and susceptibility. 
 
 If the dental profession is to keep pace with the development 
 in these fields and apply the results of investigation to the treat- 
 ment of diseases of the mouth, the study of the fundamental sciences 
 must be more thorough.
 
 CHAPTER XXV. 
 EARLY STAGES OF EMBRYOLOGY. 
 
 SINCE fertilization consists essentially in the union of the chroma- 
 tin from two cells, and as the result of the union restores the normal 
 amount of chromatin for the cells of that species, it is evident that 
 
 FIG. 251. Diagram illustrating the reduction of the chromosomes during the 
 maturation of the ovum: o, ovum; oc 1 , odcyte of the first generation; oc 2 , oocyte of 
 the second generation; p, p, polar bodies. (McMurrich.) 
 
 in some way the germ cells must be prepared for fertilization by the 
 loss of half their chromatin. This process was first observed in 
 the case of the ovum. 
 (302)
 
 MATURATION 
 
 303 
 
 Maturation. In observing fertilization of eggs of the starfish 
 and various threadworms, it was noticed that before fertilization 
 occurred the nucleus of the ovum divided with karyokinetic figures, 
 forming three small bodies known as polar bodies. This process 
 is diagrammed in Fig. 251. In reality, the ovum first divides, 
 forming one polar body; the polar body and the ovum both then 
 divide again, so that the result of the two series of division is the 
 
 FIG. 252. Diagram illustrating the reduction of the chromosomes during sperma- 
 togenesis: sc 1 , spermatocyte of the first order; sc 2 , spermatocyte of the second order; 
 sp, spermatid. (McMurrich.) 
 
 formation of four cells, one of which is functional, three disappear- 
 ing. This process is practically universal in the formation of ova 
 of both plants and animals. The cells in the ovary which form the 
 ova are called oogonia. The cells formed from these are the primary 
 oocyte. The division of this cell produces two secondary oocytes, 
 of which one disappears later. The division of the secondary oocyte 
 results in the ovum and three polar bodies. The number of chromo- 
 somes in the primary oocyte is half the number characteristic of
 
 304 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 the somatic cells, but they are made up of four pieces. In the 
 secondary oocytes they are the same number but double. In the 
 ovum and polar bodies they are the same in number and single. 
 
 Spermatogenesis.- Exactly the same series of changes occur 
 in the formation of the spermatozoa. They are illustrated in Fig. 
 252. On the outer wall of the seminiferous tubules are two forms 
 of cells, the spermatogonia and the cells of Sertoli (Fig. 253). The 
 cell of Sertoli increases in size and spreads out against the basement 
 membrane, pushing the spermatogonia away from it. They now 
 divide, forming two cells, one of which returns to the basement 
 membrane and remains as the spermatogonia, the other becomes a 
 primary spermatocyte. The primary spermatocytes divide, form- 
 ing a secondary; the secondary divide, forming spermatids, which 
 
 FIG. 253. Diagram showing stages of spermatogenesis as seen in different sections 
 of a seminiferous tubule of a rat: s, sertoli cell; sc 1 , spermatocyte of the first order; 
 sc 2 , spermatocyte of the second order; sg, spermatogone ; sp, spermatid; sz, sperma- 
 tozoon. (Von Lenhossek's diagram from McMurrich.) 
 
 develop directly into spermatozoa. By comparing the diagrams 
 they will be seen to correspond exactly with the formation of the 
 ova, except that all of the cells are small and motile. The nuclear 
 changes also correspond to those of the ova, the primary sperma- 
 tocyte having half the number of tetrad chromosomes, the second- 
 ary half the number of diad, and the spermatids half the number 
 of monad chromosomes. 
 
 Fertilization. Fertilization is essentially the same in the sexual 
 reproduction of all plants and animals. It may be easily observed 
 in the transparent cells of such animals as the starfish and the 
 threadworm, The spermatozoon enters the cytoplasm of the ova,
 
 FERTILIZATION 
 
 305 
 
 -pb 
 
 FIG. 254. Fertilization of the egg of Ascaris megalocephala, var. bivalens. (Boveri.) 
 A, the spermatozoon has entered the egg; its nucleus is shown at J ; beside it lies the 
 granular mass of "archoplasm" (attraction sphere); above are the closing phases in 
 the formation of the second polar body (two chromosomes in each nucleus). B, germ 
 nuclei (S, tf) in the reticular stage; the attraction sphere (a) contains the dividing 
 centrosome. C, chromosomes forming in the germ nuclei; the centrosome divided. 
 
 D, each germ nucleus resolved into chromosomes; attraction sphere (a) double. 
 
 E, mitotic figure forming for the first cleavage; the chromosomes (c) already split. 
 
 F, first cleavage in progress, showing divergence of the daughter chromosomes 
 toward the spindle poles (only three chromosomes shown). (Wilson.) 
 
 20
 
 306 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 where it immediately loses its characteristic form and develops 
 into a typical nucleus (Fig. 254). The ovum now has two nuclei, 
 one of which is called the male pronucleus, the other the female 
 pronucleus. These both form chromosomes, the number from each 
 being half the number typical of the species. These are arranged 
 as usual between the centrosomes. They divide longitudinally, 
 each forming two, one of which passes to either centrosome, where 
 a new nucleus is formed, and in the meantime the cytoplasm has 
 divided so that two cells are formed. The nuclear material of these 
 two cells has therefore been equally derived from the two parents, 
 and it is to control all of the future development of the individual. 
 
 FIG. 255. Holoblastic segmentation. Segmentation of frog diagrammatically 
 
 represented. 
 
 SEGMENTATION. 
 
 Holoblastic Segmentation. An idea of the development of the 
 embryo can perhaps best be obtained by following the development 
 of the frog. The frog's eggs are large and easily observed, and they 
 contain only a small amount of yolk or food material, which does 
 not obstruct the observation. The spherical ovum first divides 
 into hemispheres; these two cells are divided into four in a plane 
 at right angles, and the four are divided into eight by a plane at 
 right angles to the previous plane. This is best understood by 
 examining the illustration (Fig. 255). 
 
 The lines of cell division proceed in a regular way, the planes 
 passing in such direction as to multiply the number of cells by two 
 in each set of divisions. Very soon the cells around the black pole 
 show a tendency to divide more rapidly than those at the white 
 pole. At this stage the individual is made up of a hollow sphere 
 of cells with a space at the center, the cells at the upper surface
 
 HOLOBLASTIC SEGMENTATION 
 
 307 
 
 being small and rapidly dividing, those at the lower surface large 
 and slowly dividing (Fig. 256) . As this continues the sphere becomes 
 flattened on the bottom, and finally the lower surface is turned 
 inward until the sphere is converted into a hollow bag or sac made 
 up of two layers of cells, the outer of which are small, the inner 
 
 FIG. 256. Four stages in the development of amphioxus, illustrating the forma- 
 tion of the gastrula. I, the blastula, a hollow sphere of cells; those at the lower pole 
 larger than those at the upper and filled with yolk granules. II, invagination of the 
 lower pole, because of more rapid growth of cells at the upper pole. Ill, the gas- 
 trula, complete invagination; the creature is now a two-layered bag. A space 
 should be shown between the layers: bl, the mouth of the bag, or blastopore; hy, 
 inner layer of cells hypoblast; ep, outer layer of cells epiblast. IV, the gastrula 
 will now elongate; the cavity becomes the alimentary canal; the blastopore the 
 orifice at one end. 
 
 large, the two joining around the mouth of the sac. This hollow 
 bag stage is known as the gastrula. The cavity of the sac is really 
 a part of the outside world around which the cells have grown, and 
 will form the cavity of the alimentary canal. The opening of the 
 sac is known as the blastopore, and will form the anterior opening
 
 308 EARLY STAGES OF EMBRYOLOGY 
 
 into the alimentary tract from the mouth cavity. At this stage 
 the individual is made up of two kinds of cells, and is to be compared 
 in structure with the celenterates or such animals as the fresh- 
 water hydra and the coral polyp. 
 
 Formation of the Germ Layers. The cells which form the outer 
 layer of the gastrula are called the epiblast, the cells which line it 
 the hypoblast or entoblast. Where these two layers join around 
 the opening of the blastopore a ring of cells is formed which differs 
 from both in form and arrangement, and will form the mesoblast. 
 In the process of cell division from the ovum, therefore, three 
 kinds of cells have resulted which represent the first stage of 
 specialization. 
 
 Epiblast. From the cells of the epiblast will be formed: (1) 
 The epithelium of the surface of the body and all glands that con- 
 nect with it, the hair, the nails, and the enamel of the teeth; (2) 
 the epithelium lining the mouth and the nose cavities and the 
 lower part of the rectum; (3) the nervous system and all of the 
 organs of special sense. 
 
 Hypoblast. From the hypoblast will be formed: (1) The 
 epithelium lining the alimentary canal and the glands that open 
 from it; (2) the epithelium lining the larynx, trachea, and the 
 lungs; (3) the epithelium of the bladder and ureter. 
 
 Mesoblast. From the mesoblast will be formed : (1) The various 
 connective tissues, including bone, dentin, and cementum; (2) 
 the muscles, both striated and unstriated; (3) the circulatory 
 system, including the blood itself and the lymphatics; (4) the 
 lining membrane of the serous cavities of the body; (5) the kidney; 
 (6) the internal organs of reproduction. 
 
 Looking at these germ layers in another way, it may be said 
 that through the mechanism of cell division all of the chromatin 
 which is to control nerve cytoplasm has been distributed to the 
 epiblast; all that which is to contribute the muscular activity to 
 the mesoblast, and so on. 
 
 Meroblastic Segmentation. If the development of the chick is 
 compared to that of the frog they at first seem to be very different. 
 The ova of birds and reptiles are provided with a vast amount of 
 food material or yolk, which is provided by the parent for the 
 nourishment of the embryo. It has been seen that the frog's egg 
 contains a certain amount of yolk, and that the presence of yolk 
 granules retarded the cell division. In the case of the birds and 
 reptiles the yolk granules have increased until the active cytoplasm
 
 ME ROB LAST 1C SEGMENTATION 
 
 309 
 
 is left as a small disk floating on top of a sphere of yolk enclosed 
 in the yolk membrane. The white spot seen floating on the top 
 of the yolk of a hen's egg is called the germinal spot. Before fertili- 
 
 FIG. 257. Meroblastic segmentation. 
 
 zation this is a mass of protoplasm with a nucleus in the center 
 When segmentation begins it divides first into right and left halves 
 
 FIG. 258. First five stages of segmentation (rabbit's ovum), a, b, c, d, and e. 
 In a, b, and c the epiblast cells are larger than the hypoblastic ones. In e the 
 epiblast cells have become smaller and more numerous than the hypoblasts and 
 the epiblastic spheres are beginning to surround and close in the hypoblast cells: 
 z.p., zona pellucida; p.gl, polar globules; u, first epiblast cell; I, first hypoblast cell. 
 
 then divides again by a line at right angles to the first one, then 
 the four cells are converted into eight cells, as if by a circle, and 
 the process continues in this way (Fig. 257). It is best understood
 
 310 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 from the diagram. This type of segmentation is known as mero- 
 blastic, while that of the frog is holoblastic. 
 
 Mammalian Segmentation. The mammalian ova contain very 
 little yolk, as the nourishment of the embryo is provided for in 
 an entirely different way. The segmentation is holoblastic (Fig. 
 
 FIG. 259. Sections of the ovum of a rabbit during the later stages of segmentation, 
 showing the formation of the blastodermic vesicle: a, gastrula stages; ent, hypo- 
 blast enclosed by ep, epiblast; b, fluid is beginning to collect and separate the epi- 
 blast and hypoblast; c, the fluid has greatly increased in amount, the hypoblastic 
 cells adhering to the upper surface; d, the blastodermic vesicle; ect, the outer layer, 
 epiblast; ent, hypoblast, the inner layer adhering to the inner surface of the epiblast 
 at the upper surface, forming the opaque area. 
 
 258), but shows marked differences from that of the frog, and 
 characteristics similar to those of the birds and reptiles, and this 
 has been an added link to the evidence of the evolutionists, that 
 the mammalia have been derived in evolution from the reptiles. 
 
 After the first few divisions the cells of the upper pole divide 
 much more rapidly than those of the lower, and grow down over
 
 MAMMALIAN SEGMENTATION 
 
 311 
 
 FIG. 260. A series of sections through the neurenteric and notochordal canal of a 
 mole embryo: p.gr., the primitive groove; ep., epiblast; me., mesoblast; hy., hypoblast: 
 m.gr., medullary groove. (Heap.)
 
 312 EARLY STAGES OF EMBRYOLOGY 
 
 the others, enclosing them. When the large cells have been entirely 
 covered in by the small ones, the small ones continue to multiply 
 more rapidly and fluid collects inside the sphere, leaving the large 
 cells adhering to the inner surface of the small cell layer at one pole 
 of the sphere (Fig. 259). At the upper pole where the sphere is 
 made up of two layers of cells there is an opaque spot, or the "area 
 pellucida," from only part of which the embryo is developed, the 
 rest forming organs to provide it with nourishment during the 
 embryonal condition. 
 
 Starting from the center of the opaque area on the upper surface 
 of the sphere or blastula, there appears a streak known as the 
 primitive streak, caused by the appearance of a rod of cells lying 
 between the two layers, and from the side of this rod or notochord 
 a third kind of cell, different from either the large or small cell 
 layer, is formed. These three kinds of cells make up the three 
 layers of the blastoderm and represent the first step in differentia- 
 tion; or, to state it in a different way, all of the chromatin which 
 (Fig. 260) directs nerve cell activity has been sent to the outer 
 small cell layer, or epiblast, all of the chromatin which directs 
 
 LEGEND FOR PLATE XX 
 
 FIGS. 1 to 5. Diagrammatic representations of longitudinal and cross-sections of 
 hen's egg in various stages of incubation. They illustrate how the embryo is devel- 
 oped out of the area pellucida, and the yolk sac, the serosa, and the allantois out 
 of the extra-embryonal area of the germ layers. The embryo is represented much 
 too large in relation to the yolk sac. The yolk is represented in yellow and the ento- 
 derm in green, ectoderm in blue, mesoderm in red, and the black dotted lines indi- 
 cate the limit to which the inner and outer germ layers have extended over the yolk. 
 The red dots mark the limit of the mesoderm: ak, outer germ layer (blue); mw, 
 medullary ridges or folds; A 7 ", neural tube; am, amniotic fold; vo f , hof, saf, anterior, 
 posterior, and lateral amniotic folds; A, amnion, ah, amniotic cavity; S, serous 
 membrane; hu, dermal umbilicus; sf, lateral folds; kf 1, kf 2, head fold; afb, ifb, 
 outer and inner limb fold; ik, inner germ layer (green) ; ir, its margin of overgrowth; 
 dr, intestinal groove; dg, vitelline duct; al, allantois; ds, interstitial sac; du, intes- 
 tinal umbilicus; mk, middle germ layer (red); mk, parietal layer of mesoderm; mk, 
 visceral layer of mesoderm; si, lateral limits of the same; dm, vm, dorsal and ventral 
 mesenteries; th', body cavity; th 1 , </i 2 , embryonic extra-embryonic parts of the same. 
 
 FIG. 1. Cross-section through hen's egg on second day of incubation. 
 
 FIG. 2. Cross-section through hen's egg on third day of incubation. 
 
 FIG. 3. Longitudinal section through hen's egg on third day of incubation. 
 
 FIG. 4. Longitudinal section through hen's egg beginning of fourth day of 
 incubation. 
 
 FIG. 5. Longitudinal section through hen's egg on seventh day of incubation. 
 
 FIG. 6. Cross-section through embryo, first day. 
 
 FIG. 7. Diagrammatic longitudinal section through a selachian embryo. 
 
 FIG. 8 (Kollikie).' Half of a cross-section through embryo chick (two days.) 
 
 FIG. 9 (Kollikie). Cross-section through embryo chick, beginning of third day. 
 
 FIG. 10. Cross-section of chick (five days) in the region of the umbilicus. 
 
 FIG. 11. Diagrammatic longitudinal section of the embryo chick.
 
 PLATE XX 
 
 0.1 'f
 
 BRANCHIAL ARCHES 313 
 
 muscle cell activity, etc., has been sent to the new cells of the 
 third layer, or mesoblast, while the large cells of the inner layer or 
 hypoblast contain chromatin to direct most of the secretory activ- 
 ities and the formation of the epithelium of the alimentary canal. 
 
 NERVOUS SYSTEM. 
 
 Formation of Neural Canal. The epidermal cells of either side 
 of the primitive streak grow rapidly, forming two ridges with a 
 groove between them, which grows deeper and deeper until the 
 ridges bend over and join, enclosing a tube which is to be the 
 canal of the spinal cord (Fig. 261). The anterior end of this tube 
 enlarges into three bulbs which correspond to the ventricles of 
 the brain, and as they increase in size they fold over ventrally or 
 toward the center of the sphere until the first and second are at 
 right angles to the original tubular part. 
 
 As the outer layer forms the tube of the central nervous system, 
 the inner layer folds off a blind pouch from the general cavity of 
 the sphere which is to form the anterior part of the alimentary 
 canal (Plate XX). By this time development is complicated by 
 the formation of the embryonal membranes, the amnion and allan- 
 tois, but we may omit these entirely for our purposes. 
 
 The diagram from Quain's Anatomy (Figs. 262 and 263) illus- 
 trates the condition just described, showing the embryo in longi- 
 tudinal section, the bending over of the anterior end of the neural 
 canal to form the mid- and forebrain and the foregut, or esophagus, 
 a blind pouch ending anteriorly under the midbrain and posteriorly 
 opening into the cavity of the sphere now called the yolk sac. This 
 pouch is lined by hypoblast and covered by mesoblast and epiblast. 
 The heart has already begun its development in the mesoblast on 
 the ventral side of the foregut. 
 
 Branchial Arches. There now r appear what are called the gill 
 slits, openings from the foregut through its walls to the surface of 
 the embryo, which are separated by thickenings of the wall forming 
 arches around the gut known as the visceral or branchial arches, 
 at the center of each of which is found a bloodvessel. These struct- 
 ures are to be compared to the gills of a fish, which are slits through 
 the wall of the esophagus to the outside, so that water taken into 
 the mouth may pass out through the slits. At this time, too, the 
 arrangement of the bloodvessels exactly resembles that of a fish,
 
 314 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 and the individual may be said to be in the fish stage of develop- 
 ment. 
 
 hy. 
 
 FIG. 261. Stages in the conversion of the medullary groove into the neural canal. 
 From tail end of embryo of the cat: m.g., medullary groove; n.c., neural canal; 
 ch., notochord; ep., epiblast; hy., hypoblast; me., mesoblast; cce., celom; am., amnion. 
 (After Quain.) 
 
 Stomodeum. Plate XXI, from Quain's Anatomy, and Fig. 264, 
 from Hertwig's Text-book of Embryology, shows the embryo at 
 this stage and the arrangement of the bloodvessels. As the fore-
 
 PLATE XXI 
 
 First aortic arch 
 
 Auditory vesicle 
 
 Primitive jugular vein 
 Fourth aortic arch 
 
 Sixth aortic arch 
 Dorsal aorta 
 
 Cardinal vein 
 
 Digestive tube 
 
 Hind-gut 
 
 Olfactory pit 
 
 SI axillary iirocexx 
 Fir>it branchial yroove 
 Mandibular arch 
 
 Bulbil* eordin 
 A triu~ ;i 
 Duct of Cuvicr 
 Ventricle 
 
 Allantois 
 Umbilical artery 
 
 Profile View of a Human Embryo Estimated at Twenty-one 
 Days Old. (After His) 
 
 Si 
 
 .lowing branchial arches and relation to bloodvessels.
 
 STOMODEUM 
 
 315 
 
 brain grows ventrally, the first visceral arch, or mandibular arch, 
 also grows in the same direction, and the space between the inferior 
 surface of the forebrain and the upper surface of the first arch is 
 the beginning of the mouth and nose cavities, now called the stomo- 
 deum. From the base of the mandibular arch is seen also the 
 rounded bud, which is beginning to grow forward along the base 
 of the forebrain to form part of the maxillary arch, and finally the 
 upper jaw. At this time also the area which is to develop the 
 
 ,4 m n i 
 
 -Allantois 
 Hind-gut 
 
 FIG. 262.- Diagram of a longitudinal section of a mammalian embryo. Very 
 early, showing the folding off of the embryo. (After Quain.) 
 
 sense of smell appears on each side at the outer and lower portion 
 of the forebrain. The olfactory areas grow out of the base of the 
 forebrain, at first being on the outside of the head and in the later 
 development being enclosed, leaving an opening to the surface 
 the nostril. 
 
 If we have gained a correct idea of the conditions just described 
 by means of the pictures, it will be understood that by the growing 
 forward of the mandibular arch there is left an almost cubical
 
 316 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 space between the lower surface of the fore- and midbrain and the 
 upper surface of the mandibular arch (Fig. 264). This is a part 
 
 FIG. 263. Median sections through the head of embryo rabbits five (A) and six 
 (B) millimeters long: A, the opening from the foregut has not yet been made; 
 B, the faucial opening is shown at /; c, first brain vesicle; me, midbrain vesicle; 
 mo, medulla oblongata; m, medullary epiblast; if, infundibulum; sp.e, sphenoeth- 
 noidal, be, sphenoidal, and sp.o, spheno-occipital parts of the basal cranii; i, foregut; 
 ch, notochord; py, buccal pituitary involution; am, amnion; h, heart. 
 
 of the outside world, and is enclosed to form the mouth and nose 
 cavities. This process is best understood if we think of the develop- 
 
 FIG. 264. Embryo showing brachial arches and stomodeum. 
 
 ment from the anterior end of the forebrain of a process which 
 may be described as a curtain dropping down, making a central
 
 STOMODEUM 
 
 317 
 
 piece, and the bud from the mandibular arch on each side growing 
 forward to unite \vith it, leaving a slit between them and the man- 
 
 Lens. 
 
 Olfactory 
 pit. 
 
 Ma.rillary process. 
 
 Mandibular arch. 
 
 Hyo-mandibular cleft. 
 
 Auditory vesicle. 
 Hyoid arch. 
 
 Thyro-hyoid arch. 
 
 Sinus 
 
 -' prsecervicalis 
 
 FIG. 265. The beginning of the mandibular arch and the maxillary buds. 
 
 Cerebral hemisphere. 
 
 Fronto-nasal-. 
 process. 
 
 Stomodseum. ^m 
 
 Lateral nasal jirocess. 
 
 -Eye. 
 
 ~~Processus globularis. 
 Maxillary process. 
 
 Mandibular arch. 
 Hyo-mandibular cleft. 
 
 FIG. 266. An embryo a little older than Fig. 265. Viewed from in front. Showing 
 development of maxillary buds and frontonasal process. 
 
 dibular arch which will be the mouth. In order to get a correct 
 idea of this process it must be followed somewhat more minutely.
 
 318 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 Frontonasal Process. As the frontonasal process develops it 
 is made up of four rather bulb-like portions (Figs. 265 and 266), 
 
 FIG. 267. Embryo, a little older than Fig. 266. A, front view, frontonasal 
 process, and maxillary buds about to unite: 1, lateral nasal part of frontonasal 
 process; 2, maxillary bud; 3, mandibular arch; 4, hyoid arch. B, the same embryo 
 with the mandibular arch removed: 1, horizontal growth of the maxillary bud; 
 2, lateral nasal process; 3, mesial nasal process; 4, globular processes which form 
 the horizontal part of the intermaxillary bone. 
 
 two occupying the center and which develop into the intermaxil- 
 lary bone containing the incisor teeth and the center of the lip; 
 and two side or lateral processes which grow out around the olfac- 
 
 FIG. 268. Head of an embryo of about seven weeks. (His.) The external nasal 
 processes have united with the maxillary and globular processes to shut off the 
 olfactory pit from the orifice of the mouth. 
 
 tory area and form the alse of the nose surrounding the nostril. 
 These do not unite again with the central parts, but the end stops
 
 SEPARATION OF MOUTH AND NOSE CAVITY 
 
 319 
 
 over the point where the maxillary bud unites with the central 
 process (Figs. 266 and 267). A failure of union causes the deformity 
 of hare-lip, the opening in the lip extending to one, or, if double, 
 to both nostrils. 
 
 When the central part of the frontonasal process has united 
 with the maxillary bud on each side the arch of the upper jaw is 
 complete and the original cubical space or stomodeum is enclosed, 
 leaving only the slit between the maxillary and mandibulary arches 
 which is to form the mouth; but the enclosed space is in one cham- 
 ber, there being no separation between the mouth and nose cavities, 
 
 Palatal process of pro- 
 cessus globularis. 
 
 Palatal part of maxil- 
 lary process. 
 
 Maxillary 
 process 
 
 Processes globularis. 
 \ 
 
 Mouth of olfactory 
 pit, or nostril. 
 
 Lens. 
 
 .Eye. 
 
 Mouth 
 cavity. 
 
 FIG. 269. The head of an embryo with the mandibular arch removed. Looking 
 up from the mouth into the nose cavity. The union of the globular processes forming 
 the anterior part of the palate, and the horizontal ingrowths from the maxillary 
 buds, showing the way in which they unite from before backward, separating the 
 nose from the mouth cavity. 
 
 The time of this development in the human embryo may be placed 
 at about the fourth week. 
 
 Separation of Mouth and Nose Cavity. The separation of the 
 mouth and nose cavities occurs by the development of horizontal 
 ingrowths from the three parts making up the maxilla and begin- 
 ning at the center and progressing backward. First, a small trian- 
 gular piece from the central part of globular processes of the fronto- 
 nasal process, this uniting with the horizontal or palatal process 
 of the maxillary buds on each side until these reach the apex of 
 the triangle, which will be the intermaxillary bone, just a little 
 way back in the palate, and from here backward they unite with 
 their fellow of the opposite side. This is best seen by removing
 
 320 EARLY STAGES OF EMBRYOLOGY 
 
 the mandibular arch and viewing the parts from below (Fig. 269, 
 from Hertwig's Embryology). 
 
 The deformity of cleft palate is then a later development than 
 that of hare-lip, and either may occur without the other, though 
 they are usually found together. The cleft of the palate usually 
 turns to one side at the front, running out between the cuspid and 
 lateral unless it is double, when a detached piece is found in the 
 center in front, containing the incisors. As soon as the mouth and 
 nose cavities are separated and as fast as bone is formed in the 
 jaws most of the space is occupied by the tooth germs.
 
 CHAPTER XXVI. 
 THE DEVELOPMENT OF THE TOOTH GERM. 
 
 The Dental Ridge. By the middle of the second month of develop- 
 ment the arches of both upper and lower jaws are completed, and the 
 palate has separated the nose and mouth cavities. The first indica- 
 tion of the development of the teeth is the multiplication of the cells 
 
 . 
 
 FIG. 270. The dental ridge. A section through the mandible of a pig embryo 
 at the lower edge, two spicules of bone beginning to form; to the right Meckel's 
 cartilage. 
 
 of the epiblast in a curved line on the crest of each arch in the area 
 which is to be occupied by the teeth. By this multiplication of 
 cells the epiderm is piled up in a ridge, projecting above the surface, 
 and at the same time the deep layer of the epiblast is forced down 
 into the underlying mesoderm (Fig. 270). This structure is known 
 21 (321)
 
 322 THE DEVELOPMENT OF THE TOOTH GERM 
 
 as the dental ridge. In sections the cells piled up above the surface 
 are usually washed off more or less by the reagents, but the depres- 
 sion into the mesoderm is shown. On the lingual surface of this 
 ridge, in the part embedded in the mesoderm, the cells of the Mal- 
 pighian layer grow out lingually at right angles to the ridge, form- 
 ing a continuous shelf known as the dental lamina (Fig. 271). It 
 is important to remember that the lamina is continuous along the 
 entire extent of the ridge. 
 
 FIG. 271. The dental ridge and dental lamina. 
 
 The Enamel Organ.- From ten points on the surface of the lamina 
 little buds of epiblast start and grow down into the mesoderm, 
 increasing in size and becoming bulbous at the deep end. The 
 bulbous portion gradually becomes flattened. At this stage the 
 bulb is composed of an outer layer of columnar cells, continuous 
 with the Malpighian layer of the ridge and a central mass of large 
 polyhedral cells (Fig. 272). As the bud continues to grow into 
 the mesoderm, the mesodermic tissue below it begins to condense 
 and the cells of the upper portion of the bulb, growing more rapidly, 
 convert the bulb into a two-layered bag.
 
 THE TOOTH GERM 
 
 323 
 
 The Dental Papillae. The cells in the condensed mesoderm 
 multiply and grow up into the cavity of this cap, forming the begin- 
 hing of the dental papillae. This stage is represented in Figs. 273 
 and 274, in which the enamel organ is seen connected with the 
 lamina by a cord of epithelial cells, and made up of an outer layer 
 of columnar cells known as the outer tunic, and an inner layer of 
 columnar cells lying next to the dental papillae, known as the inner 
 tunic. The polyhedral cells between the two layers fill the central 
 part of the enamel organ and have taken on a peculiar appearance, 
 which has given to them the name of the stellate reticulum. The 
 
 FIG. 272. A section through the mandibular arch: E, enamel organ; D, begin- 
 ning of the dental papilla; B, bone; F, fold from the side of the mandible to the base 
 of the tongue covering the beginning of the sublingual gland; T, tongue. 
 
 development of the tooth germ now progresses until the dental 
 papilla has taken on the typical form of the tooth. The fully formed 
 enamel organ for an incisor of a sheep is shown in Fig. 275. The 
 cord which connects the outer tunic with the surface epithelium is 
 not shown in this section. 
 
 The Tooth Germ. The tooth germ is composed of the enamel 
 organ, made up of the outer tunic, the inner tunic, and the stellate 
 reticulum, covering the dental papillae. From the base of the 
 papillee fibrous tissue develops, growing upward around the entire 
 tooth germ and enclosing it in a definite wall or sac of fibrous tissue. 
 This is known as the dental follicle, or the follicle wall.
 
 324 
 
 THE DEVELOPMENT OF THE TOOTH GERM 
 
 The Dental Follicle. This term has been used to indicate not 
 simply the connective-tissue wall, but all of the structure enclosed 
 in it. This use of the term, however, is confusing, and the term 
 should be confined to the fibrous sac. By the end of the twelfth 
 week the follicle wall has grown up so as to enclose the enamel 
 
 FIG. 273. The enamel organ. The outer tunic connected to the lamina by the cord; 
 the dental papilla growing up into the cap. The spaces are shrinkage spaces. 
 
 organ, and the epithelial cord which has connected it with the 
 surface is broken. 
 
 Tooth Germs of the Permanent Tooth. Before the epithelial cord 
 is broken, from some point on the lingual surface of the outer tunic 
 or along the cord a bud of epithelial cells growls out and turns 
 down into the mesoderm, passing over the follicle wall (Fig. 276). 
 This continues to grow downward until it has reached the position 
 below and to the lingual of the tooth germ for the temporary tooth,
 
 BEGINNING OF CALCIFICATION 
 
 325 
 
 where it develops into the enamel organ for the corresponding per- 
 manent tooth. It goes through the same changes of form as has 
 been seen in the temporary teeth. 
 
 Beginning of Calcification. About the sixteenth week the tooth 
 germs of all the temporary teeth have been completely enclosed 
 in their follicles and the enamel organ for the corresponding per- 
 manent teeth have begun their development (Fig. 277). This 
 illustration shows a section through the lower jaw of a pig, and 
 
 FIG. 274. The enamel organ, a little older than Fig. 273. It shows the outer tunic, 
 the inner tunic, and the stellate reticulum. The dental papilla in the hollow of 
 the cap. The spaces are caused by shrinkage. 
 
 exhibits the tooth germs for two incisors at about the stage of the 
 closing of the follicle walls. The buds for the permanent teeth are 
 seen on the lingual, and the formation of enamel and dentin is 
 just beginning in the temporary teeth. Notice the remains of 
 Meckel's cartilage, and the extension of endomembranous bone 
 formation which is just beginning to form a periosteum on its 
 surface. The bone has grown around Meckel's cartilage and around 
 the tooth germs on the buccal and lingual, enclosing them in an open
 
 326 
 
 THE DEVELOPMENT OF THE TOOTH GERM 
 
 groove, which will later be completed and divided into separate 
 crypts for each tooth. Fig. 278 is from a similar specimen in the 
 region of a temporary molar. The dental papilla is taking on the 
 form of a crown and the formation of enamel and dentin is ready 
 to begin. The cells on the outer layer of the dental papilla have 
 developed into odontoblasts, forming a single layer of columnar 
 
 FIG. 275. The tooth germ, from the mandible of a sheep. The enamel organ shows 
 the outer tunic, inner tunic, and stellate reticulum. The dental papilla projects into 
 the enamel organ. The follicle is attached to the base of the dental papilla and 
 surrounds the enamel organ. The spicules of bone form the crypt wall. 
 
 cells lying in contact with the inner tunica of the enamel organ. 
 Here the formation of enamel and dentin begins, the dentin slightly 
 preceding the enamel. The odontoblasts form and calcify dentin 
 matrix from without inward. The cells of the inner tunic or amelo- 
 blasts form and calcify the enamel rods and cementing substance, 
 progressing from within outward. The line upon which the odonto-
 
 FIRST PERMANENT MOLAR 
 
 327 
 
 blasts and ameloblasts lie in contact therefore will become the 
 dento-enamel junction. The formation of dentin and enamel begin 
 at separate points, which are at first very close together, but are 
 
 FIG. 276. The tooth germ showing the bud for the permanent tooth at P. Cal, 
 cification is just beginning: F, follicle wall; D, dental papilla; T, inner tunic; T'- 
 outer tunic; S, stellate reticulum; O, odontoblasts; A, ameloblasts; B, bone. 
 
 carried farther apart by the growth of the dental papilla, until they 
 have progressed along the dento-enamel junction and unite, when 
 the increase in the diameter of the dental papilla is stopped. This, 
 perhaps, will be better understood by studying Figs. 82 to 87. 
 
 First Permanent Molar. The origin and development of the first 
 permanent molar differs from that of all the other permanent
 
 328 THE DEVELOPMENT OF THE TOOTH GERM 
 
 SM 
 
 'V 
 
 FIG. 277. A section through the lower jaw of a embryo pig, showing germs of 
 
 two incisors. 
 
 FIG. 278. Germ of a premolar from an embryo pig.
 
 FIRST PERMANENT MOLAR 
 
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 330 THE DEVELOPMENT OF THE TOOTH GERM 
 
 teeth in important respects. It is the only permanent tooth whose 
 enamel organ springs directly from the dental lamina in the same 
 way as those for the temporary teeth. It is the only permanent 
 tooth whose crown is calcified before the individual is thrown upon 
 its own resources for the obtaining of nourishment. Nature seems 
 to have taken special precautions in the formation of this most 
 important tooth. 
 
 About the seventeenth week, at a point on the dental lamina, 
 posterior to the enamel organs of the temporary teeth, a bud starts 
 to grow down into the mesoderm, which develops into the enamel 
 organ for the first molar, and by the ninth month the follicle is 
 complete and calcification has begun. 
 
 The Origin of the Second and Third Molars. The enamel organ 
 for the second molar is formed from a bud given off from the outer 
 tunic of the enamel organ of the first molar. The enamel organ 
 for the third molar is formed from a bud given off from the outer 
 tunic of the enamel organ of the second, at about the third year. 
 
 Chronology. The development of the teeth was first investi- 
 gated by Lagros and Magitot (about 1865). Since that time their 
 work has been repeated and verified by several investigators. About 
 1880 Dr. Black repeated the entire work of Magitot, and some of 
 his illustrations were used by Dr. Dean in his Translation of Magi- 
 tot Memoir. Magitot's table, showing the chronology of tooth 
 development, is given on page 329. 
 
 The previous pages are to be considered as a series of definitions, 
 and descriptions of structures, and now the student is assumed to 
 have some idea of what is meant when the "dental ridge," or the 
 "dental papilla " is mentioned. 
 
 In embryology so many things are going on at the same time and 
 the changes are so rapid that it is difficult, especially from written 
 description, to obtain a clear idea of the process. Unfortunately 
 a moving picture of the development of the tooth cannot be made 
 by direct photography as has been done with the growth of plants 
 and the opening of flowers, but it is important to visualize the process 
 as would be done by a moving picture, the present description is 
 intended to connect and relate in a most elementary way some of the 
 most important facts. 
 
 The First Indication of Tooth Development. The first indication of 
 tooth development is the multiplication of epidermal cells about 
 the maxillary and mandibular arches. This produces a cord or rod 
 of epiblastic cells projecting above the surface of the jaw arch and
 
 THE FOLLICLE WALL 331 
 
 extending into the mesoderm of the body of the arch. The extension 
 into the mesoderm is more or less vertical to the surface of the primi- 
 tive jaw. This is the "dental ridge." On the lingual side of this 
 structure the epidermal cells grow out forming a layer or shelf pro- 
 jecting from the lingual side of the ridge and extending as far as the 
 ridge itself. This newgrowth is at first nearly at right angles to the 
 axis of the ridge, but the tip of it turns down into the mesoderm, 
 becoming more and more parallel with the axis of the original dental 
 ridge. This is the "dental lamina." 
 
 Early in the development of the lamina at ten points in each arch, 
 epidermal buds start from the edge of the lamina to form the enamel 
 organs for the ten temporary teeth. When these buds start they are 
 springing from the edge of the lamina, but after the formation of the 
 enamel organs for the temporary teeth has started the growth of the 
 lamina continues growing down to the lingual of the developing 
 temporary tooth germs. The extent and continuity of this develop- 
 ment seems to be different in different species. A true mental picture 
 of this process will explain the conflicting statements as to the origin 
 of the enamel organs for the permanent teeth, which correspond to 
 or replace the temporary ones. The enamel organs for these teeth 
 are said by different authors (1) to arise from the outer tunic of the 
 enamel organ of the temporary tooth; (2) from the cord connecting 
 the outer tunic of the temporary tooth with the surface epithelium; 
 (3) or direct from the lamina. 
 
 Enamel Organ. As soon as the enamel organ begins to grow down 
 into the mesoderm. There is a response in the mesoderm below it 
 resulting in a change in the character of the cells, and the develop- 
 ment of the dental papilla. The epithelial cells of the inner tunic 
 assume the form of ameloblasts and the mesoblastic cells of the outer 
 surface of the dental papilla become columnar and take the form of 
 odontoblasts. This specialization begins at the tip of the dental 
 papilla and at the points that will be the beginnings of calcification. 
 This specialization spreads from these points along the surface of the 
 papilla. The formation of enamel begins while the enamel organ is 
 still in its typical form, that is, while the outer tunic is complete 
 and is still connected with the lamina by a cord of epithelial cells, 
 but almost immediately after the formation of enamel and dentin 
 begins, there are important changes. 
 
 The Follicle Wall. As soon as the dental papilla and enamel organ 
 begins to take on their full form, there occurs differentiation of tissue 
 in the mesoderm and the formation of fibrous tissue. This begins
 
 332 THE DEVELOPMENT OF THE TOOTH GERM 
 
 at or near the base of the papilla, but rapidly extends upward 
 (incisally) passing outside of the outer tunic inclosing both structures 
 in a fibrous sac. When this formation of fibrous tissue reaches the 
 incisal extremity of the enamel organ and approaches the point 
 from which the cord of epithelial cells extends to the lamina, the cord 
 is broken and the enamel organ is no longer connected with the 
 surface. At this time four important things happen : (1) The begin- 
 ning of calcification of enamel and dentin; (2) the breaking up of the 
 outer tunic of the enamel organ which begins at the point where the 
 cord was broken; (3) a marked proliferation of epithelial cords and 
 masses arising from the cells which formed the cord; (4) the begin- 
 ning of the bud to form the enamel organ for the successional 
 tooth. 
 
 The Breaking up of the Outer Tunic. When the follicle wall closes 
 over the incisal extremity of the enamel organ, there appears on 
 the outer surface of the outer tunic of the enamel organ little 
 rounded projections of epithelial cells, and the layer is broken up. 
 At the same time there is the formation of capillary bloodvessels 
 from the follicle wall, which carry the remains of the outer tunic 
 down against the inner tunic to form the stratum intermedium 
 (Fig. 238). There is an intimate relation between capillary blood- 
 vessels and the stratum intermedium. Leon Williams considered 
 that the cells of this layer take up materials from the blood and 
 elaborate them to be used by the ameloblasts in the calcification of 
 enamel. Enamel is formed only as far as the stratum intermedium 
 is formed, although the inner tunic of the enamel organ extends 
 apically along the dental papilla toward the end of the root as far as 
 dentin is formed. 
 
 The Breaking up of the Epithelial Cord. After the closing of the 
 follicle wall the cells which formed the cord multiply and are mixed 
 with fibrous tissue. This is no longer a continuous cord of epithelial 
 cells, but irregular strings and masses of epithelial cells lying in the 
 fibrous tissue. This has been called the cingulum, extending from 
 the follicle wall to the surface epithelium. 
 
 It often happens that the epithelial masses take on globular 
 form and it is probable that occasionally one of these may develop 
 into an enamel organ, and lead to the formation of a supernumerary 
 temporary tooth. 
 
 The hud for the corresponding permanent tooth grows downward 
 (apically) along the lingual side of the germ of the temporary tooth 
 outside of its follicle wall, until it conies to a position below and to the
 
 ORIGIN OF THE SECOND AND THIRD MOLARS 333 
 
 lingual of it, where it goes through exactly the same changes that 
 have taken place in the development of the temporary one. 
 
 At the time the follicle wall closes over the enamel of the per- 
 manent tooth there occurs a similar, but usually more marked and 
 extensive proliferation of epithelium, and the origin of supernumer- 
 ary permanent teeth is so explained. The supernumerary would 
 develop between the temporary and the permanent tooth, and as a 
 rule it is found clinically that in such cases the first tooth to erupt 
 after the loss of the temporary one is the supernumerary and the 
 last one the typical tooth. 
 
 Origin of the Second and Third Molars. If one can visualize the 
 process that has been described it will be realized that it is quite 
 difficult from the appearances of a few sections to determine whether 
 the enamel organs for the second and third molars which have no 
 temporary predecessors arise from a bud from the outer tunic of 
 the preceding (approximating) molar, or whether there is an exten- 
 sion of the lamina distally from which the buds are formed.
 
 CHAPTER XXVII. 
 
 THE RELATION OF THE TEETH TO THE DEVELOP- 
 MENT OF THE FACE. 
 
 AT birth the jaws contain all of the temporary teeth and the 
 first molars in a partially-formed condition, and the follicles for 
 all of the permanent teeth except the second and third molars. 
 These very nearly fill the substance of the bone. In the growth of 
 the bones of the face and the changes that occur in the transfor- 
 mation of the child to the adult face, the teeth play a most important 
 role. 
 
 Before considering this subject in detail it is necessary to recall 
 in this connection some things that have already been emphasized. 
 
 RELATION OF THE TEETH TO THE BONE. 
 
 In evolution the teeth originally had no connection with the 
 bone, it being formed later for their support. In embryology the 
 tooth is formed first, and the bone formed around it. In this way 
 the development of the individual repeats evolution. In the study 
 of the bone it has been emphasized that the connective tissues 
 have been specialized to meet mechanical conditions, and that 
 both ontogenetically and phylogenetically they are formed in 
 response to mechanical stimuli. The mutations of connective tissue 
 have been dwelt upon, and especially the fact that a bone as an 
 organ of support always contains fibrous tissue, and that there is 
 a continual oscillation between formation and destruction, by means 
 of which it is perfectly adapted to its mechanical environment. 
 The transformations of bone in bone growth have been pointed 
 out, and these will be still more carefully studied in connection 
 with the growth of the bones of the face. 
 
 Some years ago the author undertook a study of the structure 
 and growth of the jaws and alveolar process, which resulted in 
 very important modifications of the conceptions of the matter as 
 given by standard texts. Tomes describes the process of develop- 
 ment as essentially an addition at the posterior portions of the 
 (334)
 
 RELATION OF THE TEETH TO THE BONE 
 
 335 
 
 jaws to make room for the successively developed permanent 
 molars, and illustrates the process in diagrams (Fig. 279). l The 
 following quotation states his view : 
 
 "But the main increase in the size of the jaw has been in the 
 direction of backward elongation; in this, as Kolliker first pointed 
 out, the thick articular cartilage plays an important part. The 
 manner in which the jaw is formed might also be described as waste- 
 ful; a very large amount of bone is formed which is subsequently, 
 at no distant date, removed again by absorption; or we might 
 compare it to a modelling process, in which thick, comparatively 
 
 FIG. 279. Tomes' diagram of development of mandible from infant to adult. 
 
 shapeless masses are dabbed on to be trimmed and pared down 
 into form. 
 
 "To bring it more clearly home to the student's mind, if all the 
 bone ever formed were to remain, the coronoid process would 
 extend from the condyle to the region of the first bicuspid, and 
 all the teeth behind that would be buried in its base; there would 
 be no neck beneath the condyle, but the internal oblique line 
 would be a thick bar corresponding in width with the condyle. 
 It is necessary to fully realize that the articular surface with its 
 
 1 Tomes' Dental Anatomy, p. 208.
 
 336 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 cartilage has successively occupied every spot along this line; 
 and as it progresses backward by the deposition of fresh bone in 
 its cartilage, it had been followed up by the process of absorption, 
 removing all that was redundant." 
 
 In a similar way in any maxilla, the temporary dentition is shown 
 to occupy about the same space as the permanent teeth, as far as 
 the second bicuspid, and the adult is supposed to be formed from 
 the child by the building on of the bone at the back as the molars 
 are formed. 
 
 This conception is fundamentally misleading, for if the infant 
 mandible were to be shown in the relation to that of the adult in 
 three dimensions of space, it would be found to be above and 
 entirely within the adult mandible, and no part of the bone which 
 constituted the infant jaw is present in the adult. In the upper, 
 if the temporary teeth at two years were figured in relation to 
 those of the adult, they would be placed somewhere up in the nasal 
 cavity. 
 
 The conditions are more correctly stated by saying that forces 
 exerted at the posterior portions of the jaw through the develop- 
 ment of the successive molars cause the bone to grow downward, 
 forward, and outward in the upper arch, upward, forward, and 
 outward in the lower, carrying the bone into an entirely new posi- 
 tion in space. 
 
 In this process the peridental membrane, periosteum, and 
 articular cartilage all play their part, but all the bone posterior to 
 the second bicuspid cannot be thought of as having been formed 
 by the articular cartilage and modelled into form by the periosteum, 
 as might be inferred from Tomes' statement. 
 
 Structure of Maxillae and Mandible. Before attempting to follow 
 the growth of the bone in the development of the face, the arrange- 
 ment and distribution of the varieties of bone in the structure of 
 the mandible and maxillse should be carefully studied. 
 
 Cortical Plate. The outer surface of these bones is formed of 
 a compact layer composed partly of subperiosteal and partly of 
 Haversian system bone. This varies greatly in thickness, depend- 
 ing upon the stress to be sustained. It is called the cortical plate. 
 
 Cancellous Bone. The center of the bone is cancellous in charac- 
 ter and made up of thin plates of lamella? arranged around large 
 medullary spaces. The direction and arrangement of these plates 
 is determined by the forces received on the cortical plates and the 
 directions of stress to which they are subjected. This was pointed
 
 RELATION OF THE TEETH TO THE BONE 337 
 
 out some years ago by Walkoff in an elaborate study of the bones 
 by the use of the z-rays. By this means he showed that the plates 
 of cancellous bone in certain areas had a definite arrangement which 
 was related to the attachments of certain muscles. From the 
 examination of sections of the mandible it will be found that not 
 only is the general form of the bone determined by the forces to 
 which it has been subjected, but also that its minute inner structure 
 is definitely arranged with reference to these forces. The direction 
 and arrangement of the plates of cancellous bone are continually 
 
 FIG. 280. The distribution of bone in the alveolar process. 
 
 changed and rebuilt to readjust them to the support of new condi- 
 tions (Fig. 326). 
 
 Cribriform Plates. The alveoli or sockets into which the roots 
 of the teeth fit are bounded by a thin, definite wall, which is pierced 
 by a great many openings. These have been called the cribriform 
 plates, or sieve-like plates. They unite the cortical plates of the 
 bone at the border of the alveolar process, and are fused with it, 
 on their labial and lingual sides. The cribriform plates forming 
 the walls of the alveoli are really made up of a thin layer of sub- 
 22
 
 338 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 peridental bone, which has been built on to the plates of cancellous 
 bone, to attach the fibers of the peridental membrane (Fig. 213). 
 Within the substance of the bone and surrounding the course of 
 the inferior dental artery and nerve is found what Cryer has called 
 the cribriform tube. This extends from the point where the arteries 
 and vein enter the substance of the bone on the lingual surface of 
 the ramus, posterior to the alveolar process and below the oblique 
 line, and extends through the cancellous portion of the body of 
 the bone, emerging at the mental foramina. It is really a rather 
 definite arrangement of the plates of cancellous bone around the 
 vessels and the nerves. 
 
 FIG. 281. Skull of orang-outang. 
 
 Alveolar Process. If the adult alveolar process as seen in the 
 skull is examined, it is apparent that the bone is arranged so as 
 to give the greatest support with the least possible bulk, and where 
 there is an increase in bulk it is to meet some special force (Fig. 
 280). The incisors and cuspids are used chiefly to bite off pieces 
 of food, and when the food cannot be bitten it is torn and wrenched 
 away. This puts a heavy strain in all directions on the roots of 
 the teeth, which must be supported by the bone. For this reason 
 the roots of the incisors are usually well covered with bone through 
 their entire length. The cuspid root is long and the upper portion 
 of it so w r ell supported in the bone at the side of the nose and toward 
 the orbit that the most convex portion of it is sometimes uncovered. 
 In animals that use the incisors largely for tearing, wrenching, and 
 fighting, the bone is greatly thickened over the incisal roots, as is 
 shown in the skull of the orang-outang (Fig. 281).
 
 RELATION OF THE TEETH TO THE BONE 
 
 339 
 
 In the upper molars the spreading of the three roots gives abun- 
 dant support against the direct forces of occlusion. The grinding 
 motions bring lateral pressure against the inclined planes of the 
 cusps, which is met by a thickening of the process in its occlusal 
 
 FIGS. 282 and 283. Human mandible, showing form of the bone and the positions 
 from which sections were cut. 
 
 third (Fig. 280), forming a heavier ring of bone, while the buccal 
 roots are often exposed in their middle third. In the molars the 
 buccal incline of the lingual cusps of the upper occlude with the 
 lingual incline of the buccal cusps of the lower when the jaws are 
 brought squarely together, and in the grinding motions the outward 
 pressure on the lower molars is supported by the great mass of the 
 
 FIG. 284. Human mandible, showing form of the bone and the positions from 
 which sections were cut. 
 
 body of the bone, while the inward pressure is supported by a 
 thickening of the occlusal third, as the entire alveolar process 
 projects lingually from the body of the bone. In the examination 
 of any collection of skulls, the amount and arrangement of the
 
 340 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 bone of the alveolar process will be found to be an indication of 
 the masticatory habits of the individual. 
 
 In examining the sections through the bone of the alveolar 
 process, the adaptation of the arrangement of bone to the force 
 to be sustained should be constantly kept in mind. 
 
 Influence of Mechanical Conditions in Evolution. Professor E. D. 
 Cope, 1 in a long treatise on "The Mechanical Causes of the Develop- 
 ment of the Hard Parts in Mammals," has 
 elaborated the fact that the bones of the 
 skeletons of all mammals have been influ- 
 enced in their development by mechanical 
 conditions, and that their present forms 
 are adaptations to physical environment. 
 In this he states, as a general principle 
 of structure, that the bone is most dense, 
 but least in amount, on the side in the 
 direction toward which forces have been 
 exerted in development, and less dense, 
 but greater in amount, on the sides from 
 which the forces have been exerted. 
 These statements should be applied in 
 the study of all the sections shown. 
 
 An old dry mandible was sawed through 
 in the positions indicated in the illustra- 
 tion (Figs. 282, 283, and 284). 
 
 The portion containing the bicuspid 
 and molar on the left side was ground 
 through the molar to obtain a section 
 parallel with the axis of the tooth. The 
 portion between the alveolus of the 
 cuspid and second bicuspid on the left 
 side was ground vertically through the 
 area where the first bicuspid had been 
 (Fig. 285). The portion on the right 
 
 side containing the two bicuspids and molar was ground to give 
 three sections at right angles to the roots one in the gingival 
 third, one about the middle of the root, and one just at their apices 
 (Fig. 286). The distal portions of the bone were decalcified and 
 sections cut through the alveoli of the second and third molars 
 (Figs. 287 and 288). 
 
 1 Journal of Morphology, 1888. 
 
 FIG. 285. Ground sec- 
 tion through the mandible 
 where the bicuspid had 
 been extracted.
 
 RELATION OF THE TEETH TO THE BONE 341 
 
 The Distribution of Bone in the Mandible. In Chapter XVII, on 
 Bone, it was stated that the arrangement of the layers in the tissue 
 could be read as a record of the manner of formation. In the exam- 
 ination of these sections the arrangement of the lamella? is to be 
 
 FIG. 286. Transverse sections through the roots of two bicuspids and the first 
 molar, showing distribution of bone. 
 
 studied in this way, as well as the distribution of the varieties of 
 bone. Where the bicuspid had been extracted the alveolus has 
 been filled with fairly compact bone, rounding over the border of 
 the process. The section ground through this position shows the
 
 342 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 FIG. 287. Decalcified sections through the molar region.
 
 RELATION OF THE TEETH TO THE BONE 
 
 343 
 
 FIG. 288. Decalcified sections through the alveoli of the second and third molars.
 
 344 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 buccal and lingual cortical plates in U shape. The two plates are 
 braced together across the central portion by spicules of cancellous 
 bone. At the occlusal border the outline of the old alveolus can 
 still be seen by studying the section carefully with the microscope. 
 After the extraction of the tooth the socket was first filled with 
 connective tissue, which was later transformed into bone, joining 
 
 FIG. 289. A section ground through the first molar. 
 
 that of the alveolar wall. Near the lower border, the subperi- 
 osteal bone is found to be very thick, the bone evidently grow- 
 ing in that direction. Near the occlusal border on the lingual 
 side, there have evidently been absorptions of the surface, removing 
 the Ilaversian system bone, and then a few layers of subperiosteal 
 bone have been reformed on the surface.
 
 RELATION OF THE TEETH TO THE BONE 
 
 345 
 
 Fig. 289 shows a section ground through the molar. The cribri- 
 form plates lining the alveoli join the cortical plates at the border 
 of the process. On the lingual side the wall of the process is very 
 
 FIG. 290. The buccal pk 
 
 FIG. 291. The lingual plate from Fig. 286. 
 
 thin, but is thickened in the occlusal third to support the tooth 
 against force exerted lingually. On the buccal side the cribriform 
 plate of the alveolar wall is connected with the cortical plate by
 
 346 THE TEETH AND DEVELOPMENT OF ^THE FACE 
 
 spicules of cancellous bone. Below the apex of the root the cortical 
 plates are connected by cancellous bone in which the medullary 
 spaces are much larger. The same arrangement of the cortical 
 plate and its bracing is shown in Fig. 287, which cuts between the 
 alveoli of the second and third molar. Fig. 327 and Plate XIX 
 should be studied in this connection, remembering that the bone 
 has been formed and shaped by formation of subperiosteal bone on 
 its surface and subperidental bone at the border of the process and 
 their transformation into Haversian system and cancellous bone. 
 
 Fig. 286 is cut transversely. Notice that the gingival section 
 has been turned over in mounting. Observe the cribriform plates 
 forming the walls of the alveoli, and the way these are braced 
 against each other and the cortical plates by bands of cancellous 
 
 FIG. 292. The bone between the alveoli of the mesial and distal roots of the first 
 molar, from Fig. 286. 
 
 bone. In accordance with the principles noted, the buccal plate 
 is thin and very compact, while the lingual plate is much thicker, 
 but more open in structure, and the direction of growth has been 
 toward the buccal as the arch of the jaw increased in size. Fig. 
 290 shows the buccal plate with higher magnifications, Fig. 291 
 the lingual plate, and Fig. 292 the bone separating the alveoli 
 from the mesial and distal roots of the molar. The third figure of 
 this series shows only the tip of the distal root of the molar, but the 
 arrangement of plates of cancellous bone between the cortical 
 plates is nicely shown. 
 
 The Maxilla. In the maxilla the arrangement is exactly on the 
 same plan, the details being different because of the difference in 
 the shape of the bone.
 
 THE GROWTH OF THE JAWS 347 
 
 THE GROWTH OF THE JAWS. 
 
 It has long been noted that at birth the mandible is straight, 
 and with the eruption of the teeth the ramus develops and the body 
 increases in size. In this process the thickness of the bone is 
 increased from the mental foramina to the alveolar border, and the 
 body of the bone approaches a right angle with the ramus. When 
 the teeth are lost or lose their function the alveolar process is 
 destroyed and the bone reduced in thickness from above downward 
 until the mental foramen comes to lie on the upper surface of the 
 bone. The mandible performs two functions, a respiratory and a 
 masticatory function, and it should be remembered that these 
 are influential in its development. The object of this section is 
 to give some conception of the direction of growth in the devel- 
 
 FIG. 293. Skull at birth. 
 
 opment of the bones of the face and the way in which the changes 
 are brought about. 
 
 This can best be done by studying the series of skulls from child- 
 hood to old age, in which the outer cortical plate has been removed 
 so as to show the developing teeth in their crypts and the relation 
 of the forming teeth to those already in occlusion (Figs. 293 to 
 307). At birth all of the teeth except the second and third molars 
 have begun to develop, and their tooth germs are lying embedded 
 in the cancellous substance of the maxillae. In the upper jaw they 
 occupy almost all of the space to the floor of the nose and orbit, 
 and there is little if any indication of the maxillary sinus (Fig. 293)
 
 348 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 Each tooth germ is enclosed in a separate crypt, the wall of which 
 is formed by a cribriform plate. The walls of the crypts are braced 
 
 FIG. 294. Maxillae at about eight months after birth, showing the unerupted 
 
 tooth. 
 
 FIG. 295. Maxillae at about one year. 
 
 against each other and the cortical plates of the maxillse by spicules 
 of cancellous bone surrounding medullary spaces. As the tooth
 
 THE GROWTH OF THE JAWS 349 
 
 develops within its crypt, pressure is exerted and the crypt wall 
 is pushed backward through the cancellous bone. 
 
 Growth Force. The force exerted by the growing tooth is the 
 result of the multiplication of cells in the tooth germ, and is exactly 
 comparable to the forces exerted by multiplication of cells in any 
 position. For instance, the force exerted by the multiplication of 
 the cells in a rootlet of a plant is sufficient to force pebbles aside 
 and make an opening through hard packed earth. Some attempts 
 have been made to measure the amount of force, but we can only 
 say that it appears to be considerable, acting through short range. 
 
 FIG. 296. Maxillae at one and one-half years. 
 
 How this force is generated has been a matter of much speculation 
 and investigation. It shows some points of similarity with the 
 swelling of wood fibers when water is added. It apparently is 
 related to osmosis, and has some direct relations to blood-pressure. 
 It is certainly a very complicated matter, with chemical affinities 
 at the bottom of it. 
 
 Forces Influencing Bone Growth. While the growing tooth germs 
 are producing force which causes conditions of stress of the cortical 
 plates, the growth of the tissues within the mouth the tongue 
 and the associated organs is exerting pressure upon the lingual
 
 350 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 surfaces of the bone. The muscles attached to their surfaces trans- 
 mit force to the bone through the periosteum, and the functions 
 of mastication, deglutition, and respiration are acting upon them. 
 All of these are mechanical stimuli, to which the connective-tissue 
 cells respond. In all the process of development the growth is 
 the result of all the forces to which the bones are subjected, per- 
 fectly distributed through the substance of the bone by the agency 
 of normal occlusion. Any lack of harmony in the proportion of 
 
 FIG. 297. Maxillae in the second year, showing the relation of the erupting teeth. 
 Note the relation of the crypt of the second molar to the inferior dental canal. 
 
 these forces may allow the teeth to meet, when they erupt, outside 
 of the normal influence of their cusps, causing the beginning of 
 malocclusion. Any malocclusion disturbs the balance in the 
 distribution of forces, and results in a disturbance of the develop- 
 ment of bone, which progresses during the entire period of devel- 
 opment. This must result in the lack of balance in the proportions 
 of the features which will be proportionate to the malocclusion. 
 It has been natural and almost inevitable, because of their hard-
 
 THE GROWTH OF THE JAWS 
 
 351 
 
 FIG. 298. The complete temporary dentition (about three years), showing the rela- 
 tion of the developing permanent teeth. 
 
 FIG. 299. The complete temporary dentition and the first permanent molar. Note 
 the relation of the bicuspids to the temporary molars. (In the seventh year.)
 
 352 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 ness, to think of bones as solid and unchanging. In the study of 
 these skulls the bones of the face must be viewed not as solid and 
 rigid, but as containing millions of active cells which are continually 
 building and rebuilding their substance. 
 
 FIG. 300. Front view of the skull shown in Fig. 299. Note the relation of the per- 
 manent incisors and cuspids to each other and the roots of the temporary teeth. 
 
 Usually somewhere between the seventh and ninth months after 
 birth the growth of the central incisors causes the absorption of 
 the roof of their crypts, and the tooth moves occlusally, cutting 
 through the soft tissues (Fig. 294). The formation of cementum 
 on the surface of the root and of bone on the wall of the crypt 
 attach the connective-tissue fibers and form the beginning of the
 
 THE GROWTH OF THE JAWS 353 
 
 peridental membrane. As the tooth moves occlusally the bone 
 grows up around it from the circumference of the crypt wall, con- 
 verting it into the wall of the alveolus. The root is not fully formed 
 and the conical pulp filling the funnel-like end exerts force by the 
 
 FIG. 301. Dentition in the eighth year. Note the position of the cuspids and com- 
 pare with Fig. 303. 
 
 multiplication of cells and the blood-pressure, which cause the 
 tooth to move occlusally and the bone to grow in that direction. 
 At the same time the pressure of tongue and lips exerts pressure on 
 the surfaces of the tooth and bone, influencing the direction of 
 bone growth. The jaw increases in thickness in the occlusal direc- 
 23
 
 354 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 tion and grows forward and outward. At the same time the growth 
 'of each successively distal tooth is exerting pressure upon those 
 already erupted, causing them to move farther in the occlusal 
 direction. In Figs. 296 and 297 notice the way in which the crypt 
 walls are pushed downward by the development of the tooth root 
 
 FIG. 302. The left side of the skull, shown in Fig. 301. 
 
 until the inferior dental nerve lies between the floor of the crypt 
 and the cortical plate of the lower border. In this way enough 
 pressure may be produced to cause reflex nervous symptoms, which 
 commonly precede the eruption of the temporary molars, and so 
 development continues until all of the temporary teeth are in
 
 THE GROWTH OF THE JAWS 355 
 
 place. About the sixth year the first permanent molars take their 
 place at the distal of the temporary teeth and their cusps interlock 
 (Fig. 299). The importance of these teeth can scarcely be, over- 
 stated. They are not only to be the chief means of mastication 
 
 FIG. 303. Dentition in the eleventh year. Note the growth of the cuspids and 
 bicuspids. The second molar is about to erupt. 
 
 during the period in which the temporary teeth are lost and replaced 
 by their successors, but they are to maintain the relation of the 
 jaws to each other. The way in which these teeth lock determines 
 the balance between the forces exerted by the action of the muscles
 
 356 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 attached in the region of the ramus, and those in the region of the 
 symphysis (Fig. 299). 
 
 A deviation from the normal relation of these teeth will entirely 
 change the direction of the forces, and will be manifested by a 
 modification in the development in the bone. In the skull at this 
 period the bicuspids are seen lying below the temporary molars, 
 and the second molar developing at the distal of the first. Their 
 
 FIG. 304. Dentition in the thirteenth year. Note the relation of the bicuspid 
 crown to the roots of the lower temporary molar. 
 
 growth is transmitted through the teeth to the alveolar process, 
 and the addition of bone results. The same skull viewed from in 
 front (Fig. 300) shows the relation of the permanent incisors and 
 cuspids to the temporary ones. In the lower jaw the temporary 
 centrals have been lost and the permanent ones are forcing their 
 way between the temporary laterals. The crowns of the centrals 
 are wider than those of the teeth that were lost, and they conse-
 
 THE GROWTH OF THE JAWS 357 
 
 quently exert pressure upon the mesial surfaces of the laterals, 
 pushing them apart and carrying them upward and forward. 
 
 Study the relation of the lower centrals, laterals, and cuspids 
 in the development of the arches at from six to ten years. Notice 
 that the roots of the central are not fully formed, that the lateral 
 
 FIG. 305. The dentition of a young adult. The third molars have not erupted. 
 (About fifteen years.) 
 
 lies to the lingual of the temporary lateral root, and with its mesio- 
 occlusal angle below the distal surface of the central. The develop- 
 ment of the cuspid has pushed the crypt floor through the cancellous 
 bone until it has reached the solid cortical plate, and still the forma- 
 tion of the crown is not quite completed. The six teeth form a
 
 358 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 FIG. 306. Adult dentition. Note the distance from the apices of the incisors to 
 the lower border of the mandible and the floor of the nose. 
 
 FIG. 307. Edentulous jaws, showing loss of alveolar process.
 
 THE GROWTH OF THE JAWS 359 
 
 triangle of which the centrals are the apex, and the cortical plates 
 from cuspid to cuspid the base. The completion of the roots of 
 
 FIGS 308 and 309 were photographed in the same relative size, to show the amount \^ 
 and direction of growth, with the development of the full permanent dentition. 
 
 these teeth will carry the temporary teeth, alveolar process and 
 all, upward, forward, and outward, thus increasing the distance
 
 360 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 from the mental foramen to the symphysis and enlarging the arc 
 of the jaw from cuspid to cuspid. 
 
 FIGS. 310 and 311 were photographed in the same relative size, to show the amount 
 and direction of growth, with the development of the full permanent dentition. 
 
 In the same skull notice the relation of the upper incisors and 
 cuspids to the corresponding temporary teeth. They lie to the 
 lingual of the roots of the temporary teeth, the lateral a little to
 
 THE GROWTH OF THE JAWS 
 
 361 
 
 FKJS. 312 and 313 were photographed in the same relative size, to show the amount 
 and direction of growth, with the development of the full permanent dentition.
 
 362 THE TEETH AND DEVELOPMEXT'OF THE PACK 
 
 FIGS. 314 and 315 were photographed in the same relative size, to show the amount 
 and direction of growth, with the development of the full permanent dentition.
 
 363 
 
 the lingual of the central and cuspid. The cuspid has pushed 
 back the floor of its crypt until it is braced against the solid bone 
 at the base of the malar process. The growth of these teeth will 
 first cause the temporary teeth to move occlusally, the bone grow- 
 ing from the border of the process to follow them. In this growth 
 the distance from cuspid to cuspid is increased and spaces appear 
 between the temporary incisors some time before they are lost. 
 
 If such spaces do not appear, the development is not progress- 
 ing normally, and artificial force should be applied to stimulate 
 bone growth. If this is not done the permanent teeth are sure to 
 come in more or less rotated and out of position. 
 
 In Figs. 301 and 302 the incisors have been pushed off and the 
 permanent ones are beginning to move occlusally. Notice the 
 relation of the floor of the crypt to the floor of the nose, and the 
 root has scarcely begun to develop. In the adult skull (Fig. 306) 
 there is almost as much space from the apex of the root to the floor 
 of the nose as there is now from the border of the alveolar process 
 to the floor of the nose. The result of the growth of the cuspids' 
 roots is shown by comparing Figs. 300 and 301 with Fig. 303. 
 
 The Importance of Proximal Contact. The proper contact of the 
 teeth upon their proximal surfaces is necessary for this develop- 
 ment. If, for instance, the mesial angle of the lower lateral fails 
 to engage with the distal surface of the central, but slips by to the 
 lingual, the growth of the cuspid will push it farther and farther 
 past the central instead of enlarging the arch. One of the cogs in 
 the mechanism has slipped, and the growth of bone cannot later be 
 expected to make room for the crowded teeth. 
 
 In the next stage of growth the increase in size is from the mental 
 foramen to the ramus, and is largely influenced by the development 
 of the roots of the bicuspids and the second molars. Figs. 302 
 and 303 show the relation of the second molar to the distal surface 
 of the first, and it will be seen that its grow T th exerts force upon the 
 first molar, and this is transmitted through the arch by means of 
 proximal contact. Notice the inclination of the bicuspid roots, 
 which help to carry the growth in the same direction. 
 
 After the second molar is in place the growth of the third should 
 exert the same force and room be provided for it (Fig. 304). The 
 muscular action of the lips and tongue are specially important 
 in these last stages of growth, and particularly the forces that are 
 generated by the action of the muscles in respiration and deglutition. 
 The activity of the connective-tissue cells in the bone requires
 
 364 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 mechanical stimuli for their maintenance, and as the muscular 
 action is vigorous or deficient, the growth of bone will be full and 
 normal or imperfect and unbalanced. It appears often that the 
 bone activity becomes so sluggish that the growth of the third 
 molar cannot produce the effect it should, and it remains impacted. 
 A comparison of figures will show that while room has been made 
 for the third molar, all of the upper teeth have moved downward, 
 forward, and outward, and the lower ones upward, forward, and 
 outward. Compare the distance from the apex of the incisor 
 
 FIG. 316. Two years. 
 
 FIG. 317. Three years. 
 
 FIG. 318. Six years. 
 
 FIG. 319. Ten years. 
 
 Maxillae photographed from the median line in the same relative size, to show the 
 amount and direction of growth.
 
 THE GROWTH OF THE JAWS 
 
 365 
 
 FIG. 320. Twelve years. 
 
 FIG. 321. Adult. 
 
 Maxillae photographed from the median line in the same relative size, to show the 
 amount and direction of growth. 
 
 FIG. 322. Bone from the buccal plate of the mandible of a young sheep, showing 
 transformations of bone: 1, subperiosteal bone; 2, Haversian system bone; 3, 
 Haversian system bone becoming cancellous.
 
 366 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 roots to the floor of the nose and the lower border of the mandible 
 in Figs. 305 and 306. 
 
 FIG. 323.- The record in the arrangement of the lamellae of the growth of the man- 
 dibles. A decalcified section from near the lower border of a human mandible. 
 
 This process may be more fully realized by comparing the front 
 views of the skulls (Figs. 308 to 315). They were all photographed
 
 THE GROWTH OF THE JAWS 
 
 367 
 
 with the same lens and bellows length, so as to make the pictures 
 of the same relative size as the skulls. Notice the increase in dis- 
 tance from the floor of the nose and the floor of the orbit to the 
 edges of the upper incisors, and from the lower border of the man- 
 dible to the edge of the lower incisors. It will be seen that if the 
 infant mandible were placed in relation to the adult mandible it 
 would lie entirely within the arch and in the mouth cavity, while 
 
 FiG. 324. A decalcified section from the lingual vertical plate of a human mandible, 
 showing the arrangement of lamellae as a record of growth. 
 
 in the upper the temporary incisors in Fig. 315 would be some place 
 in the nasal cavity. In all of this growth the size of the air spaces 
 increases with the movements of the teeth, the floor of the nose 
 and palate growing downward and developing. This may be 
 shown in Figs. 316 to 321, in which the right half of the maxilla 
 has been removed from dissected skulls and photographed from 
 the median line.
 
 368 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 Tissue Changes in the Physiologic Movements of the- Teeth All 
 that has been said in regard to bone growth must be recalled in 
 
 FIG. 325. Cancellous bone from a decalcified section of a human mandible, showing 
 reconstructions to change the direction of the spicules. 
 
 order to obtain a conception of the manner in which these move- 
 ments of the teeth and the development of the bone are accom- 
 plished. Bone laid down under the periosteum and the peridental
 
 THE GROWTH OF THE JAWS 
 
 369 
 
 membrane has been transformed into Haversian system bone and 
 then made cancellous, as illustrated in Fig. 322, which is taken 
 
 FIG. 326. Decalcified section of cancellous bone from a human mandible, showing 
 absorptions and rebuildings, changing the direction of the spicules. 
 
 from the buccal plate of the mandible of a young sheep. Reversed 
 changes have also been going on, the periosteum cutting into the 
 24
 
 370 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 Haversian bone by absorption and the cancellous bone being con- 
 densed into Haversian system bone. These changes leave a record 
 in the arrangement of the lamellae, and may be studied in decalcified 
 sections (Figs. 323 to 326). Even the direction of the spicules of 
 
 FIG. 327. A longitudinal section through the tip of the alveolar process of a tem- 
 porary tooth about ready to be lost: D, dentin; Cm, cementum, showing absorption 
 and rebuilding; Pd, peridental membrane; B, bone growing occlusally at the border 
 of the process; Hb, rebuilt Haversian system bone. 
 
 cancellous bone are being constantly changed by absorptions and 
 rebuilding to adjust them to changes of stress. 
 
 While the temporary teeth are moving occlusally, bone is laid 
 down under the peridental membrane at the border of the alveolar 
 process, which is at once cut out by absorptions and replaced by
 
 THE GROWTH OF THE JAWS 
 
 371 
 
 Haversian system bone (Fig. 213). The alveolar process becomes 
 a veritable patchwork, as shown in Figs. 327 and 328. The 
 
 FIG. 328. A longitudinal section through the temporary alveolar process, which 
 is growing occlusally to follow the temporary tooth. It is from the same series as 
 Fig. 327, but shows more of the bone. Study the absorptions and rebuildings, as 
 shown in the arrangement and character of the lamella?. Pd, peridental membrane; 
 Po, periosteum.
 
 372 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 permanent tooth developing in its crypt produces conditions of 
 pressure, and osteoclasts appear in all the medullary spaces, around 
 
 and above the crypt, and through the alveolar process, as well as 
 on the crypt wall. They are more active in the medullary spaces,
 
 THE GROWTH OF THE JAWS 
 
 373 
 
 cutting away the spicules of bone, thinning and cutting apart the 
 crypt wall, and allowing it to be bent and pushed back. 
 
 2 5 
 
 Fig. 329 shows the alveolar process on the lingual side of the 
 temporary incisor, and illustrates the enlargement of the medullary 
 spaces preparatory to the eruption of the permanent tooth. Fig.
 
 374 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 330 shows the labial plate of the process, and notice that the bone is 
 being formed under the periosteum and at spots under the peridental 
 membrane, while the substance of the bone is being destroyed. 
 
 a o jj 
 
 When the tooth is finally pushed off from the gum all but a few 
 bits of the alveolar process have been destroyed, and as the per-
 
 THE GROWTH OF THE JAWS 375 
 
 manent tooth comes through, bone formation begins at the border, 
 patching on to the remains of the old process (Fig. 331). r- 
 
 In studying the absorption of bone around the crypt walls, it 
 has been noted that the osteoclasts appear first in the cancellous 
 bone (Figs. 214 and 215), surrounding the crypts and outside of 
 it. Absorptions here remove the spicules which brace the crypt 
 wall, and cut through the wall in such a way as to allow it to be 
 pushed back through the weakened substance. In the same way 
 in the movements of the teeth, absorptions appear first in the 
 spaces outside of the cribriform plates of the alveoli, until the 
 remaining bone is weakened sufficiently to spring under the press- 
 ure. All of the sections of the mandible should be studied as a 
 recoid of these bone transformations, and especially in orthodontia 
 it should be remembered that appliances are used not to push the 
 teeth through the bone as a post would be pushed through the 
 mud, but to supply mechanical stimuli to living cells whose activity 
 will result in bone growth, carrying the teeth into their proper 
 positions, and finally, that teeth will remain only in the position in 
 which all of the forces to which it is subjected are balanced.
 
 PART II. 
 
 DIRECTIONS FOR LABORATORY WORK. 
 
 (TWENTY-FIVE PERIODS IN THE LABORATORY ) 
 
 PRELIMINARY. 
 
 IT is assumed in this work that the student has had a course in 
 general histology, including laboratory work, that he is familiar 
 with the technique of handling the microscope, the technique of 
 staining and mounting sections, and that he is able to recognize 
 at once the elementary tissues. The same outfit is required as for 
 general histology, including slides and blank labels for them; 
 cover-glasses; teasing needles; forceps; section lifter; a tube of 
 balsam; a funnel; pipette; filter paper and lens paper; 6 one-ounce 
 reagent bottles containing xylol, absolute, 95, and 70 per cent, 
 alcohols, hematoxylin, and eosin; at least two chip butter dishes 
 that can be used for staining; a box for the slides; a note-book; a 
 hard and a soft drawing pencil; a good eraser; and a piece of clean 
 soft linen for wiping slides and cover-glasses. 
 
 Teeth for Grinding. It is difficult to obtain satisfactory teeth 
 for the grinding of microscopic sections, and the student should 
 bring to the laboratory a number of suitable teeth from which 
 selection can be made. Old, dry teeth are absolutely useless for 
 the purpose, however perfect their structure may have been. When 
 a tooth has been extracted for some time the tissues dry out, giving 
 up a considerable amount of water, and consequently shrink. 
 The shrinkage of dentin and enamel is unequal, and the result is 
 a cracking of the tissue. The observation of the teeth in any skul 
 will reveal cracks in the enamel that may be seen with the naked 
 eye, the tooth often splitting lengthwise. Besides the cracks that 
 can be seen, the tissue is full of microscopic cracks. When the 
 grinding of sections from such teeth is attempted, before the sec- 
 tion is reduced to sufficient thinness for microscopic observation, 
 
 (377)
 
 378 DIRECTIONS FOR LABORATORY WORK 
 
 the enamel will break to pieces and be lost. A tooth that is to be 
 used for grinding must be placed in solution as soon as it is extracted, 
 and never at any stage of the process be allowed to dry, until ready for 
 mounting. Any solution that will prevent decomposition will do 
 for this purpose. The best that I have found is a 4 per cent, formal- 
 dehyde in 50 per cent, alcohol. This may be roughly prepared by 
 diluting 95 per cent, alcohol with an equal volume of water and 
 adding one part of formalin to nine parts of the diluted alcohol : 
 
 Alcohol 45 c.c. 
 
 Water 45 c.c. 
 
 Formalin 9 c.c. 
 
 This solution not only prevents the drying, but has a hardening 
 action on the organic matter, which facilitates the grinding. Teeth 
 may be preserved in this indefinitely. 
 
 Teeth Required. From his collection the student should select 
 for grinding an incisor or cuspid, a bicuspid, and a molar. The 
 teeth should be free from caries and their crowns as perfect as 
 possible. 
 
 The Relation of the Section to the Crown. The practical value of 
 the study of ground sections depends upon obtaining from them a 
 knowledge of enamel-rod directions in relation to the tooth crown 
 as well as the section. In operating the teeth are looked at from 
 their outside surface, but the operator needs to see in the enamel 
 not simply a hard and extremely dense tissue, but a tissue made up 
 of minute rods whose general direction he knows beforehand. If 
 a tooth is selected and a section cut from it in a know r n position, 
 and the relation of the section to the crown remembered, the direc- 
 tion of enamel rods can be placed in relation to the entire crow r n as 
 well as to the section. This is one of the objects to be sought in 
 the making of the outline drawings. 
 
 Location of the Section. Having selected the teeth for grinding, 
 the next step is to locate the position and direction of the section. 
 This must be so placed as to cut the enamel rods in their length. 
 The section from the incisor or cuspid should be ground labiolin- 
 gually, but the section from the molar and bicuspid may be ground 
 either buccolingually, mesiodistally, or diagonally. The surface 
 of the tooth should be considered, and the section placed in an 
 area in which the student desires to discover the enamel-rod direc- 
 tions and the structure of the tissue. The line of the section should 
 now be marked on the tooth with India ink and a fine pen.
 
 PREPARATION OF GROUND SECTIONS OF TEETH 379 
 
 The Drawings of the Teeth. After marking the position of the 
 section the tooth should be carefully and accurately drawn, showing 
 the position of the section as seen from the axial and occlusal 
 surfaces. 
 
 Grinding of the Section. Every institution should have a machine 
 for the preparation of ground sections, but such a machine is too 
 delicate an instrument to be handled by students. In the appendix 
 will be found a chapter written by Dr. Black describing the grinding 
 machine and the technique of its use. If one is available, the student 
 may have his sections ground for him and returned ready to mount, 
 or he may grind them himself, using the following technique: 
 
 Preparation of Ground Sections of the Teeth. For this work the 
 student should have two large corundum stones not less than four 
 inches in diameter, one of "C" and one of "E" grit. Corundum 
 is very much better than carborundum for this purpose. In grind- 
 ing the stone should be kept revolving slowly and moistened with 
 a stream of water. Holding the tooth against the flat side of the 
 coarse stone with the fingers, the tissues should be rapidly ground 
 away until the position marked for the section is reached, when the 
 fine stone should be substituted and the grinding continued just 
 enough to remove the scratches. The surface should now be 
 polished on the Arkansas stone until a very perfect surface has 
 been obtained. Wash the specimen clean and immerse in several 
 changes of 95 per cent, alcohol, and leave in absolute alcohol in 
 a closed bottle for several hours or over night. Harden a drop of 
 balsam on the center of a clean slide by warming it over a Bunsen 
 burner to evaporate the xylol. When the slide is cool the balsam 
 should be neither sticky nor brittle. Now remove the tooth from 
 the alcohol, wipe it dry, and, placing it on the balsam with the 
 polished surface next to the glass, gently warm the slide until the 
 balsam is thoroughly softened, and press the tooth down against 
 the glass and clamp it firmly in position, using a spring clip. Set 
 it away to harden thoroughly, when the grinding may be continued. 
 
 Holding the slide parallel with the surface of the coarse stone, 
 the tissues may be rapidly removed until the section is about as 
 thin as a calling card, when the fine stone should be substituted 
 and the section reduced to the required thinness. It should not be 
 more than twenty microns in thickness. In the final stages progress 
 of the grinding may be followed with a hand magnifying glass. 
 Finally the surface should be polished on an Arkansas stone. The 
 specimen should now be washed with alcohol, the balsam removed
 
 380 DIRECTIONS FOR LABORATORY WORK 
 
 with xylol, and brought to the laboratory in 95 per cent, alcohol, 
 where it is to be etched and mounted according to the directions. 
 
 Every step in the above technique is important and must be 
 followed with minute care and accuracy. Not least important is 
 the cleaning of the slide. It sometimes happens that the section 
 will be loosened from the glass before the grinding is completed. 
 This is usually due to some fault in the technique. When it happens 
 it is best to finish the grinding without attempting to refasten the 
 section to the slide. To do this the section should be held against 
 the flat side of the stone, using a fine-grained cork, a piece of box- 
 wood, or some similar material. The danger of breaking the sec- 
 tion, however, is much greater. 
 
 The Preparation of Transverse Sections of the Root. For this 
 purpose one of the flattened roots furnishes the best material, as, 
 for instance, the mesial root of a lower molar, the root of a lower 
 bicuspid, or of an upper second bicuspid. Holding the root in a 
 vise by the remains of the crown, with a metal saw, saw off the 
 tip of the root, removing an eighth of an inch or less. Then saw off 
 as thin a slice as possible. In the same way saw out at least two 
 other sections, one from the gingival and one from the middle third 
 of the root. These should be dropped into a bottle of formalin- 
 alcohol until the grinding is completed. The grinding is easily 
 accomplished on the flat side of the corundum stone, holding the 
 section on the finger or under a cork. The last grinding should be 
 done on the fine Arkansas stone. 
 
 Transverse sections of the root are easily ground and can be 
 made very thin. 
 
 Manner of Working in the Laboratory. In no place in the world 
 can time be wasted more easily than in the histological laboratory. 
 The student should take the attitude of an original investigator 
 and study out the material for himself as far as possible, remember- 
 ing that he has a far better opportunity than the man who worked 
 out the details of these structures. He must constantly try to 
 picture the structure, and imagine how it would appear if sectioned 
 in another direction. 
 
 Drawings. Drawings from the microscope are made not simply 
 to occupy the student's time, nor as a record of what he has done, 
 but to make observation more accurate and detailed, and to fix 
 the impressions of structure more perfectly in mind. Many stu- 
 dents excuse themselves for careless and slovenly work by saying 
 that they are not artists. Anyone without any knowledge of the
 
 USE OF DIRECTIONS FOR LABORATORY WORK 381 
 
 principles of art can in a very short time acquire the ability to make 
 excellent microscopic drawings. A few principles of procedure will 
 help greatly. The first of all is that a light line can always be 
 made darker, therefore the drawing should always be kept light 
 until the later stages. 
 
 After selecting a field, draw lightly the outline of the principal 
 masses and then the outlines of the smaller ones. In this way the 
 proportion of objects in the field and their relation to each other 
 can be maintained. Never draw any detail such as individual 
 cells, nuclei, etc., until all of the outlines are completed. Then 
 work in the details in the darker colored areas. The making of 
 the outlines is by far the most important stage in the drawings. 
 
 Each outfit should contain a 6 H and an H-B pencil and a good 
 eraser, which must be kept clean. The pencils should be kept 
 sharp and always used with a light touch upon the paper. The 
 beginner always tends to start his drawing by making a circle. 
 This should be avoided, for it is objects that are being studied, 
 not fields, and in many cases the object cannot be bounded by a 
 circle. There is also a tendency to represent the object smaller 
 on the paper than it appears in the field. 
 
 The prime qualities in a microscopic drawing are accuracy and 
 correctness of detail. The drawings are made to show all the detail 
 of structure that can be observed. It often happens that a draw- 
 ing that looks very well shows very little knowledge of the structure 
 of the tissue which it represents. 
 
 Stencilled Laboratory Notes. In fifteen years of teaching the 
 author has found stencilled notes on the daily work in the labora- 
 tory of very great assistance. There are always variations in the 
 appearance of the material which cannot be anticipated before 
 the sections are cut. Very often something will be seen unusually 
 well that would not be mentioned in the text-book. Different 
 stains may have been used which would change the appearance of 
 the tissues, and for all of these things and many others daily notes 
 are very convenient. 
 
 USE OF DIRECTIONS FOR LABORATORY WORK. 
 
 At the beginning of the laboratory period the first thing to be 
 done is to read through the directions for the day's work. The amount 
 of work for the day is then clearly in mind, and all the steps in any
 
 382 DIRECTIONS FOR LABORATORY WORK 
 
 procedure that is to be undertaken are .understood at the begin- 
 ning. It is necessary to divide the time available, so as to accom- 
 plish the work indicated for the day. 
 
 PERIOD I. 
 
 Drawings of Tooth Surfaces Showing the Position of Sections. 
 
 The object of these drawings is to show the relation of the section 
 to the crown from which it is ground, so that in studying the 
 enamel-rod directions as seen in the sections, they may be referred 
 to the entire crown. The drawings should be made from five to 
 ten times natural size, and must be made accurately to scale (Fig. 
 332). Measure the length and the breadth of the tooth and lay 
 out a rectangle, say eight times these dimensions, to serve as a 
 guide in drawing. If the tooth is marked for a buccolingual section, 
 stick the apex of the root on a bit of wax and place the tooth on 
 the table with the buccal surface toward you. Do not change its 
 position until the drawing is completed, for to do so would change 
 lights and shadows. After getting the outline accurately, work 
 in the shadows so as to give the drawing roundness. Remember 
 in doing this that you can always make it darker, but you cannot 
 erase without injuring the neatness of the drawing. When the 
 drawings are completed the section is ready for grinding, which 
 must be done outside of the laboratory, following the directions 
 in Introduction to Part II. 
 
 PERIOD H. 
 
 Etching and Mounting of Ground Sections. At the desk will be 
 found 1 per cent, hydrochloric acid, dilute ammonia, and vaseline, 
 which are the only reagents not included in the outfit and required 
 for this work. The sections are brought to the laboratory ground 
 and ready to mount. Fill one of the dishes with water and carefully 
 wash the specimen free from all debris of grinding. Dry the sec- 
 tion between filter papers, so as to remove all moisture from the 
 surface. Fill one dish with 1 per cent, hydrochloric acid, and the 
 other with dilute ammonia. Put a very little vaseline upon the 
 tip of the finger, and holding the section by the root portion, cover 
 one surface of the crown portion with a very thin layer of it. In 
 doing this the vaseline should be wiped from the center toward 
 the edges of the section, so as to prevent it from running over on
 
 ETCHING AND MOUNTING OF GROUND SECTIONS 383 
 
 to the other surface. The vaseline is to confine the action of the 
 acid to one surface of the enamel. Holding the section by the 
 root portion, immerse the crown in the dilute acid for thirty seconds, 
 
 DISTAL SURFACE 
 
 BUCCAL 
 
 OCCLUSAL SURFACE 
 BUCCAL. MARGIN 
 
 DISTAL M. 
 
 MESIAL M. 
 
 LING 
 
 LINGUAL M. 
 
 8 DIAMETERS 
 
 FIG. 332. Drawing of occlusal and axial surfaces of a tooth to show the relation 
 of the section to the tooth. (Drawn by W. A. Offil, 1910.) 
 
 or until minute bubbles can be seen forming upon the surface. 
 Remove and immerse at once in the dilute ammonia for a minute. 
 Remove the vaseline by carefully wiping the section with absolute 
 alcohol or ether, and immerse in 95 per cent, alcohol. In this it
 
 384 DIRECTIONS FOR LABORATORY WORK 
 
 should remain while the slide and cover-glass are being prepared. 
 Obtain from the desk a cover-glass long enough to cover the entire 
 section and carefully clean both slide and cover-glass. On the 
 center of the slide place a drop of balsam that is as long as the 
 section. Holding the slide over a Bunsen burner or alcohol flame, 
 warm it gently so as to evaporate the xylol. In this process the 
 drop will spread out over the slide and the direction of spreading 
 may be guided by the heat. Allow the slide to cool and test the 
 hardness of the balsam with a teasing needle or the finger-nail. 
 When cold the balsam should be just soft enough to take the 
 imprint of the needle or nail, but not be sticky. If it is sticky it 
 must be reheated; if, on the other hand, it is brittle enough to 
 chip, it must be scraped off from the slide and the process tried 
 again. In the same way prepare a film of balsam on the cover- 
 glass. Remove the section from the 95 per cent, alcohol and dry 
 it for a few minutes in the air (after wiping with filter paper). 
 Place the section, etched side up, upon the balsam on the slide, 
 and place the cover-glass on it, balsam side down. Warm the slide 
 gently over the flame, while pressing the cover-glass down with 
 the handle of a teasing needle. As the balsam is w r armed, the 
 slide and cover-glass are brought together, forcing the balsam 
 out to the edge of the cover-glass in all directions. All excessive 
 balsam should be squeezed out at the edges. Place on the cover- 
 glass a small piece of blotting paper or a layer of cork, adjust some 
 sort of a spring clip and put the section away until the balsam is 
 entirely hard. When the balsam is entirely hard the excess may be 
 removed by gently scraping with a knife-blade and wiping with 
 xylol. The section should now be labelled w r ith the name of the 
 tooth, the direction and position of the section, the student's name 
 and number, and the date. 
 
 The mounting in hard balsam greatly improves the value of the 
 section, for the dentinal tubules and the lacunae of the cementum 
 are left filled with air and can be more easily studied. Sections 
 may, ho\vever, be mounted in the ordinary way, in soft balsam. 
 If the section is broken or extremely thin, soft balsam should be 
 used. 
 
 PERIOD HI. 
 
 Outline Drawings from Ground Sections. The object of the out- 
 line drawing is the study of the dental tissues, their distribution, 
 portion of the tooth formed by each, their relation to each other,
 
 OUTLINE DRAWINGS FROM GROUND SECTIONS 385 
 
 and the coarser points of their structure. To get the value from 
 this work the drawings must be made very accurately to scale and 
 as large as the note-book page will allow. With the Boley gauge 
 or a millimeter rule measure accurately the length of the section, 
 multiply this by eight or ten, and mark the length on a page of the 
 drawing book. Measure the width of the section at the point of 
 the greatest diameter and multiply this by the same factor. Using 
 this for the width and the previous measurement for the length, 
 lightly draw a rectangle, which is to be used as a guide in the con- 
 struction of the drawing. The success of the drawing now depends 
 on the accuracy and number of the measurements. 
 
 First measure the vertical distance from the incisal edge to the 
 gingival line on one side of the section, and then on the other, and 
 mark these on the sides of the rectangle. This will give the rela- 
 tive length of root and crown and the difference, if any, in position 
 of the gingival line on the two sides. Measure the vertical dis- 
 tance from the most prominent point on the axial surface to the 
 incisal edge or the tips of the cusps, and so on, making every 
 measurement that can help in the formation of the drawing. In 
 this way the outline of the section should first be traced inside the 
 rectangle, then the dento-enamel junction, then the pulp chamber 
 is shown, and finally the cementum. Before drawing the outline 
 of the cementum, the section should be placed under the micro- 
 scope, using the low power, and the cementum should be observed, 
 studying it from the gingival line on one side of the section to the 
 gingival line on the other. 
 
 It would be a waste of time to attempt to fill in the structure of 
 the tissue of the entire outline, and only certain things are to be 
 shown in these drawings. For that reason fill in three portions of 
 enamel and dentin and three portions of cementum and dentin, 
 using the low-power objective. Study first the bands of Retzius 
 (page 68), and lightly indicate their direction. Study the enamel- 
 rod direction, beginning at the gingival line at one side and follow- 
 ing it around the crown to the other side. In a portion at the 
 incisal edge, or on the occlusal surface, indicate the rod directions, 
 and in the same way show them in a portion near the center of 
 the axial surface on one side and near the gingival line. Follow 
 the dentinal tubules which end next to the portions of enamel 
 which have been filled in to the point where they open into the 
 pulp chamber, and indicate their direction (page 139). In the same 
 way fill in three portions of the cementum and the dentin under 
 25
 
 386 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 them one in the gingival line, one near the middle of the root, 
 and one in the region of the apex (Fig. 333). 
 
 ENAMEL 
 --.ENAMEL RODS 
 
 '-DENTINE (TUBULES} 
 
 -PULP CHAMBER 
 
 CEMENTUM 
 
 FIG. 333. Outline drawing of longitudinal section, made as a study of the dental 
 tissue. (Drawn by E. J. Schmidt.) 
 
 If any portion of the section has been lost in grinding, that 
 portion should be indicated by dotted lines, and in the same way, 
 if a portion of the crown has been lost by wear, the original form 
 may be added in dotted lines.
 
 MINUTE STUDY &F .ENAMEL AND DENTIN 387 
 
 Outline drawings should be made from each, of the three classes 
 of teeth one from the incisor or cuspid, one from a bicuspid, 
 and one from a molar, and a laboratory period should be devoted 
 to each drawing. 
 
 PERIOD IV. 
 
 Isolated Enamel Rods. Obtain from the desk a fragment of 
 enamel which has been broken in the direction of the rods. Place 
 a drop of distilled water or glycerin on the center of a clean slide. 
 Moisten the broken surface with a drop of water and lightly scrape 
 it with the blade of a broad, sharp, chisel, holding the edge parallel 
 with the surface and the shaft at right angles to it. Dip the edge 
 of the chisel in the drop of liquid on the slide, and the scrapings 
 will be left. Cover with a cover-glass and study with the high 
 power, using a small diaphragm. Fragments of enamel will be 
 found made up of broken rods, some single and others in groups. 
 Note the diameter of the rods and the appearance of the cross- 
 markings, which will be seen if the light is properly adjusted. 
 Draw as seen with the high power. 
 
 Repeat this operation, using enamel that has been immersed in 
 1 per cent, hydrochloric acid for a number of hours. Compare 
 the appearance of the rods with those of the former specimen and 
 make a drawing as seen with the high power. 
 
 Find an old tooth with a large carious cavity, remove the softened 
 dentin without touching the enamel if possible. Lightly scrape 
 the whitened inner surface of the enamel next to the cavity and 
 mount the scrapings as before. Compare the appearance of these 
 rods isolated by the action of caries with those of the previous 
 specimen. Notice that the cross-markings are more distinct and 
 the expansions and constrictions of the rods more prominent. 
 Draw a few of the rods as seen with the high power, using the small 
 diaphragm. 
 
 PERIOD V. 
 
 Minute Study of the Enamel and Dentin. Select a field from 
 one of the ground sections where the specimen is very thin, and, 
 if possible, where the entire thickness of the enamel plate can be 
 seen in one field with the f objective. To select this field all of the 
 enamel in the three sections should be carefully studied with the 
 low power, and the one chosen in which the rods can be seen best 
 and can be most easily drawn. Having selected the field, study
 
 388 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 the enamel with the high power, beginning at the dento-enamel 
 junction. Note the form of the dento-enamel junction and the 
 relation of the two tissues at this point. Note the diameter of the 
 
 DENTO-ENAMEL JCT 
 
 FIG. 334. High-power drawing of the enamel. (Drawn by A. B. Hopper, 
 
 1902-03.) 
 
 enamel rods and estimate it, using a red blood corpuscle as a stand- 
 ard of measurement. Note the striation of the enamel (page 67). 
 Using both the low and the high power, draw as accurately as
 
 MINUTE STUDY OF CEMENTUM AND DENTIN 389 
 
 possible the enamel from the surface to the dento-enamel junction, 
 showing all the details of structure that can be made out. 
 
 The drawing should be made as long as the page will allow, and 
 need not be more than an inch wide, and should include just enough 
 of the dentin to show the dento-enamel junction and the character 
 of the dentin at that point (Fig. 334). Notice the diameter of the 
 dentinal tubules, comparing them with the red blood corpuscles 
 and the enamel rods. Note the amount of matrix that separates 
 the tubules. Observe the forking and the anastomosis of the 
 tubules as they approach the enamel, and follow them as far as 
 possible. 
 
 PERIOD VI. 
 
 Minute Study of the Cementum and Dentin. With the low power 
 study the cementum in the three specimens, looking for all the 
 details of structure that can be made out (see page 154). In the 
 gingival portions and often well toward the apex, especially if the 
 tooth is from a young person, the cementum will be very thin and 
 almost structureless in appearance. With the high power, fine 
 lines parallel with the surface may be seen, which indicate the 
 lamellae. In the apical portion the cementum becomes much 
 thicker, and it will be seen that each layer is thicker and conse- 
 quently more easily seen. Little black spots looking like spiders 
 will be found in larger or smaller numbers. These are the lacunae 
 with the canaliculi radiating from them. They were filled in life 
 by cement corpuscles. Look for embedded fibers of the peridental 
 membrane. In all of this work each field should be studied with 
 both the low and the high power. 
 
 The inner layer of the cementum next to the dentin is clear and 
 structureless, and the dentin adjoining it appears with the low 
 power as a granular layer known as "the granular layer of Tomes." 
 Studied with the high power, the appearance will be seen to be 
 caused by irregular spaces in the dentin matrix communicating 
 with the dentinal tubules and filled in life with protoplasm of the 
 fibrils. Compare the dentin in the root with that in the crown 
 (page 143). 
 
 After studying all the cementum in the three sections, select 
 three fields, one from the gingival, one from the middle, and one 
 from the apical portion of the root, and draw the tissues from the 
 surface of the root to the pulp chamber. Show all the details of 
 structure that can be made out with both low and high powers
 
 390 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 (Fig. 335). With the high power search the cementum for the 
 record of absorptions which have been refilled by cementum. 
 
 Mti>\ 
 
 --CEMENTUM 
 
 f DENTINE 
 
 GRANULAR LAYER 
 OF TOMES 
 
 PULP CHAMBER 
 
 FIG. 335. Cementum and dentin. (Drawn by H. J. Lund and A. E. Hopper.) 
 
 PERIOD VII. 
 
 Drawings of Typical Cavity Walls. From the molar or bicuspid 
 section select a field in the region of a groove or pit. Imagine a 
 cavity to be prepared in this position. To help in this, an ink 
 line may be made on the cover-glass by using a fine pen and India
 
 STUDY OF SECONDARY DENTIN AND CEMENTUM 391 
 
 ink, or ordinary ink to which a little sugar has been added. Now, 
 using both the high and the low power, study the direction of the 
 enamel rods as they appear in the line of the cavity wall, and make 
 a drawing showing the structural requirements for a good wall in 
 this position. From any one of the three sections select a field 
 in the gingival third of the labial or buccal surface and indicate 
 the line of a cavity wall in the same way. Study with the low and 
 the high powers the direction of the enamel rods as they appear 
 in the line of the walls of the cavity, and make a drawing showing 
 the structural requirements for good walls in these positions (page 
 107). 
 
 PERIOD vm. 
 
 Outline Drawings from Transverse Sections of the Root. The 
 ground sections of the root have been prepared and should be 
 brought to the laboratory in solution, ready for mounting. The 
 three sections should be mounted together under one cover-glass, 
 using balsam about the consistence of molasses. The sections may 
 be studied at once, but after the day's w r ork upon them they should 
 have a spring clip adjusted to the cover-glass and be put aw^ay 
 until the balsam is thoroughly hard, otherwise they may work 
 out to the edge of the cover-glass. With the millimeter gauge 
 measure the length and breadth of each section, multiply the 
 measurements by twenty, and lay off a rectangle as in making the 
 longitudinal drawings. Draw the outline of the section and the 
 pulp chamber as accurately as possible before studying the section 
 with the microscope. With the low power follow the dento- 
 cemental junction around each section and draw it into the outline. 
 Fill in half of each section, showing the direction of the dentinal 
 tubules, the position and character of the granular layer of Tomes, 
 the number and positions of the lacunse, and the other structural 
 characteristics of the cementum. In this study the record of the 
 reduction of size of the pulp chamber which may be noted by changes 
 in the direction and the character of the dentinal tubules (page 
 152). Label the section with the name of the root from which it 
 was ground, your name, and the date. 
 
 PERIOD IX. 
 
 Study of Secondary Dentin and Cementum. With the low power 
 find a field where there is a distinct demarcation between dentin
 
 392 DIRECTIONS FOR LABORATORY WORK 
 
 of earlier and later formation, and draw it accurately with the 
 high power. Compare the size of the tubules, their number, their 
 direction, and their diameter in the earlier with the later formed 
 dentin. Is there any connection between the tubules of the two 
 portions? Find a similar field from a longitudinal section and 
 study in the same way, making an accurate drawing. 
 
 Search all of the ground sections with the low power until a field 
 is found where the dentinal tubules are cut transversely. Adjust 
 the high-power objective and study the field. Notice that by 
 focussing up and down with the fine adjustment the tubules seem 
 to move in a circle, showing the spiral course through the matrix. 
 Using a red blood corpuscle as a standard, note the size of the 
 subules, their distribution in the matrix, and the amount of matrix 
 teparating them. Look for the appearance of Newman's sheath, 
 which is that portion of the matrix forming the immediate wall 
 of the tubule. Draw accurately one field as seen with the high 
 power. Study the cementum from all the ground sections for an 
 area showing absorption and rebuilding, and if found, draw one 
 field with the high power. Draw five or six lacunae with their 
 canaliculi as seen with the high power, selecting as great a variety 
 of forms as possible. 
 
 PERIOD X. 
 
 Ground Sections of Bone. From a shaft of a femur or humerus 
 saw a disk about one-quarter of an inch thick. In doing this 
 notice the appearance of the marrow cavity especially as you look 
 into it toward the articular ends. Saw the disk into sectors with 
 an arc of about a quarter of an inch on the outer surface. From 
 this piece saw two thin slices one at right angles to the axis of 
 the bone, the other parallel with it. These should be ground as 
 directed in the introduction for the grinding of transverse sections 
 of the root, and be brought to the laboratory ready to mount. 
 They should be mounted in hard balsam as described in the mount- 
 ing of longitudinal sections of the teeth. Label the slide with the 
 name of the bone from which the section is taken and the direction 
 in which it is cut. Study the transverse section with the low 
 power, working out the arrangement of the lamellse and the dis- 
 tribution of the subperiosteal and Haversian system bone (p. 
 213). Draw the tissue from the surface of the bone to the marrow 
 cavity. This drawing should be not more than an inch wide and
 
 STUDY OF SUBPERIOSTEAL BONE AND CEMENTUM 393 
 
 the full length of the page. With the high-power objective draw 
 one or two Haversian systems. 
 
 Study the arrangement of the Haversian canals as seen in the 
 longitudinal sections. With the high power draw at least three 
 lacunae, showing one cut lengthwise, one transversely, and one as 
 seen from above. 
 
 PERIOD XI. 
 
 Decalcified Bone. One of the bones from a small animal has 
 been decalcified, embedded, sectioned, and stained with hematoxylin 
 and eosin. Receive from the desk two sections, one of which is 
 cut longitudinally, the other transversely. Mount in balsam in 
 the usual way. Label the slide with the name of the animal, the 
 bone from which it is cut, and the direction of the section. Study 
 the transverse section with the low power, noting the bone cor- 
 puscles in the lacunae, the tissue in the Haversian canals, and the 
 marrow. With the high power draw one field showing two or three 
 Haversian systems, one of which has been partially destroyed in 
 the building of another. Draw with the high power one field from 
 the marrow cavity. From the longitudinal section draw, with the 
 high power, one field showing osteoblasts in a medullary space. 
 
 PERIOD XH. 
 
 Comparative Study of Subperiosteal Bone and Cementum. For 
 
 this day's work the previously mounted sections must be used, 
 the longitudinal sections of the teeth, the transverse sections of 
 the root, the ground and decalcified sections of bone. Study the 
 cementum and the subperiosteal bone as shown in these sections 
 and make one drawing of cementum and one drawing of sub- 
 periosteal bone to show the comparison in structure. Compare 
 the regularity in form and arrangement of the lacunae in the bone 
 with the irregularity in form and position of the lacunas in cemen- 
 tum. Note that in the bone the lacunae lie between the layers; 
 in the cementum they may be between the layers or entirely within 
 a single layer. Compare the regularity in the arrangement and 
 thickness of layers with the corresponding irregularity in cementum. 
 Note the size, number and arrangement of the canaliculi radiating 
 from the lacunae in bone, and compare them with the canaliculi 
 of the cementum.
 
 394 DIRECTIONS FOR LABORATORY WORK 
 
 PERIOD XHI. 
 
 Dental Pulp from the Unerupted Tooth of a Sheep. An unerupted 
 molar or premolar of a yearling lamb was removed from the lower 
 jaw by splitting the bone. The pulp was pulled out of the partially- 
 formed dentin embedded in paraffin, sectioned, stained with hema- 
 toxylin and eosin. Bring to the desk a clean slide with a drop of 
 balsam upon the center of it and receive a section. Label the 
 slide: "Pulp from unerupted tooth of sheep, stained with hema- 
 toxylin and eosin." Study first with the low power. Upon the 
 circumference of the section the layer of odontoblasts may or may 
 not be shown, depending upon whether in the removal of the pulp 
 the fibrils have pulled away from the dentin, or the odontoblasts 
 have been pulled off from the surface of the pulp. They are usually 
 present, at least in spots. Note the number and arrangement of 
 the bloodvessels and the distribution of the connective-tissue 
 cells. With the low power draw a portion from the surface to the 
 center, showing the layer of odontoblasts, if present. With the 
 high power draw one field showing a bloodvessel and the connec- 
 tive-tissue cells, taking particular pains to represent their forms 
 correctly. If there are any odontoblasts present draw one field 
 showing them and the layer of Weil (see page 167). 
 
 PERIOD XIV. 
 
 Dental Pulp, Normal Human. A number of human teeth were 
 cracked immediately after extraction and the pulps removed from 
 the pulp chambers. They were embedded in one block of paraffin, 
 sectioned, stained with hematoxylin and eosin, and are ready to 
 be given out. Bring to the desk a clean slide with a drop of balsam 
 on the center and receive a section. Label the slide: "Trans- 
 verse section of pulp from human teeth." There will be several 
 sections in this specimen, each from a separate pulp. With the 
 low power follow the circumference of each section, looking for 
 places where odontoblasts are present. Find the best field in the 
 specimen and draw the layer of odontoblasts as seen with the 
 high power. Notice the fibrils which have been pulled out of the 
 dentinal tubules projecting from the ends of the odontoblasts. 
 If the section is parallel with the long axis of the cells, they will 
 appear as tall columnar cells with a nucleus in the deeper end. If 
 it is oblique to their axis the layer may appear as two or three
 
 ENDOCHONDRAL BONE FORMATION 395 
 
 layers of oval cells. Just beyond the odontoblasts the layer of 
 Weil will be seen, usually appearing as a clearer layer containing 
 few cells and about half as wide as the odontoblasts. Beyond this 
 the connective-tissue cells are thickly placed for a short distance, 
 and still deeper they are more widely scattered and about evenly 
 distributed in the rest of the pulp. 
 
 With the high power draw one field to show the form of the 
 connective-tissue cells of the pulp. With the low power study the 
 distribution of the bloodvessels in all of the sections. Select the 
 best section and draw the entire section to show the size, number, 
 and arrangement of the large bloodvessels. With the high power 
 draw a single field to show accurately the structure of a bloodvessel 
 wall. 
 
 PERIOD XV. 
 
 Dental Pulp, Pathologic Human. By the cooperation of the man 
 in charge of the extracting room, or an extracting specialist, teeth 
 with living but inflamed or hyperemic pulps were dropped as 
 soon as extracted into a fixing fluid. The teeth w r ere afterward 
 cracked and the pulps removed, embedded, and sectioned as 
 before. Bring to the desk a clean slide with a drop of balsam on 
 its center and receive a section. Label the slide: "Pathologic 
 pulp from human tooth stained with hematoxylin and eosin." 
 Follow the same routine in studying these specimens as in the 
 case of the normal pulp. It is impossible to tell just what condi- 
 tions will be present. Compare the size and number of the blood- 
 vessels with those in the normal tissue, and the character and 
 distribution of the cellular elements. Look for nodules of calco- 
 globuli, especially in the inflammatory specimens, and make a 
 diagnosis of the condition, as show r n in the specimen. See the 
 chapter on the Structural Changes in the Pulp and Pathological 
 Conditions for further assistance on the work in this material. 
 
 PERIOD XVI. 
 
 Endochondral Bone Formation. A forming bone from a human 
 fetus has been embedded, sectioned, and stained with hematoxylin 
 and eosin. Receive a section from the desk and mount as usual. 
 Study the specimen with the low power, identifying first the gen- 
 eral arrangement of the tissues, following from the unchanged
 
 396 DIRECTIONS FOR LABORATORY WORK 
 
 cartilage to the development of bone. Notice the subperiosteal 
 layers on the surface. Make a sketch of a sufficient part of the 
 section to show the changes from the typical hyaline cartilage to 
 the young bone. With the high power draw one field from a primary 
 marrow cavity, showing osteoblasts laying down lamellae on one 
 of the spicules, and one field showing osteoclasts. 
 
 PERIOD XVH. 
 
 Bone Growth. A piece of a long bone from a very young animal 
 has been embedded and sectioned transversely to the shaft. Sec- 
 tions have been stained in hematoxylin and eosin, to be mounted 
 as usual. Label the slide: "Growing bone cut transversely, 
 stained with hematoxylin and eosin." Study first with the low 
 power. On the surface of the section will be seen the periosteum, 
 in which the fibrous and osteogenetic layers can be easily recog- 
 nized. Bone formation is actively going on, laying down lamellae 
 under the periosteum which are being transformed into Haversian 
 system bone. With the low power draw a portion of the section 
 from the periosteum to the center of the bone. With the high 
 power draw a field showing the osteoblasts of the periosteum, a 
 field showing the absorption of subperiosteal bone to form a medul- 
 lary space, and a field showing osteoblasts in a medullary space. 
 
 PERIOD xvm. 
 
 Periosteum from Attached Portion. From a young kitten a por- 
 tion of a bone in a region to which muscles are attached to the 
 periosteum was carefully dissected out, removing the attached 
 muscle, and the tissue embedded in celloidin, the sections cut 
 parallel to the axis of the bone and perpendicular to its surface. 
 They have been stained in hematoxylin and eosin, and are ready 
 to be given out. Receive a section and mount as usual. Label: 
 "Periosteum from attached portion, stained in hematoxylin and 
 eosin." Study the specimen first with the low power. The outer 
 fibrous layer of the periosteum will be seen with the muscle fibers 
 attached to it and the osteogenetic layer with the greater number 
 of cells taking the stain more deeply. Draw with the low power, 
 showing the tissues from the surface of the periosteum well into 
 the substance of the bone. With the high power study the attach-
 
 GINGIVUS AND GUM TISSUE 397 
 
 ment of the muscle fibers to the outer layer of the periosteum, 
 the character and arrangement of the fibers of the outer layer, 
 the interlacing of the fibers of the outer and inner layer, the cells, 
 and especially the osteoblasts of the inner layer and the penetrating 
 fibers that are built into the bone. Draw the thickness of the 
 periosteum as seen with the high power, showing the details of 
 structure as accurately as possible. 
 
 PERIOD XIX. 
 
 Gingivus and Gum Tissue. The gingivus and gum tissue covering 
 the alveolar process down to the point of reflection on to the cheek 
 was dissected away from the teeth and jaw of a sheep. The tissue 
 was embedded in paraffin and sectioned parallel with the long axis 
 of the tooth. The sections have been stained with hematoxylin 
 and Van Gieson, and are ready to mount. Bring to the desk a 
 clean slide with a drop of balsam on the center and receive a speci- 
 men. Label the section: "Gingivus from a sheep, stained with 
 hematoxylin and Van Gieson." By this staining the cellular 
 elements will have a brownish color, the nuclei dark, the protoplasm 
 lighter, the white fibers should be bright red, and the elastic fibers 
 yellowish. It is a specially good stain for connective tissue. Study 
 with the low power. The epithelium will be stained a brownish 
 yellow or purple. It is a stratified squamous epithelium made up 
 of many layers of cells and with a distinct horny or corneous layer 
 on the surface from the crest of the gingivus to the point where 
 the mucous membrane is reflected on to the cheek, or where it 
 ceases to be attached to the gum. This layer is yellowish in color, 
 and is made up of closely packed scales having no nuclei. They 
 are the remains of epithelial cells from which the protoplasm is 
 gone, leaving only the horny material which it had produced. The 
 portion of the epithelial lining the gingival space has no corneous 
 layer, nuclei being seen in the cells at the surface. The cells are 
 larger and more loosely placed. The connective-tissue papillae 
 and the projections of epithelium which are between them are 
 extremely long. In the epithelium covering the alveolar process 
 the connective-tissue papillae are broader and not so deep, and the 
 cells are much more compactly arranged. At the point of reflec- 
 tion on the cheek the epithelium changes its character abruptly, 
 the corneous layer disappears, the surface cells showing nuclei,
 
 398 DIRECTIONS FOR LABORATORY WORK 
 
 the epithelial layer is thicker and made up of larger and more 
 loosely placed cells. This change in the structure explains why 
 the epithelium is easily broken where a movable portion of the 
 membrane passes over the edge of an artificial denture. When 
 an infection reaches the connective tissue a sore is produced that 
 requires some time to heal. 
 
 Study the connective tissue, which is made up of coarse, wavy 
 bundles of white fibers taking the red stain. In the gum tissue, 
 that is, the portion of the section covering the alveolar process, 
 the bundles are very large and form a very coarse network. Beyond 
 the point of reflection the bundles are finer and more delicate 
 in their arrangement. Elastic fibers take the yellowish stain. 
 Notice the bloodvessels in the connective tissue and the capillaries 
 in the papillae. With the low power draw the entire section so as 
 to show the character of the epithelium and the fibrous tissue in 
 the three parts. 
 
 With the high power draw the thickness of the epithelium lining 
 the gingival space and at the point where the membrane is reflected 
 to the cheek. 
 
 PERIOD XX. 
 
 Peridental Membrane, Transverse Gingival. The lower jaw of a 
 young sheep was sawed through between the teeth, cutting the 
 jaw into blocks each containing two teeth. The crowns were broken 
 off or opened so as to admit the fluids to the pulp tissue. The tissues 
 were decalcified, embedded, and sectioned at right angles to the 
 axis of the tooth. They are cut from the gingival portion, and have 
 been stained with hematoxylin and eosin. Receive a section and 
 mount as usual. Label the slide: "Peridental membrane, trans- 
 verse gingival, stained with hematoxylin and eosin." A similar 
 block of tissue preserved in alcohol will be found at the desk. This 
 should be observed so as to study out the relation of the section 
 to the gross appearance of the tissue. 
 
 Holding the section to the light, observe the distribution of the 
 tissue. Two roots will be seen cut across. Observe the epithelium 
 on the labial and the lingual, and possibly also that lining the 
 gingival space lying next to the root of one of the teeth. By the 
 aid of the low power sketch the outline of the entire section to 
 show the distribution of the tissues. Note the demarcation where 
 the finer fibers of the peridental membrane unite with the coarser
 
 PERIDENTAL MEMBRANE 399 
 
 mat of gum tissue. Beginning at the center of the labial surface, 
 follow the fibers springing from the cementum to where they are 
 lost in the gum tissue or attached to the approximating tooth. 
 Draw the portion of the membrane between the two roots, accu- 
 rately representing the arrangement of the fibers. The epithelial 
 structures will be seen lying between the fibers close to the cemen- 
 tum, and should be shown in the drawing (p. 248). 
 
 With the high power study the cementoblasts and the epithelial 
 structures. Make a drawing of one field, showing all the details 
 of structure as accurately as possible. 
 
 With the high power draw one field showing the fibrous tissue 
 between the roots and the relation of the fibroblasts to them. 
 This field should include a bloodvessel. 
 
 PERIOD XXI. 
 
 Peridental Membrane, Alveolar Portion, Transverse. The sections 
 for this work have been cut from the same block as the preceding, 
 but are in the occlusal third of the alveolar portion and as close 
 to the border of the alveolar process as possible. Receive a section. 
 Mount as usual and label the slide: "Peridental membrane, 
 alveolar portion, transverse, stained with hematoxylin and eosin." 
 
 Study the general arrangement of the tissues and make a sketch 
 as in the case of the previous specimen. Note the muscle fibers 
 from the muscles of the lip attached to the periosteum on the 
 labial surface of the process, the bone of the labial plate, the sep- 
 tum separating the alveoli, the peridental membrane filling the 
 space between the bone and the surface of the root, the layers of 
 the cementum, the dentin and the pulp. 
 
 After studying the specimen with the low power as carefully as 
 possible, draw the peridental membrane surrounding one root, 
 including the thickness of the labial plate of bone with its perios- 
 teum and a part of the lingual plate. In this drawing represent 
 accurately the fibers of the peridental membrane, their arrangement 
 in the bundles, and the relation of the bundles to each other and 
 the bloodvessels. To do this the fine adjustment must be used to 
 obtain ideas of the third dimension of space. With the high power 
 draw one field from the wall of the alveolus, showing the attach- 
 ment of the fibers to the bone, the osteoblasts on the surface of 
 the bone, and the other cellular elements. This field should include
 
 400 DIRECTIONS FOR LABORATORY WORK 
 
 a bloodvessel. With the high power draw the thickness of the 
 cementum at some point where a specially strong bundle of fibers 
 is attached. This should show the fibers embedded in the cemen- 
 tum, cementoblasts on the surface, and the branching and inter- 
 lacing of the bundles. 
 
 PERIOD XXII. 
 
 Longitudinal Section of the Peridental Membrane. The lower 
 incisor of a young sheep was removed from the jaw by sawing 
 through between the teeth, leaving two teeth in each block. The 
 crow r ns of the teeth were broken off near the level of the gum so 
 as to admit the reagents to the pulp chamber. The tissues decalci- 
 fied, embedded in celloidin, and sectioned. They were cut through 
 from labial to lingual, and only the ones from the central portion 
 used. They have been stained in hematoxylin and eosin and are 
 ready to mount. Mount the section as usual and label the slide: 
 "Longitudinal section through the peridental membrane of a 
 sheep, labiolingual, stained in hematoxylin and eosin." First hold 
 the section up to the light and note the relation of the tooth to the 
 bone and the soft tissues. Study the section with the low power 
 and make a sketch showing the general distribution of the tissues. 
 Show the pulp chamber, dentin and cementum, bone, periosteum, 
 gum tissue, and epithelium. Do not attempt to fill in the drawing 
 more than diagrammatically, for it would require too much time. 
 The object of the drawing is to get the general relation of the 
 tissue before studying parts of it in detail. Compare the form of 
 the labial and the lingual gingivus and make a drawing of the 
 lingual, showing the details of structure as far as the border of the 
 process and as accurately as possible. With the high power draw 
 the thickness of the epithelium lining the gingival space. Study 
 the fibers in the occlusal third of the alveolar process and make a 
 drawing to represent them accurately, showing the cementum 
 at one side and the bone at the other. The entire length of the 
 root can seldom be got in one section on account of the curve of 
 the tooth, so that the fibers can probably be studied to advantage 
 in the occlusal third of the alveolar process only. Draw one field 
 with the high power showing the bloodvessels.
 
 TOOTH GERM 401 
 
 PERIOD XXIII. 
 
 Tooth Germ. The head of an embryo pig was embedded in 
 paraffin and sectioned at right angles to the snout. The sections 
 begin in the region of the incisors and far enough back to cut through 
 the nose cavity. They have been stained in hematoxylin and eosin. 
 Bring to the desk a clean slide and receive a section. Label the 
 slide: "Tooth germ, stained with hematoxylin and eosin." 
 
 The general form of the section will depend on the position of 
 the section through the head. At the desk is the head of a similar 
 embryo preserved in alcohol. This should be observed so as to 
 determine from the section its relation to the head. By holding 
 the section to the light and the use of the low power, make a sketch 
 of the entire section. Note the epiblast covering the outer surface 
 and lining the nose and mouth cavity. The mass which is to form 
 the tongue lying between the roof of the mouth and the mandibular 
 arch. If the section is in front of the angle of the mouth there will 
 be no connection between the upper and lower parts of the section. 
 Notice the separation of the nose cavity into right and left by a 
 septum containing cartilage, and the projections of cartilage from 
 the side walls which will form the turbinate bones. On either side 
 of the septum where it joins the palate will be seen little structures 
 known as Jacobson's organ, which later disappear. Notice Meckel's 
 cartilage in the mesodermic mass of the mandible. In the epiderm 
 of the outer surface the beginning of the formation of hairs are to 
 be seen. 
 
 ^Yith the low power follow the epiderm lining the mouth cavity 
 and look for the tooth germ. In each section there are four chances 
 for tooth germs, one on either side in the upper and lower arches. 
 Select the best one and draw it as seen with the low power. The 
 appearance will depend entirely upon the stage of development. 
 
 \Yith the high power draw enough of the enamel organ to show 
 the arrangement of the cells in the outer and inner tunics and the 
 stellate reticulum. 
 
 PERIOD XXIV. 
 
 Tooth Germ. Sections have been prepared in the same way as 
 in the preceding, but from the head of an older embryo, in which 
 the tooth germs are completely formed and calcification is ready 
 to begin. 
 26
 
 402 DIRECTIONS FOR LABORATORY WORK 
 
 Receive a section, mount, and label as before, and draw the 
 outline of the entire section. Note the changes in form and in 
 the tissue elements from the previous section. Bone formation 
 has begun both in the mandible and the maxilla. The amount and 
 distribution of this should be carefully studied. 
 
 With the low power draw the entire tooth germ, selecting the 
 most typical one in the section. With the high power draw one 
 field showing ameloblasts, odontoblasts, and a portion of the 
 papillae. Find a field in which bone formation is going on and 
 draw it accurately with the high power.
 
 APPENDIX CHAPTER I. 
 
 THE GRINDING OF MICROSCOPIC SPECIMENS, USING 
 THE GRINDING MACHINE. 
 
 BY G. V. BLACK, M.D., D.D.S., Sc.D., LL.D. 
 
 The Machine. The basis of this machine is the larger watch- 
 maker's lathe known as No. 2. It must swing 4 inches, the length 
 of the bed must be 12 inches, and be good and solid. A test should 
 be made of the alignment of the lathe head to see that this is exact. 
 If there is any inaccuracy, another lathe should be selected. The 
 power should consist of one of the largest and strongest electric 
 lathes, or motors, made for the use of dentists. This power should 
 be transmitted to the lathe through an overhead shaft of a length 
 that will give good room to operate the lathe without the motor 
 being in the way. A pulley may be placed on the left end of the 
 shaft of the motor on one of the brass carriers for grinding wheels. 
 This pulley should carry a good quarter-inch round leather belt. 
 Its diameter should be 2^ inches. The pulley on the right hand 
 end of the shaft above should be 5 inches. This will reduce the 
 speed one-half and double the power. On the left end of the shaft 
 should be placed a copy reversed of the pulley on the lathe- 
 head, which has 4 grooves. This gives good varieties of speed 
 with each speed of the motor. Another small pulley will be placed 
 near the center of the length of the overhead shaft, the purpose 
 of which will be explained later (Figs. 336 and 337). 
 
 The grinding apparatus is built upon a base fitted to the lathe 
 bed in the same way as the lathe head, or tail-piece. It has one 
 main shaft parallel with the lathe bed, in good and sufficient bear- 
 ings to maintain accuracy of alignment and perfect steadiness 
 for long-continued usage (see Figs. 336 and 337). This shaft moves 
 freely lengthwise, or back and forward, while turning slowly in 
 its bearings. On the end of this shaft next to the lathe head the 
 forward end there is a larger portion, or ring, and this end ter- 
 
 (403)
 
 404 
 
 APPENDIX CHAPTER I 
 
 minates in a threaded nipple, upon which the removable grinding 
 disks are screwed firmly against the face of this larger ring, to 
 
 FIQ. 336 
 
 FIGS. 336 and 337.- -A general view of the grinding machine, showing particularly 
 the arrangement for transmitting the power from the electric motor to the machine 
 that does the work. All of this may be made out by reference to the picture while 
 following the text. The bed of the little lathe on the left hand is 12j inches long, 
 which gives a good idea of the general dimensions. 
 
 The water is delivered to the grinding stone from a rubber bag or bucket hung on 
 the frame above through a rubber tube to the metal tube on a movable stand, which 
 may be so placed as to bring the brush at its end against the stone. This stand and 
 brush are better seen in Fig. 337. 
 
 secure accuracy of adjustment, 
 more fully explained later. 
 
 The use of these disks will be
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 
 
 405 
 
 On the rear end of this shaft, just back of its rear bearing and 
 abutting against it, a large movable nut is placed. This is pro- 
 vided with a thumb screw by which it is made fast at any point 
 desired. Turning this forward pulls the shaft back from the grind- 
 ing stone. Turning it backward allows the shaft to move forward 
 
 FIG. 337 
 
 against the stone. It has also a finger reaching back over a grad- 
 uated disk just to its rear. This disk is made fast on the shaft, 
 and the two together constitute the micrometer, by which the 
 thickness to which specimens are ground is measured. The movable 
 nut has 40 threads to the inch. The graduation of the disk is on 
 the same principle as that on the screw calipers used by machinists
 
 406 
 
 APPENDIX CHAPTER 1 
 
 for fine measurements one-thousandth of an inch but as this 
 disk is If inches in diameter, the graduations of thousandths are so 
 
 FIG. 338 
 
 FIGS. 338 and 339. The lathe with the grinding machine mounted upon it in position 
 for work. On the left next to the lathe head is the grinding stone surrounded by the 
 spatter guard, which gathers all of the water from the wheel and delivers it through 
 its hollow post into a rubber tube below the lathe bed, which conveys it to a con- 
 veniently placed receptacle. The water comes from a rubber bag or bucket hung 
 on the overhead frame (see Fig. 336) through a rubber tube to the metal tube mounted 
 on a movable stand so that the brush through which it passes may be placed against 
 the stone. The grinding machine proper is secured to the lathe bed by the larger 
 thumb-screw seen below. The point-finder is seen at the foot of the spatter guard, 
 and is secured by the middle thumb-screw seen below the lathe bed. 
 
 The shaft of the grinding machine (6 inches long) runs through its whole length, 
 but is completely covered in by its housings to protect its bearings from grit, except 
 at its forward end (next to the grinding stone). This part is protected by a swaddle 
 held by a ring, which keeps the working bearing clean. On this end the grinding 
 disk is seen almost touching the stone. The micrometer is on the other end of the 
 shaft next back of the frame of the grinding machine. Next back of this is a toothed 
 wheel made fast to the shaft. This is actuated by the middle one of the belts descend- 
 ing from overhead (Fig. 336, the left hand belt in Fig. 337). This belt passes over 
 a wheel hidden from view and through a small worm shaft turns the main shaft. 
 Pressure for the grinding is supplied by a plunger actuated by a spiral spring -seen 
 at the extreme right-hand end.
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 
 
 407 
 
 wide that one-quarter of one-thousandth may readily be used. It 
 differs in plan, in that both the graduation and the parallel lines 
 
 are placed upon this disk. On the machinist's micrometer the 
 lines are placed on the shaft and the graduations on the nut. The 
 graduation is read from the side of the finger on the movable nut,
 
 408 APPENDIX CHAPTER I 
 
 and the lines are read from its end. It is a very perfect micrometer 
 (Figs. 338 and 339). 
 
 The forward movement of the shaft when grinding, and also 
 the pressure exerted upon the stone, are furnished by a tail-piece 
 placed behind it and attached to the lathe bed. This has a plunger 
 actuated by a spiral spring, which pushes the shaft forward against 
 the stone. The amount of pressure exerted in the grinding is con- 
 trolled by the amount of compression of this spring in fixing the 
 piece to the lathe bed. It may be much or little, as desired. Usually 
 very little pressure is used. When the movable nut has come 
 against the frame in which this shaft turns, the machine may 
 continue to run, but the forward movement of the shaft stops and 
 the grinding ceases in consequence. Therefore there is no danger 
 of grinding a specimen thinner than the measurement fixed upon. 
 The further arrangement for finding this measurement will be 
 described later. 
 
 On the rear portion of the graduated disk, or wheel, a portion 
 or space is toothed, and connected with a worm pinion or threaded 
 shaft by which the main shaft is turned in its bearings. A belt is 
 attached over a wheel on the end of this worm shaft, and extends 
 to the third wheel, previously mentioned, on the overhead shaft. 
 When this belt is adjusted and the motor started, it causes the main 
 shaft in the grinding machine proper to turn slowly on its axis, 
 while being pressed against the stone by the tail-piece. By this 
 arrangement every part of the specimen fixed on the' grinding disk 
 is brought successively against every part of the rapidly revolving 
 stone, and is cut perfectly level in all of its parts. 
 
 The Grinding Disks. The grinding disks are of brass, accurately 
 turned f inch thick, and If inches in diameter. They have a 
 threaded hole | inch deep in the back to fix them to the nipple on 
 the forward end of the shaft of the grinding machine. A machine 
 should have a half-dozen or more of these, lettered or numbered 
 on the edge, so that records of each may be made when measuring 
 preparatory to mounting specimens for grinding. As the mounting 
 of specimens on others of these may proceed while the grinding 
 on one is going on (for the machine, being automatic, needs little 
 attention), this number at the least is necessary for rapid work. 
 
 The machine may be stopped and the disk removed from the 
 shaft by a few backward turns, the progress of the grinding exam- 
 ined, the disk returned for further grinding, etc., at any time 
 during the progress of the work. The face of the disk, which
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 409 
 
 should be perfectly flat and parallel with the face of the stone, 
 should always be perfectly bright, so as to reflect light through 
 the specimen when it becomes thin. This enables one to judge 
 very closely of the thickness by the eye (after sufficient practice), 
 that sometimes proves a valuable check on the setting of the 
 measurement in the beginning. 
 
 The Point-finder. This is a piece of steel one-eighth of an inch 
 thick, fitted to the lathe bed and set against the face of the lathe 
 head, and made fast by a thumb-screw passing through the lathe 
 bed from below. It has a strong arm which passes around other 
 fixtures between the lathe head and the forward end of the base 
 of the grinding machine. It is provided with a set-screw, by which 
 a range of variation can be made in the distance of the forward 
 end of the frame of the grinding machine from the lathe head. 
 When this is in place and the measurement of a disk has been made 
 and recorded for the grinding of a specimen to a specified thickness, 
 the machine may be taken to pieces and set up again and the 
 grinding proceed without fear of disturbing the measurement, so 
 long as the set-screw in the point-finder is not moved. It is often 
 necessary during grinding to loosen the grinding machine from the 
 lathe bed, slide it back to adjust something, to remove disks for 
 examination of the progress of the work, etc. This point-finder, 
 by preserving the distance between the lathe head and the grinding 
 machine, enables one to do this at will, and again find his exact 
 point of measurement simply by sliding the frame of the grinding 
 machine forward against the set-screw of the point-finder. This 
 little device seems absolutely necessary to the highest usefulness 
 of the machine. 
 
 Lap Wheels and Grinding Stones. I began my work of grinding 
 specimens by the use of lap wheels, but soon discarded them because 
 they were dirty. They cut much quicker than stones, however, 
 and may be used for the bulk of the work when much grinding of 
 very hard material is to be done. They are not necessary in grind- 
 ing teeth, bone, etc., but in grinding the harder fossils, especially 
 those impregnated with the silica, and in some geological work 
 they become necessary. 
 
 The best lap wheel I have used is an aluminum wheel. Brass 
 or iron will do the work, but aluminum holds the grit better, cuts 
 with lighter pressure, and does the work quicker. In using these 
 I have fed them continuously by hand with carborundum powder 
 in soapy water, using a brush.
 
 410 APPENDIX CHAPTER I 
 
 The Stones. Anyone who is doing much grinding should have 
 a good supply of stones. I have a pair of carborundum wheels, 
 a pair of emery wheels, a pair of India oil stones, and a pair of 
 Arkansas stones. In each of these pairs one is fine and the other 
 coarser grit. Every stone is dressed to a perfect face on the lathe 
 head where it is to do its work, with a black diamond held in the 
 slide rest. 
 
 These stones, when put in good shape, seem capable of doing an 
 unlimited amount of work. The conditions of the grinding pre- 
 vents them from getting out of true. All that seems necessary is 
 to roughen them a bit with a picking wheel when they become 
 too smooth to cut well. For this purpose a much smaller picking 
 tool than the smallest sold for the general mechanical uses seems 
 desirable. This picking wheel has sharp teeth of the hardest steel 
 possible on its periphery. It is held in a handle in such form that 
 the wheel is free to turn. In use it is held against the rapidly 
 rotating stone and slowly passed over its entire surface. It may 
 be held in the hand aided by a tool rest, or may be arranged for 
 use in the slide rest, which is the better form for this work. 
 
 Watering the Stones. In grinding, the stones are kept wet in 
 running ice water. A balsam that is too soft to hold a specimen for 
 grinding in water at room temperature will hold it perfectly in 
 ice water, because it is much harder when cold. For this purpose, 
 a receptacle for ice is hung on the frame that holds the overhead 
 shaft, and filled with bits of ice and then filled with water. Both 
 the ice and the water must be clean, for the opening in the tube 
 where it passes the valve which regulates the flow is very small, 
 and a little bit of dirt or trash might stop the flow. In this case 
 the specimen being ground would be burned instantly. A bucket, 
 or a large rubber bag, will answer for this purpose. Then an ordi- 
 nary rubber tube answers to conduct the water. It is best to have 
 this rubber tube to connect with a metal tube mounted on a stand 
 that may be placed in any position wanted to deliver the water 
 to the stone. This metallic tube is provided with a valve for the 
 regulation of the flow. In its final end it should be provided with 
 a brush of rather long bristles, into which the water is delivered 
 and spread upon the stone. This brush is made upon a short tube 
 fitted into the end of the metal tube. To make this brush, first 
 cover the plain part of the small brass tube with thick shellac 
 dissolved in absolute alcohol. Place a layer of the bristles around 
 it and wrap them tightly with a fine, strong thread. Then place
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 411 
 
 more shellac over this and another layer of bristles. Continue this 
 until the brush is large enough. Then wrap thoroughly with a 
 cord in shellac, let it dry, and then trim it up. Two of these have 
 served for four years of fairly hard usage. 
 
 Waste Water. A spatter guard is made by bending a f-inch 
 round brass tube into a circle, the inner diameter of which is the 
 size of the stones used, and brazing the ends together solidly. 
 Then this is fixed in the lathe and one-fourth of its inner circular 
 diameter is turned away. The grinding stones will then go inside 
 this. Then this piece is provided with a foot and hollow post and 
 fitted to the lathe bed with a washer and nut, the same as other 
 pieces are attached. This catches all waste water and through a 
 rubber tube attached to the end of its hollow post under the lathe 
 bed delivers it into a receptacle so placed by the table as to receive 
 it. This prevents all of the spattering of water which would be 
 thrown from a rapidly revolving wheel without it. If it should 
 be inclined to run over when a very full stream is wanted, a piece 
 of rubber dam may be stretched over the foot and pulled to its 
 upper end. This may be caught under the guard in fastening it 
 to the lathe bed, and will deliver any overflow into a receptacle 
 placed to receive it. In this way nothing is wet or spattered with 
 water. 
 
 Preparation of Material. In the preparation of material, such 
 as teeth, bone, etc., in histological work of ordinary delicacy, the 
 specimen is first ground flat on one side by hand on a rough stone 
 4 inches in diameter, on the motor, and finished perfectly flat on 
 one of the finer stones on the lathe head. The piece is then washed 
 clean and placed in absolute alcohol for a sufficient time to remove 
 all traces of water, or, when cracking or injury from shrinkage is 
 not feared, it may be dried in the warming box. Then when dried 
 and warmed to about 120 F., it is ready to mount with balsam on 
 the grinding disk for grinding. 
 
 Management of Balsam. I suppose the management of balsam 
 will always be a difficult problem with many persons. Many, 
 however, learn it quickly. One may take the dry balsam and 
 dissolve it in xylol, and filter it at a high temperature, say 110 
 or 120 F. Or one may use the prepared balsam for microscopic 
 mountings. In either case it must be evaporated until stiff enough 
 so that it will move rather sluggishly at 110 F., but will be fluid 
 at 120 or 130 F.
 
 412 
 
 APPENDIX CHAPTER I 
 
 Spiders and Dogs. For using this another bit of apparatus is 
 necessary. A circular piece of steel made flat on the upper surface 
 is mounted on three legs 1| to 2 inches high. The steel disk should 
 have two rows of holes around its periphery, the one row f inch 
 inside the other. A hard-rolled tool-steel wire, or rod -^ inch in 
 
 FIG. 340. The spider with a grinding disk upon it and a specimen laid on and 
 secured by bent rods called dogs. When these dogs are placed and pressed down 
 through the holes in the disk of the spider, they hold fast. With a little pressure 
 of the finger outward on the end of the rod below the disk of the spider, the dog 
 slips up and is loose. The disk of the spider is three inches in diameter. 
 
 diameter, should exactly fit these holes. These rods should now 
 be bent at right angles with a short nib on the end, bent again at 
 right angles, so that it will point downward when the free end of 
 the rod is set into one of the holes. The length between these two 
 angles should vary from f to 1| inches in three dozen or more
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 413 
 
 pieces which should be prepared. The end which goes in the 
 holes should be cut so that it will not quite reach the surface of 
 the table when dropped into the holes with the end of the nib on 
 the surface of the circular plate. These rods are called "dogs" 
 (Fig. 340). 
 
 With this arrangement a warming box arranged with a thermo- 
 stat to maintain an even temperature, sufficiently high to soften 
 the stiff balsam, is used. The specimen, the balsam, the grinding 
 disk, and the "spider" are placed inside, and allowed to rest until 
 they have reached the temperature desired. Then working quickly, 
 a sufficient amount of balsam is placed on the grinding disk, and 
 the specimen laid on it. This should be pressed down until it is 
 seen that all space under it is filled with balsam, but no considerable 
 excess should be used. It is well if this rest so in the warming 
 box for fifteen minutes for the balsam to soak well into the speci- 
 men. Then the grinding disk, with the specimens, should be laid 
 on the spider and one of the dogs dropped into one of the holes 
 in the steel plate, that will bring its nib on to a part of the speci- 
 men chosen. Then another, and still another, should be placed, 
 each with its nib on a different part of the specimen, so that every 
 part of it may be pressed flat on the disk. More dogs should be 
 added if necessary. Now each in turn is pressed down a little, 
 one after another, until all are exerting about all the force the 
 spring of the rods will exert without permanently bending them. 
 In this condition the whole thing is again enclosed in the warming 
 box. 
 
 At this time any number of specimens of teeth or bits of teeth, 
 bone, etc., that the face of the disk will hold may be placed on the 
 disk, and all may be ground together. Four to six lengthwise 
 sections of incisor or cuspid teeth may be placed at once, or eight 
 to twelve cross-sections. It seems to be best practice, however, 
 not to load the disk too heavily. Four lengthwise sections will 
 grind better than six, as a rule. 
 
 Now, after the loaded disk had remained in the warming box 
 until all balsam that will come has been squeezed out from under 
 the specimens, all excess of balsam should be very carefully removed, 
 or wiped away, close up against the specimens. Nothing clogs a 
 stone and stops its cutting more effectually than balsam smeared 
 over it, and every excess that may come against the stone should 
 be got out of the way. 
 
 When this is done the whole thing should be returned to the
 
 414 APPENDIX CHAPTER I 
 
 warming box for from one to four hours, so that it may dry some 
 about the margins at least. Then it may be removed from the 
 warming box and allowed to cool, and await convenience in grind- 
 ing. It should, however, remain secured on the spider by the 
 dogs if it is to wait more than a few hours, for the disposition 
 of dentin to warp in drying may pull some part of the specimen 
 from the disk. Under these conditions, two or three days, or a 
 week, will do no harm. 
 
 When the grinding is completed, the disk is removed from the 
 machine and the specimens flushed with clean water, and dried by 
 the pressure of a soft napkin folded to several thicknesses, or clean 
 pieces of waste cotton fabric may be used. Then the disk with 
 its specimens should be laid in a dish and sufficient xylol added 
 to cover it, and allowed to rest until the balsam has been dissolved 
 and the specimens released. This will usually require from twenty 
 to thirty minutes, or sometimes as much as an hour. When the 
 specimens are very thin they loosen much quicker than when 
 thick. Any material not penetrated by xylol, as silicified petri- 
 factions and stones, require much more time. 
 
 When the specimens have loosened, they are ready for permanent 
 mounting for microscopic study. 
 
 Rapidity of Grinding. In order to make rapid progress in grinding 
 specimens, one should have six to ten grinding disks, nearly as 
 many spiders, and a large supply of dogs. The machine is so 
 nearly automatic in its action that it needs but little watching, 
 so that the preparation may be going on while the grinding is in 
 progress. One of the principal points that needs attention is the 
 flow of water. But if the water and ice placed in the receptacle 
 are clean and free from dirt or trash that may stop the flow of 
 water, the only care is that the quantity of water is kept up. The 
 vessel should be large enough to hold a supply for several hours. 
 If the stone should run dry, the specimen would be destroyed in 
 a few seconds. 
 
 Setting the Measurement of Grinding Disks. When beginning 
 any considerable series of grindings, the first thing of importance 
 is to try out and obtain a record of the measurements of each 
 grinding disk for the particular stone that may be selected for 
 finishing. I find that most persons, after some practice, prefer 
 to use a fine stone for the entire grind. In grinding teeth, after 
 roughing down the surface that is to form the specimen, the back 
 is also ground away to a flat surface that will better accommodate
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 415 
 
 the placing of dogs in mounting on the grinding disks. These may 
 be made quite thin and reduce the grinding with the fine stone. 
 Then the stone selected is placed in the lathe head, seeing to it 
 carefully that the face of the stone is clean. Then the grinding 
 machine is brought up in contact with the set-screw of the point- 
 finder. The tail-piece is placed in position and pushed up so as to 
 make some pressure on the shaft. Then, with the large nut the 
 shaft is so adjusted that the grinding disk being tried comes close 
 to the stone but does not touch it. Now start the machine and 
 note the running carefully, and while doing so catch the adjusting 
 nut of the micrometer and move it one-thousandth at a time, and 
 listen for the first touch of the disk to the stone. The moment 
 this is heard, quickly reverse the movement of the adjusting nut 
 and separate the disk from the stone. Try this again and again, 
 until you feel very certain of having detected the first touch of 
 the stone on the disk by moving the adjusting nut half or a quarter 
 of ToVo" inch. At last, while it is touching, stop the machine in a 
 position to see the finger on the adjusting nut, and read the measure- 
 ment and enter it on your record for that disk. In setting for a 
 grind with this disk, turn the adjusting nut so as to draw the 
 grinding disk back from the stone T"oVo inch. When the specimens 
 to be ground are mounted on this disk, place it back on the machine, 
 start it, seeing that the iced water is running first, and let it run 
 until it ceases to cut, which it will do when the forward movement 
 of the shaft is stopped by the contact of the adjusting nut of the 
 micrometer with the rear bearing of the shaft. 
 
 Then remove the disk and examine the specimens carefully. If 
 the placement has been accurate, the specimens will be too thick. 
 Replace the disk carefully and turn the nut forward so as to grind 
 one-thousandth of an inch thinner, or one may do only a half of 
 one-thousandth at a time. Repeat this until the section seems to 
 be thin enough. Then remove and mount the sections and judge 
 them with the microscope. By this time one will have arrived at 
 an accurate measurement of this disk, and the record will be trust- 
 worthy for other grinds, and will not have to be repeated until 
 the wearing of the stone begins to leave the specimens a bit thick. 
 Then a half-thousandth of an inch will bring it right. And so on, 
 and on. Each disk will be treated in the same way for each stone 
 used, and if one is doing much grinding all will be running on their 
 records, and all go smoothly. Recently a man who was grinding 
 sections of teeth for me made all of the preparations, preparatory
 
 416 APPENDIX CHAPTER I 
 
 grindings, and disk mounts, ground and removed from the disks 
 ready for mounting forty full-length sections of central incisors 
 in six hours, and had his lunch during the time. Every section was 
 complete, was even in thickness in every part, and all practically 
 the same thickness a thickness chosen for the special studies in 
 hand. 
 
 Grinding Frail Material. While the machine facilitates the 
 production of the more ordinary sections to such a degree as to 
 be indispensable to one having many grindings to do, it is in 
 the production of sections of very frail material that the grinding 
 machine stands out as vastly superior to other methods of grinding. 
 In the study of caries of enamel in which disintegration has ren- 
 dered the remaining tissue very frail and likely to fall to pieces 
 before it is sufficiently thin, we may obtain the required thinness 
 and yet retain all of the tissue. I have also produced exceedingly 
 fine sections of salivary calculus, and equally good sections from 
 small crumbs of serumal calculus. The production of these is 
 slow, but fairly certain of good results. 
 
 Also in grinding sections of fossil teeth, fossil woods, and the like, 
 in which very fine sections are too brittle to be handled in any 
 way except as stuck to glass, the machine gives excellent results. 
 In geological work it practically removes the difficulties. Good 
 sections of the very brittle stones can be made with fair safety by 
 grinding on the cover-glass. 
 
 Plans for Grinding Frail Material. Much very desirable material 
 for microscopic investigation will be found that is so frail, or at 
 lea c t so brittle, when reduced to sections thin enough for microscopic 
 investigation, that it will crumble to pieces, either in the grinding 
 or in the mounting, by the ordinary processes. For grinding and 
 mounting such material the following processes have been slowly 
 evolved. These may be divided into the balsam process and the 
 shellac process. Such material that, when made fast to a cover- 
 glass and ground in hard balsam, is not liable to go to pieces when 
 this hard balsam is softened by sticking the specimen and glass 
 cover to a glass slide may be ground in hard balsam. If, however, 
 the different parts are liable to separate and change position when 
 the balsam softens, shellac should be used for the grinding. I have 
 had some very sorrowful failures in grinding rare specimens of 
 enamel that had no cementing substance betw r een the enamel rods 
 in hardened balsam. For when the softer balsam was added to 
 mount the specimen on the glass slide, the hard balsam was softened
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 417 
 
 and the enamel rods floated out of position. All such material as 
 will not hold together strongly enough to prevent this should be 
 ground in shellac. 
 
 To grind in hard balsam, the one side of the specimen may be 
 ground flat on the rough stone and then dried out in absolute 
 alcohol. Then the ground side should be saturated to sufficient 
 depth with soft balsam, and laid aside until the balsam has become 
 hard enough to grind smoothly. Then the grinding and polishing 
 of this first side should be completed by grinding away all balsam 
 from the immediate surface, and sufficiently into the substance 
 of the specimen to produce a clean, smooth surface of the material. 
 When this has been done, and the surface dried, it should be 
 mounted on an ordinary cover-glass, the thickness of which should 
 have been measured and recorded. In this mounting the cover- 
 glass should be laid on a spider and weight enough placed upon it 
 to insure a perfect fit of the surface of the glass. This should be 
 subjected to about 120 F. heat for from one to five or six hours, 
 for the purpose of expressing the last bit of balsam possible from 
 between the specimen and the cover-glass. Then it may rest, 
 awaiting the convenience of the operator, for several days, but the 
 balsam must not be allowed to become "brittle hard," because in 
 that case it loses toughness. All excess of balsam about the margins 
 of the specimen should be carefully removed to facilitate the 
 hardening of that which remains, and especially so that it may not 
 come in contact with the grinding stone, stick to its surface, and 
 interfere with the cutting. 
 
 Good judgment must be acquired by practice as to the hardening 
 of balsam and shellac in these grinding processes. The best idea 
 of it that can be given in words is this. The balsam or the shellac 
 must have become firm enough so that it will not drag or allow the 
 specimen to move while grinding in iced water. Neither must it 
 become hard enough to become brittle, /or then it becomes liable to 
 break. 
 
 When ready, the specimen is mounted on the grinding disk. 
 This is done by first cleansing the disk, finishing with xylol, and 
 then sealing the cover-glass to this with soft balsam. This should 
 be placed on the spider and well weighted down with dogs. All 
 excess of balsam should be carefully w r iped away from the margins 
 of the cover-glass. This may be quickly dried at 120 F., or more 
 slowly at room temperature. It should, however, be warmed for 
 a half-hour or more, for the purpose of expressing as much balsam 
 27
 
 418 APPENDIX CHAPTER I 
 
 as possible. This cover-glass will be well held for grinding in 
 iced water with only a little drying about the margins, if all excess 
 of balsam is cleaned away closely. The balsam should not become 
 very hard. 
 
 If the specimen is of considerable bulk and of a quality of material 
 that can be cut with a steel saw, the disk may be caught in a vise 
 "with leather-cushioned jaws to avoid bruising," and the bulk 
 of the material removed with a jeweler's saw, leaving only a 
 moderately thin section for grinding. Or if the material is very 
 hard, as stones, silicified fossils, etc., the disks may be mounted 
 upon the slide rest and cut with the slicing disks, to be described 
 later. 
 
 The specimen is now ready for the final grinding. The record 
 for measurement with the particular stone to be used in finishing 
 has been made, tried out on unimportant material, and the cover- 
 glass has been measured and its record made. With this data, 
 the disk is screwed to its place, the micrometer turned to the 
 proper measurement for the finish, the iced water arranged, the 
 machine set in motion, and it will do the rest. When coarser 
 stones are used for cutting away considerable material, I find those 
 with just a little experience prefer to gauge the amount of the 
 cutting by the eye for the coarse stone. 
 
 Removal of the Cover-glass from the Disk. I remove the cover- 
 glass with the specimen from the grinding disk in two different 
 ways, as seems at the time best. 
 
 First, the grinding disk is placed on a heated piece of metal 
 that will warm the grinding disk quickly. Have a stick of rather 
 soft wood ready, the end of which is cut to a rather sharp angle 
 and thinned down almost in the form of a blade. When the grind- 
 ing disk begins to warm, catch the margin of the cover-glass with 
 the end of the stick and begin to make steady pressure. As the 
 disk warms, so as to soften the balsam, the cover-glass will begin 
 to move under the steady pressure, slowly at first, but more rapidly 
 later, and will slide off the grinding disk before the specimen is 
 loosened. For this plan the cover-glass should be pretty strong, 
 one and one-half to two thousandths of an inch thick. Otherwise 
 there will be great danger of breaking it. It is well in some cases 
 to run just a little xylol around the margins of the cover-glass 
 and partially dissolve the balsam that has become driest before 
 the heating. Great care must be taken not to allow the xylol to 
 spread on to the specimen, for it would loosen it very quickly.
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 419 
 
 The specimen is then turned downward and placed on a tiny 
 drop of balsam on a glass slide, and quickly pressed down close 
 and level. As the new balsam will soften the old, it should not be 
 moved further than to quickly apply a light spring clip to hold it 
 steady. The parts of the specimen are less likely to move if this 
 is laid on ice for an hour or more. 
 
 The Use of Shellac. In the second plan shellac is used instead 
 of balsam for hardening the specimen and holding its parts together 
 in the first grinding. This part of the work is otherwise done in 
 the same way. The drying of the shellac requires more time usually 
 than the balsam. 
 
 The attachment of the cover-glass to the grinding disk is done 
 in the same way as when balsam is used to hold the specimen on 
 the cover-glass that is, with balsam. The grinding proceeds 
 similarly in every respect. 
 
 In the removal of the cover-glass from the grinding disk, and 
 mounting the specimen, comes the important differences in the 
 two processes. Xylol dissolves balsam very quickly. But xylol 
 does not dissolve shellac at all. Therefore, instead of pushing the 
 cover-glass of the grinding disk, the disk is laid in xylol and the 
 balsam dissolved out. In this there is no danger of detaching or 
 moving the specimen if the handling is careful. When cleaned, it 
 is inverted upon a glass slide on a drop of balsam without fear of 
 movement of parts of the specimen, no matter how frail. 
 
 The Preparation of Shellac. To keep shellac in condition for 
 this work has some difficulties. The dry scales should be dis- 
 solved in absolute alcohol so as to make a moderately thick varnish. 
 It should then be filtered at a temperature of 110 to 120 F., or 
 be made thinner and filtered at room temperature. Great care 
 should be exercised to keep the filtrate from exposure to a damp 
 atmosphere, for it absorbs water readily and then will throw down 
 fine crystals, which destroy its value for microscopic purposes. 
 
 After being filtered it should be evaporated in a close warming 
 box in about 110 to 120 F., to the consistence of syrup. In doing 
 this it is well to divide the supply into two or three grades a 
 thinner, medium, and a thicker solution. The thinner solution 
 will be used for saturating frail specimens before any cutting is 
 done. The thicker solutions for attaching specimens to the cover- 
 glass for grinding. The medium solution for either purpose, as the 
 material may seem to require.
 
 420 APPENDIX CHAPTER I 
 
 The Grinding from Crumbled Material. There is often important 
 material for investigation that can be had only in very small crumbs, 
 or broken pieces, such as serumal calculus, sands, crumbled bits 
 of strange stones, or mixtures of such material as is found in some 
 of the coarser sands. These, on microscopic investigation, may 
 tell important stories as to their origin and throw important light 
 upon geological questions. In addition to the ordinary microscopic 
 observation, the polariscope may be turned on these, and reveal 
 important facts as to their origin and structure. Also many things 
 will be found in botanical work, such as obtaining sections of small 
 seeds, and the like, which will give important information. 
 
 Having done a few of these grindings, especially of the very 
 frail dental material, such as serumal calculus, extremely frail 
 fossil teeth, etc., plans of work more or less well adapted have been 
 developed. 
 
 For instance, I have obtained excellent sections of serumal 
 calculus, which can be had only in small crumbs or flakes, in this 
 wise: A small collection of these bits are first immersed for a 
 time in absolute alcohol, or until all air has been removed if they 
 are dry, or if they are freshly gathered, until all water has been 
 removed. Then a cover-glass is prepared by covering its central 
 part with the thicker solution of shellac, and these crumbs are 
 placed in this, in what seems to be the best position for obtaining 
 sections. These are allowed to soak full of the shellac, under a 
 close cover, and then uncovered to dry up. Then, if some of the 
 pieces seem to need it, more shellac is added from time to time, 
 until the embedding seems sufficient. This may be dried at room 
 temperature, or in the warming oven at 110 to 120 F. Shellac 
 should not be subjected to much higher temperatures for a con- 
 siderable time, because continued high temperature for many days 
 together seems to injure the strength. 
 
 When this is sufficiently hard for smooth grinding, and before 
 it has become too brittle (determining this point requires some 
 experience), the preparation is cemented to the grinding disk with 
 balsam and ground to such a point as seems most favorable for 
 obtaining sections. This point is to be determined by frequent 
 removal of the disk from the machine and examination of the 
 exposed surfaces of the several pieces. 
 
 When this part is done, the cover-glass is dissolved off of the 
 grinding disk by xylol. Then another cover-glass is attached to 
 the surface with the least possible amount of shellac. This in turn is
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 421 
 
 dried to the right consistence. Then the last cover-glass placed 
 that is, the one on the side that has been ground is secured to the 
 grinding disk with balsam. When this has set it is placed on the 
 machine and the first cover-glass is ground away and the section 
 ground to the required thinness. They are again dissolved off of 
 the grinding disk, and may be at once mounted in balsam on the 
 microscopic slide. 
 
 Difficulties in Grinding. In the grinding of material enveloped 
 in shellac, or in balsam, either of these materials are apt to gum up 
 the stone and stop the cutting, or render the grinding very slow. 
 When this is from balsam, it may be quickly removed after drying 
 the stone by washing with xylol on a brush, or a bit of cloth, while 
 the stone is slowly revolved. 
 
 When clogged with shellac, the washing is done with absolute 
 alcohol. This requires much more time, and some advantage 
 may be obtained by using pumice stone with the cloth or with 
 cork. After rubbing with pumice stone, a very thorough washing 
 with alcohol should be made to remove the last particles of pumice, 
 before rebeginning the grinding. Even with this, the ground 
 surface is apt to be rough or scratched for a time by particles of 
 the pumice lodged on the stone. These will soon disappear, how- 
 ever. Yet the pumice should not be used in the last portion of the 
 grinding. 
 
 With much grinding of hard substances, the surfaces of the 
 stones become worn so smooth that they do not cut w T ell. Then 
 the picking tool should be run over the surface until it is perceptibly 
 roughened. This will cause the stone to cut. briskly for a con- 
 siderable time, and at first following such sharpening the 
 ground surface of the specimen is likely to be full of scratches. 
 In that case a smooth stone should be used for the finishing. 
 
 Much care should be taken in keeping the stones in good condi- 
 tion. Except in the ways mentioned, no dirt or grit should be 
 allowed to come in contact with their surfaces. A single particle 
 of grit lodged in the surface of the stone will fill the whole surface 
 of the ground section with scratches. Although I shut up my 
 stones in a close-fitting drawer, I find it necessary to cover each 
 with a close-fitting cloth that is so closely woven as to exclude 
 all dust. 
 
 In taking care of the machine itself, one cannot be too careful. 
 All of the bearings of the lathe head and of the grinding machine 
 should be swaddled with candle wick saturated with oil to prevent
 
 422 APPENDIX CHAPTER I 
 
 the ingress of gritty particles. This is especially needful when 
 using the aluminum saws and feeding them with carborundum 
 powder. Then every bearing about the whole machine should be 
 especially protected to prevent the possibility of getting grit in 
 the bearings. Carelessness in such a matter will quickly ruin a 
 fine bit of mechanism. But with this care, such a machine should 
 continue to do its work well for a lifetime (Figs. 341 and 342). 
 
 FIG. 341 
 
 FIGS. 341 and 342. Arrangement for slicing very hard material. Fig. 341 is the more 
 ordinary view of the machine with the slide rest and object holder in position. In 
 Fig. 342 the lathe is turned about to give a better view of the slide rest, object holder, 
 spatter guard, and aluminum disk. In these illustrations the slotted tube is used 
 (see text) to hold the object being cut. Notice that the disk used for cutting is sur- 
 rounded by a spatter guard which is open for a space at one side so that the periphery 
 of the disk may be used in cutting. This guard gathers all water and grit used in 
 cutting, and delivers it into the pan below through its hollow post. When doing 
 this kind of work all of the bearings of the machine should be carefully wrapped 
 (swaddled) to keep them safe from intrusion of grit. 
 
 The Slicing Mechanism. This is an arrangement for slicing 
 very hard substances which cannot be cut with the ordinary steel 
 saw such as the enamel of teeth, silicified fossils, rocks, etc. This 
 consists of an aluminum disk fitted to the lathe head, and sur- 
 rounded by a special form of spatter guard that admits of the 
 use of the periphery for cutting, and an object holder fixed upon 
 the slide rest of the lathe. The object holder consists of a clamp 
 that grasps a brass tube slotted at the free end in which teeth, or 
 other objects may be made fast with plaster of Paris or sealing wax 
 for slicing. Or in place of this a brass mandril, upon the end of
 
 THE GRINDING OF MICROSCOPIC SPECIMENS 
 
 423 
 
 which there is a threaded nipple by which any of the grinding 
 disks may be attached. These are fixed in the position of the 
 ordinary tool post, and maybe swung horizontally to any possible 
 position in relation to the aluminum disk. An object can there- 
 fore be so placed on the disk as to be cut in any direction desired. 
 Usually these are fixed upon the disk with sealing wax. In using 
 the aluminum disk it is fed with carborundum po\vder suspended 
 in soapy water to give it some stickiness. This is applied with 
 a brush by^hand, and is kept going so constantly as to prevent the 
 disk from running dry. The ordinary aluminum plate, of twenty- 
 
 FIG. 342 
 
 four to thirty gauge, may be used for making these. They are first 
 cut in circles by hand, as large as the lathe will swing (4 inches), 
 and then are cut down to 3^ inches with a tool in the slide rest. 
 These are quickly made when wanted. They wear out rapidly, 
 and yet one of them will do much cutting of very hard substances, 
 and do it accurately and delicately. Rings may readily be cut 
 from the ordinary test-tubes without special danger of breaking. 
 The crown of a molar tooth may be cut into many slices; fossil 
 teeth, silicified fossil woods, stones, etc., may readily be sliced as 
 thin as they can be handled in the after- work of preparation.
 
 APPENDIX CHAPTER II. 
 
 THE THEORY OF HISTOLOGICAL TECHNIQUE. 
 
 THE first requirement of histological technique is to obtain a 
 general view of the theory of procedure. Many beginners make 
 the mistake of supposing that directions for histological technique 
 can be followed like the receipts of a cook book, or the directions 
 for an experiment in chemistry. This is very seldom the case, and 
 while it is always necessary to follow directions accurately, it is 
 still more necessary to follow them intelligently. All histological 
 methods require judgment. For instance, the length of time 
 required for xylol to replace absolute alcohol in a block of tissue 
 which is to be embedded depends upon the size of the piece, the 
 character of the tissue, the temperature, and possibly some other 
 factors. It is therefore impossible to say exactly what time would 
 be required, and the experimenter must use the judgment which 
 has been acquired as the result of experiment. In the same way no 
 experimenter can make up a stain and be sure that it will work 
 exactly like the last lot made by the same formula until he has 
 tried it. Even with the same stain the length of time required for 
 staining a section depends upon the thickness of the section, the 
 character of the tissue, and the preliminary technique it has been 
 through. So that all time directions must be considered as approx- 
 imate, and to be successful the experimenter must study, first, the 
 object to be obtained by the use of each reagent, and the peculiar 
 action of the reagent upon the tissue. 
 
 For observation with the compound microscope transmitted 
 light is ordinarily used. The object must therefore be thin and 
 transparent enough to allow the light to pass through it. The 
 higher the magnification the smaller the field, that is, the smaller 
 the portion of the tissue that can be seen at one time, and the 
 less depth of focus, and consequently the thinner the sections 
 must be. A section that would be excellent for study with the f 
 objective may be almost valueless under a T V, and sections that 
 
 (424)
 
 THE THEORY OF HISTOLOGICAL TECHNIQUE 425 
 
 are splendid under the T \- might be of little value under the f . 
 In other words, the thickness of the section should be related to 
 the magnification with which it is to be studied, and to the size 
 of the structural elements which make up the tissue. For the 
 study of the organs and tissues of multicellular organisms there 
 are three general methods (1) teasing, (2) maceration, and (3) 
 sectioning. 
 
 Teasing. In this method a small portion of the living tissue 
 is torn apart with two needles in a drop of normal salt solution or 
 some indifferent medium which will not affect the tissue. In this 
 way it is spread into a thin film and squeezed a. little between 
 a slide and cover-glass so as to separate the structural elements 
 when they may be directly observed. Of course, in studying such 
 a preparation it must be remembered that the tissue has been 
 forcibly torn apart and effects of violence must be looked for. 
 These often bring out facts of structure which would not other- 
 wise be as easily seen. After teasing the living tissue, staining 
 agents may be used to facilitate the study of structure. The fresh 
 tissues are often so transparent and made up of substances of so 
 near the same refracting index that very little structure can be 
 made out without the use of staining agents. It must be borne 
 in mind that staining agents are of two classes, diffuse and selective. 
 A diffuse stain gives an even color to all of the tissue and facilitates 
 the study chiefly by rendering it less transparent. A selective 
 stain combines more readily with one portion of the tissue than 
 another, rendering it more conspicuous. Selective stains therefore 
 must be thought of as chemical agents which combine with parts 
 of the cell or tissue and demonstrate chemical differences in the 
 structural elements. For instance, basic anilines react with the 
 chromatin of the nucleus, producing a colored compound. The 
 stain may then be washed out of the section, leaving only the 
 nuclei colored. Acid anilines in general are diffusive stains giving 
 a general color to the cytoplasm. In a similar way certain stains 
 will react only or chiefly with intercellular substances, rendering 
 them more conspicuous. For staining freshly teased specimens 
 methyl green, the formula for which w r ill be found under the para- 
 graph on stains, is an excellent agent. Teased specimens are never 
 very permanent, though they may be preserved for a considerable 
 length of time by mounting in glycerin or glycerin jelly and putting 
 a ring of varnish or white lead around the edge of the cover-glass 
 so as to prevent evaporation.
 
 426 APPENDIX CHAPTER II 
 
 Maceration. When an organ is composed of more than one 
 tissue the structural elements may be separated by selecting an 
 agent which will act upon one and not upon the others; for instance, 
 the muscle fibers of a voluntary muscle may be separated by 
 treating a piece of tissue with dilute alkali, which will soften and 
 dissolve the connective tissue, allowing the muscle fibers to sepa- 
 rate. In a similar way dilute alcohol will soften the cementing 
 substance between the epithelial cells. By first treating a piece 
 of tissue with the proper agent and then teasing, the form of the 
 structural elements of the tissue can be made out. By treating 
 a portion of connective tissue containing both white and elastic 
 fibers with dilute hydrochloric or acetic acid, which dissolves the 
 white fibers, elastic fibers which could otherwise not be seen may 
 be made out. Macerating and teasing methods are of great assis- 
 tance to the study of tissues in sections, and it would be often very 
 difficult to obtain true ideas of structure from sections without 
 their assistance. 
 
 Sectioning. For the study of the structural elements in their 
 relation to each other in the tissue sectioning is the one method. 
 As they exist in the body, however, some of the tissues are too soft 
 and others too hard to allow the cutting of a thin enough slice 
 without disturbing the relation of the structural elements. They 
 must therefore be put through rather an elaborate process in which 
 the object of every step must be understood. 
 
 Dissecting. First of all, the material for histological work must 
 be absolutely fresh, that is, living. It must be remembered that 
 living cytoplasm is chemically different from dead cytoplasm, and 
 as soon as death occurs postmortem changes begin which gradually 
 destroy the structure. The period from death to the beginning of 
 histological methods of preparation should be measured in minutes, 
 not in hours. Tissues that have been dead for a few hours will 
 not react with the staining agents so as to produce the brilliant 
 specimens that can be obtained from fresh material, and often a 
 few days will render material entirely useless except for the grosser 
 anatomical relations. The specimens to be studied should be 
 dissected while the cells of the tissue are still alive, and in doing 
 so the greatest care should be used not to disturb the relation of 
 the tissues. 
 
 Fixing. Histologically this word means killing. After dissect- 
 ing out the tissue to be studied, and while the cells are still alive, 
 it must be immersed in some liquid that will kill the cells and fix
 
 THE THEORY OF HISTOLOGICAL TECHNIQUE 427 
 
 their structure as when alive. The pieces should be made small 
 enough for the fixing agent to penetrate them rapidly, and the size 
 of the piece that can be used depends upon the density of the 
 tissue, its character, and the nature of the reagent. Some fixing 
 agents are very much more penetrating than others. All fixing 
 agents coagulate or set the cytoplasm and tend to prevent shrinkage. 
 The success of all the following steps and the value of the specimen 
 for the study of detail of structure depend upon the perfection of 
 fixation. 
 
 The fixing agents most commonly used are bichloride of mer- 
 cury, potassium chromate or chromic acid, osrnic acid, alcohol, 
 and formalin. The formulas for the same will be found on pages 
 439 and 441. 
 
 Hardening. Since all the fixing agents coagulate living cyto- 
 plasm, they are also to a greater or less extent hardening agents, 
 and after fixing tissues may be handled with less danger of dis- 
 turbing the relation of the structural elements. Some fixing 
 agents, especially chromic fluids, may be continued in their action 
 as hardening agents until the tissue has attained the proper consis- 
 tency for sectioning, but, as a rule, it is necessary to use other agents 
 for this purpose. In all cases the fixing agent must be thoroughly 
 washed out of the tissue before the process is continued. Alcohol 
 is the universal hardening agent, and at the same time it removes 
 the water from the tissue. In carrying tissues from w r ater to alcohol 
 several grades must always be used, and the more delicate the 
 tissue the more gradual must be the changes. If a piece of tissue 
 is taken from water and placed in 95 per cent, alcohol, the diffusing 
 currents will be so strong as to disturb structure and at the same 
 time the hardening action is so energetic as to produce shrinkage. 
 From water a tissue should never be placed in alcohol stronger 
 than 70 per cent., where it should be allowed to remain for twenty- 
 four hours. From 70 per cent, it may be taken to 95 per cent, 
 for the same length of time, and from 95 per cent, to absolute, 
 which will entirely remove the water and prepare the tissue for 
 embedding. If the tissue is very delicate, it should be placed in 
 water, then in 50 per cent, alcohol, and carried through in grades 
 of 10 per cent, to 95 per cent. 
 
 Embedding. In order to cut thin sections of tissue the piece 
 must be surrounded and infiltrated with some firm substance which 
 will not only support the entire piece, but will soak through the 
 tissue, filling all intercellular spaces and supporting the individual
 
 428 APPENDIX CHAPTER II 
 
 structural elements. At the same time the embedding material 
 is used to fasten the tissue firmly to a block of fiber or wood which 
 can be grasped in the clamp of the sectioning machine. Two kinds 
 of material are used for this purpose. Substances that are fluid 
 when warm, and solid when cold, as paraffin, or substances which 
 may be dissolved in volatile liquid and are solidified by evapora- 
 tion, as celloidin. In both of these methods the substances, as a 
 rule, are either oily or insoluble in water, and therefore the tissue 
 must be thoroughly dehydrated that is, have all the water removed 
 from it before it is placed in the embedding material. To accom- 
 plish this there should be at least one change of absolute alcohol. 
 From the absolute alcohol the tissue should be placed in a fluid 
 which is a solvent for the embedding material, so that it will pene- 
 trate the tissue more perfectly and rapidly. Heat is always injurious 
 to the tissue, and in embedding in paraffin, therefore, the tissue 
 should be kept in the melted paraffin for the shortest possible time 
 and paraffin of as low a melting-point as is consistent with suffi- 
 cient hardness for cutting should be used. In embedding by 
 evaporation the evaporation should not be too rapid or the shrinkage 
 will be increased. Tissues may be kept blocked and ready to cut 
 for a long time, but as a general principle the shorter the time the 
 more perfect will be the specimen. 
 
 Sectioning. For sectioning some sort of machine is necessary, 
 and many kinds have been designed, the general principles of 
 which are all the same. They consist of a clamp which holds the 
 knife and a clamp which holds the specimen, and can be adjusted 
 in such a way as to bring the specimen in proper relation to the 
 knife. The position of the specimen is advanced by a micrometer 
 screw so that sections of any desired thickness may be sliced. The 
 delicate part of this machine is the micrometer screw. The essen- 
 tial to the success of its working is the sharpness of the razor, 
 and for such specimens as decalcified bone the razor must be heavy 
 and strong, so that the edge will not spring in cutting the hard 
 tissue. 
 
 Staining. The detail of staining process will be described in 
 the next chapter, but it must be remembered that stains, as a rule, 
 are water solutions and the sections must be carried through the 
 grades of alcohol to water before they are ready for the stain. 
 After staining they must be carried back through the grades of 
 alcohol, so as to remove the water entirely before they can be 
 mounted in balsam, which is not soluble in water.
 
 THE THEORY OF HISTOLOGICAL TECHNIQUE 429 
 
 Mounting. Except in serial work, but one specimen should be 
 placed on a slide, and this should be in the center, leaving room at 
 either end for a label. In serial work the sections may be placed 
 at one end of the slide, preferably the left hand, leaving room at 
 the right for one label. 
 
 Labelling. Nothing in histological technique is more important 
 than labelling, especially in all research work. Through every 
 step of the process the specimen must be kept track of, and a 
 mixing of labels may spoil months of work. A laboratory note-book 
 containing a record of all material and work should always be on 
 the tables. I have found a system of date and number convenient. 
 For instance, on June 4 a number of specimens are dissecte'd out; 
 in the note-book the record of the source of the tissue is made; the 
 first piece is placed in a bottle of fixing fluid and the bottle labelled 
 6-4-1911, No. 1; the second, 6-4-1911, No. 2, and so on. In the 
 note-book the description of each block and the date and the hour 
 when it was placed in the fluid is recorded. In this way the tissue 
 may be carried clear through recording each step in the process, 
 and when it is sectioned and mounted we can follow its history in 
 the note-book. Every slide should be labelled first with the date 
 and the block number so as to follow its technique; second, the 
 name of the tissue, and third, the kind of staining. This should 
 be placed on the right-hand label, leaving the left-hand label for 
 index and file number if the section is preserved. 
 
 Indexing and Filing. Many beginners make the mistake of not 
 indexing and filing their slides. They think because they have 
 only a few, that they can easily find anything they want, and that 
 they will wait until they have a larger number before they begin 
 a system, but when a large number have piled up they can never 
 find time to go back and arrange them as they should be. And 
 only one who has failed in this way knows the annoyance of looking 
 through hundreds of slides to find one that he knows he has some 
 place.
 
 APPENDIX CHAPTER III. 
 
 GENERAL HISTOLOGICAL METHODS. 
 
 Fixing. As has been seen from the preceding chapter, fixing is 
 the first and one of the most important steps in all histological 
 methods. No degree of care in the latter steps can make up for 
 any imperfection in it. As a general statement all fixing agents 
 have advantages and disadvantages, so that in research work several 
 should be tried and their results compared. For class-room work, 
 however, minute details are not so important. Certain general 
 principles may be stated. Bichloride of mercury is especially 
 adapted to the fixing of epithelium of the mucous membrane. It, 
 however, does not penetrate rapidly, and small pieces must be 
 used. Crystals are liable to form in the tissue, and special precau- 
 tions must be taken for their removal. Flemming's and Zenker's 
 fluids and the fluids containing osmic acid are used chiefly in 
 research. For class work the author uses Miiller's fluid and Miiller's 
 fluid and formalin almost entirely. Stains are apt to work better 
 after chromic fixing fluids. The formulas for several of the best 
 fixing agents with directions for their use are found in the last 
 chapter. 
 
 Washing. Except for special purposes, fixing fluids are washed 
 out of the tissues in running water, and they should be thoroughly 
 removed. For this purpose the author has made a galvanized 
 iron tank in which a gauze tray divided into small gauze compart- 
 ments is suspended. The water is brought into the tank through 
 a rubber tube with the mouth resting on the bottom, and leaves 
 through a spout at the top to which another tube can be attached. 
 In this way a large number of specimens can be washed at once 
 and their identity followed. 
 
 Preserving Tissues. After washing, the tissues should be carried 
 through the grades of alcohol, and may be preserved for a con- 
 siderable time in 80 per cent, alcohol, but it should be changed 
 occasionally. 
 (430/
 
 GENERAL HISTOLOGICAL METHODS 431 
 
 Choice of Sectioning Methods. The choice between paraffin and 
 celloidin for embedding depends upon the character of the section 
 desired and the nature of the tissue. Small objects and those of 
 delicate structure, such as embryos, dental pulps, etc., are best 
 sectioned in paraffin. Large pieces and blocks containing tissues 
 of different densities are more easily cut in celloidin. Paraffin 
 can be cut much thinner than celloidin, and is therefore preferable 
 for the minute study of cell structures with the high power. Cel- 
 loidin sections are more easily stained and are easier handled and 
 therefore preferable for the study of the arrangement of tissues 
 with low powers. The author prefers celloidin sections for class 
 work whenever possible. 
 
 Embedding in Paraffin. Tissues fixed and washed are taken from 
 80 per cent, alcohol and placed in 95 per cent, for twenty-four 
 hours, then in absolute alcohol for the same same length of time, 
 and the absolute alcohol should be changed once during this period, 
 from absolute alcohol to xylol, in which the tissue should remain 
 until it is clear and translucent. The time in xylol should be as 
 short as possible, as it has a hardening action. From xylol it is 
 placed in a solution of paraffin in xylol, and from this to soft paraffin 
 in the paraffin oven, at a temperature of not over 52 or 53 C. In 
 this it should remain from one-half to six hours, when it is trans- 
 ferred to hard paraffin in the oven for the same length of time. 
 The time in the oven should always be as short as is consistent 
 with a perfect infiltration. After sufficient time in hard paraffin 
 the tissue is blocked in the following way: A mold is made by 
 placing L-shaped pieces of metal together on a flat slab. These 
 are manufactured for the purpose. Melted paraffin is poured in 
 the mold and the tissue arranged in it, placing it so that the sec- 
 tions will cut in the direction desired. A film of paraffin will harden 
 at once on the slab and the tissue can be placed very nicely with 
 the needles. As soon as a film has formed over the surface the 
 slab with the mould should be immersed in cold \vater, so as to 
 harden the paraffin as quickly as possible. When cold, sections 
 may be cut at once or the block may be preserved in a pasteboard 
 carton properly labelled. As a rule, paraffin sections should be 
 cut as soon as possible. 
 
 Paraffin. The paraffin for embedding tissues must be of the 
 best quality. That prepared for this purpose by Griibler is prefer- 
 able. It should be of two grades, that melting at 45 C., and that 
 melting at 54 C. The hard paraffin is mixed with the softer,
 
 432 APPENDIX CHAPTER III 
 
 so as to give a melting-point at about 52. In winter softer paraffin 
 should be used than in summer, as the cutting quality depends 
 upon the adjustment of the paraffin to the temperature of the room. 
 If the paraffin is too hard the sections are liable to tear and curl; 
 if it is too soft, the structure of the tissue will be disturbed in 
 cutting. Perfect infiltration is always necessary for good sections. 
 Chloroform or oil of cedar may be substituted for xylol in this 
 process. Xylol is most rapid, but has some disadvantages in its 
 action on the tissues, especially if left too long. 
 
 Cutting Paraffin Sections. If the specimen has been placed at 
 one end of the block, the other end of the paraffin may be clamped 
 in the microtome. If the piece is too small, it should be fastened 
 to a block of vulcanized fiber with melted paraffin and the fiber 
 block clamped in the specimen holder. With a sharp scalpel the 
 excess of paraffin around the specimen should be trimmed off, 
 leaving the block in a rectangular form. The microtome knife 
 is placed at right angles to the microtome bed, and the side of the 
 block should be parallel with the blade. The specimen should 
 be brought up just to the edge and the first section cut. The knife 
 should be moved with a quick, sharp motion, as paraffin sections 
 are chopped when the knife is in this position. The knife is pushed 
 back, the block lifted with the micrometer screw so as to give a 
 section of the proper thickness, and the second section cut. If 
 the paraffin is of the proper consistency and the block has been 
 properly trimmed, the edge of the second section will stick to the 
 first and the sections stretch out over the knife in a ribbon. The 
 ribbons may be transferred to a piece of clean white paper and 
 complete series of sections cut. When series are not required larger 
 specimens are often cut better by placing the blade of the knife 
 obliquely and drawing it with a slow, even motion through the 
 block. If the sections show a tendency to roll up when the corner 
 of the section begins to curl over the edge of the knife, it may be 
 caught with the tip of a camel's-hair brush and so section after 
 section transferred to the paper. Paraffin sections should cut at 
 a thickness of from seven to ten microns, but sections as thin as 
 one micron may be cut from small blocks under ideal conditions. 
 
 Handling of Paraffin Sections. For staining, paraffin sections 
 must be fastened to the slide or cover-glass. If a few sections 
 are to be cut the slide is preferable; if many sections, as in the 
 preparation of class work, square cover-glasses should be used. 
 In either case the glass must be absolutely clean. A stock of per-
 
 GENERAL HISTOLOGICAL METHODS 433 
 
 fectly clean slides and cover-glasses should always be kept on hand 
 (see p. 439). A thin film of albumin fixative is spread upon the 
 glass; this film must be as thin as possible. The best way to spread 
 it is to put a drop of fixative on a glass slab or an ordinary slide, 
 touch the edge of the drop with the end of the little finger and 
 spread it over the cover-glass, wiping oft 7 all that can be removed 
 with the finger. Lay the cover-glasses film side up on a piece of 
 paper until the required number have been prepared. As each 
 section is cut it is laid on a cover-glass, straightened, and pressed 
 down with a camel's-hair brush. If the sections curl or wrinkle 
 they should be floated on water warmed just enough to soften the 
 paraffin but not melt it. As each section is cut it should be dropped 
 on the top of the water, where it will straighten out. When a 
 number have been placed on the surface of the water they may 
 be picked up by holding the cover-glass in the point of the pliers 
 and slipping it underneath the section and lifting it as on a section 
 
 FIG. 343. Morris staining dish. 
 
 lifter. The water is drained off and the cover-glass placed in the 
 groove of the tray of a Morris staining dish, 1 shown in Fig. 343. 
 Each tray will hold about thirty cover-glasses. They must now 
 be thoroughly dried by leaving them over night at room tem- 
 perature or for a shorter time in a warm oven, which should not 
 be hot enough to melt the paraffin. When dry, each cover-glass 
 should be picked up in the pliers and passed quickly through the 
 middle of a Bunsen flame, so as to coagulate the albumin, or they 
 may all be fixed at once in an oven. Heat that will just melt the 
 paraffin will coagulate the albumin and hold the section on the 
 glass. By means of a little wire basket the tray with the thirty 
 cover-glasses may now be carried from one dish to another through 
 the following necessary reagents. First, a minute or two in xylol 
 to remove the paraffin; then absolute alcohol, then 70 per cent.; 
 then water; Delafield's hematoxylin for five minutes; distilled 
 water to wash off the stain; acid alcohol (70 per cent, alcohol to 
 which 2 or 3 drops of hydrochloric acid has been added to every 
 
 1 These are manufactured by Bausch & Lomb, 
 28
 
 434 APPENDIX CHAPTER III 
 
 100 c.c. of alcohol); again washed in tap water to remove and 
 neutralize the acid (some prefer alcohol to which a few drops of 
 ammonia have been added); 70 per cent, alcohol; eosin for thirty 
 seconds; 70 per cent, alcohol, then 95 per cent., then absolute, 
 and finally xylol. From the xylol the sections may be mounted 
 or given out to the class. For class work a student brings to the 
 desk a clean slide with a drop of balsam on the centre and receives 
 a section. 
 Summary of Paraffin Method. 
 
 Tissues in 80 per cent, alcohol. 
 
 95 per cent, alcohol, twenty-four hours. 
 
 Absolute alcohol (changed once), twenty-four hours. 
 
 Xylol, one-half to six hours. 
 
 Xylol and paraffin, one-half hour. 
 
 Soft paraffin, one-half to six hours. 
 
 Hard paraffin, one to six hours. 
 
 Block. 
 
 Section. 
 
 Fix on glass. 
 
 Heat. 
 
 Xylol, one minute. 
 
 Absolute alcohol, one minute. 
 
 95 per cent, alcohol, same. 
 
 70 per cent, alcohol, same. 
 
 Distilled water. 
 
 Hematoxylin, five to ten minutes 
 
 Tap water. 
 
 Acid alcohol. 
 
 Tap water or ammonia alcohol. 
 
 70 per cent, alcohol. 
 
 Eosin, thirty seconds. 
 
 70 per cent, alcohol. 
 
 95 per cent, alcohol. 
 
 Absolute alcohol. 
 
 Xylol. 
 
 Mount in balsam. 
 
 Label. 
 
 Celloidin Method. Tissues fixed and washed are taken from 80 
 per cent, alcohol and placed in 95 per cent, for twenty-four hours; 
 then in absolute alcohol for the same length of time, changing 
 the alcohol once. Then into a mixture of absolute alcohol and
 
 GENERAL HISTOLOGICAL METHODS 435 
 
 ether for twenty-four hours, from this into a thin solution of cel- 
 loidin, in which they should remain for from two days to a week. 
 From the thin solution they should be placed in a thick celloidin 
 solution, about the consistency of syrup, for the same length of 
 time. The tissues may be kept in the celloidin solution indefinitely 
 without injury, and if the tissue is difficult to infiltrate it may be 
 of advantage to leave them in these solutions for weeks or months. 
 In this case the bottles must of course be perfectly corked to prevent 
 evaporation. 
 
 Blocking of Celloidin Material. There are several methods for 
 blocking celloidin materials, of which the author prefers the fol- 
 lowing: Thick celloidin is poured into a Stender dish or a small 
 Petri dish until there is enough to abundantly cover the specimens, 
 which are arranged on the bottom of the dish. A match or bit of 
 cork is placed under the edge of the cover so as to allow slow 
 evaporation. In a day or two the celloidin will attain the consis- 
 tence of a thick jelly. A knife is now passed around each tissue 
 and the celloidin containing the specimen lifted out, and the excess 
 of celloidin is trimmed away. A vulcanized fiber block has one 
 surface dipped into the thick celloidin and the specimen arranged 
 upon it. Thick celloidin is now added to surround and cover the 
 tissue with its adherent celloidin. As soon as this is hardened so 
 as to form a film it is dropped into 80 per cent, alcohol to harden 
 the entire mass. In this it must remain at least twenty-four hours 
 before it can be sectioned. Tissues embedded in celloidin may be 
 kept for years in 80 per cent, alcohol blocked and ready to cut 
 without great injury to the tissues. 
 
 Celloidin solutions for embedding should be kept in two grades 
 and labelled "thick" and "thin" celloidin. The latter should be 
 quite fluid, the former about a syrup consistence. Scherring's 
 celloidin is furnished in two forms, in shreds and granules. The 
 former will dissolve more rapidly. About half an ounce is placed 
 in a large-mouthed bottle, and a mixture of equal parts of absolute 
 alcohol and ether added. It dissolves slowly and should be shaken 
 frequently. When this solution is sufficiently thick, part may be 
 poured into another bottle and diluted with sufficient absolute 
 alcohol and ether for the thin solution, while the thicker portion 
 is poured into a bottle for the thick solution, and absolute alcohol 
 and ether may be added to the stock bottle to dissolve the residue. 
 When blocking tissues as described above the trimmings are dropped 
 back into the stock bottle.
 
 436 APPENDIX CHAPTER III 
 
 Cutting Celloidin Sections. The fiber block is clamped in the 
 specimen holder and adjusted. The knife is set diagonally so as 
 to cut with a drawing motion, and both the knife and the block 
 are kept flooded with 80 per cent, alcohol. The sections may be 
 allowed to pile up on the knife, and after eight or ten are cut they 
 are slid off with a camel's-hair brush on to a section lifter and 
 transferred to 80 per cent, alcohol, in which they may be kept for 
 some time. 
 
 Staining Celloidin Sections. For transferring celloidin sections 
 the most convenient thing is a small tea-strainer with a handle. 
 These may be got for a few cents at any hardware store. By means 
 of this the sections are transferred to 70 per cent, alcohol, from this 
 to distilled water, and are stained from five to ten minutes in Dela- 
 field's hematoxylin. The stain is then washed off with tap water, 
 destained with acid alcohol, washed in tap water or ammonia 
 alcohol, stained thirty seconds in eosin, washed with 70 per cent, 
 alcohol, from this to 95 per cent., in which they should be given 
 two or three changes. From this they are transferred to beech- 
 wood creosote or some other clearing agent (see p. 445), and in this 
 they may be kept until they are ready to mount or to be given out 
 to the class. For class work the student brings to the desk a clean 
 slide, and a section is placed upon the center of it. After blotting 
 off the excess of oil he adds a drop of balsam, covers with a cover- 
 glass, and labels the specimen. 
 Summary of Celloidin Method. 
 
 Tissues in 80 per cent, alcohol. 
 
 95 per cent, alcohol, twenty-four hours. 
 
 Absolute alcohol, changed twice, twenty-four hours. 
 
 Absolute alcohol and ether, twenty-four hours. 
 
 Thin celloidin, two days to a week. 
 
 Thick celloidin, the same. 
 
 Evaporate. 
 
 Block. 
 
 80 per cent, alcohol to harden or store. 
 
 Sections cut in 80 per cent, alcohol. 
 
 70 per cent, alcohol, one minute. 
 
 Distilled water. 
 
 Hematoxylin, five to ten minutes. 
 
 Tap water. 
 
 Acid alcohol. 
 
 Tap water or ammonia alcohol.
 
 GENERAL HISTOLOGICAL METHODS 437 
 
 70 per cent, alcohol. 
 
 Eosin, one miuute. 
 
 70 per cent, alcohol to wash. 
 
 95 per cent, alcohol, changed twice. 
 
 Creosote. 
 
 Mount in balsam. 
 
 Label. 
 
 Serial Sections with Celloidin. It is difficult to cut series of sec- 
 tions with the celloidin method. The simplest process, and one 
 used with success, is to carry the sections in order from the knife 
 to the slide, arranging three or four at one end of it and leaving 
 room for a label. Strips of porous tissue paper are cut the proper 
 size and one laid over the sections to hold them in place. A thread 
 is then lightly wrapped around the slide and paper, when they may 
 be carried through the necessary agents for staining, in Naples 
 jars. After they are cleared the paper is removed, the excess of 
 the oil blotted off, the balsam put upon the section and covered 
 with a long cover-glass. 
 
 SPECIAL METHODS. 
 
 Dental Pulp. The unerupted premolars from a young sheep 
 furnish excellent material for the study of the dental pulp. The 
 jaw r s of sheep slaughtered for spring lamb can be easily obtained 
 from the stockyards, and while still warm are placed in Miiller's 
 fluid and formalin, in which they are taken to the laboratory. 
 The temporary incisors are still in place and may be used for peri- 
 dental membrane material. 
 
 With the bone forceps the cortical plate is removed and the 
 unerupted teeth dissected from their crypts. By grasping the 
 base of the dental papillae with the pliers the pulp may be pulled 
 out of the dentin. They should then be replaced in Miiller's fluid 
 and formalin for twenty-four hours, when they may be carried 
 through the usual process, embedded in paraffin, and sectioned. 
 
 Human Pulps. By the cooperation of the extracting room human 
 pulps for histological work may be obtained. As soon as extracted 
 the tooth should be wrapped in a gauze napkin, placed in the 
 jaws of a heavy vise, which is carefully tightened until the tooth 
 cracks. The same thing may be accomplished by a heavy hammer 
 on an anvil. A few trials of this will enable one to crack the tooth 
 so that the pulps may be easily removed without injury. The
 
 438 APPENDIX CHAPTER III 
 
 cracked tooth is put in Miiller's fluid and formalin for twenty- 
 four hours, when the pieces of dentin are removed and the pulp 
 carefully lifted out of the pulp chamber. It is then carried through 
 the regular process, embedded in paraffin, and sectioned. If the 
 teeth are not perfect clinical history should be noted. 
 
 Periosteum. Young kittens that have not attained their full 
 growth may be used for this purpose. The bone should be very 
 carefully dissected so as not to injure the periosteum and then 
 sawed in pieces, using a fine metal saw. It is usually best simply 
 to saw it in two at the middle of the shaft and to fix it in Miiller's 
 fluid and formalin. After fixing and washing, it should be cut in 
 small pieces and decalcified in 2 to 5 per cent, nitric acid. A com- 
 paratively large volume of acid should be used and a pad of cotton 
 placed in the lower half of the bottle, or the tissue suspended by a 
 thread. It is best to change the acid once a day. Decalcifi cation 
 may require from two days to a week, and should be tested by 
 passing sharp needles through the tissues. As soon as decalcified 
 the tissue should be washed for twenty-four hours in running 
 water, carried through the grades of alcohol, and embedded in 
 celloidin. The sections should be cut at right angles to the shaft. 
 
 Peridental Membrane. For class work the peridental membranes 
 of sheep are the best for study, as their fibers are large and their 
 direction easily observed. They are much better than those of 
 either cat or dog, in which the fibers are much finer and the bone 
 more dense. The jaws are brought from the stockyards in Miiller's 
 fluid and formalin, the crowns broken off at the level of the gum 
 so as to expose the pulp chamber, and the jaws sawed through so 
 as to leave two teeth in each block, after which they are replaced 
 in Miiller's fluid and formalin for two days, decalcified in nitric 
 acid, and thoroughly washed. They may now be cut into small 
 blocks for transverse sections and embedded in celloidin. 
 
 Embryological Material. For the study of the tooth germ in 
 class work embryo pigs of all ages are easily obtained. The entire 
 embryo should be at once placed in Miiller's fluid or a saturated 
 solution of picric acid and water. In Miiller's fluid they should 
 remain a week; in picric acid, forty-eight hours. After fixing, 
 the heads are cut off, thoroughly washed, carried through the 
 grades of alcohol, and embedded in paraffin.
 
 APPENDIX CHAPTER IV. 
 
 FIXING AGENTS AND STAINING SOLUTIONS. 
 
 Cleaning of Slides and Cover-glasses. Slides or cover-glasses on 
 which paraffin sections are to be mounted must be absolutely 
 clean. They should be dropped in strong sulphuric acid and allowed 
 to remain a few minutes. The acid should then be poured off and 
 thoroughly removed with water, and strong acetic acid poured on. 
 After remaining a few minutes wash the acid off thoroughly and 
 wipe from alcohol. Keep ready for use in a clean box. 
 
 Meyer's Fixative. The white of an egg is chopped with a pair 
 of scissors and filtered through muslin, diluted with an equal volume 
 of glycerin, and a little sodium oxalate added to prevent decom- 
 position. 
 
 FIXING AGENTS. 
 
 Flemming's Solution. A good solution for fixing nuclear struct- 
 ures is the chromic acid solution of Flemming: 
 
 Parts. 
 
 Osmic acid, 1 per cent, aqueous solution 10 
 
 Chromic acid, 1 per cent, aqueous solution 25 
 
 Glacial acetic acid, 1 per cent, aqueous solution 10 
 
 Distilled water 55 
 
 Small pieces are fixed in a small quantity of the fluid for at least 
 twenty-four hours. They are then washed for the same number 
 of hours in running water and passed through 50, 75, and 80 per 
 cent, each twenty-four hours into 90 per cent, alcohol. 
 
 A stronger solution is made as follows: 
 
 Parts 
 
 Osmic acid, 2 per cent, aqueous solution 4 
 
 Chromic acid, 1 per cent, aqueous solution 15 
 
 Glacial acetic acid 1 
 
 Fol's Solution. A modification of Flemming's solution. 
 
 Parti 
 
 Osmic acid, 1 per cent, aqueous solution 2 
 
 Chromic acid, 1 per cent, aqueous solution 25 
 
 Glacial acetic acid, 2 per cent, aqueous solution 5 
 
 Distilled water 68 
 
 (439)
 
 440 APPENDIX CHAPTER IV 
 
 Corrosive Sublimate. An excellent fixing fluid is made by satu- 
 rating distilled water with corrosive sublimate. Small pieces 
 about 0.5 cm. in diameter are immersed in this fluid for from three 
 to twenty-four hours, then washed in running water for twenty- 
 four hours, and then transferred into 70 per cent, alcohol. After 
 twenty-four hours the tissues are placed in 80 per cent, for the same 
 length of time and then preserved in 90 per cent. It often occurs 
 that after changes in temperature crystals of sublimate are formed 
 on the surface or in the interior of the object. For their removal 
 a few drops of iodine and potassium iodide are added to the alcohol 
 (P. Mayer). It is a matter of indifference whether the 70 per cent., 
 80 per cent., or 90 per cent, alcohol is thus iodized. In future 
 treatment of the object, as well as in sectioning, any such crystals 
 of sublimate will not be found to be a hindrance. In the case of 
 delicate objects it is better to undertake their removal after sec- 
 tioning by adding iodine to the absolute alcohol then used. 
 
 Acetic Sublimate Solution. An excellent solution specially used 
 for embryonic tissues and for organs containing only a small quan- 
 tity of connective tissue. To a saturated aqueous solution of sub- 
 limate, 5 to 10 per cent, of glacial acetic acid is added. After 
 remaining two to three hours or more in this solution, the objects 
 are transferred to 35 per cent, alcohol and then passed through 
 the higher grades of alcohol. 
 
 Picric Acid. Small and medium-sized objects (up to 1 c.c.) are 
 fixed in twenty-four hours in a saturated aqueous solution of picric 
 acid (about 0.75 per cent.). Objects of considerable size may be 
 left in this solution for weeks without detriment. The tissues 
 are then transferred to 70 or 80 per cent, alcohol, in which they 
 remain until the alcohol is not colored by the picric acid. Instead 
 of a pure solution of picric acid, the picrosulphuric acid of Kleinen- 
 berg, or the picric acid of P. Mayer may be used. Picrosulphuric 
 acid is made as follows: 1 c.c. of concentrated sulphuric acid is 
 added to 100 c.c. of a . saturated aqueous picric acid solution. 
 Allow this to stand for twenty-four hours and dilute with double 
 its volume of distilled water. The picric acid solution is made by 
 adding 2 c.c. of pure nitric acid to 100 c.c. of saturated picric acid 
 solution. Filter after standing for twenty-four hours. 
 
 Chromic Acid. Chromic acid is used in a 3 to 1 per cent, aqueous 
 solution. Small pieces are fixed for twenty-four hours, larger ones 
 for a longer time. The quantity of the fixing fluid should equal at 
 least more than fifty times the volume of the tissues to be fixed.
 
 FIXING AGENTS 441 
 
 After fixing, objects must be washed for at least twenty-four hours 
 in running water, then through the grades of alcohols, and preserved 
 in 80 per cent. Two to 3 drops of formic acid to every 100 c.c. of 
 chromic acid solution improve their fixing properties. 
 MiiUer's Fluid. 
 
 Potassium bichromate 2 to 2.5 grams 
 
 Sodium sulphate 1 gram 
 
 Water 100 c.c. 
 
 This solution requires a long time for fixing, at least several 
 weeks, and for large pieces several months. During the first few 
 weeks the solution should be changed every three or four days 
 and later once a week, until it remains clear. Tissues should be 
 thoroughly washed in running water at least twenty-four hours. 
 For some special purposes it is better to wash in alcohol. Tissues 
 should be carried through the grades and preserved in 80 per cent, 
 alcohol. While tissues are in MiiUer's fluid they should be kept in 
 the dark. 
 
 Miiller's Fluid and Formalin. 
 
 Muller's fluid 100 c.c. 
 
 Formalin 10 c.c. 
 
 The addition of formalin to Muller's fluid greatly hastens fixa- 
 tion. It is an excellent agent of great penetrating power, and tissues 
 stain very well after it. Twenty-four hours will fix tissues of ordi- 
 nary size, though they may be left longer without damage. Bone 
 fixed too long in formalin is liable to be hard to cut. 
 
 Zenker's Fluid. 
 
 Grams. 
 
 Potassium bichromate 2.5 
 
 Sodium sulphate 1.0 
 
 Corrosive sublimate 5.0 
 
 Glacial acetic acid 5.0 
 
 Water 100.0 
 
 Add the glacial acid in proper proportion to the quantity of the 
 solution to be used, and not to the stock solution. Allow the tissues 
 to remain in this solution for from six to twenty-four hours. Then 
 wash in running water for from twelve to twenty-four hours and 
 transfer to gradually concentrated alcohol. Crystals of sublimate 
 which may be present are removed with iodized alcohol. Zenker's 
 fluid penetrates easily and fixes nuclear and protoplasmic structures 
 equally well without decreasing the staining qualities of the elements.
 
 442 APPENDIX CHAPTER IV 
 
 Formalin. Of recent years formalin, which is a 4 per cent, solu- 
 tion of the gas formaldehyde in water, has been much used as a 
 fixing fluid. Make a solution by adding 10 parts of formalin to 90 
 parts of water or normal saline solution. Small pieces of tissue 
 should remain in this for from twelve to twenty-four hours, larger 
 pieces a number of days or weeks, and then transfer to 90 per cent, 
 alcohol. 
 
 STAINING AGENTS. 
 Delafield's Hematoxylin. 
 
 Hematoxylin crystals 4 grams 
 
 Absolute alcohol 25 c.c. 
 
 Ammonia alum, aqueous solution 400 c.c. 
 
 Methyl alcohol 100 c.c. 
 
 Glycerin 100 c.c. 
 
 Dissolve hematoxylin crystals in absolute alcohol and add to 
 the alum solution, place in an open vessel for four days, then filter 
 and add the methyl alcohol and glycerin. 
 
 Hemalum (Mayer, 91).- One gram of hematin is dissolved by 
 heating in 50 c.c. of absolute alcohol. This is poured into a solu- 
 tion of 50 grams^of alum in 1 liter of distilled water and the whole 
 well stirred. A thymol crystal is added to prevent the growth of 
 fungus/lyThe advantages of hemalum is as follows: The stain may 
 be used immediately after its preparation, it stains quickly, never 
 overstating, especially when diluted with water, and penetrates 
 deeply, making it useful for staining in bulk. After staining sec- 
 tions or tissues are washed in distilled water. 
 
 Safranin. 
 
 Safranin 1 gram 
 
 Absolute alcohol 10 c.c. 
 
 Aniline water 90 c.c. 
 
 Aniline water is prepared by shaking up 5 c.c. to 8 c.c. of aniline 
 oil in 100 c.c. of distilled water and filtered through a wet filter. 
 Dissolve the safranin in the aniline water and add the alcohol. 
 Filter before using. 
 
 Stain sections fixed in Flemming's solution for twenty-four 
 hours and decolorize with a weak solution of hydrochloric acid in 
 absolute alcohol (1 to 1000). After a varying period of time, 
 usually only a few minutes, all the tissue elements will be found 
 to have become bleached, only the chromatin of the nucleus retain- 
 ing the color.
 
 STAINING AGENTS 443 
 
 Methyl Green. Stains very quickly. One gram is dissolved in 
 100 c.c. of distilled water to which 25 c.c. of absolute alcohol is 
 added. Rinse the sections in water, then place in 70 per cent, 
 alcohol for a few minutes, transfer to absolute alcohol for a minute, 
 etc. 
 
 Hematoxylin. Van Gieson's Acid Fuchsin-Picric Acid Solution. 
 Stain in any of the hematoxylin solutions, and after rinsing sec- 
 tions in water counter-stain in the following: 
 
 Acid fuchsin, 1 per cent, aqueous solution 5 c.c. 
 
 Picric acid, saturated aqueous solution 100 c.c. 
 
 Dilute with an equal quantity of water before using. The hema- 
 toxylin stained sections remain in the solution from one to two 
 minutes, are then rinsed in water, dehydrated, and cleared. 
 
 Hematoxylin-Eosin. Sections already stained in hematoxylin 
 are placed for two to five minutes in a 1 to 2 per cent, aqueous 
 solution of eosin or in a 1 per cent, solution of eosin in a 60 per 
 cent, solution of alcohol. They are then washed in water until 
 free from the stain, after which they remain for a short time in 
 absolute alcohol. In place of the eosin solution a 1 per cent, aqueous 
 solution of benzopurpurin may be used for the following solution 
 of erythrosin (Held). 
 
 Erythrosin 1 gram 
 
 Distilled water 150 c.c. 
 
 Glacial acetic acid 3 drops 
 
 Silver Nitrate Method. Especially useful for staining intercellular 
 substances of epithelium, endothelium, and mesothelium, and 
 the ground substance of connective tissues. It may be used on 
 either fresh or fixed tissues, fresh tissue, however, being more 
 satisfactory. Spread the tissues to be stained in thin layers; 
 immerse in a 0.5 to 1 per cent, solution of silver nitrate from ten 
 to fifteen minutes; rinse in distilled water and place in fresh dis- 
 tilled water or 70 per cent, alcohol or a 4 per cent, solution of for- 
 malin and expose to direct sunlight until they assume a brown color. 
 The sunlight reduces the silver in the form of fine particles which 
 appear black on being examined with transmitted light. The 
 preparations thus obtained may be examined in glycerin or dehy- 
 drated and mounted in balsam. 
 
 Glycerin. To mount in glycerin transfer the sections from water 
 to the slide, cover with a drop of glycerin, and apply the cover-slip.
 
 444 APPENDIX CHAPTER IV 
 
 Sections colored with a stain that would be injured by contact 
 with alcohol and where clearing is not especially necessary are 
 mounted this way. 
 
 Farrant's Gum Glycerin. In place of pure glycerin the following 
 mixture may be used : 
 
 Glycerin 50 c.c. 
 
 Water 50 c.c. 
 
 Gum arabic (powder) 50 grams 
 
 Arsenous acid 1 gram 
 
 Dissolve the arsenous acid in water. Place the gum arabic in a 
 glass mortar and mix it with the water, then add the glycerin. 
 Filter through a wet filter paper or through fine muslin. To pre- 
 serve such preparations for any length of time the cover-glasses 
 must be so fixed as to shut off the glycerin from the air. For this 
 purpose cements or varnishes are used, by painting over the edges 
 of the cover-glass. These masses adhere to the glass, harden, and 
 fasten the cover-glass firmly to the slide, hermetically sealing the 
 object. Kronig's is one of the best formulas for varnish, and is 
 made as follows: Melt 2 parts of wax and stir in 7 to 9 parts of 
 colophpnium and filter the mass hot. Before employing an oil 
 immersion lens it is best to paint the edges with an alcoholic solution 
 of shellac. 
 
 Silver Nitrate. In thin membranes and sections the vessel walls 
 can be rendered distinct by silver impregnation, which brings out 
 the outlines of their endothelial cells. This may be done either 
 by injecting the vessel with a 1 per cent, solution of silver nitrate, 
 or with a 0.25 per cent, solution of silver nitrate in gelatin. This 
 method is of advantage, since after hardening the capillaries of 
 the injected tissues appear slightly distended. Organs thus treated 
 can be sectioned, but the endothelial mosaic of the vessels does 
 not appear definitely until the sections have been exposed to sun- 
 light. 
 
 The injections of lymph channels, lymph vessels, and lymph 
 spaces is usually done by puncture. A pointed cannula is thrust 
 into the tissue and the syringe empties by a slight but constant 
 pressure. The injected fluid spreads by means of the channels 
 offering the least resistance. For this purpose it is best to use 
 aqueous solution of Berlin blue or silver nitrate, as the thicker 
 gelatin solutions cause tearing of the tissues.
 
 STAINING AGENTS 445 
 
 Clearing Agents. Clearing agents are substances of high refract- 
 ing index, mostly oils, which are used to displace alcohol and pre- 
 pare tissues for embedding and sections for mounting in balsam. 
 
 Clearing agents for embedding in paraffin must be miscible with 
 alcohol and solvents for paraffin. They are called clearing agents 
 because the tissues become translucent and clear in them. Xylol 
 is the most rapid and probably most used agent. It has, however, 
 a hardening action on the tissues, especially if they remain too 
 long in it. Pure oil of cedarwood when free from turpentine is 
 an excellent agent. Chloroform has been largely used for the same 
 purpose. 
 
 Before celloidin sections are mounted in balsam they must be 
 cleared. For this purpose an oil that will mix with 95 per cent, 
 alcohol is desirable, as absolute alcohol softens the celloidin. The 
 oil used must not dissolve the celloidin, and should not dissolve 
 the stain. Beechwood creosote is an excellent agent, and has been 
 largely used. It clears sections rapidly from 95 per cent, alcohol. 
 Oil of bergamot is an excellent agent, also oil of origanum; but in 
 the latter the oleum origani cretici and not the oleum origani 
 gallici must be used. A mixture of equal parts of oil of bergamot 
 and beechwood creosote has been used satisfactorily, and is an 
 excellent agent. A cheaper mixture is made of equal parts of 
 phenol, oil of origanum, and oil of cedarwood.
 
 INDEX. 
 
 ABSCESS, absorption of roots before and 
 
 after, 284 
 Absorbent organ, 282 
 
 osteoclasts as, 287 
 Absorption of bone, 276 
 of cementum, 163 
 of deciduous tooth roots, 279 
 
 causes of, 275 
 of dentin, 280 
 of enamel, 280 
 of implanted teeth, 282 
 of permanent tooth roots, 284 
 Acetic acid and sublimate for fixing, 440 
 Acrodont teeth, 235 
 Alveolar bone, 338 
 
 relation of, to mandible, 27 
 
 to teeth, 26 
 
 removal of, physiologically, 27 
 crest group of fibers, 241 
 division of peridental membrane, 
 
 239 
 
 Alzheimer, 279 
 Ameloblasts, 327 
 Amphioxus, 19 
 Anolpgies, definition of, 23 
 
 illustration of, 23 
 Antrum of Highmore. See Maxillary 
 
 sinus. 
 Apical division of peridental membrane, 
 
 239 
 
 group of fibers, 241 
 Arey, 276 
 Aristotle, 183 
 Attachment of teeth, 230 
 by ankylosis, 233 
 fibrous membrane, 231 
 hinge joint, 231 
 insertion in a socket, 235 
 
 B 
 
 BALSAM in grinding, 417 
 Bibra, von, on enamel, 38 
 Black on absorption of roots, 286 
 
 on epithelial cords, 260 
 
 on periosteum, 222 
 Bland-Sutton on absorptions, 277 
 
 Blastoderm formation, 310 
 
 layers of, 308 
 Blastula, 307 
 Blocking of celloidin material, 434 
 
 of paraffin material, 431 
 Blood supply of ameloblasts, 283 
 of osteoclasts, 280 
 of pulp, 171 
 Bloodvessels in cementum, 153 
 
 of peridental membrane, 267 
 
 of pulp, 171 
 Bohm, 277 
 Bone, 209 
 
 and cementum compared, 393 
 
 arrangement of, 213 
 
 canaliculi, 211 
 
 cancellous, 336 
 
 compact, 213 
 
 construction and destruction in 
 bone building, 214 
 
 Cope on, 340 
 
 corpuscles of, 211 
 
 decalcified, 393 
 
 definition of, 209 
 
 distribution in mandible, 341 
 
 endochondral, 216 
 
 endomembranous, 218 
 
 fibers of Sharpey in, 160 
 
 formation and growth of, 216 
 
 ground sections of, 392 
 
 growth of, 219 
 
 Haver sian system, 211 
 
 influences of mechanical forces on, 
 350 
 
 interstitial, 214 
 
 lacunae of, 211 
 
 compared with lacunae of 
 cementum, 158 
 
 matrix of, 210 
 
 osteoclasts in, 214, 217 
 
 periosteal buds in, 217 
 
 relation of teeth to, 26, 334 
 
 structural elements of, 209 
 
 subperiosteal, 211 
 
 compared with cementum, 394 
 
 varieties of, 211 
 
 Volkman's canals in, 211 
 Branchial arches, 313 
 
 arteries, 313 
 
 clefts, 313 
 
 (447)
 
 448 
 
 INDEX 
 
 Branching of dentinal tubules in crown, 
 
 139 
 
 in root, 143 
 Bredichin, 276 
 Brooks, Dr., 201 
 Burchard and Inglis, 285 
 
 CALCIFICATION, beginning of, of teeth, 
 325 
 
 of bone, 216 
 
 of cememtum, 153 
 
 of dentin, 326 
 
 of enamel, 326 
 
 Calcium carbonate in enamel and den- 
 tin, 38, 136 
 
 fluoride in enamel and dentin, 38, 
 136 
 
 phosphate in enamel and dentin, 
 
 38, 136 
 Canaliculi of bone, 211 
 
 of cementum, 158 
 Caries of dentin, 59 
 
 of enamel, 48 
 
 granular layer of Tomes and, 146 
 
 intensity and liability of, 60 
 
 interglobular spaces and, 149 
 
 secondary or backward, 62 
 
 stages in progress of, 56. 
 Carotid, internal, 189 
 Cartilage in bone formation, 216 
 
 in enamel analysis, 38 
 
 Meckel's, 325 
 Causch, 277 
 Cavities, classes of, 96 
 
 relation of, to marginal ridges, 127 
 
 rod direction in preparation of, 97 
 
 structural requirements of, 90 
 Cavo-surface angle, 90 
 
 requirements in preparation 
 
 of, 90 
 Cell division, 300 
 
 theory of, 229 
 
 walls, 202 
 
 Celloidin, blocking of, 434 
 method of, 437 
 
 cutting of, 436 
 
 serial sections of, 437 
 
 staining of, 436 
 
 stock solutions of, 435 
 Cementing substance, 43 
 Cement oblasts, 251 
 Cementum, 153 
 
 absorption of, 163, 278 
 
 compared with bone, 293 
 
 corpuscles of, 160 
 
 definition of, 153 
 
 development of, 153 
 
 distribution of, 32 
 
 Cementum, function of, 153 
 Haversian canals in, 153 
 histogenesis of, 154 
 imbedded fibers in, 160 
 lacunae of, 158 
 
 compared with those of bone, 
 
 158 
 
 lamellae of, 154 
 structural elements of, 154 
 Cervical division of peridental mem- 
 brane, 239 
 Chemical composition of dentin, 136 
 
 of enamel, 38 
 ideas, 301 
 
 Chromic acid for fixing, 440 
 Chromosomes, vehicles of transmission, 
 
 301 
 
 Chronology of dental follicle, 329 
 Cleaning slides, 439 
 Clearing agents, 445 
 Cleft palate, 319 
 Compensating canals, 284 
 Connective tissue, 203 
 
 cells becoming phagocytic, 279 
 chemical relation of formed 
 materials to cytoplasm, 207 
 mutations of, 203 
 response to chemical changes, 
 
 207 
 
 Cope on bone, 340 
 Corrosive sublimate for fixing, 440 
 Cortical plates, 336 
 Creosote, 445 
 Cribriform plates, 331 
 Cryer, 338 
 Czermak on interglobular spaces, 146 
 
 D 
 
 DAUTSCHAKOFF, 276 
 Decalcification of bone, 438 
 
 by osteoblasts, 278 
 
 in osteomalacia, 276 
 
 of teeth, 438 
 
 Delafield and Pruden, 277 
 Dental caries, 48 
 
 of dentin, 59 
 of enamel, 48 
 
 follicle, 324, 331 
 
 lamina, 331 
 
 ligament, 342 
 
 papilla, 323 
 
 pulp, 164 
 
 ridge, 321, 331 
 Dentin, 135 
 
 absorptions of, 281 
 
 calcification of, 326 
 
 caries of, 59 
 
 changes with aye, 137 
 
 chemical analysis of, 136
 
 INDEX 
 
 449 
 
 Dentin, clear layer of, 145 
 defects in, 146 
 definition of, 135 
 development of, 326 
 distribution of, 29 
 fibrils of, 144 
 
 formative cells of, 113, 166 
 function of, 135 
 granular layer of, 145 
 histogenesis of, 135 
 interglobular spaces in, 146 
 lines of Schreger, 149 
 matrix of, 136 
 secondary, 150 
 sheaths of Newman, 137 
 structural elements of, 135 
 tubules of, 138 
 
 branching of, 143 
 caries in, 61 
 curves of, 139 
 diameter of, 138 
 direction of, in crown, 139 
 
 in root, 143 
 
 Dento-cemental junction, 144 
 Dento-enamel junction, 31, 139 
 and caries, 62 
 characteristics of, 143 
 sensitiveness of, 143 
 Dermal scales, 22, 230 
 Descriptive terms, 34 
 Dewey, Dr. Kaethe, 175, 197 
 Dissecting, 426 
 
 Dog teeth, absorptions of, 279 
 Duval, 277 
 
 ECTODERM. See Epiblast. 
 Embryology, 302 
 
 biological considerations funda- 
 mental, 298 
 
 chemical ideas related to, 301 
 
 earlv stages of, 302 
 
 of teeth, 321 
 Enamel, 28, 37 
 
 abrasion of, 96 
 
 absorption of, 280 
 
 action of acid on, 45 
 
 appearances of, 67 
 
 areas of weakness, 125 
 
 bands of Retzius, 70 
 
 blood supply of formative cells, 
 116, 284 
 
 calcification of, 326 
 
 caps, 113 
 
 cavity walls in, 90 
 
 cementing substances of, 43 
 
 characteristics of, 63 
 
 chemical composition of, 38 
 
 cleavage of, 84 
 29 
 
 Enamel cuticle, 74 
 defects in, 113 
 degree of calcification of, 38 
 development of, 326 
 differences between, and other cal- 
 cified tissue, 37 
 rods and cementing sub- 
 stance, 43 
 
 direction of rods, 41, 78 
 distribution of, 28 
 effect of caries on, 48 
 of elephant's tusk, 29 
 etching of, 45 
 function of, 28 
 gnarled, 64 
 of herbivora teeth, 34 
 histogenesis of, 37 
 hypoplasia of, 82 
 incremental lines of, 70 
 lines of Schreger in, 73 
 Nasmyth's membrane, 74 
 organ, 322 
 
 ameloblasts of, 327 
 
 blood supply of, 283 
 
 development of, 322 
 
 effect on mesenchymal tissue, 
 322 
 
 loss of, 40 
 
 of molar teeth, permanent, 327 
 
 remains of, 261 
 
 stellate reticulum of, 323 
 
 stratum intermedium of, 332 
 
 tunics of, 323 
 origin of, 37 
 planing of, 87 
 refraction of, 43 
 
 relation of formative organ to, 40 
 relative solubility of, 44 
 
 strength of rods and cement 
 
 substance, 43 
 of rodent teeth, 34 
 rods, 41 
 
 diameter of, 41 
 
 direction of, 41, 78 
 
 length of, 42 
 
 refraction of, 43 
 
 size of, 41 
 spindles, 76 
 straight, 64 
 stratification of, 68 
 striation of, 67 
 structural defects of, 113 
 
 elements of, 37 
 Tomes on, 39 
 Williams on, 39 
 Endoskeleton, 19 
 Endothelial cells as phagocytes, 277 
 
 Mallory on, .277 
 relation to nervous system, 22 
 Epiblast, 308 
 Epithelial cords, 260
 
 450 
 
 INDEX 
 
 Epithelial cords, arrangement of, 261 
 
 of cells in, 263 
 Black on, 260 
 derivation of, 261 
 distribution of, 261 
 as lymphatics, 260 
 von Brunn on, 260 
 
 Eruption of teeth, 275 
 
 Etching of enamel, 45 
 
 Eustachius, 183 
 
 Exoskeleton, 19 
 
 FACIAL artery, 197 
 
 Farrant's gum glycerin, 444 
 
 Fastening teeth to disks, 413 
 
 Fat in dentin, 136 
 in enamel, 38 
 
 Fertilization, 304 
 
 Fibers of peridental membrane, 240 
 classification of, 241 
 imbedded in alveolar 
 
 process, 241 
 in cementum, 160 
 
 Fibrils of odontoblasts, 144, 168 
 
 Fibroblasts in peridental membrane, 
 250 
 
 Filiform papillae, 293 
 
 Fischer, 277 
 
 Fixatives, 427, 430 
 
 Fixing, 426 
 
 Flemming's solution, 439 
 
 Follicle, dental, 324, 331 
 
 Fol's solution, 439 
 
 Foramen, apical, 171 
 
 Forces influencing bone growths, 340, 
 349 
 
 Frontal nasal process, 318 
 
 Fungiform papillae, 293 
 
 G 
 
 GASTRULA, 307 
 Germ layers, 308 
 Giant cells, 277 
 Gilmer, 282 
 GingivgD, 244 
 
 lymph vessels of, 195 
 Gingival division of root, 238 
 group of fibers, 241 
 space, epithelium of, 264 
 Glands of Serres, 264 
 
 of tongue, 290 
 Glycerin for mounting, 443 
 Gomphosis, 236 
 Granular layer of Tomes, 145 
 
 difficulty of staining, 145 
 invisibility of, in haema- 
 toxylin and eosin 
 stain, 145 
 
 Granular layer of Tomes, Skillen's stain 
 
 for, 145 
 
 Grinding of crumbled material, 420 
 disks, 408 
 
 in hard balsam, 416 
 of frail material, 416 
 machine, 403 
 
 clogging of stones of, 421 
 fastening teeth to disks of, 413 
 lap wheels for, 409 
 point finder of, 409 
 preparation of shellac for, 419 
 slicing mechanism for, 422 
 spatter guards for, 411 
 spiders and dogs for, 412 
 stones for, 410 
 watering stones of, 410 
 rapidity of, 414 
 removal of cover-glass from disk 
 
 of, 418 
 of tooth sections, 379 
 
 process of, 410 
 Growth force, 349 
 Gubernaculum dentis, 275 
 Gum, 289 
 
 epithelium of, 288 
 fibers of, 289 
 
 H^MALTJM, 442 
 
 Haematoxylin and eosin, 443 
 
 Delafield's, 442 
 
 failure to stain granular layer of 
 
 Tomes, 145 
 
 Hair compared with tooth, 24 
 Hardening, 427 
 Hare lip, 320 
 Hassin, 279 
 Haversian systems of bone, 211 
 
 of cementum, 153 
 Hertwig's embryology, 230, 320 
 Hess, 284 
 Hinged teeth, 231 
 Histological technic, 424 
 Holoblastic segmentation, 306 
 Homology, 19, 23 
 Horizontal group of fibers, 241 
 Howell, 276 
 Howship's lacunas, 255 
 Huber on pulpal nerves, 177 
 Huxley, 75 
 Hypoblast, 308 
 Hypoplasia of enamel, 82 
 
 IMPLANTED teeth, absorbed, 282 
 Incremental lines, 70
 
 INDEX 
 
 451 
 
 Indexing and filing, 427 
 Inferior dental nerve, 354 
 Inglis, 285 
 Intercellular substances, 200 
 
 in pulp, 171 
 
 kinds of, 202 
 relation of cells to, 201 
 Interglobular spaces, 146 
 
 Czermak on, 146 
 Intermaxillary bone, 319 
 
 JACKSON, 276 
 
 Jaws, changes with age, 26 
 
 growth of, 347 
 Jugular, internal, 189 
 
 KERATINIZED scales, 19 
 Kolliker on osteoclasts, 276 
 Krause, 183 
 
 LABELLING of slides, 429 
 Laboratory methods, 430 
 
 directions for students, 381 
 Lacunse of bone, 211 
 of cementum, 158 
 
 compared with bone, 393 
 Lamellae of cementum, 154 
 Lansit, 19 
 Lap-wheels, 409 
 Lateral nasal process, 318 
 Layer of Weil, 171 
 Leukocytes in lymph stream, 183 
 
 as origin of osteoclasts, 277 
 Ligamentum circulare, dental ligament, 
 
 242 
 penetrated by lymph channels, 
 
 195 
 
 Lingual tonsils, 297 
 Lymphatics, central trunks of, 199 
 character of fluid of, 183 
 coagulation of fluid of, 181 
 collecting trunks of, 184, 186 
 descending cervical chain of, 190 
 Dewey's work on, 197 
 discovery of, 183 
 Eustachius, 183 
 external glands of, 190 
 function of, 181 
 of gingivae, 195 
 of head and neck, 186 
 
 mastoid group, 188 
 parotid and subparotid 
 group, 189 
 
 Lymphatics of head and neck, retro- 
 pharyngeal group, 190 
 submaxillary group, 189 
 submental group, 190 
 suboccipital group, 188 
 internal glands, 190 
 of lips, 192 
 
 lymphatic duct (right), 182 
 Massa on, 183 
 of mouth and gums, 193 
 
 inner surface of man- 
 dible, 193 
 of maxillae, 193 
 outer surface of man- 
 dible, 193 
 of maxillae, 194 
 network of origin, 184, 186 
 nodes or glands, 186 
 parts of, 183 
 
 of peri dental membrane, 195 
 of pulp, 197 
 Schweitzer on, 197 
 substernomastoid glands, 190 
 thoracic duct, 182 
 of tongue, 197 
 
 anterior apical, 197 
 marginal, 198 
 median or central, 199 
 posterior or basal, 198 
 
 M 
 
 MACERATION, 476 
 
 Magitot, 330 
 
 Magnesium phosphate in dentin, 136 
 
 in enamel, 38 
 Mallory, 277 
 
 Mammalian segmentation, 310 
 Mandible, buds of, 317 
 
 distribution of bone of, 341 
 
 growth of, 335 
 
 structure of, 336 
 Marginal ridges as areas of weakness, 
 
 127 
 
 Massa, 183 
 Matrix of bone, 210 
 
 of dentin, 136 
 Maturation, 303 
 Maxilla, palatal process of, 319 
 
 structure of, 336 
 Maxillary sinus, 347 
 McMurrich, 304 
 Meckel's cartilage, 325 
 Membrana eboris, 169 
 Meroblastic segmentation, 308 
 Methods of embedding, 431, 435 
 Methyl green, 443 
 Meyer's fixative, 439 
 Miller on caries, 57 
 Molar, permanent origin of, 327
 
 452 
 
 Morris' staining dish, 433 
 
 Mounting of specimens, 429 
 
 Mouth cavity, 288, 314, 319 
 epithelium of, 288 
 formation of, 314, 319 
 glands of, 290 
 mucous membrane of, 288 
 nerve endings in, 290 
 separation from nose cavity, 
 
 319 
 
 submucosa of, 289 
 taste-buds of, 295 
 tongue, 291 
 
 Mucous glands, 290 
 
 M tillers fluid, 441 
 
 Mummery, 177 
 
 N 
 
 NASMYTH'S membrane, 74 
 Nerve fibers in dentin, 178 
 
 in peridental membrane, 271 
 
 in pulp, 177 
 Newman's sheaths, 137 
 Nissl, 279 
 Northwestern University Dental 
 
 School, 274, 280 
 Notochord, 19 
 
 OBLIQUE group of fibers, 241 
 Odontoblasts, 113, 166 
 Oil of bergamot, 445 
 
 of cedarwood, 445 
 
 of origanum, 455 
 
 Oocytes, primary and secondary, 303 
 Oogonia, 303 
 
 Osteoblastsof peridental membrane,254 
 Osteoclasts, 276 
 
 as absorbent organ, 287 
 
 in burrowing canals, 255 
 
 in cementum, 278 
 
 in dentin, 281 
 
 function of, 276 
 
 origin of 276 
 
 in peridental membrane, 278 
 Owen's odontography, 75, 150 
 
 PALATE, formation of, 319 
 
 soft, 295 
 Papillae of gingivse, 266 
 
 of gum, 289 
 
 of lip, 291 
 
 of tongue, 293 
 Paraffin, cutting of 432 
 
 Paraffin, embedding in, 431 
 kinds of, 431 
 
 method, summary of, 434 
 Pathological absorption of permanent 
 
 tooth roots, 282 
 Paul, 75 
 
 Peridental membrane, 237 
 absorption of, 278 
 arrangement of fibers of, 240 
 blood supply of, 267 
 cellular elements of, 250 
 cementoblasts in, 251 
 changes in, with age, 272 
 classification of fibers of, 240 
 
 Black on, 241 
 comparison with capsules of 
 
 organ, 238 
 with periosteum, 238 
 definition of, 237 
 division of, 238 
 epithelial structure in, 260 
 Black on, 260 
 von Brunn on, 260 
 fibroblasts in, 250 
 fibrous tissue of, 240 
 function of, 240 
 gland of Serres, 264 
 lymphatics of, 195, 270 
 nerves of, 271 
 nomenclature of, 237 
 Pacinian corpuscles in, 271 
 practical consideration of, 274 
 preparation of material in, 274 
 principal fibers of, 240 
 
 classification of, 241 
 relation of cementoblasts in, 
 
 to cure of pockets, 253 
 structural elements of, 240 
 Periosteum, appearance of, 223 
 attached complex, 229 
 
 simple, 227 
 Black on, 222 
 classification of, 222 
 definition of, 222 
 function of, 222 
 layers of, 224 
 
 relation of attachment of, to bur- 
 rowing pus, 224 
 structural elements of, 224 
 unattached complex, 226 
 
 simple, 225 
 Permanent molars, first, 327 
 
 second and third, 330 
 teeth, origin of, 324 
 Physiological absorption of tooth root, 
 
 279 
 
 Picric acid, 440 
 Placoid scales, 22 
 Pleurodont, 235 
 Point finder, 409 
 Polar bodies, 303
 
 INDEX 
 
 453 
 
 Prentiss, 276 
 
 Preparation of material, 411 
 Preserving, 430 
 Processes globularis, 319 
 Pulp, 164 
 
 arteries of, 171 
 
 cells, connective tissue, 170 
 specialized, 166 
 
 function of, 164 
 
 intercellular substance of, 171 
 
 layer of Weil, 171 
 
 lymphatics of, 175 
 
 membrana eboris of, 169 
 
 QUAIN, 314 
 
 RAPIDITY of grinding, 414 
 Reattachment of tissue to tooth roots, 
 
 253 
 Relation of enamel to formative organ, 
 
 40 
 
 of nucleus to cytoplasm, 299 
 of tooth to bone, 26 
 Removal of cover-glass from grinding 
 
 disk, 418 
 
 Retzius, bands of, 70 
 Rose, 179 
 
 S 
 
 SAFEANIN, 442 
 
 Salter, 168, 283 
 
 Schaffer, 276 
 
 Schweitzer, 197 
 
 Secondary curves of dentinal tubules, 
 
 139 
 dentin, 150 
 
 and cementum, study of, 391 
 tubules of, 150 
 Sectioning methods, 431 
 Segmentation, 306 
 holoblastic, 306 
 mammalian, 310 
 meroblastic, 308 
 Serial sections, 437 
 Serres, gland of, 264 
 Sertoli, 304 
 Sharpey's fibers, 160 
 Sheaths of Newman, 137 
 Sheep teeth, absorption of, 278 
 Silver nitrate, 443, 444 
 injection, 444 
 
 Skillen's stain of granular layer of 
 Tomes, 145 
 
 Slicing mechanism, 422 
 Southwell's experiment, 146 
 Spatter guard, 411 
 Spermatids, 304 
 Spermatocyte, 304 
 Spermatogenesis, 304 
 Spermatogonia, 304 
 Spermatozoa, 304 
 Staining agents, 442 
 
 of celloidin sections, 436 
 Stellate reticulum, 323 
 Stohr, 228 
 Stowell, 171 
 
 Stratum intermedium, 332 
 Subperiosteal bone, 211 
 
 and cementum, 393 
 
 TASTE-BUDS, 295 
 Teasing, 42^ 
 
 Teeth, attachment of, 230 
 for grinding, 377 
 relation of, to bone, 26 
 Thecodont attachment, 235 
 Tissue changes with movement of 
 
 teeth, 368 
 
 Tomes, Charles, 39, 230 
 granular laver of, 145 
 John, 145, 276 
 Tongue, 290 
 
 epithelium of, 294 
 glands of, 290 
 muscles of, 291 
 papillae of, 293 
 
 circumvallate, 293 
 filiform, 293 
 fungiform, 293 
 taste-buds of, 295 
 tonsils of, 297 
 Tonsils, 296 
 lingual, 297 
 palatal, 297 
 pharyngeal, 297 
 Tooth attachment, 230 
 ankylosis, 233 
 fibrous, 231 
 gomphosis, 236 
 hinge-joint, 231 
 germs, beginning of formation of, 
 
 325 
 
 of permanent teeth, 324 
 origin of, 324 
 time of, 325 
 
 of temporary teeth, 322 
 Traneeptal group of fibers, 241 
 Transmission, vehicle of, 301 
 Transverse sections of tooth roots, 380
 
 454 
 
 INDEX 
 
 VITAL function of peridental mer 
 
 brane, 241 
 of pulp, 164 
 
 Von Bibra enamel analysis, 38 
 Von Brunn and epithelial cords, 260 
 Von Giesen stain, 443 
 
 W 
 
 WALDEYER, 75 
 Walkoff, 337 
 
 Washing of tissue, 430 
 
 Waste water, 411 
 
 Water of crystallization in enamel, 39 
 
 Watering stones in grinding, 410 
 
 Weed, 279 
 
 Wegener, 276 
 
 Weil, 171 
 
 Williams on enamel, 39 
 
 Wilson, 305 
 
 ZENKER'S solution, 441
 
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