BIOLOGY 
 
 R 
 9 
 
OXFORD MEDICAL PUBLICATIONS 
 
 THE MICROSCOPIC 
 ANATOMY OF THE TEETH 
 
PUBLISHED BY THE JOINT COMMITTEE OF 
 
 HENRY FROWDE, HODDER & STOUGHTON 
 
 17, WARWICK SQUARE 
 
 LONDON, B.C. 4 
 
OXFORD MEDICAL PUBLICATIONS 
 
 THE MICROSCOPIC 
 ANATOMY OF THE TEETH 
 
 BY 
 
 J. HOWARD MUMMERY 
 
 D.Sc. (PENN.), M.R.C.S., L.D.S. ENG. 
 
 LONDON 
 
 HENRY FROWDE HODDER & STOUGHTON 
 
 OXFORD UNIVERSITY PRESS WARWICK SQUARE, E.C. 
 
 1919 
 
Q L1L 
 
 M9 
 
 BIOLOG* 
 
 UBRARY 
 
 G 
 
TO 
 
 SIR EDWARD SHARPEY SCHAFER, F.R.S. 
 
 IN ACKNOWLEDGEMENT OF MANY KIND SUGGESTIONS 
 
 AND IN REMEMBRANCE OF PAST ASSOCIATION 
 
 UNDER THE GENIAL INFLUENCE OF THAT 
 
 GREAT TEACHER OF PHYSIOLOGY 
 
 PROFESSOR WILLIAM SHARPEY 
 
 
 462538 
 
e Science may appear to lose influence when the fallacy of a prevailing 
 hypothesis is demonstrated ; but it holds a treasured reputation for 
 honesty of purpose by frankly acknowledging and registering its 
 mistakes.' J. W. MELLOE. 
 
 ' If a man will begin with certainties, he shall end in doubts ; but if 
 he will be content to begin with doubts, he shall end in certainties/ 
 FRANCIS BACON. 
 
PREFACE 
 
 IN the following pages I have endeavoured to bring up to 
 date, as far as possible, our knowledge of the microscopic 
 anatomy or histology of the teeth. 
 
 No doubt much remains obscure, and there are very 
 many points which are still matters of controversy, but 
 modern methods of research have done much to clear up 
 a great deal of this obscurity, which must, however, always 
 attend the study of such difficult tissues to investigate as 
 the dentine and enamel of the teeth. 
 
 Sir Charles Tomes's Dental Anatomy is still the acknow- 
 ledged and authoritative text -book for the wide field which 
 it covers, and the present work is confined to dental histology, 
 in which there are many points which have not been fully 
 considered in recent English text-books on the subject. 
 
 In presenting the results of my own investigations, which 
 have occupied me for many years, I hope I have adequately 
 acknowledged the work of others on the same subject. It 
 has been difficult during the last four strenuous years to 
 obtain papers and communications from abroad, and if 
 I have overlooked any important research I can only express 
 my regret for any such omission. 
 
 It may perhaps be considered that I have hardly given 
 sufficient space to the historical aspect of histological 
 research, but I have avoided referring at great length to 
 obsolete views and those now not generally received, in 
 order that the student may obtain a clearer understanding 
 of the present state of knowledge and of the different theories 
 that chiefly hold their ground at the present day. 
 
 To the pioneers in dental histology, and in England 
 especially to Sir John Tomes and Mr. James Salter, we owe 
 a great debt of gratitude. These early investigators obtained 
 
viii PREFACE 
 
 the most important results with the comparatively imperfect 
 methods of research then at their disposal, and stimulated 
 and guided the work of those who followed them, whose 
 facilities with perfected modern methods have been very 
 much greater ; results which might never have been obtained 
 had the way not been pointed out by their predecessors in 
 the same field of knowledge. 
 
 I desire to most gratefully acknowledge my indebtedness 
 to friends who have given me assistance in carrying out this 
 work ; especially to my friend Mr. Montagu Hopson, to whom 
 I owe many valuable suggestions, and whose kind help in 
 reading the proof-sheets and assisting in the compilation of 
 the index has been of the greatest value. 
 
 I also wish to acknowledge my indebtedness to Professor 
 Symington, F.R.S., for his kindness in supplying me with 
 valuable material and permission to reproduce a photograph 
 from the Atlas of Skiagrams ; to Sir E. Sharpey Schafer, 
 F.R.S., for the use of an illustration from the Microscopic 
 Anatomy ; to Sir E. Ray Lankester, F.R.S., for permission 
 to reproduce a figure from the Quarterly Journal of Micro- 
 scopical Science ; to Professor Osborn for the use of two 
 figures from his work on The Evolution of the Mammalian 
 Molar Teeth ; and to the publishers of Leduc's Mechanism 
 of Life for permission to reproduce an illustration from that 
 work. 
 
 To Mr. John Humphreys of Birmingham I am indebted 
 for the loan of several important preparations from his 
 valuable collection. 
 
 The illustrations, when not otherwise stated, are from my 
 own photographs and preparations. 
 
 J. HOWARD MUMMERY. 
 
 70 ALBERT BRIDGE ROAD, S.W. 
 
TABLE OF CONTENTS 
 
 CHAPTER PAGE 
 
 PREFACE ....... vii 
 
 INTRODUCTION ..... 3 
 
 I. DEVELOPMENT OF THE TEETH IN MAMMALIA 13 
 Evolution of the Human Molar " ^ .32 
 
 II. ENAMEL . . . ... 46 
 
 Tubular Enamel . . ,\ . . 84 
 The Enamel of Rodents . . .106 
 
 III. DEVELOPMENT AND CALCIFICATION OF THE 
 
 ENAMEL . . . ... 119 
 
 IV. THE DENTAL PULP . . . . i, 197 
 
 V. DENTINE . . . . ... 236 
 
 Development . . . . .262 
 
 Calcification . . . " . . . 278 
 
 VI. CEMENT . . . '.' . . . 287 
 
 Development . . . . . . 294 
 
 Calcification . '[ . . . 298 
 
 Absorption . . . , . .299 
 
 VII. THE PERIODONTAL MEMBRANE . ; . 305 
 The Gum . . . . ... 309 
 
 VIII. THE TOOTH FOLLICLE AND ITS CONNEXIONS 311 
 
 IX. NASMYTH'S MEMBRANE .... 333 
 Eruption of the Teeth . . . .346 
 
 X. THE ATTACHMENT OF TEETH . . . 352 
 XI. HORNY TEETH ...... 366 
 
 INDEX 871 
 
 MUMMERY Ti 
 
INTRODUCTION 
 
 IN a work on the microscopic anatomy of the teeth it 
 may perhaps be considered necessary to define exactly what 
 is meant by a tooth. This might seem at first sight an easy 
 matter, but in formulating any exact definition we meet 
 with many difficulties. 
 
 It may be said that if we except the bill of the bird, all 
 horny or calcified organs which occupy the commencement 
 of the alimentary canal may be looked upon as teeth, and 
 that their primary function is prehension. But such a defini- 
 tion does not cover the whole ground, for some teeth never 
 erupt, as the incisors of the female Narwhal ; teeth are 
 found in ovarian cysts, while the placoid scales of the 
 Sharks are identical in structure with true teeth. 
 
 Although the bill of the bird does not appear to fall 
 under the category of teeth, birds may be looked upon as 
 modified reptiles, and there is little doubt that they are 
 descended from forms which possessed true teeth, but these 
 were lost before Tertiary times. 
 
 The fossil birds described by Marsh were furnished with 
 teeth, and the Archseopteryx was a toothed animal with 
 strong affinities to both birds and reptiles. Among recent 
 birds Carl Rose has described the formation of a tooth-band 
 in a tern (Sterna Wilsoni), although it is quite rudimentary," 
 and no teeth are derived from it (6). 
 
 Although the primary function of teeth is prehension, 
 they are also used in many instances as weapons of offence 
 and defence, as the tusks of the Boar ; and those that are 
 developed exclusively in the male serve as sexual weapons. 
 The enormous canines of the Walrus assist in locomotion 
 and grasping, as well as in combat, and the greatly developed 
 incisors forming the tusks of the Elephant are used in 
 uprooting trees and carrying heavy weights, while the Beaver 
 employs its sharp incisors in felling timber to form its dams. 
 In the human being teeth serve as aids to speech. 
 
 These organs are variously adapted by their form and 
 
 B2 
 
4 MICROSCOPIC 1 ANATOMY OF THE TEETH 
 
 structure to their several purposes ; the pointed, more or 
 less conical teeth being employed for the seizure of prey, 
 both the sharp pointed teeth of many fishes and the more 
 powerful canines of Carnivora. The incisors are modified for 
 dividing and tearing, and the molars both for this purpose, 
 and also, in many animals, for the grinding and trituration 
 of the food. 
 
 In all these modifications they are most perfectly adapted 
 to their chief purpose, the seizure and preparation of the 
 food. 
 
 As resisting power and a certain degree of hardness are 
 essential to the functions performed by the teeth, they are 
 usually calcified, that is, permeated by, or impregnated 
 with, inorganic salts. Some teeth, however, persist through- 
 out life as horny structures, produced from the stratum 
 corneum of the oral epithelium, as the teeth of the Cyclo- 
 stomata (Petromyzon, Myxine, and Bdellostoma) in fishes, 
 and the adult structures in Ornithorhynchus which take the 
 place of teeth, although more correctly described as horny 
 plates. 
 
 While confined in the higher forms to the maxillary, 
 premaxillary, and mandibular bones, teeth are found, 
 especially in fish, in many other positions, as on the pre- 
 mandibular, vomer, pterygoid, and pharyngeal bones, and 
 upon the branchial arches. In many fish the whole mouth 
 bristles with teeth, and they are even found upon the tongue 
 in the parasitic Myxine. Horny structures are also present 
 on the tongue of Ornithorhynchus. 
 
 Three chief tissues constitute the structure of the com- 
 pleted tooth : dentine, enamel, and cement. 
 
 The bulk of the tooth is made up of dentine, a hard 
 (falcified substance penetrated in different degrees by 
 channels ; an exceedingly hard external layer, the enamel, 
 is present in most teeth and covers the exposed surface of 
 the crown and the roots, and in some instances the crowns 
 of the tooth are coated with cement, which, as its name 
 indicates, serves in many teeth to bind together the other 
 two tissues, as shown in the Ungulates, where the cement 
 in some forms in early stages a complete investment of the 
 crown, and when subjected to wear, occupies the intervals 
 
INTRODUCTION 5 
 
 between the layers of enamel and dentine, and wearing 
 down more rapidly than either of these, maintains a sharp 
 cutting edge or surface to the tooth, as is most conspicuously 
 seen in the Rodents. 
 
 The calcined portion of the tooth surrounds a cavity 
 occupied by the pulp, which in a foundation of connective- 
 tissue fibres supports the blood-vessels and nerves and the 
 formative cells of the dentine, and persists during the life 
 of the tooth. 
 
 The mammalian tooth is implanted in a socket within 
 that portion of the bone of the jaw which is especially 
 developed to receive it, named the alveolus. The tooth is 
 attached within the socket by a fibrous membrane, the 
 periodontal membrane or ligament, wiiich is continuous with 
 the periosteum of the alveolar bone on its outer aspect. All 
 teeth are not, however, implanted in sockets, being in many 
 animals attached by anchylosis, the tissue of the tooth 
 merging into that of the bone ; but, as shown by C. S. Tomes, 
 this attachment in most cases is not direct to the bone of 
 the jaw, but to a separate process of bone, the bone of 
 attachment, which is analogous to the alveolar process of 
 human teeth. 
 
 Various modifications of these modes of attachment are 
 found in the animal kingdom, which will be considered in 
 another place, and show a remarkable adaptation to the 
 habits and mode of life of the different species. Especially 
 with the class of fishes is this very noticeable, the varieties 
 of adaptive modification being so numerous that a large 
 number have probably not yet been described. 
 
 Teeth are called dermal appendages, and both in structure 
 and development are closely related to the skin. This is 
 especially well seen in the Sharks, where it is very evident 
 that the teeth and the dermal spines are similar structures, 
 the teeth passing by almost imperceptible gradations into 
 the dermal spines and appendages of the skin. The close 
 relation of the teeth and hair is another evidence of their 
 dermal nature. In many instances this interdependence of 
 the teeth and hair has been manifested ; a great excess of 
 hair being accompanied by abnormalities, diminution of 
 number, or absence of teeth*. 
 
6 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Teeth are developed from the mucous membrane of the 
 mouth, and as stated by Beddard (2) : ' Developmentally 
 and histologically there are (in Mammalia) no fundamental 
 divergencies from the teeth of Vertebrates lower in the scale. 
 The teeth originate quite independently of the jaws with 
 which they are, later, so intimately connected, the inde- 
 pendence of origin being one of the facts upon which the 
 current theory of the nature of teeth is founded. The scales 
 of the Elasmobranch fishes consist of a cap of enamel upon 
 a base of dentine, the former being derived from the epi- 
 dermis and modelled upon a papilla of the dermis, whose 
 cells secrete the dentine. The fact that similar structures 
 arise within the mouth (i. e. the teeth) is explicable when it 
 is remembered that the mouth itself is a late invagination 
 from the outside of the body, and therefore the retention 
 by its tissues of the capacity to produce such structures is 
 not remarkable.' 
 
 To the comparative anatomist and anthropologist, teeth 
 are of the greatest importance, as from their indestructible 
 nature they have survived the vicissitudes of time and 
 destructive agencies in the , most remarkable manner. 
 When found either alone or in association with remains 
 of a more perishable nature, they have indicated by 
 their form and structure the nature of the food and the 
 habits and affinities of long extinct forms, and have served 
 to forge the links of the evolutionary chain, from the 
 remotest appearance of vertebrate life on the earth to the 
 present time. 
 
 The varieties of combinations of the different tissues, the 
 intricate patterns assumed in different teeth, and the 
 differences in the intimate structure of the dentine and 
 enamel, render the study of these organs throughout the 
 animal kingdom a source of unfailing interest and of the 
 greatest importance in all endeavours to obtain a better 
 comprehension of the evolutionary development of organized 
 beings. 
 
 They render it still more evident that, so far as the 
 organic world is concerned, the doctrine of evolution as 
 stated by Darwin rests upon a sure foundation. The sur- 
 vival of the fittest is a demonstrable fact, and the principal, 
 
INTRODUCTION 7 
 
 although, as Darwin himself said, not the sole agency by 
 which this is brought about, is natural selection. 
 
 It should not, however, be forgotten that the great 
 principle enunciated by Darwin which laid the foundation 
 of the modern science of evolution was descent with modifica- 
 tion. Darwin's theory is too often spoken of as the theory 
 of natural selection, but this was only an explanation of the 
 method by which he considered the change of one species 
 into another was chiefly brought about. As stated by 
 De Vries, * The theory of descent remains unshaken even 
 if our conception concerning the mode of descent prove 
 to be in need of revision ' (8). 
 
 The law of natural selection is founded on the fact that 
 variations arise among groups of organisms, and those 
 variations which are of advantage to the organism in the 
 struggle for existence are perpetuated by heredity. Darwin 
 states in the Origin of Species (3 a), ' natural selection acts 
 solely by accumulating slight, successive, favourable varia- 
 tions, it can produce no great and sudden modifications/ 
 but Darwin considered that natural selection, while it has 
 been the principal agent, has not been the sole influence in 
 the evolution of species ; he considered it was aided in an 
 important manner by the inherited effect of the use and 
 disuse of parts. 
 
 The discoveries of Mendel (5) have, however, of late years 
 considerably modified several of Darwin's conclusions. 
 These important discoveries were for a long time passed 
 over in silence, but have lately obtained great prominence 
 among evolutionists, and they explain much in the doctrine 
 of heredity which has hitherto been obscure. Mendel's 
 researches -were repeated and confirmed by De Vries, 
 Torrens, and Tschermak, and published by them in 1900. 
 
 Mendel's first series of experiments were made on the 
 edible pea. A variety of this pea with tall stems was crossed 
 with a short-stemmed variety. The next generation, F r 
 showed all long-stemmed plants, and this tallness he described 
 as the ' Dominant ' character. These tall plants, when self- 
 fertilized, gave in the next generation, F 2 , both tall and short 
 plants in a definite numerical relation to one another, this 
 being in the ratio of, approximately, three tall to one short. 
 
8 MICROSCOPIC ANATOMY OF THE TEETH 
 
 These short plants he called the Recessives. 
 
 The recessives when self -fertilized gave pure short plants, 
 the short variety remaining pure in succeeding generations. 
 
 The dominants, however, continued to yield both pure 
 tall plants and also mixed plants, showing in successive 
 generations the same proportion of three tall to one short. 
 
 As stated by Bateson (1), ' the whole F 2 generation, there- 
 fore, formed by self-fertilization of the original hybrid, con- 
 sists of three kinds of plants : 
 
 25 per cent. 50 per cent. 25 per cent, 
 
 pure dominants impure dominants pure recessives 
 
 or three dominants to one recessive.' 
 
 It is thus seen that a permanent pure variety arises from 
 the mixed variety, and in this instance a fixed law of varia- 
 tion is established. 
 
 Many experiments demonstrate that these laws are 
 applicable to all living organisms, both vegetable and animal. 
 As Bateson says : * The fact of segregation was the great 
 discovery which Mendel made segregation being the dis- 
 sociation of characters from each other in the course of the 
 formation of the germ.' 
 
 Natural selection acts on variations, but how these varia- 
 tions occurred had always previously been an obscure and 
 undetermined problem. 
 
 Mendel's researches have, to a great extent, contributed 
 to the solution of this problem, and although much yet 
 remains obscure, this discovery has given an impulse to the 
 study of heredity which cannot fail to have the most far- 
 reaching and important results. 
 
 As the author above referred to says : ' There is nothing 
 in Mendelian discovery which runs counter to the cardinal 
 doctrine that species have arisen by means of Natural 
 Selection,' or ' the preservation of favoured races in the 
 struggle for life ', but, as he points out, the scope of natural 
 selection is closely limited by the laws of variation, and 
 Darwin would have welcomed the work of Mendel Avith 
 delight, as explaining much which he himself felt to be 
 obscure. 
 
INTRODUCTION 9 
 
 The bearing of these laws on the phenomena of colour 
 variation has been studied in many plants and animals, and 
 there is little doubt they will be of great value in the study 
 of the heredity of disease as well. As to what influence 
 these laws may have had on the evolution of the forms of 
 teeth, we are quite without evidence. 
 
 Other factors have also to be considered, as sexual selection 
 and concomitant variation. 
 
 Sexual selection is illustrated by the fact that those 
 individuals among the males which possess certain physical 
 advantages over their fellows, will be able to obtain the 
 mastery in their fights for the possession of the females, as 
 by the provision of stronger and better-developed horns 
 or tusks ; and their superior physical development would 
 be inherited. 
 
 Concomitant variation or correlation of growth is shown 
 in the development of one organ at the expense of another 
 an animal with very large horns having suppressed canines, 
 and one with greatly developed tusks showing absence or 
 diminution of horns. 
 
 One illustration of the truth of the doctrine of evolution 
 is the existence, in the course of development of the higher 
 forms, of vestigial remains of organs constantly present in 
 the adult forms of early progenitors^ as instanced by Darwin 
 in the teeth of Ungulates (36): 
 
 ' The calf has inherited teeth, which never cut through 
 the gums of the upper jaw, from an early progenitor having 
 well-developed teeth ; and we may believe that the teeth 
 in the mature animal were formerly reduced by disuse, 
 owing to the tongue and palate, or lips, having become 
 excellently fitted through natural selection to browse with- 
 out their aid ; whereas in the calf the teeth have been left 
 unaffected, and on the principle of inheritance at corre- 
 sponding ages have been inherited from a remote period to 
 the present day. 
 
 * Disuse, aided sometimes by natural selection, will often 
 have reduced organs when rendered useless under changed 
 habits or conditions of life ; and we can understand on this 
 view the existence of rudimentary organs.' 
 
 Darwin established the great principle of continuity 
 
10 MICROSCOPIC .ANATOMY OF THE TEETH 
 
 throughout the organic world, and as Professor Huxley says : 
 ' The primordial germs of a man, a dog, a bird, a fish, 
 a beetle, a snail, and a polype, are in no essential structural 
 respects distinguishable. In this broad sense it may with 
 truth be said that all living animals, and all those dead 
 faunae which geology reveals, are bound together by an all- 
 pervading unity of organization." 
 
 Especially since Darwin's time, the study of the cell and 
 its relations to heredity has greatly occupied the attention 
 of naturalists, and ' microscopic anatomy has shown us the 
 nature of the material on which organic evolution has 
 operated '. 
 
 We have to distinguish between the ' somatic ' or body 
 cells and the ' germ ' cells. 
 
 The somatic cells build up the body of the individual, the 
 germ cells are the racial cells, and the phenomena of heredity 
 depend on their continuance from one generation to another. 
 The germ cell, according to Weismann, is immortal ; the 
 somatic cell perishes with the individual body. 
 
 As Wilson says (9) : ' The cell theory must be placed beside 
 the evolution theory as one of the foundation stones of 
 modern biology these two great generalizations have been 
 developed along widely different lines of research, and have 
 only within a very recent period met upon a common 
 ground.' The cell nucleus is shown to be the vehicle of 
 inheritance, and, as this author further states, * thus the 
 wonderful truth became manifest that a single cell may 
 contain within its microscopic compass the sum total of 
 the heritage of the species. The death of the individual 
 involves no breach of continuity in the series of cell divisions 
 by which the life of the race flows onwards the individual 
 body dies, it is true, but the germ cells live on, carrying 
 with them, as it were, the traditions of the race from which 
 they have sprung, and handing them on to their descendants ; 
 the body is merely the carrier of the germ cells which are 
 held in trust for coming generations.' 
 
 Another question which has led to much controversy is 
 whether characters acquired during the lifetime of the 
 individual are capable of being transmitted to the offspring. 
 This much-debated theory was formulated by Lamarck in 
 
INTRODUCTION 11 
 
 the early part of the nineteenth century, and Darwin, in 
 order to account for the transmission of acquired characters, 
 founded his theory of ' pangenesis ', by which he considered 
 the germ cells were compounded of minute germs from every 
 part of the body, and he thus accounted for the transmission 
 of acquired as well as congenital variations ; but this theory 
 has been abandoned by subsequent authorities. 
 
 The theory that characters acquired during the life of the 
 individual are transmitted by heredity has met with very 
 little acceptance in Great Britain or in Germany, but has 
 many supporters in France and especially in America, where 
 the school of neo-Lamarckians, supported by such eminent 
 naturalists as Cope, Ryder, and Osborn, still maintains 
 a prominent position. 
 
 Weismann, who was one of the chief opponents of this 
 theory, considers that ' not one of the asserted cases of 
 transmission of acquired characters will stand the test of 
 rigid scientific scrutiny '. 
 
 * It is impossible ', he says, ' that acquired characters 
 should be transmitted, for it is inconceivable that definite 
 changes in the body or " soma " should so affect the proto- 
 plasm of the germ cells as to cause corresponding changes 
 to appear in the offspring.' 
 
 But all these researches, which have so much occupied the 
 attention of a host of investigators for the past fifty years, 
 have not brought us any nearer to an understanding of the 
 origin of life. 
 
 If we admit the possibility of spontaneous generation in 
 the earliest conditions of the globe, we cannot form any 
 conception of such a process we fail to comprehend the ener- 
 gizing force which originated such beginning. If we study 
 the production of the remarkable osmotic growths produced 
 by Leduc and others from inorganic salts in a colloid, we 
 cannot be sure that these curious forms, similar as they are 
 to the products of living organisms, show anything but an 
 analogy to, and are not identical with them. 
 
 If we accept Lord Kelvin's hypothesis, ' that life may be, 
 and may have been, disseminated across the bounds of space 
 throughout the solar system and the whole universe' (7), 
 this brings us no nearer to the origin of life, and we can 
 
12 MICROSCOPIC ANATOMY OF THE TEETH 
 
 only say with Hutton, we find * no vestige of a beginning, 
 no prospect of an end \ l 
 
 1 For an interesting description of Arrhenius' view of the mode by which 
 minute organic particles might be supposed to be disseminated into space 
 from our atmosphere, see Growth and Form, by D'Arcy W. Thompson, 
 pp. 48 and 49. It has, however, been suggested that the nature and 
 intensity of the light outside our atmosphere would be rapidly destructive 
 to all forms of life, and as Sir Ray Lankester says in a recent work (4) : 
 ' Hence Sir James Dewar argues that, whilst it would appear that the 
 extreme cold of space would not kill a minute living germ and prevent 
 it passing from planet to planet or from remotest space to our earth, 
 yet one thing which is more abundant in space than within the shell of our 
 atmosphere is absolutely destructive to such minute particles of living 
 matter, even when hard frozen, and that is intense light, the ultra-visible 
 vibrations of smallest wave length.' 
 
 REFERENCES 
 
 1. Bateson. MendeVs Principles of Heredity. Camb., 1909. 
 
 2. Beddard, F. E. Camb. Nat. Hist., x. 43-4. 
 
 3. Darwin, C. (a) Origin of Species, p. 282 ; (6) ibid., p. 292. 
 
 4. Lankester, E. R. Diversions of a Naturalist, p. 159. 
 
 5. Mendel, Gregor Johann. Versuche uber Pflanzen-Hybriden : Verb. 
 Naturf. Ver. in Brunn, Bd. 10, 1865. Translation in Journal Roy. 
 Horticult. Soc., 1901, xxv, xxvi. 
 
 6. Rose, C. ' liber die Zahnleiste und die Eischwiele der Sauropsiden.' 
 Anat. Anzeig., 1892, Xos. 23 and 24, p. 749. 
 
 7. Thompson, D'Arcy W. Growth ami Form. Camb., 1917. 
 
 8. De Vries, Hugo. Plant Breeding, London, 1907, p. 3. 
 
 9. Wilson, E. B. The Cell in Development and Inheritance. London and 
 New York, 1896. 
 
CHAPTER I 
 DEVELOPMENT OF THE TEETH IN MAMMALIA 
 
 THE two principal tissues of the teeth are enamel and 
 dentine, enamel being derived from the ectodermic epi- 
 thelium of the mouth and dentine from the mesodermic 
 submucous tissue. 
 
 The enamel organ is the first part of the tooth germ to 
 appear, the dentine papilla arising later, although it is the 
 first to show commencing calcification. 
 
 While, however, the appearance of an epithelial inflection Epithelial 
 is the most constant indication of tooth formation, it does mflectlon 
 not necessarily point to the future formation of enamel. 
 
 Tooth germs are developed, not upon the surface, but 
 always at some little distance within the tissues, and in 
 their simplest form consist of dentine and enamel only, bnt 
 the future tooth is determined by the epithelial inflection 
 rather than by the dentine germ, which is a subsequent 
 production of the mesodermic tissue. 
 
 It is only in comparatively recent times that a clear 
 understanding of the mode of development of the teeth has 
 been arrived at. 
 
 The views held by Goodsir (7) were for long accepted as 
 giving a true explanation of the process of tooth develop- 
 ment, and were adopted by Professor Owen and many other 
 eminent authorities. 
 
 Goodsir considered that the first indication of tooth Views of 
 formation was the appearance of a groove which he called 
 ' the primitive dental groove ', and that this was followed 
 by a papillary stage, a follicular, and an eruptive stage. 
 This primitive dental groove we now know was not a groove 
 but a band. The epithelial band is formed from the cells 
 of the deeper layer of the epithelium of the mouth, and is, 
 by the proliferation of these cells, prolonged downwards 
 into the mesodermic tissue as a continuous band, passing 
 horizontally along the line of the future jawbones. The 
 
14 MICROSCOPIC ANATOMY OF THE TEETH 
 
 separation of the cells of the mesoderm by the intrusion of 
 the epithelial band forms a groove (the primitive dental 
 groove of Goodsir), but it is not hollow, but filled with the 
 cells of this epithelial band. 
 
 Goodsir 's views were correct up to a certain point, but 
 the methods of preparation then available did not enable 
 him to appreciate the all-important part taken by the 
 epithelial cells in this process, as these cells were washed 
 out of the depression in the mesodermic tissue produced by 
 the ingrowth of the epithelium, which then appeared as 
 a hollow groove. As pointed out by Schafer (24), Kolliker 
 showed the importance of the cells of the deeper layer of the 
 epithelium of the mouth in the production of the primitive 
 dental lamina from which the enamel organs of the teeth 
 are developed, as although attention had already been drawn 
 to this by Huxley (10) and Marcusen (17), it had not been 
 generally accepted. In fact Huxley accepted the views of 
 Goodsir, except with regard to the development of the teeth 
 in fishes and reptiles, which he did not consider could be 
 explained by this view. 
 
 The explanation of the mode of development of the teeth 
 in the Mammalia, now generally accepted as the true one, 
 is founded upon the researches of Kolliker (12), Leche (14), 
 Rose, and many others, and has been demonstrated by 
 Rose (19) in the series of illustrative models w r hich he 
 prepared. 
 
 There are certain differences in the development of the 
 teeth in reptiles and fishes which will be considered else- 
 where. 
 
 At about the thirty-fourth to the fortieth day of intra- 
 uterine life, when the embryo is from 12 to 15 mm. long, 
 before the commencement of ossification, the lower jaw 
 being represented by Meckel's cartilage only, the first 
 indication of tooth formation occurs. 
 
 This consists of an ingrowth of the deeper layers of the 
 epithelium of the mouth, forming a band which follows the 
 line of the future alveolar margin of the jaw. While it is 
 usual to speak of an ingrowth of the epithelium, it is neces- 
 sary to remember that in the embryonic jaw we are not 
 looking at a completed tissue in which the only active 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 15 
 
 growth is that of the epithelial cells, but at a structure 
 which is rapidly enlarging in every direction ; upgrowth and 
 downgrowth are proceeding at the same time, and the 
 tissues of the embryonic jaw are increasing in thickness as 
 well, so that the tissue near the surface becomes more 
 deeply situated, owing to the increase of growth in all 
 directions around it. 
 
 It is thus seen that the usual description of an ingrowth 
 or downgrowth of the cells is not strictly correct, and it 
 would be more accurate to speak of it as a proliferation of 
 the deeper cells of the epithelium, which become deeply 
 situated on account of the growth of the neighbouring 
 mesodermic cells. 
 
 Really the whole surface of the epithelium is raised, and 
 the general proliferation of cells being associated with the 
 growth of the jaw tissues on every side, the parts are in- 
 creased in size and not diminished, as might be suggested 
 by the usuaj description. To avoid confusion, however, 
 this proliferation of cells will be spoken of as an ingrowth or 
 downgrowth. 
 
 In some mammalian germs, more especially in Ungulates, Primitive 
 there is a more or less marked heaping up of the epithelium 
 over the situation of the ingrowth. This ingrowth, the 
 'primitive dental lamina ', divides (according to Rose on 
 the forty-eighth day) into two separate bands lying 
 rectangularly to one another, the one passing perpen- 
 dicularly into the jaw (the labio-dental lamina), the other 
 more or less horizontally backwards, constituting the true 
 tooth-band or ' dental lamina '. 
 
 The labio-dental lamina is an ingrowth of the epithelium Labio- 
 between the lip and the jaw, the cells of which atrophy and 
 form an open groove extending along the developing jaw, 
 between the tooth- band and the opening of the mouth in 
 front, and the tooth-band and the buccal wall farther back. 
 There is some difference of opinion with regard to the time 
 and mode of differentiation of the labio-dental lamina and 
 the dental lamina. These divisions of the primitive dental 
 lamina arise, according to Rose, simultaneously. Baume 
 (2) is of opinion that the dental lamina (or tooth-band) is 
 differentiated from the previously formed labio-dental lamina, 
 
16 MICROSCOPIC ANATOMY OF THE TEETH 
 
 while Leche (14) considers that the two laminae are formed 
 simultaneously and independently. 1 
 
 Tooth- The dental lamina or tooth-band with which we are more 
 imme'diately concerned does not pass quite horizontally into 
 the jaw, but is curved, in the maxilla backwards and up- 
 wards, in the mandible backwards and downwards. Little 
 thickenings or buds soon appear upon the dental lamina 
 where teeth are to be formed ; these thickenings, however, 
 do not arise upon the free lower margin of the lamina, but 
 on its labial aspect just short of that margin. The thickened 
 portions grow down into the submucous tissue beneath, 
 forming a cap over the mesodermic dental papilla, and form 
 the enamel organs of the milk teeth (fig. 1). 
 
 In this figure the relations of the mesodermic dentine 
 papilla with the epithelium are shown diagrammatically. 
 
 As the enamel organs are not given off from the free edge 
 of the lamina, there is left a free growing edge above and 
 behind the milk-tooth germs in the upper jaw, below and 
 behind them in the lower jaw, and from this produced 
 portion of the tooth-band the successional permanent teeth 
 are developed. 
 
 These latter then are not produced from the neck of the 
 enamel organ, as was formerly taught, but from the further 
 growing margin of the parent tooth lamina. As the develop- 
 ing jaws continue to grow backwards, the portion of the 
 dental lamina beyond the limits of the forming deciduous 
 teeth is prolonged also in the same direction, and gives rise 
 to the permanent molars. When further developed, the 
 enamel organs become separated from the dental lamina, 
 and are only attached to it by means of connecting bridges 
 which undergo perforation and absorption, the further 
 development of the tooth proceeding independently of the 
 dental lamina. At birth the connecting bands between the 
 milk incisors have nearly disappeared, while those between 
 the first and second temporary molars are still uninterrupted. 
 
 1 For the sake of clearness of description the nomenclature given by 
 Professor Schafer .(Microscopic Anatomy) is adopted. The primitive 
 Zahnleiste is called the primitive dental lamina. The Lippenfurche of 
 Rose is called the labio-dental lamina, and the Zahnleiste of Rose the 
 dental lamina or tooth-band. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 17 
 
 The breaking up of the dental lamina and the connecting 
 bridges gives rise to little separated portions or islands of 
 epithelium, the so-called ' glands of Serres ' (20). As will be 
 shown, however, in discussing the structure of the follicle Connoct- 
 (p. 311), the remains of the epithelial bridges or necks of the blf.i, es 
 
 
 
 FIG. 1. Diagrams illustrating development of mammalian teeth (adapted 
 from Gegenbaur). ep. Epithelium of mouth; /. furrow (not always 
 present); I dental lamina or tooth- band ; V. its continuation to form 
 the permanent tooth ; eo. enamel organ cells ; ex. epithelium ; s stellate 
 reticulum ; e. enamel ; a. ameloblasts ; d. dentine ; p. papilla ; b. bone ; 
 m. germ of permanent tooth : c. capsule. 
 
 enamel organ do not all atrophy there is also a prolifera- 
 tion and further development of these cells which takes 
 place within the connective tissue of the follicle. 
 
 On the surface of the epithelium of the jaw is a shallow Tooth 
 groove, the ' tooth furrow ', which marks the connexion of 
 the tooth-band with the oral epithelium ; this furrow runs 
 
 furrow . 
 
18 MICROSCOPIC ANATOMY OF THE TEETH 
 
 chiefly on the outer side of the wall of the jaw, but in the 
 incisor region, on the top of the jaw wall. As Rose says : 
 ' This tooth furrow (which, as already mentioned, indicates 
 the line of demarcation of the tooth-band from the epithelium 
 of the jaw) is in some places fairly deep, in others scarcely 
 indicated, in places even, obviously double, according to 
 the more or less irregular disposition of the tooth-band.' 
 Rose's Rose prepared a series of wax models built up by the 
 
 B s< ingenious method of Born, which show the important points 
 in this explanation of the mode of development of the teeth, 
 which is now generally accepted as being in most particulars 
 the correct one. 
 
 FIG. 2. From the model of the mandible of an embryo of nine weeks. 
 a. tooth- band ; 6. lip furrow band ; c. lip furrow. 
 
 These models are made to scale, from photographs of 
 serial sections, magnified to the same degree and copied on 
 sheets of wax, which are moulded into a solid model by 
 melting their edges. 
 
 Three of these models are figured in the accompanying 
 illustrations, and a description of them may assist in obtain- 
 ing a clear understanding of the most important points in 
 mammalian tooth development. 
 
 Fig. 2 shows a photograph of the model of the epithelial 
 surface of the mandible of an embryo of nine weeks, 2-5 cm. 
 in length. The opening of the mouth is represented with 
 part of the epithelium of the lip and of the mucous mem- 
 brane of the mouth. The tooth-band is seen as a curved 
 band directed backwards ; in the mandible at this stage it 
 shows an undulating margin and enlargements at intervals, 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 19 
 
 the enlargements being the first indication of the separation 7 
 of the original enamel organs. 1 
 
 Fig. 3 is a photograph of the model of the maxilla of 
 an embryo, fourteen weeks old, 11^- cm. in length. The 
 epithelial structures only are seen, looking down upon the 
 upper or cranial aspect of the epithelium of the mouth and 
 into the hollow caps of the epithelial enamel organs, which 
 are advancing in a vertical direction towards the observer, 
 
 FIG. 3. From the model of the maxilla of an embryo of fourteen weeks. 
 The epithelial structures viewed from above. The germs of the ten deci- 
 duous teeth are seen. d. lip furrow ; c. lip furrow band ; 6. tooth germ ; 
 a. tooth-band ; of. prolongation of tooth-band to form permanent teeth. 
 
 the free margin of the tooth-band also advancing upwards 
 behind and beyond the enamel organs. 
 
 These enamel germs have been aptly compared to swallows' 
 nests attached to the flat surface of a board ; they are seen 
 to be attached to the labial surface of the tooth-band and 
 not to its free growing margin. The mesodermic tissues are 
 not represented in the model, being supposed to be stripped 
 off from the epithelial structures. 
 
 1 A set of these models is to be seen in the Odontological section of the 
 Hunterian Museum of the Royal College of Surgeons. Details of the mode 
 of production of the models are given in Rose's original paper (19) and 
 in the author's communication to the Odontological Society's Trans., 
 May 1893. 
 
 C 2 
 
20 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Fig. 4 shows the left half of the mandible of a seventeen- 
 weeks' embryo, 18 cm. long. The enamel organs of the five 
 temporary teeth of that side are seen growing from the 
 tooth-band, which shows indications of absorption between 
 the germs. The prolongation of the tooth-band backwards, 
 which gives origin to the first permanent molar, is very 
 clearly seen. It is evidently not produced from the neck of 
 the enamel organ of the temporary tooth, as formerly taught, 
 but from a further backward growth of the original tooth- 
 band. 1 
 
 FIG. 4. Model of left half of mandible of a seventeen weeks' old foetus, 
 c. lip furrow ; &. lip furrow band ; a. tooth-band ; a', prolongation of 
 tooth-band to form first permanent molar. 
 
 From these models and the foregoing description, it can 
 be easily understood that in the process of breaking up and 
 absorption of the tooth-band between the forming enamel 
 germs, remnants of this epithelial lamina may persist, and 
 give rise to such irregularities as supernumerary teeth, 
 odontomes, &c., other portions becoming degenerated and 
 not absorbed, giving rise to such abnormal structures as 
 cysts and epithelial pearls. 
 
 The question of the existence of pre-milk and post- 
 permanent teeth is closely connected with this mode of 
 development of the teeth. 
 
 Pre-milk teeth would be produced by buds given off from 
 
 1 To understand clearly the relations of the parts in this model the page 
 containing the illustration should be held above the head (the lower 
 margin of the page forward), and viewed from below and from the front. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 21 
 
 the tooth-band in front of the buds for the milk teeth 
 These have often been described in developing teeth, but 
 are probably never calcified. Post-permanent teeth are 
 considered to be due to the downgrowth of the persisting 
 remains of the dental lamina or tooth-band, at the back of 
 the jaw. 
 
 C. S. Tomes (27 a) says: 'A post-permanent set is repre- Post-per 
 sented in some animals by bands beyond the permanent ^fpre- 
 tooth germs, but these never calcify,' and he further says lacteal 
 that he ' regards both pre-milk and post-permanent rudiments 8 
 as at best hypothetical, and the evidence insufficient to 
 establish their existence ' ; but Marett Tims says : ' There 
 is now less reason for hesitation in accepting the evidence of 
 the pre-milk vestiges than was formerly the case. Doubt 
 may still exist as to the value of the post-permanent 
 downgrowths of the dental lamina.' 
 
 In the further course of development, the successional 
 permanent teeth grow more deeply into the jaw, ingrowing 
 septa of bone separate them from the milk teeth, and they 
 come to have an alveolus of their own. 
 
 Immediately following the formation of an enamel organ Dentine 
 from the epithelial dental lamina, the dentine germ arises papl 
 and is seen in the form of a papilla filling up the concavity 
 of the enamel organ. 
 
 As previously stated, the production of the dentine papilla 
 seems to be- determined by the formation of an enamel 
 organ, and the dentine germ is not a true papilla, but a con- 
 densation and proliferation of the cells of the mesoderm in 
 this situation, first appearing as an opacity in the sub- 
 mucous tissue. 
 
 Leche holds that this may be due to the crowding of the 
 cells by the surrounding growth of the epithelial cells of the 
 enamel organ. Dursy (6) considers that this opacity forms 
 a band extending all around the jaws, the prominences only 
 arising where teeth are to be formed, and the intermediate 
 portions of the opaque band becoming absorbed in the same 
 manner as are the connecting bridges of the epithelial dental 
 lamina. 
 
 The lower and lateral margins of the dentine germ are 
 later prolonged upwards, surrounding the enamel organ and 
 
S * 2 
 
 1 1 
 
 Q 
 
 o m 
 
 l-a* 
 
 1 H I 
 
 .S 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 23 
 
 meeting over its upper surface. It thus becomes closed in 
 by the mesodermic tissue, which forms the sac of the develop- 
 ing tooth. 
 
 The tooth-sac of the developing permanent tooth is Tooth- 
 closed in by a bony shell, except at its uppermost point, sac * 
 where it is pierced by a foramen which opens upon the 
 gum behind the corresponding milk tooth. From the sac 
 a fibrous band passes through this foramen and becomes 
 blended with the gum behind the milk tooth. This band 
 or cord is called the ' gubernaculum ' or rudder, as it was Gubema- 
 supposed to guide or direct the course of the erupting tooth, culum. 
 There is, however, no distinct canal, but bands of connective- 
 tissue fibres enclosing strands of epithelium. 
 
 Its position is marked by the foramina, which are seen in 
 the bone immediately behind each of the temporary teeth. 
 Malassez (16), in a paper ' On the Structure of the Guber- 
 naculum dentis ', says : ' The teeth of replacement are con- 
 tained in a bony cavity which is prolonged in the form of 
 a canal to the alveolar border, where it opens at the inner 
 side of the milk tooth. The dental follicle prolongs into the 
 canal a kind of cord which continues until lost in the fibrous 
 tissue of the gingival margin.' It was considered by 
 Delabarre (5) and Serres (20) to be hollow and to guide the 
 tooth in eruption. Sappey (21) said it contained 'the last 
 remnants of the epithelial proliferations '. Malassez con- 
 siders there is no part of the gubernaculum in which a canal 
 exists, but it is made up of connective-tissue fibres, mostly 
 arranged longitudinally, but the point to which he would 
 draw particular attention is that the connective tissue 
 encloses numerous epithelial strands. In the deep part the 
 epithelial elements are more abundant and form a ' rich 
 network ' and ' lateral buds in the form of clubs ' are to be 
 seen. ' In the most superficial part of the gubernaculum 
 they are, on the contrary, less numerous, less rectilinear, 
 and more rarely anastomosed.' He also says that these 
 tracts of epithelium can be seen to proceed from the ' corre- 
 sponding enamel cord of the enamel organ ', which he 
 considers not only persists, but ' even proliferates with great 
 activity in the neighbourhood of the tooth of replacement '. 
 and his conclusion is that ' this fact leads us to surmise 
 
24 MICROSCOPIC ANATOMY OF THE TEETH 
 
 that the epithelial masses play a certain role in the eruption 
 of the corresponding tooth '. 
 
 These observations will be further considered in treating 
 of the dental follicle (see p. 317), but Warwick James (11) 
 has lately expounded a similar view in connexion with the 
 eruption of the temporary teeth. He considers that the 
 epithelium ' directs the tooth to its position in the gum ', 
 and that the ' path of eruption is prepared by the epi- 
 Epitheliai fchelium '. The principal agents in providing this path are 
 the 'epithelial coils' or 'globes epidermiques ', which, by 
 opening out and disrupting, form spaces in the connective 
 tissue of the follicle in the course of the erupting tooth. 
 The epithelial coils will be further considered in another 
 place (p. 312). 
 
 The tooth-sacs, shown in fig. 5 as they appear at birth, 
 consist of an outer and an inner coat, the outer connected 
 with the periosteum, and the inner coat richly supplied 
 with blood-vessels and separated from the outer by a thin 
 layer of jelly-like connective tissue. The extreme vascularity 
 of the inner coat ' doubtless has relation to the nutrition of 
 the enamel organ ' (Schafer). 
 
 From the tooth-sac, which is seen to take its origin from 
 the tissue of the mesoderm, the cement and periodontal 
 membrane are formed, and in those animals which possess 
 coronal cement, an investing cap of cement also. 
 
 Fig. 5, drawn by the author from a preparation by 
 Professor Symington, shows the tooth- sacs in the left half 
 of the mandible at birth, and in figs. 6 and 7 are shown the 
 stages of calcification of the milk teeth at birth. It is seen 
 that the calcified cusps of the molars have become fused 
 together, and the calcification of the first permanent molar 
 appears as a tiny triangle in its crypt. 
 
 Fig. 8, a skiagram from Professor Symington's atlas (23) y 
 shows very distinctly the stages of calcification at the 
 period of birth. The crypts of the teeth are seen and the 
 fusion of the calcifying cusps of the temporary molars i& 
 well shown, also the single calcified point of one cusp of 
 the first permanent molar. As pointed out by Professor 
 Symington, there is no important difference between this 
 specimen (one month old) and that of the newly-born 
 
PLATE I. 
 
 fr 
 
 Germ of Human Temporary Molar in the crypt. The extension of the tooth 
 band for the formation of the permanent tooth is seen on the left. x 50. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 25 
 
 infant, but the calcification, being slightly further advanced, 
 gives a better image on the photographic plate, and has 
 therefore been chosen as an illustration. 
 
 FIG. 8. Right side of jaws of male infant one month old. First molar 
 more calcified than second molar. Independent deposits on the second 
 molar are well shown ; they become gradually united by extension of the 
 calcification. A single calcified tip of one cusp of the first permanent 
 molar only is seen. There is no important difference between this stage 
 and the condition af birth. From Symington and Rankin's Atlas of 
 Skiagrams. 
 
 The first indication of tooth formation is the differentia- Summary, 
 tion of a band of epithelial tissue just beneath the surface 
 of the forming jaw : this is the primitive dental lamina. 
 
 The primitive dental lamina separates at an early stage 
 into two laminas at right angles, or nearly so, to one another 
 
26 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the vertically directed labio-dental lamina and the hori- 
 zontally placed dental lamina or tooth-band. 
 
 Prominences arise on the tooth-band, near to, but not 
 at the margin of the lamina, on its labial aspect. These 
 prominences become the enamel organs of the milk teeth. 
 The free margin of the lamina behind these germs gives rise 
 to the sticcessional permanent teeth, the permanent molars 
 being formed from that portion of the lamina which grows 
 backwards beyond the limits of the milk dentition. 
 
 The germs of the teeth become separated and detached 
 from the lamina by the absorption of the connecting bridges. 
 
 Each dentine germ or papilla is formed beneath the cap of 
 the enamel organ, and is produced in the mesodermic tissue, 
 which, according to Dursy, is differentiated as a continuous 
 opaque band round the jaws, corresponding to the epithelial 
 dental lamina from which the enamel is formed. 
 
 These papillae appear in the band in the positions corre- 
 sponding to those of the future teeth, while the intermediate 
 portions become atrophied and disappear like the bridges 
 of the epithelial lamina. 
 
 The margins of the dentine germ are described as grow- 
 ing up and around the epithelial enamel organ, forming 
 the tooth-sac from which the cement and periodontal 
 membrane are produced. It is doubtful if this is a correct 
 description, and this enclosure of the enamel germ may 
 with more probability be considered to be due to the con- 
 densation and proliferation of the surrounding connective 
 tissue. 
 
 The histology of the enamel and dentine organs and the 
 tooth follicle will be described in another chapter. 
 
 The accompanying table carries the development of the 
 teeth up to the time of birth. The eruption of the permanent 
 teeth will be better considered in works on dental anatomy ; 
 no work on the histology of the teeth would, however, be 
 complete without some account of the development of these 
 organs. 
 
 In fig. 9 is shown the tooth germs in their crypts in the 
 upper jaw of a foetal pig, and in fig. 10 a more advanced 
 germ from the pig in which the calcification of the dentine 
 and enamel is further advanced. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 27 
 
 FIG. 9. Upper jaw of pig. The developing teeth in situ. 
 
 FIG. 10. Tooth germ of pig within the crypt. 
 
28 MICROSCOPIC ANATOMY OF THE TEETH 
 
 
 
 
 a 
 
 
 
 &2 
 
 1 I 
 
 s s 
 
 1 
 
 * 
 
 
 
 O <M 
 
 i ^ 
 
 I 
 
 
 CSJ JO 
 
 Tf 
 
29 
 
30 MICROSCOPIC ANATOMY OF THE TEETH 
 
 At birth the sacs of the first permanent molars and those 
 of all the temporary teeth are fully formed (fig. 5), the 
 crowns of the temporary incisors are calcified, and the tip 
 of the canine and the separately formed cusps of the two 
 temporary molars are united. One cusp of the first per- 
 manent molar is calcified and seen as a minute triangle 
 (figs. 6 and 7). 
 
 Tooth Development in Reptiles and Fish 
 
 In Reptiles the new tooth germs are, without doubt, 
 developed from the tooth-band as in Mammalia, the succes- 
 sional teeth being given off from the continued ingrowth 
 of this band. In the osseous fish, where it is difficult to 
 trace the connexion of the germs with the tooth-band, it has 
 been considered that these germs may arise independently, 
 and Heincke has stated that new enamel organs may be 
 derived from older ones ; but this hardly seems consistent 
 with the received views of tooth development in the Verte- 
 brata, new teeth being derived, not from the enamel organ 
 of a previously formed tooth, but from the extension 
 beyond the tooth of the tooth-band or dental lamina. In 
 Elasmobranch fishes the connexion of the newly-formed 
 teeth with a common tooth-band is very evident. As 
 C. S. Tomes says (27), in the osseous fish ' no obvious con- 
 nexion between the germs of teeth of different ages is seen ', 
 and it appears much more probable that his explanation is 
 the correct one where he says ' it is likely that the germs 
 so soon become detached that their origin from a common 
 tooth-band is masked '. 
 
 Rose considers that in all the lower Vertebrates up to the 
 Urodelse (frogs, newts, &c.), the earliest tooth germ appears 
 as an upstanding papilla of the mucous membrane of the 
 mouth (beneath the epithelium), raised above the surface 
 of the surrounding mesodermic tissue. 
 
 Leche denies the existence of an upstanding papilla in 
 the tooth germs of the osseous fish, but Rose considers that 
 it is easily overlooked in the very minute enamel organs of 
 fish embryos, as he has found it in all the osseous fish which 
 he has examined, as well as in reptiles. This author con- 
 siders that this indicates the origin of the jaw teeth from 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 31 
 
 calcified skin scales the placoid scales of Elasmobranch 
 fish described by Hertwig (see p. 177). 
 
 By the placoid stage of tooth development, Rose under- Placoid 
 stands the condition in which the earliest tooth germs in stage * 
 the region of the mucous membrane of the jaws develop and 
 proliferate over the deeper lying cells, as do the placoid 
 scales of the Sharks or the papillae of the skin in Mammalia. 
 
 The epithelium of the jaw is somewhat thickened in 
 places, and beneath these thickenings the round cells of the 
 connective tissue have accumulated. 
 
 In distinct circumscribed areas the growth of the epi- 
 thelium is more pronounced, and the papillae of the placoid 
 germs are seen as lenticular epithelial growths. As in 
 Mammalia, the epithelial tissue is the true form-determining 
 element of the tooth germ. 
 
 He affirms that in Lepidosteus and the Salmonidae the 
 earliest placoid germs by further growth sink deeper and 
 deeper into the connective tissue of the jaw, and exhibit 
 an intermediate stage between the placoid and the mam- 
 malian form of tooth development. 
 
 In this, which he calls the conical stage, the whole mucous conical 
 membrane of the jaw is concerned, and its epithelial layer Sta 8 e - 
 contributes for each tooth germ a. definite epithelial process, 
 which in the later-formed teeth dips deeply into the mucous 
 membrane, as do the epithelial sheaths of the hair follicles. 
 Rose (19 c, d) would thus describe three stages in tooth de- 
 velopment : 
 
 1. The placoid stage. In this, the germs are seen as 
 upstanding papillae of the mucous membrane beneath 
 definite thickenings of the epithelial cells which invest 
 them. 
 
 2. A second or conical stage in which the germs sink 
 deeply into the mucous membrane enclosed by a distinct 
 epithelial sheath, as seen in the human hair follicle, and the 
 whole surface of the mucous membrane of the jaw is con- 
 cerned in tooth formation. 
 
 3. The tooth-band stage, as seen in Mammalia, where the 
 epithelial germs are given off from a definite tooth-band, 
 and it is this differentiated portion of the epithelium only 
 which is concerned in tooth formation. 
 
32 MICROSCOPIC ANATOMY OF THE TEETH 
 
 The Evolution of the Human Molar 
 
 Teeth, being often the only perfectly preserved remains of 
 early ancestors of the Mammalia, are of very great impor- 
 tance in the study of the evolution of the various existing 
 forms, and many different views have been held as to the 
 mode in which the highly complicated molars of existing 
 mammals have been evolved from simpler types. 
 
 Although this is a subject which comes more appropriately 
 under the heading of Dental Anatomy, recent researches on 
 certain structures in the enamel organ of Mammalia, which 
 have a strong bearing on the development and evolution of 
 the different forms of teeth, render it necessary to review, 
 however briefly, the principal theories which have been 
 brought forward to account for the origin of these different 
 forms. 
 
 Heterodont When teeth are similar in form and size throughout the 
 and homo- ser i eSj as j n the Dolphin, the dentition is spoken of as 
 homodont ; when, on the other hand, they vary in different 
 parts of the mouth it is described as heterodont. 
 
 Mono- It was stated by Owen that homodont animals usually 
 
 phyodont have but one set of teeth, or are monophyodont. Heterodonts 
 phyodont. possess two sets of teeth, constituting a permanent and a 
 milk dentition, and are spoken of as diphyodont. 
 
 There are exceptions to this rule, however, and it was shown 
 by C. S. Tomes (27 b) that in the Armadillo (Tatusia peba), 
 with a homodont dentition, there are both milk and perma- 
 nent teeth ; it is truly diphyodont, and in many homodont 
 animals traces of a milk dentition have been found, and, as 
 stated by Marett Tims, there are very few animals which 
 can be considered to be truly monophyodont, modern 
 methods of research having shown that in most cases hitherto 
 regarded as monophyodont, functionless representatives of 
 other dentitions are to be found, and the term can only be 
 consistently retained to describe a single functional dentition 
 as in the toothed whales. 
 
 Fish and reptiles are poly phyodont, having a continuous 
 succession of teeth, but in mammals never more than two 
 sets of teeth are developed, although in them also vestiges 
 of additional series have been described by Leche and others . 
 
PLATE II. 
 
 me" 
 
 pa- me. 
 
 pa, me. 
 
 ps. me. 
 
 irtc" en" 
 
 me" 
 
 EVOLUTION OF THE CUSPS OF THE HUMAN MOLARS. (From Osborn.) 
 
 FIG. A. Evolution of lower molar cusps. i. Simple conical reptilian tooth. 
 2. Dromotherium. 3. Microconodon. 4. Spalacotherium. 
 
 5. Amphitherium. 
 
 FIG. B. Evolution of upper molar cusps. I. Anaptomorphus. 2. An upper 
 Eocene Monkey. 3. Esquimo. 4. Negro. (See addition of 
 "talon" hy, to " trigon," composed of /a, pr, me.} 
 
 FIG. C. Lower molar cusps. I. In Miacis. 2. Anaptomorphus. 3. Homo. 
 (The cusps of the trigon and trigonid are coloured red, those of 
 the talon and talonid are coloured blue.) 
 
 To face p. 32. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 33 
 
 Reptiles have usually teeth of a simple conical form, 
 while the Mammalia have multicuspidate teeth, and different 
 views have been held as to the manner in which the cusps 
 of the higher forms have been evolved. 
 
 According to one view, first formulated by Ameghino (1) Con- 
 and strongly supported by Kiikenthal (13) and Rose (9), the 
 multiple cusped teeth of Mammalia have been produced by 
 the union or concrescence of single conical teeth united to 
 form a single tooth, made up of these separate elements, 
 each cusp of a compound tooth corresponding to the single 
 conical reptilian tooth. 
 
 This union of the conical teeth might take place either in 
 the antero-posterior (mesio -distal) direction by the union 
 of teeth of the same series, thus shortening the jaw, or in 
 the transverse or bucco-lingual direction, as suggested by 
 Kiikenthal, by the union of the teeth of different series. This 
 has been called the concrescence theory. 
 
 The basal ridge or cingulum is a ridge which surrounds the Cingulu 
 tooth at the neck, and is considered by many to play a very 
 important part in the production of cusps. Speaking of this 
 structure, Osborn (8) says : ' By its disappearance in some 
 regions and by its elevation into prominences in others, the 
 form of a tooth may be prof oundly modified, and it thus comes 
 to be regarded as a sort of mother of cusps.' As shown by 
 C. S. Tomes (27), the cingulum is well developed in most of 
 the Insectivora, where the crowns ( often bristle with sharp 
 points ', which are produced by the elevation of the cingu- 
 lum, and these points often exceed in length the principal 
 cusps of the tooth. Marett Tims considers that the develop- 
 ment of cusps from the cingulum explains to a great extent 
 the evolution of complex tooth forms. It would certainly 
 appear that accessory cusps arise from the cingulum. 
 
 The Tritubercular Theory 
 
 Professor Cope (4) in 1879 discovered a large series of fossil . 
 mammals in deposits of the Tertiary Age at Puerco Canon 
 in New Mexico. These small animals showed a general 
 similarity of form in the molar teeth, even in those of 
 apparently different habits. These teeth are made up 
 of * three main tubercles on the crowns of both upper and 
 
 MUMMERY 
 
34 MICROSCOPIC ANATOMY OF THE TEETH 
 
 lower molars, disposed in triangles '. He concluded that 
 * this tritubercular type was ancestral to many if not all 
 of the higher types of molar teeth ', and considered that it 
 arose from a single conical type or reptilian form of tooth 
 by the addition of added denticles. 
 
 It is seen that the concrescence theory also derives the 
 molar teeth from a single reptilian cone, but in a different 
 manner, the supporters of this theory considering that by 
 a shortening of the jaw the multiple conical teeth of the 
 reptiles underwent a clustering or concrescence, forming 
 multicuspidate teeth, and that tritubercular teeth were 
 not the earliest type derived from the single reptilian cone. 
 They thus consider that a molar tooth with numerous 
 tubercles was the more primitive form. 
 
 According to the tritubercular theory, the primitive form 
 from which the mammalian molar arose was a single cusp, 
 and additional cusps have been successively added to this. 
 The tooth with a single conical crown has not hitherto 
 been found in any known ancestor of the Mammalia, but 
 is described as the haplodont type (Plate II, fig. AJ. 
 
 A single main cone, however, furnished with two small 
 accessory cusps, occurs in the fossil Dromotherium (fig. A 2 ), 
 and is described as the protodont type. A central cone with 
 two well-developed accessory cusps gives the triconodont 
 type. 
 
 Cusps of This form is represented in the extinct Triconodon (fig. A 3 ), 
 
 bercidar anc ^ ^ rorQ this we P ass * ^ ne tritubercular type (fig. AJ, 
 
 tooth. in which there is a triangular crown with three main cusps, 
 
 called respectively the protocone, paracone, and metacone, 
 
 the protocone being internal in upper molars, external in 
 
 lower molars. The corresponding cusps in the lower jaw 
 
 are distinguished by the affix -id, being called protoconid, 
 
 paraconid, and metaconid respectively. 
 
 Trigon According to Cope and Osborn (18) the molar is made up 
 on ' of the primitive triangle or trigon and the added ' heel ' or 
 ta'on (fig. A-). 
 
 The trigon or primitive triangle is the sectorial or tearing 
 element of the mammalian tooth ; the talon, the crushing 
 or masticating element. 
 
 Upper Molars. According to Osborn, in the early lemurs 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 35 
 
 and monkeys the upper teeth were, almost without excep- 
 tion, triangular. In the late Eocene or early Miocene Age, 
 the spur or talon was developed, and the triangular became 
 a quadricuspidate tooth. 
 
 In Plate II, fig. B, which is also adapted from Osborn, the 
 cusps of the trigon are coloured red, and those of the talon 
 blue. In Anaptomorphus (fig. B T ) , a lower Eocene form allied 
 to Tarsius, a lemurine animal, the trigon only is present, the 
 protocone being internal and the paracone and metacone, 
 forming the base of the triangle, external. 
 
 In fig. B 2 , a molar of an upper Eocene monkey, in addition 
 to the trigon a cusp has appeared, the hypocone, from an 
 upgrowth of the cingulum, and forms the talon of the upper 
 molars. 
 
 In fig. B 3 , the molar of an Eskimo, the hypocone is very 
 fully developed, as, owing to the nature of the food of these 
 people, the teeth are very little used in mastication. 
 
 Fig. B 4 , from a negro, shows the cusps of the trigon and 
 talon as seen, in the normal human upper molar tooth. 
 
 It is thus seen that the four upper molar cusps represent 
 the three cusps of the original tritubercular tooth (the 
 trigon) and one cusp of the added talon, the hypocone. 
 
 Upper Molars. 
 
 Anterior palatal Protocone ) _ . 
 . Primitive triangle or 
 
 Anterior buccal Paracone Y T . 
 
 Posterior buccal Metacone j 
 
 Posterior palatal Hypocone Primitive heel or Talon. 
 
 Lower Molars. Although in the early ancestors of man 
 the three elements of the trigon were represented (fig. 10), 
 in the lower molars of human teeth- the paraconid is 
 suppressed, and only the protoconid and metaconid of 
 the original triangle remain, the three other cusps of the 
 quinquecuspidate lower molar being formed by the talonid. 
 The suppression of the paraconid is considered to be due 
 to the antagonism of the upper and lower molars not allowing 
 of its persistence in man. In Miacis, a form from the lower 
 Eocene (Plate II, fig. CJ, the trigonid is seen in all the three 
 molars, and, according to Osborn, shows ' how the primitive 
 anterior portion (trigonid) of the crown was reduced to the 
 
 D 2 
 
36 MICROSCOPIC ANATOMY OF THE TEETH 
 
 level of the posterior portion (talonid) while retaining all 
 of its (three) cusps '. Fig. C 2 shows the arrangement of 
 the cusps in Anaptomorphus , the oldest lemur known, the 
 cusps coloured red representing the fully developed trigonid 
 in the first and second molars, but in the third molar the 
 paraconid has disappeared. In this animal we have a six- 
 cusped tooth representing, in the first and second molars, 
 the three cusps of the trigonid and the three of the talonid. 
 In the third molar the six cusps are -made up of two of the 
 trigonid and four of the talonid. 
 
 Trigonid In man the trigonid is represented by the protoconid and 
 an ' metaconid only, the paraconid being suppressed, and the 
 five cusps are made up of these and three cusps of the 
 talonid the hypoconid, entoconid, and hypoconulid (fig. C 3 ). 
 
 Lower Molars. 
 
 Anterior buccal Protoconid ~\ Primitive triangle or 
 Anterior lingual Metaconid / Trigonid. 
 Posterior buccal Hypoconid ^ 
 
 Posterior lingual Entoconid > Primitive heel or Talonid. 
 Posterior mesial Hypoconulid J 
 
 Origin of Scott (22) claims that the premolars have arisen by a 
 
 premolars. (jjg eren ^ p roce ss from the molars, and considers that the 
 
 internal cusps of these have arisen from the cingulum. 
 
 Cope considers that this may show ' the origin of two 
 
 identical structures by different evolutionary routes '. 
 
 Among the many critics of the tritubercular theory, 
 Forsyth Major (15) considers that the advocates of tri- 
 tuberculism have failed to show that the mammalian molar 
 can be traced back to a more and more simple form, and is 
 Polybuny. of opinion that it can be traced to a polybunous or multi- 
 tuberculate form, and that the real tritubercular pattern is 
 a more specialized secondary stage. He states that in the 
 lower Eocene strata, multitubercular teeth are found side by 
 side with the simpler forms. He would, therefore, consider 
 that the tritubercular teeth are reduced and modified forms 
 of earlier multitubercular molars. 
 
 Ameghino considers the tritubercular form to be ' the 
 result of the simplification of molars which were formerly 
 more complicated '. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 37 
 
 Osborn, in reply to this criticism, says : ' The further we 
 go back among the ancestors of the Multituberculates and 
 Rodents, the less polybunic and more tritubercular they 
 appear.' He holds that the multituberculate tooth is of 
 tritubercular origin. 
 
 Leche (14), Taeker (25), and Rose, from the study of 
 development in the embryo, agree in stating that in mar- 
 supials, ungulates, and man the first cusp to develop is the 
 paracone and not the protocone. M. F. Woodward (28) also 
 confirms this observation, and says that the paracone is 
 identical with the primitive dental germ, and the protocone 
 is an internal ledge growing out from its base, and con- 
 cludes that the paracone in upper molars corresponds to 
 the primitive reptilian cone. In the lower teeth, however, 
 the successional development corresponds with the order 
 of the cusps in the trigonid of the lower molar. 
 
 Marett Tims (26), working on the embryology of the dog, 
 comes to similar conclusions considering its molar to con- 
 sist of a primary cone, the paracone, a secondary cone, the 
 metacone, and three cusps derived from the cingulum, 
 which he considers plays a very important part in the 
 development of the mammalian molar. 
 
 Smith Woodward (29), speaking of primitive trituberculy, 
 says 'this at first sight brilliant generalization can only 
 be accepted as a convenient working hypothesis which 
 remains on its trial', and Gidley (8) concludes that 'no theory 
 involving an absolute uniformity of succession in the develop- 
 ment of complex molars will hold true for all groups of 
 animals '. He considers, however, that the nomenclature 
 proposed by Osborn is very convenient for description, and 
 saves much confusion which would be- brought about by 
 any change in the descriptive terms used. While there have 
 been a great many criticisms of the tritubercular theory, it 
 has been very ably and impartially stated by Osborn, who 
 acknowledges many difficulties in the acceptance of the 
 theory in its entirety. 
 
 The most recent investigation of the evolution of the Boik's 
 mammalian molar is that by Professor Bolk of Amster- res ^ arches - 
 dam (3). In discussing the views of this author we have 
 to distinguish between the fusion of teeth in an antero- 
 
38 MICROSCOPIC ANATOMY OF THE TEETH 
 
 posterior (mesio- distal) direction, which would be a fusion 
 of teeth of the same series, and fusion in a transverse or 
 bucco-lingual direction, which would mean the fusion of 
 teeth of different series. Professor Bolk, in his first paper (3 a) 
 on the relationship of the mammalian dentition to that of 
 the reptiles, considers that there is an intimate relation 
 between the diminution of mammalian tooth generations 
 and the complication of their crown surfaces. He says that 
 the so-called single tooth row of the reptiles is really double, 
 consisting of an outer and an inner row, and that their 
 dentition only secondarily becomes what he terms ' mono- 
 stichic ', consisting of an apparently single row. He even 
 describes a third row, but this is resorbed before eruption. 
 He considers that the diphyodont dentition of the Mam- 
 malia represents the two rows of the reptilian ancestor, thus 
 concluding that each tooth in the primates has arisen from 
 the concrescence of two tooth generations. ' The complica- 
 tion of the teeth in longitudinal (mesio -distal) direction was 
 initiated among the reptiles and inherited by the Mammalia. 
 The complication of the crowns in a transverse (bucco- 
 lingual) direction is the result of the concrescence of two 
 t^oth generations, whereby the origin of the mammalian 
 tooth from the reptilian was completed, and ' by this con- 
 crescence the multiplicity of the tooth generations was 
 suppressed'. In other words, the polyphyodont dentition 
 of the reptiles is represented by the complication of the 
 crowns of the mammalian tooth in a transverse direction. 
 
 In his second paper (3&), Bolk describes the results of 
 the microscopic examination of early tooth germs in man 
 and other primates. He also made use of the Born system 
 of modelling employed by Rose in his work on tooth develop- 
 ment (see p. 16). In this paper he describes, in the germs 
 of both deciduous and permanent teeth, the following 
 structures in connexion with the enamel organ : ( 1 ) a lateral 
 enamel ledge, (2) an enamel crypt, (3) an enamel septum, 
 (4) the enamel navel. 1 
 
 The lateral enamel ledge is a buttress or outgrowth arising 
 from each tooth on the lateral margin of the enamel organ. 
 
 1 A good summary of Professor Bolk's views is given in a review in the 
 Dental Ccsmos for 1913, vol. 55, pp. 103 and 1058. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 39 
 
 The enamel crypt is a niche enclosed laterally by the 
 lateral ledge, and its floor is formed by the top of the enamel 
 organ. 
 
 The enamel septum. Two centres are described for the 
 differentiation of the enamel pulp (or stellate reticulum) 
 a mesial or lingual and a lateral or buccal. An area of 
 undifferentiated cells forms a septum between these two 
 centres, and stretches from the external epithelium of the 
 enamel organ to the stratum intermedium, dividing the body 
 
 FIG. 11. Enamel organ of Macropus, showing division of stellate 
 reticulum into two portions. The enamel septum of Bolk. ( x 225.) 
 
 of the enamel organ into a mesial and a lateral portion 
 (see figs 11,12). 
 
 The enamel navel, as this author calls it, is a groove or 
 depression in the external epithelium of the enamel organ 
 at the point where the septum touches this layer ' This 
 groove ', he considers, ' further accentuates the division of 
 the enamel organ into a mesial and a lateral half, suggested 
 by the septum.' 
 
 Marett Tims and Hopewell Smith describe and figure in 
 the enamel organ of a Wallaby a division into two parts, 
 but are uncertain whether to interpret it as the fusion of 
 
40 MICROSCOPIC ANATOMY OF THE TEETH 
 
 two enamel organs or the subdivision of one. If considered 
 to represent the latter, this photograph (p. 369, To-mes's- 
 Dental Anatomy, 7th ed.) shows Bolk's enamel septum ami 
 the enamel groove or navel in the external epithelium. 
 
 In a preparation of the author's of the enamel germ of 
 Macropus the enamel septum is very evident, as shown in figs. 
 11 and 12. The stellate reticulum is seen to be divided into 
 two parts, and the cells in the septum are not fully differen- 
 tiated. Under higher magnification (fig. 12) the junction of 
 
 FIG. 12. Under higher magnification, showing blending of cells of the 
 enamel septum with those of the stratum intermedium. ( x 350.) 
 
 the cells of the septum with those of the stratum inter- 
 medium is well seen. In this preparation, however, the 
 septum cannot be traced to its junction with the external 
 epithelium, and the groove on the surface of the latter is 
 not visible. It does not appear that in this case the line of 
 cells can be looked upon as indicating the fusion of two 
 enamel organs, but actually represents the subdivision of 
 one, and tends to lend confirmation to Bolk's description, 
 As he finds these structures in the enamel organ of the 
 primates, Bolk considers that they indicate that each 
 primate tooth arises from the fusion of two separate germs ; 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 41 
 
 that each primate tooth is a double structure equivalent to 
 two reptilian teeth. This view is a compromise between 
 the concrescence and the differentiation theories. 
 
 * In the reptilian ancestor of the Mammalia a triconodont 
 differentiation had arisen, and two such triconodont teeth, 
 one buccal and one lingual, had become fused. The fusion 
 of these two triconodont teeth, the lingual side of one, to 
 the labial side of the other, yielding a six-cusped element as 
 the typical mammalian tooth, the varieties arising by 
 a secondary reduction of this six-cusped tooth.' 
 
 It is seen that Bolk disposes of the assumption that each 
 cusp of the mammalian tooth represents a single reptilian 
 cone-tooth. 
 
 Summary 
 
 To account for the evolution of the human molar tooth, 
 several theories have been formulated, the most important 
 of these being : 
 
 Trituberculism. 
 
 The Polybuny or Multicuspidate theory. 
 
 Concrescence. 
 
 Bolk's triconodont concrescence theory, as it might perhaps 
 be called. 
 
 The Tritubercular theory is a very important generaliza- 
 tion, and although not generally accepted in its entirety, 
 throws great l : ght upon the relations of the cusps and the 
 functional evolution of the human molar. 
 
 This theory, founded by Cope on the discoveries of early 
 mammalian remains in New Mexico, considers that the 
 principal cusps of both upper and lower molars are produced 
 by the addition of cusps to the first-formed protocone, 
 which represents the original haplodont or single cone of 
 the reptiles. These added cusps take the shape of a triangle 
 in early forms of the Mammalia, in the upper jaw the base 
 of the triangle being outwards, the protocone forming the 
 inner cusp ; in the lower molars the base of the triangle is 
 inwards, the protocone forming the outer apex of the 
 triangle. This primitive triangle is called the trigon in the Trigon 
 upper teeth, the trigonid in the lower. To this trigon Trigoni 
 another element is added, the heel or talon, the trigon 
 
42 MICROSCOPIC ANATOMY OF THE TEETH 
 
 representing the cutting or sectorial type, the talon the 
 crushing or masticating type. 
 
 ( Protocone. 
 fTrigon (upper) J Paracone. 
 
 Primitive Triangle . J 1 Metacone. 
 
 I Protoconid. 
 
 j^Trigonid (lower) -I Paraconid. 
 (Metaconid. 
 
 r Talon (upper) Hypocone. 
 
 Heel Hypoconid. 
 
 ^Talonid (lower) 
 
 Metaconid. 
 Hypoconulid. 
 
 Talon and The talonid is more developed in the lower molars forming 
 
 Talonid. **.** , , , 
 
 three ot tne nve cusps, the talon only possessing one, the 
 hypocone. In the trigonid of the lower molars the paraconid 
 is suppressed. 
 
 It has been shown, however, by several embryologists 
 that the paracone is the first to appear in the upper molars, 
 although the order of the appearance of the cusps of the 
 lower molars corresponds to the Cope-Osborn theory. The 
 Multi- other principal criticism brought against this theory is that 
 lar Theory, niulti tubercular molars are found in association with tri- 
 tubercular forms in the same geological strata, and it is 
 considered quite as probable that the cusps of higher forms 
 are produced by the suppression of cusps, and not by 
 addition to a single cone. This is ca led the Polybuny 
 or Multitubercular theory. The theory of Concrescence, 
 supported by Ameghino (1), Rose, and Kiikenthal (13), 
 supposes the cusps of human molars to have arisen by the 
 union or concrescence of simple cones. 
 
 According to Bolk, the multicuspidate teeth of the 
 primates have arisen from the fusion of ancestral reptilian 
 teeth, both antero -posteriorly and laterally, the antero- 
 posterior fusion being of teeth of the same series, the lateral 
 fusion, of teeth of separate series. In each case a triconodont 
 tooth has arisen, and these becoming laterally fused, form 
 the six-cusped molar, which he considers the typical mam- 
 malian molar tooth, further modification arising by the 
 addition or suppression of cusps. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 43 
 
 As it is generally allowed that subsidiary cusps arise 
 from the cingulum, Bolk's views would appear to combine 
 to some degree three principal views of cusp development 
 concrescence, trituberculy, and the cingulum theory. 
 
 The structures described by this author in the enamel 
 organ are of great interest in their bearing on tooth develop- 
 ment, and the confirmation of their constant presence in 
 early tooth germs would be of great value in the explanation 
 of the evolution of the teeth of primates. 
 
 The subject is such a complicated one, especially in 
 determining the homologies of the ungulate molar, and 
 there are so many side issues connected with it, that it is 
 impossible to give anything but the merest outline in 
 a work of this scope, but the author has endeavoured to 
 state as simply as possible the principal views with regard 
 to the evolution of the human molar. 
 
 The reader who wishes to pursue this interesting subject 
 further is referred to the works mentioned in the short 
 bibliography attached to this chapter, and especially to 
 Osborn's Evolution of Mammalian Molar Teeth. An interest- 
 ing paper on the evolution of human dentition by Mr. John 
 Humphreys was contributed to the Sixth International 
 Dental Congress (9). 
 
 REFERENCES 
 
 1. Ameghino, F. (a) Filogenia, 1884. 
 
 (6) 'On the Primitive Type of the Plexodont Molars of Mammals.' 
 
 Proc. Zool Soc. Lond., 1899. 
 (c) ' Sur 1'evolution des dents des mammiferes.' Bol Acad. Nac. 
 
 Ciencias en Cordoba, t. xiv, 1896, pp. 381-517. 
 
 2. Baume. ' Versuch einer Entwickelungsgeschichte des Gebisses.' 
 
 Odontologische Forschungen, 1882. 
 
 3. Bolk, L. (a) 'The Structure of the Reptilian Dentition and its Rela- 
 
 tionship to the Mammalian Dentition.' Anai. Anzeig., vol. xli, 
 Supplement, Berlin, 1912. 
 
 (6) Die Ontogenie der Primatenzdhne (Odontolog. Studien, I): Versuch 
 einer Lo'sung der Gebissprobleme, 122 pp., Jena, 1913. 
 
 4. Cope, E. D. (a) ' Systematic Catalogue of the Vertebrata of the 
 
 Eocene of New Mexico.' Rept. Geog. Explor. and Surveys, <bc., 
 Wheeler, 1875, pp. 5-7. 
 
 (6) ' On the Trituberculate Type of Molar Tooth in the Mammalia.' Pall. 
 Bull., No. 37, Proc. Amer. Phil. Soc., xxi, December 1883, pp. 324-6. 
 
44 MICROSCOPIC ANATOMY OF THE TEETH 
 
 (c) ' On the Tritubercular Molar in Human Dentition.' Jour. Morphol., 
 July 1888, p. 7. 
 
 5. Delabarre. (a) These doct.; Paris, Dec. 31, 1806. 
 
 (b) Traite de la seconde dentition, Paris, 1819. 
 
 6. Dursy, E. Entwickelungsgeschichte des Kopfes, 1869. 
 
 7. Goodsir. Edinburgh Medical and Surgical Journal, 1838. 
 
 8. Gidley, J. W. ' Evidence bearing on Tooth-Cusp Development.' 
 
 Proc. Washington Acad. Sci., vol. viii, 1906, pp. 91-110. 
 
 9. Humphreys. J. ' Report on the Evolution of the Human Dentition.' 
 
 Trans. Sixth Int. Dental Congress, 1914, pp. 23-30. 
 
 10. Huxley, T. H. ' Development of the Teeth and on the Nature and 
 
 Import of Nasmyth's Persistent Capsule.' Quar. Journ. Micr. Sci., 
 London, 1853, vol. i, pp. 149, 164. 
 
 11. James, .W. W. ' A Preliminary Note on the Eruption of the Teeth.' 
 
 Proc. Roy. Soc. of Medicine, June 1909. 
 
 12. Kolliker. Gewebelehre. 
 
 13. Kiikenthal, W. ' Ueber den Ursprung und die Entwickelung der 
 
 Saugethierzahne.' Jenaische Zeitsch. fur Natunvissenschaft, Bd. 
 xxviii, 1893. 
 
 14. Leche. ' Studien iiber die Entwicklung des Zahnsystems bei den 
 
 Saugethieren.' Morphol. Jahrb., Bd. xix,1892, p. 502; Bd. xx,p. 113. 
 
 15. Major, C. J. Forsyth. * Nageriiberreste aus Bohnerzen Siiddeutschlands 
 
 und der Schweiz, nebst Beitragen zu einer vergleichenden Odonto- 
 graphie von Ungulaten und Unguiculaten.' Palaeontographica, 
 1873, xxii. 
 
 16. Malassez, E. ' On the Structure of the Gubernaculum dentis and the 
 
 Paradentaire Theory.' Seances et memoires de la Societe de Biologie, 
 8th ser., t. iv, No. 25, 1887, pp. 446-8. 
 
 17. Marcusen. Bull, de VAcad. de Petersbourg, 1849. 
 
 18. Osborn, H. F. Evolution of Mammalian Molar Teeth. London, 1907. 
 
 19. Rose, C. (a) ' Uber die Entstehung und Formabanderungen der 
 
 menschlichen Molaren.'. Anat. Anzeiger, 1892, Nos. 13 and 14. 
 (6) ' Uber die Zahnentwicklung des Menschen.' Schweiz. Vierteljahrs- 
 schriftfiir Zahnheilkunde, Bd. ii, 1892. 
 
 (c) 'Ueber die Zahnentwickelung der Fische.' Anat. Anzeig., Bd. ix, 
 No. 21. 
 
 (d) ' Ueber die Zahnentwickelung der Reptilien.' Deutsche Monats- 
 schriftfiir Zahnheilkunde, April 1892. 
 
 20. Serres. Essai sur Vanatomie et la physiol. des dents. Paris, 1817. 
 
 21. Sappey. Traite d'anatomie descriptive, t. iv, p. 119. 
 
 22. Scott, W. B. ' The Evolution of the Premolar Teeth in the Mammals.' 
 
 Proc. Acad. Nat. Sci. Philad., 1892, pp. 405-44. 
 
 23. Symington, J., and Rankin, J. C. An Atlas of Skiagrams illustrating 
 
 the Development of the Teeth, 1908. 
 
 24. Schafer, E. A. Microscopic Anatomy. 
 
 25. Taeker, J. Zur Kenntnis der Odontogenese bei Ungulaten. Inaugural 
 
 Dissertation, Dorpat, 1892. 
 
DEVELOPMENT OF THE TEETH IN MAMMALIA 45 
 
 26. Tims, H. W. Marett* ' On the Tooth Genesis in the Canidse.' Journ. 
 (Zool.) Linnean Soc. Lond., xxv, 1896, pp. 445-80; xxviii, 1901, 
 pp. 261-90. 
 
 27- Tomes, C. S. (a) A Manual of Dental Anatomy, Human and Comparative, 
 (b) ' On the Development of the Teeth of the Newt, Glow-worm, 
 and Green Lizard.' Phil. Trans. Roy. Soc., 1876, vol. clxv, part 1, 
 pp. 285-96. 
 
 28. Woodward, M. F. Proc. Zool. Soc. Lond., 1896, pp. 588-9. 
 
 29. Woodward, A. Smith. Outlines of Vertebrate Palceontologyfor Students 
 
 of Zoology, Univ. Press, Camb., 1898, p. 269. 
 
CHAPTER II 
 ENAMEL 
 
 ENAMEL, the hardest of animal substances, containing less 
 organic matter than any other tissue of the body, enters 
 into the composition of most teeth. 
 
 It either forms the external calcified layer of the crowns 
 of teeth, as in man and many of the Mammalia, or in those 
 teeth which are covered with cement, as in the Herbivora, 
 plays a most important part in maintaining an effective 
 grinding surface. 
 
 This is especially well seen in such compound teeth as 
 those of the Elephant, Wart-hog, and Capybara. The molars 
 of these animals are at first covered with an investing 
 cap of cement, which, when the tooth comes into use, is 
 rapidly worn away, exposing the dentine ; this being more 
 easily abraded than the enamel, the latter is raised into 
 prominent ridges, and serves to maintain a most effectual 
 grinding surface to the tooth. The cutting edges of the 
 incisors of Rodents are maintained in the same manner by 
 the position and unequal wear of the three tissues. 
 
 Enamel may be confined to the tip of the tooth, as in the 
 eel, or invest the whole of the exposed surface, as in man ; 
 it is absent in some fish and in the teeth of the order Edentata 
 (Sloths, Armadillos, &c.), and is not found in many reptiles 
 and cetaceans. 
 
 It is sometimes difficult to determine whether the outer 
 layer of many teeth of fish and reptiles is really enamel. 
 It is translucent, and shows no visible structure, and might 
 be either a thin layer of dentine or of cement, for both 
 dentine and cement in very thin layers often appear 
 structureless and transparent. It has been stated, how- 
 ever, that in all teeth which have been examined an 
 enamel organ is present, as well in those teeth which are 
 provided with enamel as in those in which it is totally 
 absent. It is considered that the presence of the enamel 
 organ, which is produced from the primary inflection of the 
 
ENAMEL 47 
 
 oral epithelium, is the most constant indication of tooth 
 formation whether followed or not by calcification, and even 
 in man the epithelial cells are said to extend at the sides and 
 base of the dentine germ beyond the point at which enamel 
 ceases to be formed, as the epithelial sheath of Hertwig. 
 The author has shown, however, that there is every evidence 
 that this extension of epithelial cells is not an extension of 
 the layers of the enamel organ, but that it is derived from 
 other epithelial elements of the follicle. He has shown that, 
 in man as well as in Rodents, the enamel organ ceases at the 
 neck of the tooth where the enamel terminates. This ques- 
 tion will be further considered in describing the structure 
 and functions of the epithelial sheath of Hertwig (p. 320). 
 
 In a tissue of such extreme hardness as enamel we should Chemical 
 naturally expect to find a great preponderance of inorganic tionof U 
 salts, and some authorities have even asserted that there enamel, 
 is no animal matter in the fully calcified enamel. Von 
 Bibra's (2) analysis of enamel is as follows : 
 
 Calcium phosphate and fluoride . . 89*82 
 
 Calcium carbonate i 4-37 
 
 Magnesium phosphate ,. . . . 1-34 
 
 Other salts . . . . . 0-88 
 
 Cartilage . . . ' .' . . . 3-39 
 
 Fat . ... . . . 0-20 
 
 This analysis is seen to give : 
 
 Organic substances . . . . . 3-59 
 Inorganic substances . . . . 96-41 
 
 In the adult female Von Bibra estimates the amount of 
 animal matter as 5-97 per cent. 
 
 The two principal lime salts present in enamel are seen 
 to be calcium phosphate and calcium carbonate, but in the 
 completed tissue the calcium phosphate largely predominates, 
 Von Bibra giving 89-82 of calcium phosphate and fluoride 
 to 4-37 of calcium carbonate (the fluoride only contributing 
 about 2 per cent.), magnesium phosphate being present in 
 the proportion of about 1-3 per cent. 
 
 Hoppe-Seyler (9) considers that in human teeth the 
 phosphate and carbonate ot lime exist in combination 
 
48 MICROSCOPIC ANATOMY OF THE TEETH 
 
 (Ca lu Co.,(Po 4 ) 6 ), and that this salt forms in the finished 
 tissue 95-35 per cent, combined with magnesium phosphate 
 1-5 per cent., leaving an organic residue of 3-60 per cent. 
 
 Hoppe-Seyler also considered that the enamel in the 
 infant contains a much larger amount of organic matter, 
 and this has been given by some authors as high as 14 per 
 cent. C. S. Tomes (18 c), however, justly points out that 
 such an analysis must be of a very uncertain nature, as the 
 difficulties to be met with in obtaining a sufficient amount 
 of the incomplete enamel at birth without contamination 
 with other substances would be almost insuperable. 
 
 An analysis of the enamel of erupted or erupting young 
 growing teeth would be of the greatest value in deciding 
 the much-vexed question as to whether enamel undergoes 
 any changes after eruption resulting in further consolida- 
 tion. No such examination has, however, been recorded, 
 although there are very strong evidences that some such 
 change does take place in growing teeth. 
 
 There have been considerable discrepancies in the analysis 
 of enamel given by different authors. Tomes accounts for 
 these discrepancies by the fact that the organic matter in 
 enamel had been estimated by the loss on ignition, but 
 that the water in intimate combination with the enamel 
 had not been accounted for, and he showed that in the 
 case of elephant's enamel as much as 4 per cent, of water 
 remained after the enamel had been dried for a long time 
 at a temperature of 300 F. He concludes that ' the total 
 loss on ignition is very nearly accounted for by the water 
 given off ', and what has hitherto been considered organic 
 matter is simply water in intimate combination with the 
 lime salts, and probably with the tribasic calcium phosphate, 
 as this substance retains in combination one or more equiva- 
 lents of water which it will not part with below a red heat. 
 Tomes 's conclusions from these experiments were that 
 enamel contains no organic matter. 
 
 The most recent analysis of enamel is that given by Dr. 
 Lovatt Evans (10), who in a paper contributed to the Inter- 
 national Medical Congress in 1913 showed that the organic 
 matter in human enamel was between 1 and 2 per cent. 
 
 Ground enamel, carefully separated from every other 
 
ENAMEL 49 
 
 tissue, was weighed and treated with dilute hydrochloric 
 acid on a water bath and evaporated to dryness. To ensure 
 the complete decomposition of the carbonates this process 
 was repeated with concentrated hydrochloric acid. Frank- Frank- 
 land's method of detecting the presence of organic matter method, 
 in the residue was adopted. ' The residue is strongly heated 
 in vacua with copper oxide, and the resulting gas, consisting 
 of carbon dioxide and nitrogen, is collected and analysed. 
 From the amount of gas formed, the amount of carbon 
 in the organic matter can be deduced, and some idea of the 
 relative quantity of organic matter can also be obtained by 
 comparison of the amounts of carbon and nitrogen.' This 
 method, Dr. Lovatt Evans states, is sufficiently delicate 
 'to detect 0-0003 gramme of carbon in 3 grammes of the 
 enamel, i.e. 0-01 per cent, of carbon, or say 0-02 per cent, 
 of organic matter, if we assume that the organic matter 
 contains 50 per cent, of carbon'. The results gave an 
 organic content of 1 to 2 per cent. 
 
 We see that the two methods employed by these investi- 
 gators gave different results, although both agree in showing 
 that there is less organic matter present than the older 
 analyses yielded. 
 
 It seems very difficult, however, for other reasons, to 
 imagine with C. S. Tomes that enamel is a totally inorganic 
 tissue, but the 1 to 2 per cent, of organic matter found by 
 Dr. Lovatt Evans may be quite sufficient to account for 
 the histological evidences of the presence of organic matter 
 in enamel. 
 
 Owing to its great density and the serious interference Structure 
 with the images it presents, caused by refraction, enamel enamel!" 1 
 is one of the most difficult substances for microscopical 
 examination. The actual structure is so veiled by the dense 
 calcification, and the course of the constituent prisms is so 
 varied and complicated, that without studying the process 
 of its development it would be impossible to arrive at any 
 certain knowledge of its true histological nature. These 
 difficulties of investigation have been the cause of the many 
 contradictory statements concerning its actual structure 
 that have appeared in the various publications on the 
 histology of enamel. 
 
50 MICROSCOPIC ANATOMY OF THE TEETH 
 
 To the unaided eye enamel has a smooth, glistening, 
 almost crystalline appearance. It is seen under the micro- 
 scope to be built up of prisms or columns united by a densely 
 calcified intermediate substance, which in perfectly formed 
 enamel cannot easily be distinguished from the prisms. 
 Course The prisms pass from the outer surface of the dentine 
 
 prisms ^ ^ e free margin of the enamel, but they do not follow 
 a straight or even course, being in many places spiral in 
 arrangement and disposed at various angles. Prisms in 
 
 FIG. 13. Longitudinal section of enamel. (x400.) 
 
 transverse section can often be seen passing more or less 
 at right angles to the others (fig. 15). Their usual arrange- 
 ment, however, is in radiating and undulating lines or 
 groups of prisms, maintaining a principal direction through- 
 out their course (figs. 13, 14). The prisms or columns, each 
 of which extends from the dentine to the surface in a more 
 or less undulating course, show a marked cross-striation 
 at regular intervals, and are separated from one another 
 by the intercolumnar or interprismatic substance, which, 
 like the prisms, is highly refractive and very fully calcified. 
 
 In transverse section the prisms show oval, hexagonal, 
 or polygonal outlines (figs. 15, 16, 24), and in many places 
 
ENAMEL 
 
 51 
 
 FIG. 14. Longitudinal section of enamel (cross-striation 
 very marked). ( x 800.) 
 
 FIG. 15. Transverse section of enamel showing arcade 
 
 form of prisms. ( x 750.) 
 
 E 2 
 
52 MICROSCOPIC ANATOMY OF THE TEETH 
 
 have an arched appearance, being rounded on one aspect 
 and more or less concave on the opposite side (fig. 24), 
 as will be more fully explained later. Prisms of hexagonal 
 form are not very frequently met with in human teeth, and 
 Leon Williams describes them as usually oval or rounded 
 (see fig. 16). In marsupial enamel they appear, however, 
 to be more constantly of an hexagonal form. 
 
 Bodeker holds that there is a living protoplasmic network 
 between the enamel columns continuous with the contents 
 
 FIG. 16. Transverse section of enamel showing oval and rounded 
 sections of prisms. ( x800.) 
 
 of the dentinal tubes, but such a view is at variance with 
 all our knowledge of the structure and composition of enamel, 
 and is certainly due to a misinterpretation of appearances. 
 
 The course of the enamel prisms is more regular in some 
 orders of Mammalia ; in the Manatee they are in many places 
 arranged in straight parallel lines, and the Rodents especially 
 show curiously regular patterns which will be further 
 explained when treating of rodent enamel. 
 
 As the area of the enamel is much greater at the outer 
 surface than at the dentine junction it is supposed that there 
 must be supplemental prisms interposed, but these, owing 
 to the complicated pattern of the enamel, are difficult to 
 
ENAMEL 53 
 
 trace, and although both C. S. Tomes (18 c) and Hope well 
 Smith (8) speak of them as being present in human enamel, 
 no record of their existence has been made by either photo- 
 graph or drawing so far as can be ascertained. 
 
 It can, however, be shown that both branching and the 
 interposition of supplementary prisms are to be seen in 
 other mammalian enamels, and the probabilities are greatly 
 in favour of a similar condition being present in human 
 teeth. 
 
 It would appear that if there is any evidence of branching 
 and the occurrence of supplemental prisms, we should be 
 most likely to see it in teeth in which the spreading out of 
 the enamel is greater than in human teeth. 
 
 In the enamel of the Wart-hog (Phacochcerus) where these Supple- 
 conditions prevail it has been shown by the author in a recent ^ tary 
 communication (11 a) that the columns of prisms divide branching 
 dichotomously, and supplementary prisms are introduced p^,! m 
 between them. chcerus. 
 
 The large third molar of Phacochcerus is a compound tooth 
 made up of from twenty-four to thirty denticles, arranged 
 in three rows, each denticle consisting of dentine surrounded 
 by an investing sheath of enamel and enclosed in cement. 
 The denticles, which form columns passing deeply into the 
 jaw, are united by the cement, and the whole forms a 
 compound tooth having many points of similarity with the 
 molar tooth of the Elephant. As the tooth becomes worn 
 down by mastication, the surface shows oval rings of dentine 
 surrounded by enamel, the intervals between the denticles 
 being filled up with cement. This disposition of the 
 enamel gives a much wider area at the circumference than 
 at the dentine junction, so that in the comparatively short 
 course of the enamel columns they would come to be very 
 wide apart at the surface if they maintained the same 
 diameter throughout. A thin section of one of these den- 
 ticles, where the enamel columns are seen radiating from 
 the dentine to the cement, shows that there is no apparent 
 increase in the diameter of the prisms or of the interprismatic 
 substance (which is very abundant in Phacochcerus) as they 
 approach the surface, but it also shows very distinctly that 
 the columns of prisms divide and branch and that the 
 
54 MICROSCOPIC ANATOMY OF THE TEETH 
 
 spaces between the branches are filled with supplementary 
 prisms (fig. 17). We thus see that, at all events in one order 
 of the Mammalia, the branching and interposition of prisms 
 is at least one of the methods adopted by nature to over- 
 come this difficulty. 
 
 Variation Pickerill (14) has given another explanation of the mode 
 
 meter of ^J which he considers this difficulty of the widening 
 
 enamel of the enamel at the periphery is overcome. He denies 
 
 the existence of supplemental prisms, but says that the 
 
 FIG. 17. Enamel of Phacochcerus (Wart-hog) showing 
 branching of enamel columns. 
 
 enamel prisms taper somewhat from the surface to the den- 
 tine margin. He has measured a large number of prisms 
 across their diameter, both at the dentine margin and at the 
 enamel surface, and describes a marked increase in their 
 diameter both in native and in European races. 
 
 According to this view each enamel prism would be 
 a cone with its base at the surface of the enamel, and he 
 compares the arrangement with that of the prisms in the 
 outer layer of the Pinna shell. From a series of measure- 
 ments he gives the following average diameter of the prisms : 
 On the buccal curve of the teeth, at the amelo-dentinal 
 
ENAMEL 55 
 
 junction 0-0031 mm., at the enamel surface 0-0057 mm., 
 giving a proportion of 1 to 1-83. At the cusps the average 
 measurement at the amelo-dentinal junction was 0-0025 mm., 
 at the enamel surface 0-0065 mm. Kolliker gives the 
 diameters of enamel prisms as from 0-0064 to 0-0051 mm., 
 but does not indicate from what part of the enamel his 
 measurements were taken, and there appears to be little 
 doubt that there is a distinct variation in the diameter of 
 the prisms within the substance of the enamel. 
 
 One of the most complete researches hitherto published Leon 
 on the structure of human enamel is that of Leon Williams 
 (21), and his paper, accompanied by excellent original structur 
 photographs, demonstrates very completely the main points 
 of his observations. 
 
 He holds that the enamel consists of two distinct portions, 
 the prisms, or rods as he prefers to call them, and the sub- 
 stance between the rods, and that these are developed in 
 different manners. The calcific matter which builds up 
 the rods is a product of the ameloblast cell. The rods are 
 built up of little blocks of coalesced granules . and are 
 arranged like piles of bricks ; they show a cross -marking 
 where they meet one another, which is the cause of the 
 minute cross-striations of the enamel prisms. He describes 
 the connexion of the rods with one another laterally by 
 calcified processes. Strings or threads in the calcified 
 material are also seen in many places passing along the 
 length of the rods : these are well shown in fig. 14, and must 
 not be confused with other darker strongly marked vertical 
 lines which are due to refraction. In fig. 18 the granular 
 nature of the calcific deposit can be seen in the formed 
 enamel prism. The cross -striation is confined to the rods 
 and does not traverse the interprismatic or cement sub- 
 stance. The blocks of the enamel rods do not alternate 
 in neighbouring rods, but are opposite one another all across 
 the enamel (figs. 14, 19). The cement substance between 
 them is calcified, according to Leon Williams, independently 
 of the rods, cementing them together and forming a compact 
 tissue which in normal enamel is completely calcified and 
 contains no trace of organic matter. Fig. 20, a photo- 
 graph from a ground preparation, shows at a a detached 
 
56 MICROSCOPIC ANATOMY OF THE TEETH 
 
 segment of the enamel column which strongly suggests that 
 the enamel is built up in this manner. 
 
 Summary Perfect enamel then consists, according to this author, 
 Wiufams's ^ ^ e prisms, for which he has substituted the term ' rods ', 
 views. made up of vertical rows of calcified disks in close apposition 
 but sufficiently separated to show the cross-striation which 
 marks their line of junction, and of a densely calcified cement- 
 ing or interprismatic substance. Delicate plasmic strings 
 are to be seen running vertically within the substance of 
 
 FIG. 18. Enamel (longitudinal ) showing granular structure 
 of prisms. ( x 800. ) 
 
 the calcified rod, and calcified connecting bridges pass 
 horizontally between them (fig. 14). 
 
 The vertical rows of calcified disks forming the rods 
 are built up of minute coalesced granules. 
 
 There is little doubt that this is a true description of the 
 structure of enamel of perfect quality, but Leon Williams's 
 statement that enamel is an entirely inorganic tissue requires 
 some modification, for the reason that enamel in human 
 teeth is seldom, if ever, a perfect tissue. Imperfections in 
 structure, especially at the dentine margin, and minute 
 channels from the dentine are so frequent as scarcely to 
 be looked upon as abnormalities. 
 
ENAMEL 
 
 57 
 
 FIG. 19. Enamel (human); longitudinal stride very 
 pronounced. ( x 800. ) 
 
 FIG. 20. Enamel, longitudinal (Weil process). Shows small block 
 detached in grinding (a ). ( x SCO. ) 
 
58 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Mr. Douglas Caush (5) long ago described staining of 
 normal enamel by several methods, and Dr. von Beust (3) 
 and others have succeeded in staining portions of the 
 enamel by the alcoholic fuchsin method, and in fig. 21 both 
 the prisms and the interprismatic substance were stained 
 with anilin violet. 
 
 Leon Williams says : ' In no instance have I ever been 
 able to demonstrate the presence of stainable matter, other 
 than bacteria, in human enamel.' The author has, however, 
 
 FIG. 21. Young enamel (longitudinal) stained with anilin violet (Weil pro- 
 cess). Both prisms and interprismatic substance are stained. ( X 800.) 
 
 many preparations showing the penetration of the stain 
 into the tubes in the enamel which are prolonged from the 
 dentine, into the spindle-like bodies at the margin of the 
 dentine, and also in some places into the interprismatic 
 substance. 1 
 
 Professor Walkhoff (20), in a recent paper, concludes from 
 an examination of many specimens of enamel i^hat there 
 is scarcely a single case among civilized man in ' which he 
 cannot find considerable faults of structure in the growth 
 
 1 Von Ebner succeeded in staining young enamel, but he did not' notice 
 any true differential staining between the prisms and the cement sub- 
 stance ; sometimes one, sometimes the other, was more fully stained. 
 He made use in these experiments of unbleached shellac, the erythrolaccine 
 staining the prisms and interprismatic substance. The method is described 
 on p. 28 of his paper (6 a). 
 
ENAMEL 59 
 
 of the tissue elements and in their calcification, and he found 
 that the teeth of the anthropoid apes showed defects in the 
 structure of the enamel exactly similar to those in man. 
 
 Other explanations have been given of the cause of the Striation 
 cross-striation of the enamel prisms from that given above. 
 Von Ebner (66) holds that it is the result of the action of 
 acids , but these striae can be seen in prisms which have not been 
 treated with acids. Fragments scraped from the forming 
 enamel of a dry human tooth still in its crypt and teased 
 out in glycerine on a slide show these markings very distinctly. 
 That this cannot be due to any acid that may be present 
 in the glycerine is shown by the fact that they are quite 
 as evident when examined in alkaline Farrant solution. 
 
 Hertz described the striae as due to the intermittent 
 calcification of the enamel rods. The generally accepted 
 view is that they are due to varicosities in the prisms, but 
 this seems to be describing the condition without any 
 reference to its cause. There seems little doubt that the 
 appearance is due to varicosities, but the cause of these is 
 the mode of deposition of the lime salts in the prisms of the 
 enamel. These are made up, as Harting (7) originally said, 
 of little piles of calcospherites, and are due. as Leon Williams 
 has pointed out, to the succession of these regularly deposited 
 calcified bodies within the cytoplasmic strings which form 
 the foundation of the enamel rods, as will be more fully 
 explained when describing the calcification of enamel. 
 Fig. 22, from a preparation of the author's of developing 
 marsupial enamel teased out in glycerine, shows in several 
 places the beaded prisms made up of a regular deposit of 
 small calcified bodies and helps to confirm this view of the 
 nature of these striae. 
 
 The suggested analogy with the varicosities of voluntary 
 muscle fibres can scarcely merit serious attention, as the 
 varicosities in enamel are due to its mode of calcification 
 and are dependent upon enamel being a calcified tissue. 
 The opinion of Hannover and Hertz that they are 4ue to 
 the intermittent calcification of the enamel rods was there- 
 fore very close to the real explanation, although these 
 authors did not describe the actual mode of deposition of 
 these uniform calcified elements of the prism. 
 
60 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Form and Within recent years some researches on the form and 
 arrangement of the enamel prisms in human and some other 
 mammalian enamels have been published by E. Smreker 
 (16) and Professor von Ebner (6 a). Smreker, who was the 
 first to point out this form of modification of the enamel 
 prism, asserted that the majority of the prisms in human 
 enamel are not rounded or polygonal in transverse section, 
 but have an arched form, owing to the prisms being longitu- 
 
 FIG. 22. Marsupial enamel. Teased preparation showing calcospherites 
 in the laminse. (x 1,000.) 
 
 dinally grooved and fitting into one another. Von Ebner 
 confirms these observations in his paper published in the 
 same year (1905). Smreker compares the arrangement of 
 the prisms to that of the cells of the prickle cell layer of 
 a pavement epithelium. In the epithelium the cells are seen 
 to present convex and concave margins, the concavity being 
 dependent upon the convexity of the cell beneath (fig. 23). 
 Where one cell comes into contact with a single neighbouring 
 cell there is a single concavity, but if two or more cells are 
 in contact with a single cell the single cell will show two or 
 more concavities. Comparing the transverse section of the 
 
ENAMEL 61 
 
 enamel prisms with such cells it will be seen that this arched 
 or concavo-convex shape of the prism renders it necessary 
 to conclude that in longitudinal section it should show 
 a convex margin, and a single, double, or compound groove 
 on the reverse side (figs. 15 and 24). As in the epithelium 
 
 FIG. 23. Drawing to show the arrangement of the 
 epithelial cells in a pavement epithelium. 
 
 the interval between the cells is crossed by the prickle 
 processes, so in enamel the prisms are connected by bridges 
 which traverse the interprismatic substance which cements 
 together the interlocking prisms. 
 
 FIG. 24. Human enamel. Transverse prisms, (x 1,500.) 
 
 Such an arrangement would necessarily give a very 
 perfect adaptation and tend greatly to increase the resistance 
 of the enamel to any disrupting force. These observations 
 have received very little attention and are not even referred 
 to in recent text-books. They were considered by Walkhoff 
 to be false appearances and not to be evidence of any 
 real structure in the enamel. A careful examination of 
 suitably prepared sections, however, and the evidence 
 
DESCRIPTION OF PLATE III 
 
 Drawings from teased preparations of enamel, 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. Compare with photograph, fig. 26. 
 
 FIG. 6. From Elephant : bridges in transverse section. The inter- 
 prismatic substance appeared dark and the bridges are very conspicuous 
 as white lines. 
 
 FIG. 7. Elephant. From a section showing a wing process in the enamel. 
 Compare with photograph, fig. 27. 
 
 FIG. 8. Elephant. From a section showing ridges and grooves, r. Ridges; 
 g. grooves. Compare with photograph, fig. 27. 
 
 FIG. 9. Two prisms from Elephant, showing needle-splitting (n) and 
 inter- columnar 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 (6). 
 
 FIG. 11. Fragments of prisms seen obliquely (Elephant). Compare with 
 photograph, fig. 28. 
 
 FIG. 12. a, b, c. Wing processes and membranous layers in Macropus, 
 showing needle-splitting, wings, and bridging. 
 
 FIG. 13. Fragment of Macropus enamel. Membranous laminae at right 
 angles to direction of prisms. 
 
 FIG. 14. Wing process, human enamel. 
 
 The drawings of the teased preparations were made by direct observation 
 under the microscope, and are magnified about 400 diameters ; figs. 6 and 1, 
 from sections, are x 800. 
 
PLATE III 
 
64 MICROSCOPIC ANATOMY OF THE TEETH 
 
 afforded by teased preparations will, we think, leave little 
 doubt that, at all events in many areas of the enamel, such 
 an arrangement of the prisms exists. Von Ebner expresses 
 his surprise that the observations of Smreker have not been 
 more generally recognized, and concludes that the necessity 
 of preparing very thin sections by grinding and the great 
 difficulties encountered in the microscopic examination of 
 enamel have been the cause of this neglect. Smreker 
 prepared very thin ground sections which were carefully 
 polished, some being treated with silver nitrate and some 
 not. 
 
 It will be said that the obvious explanation of this appear- 
 ance in transverse section is that the prisms are viewed 
 obliquely, one prism appearing to slightly overlap another 
 of the same form, for if a circular disk is overlapped by 
 another it would cause an appearance of concavity in the 
 underlying disk, and, similarly, if one disk is overlapped 
 by two the lower one would have a double concave margin. 
 It would be very difficult to prove that this was not the 
 explanation by the examination of sections alone, but 
 teased preparations, in which the enamel prisms are separated 
 and isolated, would indicate if there were really any such 
 structure as that which these authors describe. 
 
 Von Ebner, who was at first inclined to think that the 
 appearances were due to the obliquity of the sections, 
 endeavoured to prove the point by this method. Fragments 
 of enamel were scraped from a tooth and allowed to fall 
 into water or glycerine, where they were separated and 
 broken up with needles and examined under the microscope. 
 Such preparations showed separated prisms with grooves 
 running the whole length of the detached portion, and other 
 separated fragments were seen in cross -section with a con- 
 cavo-convex figure. These observers also describe other 
 appearances in both teased preparations and sections, as 
 the Flugelfortsatze or wing processes of the prisms (Plate III, 
 fig. 14) and connecting processes between the columns of 
 prisms, called by Von Ebner the intercolumnar bridges. 
 
 The wing processes of the prisms may often be observed 
 in teased preparations of mammalian enamel. These are 
 projections from the prisms, appearing sometimes 
 
ENAMEL 65 
 
 as projecting obliquely directed fibres, sometimes showing . 
 a zigzag splitting like an edge of fractured glass. These 
 processes are seen to pass all across the prism as well as 
 project from its edges, and Von Ebner has figured them in 
 human enamel as enveloping the concavo-convex prisms 
 like a sheath and projecting on either side. These wing 
 processes are visible not only in forming enamel but also in 
 the completed tissue, and can be seen in thin sections as 
 well as in teased preparations. In some ground preparations 
 made by the author to investigate these observations of 
 Smreker the wing processes of the prisms are distinctly 
 visible in some parts of the sections, and, as will be explained 
 farther on, they are still more conspicuous in the enamel 
 of the elephant. Here and there in teased preparations, 
 portions of membrane-like expansions are seen attached to 
 the prisms, but Von Ebner does not consider, for the reasons 
 given in his paper, that these are identical with the wing 
 processes (Plate III, fig. 12). 
 
 The connecting processes between the prisms have been 
 described by Leon Williams, and are no doubt the processes 
 which form the intercolumnar bridges of Von Ebner. They 
 can be seen in thin sections of enamel, but are said by 
 Von Ebner not to be visible in transverse sections. A pre- 
 paration of the author's, however, shows them quite clearly 
 in transverse section, and they are still more evident in 
 Elephant enamel (Plate III, fig. 6, and fig. 29). 
 
 In all teased preparations of human and other mammalian Needle- 
 enamels the broken prisms are seen to terminate in needle- splitting. 
 like oblique points at one end and sometimes at both ends of 
 the fractured prism (Plate III, fig. 9, &c.). This appearance 
 can be seen over the whole field of the microscope, and it is 
 seldom that a direct transverse fracture can be detected. 
 This needle-like splitting was considered by Von Ebner 
 to be due to the obliquely directed wing processes, but, as 
 it is seen in marsupial enamel where the wing processes are 
 very distinct and have a direction almost at right angles 
 to them, it is a little difficult to understand that this can 
 be the correct explanation of their origin. 
 
 The direction of the splitting would appear to suggest 
 that it takes place along the lines of the Tomes' processes 
 
66 MICROSCOPIC ANATOMY OF THE TEETH 
 
 of the amelo blasts, which are prolonged into the enamel 
 and constitute the longitudinal system of organic fibres 
 which form the foundation of enamel. 
 
 In studying the wing processes the author also made use 
 of marsupial enamel, which in many respects appears to 
 give us the key to the mode of formation of higher mamma- 
 lian enamels ; it is less perfectly calcified, at all events in 
 early stages, and the structure is more easily made out 
 than in a tissue which is very rapidly calcified and in which 
 the early stages of development are soon obscured by the 
 densely deposited lime salts. Teased preparations of the 
 forming enamel of Macropus rufus were examined. The 
 wing processes were very much more evident than in any 
 other enamel investigated ; they have a more feathery 
 appearance and often pass right across groups of prisms, 
 and appear to show a transition into membrane-like expan- 
 sions (Plate III , figs. 1 2 and 13). This would appear to indicate 
 that wing processes and these calcified membranes are one 
 and the same thing, and, as before stated, the wing processes 
 and transverse fibres of the membrane-like expansions are 
 seen to pass almost at right angles to the columns of prisms 
 and their pointed needle-like fractured terminations. The 
 author was, however, unable to detect in marsupial enamel, 
 either in early or completed stages, the curious interlocking 
 of the prisms described by Smreker in human enamel. 
 The prisms in all marsupials examined show the usual 
 polygonal forms in transverse section, and no longitudinal 
 grooving could be detected in teased preparations. 
 
 It may be that the interlocked prisms indicate a more 
 highly developed and specialized structure of the enamel. 
 Inter- The intercolumnar bridges are somewhat difficult to 
 
 Detect i n human enamel ; they are shown in fig. 14 and in 
 several of Leon Williams's photographs. In the Elephant 
 they can be seen with great distinctness both in longitudinal 
 and transverse sections (11 a). A study of the microscopic 
 anatomy of the enamel of the Elephant which curiously seems 
 to have escaped the attention of histologists, convinced the 
 author that this remarkable form and arrangement of the 
 prisms is an actual fact and not a false appearance due 
 to the prisms being viewed obliquely, for in the Elephant 
 
ENAMEL 67 
 
 the prisms are very much larger than those of human 
 enamel and their structure is more easily made out both in 
 sections and in teased preparations. The average diameter 
 of the prisms of Elephant enamel is 10 to 12 ^, while that of 
 human enamel is 4 to 5 ju. 
 
 The interlocking of the prisms is more general in the Elephant 
 Elephant ; in fact, over the surface of a large section it is el 
 difficult to find a single prism that shows the usual rounded 
 or polygonal form. On examining a transverse section of 
 
 FIG. 25. Enamel of molar of Asiatic Elephant. ( x800.) 
 
 the enamel of the Elephant's molar, which includes the 
 neighbouring areas of dentine and cement, the prisms 
 are seen both in transverse and longitudinal section, the 
 former predominating, and the transversely cut prisms 
 appear at first sight to overlap one another like tiles on 
 a roof, or the scales of a fish (figs. 25 and 26). Teased pre- 
 parations of the enamel show, however, very clearly that 
 these prisms have a concavo-convex cross -section and are 
 deeply grooved posteriorly (Plate III, figs. 1, 2, 3). 
 
 The groove may be single, when there is a prominent Grooving 
 ridge to be seen on each side of it, or they may be double, as pni 
 is usually the case, when there is, in addition, a pronounced 
 
 F 2 
 
68 MICROSCOPIC ANATOMY OF THE TEETH 
 
 ridge separating the two grooves. In these teased pre- 
 parations small detached fragments are seen which show 
 the concavo-convex form in cross-section (Plate III, fig. 4), 
 and occasionally, longitudinally broken-up prisms which 
 give still more certain evidence of the structure, as they 
 show that the broken end of the prism has the above- 
 described concavo-convex form (Plate III, fig. 3). Detached 
 fragments, in which the prisms have broken up in a trans- 
 verse or oblique direction, also give very instructive views 
 
 /m 
 
 
 FIG. 26. Enamel of molar of Asiatic Elephant. ( x 800. ) 
 
 of this structure. In Plate III, fig. 10, the whole of one of 
 the arched prisms is seen in situ, with its concave margins 
 projecting free from the surrounding prisms. 
 
 In many parts of "the sections not only have the prisms 
 the tile-like appearance above described, but they show 
 a serrated margin and appear as if grooved or ridged on 
 the surface, many of them bearing a curious resemblance 
 to the frond of a maiden-hair fern (fig. 25). 
 
 In these transverse sections of the Elephant's molar, 
 where there are alternate areas of cement enamel and 
 dentine, the convexity of the prism is invariably directed 
 towards the dentine and never towards the cement on 
 
ENAMEL 69 
 
 the opposite side, in any sections examined. In longitu- 
 dinal section the prisms appear wavy, and in many parts 
 show grooves and feathered margins (fig. 27). 
 
 Fig. 28 shows very distinctly the actual arrangement of 
 these double concave prisms in the Elephant. It was 
 photographed at a crack in the enamel, and the prisms are 
 seen at the same time in both longitudinal and transverse 
 section : the fringed margins and little shining points, 
 apparently due to the broken connecting bridges, are also 
 
 FIG. 27. Enamel of molar of Indian Elephant. Longitudinal section 
 showing grooves, &c. ( x 500.) 
 
 visible. From this photograph and from many preparations 
 it would appear that the prisms of the enamel are arranged, 
 not in direct lines from the dentine, but that independently 
 of all their curves and intercrossing they maintain a slope 
 towards the dentine which it is very difficult to follow out 
 or define. 
 
 The connecting bridges in the Elephant are well shown 
 in fig. 29 in longitudinal section, and in transverse section 
 in Plate III, fig. 6. These processes are seen to pass across 
 the interprismatic substance and form connexions between 
 neighbouring columns. In the longitudinal view it is seen 
 that the connecting processes are somewhat irregular in 
 
70 MICROSCOPIC ANATOMY OF THE TEETH 
 
 form and direction, as are the connecting or wing processes 
 of the prisms. That the membranous expansions and wing 
 processes are calcified, is shown both in man and the elephant 
 by examination with polarized light, and the very irregular 
 zigzag splitting of the wing processes, sometimes seen, may 
 be due to their fracture in teased preparations. 
 
 While the needle -like splitting of the enamel in the 
 teased preparations may be due to the longitudinal fibrilla- 
 tion, as before suggested, the membrane -like fibrillar expan- 
 
 FIG. 28. Enamel of Elephant photographed at a crack, showing prisms 
 in both longitudinal and transverse section. ( x 400. ) 
 
 sions, the wing processes, and the bridges one would be 
 inclined to consider as due to the transverse fibrillation of 
 the matrix so apparent in the forming enamel of marsupials 
 where the teased preparations break up into laminae 
 (Plate III, figs. 12 and 13). 
 
 These transverse fibrillar layers must also be incorporated 
 in the substance of the calcified prism. These grooved 
 prisms are not in direct contact with one another, being 
 separated by the interprismatic substance which is, as it 
 were, flowed around them, cementing together and com- 
 pacting the whole tissue. As Leon Williams pointed out, the 
 prisms being in many places round rather than hexagonal 
 
ENAMEL 
 
 71 
 
 there must of necessity be interspaces between them. Pro- The inter- 
 fessor Walkhoff (20) considers that the cement substance of P r js matic 
 the enamel, described by Von Ebner, Leon Williams, and other 8 
 observers as the interprismatic substance, does not exist, and 
 is in reality the outer or cortical layer of the prisms, or, as 
 he calls them, iheZentralkorper. C. S. Tomes also says : ' no 
 thoroughly distinct interstitial substance exists in enamel.' 
 The evidences, however, afforded by the development of 
 enamel and the marked staining of a substance between the 
 
 FIG. 29. The intercolumnar bridges (from a teased preparation of 
 Elephant enamel ). ( x 600. ) 
 
 prisms in marsupials, point to the actual presence of such 
 a substance altogether independent of the prisms. Leon 
 Williams considers the interprismatic substance to be a sepa- 
 rate material, formed separately from the prisms, and the 
 author's own investigations on the enamel of marsupials 
 appear to confirm this view. 
 
 Another confirmation of the separate formation and 
 distinct nature of the interprismatic substance is given in 
 a recently published paper by the late Dr. G. V. Black and 
 Dr. F. S. McKay ( 1 ), in which they describe an endemic patho- 
 logical condition of the teeth occurring in certain districts 
 of the Rocky Mountains. This condition they call mottled 
 
72 MICROSCOPIC ANATOMY OF THE TEETH 
 
 teeth, from the alternate bleaching and pigmentation of the 
 enamel seen in this affection. This pathological condition 
 affects only the interprismatic substance and not the 
 prisms. The work of calcification appears to be reversed, 
 and the last-formed part of the enamel, the interprismatic 
 substance, is removed by the action of some as yet un- 
 known agent which has no effect upon the prisms. This 
 remarkable complaint, investigated by such an accurate 
 
 FIG. 30. Striae of Retzius. ( x 150.) 
 
 observer as Dr. Black on the spot, and by means of a very 
 carefully prepared series of microscopical sections, lends 
 the greatest confirmation to the view that the prisms and 
 the interprismatic substance are separate structures. The 
 question then arises, how are they distinct from one another ? 
 Is the cement substance only a persistently less completely 
 calcified element of the enamel or is it in some way of 
 a different composition ? 
 
 The so-called ' brown strise of Retzius ' (figs. 30, 31, 32) are 
 brown lines which conform to the position of the enamel cusps 
 at different stages of growth and are probably marks of their 
 stratification, an opinion held by Kolliker, Walkhoff, and 
 others. 
 
ENAMEL 
 
 73 
 
 FIG. 31. Striee of Retzius. ( X 300. 
 
 FIG. 32. Stria? of Retzius (human enamel). ( x 750.) 
 
74 MICROSCOPIC ANATOMY OF THE TEETH 
 
 They would seem to be an indication of intermittent or 
 interrupted calcific deposit. 
 
 Pickerill (14) speaks of the striae of Retzius or incremental 
 lines, and shows that they correspond with the outcrop on 
 the surface of the enamel of the imbrication lines which 
 he describes as giving rise to the furrows on the proximal 
 surfaces. Underwood (19) considers that the striae ' are due 
 to an excessive granularity of the prisms ; this granularity in 
 each prism corresponding with that of its neighbour, the 
 effect of a line is produced '. 
 
 Von Ebner considers they are due to the entrance of air 
 between the rows of prisms, but they are evident in Weil 
 preparations and in others of teeth which have not been 
 allowed to dry. In his latest paper on enamel (6 a) Von 
 Ebner says, however, ' the typical Retzius lines, found only 
 in permanent teeth, the contour lines, are dependent, 
 as are also the contour bands i n milk teeth, on the arrest 
 of an early stage of development during enamel formation '. 
 He describes the sharp brown lines as well as the brownish 
 band-like striae in dry sections as due to the entrance of 
 air between the prisms, as first stated by Baume, but he 
 speaks also of two kinds of Retzius's striae, one in which 
 the prisms are seen to be broken across and others which 
 lie deeper in the enamel. He considers that both kinds 
 of striae are different appearances of the same structural 
 condition, and in dry sections contain clefts or fissures 
 enclosing air. Leon Williams says that the appearances 
 in his photographs show that these striae must be due to 
 a pigmentary deposit, but he scarcely conveys this idea 
 in the context, where he would appear to agree with Pickerill 
 and others that they are due to incremental deposit. An 
 appearance of pigmentation would appear to be only 
 a characteristic of these incremental lines in the finished 
 enamel. Pickerill considers that ' the only rational solution 
 of such appearances is, that imbrications and striations of 
 all varieties are to be regarded as evidence of checks in the 
 secretive functions of the ameloblasts ' (14). 
 
 He does not consider that the striae of Retzius are due 
 to pigmentation, as they have a different appearance by 
 reflected and transmitted light : by reflected light appear- 
 
ENAMEL 75 
 
 ing as white bands and the intermediate substance 
 dark. 
 
 These outcrops of the prisms in any case conform to the 
 imbrication lines above described, and are in all probability 
 evidences of a stratified deposition of the enamel. 
 
 As stated by Pickerill, the incremental lines are well 
 marked in the teeth of native races, and he considers it incon- 
 ceivable, owing to their practically universal presence, that 
 they are due to any hypoplastic cause and that they must 
 be physiologically developed structures. He shows very 
 clearly the -correspondence of the striae of Retzius with the 
 imbrication lines on the outer side of the enamel. He says : 
 ' In the cervical portion, the angle of incidence ' (of these 
 lines with the surface) ' becomes progressively less and less 
 and the ridges longer and fewer until the striae become 
 parallel with the surface and the ridge ceases to be distin- 
 guishable as such, but forms part of the general contour 
 of the tooth. When the ridges cease to be apparent the striae 
 of Retzius also cease to be marked, although remaining 
 distinctly visible.' In those animals where the striae of 
 Retzius are absent, he points out that the imbrication lines 
 are also absent, and considers them absolutely dependent 
 on one another. Kolliker and Walkhoff believe that all 
 striation in enamel is due to the deposition of lime salts 
 in strata. Certainly intermittent deposit appears to be 
 indicated as the rule in the deposition of the hard tissues, 
 as bone cement, &c. (see Chapter III). 
 
 Strong evidence that the striae of Retzius are due to the 
 outcrop of lines of prisms is afforded in floated specimens 
 of Nasmyth's membrane. The impressions of the prisms 
 on the clear layer of the membrane which is in immediate 
 contact with the enamel are seen to be arranged in parallel 
 lines (fig. 33), and these impressions show distinctly that the 
 prisms which made them were raised above the surface of 
 the rest of the enamel. There can be no question here of any 
 appearances produced by grinding, such as are referred to 
 by Von Ebner and Zsigmondy (22), as the specimen from 
 which this photograph was taken was floated off the 
 enamel in acid and must necessarily give an accurate 
 view of the undisturbed surface. Pickerill was the first 
 
76 MICROSCOPIC ANATOMY OF THE TEETH 
 
 to describe these impressions on Nasmyth's membrane in 
 specimens floated off the tooth in acid after previous treat- 
 ment with silver nitrate. He speaks of the dark striations 
 visible in these preparations as corresponding ' to the fur- 
 rows between each imbrication ' and taking ' the stain more 
 deeply because the pellicle is there a little thicker '. In 
 the author's preparation, as shown in fig. 33, the ridges are 
 impressed upon the membrane from beneath and the lines 
 of the impressions of the prisms are seen to follow these 
 ridges, so that both methods of preparation show the im- 
 
 FIG. 33. Floated preparation of Nasmyth's membrane showing 
 impressions of enamel prisms in parallel lines. ( x250.) 
 
 press of the imbrication lines on the membrane. Both 
 Von Ebner and Zsigmondy in their study of the striae 
 of Retzius dwell chiefly on the different appearances in 
 ground sections of enamel. Many of these appearances 
 are produced by grinding the sections across the incremental 
 lines at different angles, and do not seem materially to affect 
 the question of the actual cause of the striae, which, there 
 can be little doubt, is the deposit of the enamel in these 
 alternating lines of growth. In hypoplastic teeth the 
 interruption of the deposit of the enamel is very clearly 
 shown. It can be seen in fig. 34 that the incremental line 
 at a is followed by one at b that stops short of the normal 
 
ENAMEL 77 
 
 surface of healthy enamel indicating that the portion 
 behind a was incompletely formed. 
 
 Other markings in the enamel are known as Schreger's Schreger' 
 lines. These lines, which appear white by reflected light, lmes 
 are said by Von Ebner to be quite invisible by transmitted 
 light ; in thin sections this would appear to be the case, 
 but in thick sections they are seen as dark cloudy bands. 
 Von Ebner considers that they depend upon the different 
 directions of contiguous groups of prisms, and Pickerill 
 speaks of them as due to the different optical densities of 
 
 FIG. 34. Hypoplastic enamel showing incremental lines, 
 a, 6. Incremental lines. ( x 45. ) 
 
 contiguous groups of prisms. They are evidently due to 
 optical phenomena and have little histological significance. 
 
 The line of junction of the enamel and dentine (fig. 34), Amelo- 
 or the amelo-dentinal junction as it is usually called, shows junction, 
 a festooned margin, the enamel terminating in rounded 
 contours, the convexities of which are directed towards 
 the dentine. 
 
 These contours are formed by the calcified substance 
 of the enamel and have a very strong resemblance to those 
 at the margin of the dentine, where the coalesced calco- 
 spherites which form the calcified dentinal matrix are 
 
78 MICROSCOPIC ANATOMY OF THE TEETH 
 
 advancing upon the odontogenic zone. The union of the 
 enamel and dentine does not, however, appear to be a very 
 intimate one, but in some mammalian teeth which have to 
 bear great strain a much more perfect union between the 
 two tissues is seen. 
 
 In a transverse section of the molar of the elephant, 
 which is made up of plates of enamel, dentine, and cement, 
 little thorn-like processes are seen projecting into the enamel 
 from both the dentine and the cement, forming a complete 
 interlocking of the tissues (fig. 35). This was first recorded 
 by Professor Miller and Dr. Dieck in the Asiatic elephant. 
 Miller also found the same structure, although not so highly 
 developed, in the African elephant, and described a similar 
 
 FIG. 35. Transverse section of molar of Elephant showing the thorn- 
 like processes. ] E. Enamel ; D. dentine ; C. cement. 
 
 condition in the enamel of the Wart-hog and Hippopotamus, 
 in which animals, however, the thorn-like processes proceed 
 only from the cement and not from the dentine. 
 
 Certain spaces, the so-called ' spindles ', are found within 
 the enamel especially at, and around, the summits of the 
 dentine cusps (figs. 36 and 37). 
 
 These are irregular areas of a more or less tubular form 
 and are found, often in great abundance, in most human 
 teeth. They mostly project at right angles to the dentine 
 surface. 
 
 Waldeyer and Hertz denied the existence of these spindles 
 on the ground that the appearances were produced by the 
 obliquity of the sections or by fissures in the enamel, but 
 it is difficult to understand how these observers can have 
 arrived at such a conclusion after the examination of even 
 a few good ground sections of human enamel. There appears 
 
ENAMEL 
 
 FIG. 36. Spindles in the enamel. From a ground section of a tooth 
 of the neolithic age. ( x 350.) 
 
 FIG. 37. Spindles in human enamel (treated with fuchsin). Dentinal tube 
 passing into spindle. One spindle contains two air-bubbles. ( x 500.) 
 
80 MICROSCOPIC ANATOMY OF THE TEETH 
 
 to be no reason for considering them to be pathological, for 
 they are seen in well-formed teeth, sound in other respects, 
 and are conspicuous in the densely calcified enamel of the 
 teeth of prehistoric races. In several sections of human 
 teeth prepared by the author by the Weil process these 
 spindles are not confined to the immediate margin of the 
 dentine, but pass very deeply into the substance of the 
 enamel. The contents of these spaces are difficult to inter- 
 pret ; they usually appear to consist of amorphous granular 
 matter, appearing quite dark, almost black, in sections. 
 Some, however, are quite clear and transparent, and when 
 stained from the dentine by the fuchsin method show a 
 clear uniform coloration. Their communication with the 
 dentine is evidenced by the passage of the stain from the 
 tubes into their interior and by bubbles of air carried 
 by the stain to their farther extremities (fig. 37). Homer 
 {15) describes and figures fine corpuscles in these spindles 
 which he considers to be nerve-end bodies forming the 
 termination of nerve fibres in the dentinal tubes. As nerve 
 fibres traverse the dentinal tubes it does not seem improbable 
 that they should also penetrate any spaces in the enamel 
 with which the tubes communicate, but although rounded 
 bodies are certainly occasionally to be seen in these spindles 
 they are hardly sufficiently definite to be convincing. The 
 author found appearances very suggestive of these fine 
 corpuscles connected by fine thread-like processes in one 
 of the spindles of a tooth from the Stone Age (fig. 36), and 
 it seems highly probable that such round bodies and their 
 connecting strands may be caused by the arrangement of 
 the granular contents of the spindles. Certainly in the 
 majority of instances they have no appearance of being 
 filled by any organic material. 
 
 Walkhoff holds that there is an absorption of the first- 
 formed dentine, and some of the tubes, escaping absorption, 
 persist as these spindles and their connecting dentinal 
 tubes. This would not appear to account for the size and 
 shape of these spaces, and it is a little difficult to understand 
 by what agencies such an absorption occurs. 
 
 As described in a paper on the* tubular enamel of mar- 
 supials (11&) the author found apparently identical spaces 
 
ENAMEL 81 
 
 or spindles in marsupial teeth. In the incisor tooth of 
 one of the Kangaroo -rats many similar spaces filled with 
 granular matter were found half-way across the enamel 
 and a few near the free margin (fig. 38). These spaces 
 communicate with dentinal tubes, and it is significant that 
 in those marsupials in which the tube system is much 
 reduced, bodies exactly like the spindles in human enamel 
 are found at the amelo -dentinal junction and also deeper 
 within its substance, and dentinal tubes are often seen 
 
 FIG. 38. One of the terminal bulbs in the enamel from the 
 same specimen shown in fig. 56. ( x250.) 
 
 passing into these spaces, crossing them and terminating 
 more deeply in the enamel. For this and other reasons dealt 
 with more fully in the section on tubular enamels one would 
 look upon these spindles as due to imperfections in the 
 calcification of the interprismatic substance. The dentinal 
 tubes certainly enter them in many places, but they are 
 not dilatations of these tubes but spaces into which they 
 enter. Von Ebner, writing in 1890, says that the fissure 
 formations in human enamel are the result of a drying up 
 or shrinking of the interprismatic cement substance (66). 
 Pickerill (14) also describes them as 'interprismatic spaces 
 
82 MICROSCOPIC ANATOMY OF THE TEETH 
 
 combined with the poor formation of the cement substance 
 and of the outer portion of the prisms, due to the enamel 
 organ at the time not having acquired a perfect function '. 
 
 That dentinal tubes pass into human enamel in places 
 there can be no doubt ; they are seen to do so in a great 
 number of preparations, both stained and unstained. This 
 is, however, doubted by Pickerill, who considers the appear- 
 ance of tubes crossing the boundary line is due to the over- 
 lapping of the dentine and enamel. 
 
 In properly stained preparations, however, these tubes 
 are seen to pass in so deeply that no such confusion 
 could possibly arise, good thin sections showing the enamel 
 and dentine quite clearly separated. Moreover, the fuchsin 
 method also shows, as referred to above, that the 
 laminae between the horizontal layers of prisms also stain. 
 Pickerill was quite unable to convince himself when 
 using the Von Beust method of staining with alcoholic 
 fuchsin, taken up from the pulp cavity by capillary attrac- 
 tion, that stained tubes passed into the enamel from the 
 dentine. It is a little difficult to understand why he failed 
 to see them, as by this method the spindles and the tubes 
 are often deeply stained and in many places air-bubbles 
 have preceded the staining fluid to the ends of the spindles. 
 He also is of opinion that ' it by no means follows that 
 because a highly volatile and deeply penetrating stain like 
 alcoholic carbol fuchsin 1 may pass occasionally across the 
 amelo-dentinal junction that serum or lymph would do the 
 same '. It surely is inconceivable that a staining solution, 
 which after all is not so highly volatile, should pass into 
 a dead tissue more perfectly than the fluids of the living 
 body in a living tissue. 
 
 It seems much more probable that the permeation by 
 serum or lymph from the circulating blood would be more 
 thorough than that obtained by any artificial stain. 
 
 If this contention were true it would nullify the value of 
 any stains, alcoholic or other, in demonstrating the existence 
 of channels or spaces in a tissue. What is usually required 
 in histological investigation, especially on the teeth, is 
 
 1 Carbol fuchsin is not employed, but a simple solution of fuchsin in 
 alcohol. 
 
ENAMEL 83 
 
 a stain which shall be capable of penetrating as perfectly 
 as possible. 
 
 In accordance with the author's views on the nature of 
 tubular enamel, the fact that the tubes of the dentine 
 occasionally penetrate the enamel in human teeth can be 
 well understood. 
 
 We find penetration of the enamel by dentinal tubes the 
 main characteristic of the tissue in the marsupials, and 
 we also see a similar penetration in other orders of the 
 higher Mammalia, as Hyracoidea, Rodentia, and Insectivora, 
 
 FIG. 39. Fibrous bundles at amelo-dentinal junction. 
 Hypoplastic enamel. ( x 50.) 
 
 which show a reversion to the condition in the marsupials, 
 and in human teeth we have an indication of survival of, 
 or reversion to, the tubular enamel of these other orders 
 of the Mammalia. 
 
 Imperfections of structure at the dentine margin are 
 frequently met with in human enamel, areas in which both 
 prisms and interprismatic substance have a coarsely granular 
 appearance. 
 
 Radiating bundles of fine fibres are also very frequently 
 seen which appear to be connected with dentinal tubes 
 entering the enamel (fig. 39). These defects of structure 
 are much more apparent at the amelo-dentinal junction 
 
 G2 
 
84 MICROSCOPIC ANATOMY OF THE TEETH 
 
 than elsewhere, the probable reason for which will be 
 considered in treating of the development of enamel. 
 
 Tubular Enamel 
 
 Tubular enamel may be described as an enamel which 
 is normally penetrated by tubes either from the dentine 
 or from its outer surface. 
 
 The penetration of the enamel by tubes from the dentine 
 is seen in many fishes, in marsupials in its most complete 
 form, and also in the Hyracoidea (in Hyrax), among Rodentia 
 in the Jerboa (Dipus), and in the Insectivora in the Shrews 
 (Sarex). 
 Tubular Penetration by tubes from the outer surface of the enamel 
 
 toFiS? is often found in fish- 
 In the Plagiostomi it is seen in Cestracion, Lamna, and in 
 
 many sharks both recent and fossil, and among osseous 
 fish, in the Gadidse, Labridae, and Sparidse or sea-breams, 
 being especially well marked in the genus Sargina, and the 
 enamel of a member of this group, the Sargus ovis or Sheep's 
 Head fish, has long been taken as the typical example of 
 this penetration from the outer surface of the enamel. 
 The outer layer of the teeth of Plagiostome fish differs in some 
 respects from the enamel of higher forms, for besides the 
 penetration by tubes it shows a marked transverse striation 
 and contains lacuna-like spaces, and in some examples, 
 as Lamna, there is a differentiated outer layer. From 
 a study of the mode of development of this structure in 
 Elasmobranch fishes, C. S. Tomes (18 d) concludes 
 that the outer hard layer in the teeth of Selachia and 
 Ganoid fish corresponds neither to the dentine nor the 
 enamel of Mammalia. Having no collagen matrix it cannot 
 be looked upon as dentine, 'while', he says, 'its organic 
 matrix is beyond question furnished by the mesoblastic 
 dentine papilla, the epiblastic ameloblasts over it are in 
 a state of development which implies that they take an 
 active part, and that the tissue is a joint production '. 
 He considers that the part played by the ameloblasts is 
 probably the elaboration of the calcifying salts in this 
 enamel-like substance. In his classification of enamels, 
 
ENAMEL 
 
 85 
 
 he places it under the heading of ' Enamels which are not 
 wholly epiblastic ', 'a tissue which is laid down by the 
 operation of epiblastic ameloblasts in a matrix which is 
 derived from a modification of the surface of the meso- 
 blastic dentine papilla. Rose is, however, inclined to look 
 upon these tissues as dentine, but on the whole of the 
 evidence afforded by the structure and development of 
 this tissue in Elasmobranchs, Tomes considers it may be 
 appropriately called ' enamel '. 
 
 en 
 
 FIG. 40. Enamel and osteodentine of Heterodontus (Ostracion). 
 Fuchsin stain by capillary attraction. ( X 50.) 
 
 In Cestracion Philippi (Heterodontus), the Port Jackson Hetero- 
 shark, tubes are seen to penetrate the outer layer from (c es tra- 
 without, as well as from the dentine. This fish is of especial CMm )- 
 interest, as the family to which it belongs, represented by 
 four species, are the sole living representatives of a family 
 of fishes which were the most characteristic and abundant 
 sharks of the Mesozoic Period. 
 
 Their rounded teeth are eminently adapted for crushing 
 the hard-shelled animals which form their food. The surface 
 of these teeth is marked by rows of little pits or depressions, 
 and a longitudinal section shows radiating lines passing 
 from the bottom of these pits into the enamel. When 
 
86 MICROSCOPIC ANATOMY OF THE TEETH 
 
 treated with alcoholic fuchsin, these radiating lines are 
 strongly coloured by the stain, and a broad band of more 
 diffuse staining passes a little way into the enamel at the 
 base of the depressions (fig. 40). 
 
 The large tubes of osteodentine which form the bulk of 
 the tooth give out tree -like branches that divide and sub- 
 divide, their ultimate fine ramifications passing outwards 
 and crossing and mingling with the tubes from the outer 
 surface. Fine tubes also pass into the enamel from all 
 parts of the outer surface, although larger and more deeply 
 penetrating in the radiating bundles which start from the 
 base of the depressions. The two systems of tubes appear 
 to communicate with one another at the inner margin of 
 the enamel by their finest subdivisions. The line of junction 
 between enamel and dentine is very imperfectly defined, 
 but in most sharks the boundary line is much more distinct. 
 
 In many sharks the tubes from the osteodentine enter the 
 enamel more or less in bundles and branch and anastomose 
 within it, forming a network. Many of the dentinal tubes enter 
 lacuna-like spaces in the enamel. The tubes from the out- 
 side enter in more or less parallel straight lines, and appear 
 to communicate with the dentinal system in the deeper part 
 of the enamel. 
 
 In Lamna cornubica (the Porbeagle shark, fig. 41) the 
 typical osteodentine core sends large branches into the 
 enamel, which also shows traces of lamination and short 
 transverse markings. The delicate striae from the outside 
 take the stain faintly and the blending of the dentine 
 and enamel at their junction is imperfectly defined as in 
 Cestracion. These markings or striae from the outside have 
 not been hitherto shown conclusively to be of a tubular 
 nature. Tomes speaks of them as striae and considers it 
 doubtful whether they are of a tubular nature or merely 
 distinctly shown prisms, and says he has never been success- 
 ful in getting coloured fluids to enter them (18 e). 
 
 The author has shown, however, that, in the sharks, in 
 the Sargus, and in all the other fishes examined for this 
 purpose, the markings are most certainly tubes, and are 
 quite independent of the columns of prisms which are also 
 seen at the margin of the enamel following a course parallel 
 
ENAMEL 87 
 
 to that of the tubes, but quite unstained. The fuchsin 
 method before referred to was adopted in this investigation, 
 and the stain was found to enter the striae from the outer 
 surface of the enamel and penetrate them to their termina- 
 tions. The prisms of the enamel are easily distinguished 
 from the tubes, showing that the enamel columns and the 
 strise are not identical, as has been suggested. 
 
 In Sargus ovis, the Sheep's-head fish of the United States, Sargus. 
 the tubes, which are very strongly marked, pass into the 
 
 f 
 
 ^'- *R? 
 
 FIG. 41. Osteodentine tooth of Lamna cornubica (Porbeagle Shark) 
 Osteodentine centre ; tubes radiating from medullary channels, e. Enamel. 
 Fuchsin stain by capillary attraction. ( x50.) 
 
 enamel at right angles to its surface, and about half-way 
 across its width bend right and left, crossing one another on 
 its inner third, and terminating at a line of dense calcifica- 
 tion which forms a dark band following the contour of the 
 dentine surface, and separated from it by a narrow clearer 
 space. The tubes do not reach the dentine in any part 
 (fig. 42). The very complicated pattern in drawings and 
 photographs of Sargus enamel is only due in part to the 
 ramifications of the tubes, which can be easily followed 
 in the stained preparations, for the densely calcified band 
 of enamel near the dentine is formed by the exceedingly 
 intricate and complicated course of the prisms. 
 
88 MICROSCOPIC ANATOMY OF THE TEETH 
 
 This is clearly shown in ground preparations, the red- 
 stained tubes being sharply relieved against the unstained 
 yellowish enamel. In his work on dental histology Mr. Hope- 
 well Smith says 'the tubes are found in the longitudinal 
 axes of the enamel rods ' ; that this is not the case is very 
 evident in the preparations here referred to the tubular 
 system is completely independent of the enamel rods as 
 shown both in the completed tissue and in teeth in the 
 course of development. This band near the dentine appears 
 
 P 
 
 FIG. 42. Sargus ovis. Incisor tooth (stained with alcoholic fuchsin by 
 capillary attraction). Within the pulp (p) are seen the vascular channels 
 shown in fig. 160 (here cut across). ( x 50.) 
 
 to be made up of prisms very intricately arranged, and it 
 is quite impossible to trace the course of individual prisms. 
 This pattern can be seen in the abundant organic matrix 
 of the enamel which is laid down in the early stages of its 
 development before any calcification has commenced. In 
 many preparations in which the enamel cap was quite soft 
 and easily cut with a knife the same pattern of twisted and 
 spirally arranged prisms was seen, and such preparations 
 took stains deeply. In the fully-formed tooth a clear area is 
 always to be seen between this layer and the dentine (fig. 43). 
 The first preparations of Sargus enamel examined by the 
 
ENAMEL 
 
 89 
 
 fuchsin staining method were from a dry preparation of the 
 upper and lower jaws of a Mediterranean species of Sargus, 
 S. noct. In this fish, unlike S. ovis, there is a very free 
 penetration of tubes from the dentine as well as from without. 
 In fig. 43 it is seen that the dentinal tubes enter the enamel 
 in radiating bundles and pass right across it, interlacing with 
 the tubes from the outside. At the dentine margin they 
 spread out a little from the close bundles and bend slightly 
 towards one another before spreading out into the enamel. 
 
 FIG. 43. Sargus noct. Completed enamel. 
 d. Dentine; e. enamel. ( x 50.) 
 
 These dentinal tubes are very abundant and deeply stained ; 
 many can be followed to their terminations close to the free 
 margin, while others are lost in small oblong bundles of tubes 
 formed by those of the outer tube system. These two 
 separate systems of tubes crossing one another in many direc- 
 tions form, with the twisted and spirally arranged prisms, 
 a more intricate and complicated pattern even than in 
 Sargus ovis. Many of the tubes from the dentine divide 
 and branch, as do also those from the outside, but the 
 majority of the tubes of both systems pass in more or less 
 even curves across the enamel, interlacing with one another. 
 This penetration by both sets of tubes does not appear to 
 
90 MICROSCOPIC ANATOMY OF THE TEETH 
 
 have been hitherto described in the Sparidae, as the species 
 examined had always been S. ovis, in which there is no 
 penetration by tubes from the dentine. Another species 
 of Sargus, S. vulgaris, also shows penetration by dentinal 
 tubes, but they only penetrate for a short distance, ter- 
 minating at the densely calcified layer of prisms above 
 described. 
 
 We see, therefore, that in the same genus (Sargina) there 
 is a very complete penetration by both systems of tubes 
 in Sargus noct, a partial penetration from the dentine and 
 
 c 
 
 FIG. 44. Sargus noct. Unerupted molar, o. Remains of enamel organ ; 
 t. open deeply stained channels in the enamel ; b. stained horizontal band ; 
 c. unstained enamel ; d. dentine. (x50.) 
 
 a full one from the outside in Sargus vulgaris, and a complete 
 suppression of the dentinal tubes in the enamel of Sargus 
 ovis, only the tubes from without being present. It is not 
 usual to find such a marked difference of structure in the 
 enamel in species belonging to the same genus. 
 
 In the Sparidse the teeth replace one another vertically 
 instead of laterally as in most fish. The successional teeth 
 lie immediately beneath those in use and within the substance 
 of the bone. 
 
 If the enamel of these unerupted teeth is examined by 
 the fuchsin method it is seen to differ very considerably in 
 appearance from that of the erupted teeth (figs. 44 and 45). 
 
ENAMEL 91 
 
 The tubes from the outside have very widely open orifices 
 and are stained deeply in broad vertical stripes within the 
 enamel ; these broad bands pass about half-way across 
 and then become fused in a deeply stained area which 
 extends horizontally across the enamel, being sharply 
 limited at the lower margin by the band of calcified prismatic 
 material previously described. If a tooth is examined 
 which is a little farther advanced towards eruption, the 
 stained horizontal band is seen to be broken up, only a few 
 
 A k 
 
 l d 
 
 FIG. 45. As fig. 44, more highly magnified, t. Stained channels ; 6. hori- 
 zontal stained band; d. dentine; e. enamel prisms. (x!30.) 
 
 patches of stained material being seen in its former position, 
 but many of the entering stained spaces are still very wide 
 and show laterally expanded areas. All stages of this 
 gradual contraction of the stained area are to be seen in 
 the molar teeth in different stages of development, to the 
 completed enamel of the tooth in wear. 
 
 From these appearances it seems impossible to avoid 
 the conclusion that these tubes have a calcifying function, 
 conveying the lime salts to the organic foundation substance 
 of the enamel previously laid down. In no instance could 
 this stained horizontal band be seen to pass beyond the 
 
92 MICROSCOPIC ANATOMY OF THE TEETH 
 
 layer of dense enamel (fig. 44, c), which would suggest that 
 this layer bordering the dentine had been calcified previously 
 to the calcification of the main bulk of the tissue. A study 
 of the development of the enamel in these fish probably 
 gives the true explanation of this appearance. As shown 
 elsewhere (p. 186), the part near the dentine appears to be 
 laid down under the influence of true ameloblasts ; after 
 its formation the ameloblasts disappear as such and the 
 remainder of the enamel is deposited by a structure which 
 has assumed the form of a vascular secreting organ. 
 
 FIG. 46. Sargus noct. Granules in tubes of enamel 
 of unerupted tooth. ( X 250. ) 
 
 In many sections of these unerupted teeth of Sargus the 
 tubes are seen to be closely packed with small dark granules 
 which do not take the stain and which would appear to be 
 the calcifying substance conveyed to the enamel (fig. 46). 
 The principal points brought out by this investigation are : 
 
 1. That the enamel of Sargus and many other fish which 
 show this outer striation is penetrated by tubes into which 
 a stain freely passes, their tubular nature being still further 
 evidenced by the presence of 'small granules which are seen 
 not only at the circumference but also deeply in the enamel 
 near their terminations. 
 
ENAMEL 93 
 
 2. That in the Sparidse a tube system from the dentine 
 is fully developed in some species, the outer and inner 
 systems of tubes existing together. 
 
 3. That a progressive calcification of the larger part of 
 the enamel takes place in its interior in teeth that have not 
 yet come into use, and that this process is continued by the 
 agency of the tubes and their contents. 
 
 In the group Scarus of the Labridse (Wrasses) there is 
 a still more complete penetration by tubes from without 
 than in Sargus. In the specimens examined the tubes took 
 
 FIG. 47. Tubular enamel of Pseudoscarus. d. Dentine ; 
 e. enamel. ( x 50.) 
 
 the stain freely, entering the enamel in parallel straight 
 lines as in Sargus, but more intricately crossed and inter- 
 woven in the deeper parts than in any of the Sparidae. 
 The enamel of the pharyngeal teeth of Scarus forms a 
 very thick investment to the tooth, and the tubes, after 
 passing in for about one-third of its width, cross one 
 another in all directions and course along the dentine 
 margin parallel to its surface, but appear to have no connexion 
 with it, at all events in the species of Scarus examined. 
 Fig. 47 shows the great abundance of these tubes and the 
 complicated pattern which they form. A very remarkable 
 peculiarity of these teeth of Scarus is the very small amount 
 of dentine in comparison with the bulk of the enamel seen 
 in most specimens, both of the pharyngeal teeth and those 
 
94 MICROSCOPIC ANATOMY OF THE TEETH 
 
 of the beak-like jaws. Large tubes resembling those of 
 a true vascular dentine are seen in the dentine and in several 
 places even entering the enamel. They not only form loops 
 at the amelo-dentinal junction which encroach upon the 
 enamel, but broad tubes pass in several places well into its 
 substance. These are not merely the small isolated loops 
 described by C. S. Tomes in Sargus, but they form a connected 
 system (fig. 48). 1 
 
 d 
 
 FIG. 48. Vascular loops in tooth of Pseudoscarus. e. Enamel ; 
 d. dentine Ground section. (x!50.) 
 
 In the maxillary teeth of Scarus, according to Von Boas 
 (4), the dentine is much more reduced than in the pharyn- 
 geal teeth, and he considers that a resorption of the dentine 
 occurs after its first formation and describes absorption 
 contours or Howship's lacunae between the enamel and 
 dentine. 
 
 In a freshly preserved specimen of Pseudoscarus, kindly 
 sent to the author by the Curator of the New York Aquarium, 
 he found confirmation of these observations of -Von Boas, 
 as shown in fig. 49, photographed from preparations from 
 this specimen. In many places absorption lacunae are seen 
 between the enamel and the dentine in the superimposed 
 teeth of the maxilla. These can also be seen in the pharyn- 
 
 1 A similar condition in the crown of the incisor teeth of Sargus ovis is 
 described on p. 258. 
 
ENAMEL 
 
 95 
 
 FIG. 49. Pseudoscarus. The maxillary denticles the dentine is almost 
 entirely absorbed, the bulk of the tooth consisting of the tubular enamel. 
 Ground section of freshly preserved preparation. ( X 60. ) 
 
 FIG. 50. Pseudoscarus. Margin of jaw showing one of the maxillary 
 denticles in use both enamel and dentine have undergone absorption. 
 Ground section from a freshly preserved specimen. ( x 60.) 
 
96 MICROSCOPIC ANATOMY OF THE TEETH 
 
 geal teeth where vascular loops encroaching on the enamel 
 are also visible. 
 
 Fig. 50 shows one of the teeth in use at the margin of 
 the maxilla, which has undergone very extensive absorption 
 of the dentine, and also of the enamel where the successional 
 denticles are advancing. Fig. 49 also shows the excavation 
 of the denticles farther back in the jaw, the dentine at a, as 
 in fig. 50, having almost entirely disappeared. Fig. 51 
 
 FIG. 51. Section of upper jaw of Pseudoscarus showing 
 arrangement of denticles in the bone. ( x 10.) 
 
 shows under lower magnification the superimposed teeth of 
 the maxillary bone of Pseudoscarus. 
 
 The Tubular Enamel of Marsupials 
 
 The tubes in marsupial enamel are entirely connected 
 with the dentinal tubes, and there is no penetration of the 
 enamel from without as in many fish. 
 
 The determination of the exact nature and significance 
 of this tubular condition is of the greatest importance in 
 the study of the development of marsupial enamel and of 
 mammalian enamel generally, and will be considered more 
 fully in the chapter on ' Development of the Enamel '. 
 
ENAMEL 97 
 
 x 
 
 Sir John Tomes (17 a) in 1849 first described the existence 
 of tubes in the enamel of marsupials, continuous with those 
 of the dentine. This fact was denied by Waldeyer and Hertz, " 
 but, as C. S. Tomes says, ' it is very difficult to understand 
 how any one looking at a section of a tooth of the Kangaroo 
 or Wallaby could have any doubts on this point '. J. Tomes, 
 moreover, showed that when marsupial enamel was de- 
 calcified the tubes were seen hanging out from the dentine 
 after the removal of the lime salts. 
 
 In all marsupials, with the exception of the rodent-like 
 Wombat, these tubes can be seen ; but they are not by any 
 means developed to the same extent in the different families 
 of marsupials, where all degrees of penetration are seen 
 from the very complete one in Macropus to the scanty 
 development of the tubes in many Phalangers. In Macropus 
 the tubes not only enter in great abundance but traverse 
 nearly the whole width of the enamel, in many places 
 terminating just beneath the outer border. 
 
 If a ground section of the enamel of Macropus be examined 
 under the microscope it is seen that the dentinal tubes pass 
 across the boundary at the amelo -dentinal junction, and 
 enter irregular dilatations or spaces, whence they are con- 
 tinued in more or less parallel lines into the enamel. A uni- 
 form bending of the tubes at an obtuse angle near the dentine 
 is a very frequent appearance in marsupials. 
 
 The dilatations above referred to vary greatly in size 
 and contour, and in some places are altogether absent, 
 the tubes passing from the dentine directly into the enamel, 
 as seen in many parts in the teeth of the Hypsiprymninse. 
 When teeth of the Wallaby or Kangaroo are examined 
 which have been injected with alcoholic fuchsin from 
 the pulp cavity, the stain is seen to have passed uninter- 
 ruptedly across the boundary and along the whole length 
 of the tubes. The interprismatic substance also shows a 
 diffuse staining in places ; in fact, in many of these sections 
 the enamel appears to be more fully stained than the 
 dentine (figs. 52 and 53 A, B). 
 
 This method of staining demonstrates very clearly the Termina- 
 continuity of the enamel tubes with the dentinal tubes. tlSesin 
 In some sections of the teeth of Bettongia (fig. 54), one of Bettongia. 
 
 MDMMEEY JJ 
 
98 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the Kangaroo rats, treated with silver nitrate by the process 
 of Ramon y Cajal, the matrix of the enamel is uniformly 
 stained of a yellowish brown, the tubes are very deeply 
 stained and vary greatly in diameter, and very fine cross - 
 branches pass horizontally at right angles to the tubes ; 
 these are also seen in some sections of the teeth of Macropus, 
 and their origin is a little difficult to understand. Similar 
 cross -branches are described by Sir John Tomes in the enamel 
 of the rodents. This uniform staining of the matrix is not 
 seen in other mammalian enamels, and would appear to 
 indicate that in these animals it is not so completely 
 calcified as in higher forms . Another appearance in the incisor 
 teeth of Bettongia has a very strong bearing upon the dis- 
 puted question of the origin of the tubes in marsupial enamel. 
 In about the middle third of the length of the long lower 
 incisor of Bettongia, many of the tubes from the dentine, after 
 passing half-way across the width of the enamel, terminate 
 in bulb-like closed ends ; these terminations being directed 
 towards the enamel surface and not towards the dentine 
 would appear to indicate that they are true dentinal tubes, 
 and not tubes of enamel origin which have become con- 
 nected with them, according to the view of the nature of 
 the tubes in marsupial enamel held by C. S. Tomes (figs. 38 
 and 55). 
 
 In the Cuscus (Phalanger orientalis) the tube system is 
 very much reduced, but similar bulb -like terminations are 
 seen, many of which are quite near the dentine and others 
 farther within the enamel substance. These bulb-like 
 endings have a great similarity to the so-called spindles seen 
 in human teeth (see p. 78), and probably have a similar 
 origin in areas of imperfect calcification of the interprismatic 
 substance. A comparison of figs. 37 and 56 will demonstrate 
 the great similarity between the spindles in human enamel 
 and these expanded terminations of the tubes in marsupials. 
 In the Phalanger (fig. 56 A) the tubes from the dentine 
 are in many places seen to enter these spaces, and after 
 traversing them to pass out on their distal ends and terminate 
 deeper in the enamel. This is also occasionally seen in the 
 similar spaces in human teeth, but tubes so prolonged do 
 not pass in so deeply as in marsupials (fig. 56 B). 
 
ENAMEL 
 
 99 
 
 FIG. 52 Dentine and enamel of Macropus injected with fuchsin from 
 the pulp cavity, d. Dentine ; e. enamel. ( x250.) 
 
 A B 
 
 FIG. 53. A. Enamel of Macropus; tubes at enamel margin. Fuchsin 
 injected preparation, e. Enamel ; d. dentine. ( x 700.) 
 
 B. Showing the escape of the fuchsin stain from the tube into the 
 surrounding imperfectly calcined fibrillar basis substance. ( x250.) 
 
100 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Transverse sections of the enamel of Macropus which 
 had been treated by the fuchsin method showed that the 
 interprismatic substance is abundant and is deeply stained, 
 and in some places there is a faint staining of the prisms 
 themselves (fig. 57). The transverse sections of the tubes 
 are much more deeply stained than the interprismatic 
 substance, and are seen to be lying within this interprismatic 
 substance and not within the prisms. In those sections 
 that are directly transverse, the author could find no tubes 
 
 FIG. 54. Passage of dentinal tubes into the enamel in Bettongia. 
 Weil ground section, e. Enamel ; d. dentine. ( x 150. ) 
 
 that could be considered to be within the prisms. Here 
 and there one is seen which at first sight might be 
 thought to be within the prism, but careful focusing 
 shows that this appearance is caused by the spiral course 
 of many of the tubes which are seen through the thickness 
 of the sections. In very thin preparations, which are very 
 difficult to obtain, the author cannot but consider that the 
 tubes are indubitably shown to lie within the stained 
 interprismatic substance. 
 
 C. S. Tomes describes transverse sections of marsupial 
 enamel in which he says that three-fourths of the tubes 
 appear clearly to be in the substance of the prisms, the 
 
ENAMEL 
 
 FIG. 55. Spindle-like bodies terminating dentinal tubes within 
 the enamel of Bettongia. Ground section. ( x 50.) 
 
 e 
 
 A B 
 
 
 
 FIG. 56. A. Longitudinal section at apex of dentine cusp of Phalanger 
 orientalis showing tubes passing into enamel and others entering the 
 spindle-shaped spaces, d. Dentine ; e. enamel. ( x 130.) 
 
 B. Section in same position of human temporary molar injected with 
 fuchsin from pulp, (x 130.) 
 
102". : MICROSCOPIC ANATOMY OF THE TEETH 
 
 remaining fourth appearing as though they were between 
 them, but he considers it is a priori highly improbable that 
 they occupy both positions. Von Ebner (66) considers that 
 the tubes lie between the prisms in the interprismatic spaces, 
 and his figure of transverse marsupial enamel shows, accord- 
 ing to C. S. Tomes, that all but one appear to do so ; the 
 drawing, however, scarcely indicates that one lies within 
 the prism, and any appearance of its so doing may probably 
 
 FIG. 57. Transverse section of enamel of Macro pus (injected with 
 fuchsin from the pulp cavity). Interprismatic substance strongly stained 
 and transverse sections of enamel tubes more deeply stained and within 
 the interprismatic substance. ( x 800. ) 
 
 be fully explained as being due to the thickness of the 
 section. 
 
 J. Tomes (17 a) and Kolliker considered there was no doubt 
 that the dentinal tubes pass into the enamel. This is denied 
 by Waldeyer. Von Ebner agrees with Kolliker and J. Tomes. 
 The passage of tubes from the dentine to the enamel is held 
 by Von Ebner to be analogous, to the ingrowth of nerve 
 fibres into epithelium. He says it is evident that in young 
 marsupial teeth the canals run in the interprismatic sub- 
 stance. By decalcification experiments he showed that the 
 tubes held together and had much the appearance of elastic 
 
ENAMEL 103 
 
 fibres, and he states that one could see by such experiments 
 that the floating tubes are isolated twisted objects and are 
 not held together by any membrane. In the author's 
 decalcification experiment to be presently described, a 
 distinct fibrillar basis to the enamel remained, in which 
 these tubes lay. 
 
 From an examination of his own stained preparations 
 the author considers there can be no doubt that the tubes 
 lie between and not within the prisms. Professor Von Ebner 
 and Professor Paul both consider that the tubes lie between 
 the prisms, but C. S. Tomes looks upon the tubes of 
 marsupial enamel as canals in the centre of the prisms, 
 considering that calcification in marsupial and other mam- 
 malian enamels takes place centripetally, leading in some 
 cases to the complete obliteration of the central channel as 
 in human teeth, in others not reaching the centre and so 
 leaving tubes within the prisms ; there is a squeezing up, 
 as it were, of the central canal by the. advancing calcification. 
 According to this view there would be no true interprismatic 
 substance in enamel, calcification with regard to the prisms 
 proceeding from without inwards. The tubes in the enamel 
 of marsupials are, he considers, entirely a product of the 
 enamel organ and cannot properly be called dentinal tubes, 
 and their connexion with the dentinal tubes he would look 
 upon as a joining up of the enamel tubes with those of the 
 dentine. 
 
 One cannot but consider, however, that the evidences 
 given by finished structure and development are very 
 strongly in favour of the existence of a very distinct inter- 
 prismatic substance, and further corroborative evidence 
 has lately been advanced in the paper by the late Dr. Black 
 on the curious endemic disease of the teeth in the Rocky 
 Mountains above referred to, where the interprismatic 
 substance appears to be the sole part of the enamel affected 
 (see p. 71). 
 
 It would appear that the organic basis of marsupial 
 enamel is more persistent than that of the enamel of the 
 higher mammalia. 
 
 If a piece of a ground section of marsupial enamel and 
 dentine is decalcified on a slide, with suitable precautions, 
 
104 MICROSCOPIC ANATOMY OF THE TEETH 
 
 to prevent the disturbance and washing away of the dissolv- 
 ing enamel which usually occurs in these experiments, 
 a residue, indestructible by acids, is left which retains the 
 form and arrangement of the calcified tissue faintly 
 indicated on the slide. In the author's experiment a thin 
 ground section of the enamel and dentine of the Kangaroo 
 (Macropus rufus) was stained with toluidin blue and 
 placed in a drop of Farrant's solution on a microscope 
 slide. Strong hydrochloric acid was added drop by drop to 
 the Farrant solution and the action watched under the 
 microscope. No cover-glass was used and the specimen 
 was not disturbed by the action of confined bubbles of gas. 
 Decalcification proceeded very rapidly, and, when all the 
 calcified enamel had disappeared, the tubes were seen lying 
 in a faintly -tinted fibrillar stroma and had retained their 
 positions as in the formed tissue. The whole contour of 
 the piece of enamel could be traced upon the slide, and the 
 tubes were so little disturbed that their characteristic bending 
 in parallel lines near the dentine, so commonly seen in the 
 enamel of Macropus, was quite evident. The decalcification 
 experiment described by Sir John Tomes was carried out 
 in a watery medium and the delicate stroma was not retained 
 as in the dense medium employed by the author, but he was 
 able to demonstrate that the tubes passing from the dentine 
 to the enamel retained their form and could be seen waving 
 about in the acid solution. 
 
 Further evidence of the delayed or only partial disappear- 
 ance of the organic basis of marsupial enamel is also seen 
 in the stained preparations of the completed tissue. As 
 shown in fig. 53 B, in many places the stain has penetrated 
 from the interprismatic spaces into a delicate fibrillar 
 structure within the matrix of the enamel ; this would 
 appear to be a portion of the organic basis which has escaped 
 calcification and has taken the stain as freely as the inter- 
 prismatic spaces themselves, which certainly in Macropus 
 appear from their staining reaction to have undergone in 
 many places very little or no calcification. 
 
 Enamel of The enamel of Hyrax (the biblical Coney) is very com- 
 
 yrax. pi e t e ly penetrated by tubes from the dentine which pass in 
 
 many places all across it to the outer margin. At the June- 
 
ENAMEL 105 
 
 tion with the dentine there is no appearance of the spaces 
 into which the tubes pass, so conspicuous in the enamel of 
 Macropus. The tubes bend slightly and often branch at 
 their entry into the enamel, but otherwise pass uninter- 
 ruptedly across the boundary. In other respects the enamel 
 is very similar to that of marsupials, but the absence of 
 the dilatations points probably to a more complete calcifica- 
 tion of the interprismatic material in Hyrax, a point which 
 will be further considered in treating of the development 
 and calcification of the enamel in marsupials. 
 
 As previously mentioned, penetration of the enamel by 
 tubes continuous with those from the dentine is also seen 
 in the shrews (Sorex), among the Insectivora, and in the 
 Rodents in Jerboa. Slight penetration of the enamel from 
 the dentine is also seen in many Garni vora (18/), and has 
 been described by the author in the molar tooth of the 
 Elephant (1 1 a), and may often be seen in human teeth where 
 occasionally scattered tubes penetrate to a considerable depth. 
 
 These examples of tubular enamel, as Tomes says, ' dis- 
 tinctly point to this penetration of the epiblastic enamel 
 by tubes continuous with those of the mesoblastic dentine, 
 being a primitive character, to which some slight tendency 
 to revert has not been quite lost by placental mammals '. 
 
 Owing to some resemblances to the dentition of mar- Oeodonts. 
 supials found in the teeth of Creodonts, C. S. Tomes was 
 led to examine numerous teeth of this fossil sub-order (the 
 members of which are considered by most authorities to be 
 the ancestors of the Carnivora, and possibly also of the 
 Insectivora), and to determine if the histological characters 
 of the teeth showed any resemblance to those of marsupials. 
 The conclusions he arrived at from the examination of teeth 
 of Hyenodon, Mesonyx, and many other forms, were that, 
 so far as the structure of the enamel was concerned, none 
 of these animals show any greater resemblance to mar- 
 supials than do the recent Carnivora, but the patterns of 
 the enamel in Creodonts closely resemble those of the 
 Carnivora. Tubular enamel was not found in the teeth of 
 the Creodonts examined, and so far as the histological 
 evidence is concerned, they do not show any affinity to 
 marsupials (18/). 
 
106 MICROSCOPIC ANATOMY OF THE TEETH 
 
 The Enamel of Rodents 
 
 There are certain peculiarities in the enamel of the 
 Rodents which render it advisable to consider its structure 
 in a separate section. 
 
 The incisors of Rodents are of great length and very large 
 in proportion to the other teeth ; they extend a long way 
 back in the jaws and describe a sharper curvature in the 
 upper than in the lower jaw. 
 
 The exposed portions of these incisors terminate in a chisel- 
 shaped edge produced by the unequal wear of the three 
 tissues which make up the tooth. The dentine core is 
 coated with enamel, which is confined to the outer or 
 anterior surface, and with cement beneath. The cement 
 does not cover the surface of the enamel as was originally 
 stated by Owen, but in the Rodent-like marsupial, the 
 Wombat, the enamel has an external covering of cement. 
 The unequal wear of the three tissues gives rise to the sharp 
 cutting edge, the cement wearing down most rapidly, the 
 dentine less so, and the enamel least, so that the latter 
 tissue projecting beyond the others forms a sharp chisel 
 edge to the tooth. As the wear rapidly proceeds at this 
 free edge the tooth is maintained at the same level by 
 constant advance from behind, the additional growth taking 
 place during the whole life of the animal from its persistent 
 pulp, and the curvature of the teeth in the jaws preventing 
 injury to this pulp from direct pressure. The incisor teeth 
 of many rodents show a deep orange coloration of the 
 enamel on the outside. This pigmentation is confined chiefly 
 to the outer surface, but penetrates some little distance 
 within the enamel substance, gradually fading away towards 
 the dentine. This coloration is very strongly marked in 
 the Porcupine, Beaver, and Squirrel, but is absent in the 
 Hares and Rabbits. The molar teeth of the Capybara and 
 Beaver and of the Hares and Rabbits also grow from 
 persistent pulps. In the Capybara (HydrocJicerus) the large 
 molar tooth is built up of a series of plates or laminse, each 
 consisting of dentine surrounded by enamel, the separate 
 plates being united by cement. The wear of the tissues 
 being unequal, as in the incisors, a sharp grinding surface 
 
ENAMEL 107 
 
 is maintained by the projection of the ridges of enamel. 
 Each plate has its own separate pulp cavity with a distinct 
 persistent pulp, thus differing from the molars of the 
 Elephant, which show a similar disposition of the tissues on 
 the grinding surface, but not growing from a persistent 
 pulp the plates have a common pulp cavity. 
 
 As the incisor teeth of Rodents are continually growing, 
 it can be easily understood how in the case of fracture or 
 dislocation of the exposed part of one tooth the opposing 
 tooth will continue to grow forward following the curve 
 
 FIG. 58. Skull of Rat (Mus decumanus). Dislocation and overgrowth 
 of incisors. The right upper incisor penetrates the skull. (From a specimen 
 lent to the author by Mr. Montagu Hopson. ) 
 
 within its socket, and various malformations may result. 
 Instances are recorded in rats and rabbits where, one incisor 
 not being opposed owing to accident, it has elongated to 
 such an extent that it has penetrated the skull of the 
 animal. 
 
 Fig. 58 shows the effects of fracture and dislocation of the 
 incisors of a rat, photographed from a specimen kindly 
 lent to the author by Mr. Montagu Hopson. 
 
 The course of the enamel prisms in human enamel is 
 difficult to define, and their spiral twistings and longitudinal 
 curvatures render it very difficult to follow their actual 
 direction in any one part, but the general trend of the prisms, 
 
108 MICROSCOPIC ANATOMY OF THE TEETH 
 
 independently of all these secondary curvatures, is from 
 the dentine to the enamel surface. In the Manatee the 
 prisms in many parts have a straight course from dentine 
 to surface, but this is the rarest condition in the enamel of 
 mammalia. The enamel of Rodents is in the majority of 
 species characterized by its arrangement on the inner side 
 in decussating layers, while on the outside the layers are 
 parallel to one another. This division into two layers was 
 pointed out by Professor Owen (12), but the most complete 
 research on the structure of Rodent enamel was carried 
 out by Sir John Tomes and described in a paper read before 
 the Royal Society in 1850 (17 b). This paper, the result of 
 a very complete investigation of the enamel of the different 
 genera and species of Rodents, showed that the structure 
 of the enamel was to be looked upon in many cases as an 
 assistance in the identification of species. In the words of 
 this author, he was enabled to show that ' the teeth of some 
 species of the order have specific characters by which they 
 can be distinguished from any of her known teeth '. ' That in 
 all Rodentia except the Leporidse (hares and rabbits) a portion 
 of the enamel has a laminated arrangement of its fibres '; 
 ' that the enamel laminae have a different and distinctive 
 character in each of the larger groups, and that the variety 
 of structure is constant throughout the members of the 
 same group.' The outer and inner portions of the enamel 
 in all Rodents (except the Leporidse) show two distinct 
 areas, the portion near the dentine exhibiting a decussation 
 or crossing of the laminae, and the outer portion a disposition 
 in parallel lines. In the different species there is a great 
 variation in the width of these layers, in the angle of inclina- 
 tion of the prisms to the dentine surface, and in the pattern 
 formed by the decussation of the inner layers. 
 
 Castoridae. In the Beaver (Castor fiber) the decussation of the inner 
 layers is very distinct, as is also the change in the direction 
 of the prisms in the outer layer. In describing the 
 arrangement in the enamel of the Beaver, to avoid confusion 
 it may be explained that by a longitudinal section is meant 
 a section taken parallel to the long axis of the tooth, by 
 a transverse section one taken across the tooth at right 
 angles to the long axis, but of course in both instances the 
 
ENAMEL 
 
 109 
 
 enamel prisms are for the most part viewed longitudinally 
 from dentine to surface. Examining, in the first place, 
 a transverse section of Beaver's enamel, the separation into 
 two layers is very distinctly seen (fig. 59). The inner portion, 
 which extends not quite half-way across the width of the 
 enamel, is traversed by lines arranged at right angles to one 
 another and making an angle of 45 with the dentine surface. 
 The crossing prisms lie in different laminae, all those of one 
 lamina passing in the same direction and being crossed by 
 
 FIG. 59. Enamel of Beaver, d. Dentine ; a. inner layer of enamel 
 decussating layers ; 6. outer layers of enamel. ( X 225. ) 
 
 those of the superimposed lamina which lie at right angles 
 to them. If it were possible to obtain a sufficiently thin 
 -section exactly parallel to the layers, the prisms would be 
 seen in the single layer to be all passing in the same direction ; 
 but it is not possible to make such a section, as the layers are 
 not in the same longitudinal plane as the dentine but are 
 inclined from the perpendicular. 
 
 A little less than half-way across the enamel each indivi- 
 dual prism, whether passing from right or left, is bent 
 upwards, and they all lie parallel to one another to the 
 surface, their direction being at an angle of about 70 to 
 the dentine surface. At the point where they alter their 
 direction they show a slight sigmoid curve. 
 
110 MICROSCOPIC ANATOMY OF THE TEETH 
 
 In a longitudinal section (fig. 60) there is quite a different 
 appearance. The prisms are arranged at an angle of 60 
 to the dentine surface and are not seen to decussate, but 
 
 e 
 
 FIG. 60. Enamel of Beaver (Castor). Longitudinal section, d. Dentine 
 e. enamel, inner layer ; e'. outer layer. ( x250.) 
 
 A B 
 
 FIG. 61. A. Diagram to show the arrangement of the prisms of the 
 
 Beaver as viewed in a transverse section. E. enamel surface ; D. dentine 
 
 surface. 
 
 B. As viewed in a longitudinal section. 
 
 are parallel to one another, the intercrossing laminae being 
 viewed on edge. At a little less than half-way across the 
 enamel they abruptly change their direction to one at an 
 angle of 30 to the dentine surface. 
 
 The structure may be perhaps better explained by the accompanying 
 diagram, fig. 61 A and B, as it is difficult to clearly comprehend the 
 
ENAMEL 111 
 
 arrangement without some such device, but it may be much better 
 understood by a rough model. If some wooden matches are strung on 
 a wire or pin exactly through the middle of their length, and are then 
 alternately placed to form a series of rectangular crosses, two arms of 
 which are placed on a level surface, the appearance of the inner layer in 
 a transverse section is represented. To allow for the obliquity of the 
 prism the crosses must be inclined backwards from the base line at an 
 angle of 60. Another match attached to the upper ends of the arms 
 of the cross and sloped backwards at an angle of 70 to the base line 
 represents a prism of the outer layer. If this model is viewed sideways 
 the appearance in longitudinal section is at once explained. The cross 
 laminae leaning backwards at an angle of 60 are seen in the longitudinal 
 view as parallel lines at an angle of 60 to the base line, and the piece 
 representing the prism of the outer layer slopes at an angle of 30. The 
 cut ends of the crossing prisms which are directed obliquely forwards 
 are not very evident in the Beaver, but are better seen in some other 
 Rodents, as the Squirrel. 
 
 A transverse section of the enamel of the Squirrel (Sciurus) Sciuridae. 
 shows a similar rectangular crossing of the laminae near the 
 dentine, but this inner layer extends across two -thirds of 
 the width of the enamel and the crossing laminae are at 
 right angles, or upright to the dentine surface, and when 
 viewed in longitudinal section the laminae are seen to be 
 arranged at right angles to the dentine surface, not inclined 
 at an angle of 60 as in the Beaver. 
 
 In the Squirrel, as previously stated, the cut ends of the 
 crossing laminae can be distinctly seen in many longitudinal 
 sections. 
 
 The prisms of the outer layer in Sciurus are bent at an 
 angle of 45 from the direction of the crossing laminae, 
 and are arranged in a single layer almost at right angles 
 to the enamel surface, but are very slightly curved, the con- 
 vexity of the curve being directed towards the cutting edge 
 of the tooth. In the Squirrel there is a very thin layer of 
 enamel, its width being according to a measurement by Sir 
 John Tomes only ^-gtli of an inch, and he also speaks of 
 the colour as confined to the outer third ; but in a specimen 
 from which the photograph, fig. 62, was taken the deep 
 orange colour extends more than half-way across, gradually 
 fading, however, towards the dentine. 
 
 There are very slight differences in the arrangement of the 
 prisms in other members of the family, but, as the author 
 
112 MICROSCOPIC ANATOMY OF THE TEETH 
 
 above named says, the enamel of Pteromys (flying squirrel) 
 and Spermophilus (the American Souslik) may be recog- 
 nized at first sight as belonging to the Sciuridse. 
 
 Hystri- In the Porcupine (Hystrix), although the portion of enamel 
 near the dentine is laminated as in other Rodents, the 
 structure is much complicated by the flexuous course of the 
 prisms. The laminae which leave the dentine at an angle of 
 80 become confluent, and, as Sir John Tomes says, the 
 enamel prisms appear as if thrown into waves, ' they pursue 
 
 FIG. 62. Enamel of Squirrel (Sciurus). Transverse section. 
 c. Enamel ; d. dentine. ( x 250. ) 
 
 a serpentine course and in the lamelliform portion describe 
 three tolerably uniform curves ', but lie parallel to one another 
 in the outer portion of the enamel as in other Rodents. 
 
 The enamel of the Capybara (Hydrochoerus) is very 
 similar in structure to that of the Porcupine, but the 
 lamellae of the inner layer are more confluent and more 
 parallel. The enamel near the dentine is crowded with 
 small rounded spaces. 
 
 The enamel of the Marmots (Arctomys) differs from that 
 of the Sciuridse, but is not wholly different in type. The 
 prisms in the first part of their course are seen in longitudinal 
 sections to lie in parallel layers at right angles to the dentine, 
 
ENAMEL 
 
 113 
 
 as in the squirrel, and extend across the inner two-fifths 
 of the enamel. In transverse section they form a diamond 
 pattern and become parallel and waved in the outer portion, 
 but in the outer layer there is also some intercrossing 
 faintly to be seen and an appearance of diamond-shaped 
 spaces. 
 
 In the Rats and Mice (Muridaa) there is also a decussation 
 of the laminae on the inner half of the enamel. The prisms 
 are, as seen in longitudinal section, at an angle of 55 to 
 
 a 
 
 d 
 
 FIG. 63. Enamel of Rat. s. Papillae and vessels in the enamel organ ; 
 a. ameloblasts ; e. enamel ; d. dentine, (x 250.) 
 
 the dentine surface and form a diamond-shaped pattern 
 in this area. They are slightly curved (fig. 63), the curvature 
 being most evident near the dentine, and in the outer part 
 of the enamel they proceed in a single layer in straight lines 
 at an angle of 25 with the surface of the dentine. Sir John 
 Tomes speaks of the lamellae as being serrated, andC. S. Tomes 
 says, ' The borders of the individual prisms are slightly 
 serrated, the serrations of contiguous rods interlocking ' (fig. 
 64). An examination of thin sections of rat enamel cut by 
 the Weil process suggests, however, that these projections are 
 not a portion of the prisms but of an interprismatic calcified 
 material which separates the prisms from one another, as the 
 
 MUMMERY j 
 
114 MICROSCOPIC ANATOMY OF THE TEETH 
 
 processes appear to pass between the prisms where viewed in 
 transverse section and to project from them in places where 
 viewed in longitudinal section. In fig. 65, at a, a single 
 transversely-cut prism is seen with a narrow process on 
 either side of it which certainly suggests the presence of 
 some intervening substance and not a serration of the prism. 
 The markedly fibrous nature of rat's enamel was drawn 
 attention to by Leon Williams in his paper on enamel (21), 
 where he says, ' Enamel from the teeth of rats and mice 
 
 FIG. 64. Enamel of Rat. Interdigitation of transverse and longitudinal 
 prisms and interprismatic substance. ( x 700. ) 
 
 shows a more marked fibrous character than any other 
 that I have examined, and the most striking feature of 
 arrangement of the fibres is the interlacing or weaving 
 together of those running in different directions like warp 
 and woof in a web ' . He compares the appearance of the 
 broken edge of the enamel to that of a torn fabric. This 
 fibrous condition of the enamel is shown in fig. 67 from 
 a ground section accidentally torn in the middle, and it is 
 also very noticeable in the outer layer of enamel close to 
 the surface, where an interlacement of fine fibres is seen 
 mostly running in a direction parallel to the surface (fig. 66). 
 Leporidae. In the Hares and Rabbits (Leporidae) there is no division 
 of the enamel into an outer and an inner layer, and the 
 prisms are not arranged in alternating lamellae. Von Ebner 
 
ENAMEL 
 
 115 
 
 describes tubes in the enamel of the hare, which have no 
 communication with the dentinal tubes. He considers that 
 these enamel tubes were originally continuations of the 
 
 FIG. 65: Enamel of Rat. Transversely cut prisms showing projection 
 of the interprismatic substance on either side. ( x 800.) 
 
 FIG. 66. Enamel of Rat near outer margin. ( x 650. ) 
 
 dentinal tubes, but have been cut off from them by a process 
 of resorption in the early stages of development, and he 
 would explain the isolated enamel tubes found in the 
 marsupial Petaurus in the same manner. 
 
 i 2 
 
116 MICROSCOPIC ANATOMY OF THE TEETH 
 
 A rodent type of dentition is found in two animals widely 
 separated from the Rodents and from each other. In 
 Australia the marsupial Wombat has long scalprif orm teeth 
 growing from persistent pulps, and in Madagascar a similar 
 dentition is found in a Lemur, the Aye-aye (Cheiromys) . In 
 both of these countries indigenous true Rodents are un- 
 known. As C. S. Tomes says (18 c), 'In past times ... a large 
 number of animals, widely apart from one another in their 
 affinities, have more or less approximated to a rodent 
 
 FIG. 67. Enamel of Rat showing fibrous nature. ( x 450. ) 
 
 dentition, and as this adaptive resemblance would give to 
 them advantages in getting access to food which was denied 
 to animals with relatively blunt and weak front teeth, it is 
 possible that some of these may have left descendants, and 
 that hence the existing Rodents may have sprung from 
 more than one stock '. 
 
 Tubes from the dentine passing into the enamel are seen 
 in abundance in the teeth of the Rodent Jerboa (Dipus), 
 and Von Ebner says it is not difficult to find tubes in the 
 enamel of other Rodents, as mouse and guinea-pig, but these 
 are not continuous with the dentinal tubes. He considers 
 that a resorption of the first -formed dentine takes place 
 whereby the ends of the dentinal tubes are left open, and 
 
ENAMEL 117 
 
 that this resorption may be due to the action of the enamel 
 cells. This resorptive process, he thinks, has cut off the 
 enamel tubes from any connexion with the dentinal tubes. 
 In man, where only very short portions of the dentinal 
 tubes enter the enamel, isolated enamel tubes are not found. 
 The crevices in human enamel he considers only occur 
 through drying or shrinking of the interprismatic substance. 
 
 REFERENCES 
 
 1. Black, G. V., and McKay, F. S. ' Mottled Teeth.' Dental Cosmos, 
 
 Feb. 1916, Iviii. 129. 
 
 2. v. Bibra. Chemische Untersuchungen uber die Knochen und ZdJine des 
 
 Menschen und der Wirbelthiere, &c., 1884. 
 
 3. v. Beust. * A Contribution to the Study of Immunity in Dental 
 
 Caries.' Dental Cosmos, 1912, liv. 659. 
 
 4. v. Boas. ' Die Zahne der Scaroiden.' Zeits. f. wissensch. Zoologie, 
 
 1879, Bd. xxxii, pp. 189-216. 
 
 5. Caush, D. ' Is there Uncalcified Tissue in the Enamel?' Dental Cosmos, v 
 
 vol. xlvii, Feb. 1905. 
 
 6. v. Ebner, V. (a) ' Uber die histologischen Veranderungen des Zahn- " 
 
 schmelzes wahrend der Erhartung, insbesondere beim Menschen.' 
 Archiv f. micr. Anat., Bd. Ixvii, 1905. 
 
 (&) ' Strittige Fragen uber den Bau des Zahnschmelzes.' Sitzungsb. v 
 d. k. Akad., Wien, 1890, Bd. xcix, pp. 57-104. 
 
 7. Harting, P. Sur la production artificielle de quelques formations 
 
 calcaires organiques, Amsterdam, 1872 ; and Quart. Journ. Micr. 
 Sci., London, 1872, vol. xx, N.S., pp. 118-23. 
 
 8. Hopewell Smith. The Histology and Patho-histology of the Teeth, 1903. 
 
 9. Hoppe-Seyler, E. Handbuch der Physiologisch- und pathologisch- 
 
 chemischen Analyse &c., Berlin, 1893 (new ed.). 
 
 10. Lovatt Evans. Transactions, International Medical Congress, 1913 
 
 (Section of Stomatology). 
 
 11. Mummery, J. H. (a) ' On the Structure and Arrangement of the v 
 
 Prisms, especially as shown in the Enamel of the Elephant.' Proc. 
 Roy. Soc. Med., vol. ix, 1916, Odontol. Section, pp. 121-38. 
 (6) ' On the Nature of the Tubes in Marsupial Enamel, and its bearing \J 
 upon Enamel Development.' Phil. Trans. Roy. Soc., Ser. B, ccv. 
 295-313. 
 
 12. Owen, R. Odontography, pp. 404-6. 
 
 13. Paul, F. ' Some Points of Interest in Dental Histology ; the Enamel 
 
 Organ.' Dental Record, 1896, xvi. 493-517. 
 
 14. Pickerill, H. P. ' The Structure of Enamel.' Dental Cosmos, Oct. 1913. < 
 
 15. Romer, O. N erven im Zahnhe.in : zahrihistologische, Studie. Frei- 
 
 burg, 1899. 
 
118 MICROSCOPIC ANATOMY OF THE TEETH 
 
 16. Smreker, E. (a) ' tiber die Form der Schmelzprismen menschlicher 
 
 Zahne,' &c. Archivfur micr. Anat., 1905, Ixvi. 18-81. 
 (6) ' tiber die Darstellung der Kittsubstanz des Schmelzes menschlicher 
 Zahne.' Anat. Anzeig., Bd. xxii, 1903. 
 
 17. Tomes, J. (a) ' On the Structure of the Dental Tissues of Marsupial 
 
 Animals, and more especially of the Enamel.' Phil. Trans. Roy. 
 Soc., London, 1849, vol. cxxxix, pt. ii, pp. 403-13. 
 (6) ' On the Structure of the Dental Tissues of the Order Rodentia.' 
 Phil. Trans. Roy. Soc., London, 1850, pt. ii, pp. 529-69. 
 
 18. Tomes, C. S. (a) ' On the Chemical Composition of Enamel.' Journ. 
 
 Physiol, Camb., 1896, xix. 217-23. 
 (6) * On the Development of Marsupial and other Tubular Enamels, 
 
 with Notes upon the Development of Enamel in General.' Phil. 
 
 Trans. Roy. Soc., 1898, clxxxix. 107-22. 
 (c) Dental Anatomy, 7th ed. 
 
 (d) ' On the Structure and Development of the Enamel of Elasmobranch 
 
 . Fishes.' Phil. Trans. Roy. Soc., 1898, Ser. B, vol. cxc, pp. 443-61. 
 
 (e) t Upon the Development of the Enamel in certain Osseous Fish.' 
 
 Phil. Trans. Roy. Soc., 1900, Ser. B, vol. cxciii, p. 42. 
 (/) ' On the Minute Structure of the Teeth of Creodonts, with especial 
 
 reference to their suggested Resemblance to Marsupials.' Proc. 
 
 Zool. Soc. Lond., 1906, i. 45-58. 
 
 19. Underwood, A. Aids to Dental Anatomy and Physiology, 3rd ed., p. 63. 
 
 20. Walkhoff, O. (a) Die Erdsalze in ihrer Bedeutung fur die Zahncaries. 
 
 Berlin, 1913. 
 (6) ' Die vermeintliche Kittsubstanz des Schmelzes.' Anat. Anzeig., 
 
 Bd. xxiii, S. 199. 
 (c) ' Beitrag zur Lehre von der Struktur des Schmelzes.' Deutsche 
 
 Monatschr. f. Zahnheilkunde, xxi. Jahrg., Dezember 1903, S. 625. 
 
 21. Williams, Leon. ' On the Formation and Structure of Dental Enamel.' 
 
 Dental Cosmos, 1896, xxxviii. 101-269 and 453 ff. 
 
 22. Zsigmondy, O. ' tiber die Retziusschen Parallelstreifen im mensch- 
 
 lichen Schmelze.' Osterreich. Zeit.fiir Stomatol., Vienna, 1913. 
 
CHAPTER III 
 
 DEVELOPMENT AND CALCIFICATION OF THE 
 ENAMEL 
 
 1. The Enamel Organ of Mammalia 
 
 As explained in Chapter I, the enamel is developed from 
 the deeper layers of the epithelium of the mouth. It is thus 
 of ectodermic origin, and the several layers of cells into 
 which this involution becomes differentiated are collectively 
 spoken of as ' the enamel organ ' (fig. 68). 
 
 This enamel organ commences as a bud-like process, 
 which grows downwards into the sub -epithelial tissue, and 
 
 FIG. 68. Diagram showing the enamel organ and its connexions 
 before the commencement of calcification, e. Epithelium of mouth 
 m. Malpighian layer ; c. connective tissue ; n. neck of enamel organ 
 ex. external epithelium ; 5. stellate reticulum ; i. stratum intermedium 
 a. ameloblasts ; p. pulp ; b. bone ; t. commencing tooth-sac. 
 
 is formed by an invagination of the Malpighian layer of the 
 oral epithelium. 
 
 The rounded end of the bud becomes indented from beneath, 
 and the sides of this indentation grow round the mesodermic 
 connective tissue which afterwards forms the dentine papilla. 
 It thus comes to have a bell shape, the connecting band 
 with the epithelium of the mouth forming the handle. 
 
 The cells constituting the walls or sides of this inflection, 
 
120 MICROSCOPIC ANATOMY OF THE TEETH 
 
 at first in contact, become separated from one another by 
 a layer of stellate cells which fills the space between them. 
 This, ' the stellate reticulum ' eventually in Mammalia, 
 occupies the greater part of the area of the enamel organ 
 during the earlier stages of enamel development. 
 
 The sides of the bell, the section of which appears as 
 a crescent, continue to grow downwards, the horns of this 
 crescent further enclosing the dentine papilla. 
 
 The investment of the dentine papilla is considered not to 
 be produced by the upward growth of this papilla, but by 
 the down growth of the epithelial enamel organ around it. 
 Those cells which immediately invest the papilla between 
 the two horns of the crescent and form the inner layer of the 
 enamel organ become columnar in shape, and are known 
 as the ameloblasts or enamel-forming cells. 
 
 The continuation of this layer on the outer side of the 
 stellate reticulum forms the external epithelium of the 
 enamel organ which separates it from the connective tissue 
 of the capsule. These cells, however, do not become 
 columnar in shape, as do those forming the internal epithe- 
 lium. A layer of rounded cells also appears, intervening 
 between the internal epithelium or layer of ameloblasts 
 and the stellate reticulum (fig. 68 (a)). This layer, called 
 the stratum intermedium, is only seen in contact with the 
 internal epithelium, and does not pass beyond the horns of 
 the crescent. 
 
 The enamel organ is thus seen to be made up of four 
 distinct layers from without inwards : 
 
 The external epithelium. 
 
 The stellate reticulum. 
 
 The stratum intermedium. 
 
 The internal epithelium. 
 
 The External Epithelium (figs. 69 to 72). This layer, which 
 forms the periphery of the enamel organ, is made up of 
 polygonal nucleated cells, which in later stages are often 
 elongated, their longer axes lying parallel to the surface. 
 The cells of this layer merge into those of the stellate 
 reticulum. 
 
 It has been debated whether the cells of this layer have 
 any distinct function beyond that of limiting the boundary 
 
DEVELOPMENT OF THE ENAMEL 
 
 121 
 
 of the enamel organ. Its intimate relations, however, with 
 the blood-vessels of the capsule would suggest that it serves 
 a much more important purpose. Dr. Paul says its ' chief 
 function seems to be the separation from the blood of the 
 constituents of enamel which are elaborated by the internal 
 epithelium ' (17). It would seem highly probable that this is 
 the function of these cells, at all events in the early stages 
 of calcification. The budding of the cells of the external 
 
 FIG. 69. External epithelium of enamel organ of Macropus showing 
 elongated fusiform cells, e. External epithelium ; s. stellate reticulum. 
 (x350.) 
 
 epithelium, to be presently described, would also indicate 
 a more important function of this layer than is usually 
 ascribed to it. The blood-vessels in the follicle are in close 
 relation with the external epithelium of the enamel organ, 
 and in places it is seen to be discontinuous, and these blood- 
 vessels are distinctly in contact with the cells of the stellate 
 reticulum (fig. 70). 
 
 In marsupials they have been seen to penetrate the ex- 
 ternal epithelium in many instances and to enter the stellate 
 reticulum. In the Wallaby (Macropus billiardieri) they have 
 been shown to do so by Marett Tims and Hopewell Smith 
 
122 MICROSCOPIC ANATOMY OF THE TEETH 
 
 (24), and Professor Poulton long ago described blood-vessels 
 in the stellate reticulum of the rat (18). 
 
 In fig. 71, from a preparation of the author's of Macropus 
 rufus, blood-vessels are seen crossing the external epithelium 
 and also lying within the stellate reticulum, an appearance 
 seen in many sections, and a photograph sent to him by 
 Mr. Law shows numerous blood-vessels in this layer. There 
 is little doubt that many preparations, said to show blood- 
 vessels in the stellate reticulum, really showed them, not 
 
 **- v 
 
 FIG. 70. The external epithelium in a human tooth germ showing 
 breaks of continuity and blood-vessels in contact with the stellate reticulum 
 (photographed from a preparation by Mr. W. James), e. External epithe- 
 lium ; 6. blood-vessels ; s. stellate reticulum. (x!50.)- 
 
 in this layer, but in the connective tissue outside the enamel 
 organ, which, when the stellate reticulum has disappeared, 
 lies in contact with the stratum intermedium, and often 
 very strongly resembles it in appearance. There is, however, 
 no longer any doubt that vessels are found, at all events in 
 marsupials, within the enamel organ. 
 
 Mr. Thornton Carter (5) has lately described blood-vessels 
 deeply within the stellate reticulum. 
 
 It was long ago shown by Malassez and by Robin and 
 Magitot, that the external epithelium of the enamel organ 
 
DEVELOPMENT OF THE ENAMEL 
 
 123 
 
 in human tooth germs is in many places not continuous, 
 but shows numerous indentations and interruptions, and 
 the blood-vessels of the capsule are in actual contact with 
 the cells of the stellate reticulum (see fig. 70). They also 
 showed that epithelial buds arise from the outer surface of 
 the external epithelium and proliferate within the follicle, 
 as shown in fig. 72 (photographed by the author from 
 a preparation by Warwick James). 
 Malassez says, ' This ' (the external layer of the enamel 
 
 
 FIG. 71. Blood-vessels penetrating the external epithelium and entering 
 the stellate reticulum in Macropus rufus. ( x 125.) 
 
 organ) ' does not form everywhere and always a continuous 
 investment, as is generally thought. One sees in it, in certain 
 regions, numerous lacunae or windows which give it the 
 appearance of an epithelial network.' 
 
 The papillary projections into the external layer of the 
 enamel organ, first described by Herrissant (14), were sup- 
 posed by him to be charged with the function of secreting 
 the enamel. They no doubt represent these vascular pro- 
 longations described by Malassez, and probably serve to 
 convey a blood supply to the stellate reticulum, but there 
 is no evidence that they have any directing influence on 
 the enamel prisms as some authors have imagined. 
 
124 MICROSCOPIC ANATOMY OF THE TEETH 
 
 This penetration by blood-vessels may, perhaps, be looked 
 upon as a vestigial survival of the condition described by 
 the author in certain families of osseous fish in which the 
 enamel organ is permeated by blood-vessels or sinuses 
 enclosed in a delicate sheath, forming a" regular layer around 
 the forming enamel, to which further reference will be made 
 in describing the development of the enamel in fish. 
 
 The Stellate Reticulum (%s. 73 and 74). There has been 
 some controversy and considerable difference of opinion as 
 
 e. 
 
 FIG. 72. Budding of the external epithelium (human tooth), s. Stellate, 
 reticulum ; e. external epithelium ; &. blood-vessels. (Photograph from a, 
 preparation of Mr. W. James.) (x 150.) 
 
 to the structure and function of this layer. A great part of 
 this uncertainty has been caused, as was first pointed out 
 by Leon Williams, by the unsuitable methods of preparation, 
 and the bare skeleton of the stellate reticulum only has been 
 seen. 
 
 The shrinkage caused by the use of alcohol in both fixing 
 and dehydration, and the further distortion caused by clear- 
 ing oils and Canada balsam, are fatal to such a delicate 
 tissue as the stellate reticulum, as they are indeed to many 
 others of a similar nature, and while these routine methods 
 are perfectly applicable to many preparations, they should 
 not be adopted in the investigation of tissues of this nature., 
 
DEVELOPMENT OF THE ENAMEL 
 
 125 
 
 A specimen fixed in formol and cut in gum on the 
 freezing microtome will, when mounted in glycerine or 
 Farrant solution, give quite a different picture, and one that 
 with little doubt gives a much better indication of the real 
 structure. 
 
 Such a preparation of the stellate reticulum (fig. 74) 
 of a mammalian enamel organ shows that it is made 
 up of large cells, showing under high powers of the micro- 
 scope a delicate reticulated structure with oval or round, 
 
 FIG. 73. Stellate reticulum. Macropus. (x600.) 
 
 very conspicuous nuclei, which in well-stained preparations 
 also show one or more nucleoli. These cells are connected 
 by Abroad processes with one another and by finer thread-like 
 processes, giving to the whole structure its characteristic 
 stellate appearance. The intervals between the cells are 
 filled with a gelatinous or colloidal material which shows 
 no visible structure, but takes a very faint stain, showing 
 that these spaces are not empty, even in the microscopical 
 preparation. Leon Williams considers that the stellate 
 structure is simply the intercellular substance which is left 
 after the removal of the cell contents, and that the stellate 
 appearance is due to the persistence of the slightly modified 
 cell wall. It would appear advisable, however, to confine 
 
126 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the term intercellular substance to the jelly-like substance 
 above referred to, and not to apply it to these extensions 
 of the cell wall. It appears very probable that this altered 
 tissue of the outer part of the cell may by its persistence 
 give rise to the stellate appearance in balsam mounted 
 sections. In such preparations the intercellular substance, 
 as well as the cells, has disappeared. In sections mounted 
 without the use of alcohol the cells themselves are seen to 
 have a stellate form with broad connecting processes or 
 bridges, the outer layer of these cells extend'ng into other 
 
 FIG. 74. Stellate reticulum. Macropus. (x 1,000.) 
 
 long, narrow processes which communicate with one another 
 over the whole area of the tissue and enclose spaces con- 
 taining an apparently homogeneous material. 
 
 It is apparently the network formed by these connecting 
 processes which forms the web-like tissue seen in the balsam 
 preparations, and which remains after the more delicate 
 tissue of which they form the boundary has fallen away. 
 
 The appearances would suggest that this outer layer and 
 its processes have undergone some change which makes 
 them more resistant than the rest of the cell, and they 
 therefore persist after the removal of the body of the cell 
 with its nucleus. 
 
DEVELOPMENT OF THE ENAMEL 127 
 
 According to Rose (21), the formation of the cells of the 
 stellate reticulum commences in the centre of the enamel 
 organ. The cells of the rete mucosum of the oral epithelium, 
 closely packed under the enamel epithelium, increase in 
 size in this situation, the nuclei becoming larger and separat- 
 ing as the cells grow larger. He says : ' Between these cells 
 a clear protoplasmic fluid is secreted, causing the appearance 
 of small intercellular spaces. These spaces increase and 
 press against the neighbouring cells to such an extent that 
 the cells are finally connected with one another only by 
 means of narrow protoplasmic bridges.' He considers that 
 they * form a wide-mouthed canal system, containing a thin 
 fluid protoplasm, apparently highly suitable for a rapid 
 circulation of nutritive fluids '. 
 
 Messrs. Underwood and Wellings (28 b) consider that ' the 
 cells (where no alcohol has been used) are round or oval, 
 with round or oval nuclei, the latter occupying most of the 
 cell. The processes which connect these cells with their 
 neighbours are wavy and not straight, and the cell envelope 
 is not drawn out where the processes are given off.' They 
 consider that the stellate appearance is artificially produced. 
 As shown, however, in fig. 74, the cells themselves are mostly 
 stellate rather than round or oval, and in some there is 
 a considerable area of cell substance around the nuclei, and 
 one would be inclined to consider that the stellate appear- 
 ance is decidedly an indication of real structure and not 
 artificially produced, although the distortion so commonly 
 apparent in preparations and in published drawings and 
 photographs might suggest this. 
 
 With regard to the functions of the stellate reticulum, there 
 is also very considerable difference of opinion. C. S. Tomes 
 says it has been supposed to have no more important 
 function than to fill up the space subsequently taken up by 
 the growing tooth, and quotes Dr. Paul in support of 
 this view. 
 
 As Rose points out, in the rapidly forming and speedily 
 worn out teeth of lower vertebrata there is no stellate 
 reticulum, the enamel organ being of very simple structure ; 
 but in higher animals this tissue is present, the teeth are 
 developed more slowly, and as the epithelial enamel organ 
 
128 MICROSCOPIC ANATOMY OP THE TEETH 
 
 i 
 
 contains no blood-vessels, nutrition must take place by 
 osmosis, and a special arrangement is necessary to carry 
 nutriment to the growing epithelial cells. He states also 
 that the development of the stellate reticulum is intimately 
 associated with the deposition of a thick enamel cap in the 
 higher vertebrata. 
 
 It certainly seems more reasonable to suppose that a tissue 
 consisting of such fully developed cells should take part in 
 the process of calcification by acting as a storehouse of 
 material and serving as a channel for the conveyance of 
 substances to the more active cells, than that its function 
 should be merely a mechanical one. 
 
 C. S. Tomes is inclined to look upon the stellate reticulum 
 as a tissue which has undergone colloid degeneration, such 
 as is seen in epithelial tumours. This view was supported 
 by Eve in a paper read before the Odontological Society of 
 Great Britain in 1885 (9). In this condition the cells often 
 become enlarged and the nuclei pressed against the cell 
 wall ; but this last appearance does not occur in satisfactory 
 preparations of the enamel organ, although it may possibly 
 be seen in shrunken preparations or in later stages of develop- 
 ment, when the stellate reticulum is disappearing and 
 degenerating, and Rose's view of its functions would appear 
 much more consistent with those of a vital, developing 
 tissue. 
 
 The Stratum Intermedium (fig. 75, s). The cells of this 
 layer communicate by their processes with those of the 
 stellate reticulum. Their nuclei are somewhat smaller than 
 those of the cells of the stellate reticulum, and when cal- 
 cification has commenced the cells and their nuclei are 
 generally seen to be slightly elongated in a direction parallel 
 with the surface of the internal epithelium, but many cells 
 retain a rounded or polygonal form. 
 
 Before the commencement of calcification, many cells of 
 the stratum intermedium are seen lying close to and often 
 between the distal ends of the ameloblasts ; but after calcifica- 
 tion has commenced they are completely shut off from these 
 cells by a membrane, the outer ameloblastic membrane of 
 Williams. 
 
 After the stellate reticulum has disappeared in the later 
 
DEVELOPMENT OF THE ENAMEL 129 
 
 stages of enamel formation, the external epithelium lies in 
 close contact with the stratum intermedium, and the blood- 
 vessels of the capsule are in close proximity to its cells. It 
 is contended by Leon Williams (30) that in the rat the 
 blood-vessels enter the stratum intermedium, and that their 
 loops dip into the spaces between the cells, from which they 
 can be pulled away, the vessels then projecting as long 
 processes, and he claims for the stratum intermedium the 
 functions of a true secreting organ in this animal. 
 
 FIG. 75. The enamel organ of Macropus at the commencement of 
 calcification, st. Stellate reticulum ; s. stratum intermedium ; a. amelo- 
 blasts; od. odontogenic zone ; o. odontoblasts. On the left calcification 
 of the dentine has just commenced and the outer ameloblastic membrane 
 appears, but is not seen on the right side where there is no calcification. 
 x400.) 
 
 Certainly the photographs accompanying his description 
 give every evidence of such an arrangement, and the con- 
 ditions found in certain fish in which blood-vessels or sinuses 
 form a conspicuous part of the enamel organ, as shown by 
 the author in a recent paper, would tend to confirm this 
 observation. Fig. 76, from a photograph of a preparation 
 by the author, shows these papillae and loops in the enamel 
 organ of the rat. 
 
 The Internal Epithelium. Ameloblast or Adamantoblast 
 
130 MICROSCOPIC ANATOMY OF THE TEETH 
 
 layer (fig. 77, a, and Plate IV at a). This, the most constant 
 and most important structure of the enamel organ, forms its 
 innermost layer, and before calcification has commenced lies 
 in immediate contact with the dentine papilla. The cells 
 of this layer are columnar in shape, with very distinct oval 
 nuclei, which lie for the most part at the distal extremity of 
 the cell, the end farthest removed from the point where the 
 earliest deposit of calcific matter takes place. Here and 
 there, however nuclei are to be seen near the proximal ends 
 
 FIG. 76. Enamel of Rat. s. Papillae and vessels in the enamel organ ; 
 a. ameloblasts ; e. enamel; d. dentine. ( x 250.) 
 
 of the cells, and in late stages of calcification in marsupials 
 very many nuclei occupy this position, and are crescentic 
 in shape, as first pointed out by C. S. Tomes (26), who also 
 described the horns of the crescent as looking as if they 
 were continued into filamentous processes. Many prepara- 
 tions of the author's seem to show that there can be little 
 doubt that these processes are connected with the nuclei 
 (fig. 85). 
 
 The nuclei of the ameloblasts have very distinct nucleoli, 
 and show a fine nuclear network in suitably stained pre- 
 parations, and the body of the cell is occupied by very 
 delicate strands of protoplasmic fibres arranged chiefly in 
 
PLATE IV. 
 
 Tooth germ of Kitten. A drawing by the author from a preparation by 
 Prof. Milroy. x 1000. 
 
 S. Stratum intermedium ; a. ameloblasts ; e. enamel ; / Tomes' processes ; 
 m v outer ameloblastic membrane ; w 2 . inner ameloblastic membrane. 
 
 To face p. 130. 
 
, : . ;, <$ 
 
DEVELOPMENT OF THE ENAMEL 
 
 131 
 
 a longitudinal direction (see Plate IV). The cells are generally 
 considered to be in close contact with one another, their 
 transverse section having an hexagonal form from mutual ap- 
 position, but Messrs. Underwood and Wellings (28 6) describe 
 intervals between them which they say appear to correspond 
 in diameter to the intercolumriar spaces in the formed enamel. 
 The proximal ends of the cells are square in shape before 
 the commencement of calcification and form a more or less 
 straight line where they are in contact with the tissue of the 
 
 
 i^y^^fjif 
 ^^m^^^ 
 
 a 
 
 o 
 
 P 
 
 FIG. 77. Macropus. Before the commencement of calcification. The 
 cells of the stratum intermedium (s) are seen to be blended with those of the 
 internal epithelium (a), r. Stellate reticulum ; s. stratum intermedium ; 
 a. amelo blasts ; o. odontoblasts ; p. pulp. (x!50.) 
 
 dentine pulp. When calcification has commenced each amelo- 
 blast is seen to be provided with a process which extends to 
 the surface of the forming enamel. This, the Tomes' process 
 of the ameloblast, is often seen very much pulled out and 
 elongated, especially in balsam mounted specimens, and 
 also has a tapered form in such preparations. 
 
 In specimens, however, which have not been decalcified 
 or dehydrated, these processes are very little narrower than 
 the cell itself. They are very distinct in marsupials, and 
 their fine fibrillar structure is well shown, this being a 
 
 K 2 
 
132 MICROSCOPIC ANATOMY OF THE TEETH 
 
 continuation of the same material seen in the ameloblast cell 
 itself. The existence of these processes has been doubted 
 by some authorities, who look upon it as an artificial product 
 caused by the dragging or pulling away of the forming 
 enamel from the ameloblast cell ; but a study of marsupial 
 enamel organs where they are continued into fibres which 
 pass across the whole area of the forming enamel, renders 
 it impossible to doubt their existence as a definite and 
 important part of the cell, as will be better understood 
 after the development of the enamel in marsupials has been 
 considered (see p. 153). 
 
 inner and In developing enamel, membranes have been described 
 amelo between the ameloblast cells and the forming enamel, and 
 blastic also between these cells and those of the stratum inter- 
 medium. 
 
 The question of the existence of membranes in these 
 situations has been the subject of much controversy. Pro- 
 fessor Huxley held the view that there was a ' membrana 
 preformativa ' between the ameloblasts and the forming 
 enamel, arid that the calcification of enamel took place 
 by the transference of crystallizable products through a 
 membrane. 
 
 This theory has been revived by Leon Williams and others. 
 Williams (30), in his study of enamel, is convinced of the 
 existence of these membranes, and has called them the outer 
 and inner ameloblastie membranes. The outer membrane 
 intervenes between the stratum intermedium and the 
 ameloblasts, and the inner ameloblastie membrane separates 
 the ameloblasts from the forming enamel, but is pierced by 
 the Tomes' processes. An examination of many sections 
 of developing teeth prepared in different ways will, we think, 
 clearly show that such membranes really exist, or certainly 
 a substance which retains its continuity when the cells have 
 fallen away from it, perhaps the thickened cell walls united 
 at their margins., Fig. 78 shows the outer ameloblastie 
 membrane in the enamel organ of Macropus, where the 
 ameloblasts have become detached from its under side and 
 it still persists as a distinct membrane, and Leon Williams 
 has published a photograph in the illustrations to his 
 paper which shows the inner ameloblastie membrane 
 
DEVELOPMENT OF THE ENAMEL 133 
 
 attached to the ameloblasts, and torn away from its con- 
 nexion with the forming enamel. Both the outer and inner 
 ameloblastic membranes are well shown on Plate IV. 
 
 Tomes is inclined to look upon these membranes as 
 non-existent, or as artificial products, but in sections which 
 have not been treated with either alcohol or acids they are 
 easily to be seen. 
 
 The membranes appear to be dependent upon the calcify- 
 ing process, for before its commencement they are not to 
 
 m 
 
 f* 
 
 FIG. 78. Macropus. Outer ameloblastic membrane (m) where amelo- 
 blasts have been detached. ( x 1,000. ) 
 
 be seen, and the cells of the stratum intermedium are 
 mingled with the ameloblasts (figs. 75 and 77), although 
 afterwards entirely shut off from them by the outer 
 ameloblastic membrane. 
 
 Processes from the cells of the stratum intermedium 
 apparently penetrate this membrane and communicate with 
 the ameloblasts. 
 
 In criticizing Leon Williams's statements, C. S. Tomes says 
 that little reliance can be placed on differential staining as 
 evidence of the existence of the membranes. No doubt 
 many conditions affect these staining results, as the previous 
 
134 MICROSCOPIC ANATOMY OF THE TEETH 
 
 fixation of the tissue, the length of time during which the 
 preparation is exposed to the staining reaction, and the 
 composition and concentration of the stain, while many com- 
 pound stains are unreliable. These membranes are, however, 
 clearly visible in unstained preparations of marsupial enamel 
 organs and in those treated with hsematoxylin, and require 
 no differential stain to make them apparent. 
 
 2. Calcification 
 
 With the exception of horny teeth which are hardened by 
 the cornification of the stratum corneum of the epithelium, 
 all teeth are calcified, or impregnated with lime salts. The 
 organic matter in which they are first laid down is per- 
 meated with the salts in different degrees, producing 
 structures of sufficient hardness to resist the severe strain 
 to which they are subjected, the calcification of enamel 
 being so complete that its original structure is considerably 
 veiled or lost, and it is the densest and hardest of all the 
 animal tissues. 
 
 The formation of enamel and the mode of deposition of 
 the lime salts have long been among the most difficult 
 problems in the histology of the teeth. 
 
 Many contradictory theories have been held on the 
 subject, and while much fresh light has been thrown upon 
 it in late years by the investigations of Rose, Von Ebner, 
 Leon Williams, and others, there, still remain many diffi- 
 culties to be overcome in arriving at a clear understanding 
 with regard to it. 
 
 Owing to the complicated courses of the enamel prisms, 
 the difficulty of obtaining sections in which the calcified 
 portion of the tissue is in normal relations with the formative 
 cells, and the great care necessary to avoid distortion of the 
 preparations and the consequent deceptive appearances 
 produced, there are few tissues that present more difficulties 
 to the investigator. 
 
 The subject of the calcification of enamel is so intimately 
 bound up with certain chemical and physical phenomena, 
 that a brief statement of these is necessary before proceed- 
 ing to a more detailed consideration of the subject, that it 
 may be made clear, at all events in some degree, what part 
 
DEVELOPMENT OF THE ENAMEL 135 
 
 these physical processes take in the phenomena under con- 
 sideration. 
 
 The author undertook some experiments with solutions 
 of lime salts in inorganic solutions, to ascertain, if possible, 
 to what extent the deposition of these bodies in the forms 
 found in calcifying tissues was produced by purely physical 
 agencies, and how far the phenomena of osmosis were con- 
 cerned in bringing this about. 
 
 Professor Leduc's experiments on the production of 
 osmotic growths in chemical solutions suggested the examina- 
 tion of the production of similar growths under the micro- 
 scope. 
 
 When a solution of silicate of potash in tap water (con- Osmotic 
 taining lime salts) was employed, into which a few particles juries 
 of copper sulphate were dropped, the phenomena were 
 easily followed under a low power of the microscope. When 
 the copper salt was dropped into the solution, the smaller 
 particles immediately gave rise to a number of small con- 
 voluted tubes, but in the larger pieces the process was less 
 rapid and more easily followed. A globular projection 
 appears at the margin of the little mass of crystals ; this is 
 still further protruded by a series of small explosive impulses, 
 and eventually the limiting membrane formed between the 
 dissolving crystalloid and the surrounding colloidal medium 
 is pushed forward in the form of a steadily growing tube. 
 These tubes can be watched as they lapidly pass across the 
 field of the microscope, some terminating in pointed ex- 
 tremities, others reaching the margin of the microscope 
 slide. The growth of these tubes is due to the rapid passing 
 of water through the membranous wall from the surround- 
 ing solution by osmosis, the distension caused by the water 
 pushing the tube forward, the resistance of the membrane 
 at the same time preventing the lateral bulging of the tube. 
 
 These membranes were first described by Traube (27), Traube's 
 who called them ' membranes of precipitation '. The most ^nesoi 
 interesting point, however, to notice in this experiment is pretipita 
 that in many places, as the tube nears the end of its growth, 
 a line is seen to be drawn across it, dividing it with a distinct 
 septum. It is difficult to explain what determines the pro- 
 duction of these transverse membranes. It can hardly be 
 
136 MICROSCOPIC ANATOMY OF THE TEETH 
 
 due to fracture, as has been suggested, for the course of the 
 tube is not altered, as one would expect to find if this were 
 the case. 
 
 That this membrane completely closes the tube is shown 
 by the fact that small crystalline particles of the copper 
 salt accumulate upon its upper or proximal surface, the 
 lower part of the tube remaining clear. 
 
 FIG. 79. Osmotic tube formed with copper sulphate in silicate of 
 potash. Osmotic membrane showing crystalline particles above and 
 calcospherites below. (x!5.) 
 
 After some hours, however, spheroidal bodies appear in 
 the clear part of the tube below the membrane, many being 
 in close contact with it (fig. 79). These bodies have all the 
 appearances of calcospherites of the radial type. They are 
 probably composed of silicate of lime, the lime salts being 
 derived from the tap water in which the silicate is dissolved. 1 
 
 1 These bodies below the membrane form also when distilled water is 
 used to dissolve the silicate, but, as Dr. Lovatt Evans says, this would 
 
DEVELOPMENT OF THE ENAMEL 137 
 
 The interesting point is that true calcospherites are 
 formed by dialysis through membranes which are formed 
 in inorganic solutions by purely physical processes. 
 
 Graham (10), as the result of his experiments on the 
 diffusion of liquids through membranes, divided substances 
 into crystalloids and colloids crystalloids having the power 
 when in solution of passing easily through membranes ; 
 colloids, on the other hand, passing through with difficulty, 
 or not at all. The particles of a colloid solution are held 
 together by a very feeble force ; they consist of very fine 
 particles in a state of suspension, rather than solution in 
 the solvent. The exact line of demarcation, however, 
 between crystalloids and colloids is not always very pro- 
 nounced, and according to Krause there is a steady transition 
 from the crystalloids to the colloids. 
 
 The living body is built up of substances essentially 
 colloidal in character. These substances may exist either 
 in the solid condition, when they are called ' gels ', or in the 
 fluid state, when they are spoken of as ' sols '. 
 
 If the colloids throughout the body are brought into the 
 condition of irreversible ' gels ', life ceases, but there are 
 conditions in which the transition of ' sols ' into ' gels ' 
 does not lead to death, but to the formation of the important 
 structures before referred to, the membranes of precipitation. 
 This partial gelation of the colloids takes place when two 
 different colloids, or a colloid and a crystalloid, come into 
 contact. 
 
 These membranes of precipitation of Traube (27) are 
 generally considered to be permeable to water only, 
 but, as has been evidenced in the organic tissues of plants, 
 salts are capable of passing through them, and Loeb l is of 
 opinion that cell walls are not impermeable to salts, but 
 there is only a difference in the rate of diffusion, many 
 salts diffusing only very slowly into the protoplasm. 
 Leduc (15) says : ' Osmotic membranes were formerly 
 called semi-permeable membranes, being regarded as 
 membranes which allow water to pass through them but 
 
 not be a proof of the absence of lime, as calcium salts are present in ordinary 
 specimens of sodium silicate. 
 
 1 Dynamics of Living Matter, 1906. 
 
138 MICROSCOPIC ANATOMY OF THE TEETH 
 
 arrest the passage of the solute. This definition is inexact, 
 since no membrane permeable to water is absolutely im- 
 permeable to the solutes. All we can say is that certain 
 membranes are more permeable to water than to the sub- 
 stances in solution, and are, moreover, very unequally 
 permeable to the substances in solution. The term " osmotic 
 membrane " should therefore in all cases replace that of 
 "semi -permeable membrane".' 
 
 In the experiment with the copper sulphate above 
 described, it was seen that although the crystals accumulated 
 on the proximal side of the membrane very rapidly, the part 
 of the tube on the distal side remained clear for several 
 hours, when the slow diffusion of the salts through the 
 membrane caused the production of the calcospherites 
 which appeared in this situation, the lime salts contained 
 in the water which had permeated the tube wall diffusing 
 through the septum. We thus see that under purely physical 
 conditions we have a dialysis of salts of lime through these 
 inorganic membranes, and this observation points to the 
 suggestion that the same thing may occur from physical 
 causes in the calcification of the various hard tissues of man 
 and animals. 
 
 How far the membranes found in the living body are the 
 result of purely physical agencies, and to what degree they 
 are affected by physiological and vital conditions, we are 
 unable to determine, but it is seen that very firm membranes 
 do arise in the absence of all organic substances, although, as 
 Professor Philip says (19), ' a purely physical theory of the 
 exchanges which take place across a living membrane is 
 inadequate ; there is a physiological as well as a physical 
 permeability ' . 
 
 The bearing of these observations will be better under- 
 stood after the calcification of enamel and dentine has been 
 considered. 
 
 In the year 1858 Rainey published his work on molecular 
 coalescence (22), showing that lime salts were deposited in 
 colloidal solutions not in a crystalline but in a globular 
 form, the globules having a very definite structure and 
 arrangement. He also showed that in many animal 
 organisms, as Mollusca and Crustacea, similar forms appeared 
 
DEVELOPMENT OF THE ENAMEL 139 
 
 during the calcification of the shell. The first experiments 
 were made with a solution of carbonate of potash in gum 
 (which contains salts of lime). The bodies so produced were 
 globular in form, showing concentric lamination and radial 
 striae. Professor Harting of Utrecht (12) independently 
 carried out a similar research, and succeeded in producing 
 a great variety of these spherical bodies in various sub- 
 stances, his most instructive experiment being one in which 
 small fragments of calcium chloride and of sodium carbonate 
 were placed on the opposite sides of a flat dish containing 
 egg albumin. The dish was left perfectly still and undis- 
 turbed for two or three weeks ; the salts diffusing through 
 the albumin from the opposite sides formed a crust upon 
 the surface. This crust was found to consist of a deposit of 
 carbonate of lime in the globular form, showing a great 
 variety of spheres and disks of varying size, and very similar 
 to those produced in gum in Rainey's experiments. He 
 found that they retained their form after they had been 
 subjected to the action of an acid, some portion of the lime 
 having become so intimately blended with the albumin 
 that it remained in combination with it. To this substance, 
 left after decalcification, Harting gave the name of ' calco- 
 globulin ', distinguishing the calcified globular bodies by 
 the name of ' calcospherites '. If a microscope slide be 
 prepared with gum containing carbonate of potash or soda, 
 and examined under the microscope, minute granules appear 
 after a short time, which increase in size by coalescence 
 until small globular bodies are formed the process of 
 molecular coalescence described by Rainey. 
 
 In this experiment the formation of the particles is con- 
 sidered to be due to surface tension ; by this agency ' the 
 first ultramicroscopic particles are brought together ' , and 
 these further coalesce into visible particles, and finally into 
 the large calcospherites. 
 
 It is surface tension that maintains the form of a drop of 
 fluid, and in considering the chemistry of colloids Ostwald 
 has shown that when a substance begins to separate out 
 from a solution it always makes its appearance first as 
 a liquid. In colloidal suspensions the material appears first 
 in a condition of solution, and as it passes from this to the 
 
140 MICROSCOPIC ANATOMY OF THE TEETH 
 
 condition of suspension, the rounded contour of the liquid 
 form produced by surface tension is maintained, while 
 a process of crystallization tends to take place throughout 
 this little liquid globule. This is the explanation which has 
 been given of the first determination of the rounded forms 
 which occur in colloidal substances, and there are many 
 phenomena which point to the correctness of this view. 
 
 The more recently investigated phenomena of adsorption 
 which are intimately associated with surface tension no 
 doubt take an important part in the processes of calcifica- 
 tion in the living body. 
 
 These phenomena depend upon the power of the adsorbed 
 substance to lower the surface tension, and itself accumu- 
 lating at the surface it leads to the formation of membranes, 
 and bears an intimate relation to the phenomena of osmosis. 1 
 
 Both Rainey and Harting found that when calcium 
 phosphate was present in the solutions in excess of the 
 carbonate, globular bodies were not formed, but the deposit 
 was crystalline, but if there was only a small proportion of 
 phosphate to carbonate, larger and more perfect spheres 
 were produced than with carbonate alone. This observation 
 has a most important bearing on the subject of the calcifica- 
 tion of the hard tissues of the teeth, as in completed dentine 
 and enamel the phosphates are largely in excess ; but this 
 question will be further considered in discussing the calcifica- 
 tion of enamel. 
 
 In the works of Harting and Rainey a full and minute 
 description is given of the various forms in which the calcific 
 matter is deposited. 
 
 Two main forms of calcospherites may, however, be con- 
 veniently distinguished those in which radial striae are 
 most evident and those especially marked by concentric 
 lamellae or rings, although there are many which show both 
 structures. In developing enamel it will be shown that the 
 deposited spherites belong to the radial system, while in 
 dentine they are of the concentric form (fig. 80). 
 
 1 For a further study of the phenomena of adsorption the following works 
 may be consulted : D'Arcy W. Thompson, Growth and Form, p. 277, &c. ; 
 Bayliss, Principles of General Physiology, pp. 54-73 ; Taylor's Chemistry 
 of Colloids, p. 221, &c. ; and W. Ostwald, Grundriss der Kolloidchemie (1909). 
 
DEVELOPMENT OF THE ENAMEL 
 
 141 
 
 In many parts of the calcareous deposit in the albumin 
 experiments, the globules are seen to have coalesced ; they 
 are often arranged in rows around a central extension filled 
 with minute spherites, and in other places where in contact, 
 have become fused into larger bodies, hour-glass-shaped and 
 double spherites being very frequent. 
 
 The larger ones show a tendency to disintegrate, the 
 central part breaking down into smaller elements (fig. 80,/). 
 In many of the larger radial spherites, beside the delicate 
 
 a e f. 
 
 FIG. 80. Calcospherites. a. Concentric form (formed in albumin); 
 b. radial form (formed in albumin); c. radial spherite from marsupial 
 forming enamel ; /. a similar one disintegrating ; d. oval spherite from 
 human developing enamel ; e. granular spherite from human enamel. 
 
 radial striae, wider spaces are seen extending from the 
 centre to the circumference, of an irregular shape, and 
 evidently indicating a splitting or dividing of the spherite 
 into sections. This is frequently seen in the disintegrating 
 globules in enamel, presently to be referred to. 
 
 The occurrence of calcospherites in the living animal may 
 be well studied in the carapace of the prawn und in the claw 
 of the crab, as was pointed out by Rainey. A fragment of 
 the transparent carapace of the prawn, mounted in glycerine 
 without any previous preparation, shows the spherites very 
 beautifully in all stages of coalescence and disintegration 
 (fig. 81), and in the crab their incorporation in the hard 
 
142 MICROSCOPIC ANATOMY OF THE TEETH 
 
 shell can be very distinctly followed in thin ground sections 
 (fig. 82). 
 
 In all these organisms, in Crustacea, Mollusca, and Brachio- 
 poda, there is a distinctly visible delicate fibrillar basis 
 substance which appears to serve as a support to the deposited 
 material secreted by the animal, and in which calcification 
 takes place. This is a clear, apparently structureless, 
 substance, and does not seem in any way to determine the 
 actual arrangement of the particles. It is not a con- 
 
 
 FIG. 81. Calcospherites in the carapace of the Prawn. ( x 100.) 
 
 nective tissue, and shows no connexion with cells. It is 
 evident in all these examples that the shell is ultimately 
 formed by the fusion of the calcospherites. 
 
 The view held by Bower bank (3) and Carpenter (4} 
 with regard to the formation of shell in the Mollusca was 
 that ' shell is an organic formation growing by interstitial 
 deposit in the same manner as in the teeth and bones of the 
 higher animals '. This view, which accorded with the 
 opinion held at the time, that enamel and dentine were 
 formed by actual deposit in the formative cells, supposed 
 that the same thing occurred in the Mollusca, and that in 
 
DEVELOPMENT OF THE ENAMEL 143 
 
 Pinna and in other shells the membrane left after decalcifica- 
 tion represented the organic cells in which the deposit of 
 the lime salts had taken place. The view now generally 
 held with regard to shell formation is that it is due to 
 a process of secretion. This is more in accordance with 
 modern views regarding the calcification of the teeth, 
 which is considered to be produced by deposit in a material 
 separated and secreted by the cells, and not by the direct 
 
 FIG. 82. Carapace of Crab showing striation and incorporation of 
 spherites. Dark ground illumination. (x500.) 
 
 conversion of the cell into the calcified substance by inter- 
 stitial deposit. The organic remainder after decalcification 
 of a shell of Pinna would not, then suggest that these 
 hexagonal outlines indicate cells, but that they were due to 
 the remains of the fibrillar organic material present in all 
 shells, in which the deposit takes place the remains of the 
 basis substance between the crystalline calcareous plates by 
 which the prisms of this shell are built up. 
 
 In the brachiopod Terebratula, the fibrillar substance 
 is very clearly to be seen in, the mantle, and also in 
 the remains of the decalcified shell there is no evidence of 
 
144 MICROSCOPIC ANATOMY OF THE TEETH 
 
 any deposit in cells but in a very strongly defined irregular 
 network of clear homogeneous material 1 (166). 
 
 Calcareous deposits are not then considered to be due to 
 the plastic agency of the living cell but to physical processes, 
 although, as stated by Professor D'Arcy Thompson, ' the 
 developing concretion is somehow or other so associated 
 with living cells that we are apt to take it for granted that 
 it owes its peculiarities of form to the constructive or plastic 
 agency of these ', and further, ' the appearance of direct 
 association with living cells, however, is apt to be fallacious, 
 for the actual precipitation takes place, as a rule, not in 
 actively living, but in dead, or at least inactive, tissue, that 
 is to say, in the " formed material " or matrix which (as 
 for instance in cartilage) accumulates round the living cells, 
 in the interspaces between these latter, or at least, as often 
 happens, in connexion with the cell wall or cell membrane, 
 rather than with the substance of the protoplasm itself ' (25). 
 
 3. Calcification of the Enamel 
 
 Two essentially different views have been held as to the 
 part taken by the cells of the enamel organ in the process 
 of calcification. By some, especially the earlier authorities, 
 it was held that enamel is produced by the actual con- 
 version of the cells into the calcified substance, and the 
 incorporation of these cells in the formed enamel. 
 
 By others it is considered that the process is one of 
 secretion by the ameloblasts, being comparable to the 
 process of shell formation in the Mollusca, where the hard 
 tissue is produced by a secretion from the mantle of the 
 living animal. 
 
 According to the first view the calcifying salts would be 
 deposited within the substance of the cell, which would thus 
 be converted into the calcified enamel, but recent views on 
 the subject would appear to suggest that the process is to 
 some extent a combination of the two methods ; that while 
 
 1 Dr. Carpenter in his work on the microscope (46), published subse- 
 quently to his original paper, is disposed to agree with Professor Huxley 
 * in the belief that the entire thickness of the shell is formed as an excretion 
 from the surface of the epidermis ' . 
 
DEVELOPMENT OF THE ENAMEL 145 
 
 the ameloblasts are receding as calcification advances, they 
 do not themselves undergo conversion into enamel, but their 
 processes, the Tomes' processes to be afterwards described, do 
 become incorporated in it and form the organic foundation of 
 the finished tissue. This will be more clearly understood when 
 the appearances seen in forming enamel have been described. 
 
 While in Mammalia the ameloblast cells are undoubtedly 
 the formative agents of the enamel, it has been pointed 
 out by C. S. Tomes in the Gadidse (26 b) among osseous 
 fish, and in Sargus and Labrus by the author, that although 
 in the earliest condition of the tooth-germ in these fish the 
 ameloblasts are distinctly present, they disappear at an 
 early stage, and their place is taken by other structures, into 
 which the enamel cells have been apparently converted. 
 
 This disappearance of the ameloblast cells, as such, is 
 especially remarkable, as even in the placoid scales of 
 Elasmobranchs (the hard prominent tubercles of the skin of 
 the sharks and rays) a layer of ameloblasts is distinctly 
 evident during their development, and enamel appears to 
 be formed by their agency, as in the true teeth. This will 
 be further referred to in considering the development of 
 the enamel in fish. 
 
 In the study of the calcification of mammalian enamel 
 we shall include that of the tubular enamel of marsupials, 
 for although the higher existing Mammalia may not be in 
 the direct line of descent from the marsupials, the structure 
 of their enamel is in all essential particulars similar to that 
 of human teeth, and the presence of tubes probably only 
 an indication of less complete calcification. The enamel is 
 formed in the same manner as in higher orders, and is, as 
 it were, a stage in the development of the teeth of more 
 advanced forms. It is more slowly and less completely 
 calcified, and consequently the stages in its formation are 
 more easily studied. 
 
 A tubular condition of the enamel is seen in Hyrax, Jerboa, 
 and the Shrews, where the enamel is in other respects similar 
 to the enamel of other orders of the higher mammalia. 
 
 It may be stated that although in man and the higher 
 mammalia no blood-vessels are seen deeply within the 
 enamel organ, it was shown by Malassez, as explained 
 
 MUMMERY 
 
146 MICROSCOPIC ANATOMY OF THE TEETH 
 
 above, that the external epithelium in man is in many 
 places discontinuous, and blood-vessels lie in direct contact 
 with the cells of the stellate reticulum (see fig. 70). It has 
 been demonstrated in many instances that beyond all doubt 
 blood-vessels do penetrate the external epithelium, and are 
 seen within the stellate reticulum of marsupials. 
 
 Mr. Thornton Carter says that in marsupials the ramifica- 
 tions of these vessels extend often as far as the stratum 
 intermedium, but his fig. 21 is from a transverse section, 
 and does not show to what depth this vascular network 
 penetrates. He states, however, that in his preparations 
 vessels are frequently seen * in contact with the cells of the 
 stratum intermedium ' (5 a). 
 
 They do not, however, form any regular system in this 
 situation, but are irregularly scattered in the reticulum, and 
 suggest that they show a vestige of an earlier condition in 
 which the presence of blood-vessels in the enamel organ 
 was a more constant and regular phenomenon, and the 
 condition gives rise to more interest in view of the observa- 
 tions of the author in recent investigations on the enamel 
 organs of the Sparidse and Labridse, to be presently referred 
 to, where a regular system of blood-vessels is seen in the 
 enamel organ, and where it evidently takes a very important 
 part in the calcifying functions. 1 
 
 If we agree with the statement of Rose (21) that the 
 development of the stellate reticulum is intimately associated 
 with the deposition of a thick enamel cap in the higher 
 vertebrata, we must assign to this layer an important 
 function in the process of calcification, and it is considered by 
 Rose that the arrangement of the cells favours osmosis and the 
 conveyance of nutriment to the growing cells of the internal 
 layers ; it may also assist in the separation of calcifying 
 salts from the blood circulating in the vessels of the capsule. 
 
 The stratum intermedium, which is in intimate relation 
 with the cells of the stellate reticulum, their processes com- 
 municating with one another, was considered by many to 
 be the recuperating layer for the cells of the internal epithe- 
 lium or ameloblasts ; but after the formation of enamel has 
 
 1 In Pseudoscarus, which is very closely allied to the Labridae, blood- 
 vessels also penetrate the enamel organ. 
 
DEVELOPMENT OF THE ENAMEL 147 
 
 commenced, these cells, as has been shown, are cut off from 
 the ameloblast layer by a separating membrane, so that 
 it seems more reasonable to suppose that the principal 
 recuperating effect they have on the ameloblast cells must 
 be due to osmosis and the passage of nutrient material and 
 lime salts through this membrane, although, as stated in 
 discussing the inner ameloblastic membrane, the penetration 
 of this separating layer by processes of the cells is quite 
 comprehensible . 
 
 A very distinct function is, however, assigned by Leon 
 Williams to the cells of the stratum intermedium in the 
 later stages of enamel formation, when the stellate reticulum 
 has disappeared and the external epithelium is in contact 
 with the cells of the stratum intermedium. 
 
 He describes blood-vessels in the stratum intermedium 
 at this stage, and that there are blood-vessels in this layer 
 is fully evidenced by the photographs in illustration of his 
 paper. 
 
 When, however, this author speaks of the incorrectness 
 of the statements of Wedl and the majority of observers 
 that no blood-vessels are seen in the cells of the enamel 
 organ proper, he scarcely conveys the true meaning of their 
 statement. In the early stages of enamel formation, when 
 the stellate reticulum intervenes between the external 
 epithelium and the stratum intermedium, no blood-vessels 
 are seen in the enamel organ except vestiges of them in the 
 stellate reticulum of marsupials, and at the outer border 
 of the stellate reticulum in man (see figs. 70 and 72). It 
 is only in the later stages of calcification that this supply of 
 blood-vessels can be seen, when the stellate reticulum has 
 disappeared and the external epithelium is in contact with 
 the stratum intermedium, and the abundant vessels of the 
 connective tissue of the follicle are in intimate relation with 
 this layer of cells. He has shown, and one cannot but think 
 very conclusively, that in the rat blood-vessels enter the 
 stratum intermedium in these later stages very abundantly, 
 and that they have a special arrangement with regard to 
 the cells. 
 
 In this later stage of calcification he says that the charac- 
 teristic forms of the cells of the stratum intermedium have 
 
 L 2 
 
148 MICROSCOPIC ANATOMY OF THE TEETH 
 
 entirely disappeared, and their place is taken by a definite 
 secreting structure. 
 
 He describes and figures capillary loops alternating with 
 epithelial papillae, and in fig. 15 in the illustrations to his 
 paper shows these ' secreting papillae ' pulled away from 
 the capillary loops, and it is very evident that the vessels 
 dip in between the papillse. 
 
 Fig. 76, from a photograph of the enamel and enamel 
 organ of the rat, from a specimen of the author's prepared 
 by the Weil process, also shows these papillae and blood- 
 vessels. 
 
 The same author also describes these papillae as being in 
 intimate relation with the ameloblast cells beneath them, 
 ' each ameloblast seeming to have a root-like process which 
 extends into and is lost in the substance of the papilla to 
 which it belongs '. 
 
 It would be evident, however, that if this is the case, the 
 outer ameloblastic membrane separating the cells of the 
 internal epithelium from those of the stratum intermedium 
 must have either disappeared, or it must be perforated by 
 * the root-like processes of the ameloblasts '. That such 
 processes do- pass through this separating layer between 
 the stratum intermedium and the ameloblasts is, we think, 
 evident in many preparations. 
 
 The present author, in his investigations on enamel develop- 
 ment in certain fish, found and figured a similar condition 
 in the enamel organ of one of the Lr.bridse (Pseudolabrus 
 japonicus), and, as shown in the paper on the subject (16 d), 
 in these fish the whole of the enamel organ appears to be 
 converted into a secreting structure, there being no trace of 
 the cells of the internal or external epithelium, unless the 
 cells within the secreting papillae represent some constituent 
 of the enamel organ of mammalia. 
 
 Leon Williams was unable to detect this definite arrange- 
 ment in other mammalian enamel organs, but only in the 
 Rodents, although in the sheep he found the cells of the 
 stratum intermedium show a more or less orderly arrange- 
 ment around capillary loops. 
 
 In some specimens of the enamel organ of Macropus the 
 cells of the stratum intermedium are seen to have lost their 
 
DEVELOPMENT OF THE ENAMEL 149 
 
 characteristic form, and large highly refractive globular- 
 bodies are seen in their place (fig. 83). 
 
 Leon Williams, in summing up this part of his subject, 
 says : ' It is now perfectly evident, however, that in the 
 development of enamel the cells of the stratum intermedium 
 play the most important part in the selection from the blood 
 of the material for the construction of this tissue. The 
 formation of enamel begins, however, before the full develop- 
 
 FIG. 83. Developing tooth of Macropus. Late stage of calcification 
 of the enamel, e. Enamel ; a. ameloblasts ; g. globular bodies in stratum 
 intermedium. No Tomes' processes are visible. ( x 700. ) 
 
 ment of the stratum intermedium, and it is highly probable, 
 as has been pointed out by previous writers, that the material 
 for the commencement of enamel formation is stored in the 
 stellate reticulum.' 
 
 The cells of the internal epithelium of the enamel organ, or Amelt/ 
 ameloblasts, are in Mammalia the active agents in the forma- l 
 tion of the enamel. These are elongated columnar cells with 
 large nuclei, which are situated in that part of the cell most 
 distant from the forming enamel (figs. 84 and 85 and Plate IV) . 
 
 The cell shows a fine reticular structure, and in thin 
 sections the fibrillar constituents are seen to be arranged 
 more or less parallel with the long axis of the cell. The 
 
150 MICROSCOPIC ANATOMY OF THE TEETH 
 
 nucleus shows a delicate nuclear network and one or more 
 nucleoli, and as Leon Williams says, ' it seems almost a fore- 
 gone conclusion that these cells must be renewed, or at least 
 that the necessary increase in number during the progress 
 of enamel development must come from cell division ' , but 
 the actual process of cell division in the ameloblasts had not, 
 however, been observed. 1 
 
 t 
 
 FIG. 84. Developing tooth of Macro pus cut without decalcifica- 
 tion. a. Ameloblasts ; t. Tomes' processes ; e. enamel ; 8. stratum 
 intermedium. ( x 700. ) 
 
 Mi 1 . Thornton Carter has, however, lately described 
 mitosis in the ameloblast cells and in those of the stratum 
 intermedium of marsupials at the stage immediately pre- 
 ceding the formation of a thin layer of dentine over the 
 pulp, but was unable to detect it in later stages. He divides 
 the life-cycle of the ameloblasts in marsupials into fourteen 
 different stages (5 a). 
 
 As first pointed out by C. S. Tomes, sometimes during 
 active deposition of the enamel the nucleus is crescentic in 
 form (26 d), and the fine fibrillar elements of the cell body 
 
 1 For the description of amitosis in the late stage? of the enamel organ 
 cells see under ' Nasmyth's Membrane ', p. 333. 
 
DEVELOPMENT OF THE ENAMEL 
 
 151 
 
 appear to be connected with the horns of the crescent, 
 which are directed towards the forming enamel (see fig. 85). 
 
 Granules and clear oval bodies are also seen within the Globules 
 
 i mi. in ame *- 
 
 cell when the deposition of enamel has commenced. Inese blasts. 
 oval bodies are variable in size and shape, and do not react 
 
 
 e 
 
 FIG. 85. Enamel organ of Macropus (cut without decalcification ). 
 s. Stellate reticulum; i. stratum intermedium ; a. ameloblasts; t. Tomes' 
 process ; e. forming enamel. (The interlacing of the fibres of the Tomes' 
 processes and the continuity of these fibres with the cytoplasmic threads 
 in the ameloblasts are seen. ) x 800. 
 
 to polarized light. They would appear to be globules of 
 material of a colloidal nature secreted by the cell, and in all 
 probability the material which is elaborated by the cell and 
 containing the calcifying salts. 
 
 These globular bodies in the ameloblasts have been 
 
152 MICROSCOPIC ANATOMY OF THE TEETH 
 
 described more especially by Arnell (2), Graf v. Spee (23), 
 Von Ebner (8), and Leon Williams. They are very evident 
 in the cells as soon as enamel deposition has commenced, 
 and that they do contain the calcifying elements there seems 
 to be very little doubt, although C. S. Tomes and others, 
 while quite convinced of the appearance of these bodies, 
 do not feel in a position to give any positive opinion as to 
 their nature. 
 
 When calcification is about to commence, the ameloblast 
 cells become separated from the cells of the stratum inter- 
 medium by a delicate membrane, although prior to the 
 commencement of the process they show no signs of such 
 separation, but the cells of the stratum intermedium are 
 in many places seen to be lying between the ameloblasts as 
 shown in figs. 75 and 77. 
 
 This membrane once formed, no such intermingling of the 
 cells of the two layers can ever be seen, but they remain 
 quite distinct from one another. 
 
 At the same time another delicate membrane-like layer 
 is formed between the ends of the ameloblasts and the 
 forming enamel. These two membranes, called by Leon 
 Williams the outer and inner ameloblastic membranes 
 respectively, are supposed to take an important part in the 
 calcifying process, and to act as dialysing membranes to 
 separate the lime salts from the albuminoid material con- 
 tained in the cells. As shown in the experiments with 
 inorganic solutions above referred to, such a process takes 
 place through the membranes of precipitation formed in 
 the solution of silicate of potash, the lime salts contained 
 in the water being separated by dialysis and deposited on 
 the distal side of the membrane as calcospherites. 
 
 Professor Huxley, as previously stated, described an inner 
 ameloblastic membrane under the name of membrana 
 preformativa, and he also considered that the calcification 
 of enamel took place ' by the transference of crystallizable 
 products through a membrane '. 
 
 In a recent paper, referred to more fully on p. 174, Mr. 
 Thornton Carter denies the existence of an outer amelo- 
 blastic membrane, but fig. 78 will, we think, give a clear 
 indication of its existence. Whether these membranes are 
 
DEVELOPMENT OF THE ENAMEL 153 
 
 separate structures or are formed by the union of the 
 limiting membrane or wall of the cell, has not been deter- 
 mined. That in the case of both the outer and inner mem- 
 branes, processes pass through them from the cells, both of 
 the stratum intermedium and the ameloblasts, there can 
 be little doubt, but the protoplasm of the cell wall is not 
 a rigid substance like a sieve, and is quite capable of acting 
 as a dialysing membrane, although these processes do pass 
 through it. 
 
 Both Tomes and Carter consider it an inexplicable 
 anomaly that the dialysing membrane should be penetrated 
 by the fibrils of the Tomes' processes ; but while Tomes 
 denies the existence of such separating membrane, Carter 
 apparently allows the existence of some such structure, but 
 not the processes of the cell, as he considers with Tomes 
 that Ave could not have a dialysing membrane which is 
 perforated by processes. In this connexion it may be 
 mentioned that Ramon y Cajal l describes the intercellular 
 bridges of epithelial cells as being covered with a prolonga- 
 tion of the cell membrane. 
 
 In order to avoid the distortion of the delicate tissues of 
 the enamel organ so frequently produced by the employ- 
 ment of such reagents as alcohol and balsam, and the 
 employment of heat in the process of embedding, the author 
 in his investigations of enamel development made use of 
 preparations hardened in formol and cut in the freezing 
 microtome. These were stained with watery solutions of 
 the dyes and mounted in glycerine or Farrant solution. 
 By this method preparations were obtained which showed 
 all the parts of the enamel organ in their proper relations 
 to one another and free from the shrinking and distortion 
 so often seen in preparations of tooth-germs. 
 
 The first notable point brought out in marsupial germs 
 prepared in this way is the great distinctness of the Tomes' 
 processes of the ameloblasts, as was pointed out by C. S. 
 Tomes (figs. 84 and 85). They are not drawn out into thin 
 lines or apparently pulled apart from the forming enamel, 
 and are seen in many places to be nearly as wide as the 
 cells from which they arise. C. S. Tomes says that these 
 1 Int. M onatschr. f. Anat. u. HistoL, iii. 1886. 
 
154 MICROSCOPIC ANATOMY OF THE TEETH 
 
 processes do not show any minute fibrillar structure, but 
 certainly such structure appears to be indicated by the 
 photographs (figs. 86 and 87). 
 
 Owing to the wide area of enamel which has only under- 
 gone partial calcification in early enamel germs of Macropus, 
 the prolongations of the Tomes' processes can be traced in 
 many sections all 'across this space to the dentine, and it 
 was no doubt these extensions of the Tomes' processes which 
 were described by Andrews, who was the first to draw atten- 
 tion to this fibrillar foundation of the calcified enamel ( 1 ) . 
 
 FIG. 86. Developing tooth of Macropus. a. Ameloblasts ; t. Tomes' 
 processes spreading out into the forming enamel (e). (x 700.) 
 
 As the ameloblasts are separated from the forming 
 enamel by a membrane, the inner ameloblastic membrane, 
 this must be pierced by the Tomes' processes, which are 
 processes of the ameloblast cell. 
 
 It has been long known that a fenestrated membrane can 
 be detached from the surface of growing enamel by acids, 
 but in some of the author's preparations, where no acids 
 were employed, such a membrane is detached where the 
 enamel is slightly separated from the cells. It is difficult 
 to say whether this fenestrated membrane shown in fig. 88 
 was in contact with the ameloblasts or with the surface of 
 the forming enamel, but it certainly does not require the 
 action of an acid to bring it into view. 
 
 A structure has been described in forming enamel which 
 
DEVELOPMENT OF THE ENAMEL 
 
 155 
 
 FIG. 87. Continuity of cytoplasmic threads of ameloblasts -with Tomes' 
 processes and forming enamel, i. Stratum intermedium ; a. ameloblasts ; 
 t. Tomes' processes ; e. enamel. ( x 700. ) 
 
 FIG. 88. Fenestrated membrane over forming enamel. Separated with- 
 out the action of acid. e. Enamel organ ; m. fenestrated membrane. 
 (X600.) 
 
156 MICROSCOPIC ANATOMY OF THE TEETH 
 
 appears to be present in all mammalian tooth-germs the 
 so-called ' honeycomb '. 
 
 At the point where the Tomes' processes enter the enamel, 
 a structure is seen resembling a section of the cells of a honey- 
 comb. This appears to consist of a substance on the border- 
 land of calcification. It is resistant to acids, and exists only 
 during the early stages of enamel formation. C. S. Tomes 
 describes this structure as identical with the fenestrated 
 membrane above described, which is raised from the surface 
 of forming enamel by the action of acids ; but, as already 
 shown, this fenestrated membrane also becomes detached 
 without the action of an acid, and would thus not appear to 
 have a very intimate connexion with the forming enamel. 
 Tomes considers that, although probably present in all 
 mammalian enamels, the honeycomb is much more con- 
 spicuous and of greater depth in marsupials (26). It is 
 much more evident in oblique sections than in those that 
 are directly transverse, and he describes the Tomes' pro- 
 cesses as entering this structure and filling up the orifices 
 between the septa, the fibres adhering to the septa when 
 the enamel is pulled away from the ameloblasts. 
 
 He consequently concludes that the process of calcifica- 
 tion is a centripetal one, commencing at the septa of the 
 honeycomb and gradually filling up the spaces between 
 them. In marsupials, the centre remaining open owing to 
 this process of gradual closing in of the space being incom- 
 plete, a tube remains in the central axis of the prism. In 
 non-tubular enamel calcification advances to the complete 
 closing and obliteration of this central tube. C. S. Tomes 
 would thus look upon the tubes in marsupial enamel as 
 entirely a product of the enamel organ, and their connexion 
 with the tubes of the dentine with which they become con- 
 tinuous as a secondary matter. 
 
 According to this view of enamel calcification, there 
 would be no true interprismatic substance. 
 
 It would appear as if the honeycomb was a structure of 
 a transitory nature, and according to the present author's 
 view it acts more as a directing structure to the fibres of 
 the Tomes' processes than as a permanent constituent of 
 the enamel prisms, for the evidence is very strong that the 
 
DEVELOPMENT OF THE ENAMEL 157 
 
 calcification of the interprismatic substance takes place, at 
 all events to a very great degree, independently from the 
 calcification of the prisms. 
 
 It can be seen in figs. 86 arid 87 that at the honeycomb 
 region the fibres of the Tomes' processes spread out in a fan 
 shape, the fibres from neighbouring processes crossing one 
 another and intermingling, to be again drawn together to 
 form the columns of the prismatic substance of the enamel. 
 We thus see that the ultimate bundles of fibres which form 
 the basis of the enamel prisms do not appear to be the 
 direct prolongations of one particular ameloblast, but are 
 contributed to by those in contact with it on either side. 
 
 As was first pointed out by Dr. Andrews of Boston, there Fibrillar 
 is a delicate organic fibrillar foundation to enamel as there 
 is in dentine, an organic network or scaffolding in which 
 calcification takes place. The fibres described by Andrews 
 were the prolongations of the Tomes' processes so clearly 
 seen in marsupial enamel, and which, as' previously described, 
 are seen to extend all across the area of forming enamel to 
 the dentine, when the process of calcification is but very 
 little advanced. 
 
 There are also other delicate fibres passing in a transverse 
 direction, which are very evident in marsupial forming enamel 
 and which are also derived from the ameloblast cells (fig. 89). 
 This fibrillar foundation is also very evident in the enamel 
 of the rat, as shown in Leon Williams's illustration on p. 22 
 of his paper taken from a preparation by the author, and 
 the fibres, as he says, ' bear a striking resemblance to the 
 effect produced by tearing a woven fabric of any sort'. 
 Further evidence of the existence of this substructure in 
 enamel is afforded by the experiment described on p. 104, 
 where a delicate membrane-like foundation was left on the 
 slide after the decalcification by strong acids, and the tubes 
 of the enamel remained undisturbed within its substance. 
 
 Also in ground sections of marsupial enamel which had 
 been treated with alcoholic fuchsin there is in many places 
 an appearance of transverse stained lines which are evidently 
 portions of the transverse fibres which have escaped calcifica- 
 tion (fig. 53 B). 
 
 The existence of a fibrillar condition in forming enamel is 
 
158 MICROSCOPIC ANATOMY OF THE TEETH 
 
 also clearly demonstrated by the following method. If an 
 unerupted tooth of Macropus is taken from its crypt when 
 the enamel forms a thin friable investment to the cap of 
 dentine, and scrapings from this enamel are allowed to fall 
 into glycerine and examined on a microscopic slide, it will 
 be seen that when they are broken up with needles the 
 fragments split into laminae which lie parallel to one another, 
 and both the longitudinal and transverse striae are very 
 clearly seen. 
 
 FIG. 89. Transverse striation in developing enamel of Macropus. 
 
 The most interesting point, however, demonstrated by 
 this method is the presence of granules and large calco- 
 spherites between and upon the laminae. 
 
 In fig. 90, from a specimen prepared in this manner, the 
 laminae are seen to have separated in the direction of the 
 columns of the prisms, and a cross striation is also very 
 evident. Large globular bodies are seen between the laminae 
 and attached to their edges, while many of the larger bodies 
 are lying free in the surrounding fluid. 
 
 Examination of a portion of these laminae with a high 
 power shows that these globular bodies are true calcospherites 
 of the radial type, and scattered granules are seen between 
 
DEVELOPMENT OF THE ENAMEL 159 
 
 them. Some of the large calcospherites are seen to be 
 splitting in a radial manner into three or four segments, and 
 are surrounded by small granules. These large calcospherites 
 vary greatly in size, but they also show the radial type of 
 formation, no concentric spherites being visible. 1 
 
 Fig. 91 is particularly instructive, for it shows that small 
 uniform calcified bodies are building up the enamel prisms, 
 while large calcospherites are lying between the laminae and 
 showing indications of breaking down into smaller particles. 
 
 FIG. 90. Fragment of enamel from developing tooth of Macropus teased 
 out in glycerine, showing laminae and spherites. ( x 140.) 
 
 These appearances would seem to indicate that the enamel 
 prisms are calcified independently of the interprismatic 
 substance, the prisms being formed by the fusion of these 
 small calcospherites into the regular blocks of calcified sub- 
 stance seen in mature enamel, while the larger bodies by 
 their fusion and disintegration form the cementing or 
 interprismatic substance. As will be shown later, however, 
 and distinctly seen in fig. 94, these larger bodies are not 
 always broken up, but become fused with one another. 
 
 1 The calcospherites in these figures may be compared with the artifi- 
 cially formed spherites shown in fig. 80. 
 
160 MICROSCOPIC ANATOMY OF THE TEETH 
 
 An examination of fragments of the enamel cap in human 
 unerupted teeth, treated in the same manner, also showed 
 the same appearances (fig. 92), but owing to the more rapid 
 and dense calcification in human teeth the calcospherites are 
 more difficult to detect, the separation of the laminae not 
 taking place to the same depth in the forming enamel as in 
 marsupials. As in the first case, large calcospherites are 
 seen free in the surrounding fluid ; but while in marsupials 
 these bodies are all round in form, in human enamel they 
 
 FIG. 91. Portion of enamel from specimen shown in fig. 90 more highly 
 magnified, showing the beading of the prisms (a) and large radial calco- 
 spherites in the laminae. ( x 1,000.) 
 
 are mostly oval, very few round ones being seen. They 
 have the same radial structure, however, and two or more 
 are frequently found fused together, forming an irregular 
 mass. 
 
 A scraping from the surface of the enamel of a human 
 tooth still in the crypt showed a large mass of calcospherites, 
 the larger ones all having this oval form (fig. 93). Large 
 scattered spherites are also seen in human enamel, which are 
 uniformly and finely granular. 
 
 It is thus seen that there are in forming enamel two 
 
DEVELOPMENT OF THE ENAMEL 
 
 161 
 
 different forms of deposit of the calcospherites : the small 
 regularly arranged bodies which are deposited in the sub- 
 stance of the longitudinally arranged prolongations of the 
 
 Fra. 92. Large spherites in human forming enamel. Teased preparation. 
 c. Calcospherites in the laminae. ( X 500.) 
 
 FIG. 93. Mass of calcospherites from surface of forming enamel of 
 temporary human molar within the tooth-sac. ( x 600.) 
 
 Tomes' processes, and which build up the prisms of the 
 enamel, and the larger scattered calcospherites between 
 the laminae, which appear to be the calcifying agents of the 
 interprismatic substance. These larger spherites are seen 
 to be disintegrating in many places, and in others the large 
 
 MUMMERY 
 
 M 
 
162 MICROSCOPIC ANATOMY OF THE TEETH 
 
 spherites become fused together and form a calcified matrix 
 for the enamel prisms. 
 
 That the interprismatic substance is formed by the fusion 
 of these larger bodies and that they can be distinguished in 
 fully formed enamel is shown by the following observation. 
 A thin ground section of a human tooth which was affected 
 by commencing caries, and showed a deep fissure in the 
 enamel, nearly but not quite reaching the dentine, was 
 examined under the microscope. Surrounding the fissure, 
 
 FIG. 94. Enamel of carious adult molar. Ground section (Weil process). 
 c. Large spherites revealed by action of the acid in caries ; p. transverse 
 sections of the prisms showing their granular structure (compare with 
 fig. 91). (x 1,000.) 
 
 the contours of large calcospherites of a radial nature, 
 similar to those seen in the teased preparations of marsupial 
 enamel, are clearly visible, the acids produced in the carious 
 process having dissolved a portion of the lime salts and 
 revealed the actual contours of the calcific deposit. It 
 would, therefore, appear that although many of these larger 
 bodies in the process of calcification break up and become 
 granular, others become fused together and still retain their 
 original form (fig. 94). 
 
 Professor Underwood (28 a) described a similar appearance 
 
DEVELOPMENT OF THE ENAMEL 163 
 
 in enamel which was not carious, but the subject of erosion, 
 the contours of the spherites being there also distinctly 
 visible, and he compared this appearance to that of the 
 interglobular spaces in dentine. 
 
 These observations would appear to be strongly corrobora- 
 tive of the view held by Leon Williams and the author, 
 that the interprismatic substance of enamel is calcified 
 independently of the prisms, for it can be distinctly seen 
 that the small regular calcifying elements of the prisms are 
 quite distinct from these large calcospherites both during 
 development and in the completed enamel. 
 
 Von Ebner does not hold with the view of Leon Williams 
 and Andrews that the prisms are calcified independently of 
 the interprismatic substance. He considers that each 
 ameloblast is provided with a separate Tomes' process, 
 which deeper in the first -formed enamel forms a separate 
 enamel prism, and that all the enamel cells together form 
 an interprismatic substance, in which the young prisms lie, 
 and by which they are surrounded. The prisms he considers 
 increase in thickness at the cost of the interprismatic 
 material, and in quite homogeneous enamel are completely 
 calcified. 
 
 He also states that thin layers of uncalcified cement sub- 
 stance extend to the free surface of the enamel, where they 
 become continuous with Nasmyth's membrane. 
 
 With regard to the existence of laminae in formed enamel, 
 which are arranged longitudinally between the dentine and 
 the surface, evidence is afforded by sections of human teeth 
 which have been treated with alcoholic fuchsin by capillary 
 attraction. The stain is seen to have spread over the 
 ground surface of the section in places, and if a section is 
 examined during the process of grinding it is seen that these 
 stained laminae are gradually ground away, portions of them 
 often remaining in the finished section. The staining of 
 these areas was long ago pointed out by Mr. Douglas 
 Caush (6), and corroborates the evidence afforded by the 
 teased preparations of marsupial enamel, which as pointed 
 out previously break up into plates or sheets at right angles 
 to the surface. 
 
 In the chapter on tubular enamel the reasons are given, 
 
 M 2 
 
164 MICROSCOPIC ANATOMY OF THE TEETH 
 
 as evidenced in the completed tissue, for considering that 
 these tubes are not an enamel product but are penetrating 
 dentinal tubes, and the evidence in favour of this view, 
 afforded by developing teeth, may now be considered. 
 
 Although, as in all other mammalian enamels described, 
 calcification in the dentine in marsupials precedes that in 
 the enamel, yet when a small layer of calcified dentine only 
 is laid down, it can be seen that the area of enamel between 
 
 
 FIG. 95. Developing tooth of Macropus cut without decalcification. 
 s. Stratum intermedium ; a. ameloblasts ; t. Tomes' processes ; e. enamel ; 
 d. calcified dentine ; o. odontogenic zone ; od. odonto blasts ; p. pulp. 
 (x75.) 
 
 the ameloblasts and the dentine is very much wider than 
 that of the dentine (fig. 95), but this space is occupied by 
 the extended fibres of the Tomes' processes, and the calcifica- 
 tion within and between them is very incomplete. In this 
 section of an advanced germ of Macropus cut without 
 decalcification, and in which there is no disturbance of the 
 tissues, it is seen that a very much wider area of enamel 
 than of dentine is laid down ; in one part measured, the 
 enamel was found to be three times the width of the calcified 
 dentine. The stain employed (methylene blue) has uniformly 
 
DEVELOPMENT OF THE ENAMEL 165 
 
 coloured the enamel a strong blue throughout the whole 
 section, showing that very little calcification has taken place, 
 and large calcospherites are seen near the dentine junction. 
 The calcified dentine is quite unstained, while the odonto- 
 genic zone and the odontoblasts are a deep blue. It is also 
 noticeable that the part of the enamel first laid down, 
 bordering on the dentine, shows signs of still more incom- 
 plete calcification than the rest of the tissue. This appears 
 a somewhat anomalous and curious condition. It would 
 appear as if the ameloblast processes, where they extend to 
 the dentine, leave spaces between them which very slowly 
 become obliterated by calcification. These spaces can be 
 clearly seen in figs. 96 (s) and 97, and lying in the spaces are 
 round calcospherites such as are seen in the teased pre- 
 parations. The presence of these spaces and of the large 
 calcospherites would appear to indicate that a gradual clos- 
 ing in upon the enamel prisms takes place, while the enamel 
 above has become more completely calcified, and may help 
 to explain the presence of the dentinal tubes in the enamel. 
 
 C. S. Tomes says : ' The first -formed layer of dentine con- 
 tains only the fine terminations of the dentinal tubes ' (26 d), 
 but an examination of many sections shows that although 
 some tubes terminate in fine branching extremities before 
 reaching the enamel junction, others, and often the majority, 
 pass as large unbranched tubes to the enamel margin, and 
 fig. 96 shows these tubes actually passing into the open 
 spaces above described. 
 
 It is therefore probable that these spaces, which form the 
 ' clumsy joint ' of Tomes, are not due to inclusions of 
 dentine matrix as suggested by Professor Paul (17), or to 
 dilatations of the tubes in this situation, but to the imperfect 
 calcification of the interprismatic substance surrounding 
 them. If such spaces were due to intrusions of dentine 
 matrix we should expect to find them well defined in early 
 stages of enamel development, but instead of these irregular 
 spaces we find an area of uncalcified tissue of an irregular 
 form. As calcification advances, these spaces around the 
 tubes are gradually encroached upon by the calcifying 
 substance, but in most marsupials not completely closed in, 
 and remain as the irregular spaces seen in finished marsupial 
 
166 MICROSCOPIC ANATOMY OF THE TEETH 
 
 enamel. In some marsupials and in the tubular enamel of 
 Hyrax the calcification is more complete in this situation, 
 and the dentinal tubes pass into the enamel without showing 
 any spaces or apparent enlargements. These spaces have 
 
 P 
 
 FIG. 96. Developing tooth of Macropus (cut without decalcification). 
 e. Enamel ; s. area of incomplete calcification ; d. calcified dentine ; 
 d'. odontogenic zone ; o. odontoblasts ; p. pulp. In several places the 
 dentinal tubes are seen passing into the area s. ( x 800.) 
 
 been previously referred to on p. 81, and this view of their 
 causation agrees in the main with those of Von Ebner and 
 Pickerill. 
 
 If, at the first formation of the dentine, the dentinal fibril 
 passes into the organic substance of the enamel, and a con- 
 nexion is thus established between the cells of the dentine 
 
DEVELOPMENT OF THE ENAMEL 167 
 
 and the enamel matrix, it can easily be understood that the 
 fibril would continue to penetrate this slightly resisting 
 substance until eventually closed round by calcification. 
 
 The chemical composition of adult enamel presents a 
 problem which it is somewhat difficult to elucidate in view 
 of these observations on calcification. 
 
 It is seen that the calcific deposit in the interprismatic 
 substance takes place in the form of large calcospherites, 
 and that, despite opinions to the contrary, the globular form 
 
 FIG. 97. Developing tooth of Macropus. e. Forming enamel ; c. calco- 
 spherites ; d. calcified dentine ; o. odontogenic zone (cut without 
 decalcification). ( x 500. ) 
 
 is the form in which lime salts are deposited in the hard 
 tissues of the teeth. 
 
 In the artificial experiments where these large calco- 
 spherites are deposited in albumin and other media, they 
 consist of carbonate of lime, and although it was shown by 
 Professor Harting that when a little phosphate of lime was 
 present with the carbonate the largest and most perfect 
 spherites were formed, if the phosphates were at all in excess 
 of the carbonates, spherites were not formed but the deposit 
 was crystalline. 
 
 The analyses of adult enamel show a very great prepon- 
 derance of phosphates over carbonates roughly 89 per cent, 
 of phosphates to 4 per cent, of carbonates. In view of this 
 
168 MICROSCOPIC ANATOMY OF THE TEETH 
 
 fact it is hard to understand how the large spherites, which in 
 all outward appearance resemble the artificially produced 
 carbonate calcospherites, are produced. We see no altera- 
 tion of the spheroidal to the crystalline form, and when the 
 structure of finished enamel is revealed by the action of 
 caries and erosion the outlines of these large spherites can 
 still be traced. 
 
 The question arises whether in the early stages of calcifica- 
 tion the carbonates preponderate ; but there are no analyses 
 of forming enamel, and they would be very difficult, if not 
 impossible, to obtain. 
 
 Professor Sims Woodhead (29), however, suggests that 
 in the early stages of bone formation, which also shows an 
 excess of phosphates in the completed tissue, the carbonates 
 may be present in excess. He says, * Newly formed bones, 
 or new bone tissue of any kind, where the cells are extremely- 
 active in building up the matrix, almost invariably have 
 a larger proportion of carbonate of lime than fully formed 
 ones, because here the active cells set free a larger proportion 
 of carbonic acid, as a result of which more phosphoric acid 
 may be replaced by the carbonic acid lime salts '. The same 
 conditions probably apply to the other calcified organic 
 tissues of the body, as enamel and dentine. The conditions 
 in young growing animals, where the metabolic changes are 
 very active, are eminently favourable to the production of 
 carbonic acid, and it is quite conceivable, and in fact 
 probable, that in the early stages of enamel and dentine 
 formation the carbonates may be greatly in excess of the 
 phosphates, and thus the deposit of lime salts as calco- 
 spherites be better understood. 
 
 As stated by C. S. Tomes, Hoppe-Seyler considers that the 
 salt in teeth and bone is a double salt, consisting of three 
 equivalents of calcium phosphate with one of calcium 
 carbonate. 
 
 Whether in this combination the influence of the car- 
 bonate may be effective in causing a globular deposit might 
 be considered ; but in whatever way we explain the chemical 
 conditions which allow of their formation, we cannot escape 
 the evidence that in both dentine and enamel the lime 
 salts are deposited in the globular form. 
 
DEVELOPMENT OF THE ENAMEL 169 
 
 There are no doubt many objections to the above explana- 
 tion, and further researches are very desirable before we can 
 have a clear understanding of the exact chemical conditions 
 in calcification, but very considerable light has lately been 
 thrown upon this question by the researches of Pauli and 
 Samec (1910) (20), referred to by Professor D'Arcy Thompson 
 in his work on Growth and Form (25), from which the following 
 quotation is taken : ' It has been shown, in the first place, 
 that the presence of albumin has a notable effect on the 
 solubility in a watery solution of calcium salts, increasing the 
 solubility of the phosphate in a marked degree, and that of 
 the carbonate in still greater proportion ; but the sulphate 
 is only very little more soluble in the presence of albumin 
 than in pure water, and the rarity of its occurrence within 
 the organism is so far accounted for. On the other hand, 
 the bodies derived from the breaking down of the albumins, 
 their " catabolic " products, such as the peptones, &c., 
 dissolve the calcium salts to a much less degree than 
 albumin itself ; and in the case of the phosphate, its solu- 
 bility in them is scarcely greater than in water. The 
 probability is, therefore, that the actual precipitation of the 
 calcium salts is not due to the direct action of carbonic 
 acid, &c., on a more soluble salt (as was at one time believed), 
 but to catabolic changes in the proteids of the organism, 
 which tend to throw down the salts already formed, 
 which had remained hitherto in albuminous solution. The 
 very slight solubility of calcium phosphate under such 
 circumstances accounts for its predominance in, for instance, 
 mammalian bone, and wherever, in short, the supply of 
 this salt has been available to the organism. 
 
 ' To sum up, we see that, whether from food or from sea- 
 water, calcium sulphate will tend to pass but little into 
 solution in the albuminoid substances of the body ; calcium 
 carbonate will enter more freely, but a considerable part 
 of it will tend to remain in solution ; while calcium phosphate 
 will pass into solution in considerable amount, but will be 
 almost wholly precipitated again, as the albumin becomes 
 broken down in the normal process of metabolism.' l 
 
 1 D'Arcy W. Thompson, Growth and Form, pp. 434 and 435. 
 
170 MICROSCOPIC ANATOMY OF THE TEETH 
 
 These observations perhaps afford a clearer insight into 
 the actual method by which calcium phosphate comes to 
 predominate in teeth and bone than any that have been 
 hitherto presented, and if fully confirmed will remove many 
 difficulties in the clear comprehension of the actual physical 
 and chemical processes in calcification. 
 
 A very interesting research has been recently undertaken by Mrs. E. Mel- 
 lanby, the first results of which are embodied in a paper published in the 
 Lancet on December 7, 1918. These experiments were undertaken to 
 throw additional light on the influence of diet on tooth formation, and 
 were carried out on the teeth of puppies. It would appear that the 
 incorporation of the proper amount of calcium salts with the teeth is 
 dependent not so much on the amount of calcium in the diet as on the 
 action of some determining factor in the food which is the cause of the 
 efficient allocation of calcium salts to the forming teeth. This determining 
 factor would appear to be, in the author's words, ' a diet containing an 
 abundance of those articles with which the fat soluble A accessory food 
 factor is associated, e.g. cod-liver oil, butter, &c.', allowing ' the develop- 
 ment in puppies of :.ound teeth '. A diet otherwise adequate, but deficient 
 in the substances with which fat soluble A is associated, brings about 
 defects in puppies' teeth. These effects are described as delay in the loss 
 of the deciduous and eruption of the permanent teeth, and defective 
 conditions of the enamel. The calcium may be present in the food in such 
 a form that it can be easily taken up by the blood, but if this accessory food 
 factor is absent, the cells of the formative organs of the teeth are unable 
 to perform their calcium-separating functions with success * owing to the 
 lack of one or more factors in the blood-stream which normally regulate 
 this function '. The further record of the results of these experiments will 
 be awaited with much interest. 
 
 Summary It is generally considered that the calcification of enamel 
 is brought about by a process of secretion from the cells 
 of the enamel organ, and not, as stated in former times, 
 by an actual conversion of the substance of the cell into 
 enamel. 
 
 The Tomes' processes of the ameloblasts, however, do 
 become incorporated in the formed enamel. 
 
 Blood-vessels are not usually seen in the enamel organ 
 of mammalia, but vestiges of these are found in the external 
 epithelium and in the stellate reticulum of some marsupials, 
 and are described by Leon Williams in the stratum inter- 
 medium in the later stages of calcification in the rat. In 
 the enamel organs of some fish, however, blood-vessels are 
 
DEVELOPMENT OF THE ENAMEL 171 
 
 regularly arranged within the enamel organ and extend from 
 the circumference to its free inner surface. 
 
 While the stellate reticulum probably serves as a store- 
 house of material for the commencement of calcification, 
 the cells of the stratum intermedium play, according to 
 many authors, the most important role in the selection of 
 material from the blood for the completion of the process. 
 
 The ameloblast cells which constitute the internal epithe- 
 lium of the enamel organ are the active agents in the elabora- 
 tion of the calcifying products. 
 
 They contain clear oval bodies which are considered to 
 consist of the colloidal material which is separated by them. 
 
 When calcification has commenced, a separating mem- 
 brane is seen between the stratum intermedium and the 
 internal epithelium and another between the cells of the 
 internal epithelium and the forming enamel. These are 
 called the external and internal ameloblastic membranes 
 respectively. 
 
 Each ameloblast is furnished with a process or prolonga- 
 tion of a fibrillar nature the Tomes' process. These pro- 
 cesses pass from the ends of the ameloblasts to the forming 
 enamel, and are incorporated in its substance, forming with 
 other transversely arranged fibres the organic network 
 or foundation substance of the enamel. This is most 
 evident in marsupial enamel, but the same structure exists 
 in other mammalian enamels, although it is much earlier 
 veiled by dense calcification. Decalcification experiments 
 confirm the existence of a fibrillar basis substance after the 
 removal of the lime salts in marsupial enamel. This organic 
 residue is derived from the ameloblast cells.. It is within the 
 substance of the longitudinal fibres 'or prolongations of the 
 Tomes' processes that the calcification of the enamel prisms 
 takes place. 
 
 The honeycomb is a fenestrated membrane-like substance 
 seen in the outer part of the forming enamel to which 
 the fibres of the Tomes' processes pass, and, according to 
 C. S. Tomes, the fibres enter the interspaces and are attached 
 to the septa. This honeycomb is considered by Tomes to 
 be identical with the fenestrated membrane raised from 
 the surface of forming enamel by acids ; it also sometimes 
 
172 MICROSCOPIC ANATOMY OF THE TEETH 
 
 becomes detached without the action of an acid. According 
 to the author's observations the fibres of the Tomes' pro- 
 cesses spread out in a fan shape at the honeycomb region, 
 portions of the fibrillar process intermingling with those 
 that surround them, and he considers that a rearrangement 
 of the fibres takes place in the honeycomb region, resulting 
 in the formation of the strands which extend in marsupials 
 from the honeycomb throughout the whole width of the 
 forming enamel to the dentine. 
 
 The forming enamel of marsupials and in a less degree 
 human enamel, when teased out in glycerine, shows that 
 the tissue is formed in distinct parallel laminae lying at right 
 angles to the surface. Longitudinal and transverse striae 
 are visible in the laminae, and large radial calcospherites 
 lie upon and between them. A regular deposit of small 
 calcified bodies of uniform size is also seen in the forming 
 enamel prisms. 
 
 The large bodies are concerned in the formation of the 
 interprismatic material, while the smaller ones build up the 
 columns of the prisms. The persistence of these large 
 calcospherites in the interprismatic substance is shown in 
 certain cases of caries and erosion in completed human 
 enamel. Many of these bodies, however, split asunder 
 and break down into small granules. 
 
 The clear bodies which are seen in the ameloblasts have 
 been described by several authors ; they are called by 
 Leon Williams the enamel globules, and he describes them 
 as ' passing down the ameloblasts and emerging from the 
 membrane beneath their inner ends ', the inner ameloblastic 
 membrane before described. On the enamel side of the 
 membrane these bodies, which are more or less regular in 
 size and shape, are arranged along the fibres of the forming 
 enamel and apparently become calcified as fine granules 
 which become fused into the small blocks which form the 
 calcified elements of the finished prisms. These blocks 
 and their constituent granules are seen in figs. 18 and 
 91. He speaks of these bodies being 'compressed into 
 disk-like shapes and sometimes partly or quite melted into 
 one another '. The interprismatic substance appears to 
 be formed independently of the prisms, and is produced 
 
DEVELOPMENT OF THE ENAMEL 173 
 
 within a substance also dialysed through the inner amelo- 
 blastic membrane. 
 
 Large irregular-sized bodies are formed in this substance 
 and by their fusion form the iriterprismatic material. As 
 shown by the present author, these bodies take the form of 
 large radial calcospherites such as are formed by carbonate 
 of lime in the artificial experiments, and they are seen to 
 be revealed in certain cases of caries and erosion. In fully 
 formed perfect mammalian enamel these elements become 
 so fused together and veiled by the dense deposit of the lime 
 salts that it is difficult or almost impossible to detect them 
 in many situations. 
 
 With regard to the origin of tubular enamel two funda- 
 mentally different opinions have been held. C. S. Tomes 
 considers that the tubes are entirely an enamel formation, 
 and are due to the calcification commencing at the septa 
 of the honeycomb and gradually closing up towards the 
 centre of what may be described as the cell of the honey- 
 comb ; in non-tubular enamels this closure is complete and 
 a solid enamel prism results ; in tubular enamels the calcifica- 
 tion does not reach the centre but remains open and uncalci- 
 fied as the tube. 
 
 The calcification of all mammalian enamel he would look 
 upon as a centripetal process, and he would deny the exis- 
 tence of a true interprismatic substance in finished enamel. 
 
 The other view is that held by Professor von Ebner and 
 the author, that the tubes of the enamel are not an enamel 
 organ product but are true dentinal tubes included in the 
 enamel. The arguments in favour of this view are given in 
 the chapter on tubular enamel so far as the finished tissue 
 is concerned. In developing enamel it is not considered 
 that there is any evidence of an open space in the honey- 
 comb, but the fibres of the Tomes' process enter, interlace, 
 and are rearranged in the honeycomb region, and the 
 deposit of the lime salts, as described by Leon Williams, 
 takes place within them as a regular deposit of calcified 
 granules arranged longitudinally, the interprismatic sub- 
 stance being separately formed and in marsupials remaining 
 in great part uncalcified for a considerable time. The spaces 
 described above between the forming enamel prisms at the 
 
174 MICROSCOPIC ANATOMY OF THE TEETH 
 
 dentine junction permit the penetration of the rapidly 
 growing dentinal fibril which eventually becomes closed 
 round by advancing calcification. 
 
 According to this view there is a distinct interprismatic 
 substance, and the process of calcification is a centrifugal one. 
 Tomes considers that the process of calcification in all mam- 
 malian enamels is made clear by the view he takes of the 
 process that tubular enamel only marks a stage in the forma- 
 tion of solid enamel prisms. The process is, however, quite 
 as clearly explained by the other view, for the solid enamels 
 would be due to the early and more complete calcification 
 of the interprismatic substance preventing the penetration 
 of the fibril from the dentine, and it appears to be more in 
 accordance with the present knowledge of the development 
 and structure of mammalian enamel. 
 
 In a recent paper (5 a) Thornton Carter has adopted 
 a new view of the nature of the tubes in marsupial enamel. 
 He considers that marsupial enamel is not tubular, but 
 that what is thought to be the tube is a colloidal product, and 
 apparently not, strictly speaking, of an organic nature, 
 and he does not consider that the Tomes' processes of the 
 ameloblasts are processes of the cell, and thus acknowledges 
 no fibrillar organic basis to enamel. He also considers 
 Leon Williams's view of the structure of the enamel prisms 
 to be erroneous. 
 
 Mr. Carter appears to consider the so-called enamel tube 
 to be a naked fibril within the septa of a honeycomb structure 
 arising ' from the coagulation or gel formation of an. organic 
 substance not usually present in the colloidal secretion shed 
 by the enamel cells of most other mammals '. He would thus 
 interpret the definite persistent structure seen in the enamel 
 of marsupials as part of the ' gel ' formation of a colloidal 
 material secreted by the ameloblasts, yet he speaks of an 
 organic substance present in this colloidal secretion which 
 he considers to be of a protein nature. One had always 
 considered that such colloidal product of secretory cells 
 contained the lime salts in suspension, and that calcification 
 took place within it. Analogy would lead us to expect 
 that the colloid would be effused into an organic basis 
 substance, and not that it would contain this invisible 
 
DEVELOPMENT OF THE ENAMEL 175 
 
 organic material in suspension. The persistence of part 
 of this colloid as a permanent * gel ' in the finished tissue 
 is not, it appears, consistent with what we know of the 
 physical processes involved in calcification. Uncalcified 
 substances in the finished tissues are usually protoplasmic, 
 and it is difficult, moreover, to see how the junction of this 
 ' gel ' substance, which he considers composes these fibrils, 
 is effected with the living protoplasmic dentinal fibrils. 
 The evidences of the actual presence of tubes in marsupial 
 enamel are, however, so complete that it is very difficult 
 to allow Mr. Carter's contention. 
 
 The passage of coloured fluids from the dentinal tubes 
 along channels in the enamel, and the fact that in dried 
 specimens these fluids run out again unless due precautions 
 are taken, seems conclusive evidence of their tubular nature. 
 In this method of staining applied to teeth in which the soft 
 parts have been fixed, the fluid does not run out, but is 
 retained by the tube contents which we have considered 
 to be of a protoplasmic nature. 
 
 Mr. Carter says there are no spaces between the prisms, 
 not even at the dentine junction, but figs. 96 and 97 scarcely 
 support this contention. The author denies the existence of 
 Tomes' processes of the ameloblasts and says, ' there is not 
 the slightest evidence of any cytoplasmic structures such as 
 Tomes and Mummery believe to be present ' ; and again, 
 ' in forming marsupial enamel there is no trace of the exis- 
 tence of such prolongations of the cytoplasm of the amelo- 
 blasts as those on which Tomes has based his theory of 
 enamel development '. 
 
 In his own investigations the present author has traced 
 the cytoplasmic threads of the ameloblasts distinctly into 
 these processes, and also can see them in numerous prepara- 
 tions Becoming incorporated in the forming enamel as 
 shown in figs. 86 a.nd 87. From these appearances it seems 
 impossible to accept Mr. Carter's explanation of them as 
 a colloidal deposit, and there is every evidence they are of 
 an organic nature and a portion of the ameloblast cell, and 
 that Mr. Tomes's view that they enter into the formation 
 of the prisms is correct. It would appear that these processes 
 being proved to be prolongations of the cell, the whole of 
 
176 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Mr. Carter's argument as to the structure of marsupial 
 enamel must fail, as it would be shown that the enamel is 
 laid down in a fibrillar organic basis substance and is not 
 merely a colloidal product. With regard to the deposition 
 of the lime salts, Mr. Carter considers Leon Williams's views 
 are erroneous, and describes a sponge- work formation with 
 large meshes in which what he considers to be the fibril is 
 
 FIG. 98. Macropus. Incorporation of Tomes' processes in 
 the forming enamel, (x 1,000.) 
 
 laid down and a further congelation of the colloid with 
 regular meshes in which the lime salts are deposited to form 
 the prisms, the regular meshes being as it were the moulds 
 in which the calcific matter is deposited. The illustrations 
 to this book we think give the strongest support to Leon 
 Williams's view. 1 
 
 Fig. 98 shows very clearly the incorporation of the 
 Tomes' processes in the forming enamel. 
 
 1 For a fuller discussion of this question see the author's reply to 
 Mr. Carter in the Proceedings of the Royal Society of Medicine, May 1918. 
 
DEVELOPMENT OF THE ENAMEL 177 
 
 Development and Calcification of the Enamel in Fish. 
 
 The first appearance of a true enamel organ in vertebrates 
 is seen in the development of the placoid scales or dermal 
 spines of the Sharks and Rays (fig. 99). 
 
 Hert wig's well-known researches on this subject demon- 
 strated that a distinct enamel organ and dentine germ is 
 concerned in the calcification of the placoid scale, and this 
 is clearly comprehensible in view of the fact before referred 
 to that teeth and dermal scales are similar structures, the one 
 merging into the other at the margin of the jaws. 
 
 FIG. 99. Vertical section through the skin of an embryo shark. C. Der- 
 mis ; c, c, c, d. layers of the dermis ; E. epidermis ; e. enamel organ ; 
 o. enamel layer ; p. papilla of the dermis. From Wiedersheim in Camb. 
 Nat. History. 
 
 These scales are developed in the mucous membrane 
 and have no developmental relation to the bones. The 
 teeth are produced in the same manner and their connexion 
 with the bones is purely secondary, so that the teeth 
 in all vertebrates may be accurately described as dermal 
 appendages. 
 
 'These external skeletal appendages are formed both 
 from the ectodermic cells and from the mesoderm. In 
 general histology the cells which secrete the hard structures 
 of both dentine and bone are termed scleroblasts, a term 
 which would include both odontoblasts and osteoblasts. 
 The scales of the bony fish (Teleostomi) and of the Dipnoi 
 (Lepidosiren, &c.) are derived solely from the dermis and 
 
178 MICROSCOPIC ANATOMY OF THE TEETH 
 
 possess no enamel cap, but the dermoid scales of the Elasmo- 
 branchii (Sharks and Rays) are composed of enamel 
 formed from the ectodermic epithelial cells, and of dentine 
 produced from the mesodermic scleroblasts. 1 The papilla 
 of the dermis forms a pulp cavity around which the dentine 
 is secreted, and the dentine is covered by a layer of true 
 enamel formed by ameloblasts. Beneath the dentine is 
 a basal plate of bone which is perforated for the passage of 
 blood-vessels to the pulp cavity. It is thus seen that placoid 
 scales and teeth are both structurally and developmentally 
 identical, but the true teeth of the sharks, which are developed 
 under the thecal fold of mucous membrane which covers 
 them, attain to a larger size and more complete differentia- 
 tion of the tissues composing them. 
 
 In the Plagiostome fish the nature of the outer layer of the 
 tooth has been the subject of much controversy, Rose and 
 several other authors considering it to be a form of dentine, 
 but Tomes, after a consideration of all the evidence, looks 
 upon it as enamel. 
 
 In the developing tooth of the sharks the cells of the 
 dentine papilla do not appear to be specialized as an odonto- 
 blast layer, but the surface of the mesoblastic dentine 
 papilla consists of a delicate fibrillar tissue, the fibres lying 
 parallel to the surface, and long processes of the underlying 
 cells, which rapidly increase in number, are continued 
 into this layer. As Tomes says, ' The great and most 
 striking peculiarity of these tooth germs lies in the fact 
 that the first apparent calcification of the true dentine, 
 whether it is a fine -tubed dentine as in Car char ias, or an 
 osteodentine as in Lamna, takes place, not at the outside of 
 the whole papilla as invariably happens in mammals, but 
 at the inner side of the specialised layer, thus cutting it off 
 from the rest of the pulp' (26 a). Beneath this specialized 
 layer the calcification of the dentine proceeds as in ordinary 
 tooth germs, and it is within this previously formed layer 
 that the calcification of the tissue considered to be enamel 
 takes place. This calcification is not, however, brought 
 
 1 It has been affirmed by KJaatsch that the scleroblasts are also of 
 epidermic origin, but this statement has not been generally received. 
 Klaatsch, M&rph. Jahrb., xvi, 1890, pp. 97 and 209. 
 
DEVELOPMENT OF THE ENAMEL 179 
 
 about by the cells of the dentine pulp, but takes place under 
 a distinct layer of large ameloblasts derived from the epi- 
 blastic epithelium of the mouth. It is seen that a deposit 
 of enamel takes place within the tissue which is previously 
 laid down by the mesoblastic dentine papilla, and we have 
 the apparent anomaly of a deposit of enamel by true enamel 
 cells within a matrix derived from the dentine papilla. 
 
 It was considered by Hertwig and Rose that these amelo- 
 blasts are concerned only in the production of a surface 
 membrane (Schmelzoberhdutchen) , but C. S. Tomes states that 
 ' no such membrane can be raised from the surface when 
 advanced in calcification'. Tomes, to whom our know- 
 ledge of the development of the enamel in Plagiostome fish 
 is chiefly due, sums up his important researches on the 
 subject as follows : 
 
 ' The outer hard layer which covers the teeth of Selachia 
 and Ganoids does not correspond exactly either with the 
 dentine or the enamel of mammalia. 
 
 ' In structure it ranges by slight gradations from the 
 simple and thoroughly enamel-like tissue met with in 
 the Rays to the complex tubular tissues ' in many sharks. 
 
 ' It is not a dentine, because it has no collagen matrix, 
 the organic tissue which it contains being easily soluble in 
 weak acids. 
 
 ' While this organic matrix is beyond question furnished 
 by the mesoblastic dentine papilla, the epiblastic amelo- 
 blasts over it are in a state of development which implies 
 that they take an active part, and that the tissue is a joint 
 production. 
 
 ' Though it is not fully demonstrable what that may be, 
 it upon the whole seems probable that they furnish it with 
 its calcifying salts. 
 
 ' Just as the entire teeth of Selachians present the problem 
 of tooth formation reduced to its simplest aspects, so this 
 layer appears to be the first introduction of enamel as 
 a separate issue, and therefore, to avoid multiplication of 
 terms, it may be appropriately called enamel.' 
 
 The structure of the enamel organ and development of in osseous 
 the enamel in osseous fish has only been thoroughly studied 
 in a few families, and much further research is necessary, 
 
 N 2 
 
180 MICROSCOPIC ANATOMY OF THE TEETH 
 
 especially in view of the very remarkable conditions which 
 have been found in many of those already examined. 
 
 An investigation of the development of the enamel in 
 the Gadidse (Cod family) was published by C. S. Tomes 
 in 1900 (266), and a special investigation of the conditions 
 in the Sparidse and Labridse (Sea-breams and Wrasses) was 
 published by the present author in 1917. 
 
 A very curious and anomalous condition is seen in the 
 enamel organ of the Cod family as described by Tomes. 
 In the early stages of development there are no marked 
 peculiarities of the enamel organ ; the odontoblast layer is 
 well developed, and there is a distinct external epithelium, 
 but no stratum intermedium or stellate reticulum separating 
 the two layers, and the enamel organ is enclosed within 
 a definite tooth-sac. In the next stage described, the 
 ameloblasts have entirely disappeared and their place is 
 taken by a delicate reticulate structure. 
 
 A considerably greater amount of forming enamel is laid 
 down than of dentine, in one measurement taken the width 
 of the dentine being 18 /^, while that of the enamel was 90 /m. 
 This, he states, ' is a reversal of the order of procedure 
 obtaining in mammals, in whom the dentine always ante- 
 dates the enamel considerably'. In marsupials, however, 
 a similar condition is seen, the width of the layer of form- 
 ing enamel far exceeding that of the dentine (see fig. 95). 
 C. S. Tomes also describes an appearance of fibrillation 
 within the delicate stroma this fibrillation being arranged 
 at right angles to the dentine and he considers that the 
 appearances are not very different from those which would 
 result from the approximation of thin- walled tubes and per- 
 haps some interstitial substance, a sort of honeycomb with 
 enormously elongated cells. He also states there are no cells 
 visible. Transverse sections are described as showing circular 
 clear spaces surrounded by a stained area in sections treated 
 with hsemalum. He describes abundant blood-vessels in the 
 walls of the tooth-sac, but states that they do not enter the 
 enamel organ. The same author points out that a peculiarity 
 in the development of the enamel in these fish is ' that 
 after the transformation of the ameloblasts into the stroma 
 there are no conspicuous cells to which can be assigned the 
 
DEVELOPMENT OP THE ENAMEL 181 
 
 function of separating out the lime salts, a function which is 
 apparently discharged by the ameloblasts in mammalian 
 tooth germs ... so that one is driven to the conclusion that, 
 the stroma once formed, it is able to draw into itself the 
 required lime salts and to deposit them at that point which 
 is furthest from the vessels '. 
 
 A paper by Mr. Thornton Carter on the development of 
 the enamel in the Hake has recently appeared. He states 
 that the cells of the external epithelium and the ameloblasts 
 ' remain in contact throughout the whole life history of the 
 enamel organ ', and describes the changes which take place 
 in the ameloblast cells. He denies the disappearance of 
 these cells and states that they do not disappear but become 
 elongated and vacuolated, and there is no evidence that the 
 ameloblasts become transformed into a stroma which 
 becomes incorporated in the enamel. For a consideration 
 of the evidences in favour of this view the reader is referred 
 to the original paper (56). 
 
 The present author has found large nucleated cells lying 
 in a stroma in the Haddock (Gadus ceglefinus), the whole 
 bearing a strong resemblance to a stellate reticulum, but not 
 being able to procure absolutely fresh material was unable 
 to carry out the investigation. The cell structure of the 
 enamel organ cannot be properly studied in imperfectly 
 fixed material, as the author found in his first attempts 
 to demonstrate the structure of the enamel organ of Sargus 
 ovis. Mr. Carter states that an interval of even five minutes 
 after death, before fixation, is sufficient to interfere with 
 the finer reactions of cells to stains, and specimens of fish 
 should be placed in the fixing solution as soon as caught. 
 
 Tomes confined his investigations chiefly to the enamel 
 organs of the Gadidse. He also examined the development 
 of the enamel in Sargus and Labrus, but was unable to 
 obtain satisfactory preparations owing to difficulties which 
 he considered ' almost insuperable ', the tooth germs being 
 so deeply placed in the bone that fixing agents did not 
 reach them. 
 
 It appeared to the author that if the jaw of a fresh 
 specimen of Sargus ovis was cut up at once and placed 
 in the fixing solution, this difficulty might be overcome, and 
 
182 MICROSCOPIC ANATOMY OF THE TEETH 
 
 through the kindness of Dr. M. Cryer of Philadelphia he 
 was able to obtain material prepared in this manner, which 
 gave very satisfactory results, and he ventures to think, 
 shed an entirely new light on the process of calcification 
 in certain fish. 
 
 From the preparations of Sargus germs which he did 
 obtain, Tomes considered that the process was in all essen- 
 tials the same as in the Gadidse. It is, however, very evident 
 from a study of the author's sections that the process is not 
 the same as described by Tomes in the Gadidse. 
 
 The dental germs of fish are so very delicate that the 
 ordinary treatment with alcohol and paraffin is quite inappli- 
 cable to them. 
 
 Specimens hardened in formol were decalcified in formic 
 acid, embedded in gum solution, and cut in the freezing 
 microtome. The use of alcohol at any stage was avoided, 
 and the specimens were mounted, after staining, in Farrant's 
 medium. Some specimens of Sargus made in the ordinary 
 manner and embedded in paraffin showed that in early 
 stages there is a distinct layer of ameloblasts and an external 
 epithelium as in the Gadidse, and that a layer of enamel had 
 been deposited under the influence of the ameloblasts. 
 
 Later germs showed the apparent disappearance of the 
 ameloblasts as in Gadidse, but although a stroma was seen 
 with parallel strise their structure was very indefinite, and 
 while there were indications of the presence of blood-vessels 
 in the enamel organ they could not be clearly distinguished. 
 The frozen sections, however, showed that the earlier ones 
 were badly shrunk from the treatment with heat and 
 alcohol, and gave a very different appearance. 
 
 In these sections there is a very clearly defined capsule 
 with abundant blood-vessels, and a structure is seen between 
 the capsule and the forming enamel which is arranged in 
 parallel lines at right angles to the surface, and does not 
 quite reach the forming enamel, from which it is separated 
 by a narrow band of delicate reticular tissue (fig. 100). 
 An examination of these parallel strise shows that they are 
 made up of blood-vessels in direct continuation with those of 
 the capsule, regularly alternating with an apparently tubular 
 substance with concave margins (figs. 101 and 103). Between 
 
DEVELOPMENT OF THE ENAMEL 183 
 
 FIG. 100. Enamel organ of Sargus ovis showing vascular channels 
 alternating with secreting tubes. b. Blood-vessels of capsule ; c. secreting 
 cells ; s. stroma ; e. position of decalcified enamel. 
 
 FIG. 101. Sargus ovis. Portion of fig. 100 more highly magnified 
 (upper portion). b. Blood-vessels of capsule ; c. secreting cells and tubes ; 
 b'. vascular tubes. 
 
184 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the blood-vessels and these tubular prolongations is a delicate 
 tissue continuous with the margins of the concavities, filling 
 up the space between the tubes and the blood-vessels, and 
 the alveoli so formed are occupied by delicate cells with 
 very distinct round nuclei. The blood-vessels, which arise 
 from those in the capsule, terminate in loops just short of 
 the inner margin of the enamel organ (fig. 102). The tubular 
 processes, on the other hand, pass into the stroma which is 
 in connexion with the forming enamel and become blended 
 
 FIG. 102. Sargus ovis. Lower portion of fig. 100. t. Termination of 
 vascular tubes ; s. opening of secreting tubes into stroma ; c. cellular 
 elements between the tubes. The delicate cellular portions shown in figs. 
 101 and 103 have been partially destroyed in cutting the section. ( x 250.) 
 
 with it, but at their outer extremities they do not reach the 
 capsule, their free ends lying between two of the blood-vessels 
 or vascular tubes, as we may more conveniently call them. 
 
 We thus see that we have here a structure in all respects 
 analogous to a secreting organ the cells between the 
 vascular and secreting tubes probably serving to separate 
 the lime salts from the circulating blood in the vascular 
 tubes, and they would be passed by the secreting tubes 
 to the interior of the enamel organ and the forming 
 enamel. 
 
DEVELOPMENT OF THE ENAMEL 185 
 
 While there is no evidence of the presence of blood- 
 vessels in the enamel organ of the Gadidae, in Sargus they 
 form a very important and regular portion of its structure. 
 In specimens where a portion of the enamel has escaped 
 decalcification it is seen that both systems of tubes stop 
 short of the forming enamel, a narrow interval separating 
 them from it. This interval is occupied by a delicate 
 reticulum filled with fine granules and larger scattered bodies 
 which appear to be calcospherites deposited in this reticular 
 structure. The tubular structure of the finished enamel of 
 
 FIG. 103. A portion of the enamel organ of Sargus ovis, showing the 
 relations of the tubular organ with the vascular tubes and the cells with 
 their nuclei which occupy the spaces between them. Abbe prism drawing. 
 ( x 290.) c. Capsule with blood-vessels ; t. secreting tubes and cell nuclei ; 
 b. blood-vessel tubes. 
 
 Sargus is not due to the actual passage of these tubular 
 structures into the forming enamel (at all events until 
 calcification reaches the inner circumference of the capsule), 
 for they do not reach it, and the pattern of the enamel is 
 laid down by the delicate organic stroma in which calcifica- 
 tion takes place, the vascular and secreting tubes and cells 
 taking the part in this calcifying process which is fulfilled 
 by the ameloblasts in Mammalia. 
 
 Tomes, in his paper, speaks of the apparent anomaly 
 in the Gadidse that the lime salts are separated from the 
 blood at a distance from the place where they are deposited. 
 This certainly does not appear to be the case in the Sparidae, 
 for the materials for calcification can be elaborated within 
 the enamel organ itself, the blood-vessels and secreting cells 
 
186 MICROSCOPIC ANATOMY OF THE TEETH 
 
 being in the immediate neighbourhood of the forming 
 enamel. Similar conditions exist in the Labridse, as will be 
 presently explained. If, however, in the Gadidse the presence 
 of cells in the enamel organ is substantiated, this anomaly 
 would cease to exist in them also, although the absence of 
 blood-vessels in this family would not bring about quite 
 the same conditions, and the deposit of the lime salts would 
 be probably due to a process of dialysis as in mammalian 
 tooth germs. 
 
 It was shown that in Sargus ovis the portion of enamel 
 next to the dentine is not traversed by any enamel tubes 
 but consists of interlaced enamel prisms. In Sargus noct 
 and 8. vulgaris it is traversed by tubes (figs. 43 and 44), 
 but these are derived from the dentine. Tomes also points 
 out that the inner layer of enamel in the Gadidse is not 
 tubular. It would appear that this is the portion of the 
 enamel laid down by the ameloblasts, the tubular portion 
 of the enamel both in Gadus and Sargus being deposited 
 by the structure which arises after the disappearance of the 
 ameloblasts. Further investigation is desirable to ascertain 
 if this secreting structure is confined to those enamels 
 which are traversed by tubes from without, as we know 
 that in many osseous fish the whole enamel appears to be 
 deposited by ameloblasts as in Mammalia. 
 
 As pointed out in a former chapter, a thin pellicle of the 
 enamel organ remains attached to the surface of the enamel 
 when it is apparently completed and just about to erupt. 
 It appears as if, at this late stage, when the calcification has 
 reached the inner surface of the capsule, the enamel is in 
 direct contact with the blood-vessels at the margin, for nuclei 
 of blood-vessels can be sometimes detected in the tubes, 
 which at this stage are widely open and very incompletely 
 calcified (fig. 46). 
 
 In studying the development of the tubular enamel of 
 the Labridse or Wrasses, the following examples were taken : 
 Tautoga onitis, the Black fish of the Atlantic coast of 
 America ; and two Japanese fish, Halichoeres pooecilopterus 
 and Pseudolabrus Japonicus. 
 
 The enamel organs of these fish also show a similar 
 secreting structure to that of Sargus, but there are some 
 
DEVELOPMENT OF THE ENAMEL 187 
 
 marked differences of arrangement in the different species 
 examined. 
 
 As in Sargus, the enamel organ is penetrated by a regular 
 system of vascular tubes which are mostly arranged in 
 a radiating manner more or less at right angles to the surface 
 (fig. 104). At the sides of the enamel organ, however, many 
 of these tubes are arranged more or less horizontally. As 
 shown in fig. 105, the blood-vessels are enclosed in a very 
 distinct sheath, and these can be teased out and separated 
 from the surrounding tissue. The separation of the masses 
 
 FIG. 104. Tooth germ of Tautoga (Labridae). 6. The vascular tubes 
 in the enamel organ ; e. points to detached uncalcified enamel ; d. dentine 
 showing section of the flange on which the enamel rests ; p. pulp. ( X 50.) 
 
 of blood corpuscles would appear to be partially due to 
 clotting, and also to the fact that the sheath appears to be 
 continued in a transverse direction across the enamel organ. 
 When a true vertical section is procured it is seen that the 
 vascular tubes are surrounded by cellular elements arranged 
 exactly like a simple tubular gland (figs. 106 and 107), and that 
 within the centres of these gland-like structures channels are 
 visible filled with granules, these channels becoming approxi- 
 mated below and passing into a similar delicate stroma to 
 that seen inSargus intervening between the enamel organ and 
 the forming enamel. This apparatus would apparently serve 
 the same purpose as the somewhat identical arrangement 
 in the enamel organ of Sargus, conveying the lime salts 
 elaborated in these gland-like structures from the blood- 
 
188 MICROSCOPIC ANATOMY OF THE TEETH 
 
 vessels to the calcifying stroma. At this stage there is 
 no vestige of an ameloblast layer or of an external epithelium, 
 the outer margins of these bodies being only separated 
 from the connective tissue of the capsule by a delicate 
 basement membrane. In the earliest germs of Tautoga, 
 as in Sargus and the Gadidse, an ameloblast layer and an 
 external epithelium are present, but very soon a remarkable 
 change takes place in the enamel organ. 
 
 At the time when a narrow layer of enamel is laid down 
 
 FIG. 105. The vascular tubes in Tautoga teased out. ( x 500. \ 
 
 and no dentine is formed, and no differentiation of any 
 odontoblast layer in the pulp, a mass of epithelial tissue is 
 seen to have penetrated the capsule from without and to 
 be enveloping the enamel organ very much as the first 
 epithelial inflection envelops the dentine papilla in mamma- 
 lian tooth germs. The external epithelium together with 
 the ameloblast layer soon disappears, and the whole of 
 the circumference of the enamel organ is occupied by 
 this gland-like tissue and the penetrating blood-vessels. 
 
 At this stage, then, we have an enamel organ made up 
 of blood-vessels or vascular tubes, gland-like tissue, and 
 a granular stroma intervening between this tissue and the 
 
DEVELOPMENT OF THE ENAMEL 
 
 189 
 
 
 FIG. 106. The enamel organ of Tautoga onitis (Labridae). The tubular 
 gland-like bodies with their ducts, g. Glands ; 6. blood-vessel tubes ; 
 e. position of forming enamel. ( x350.) 
 
 FIG. 107. As fig. 106 under higher magnification, g. Tubular gland-like 
 structures ; I. lumen ; 6. blood-vessel tubes. ( x 650.) 
 
190 MICROSCOPIC ANATOMY OF THE TEETH 
 
 calcifying enamel, and within this stroma lie numerous 
 irregularly-shaped bodies which would appear to be the first 
 deposit of the lime salts in the form of calcospherites. 
 In Tautoga it appears that not only is the enamel in advance 
 of the dentine in the calcifying process, but a considerable 
 amount of enamel is laid down before any dentine is formed. 
 The line of demarcation between the enamel and the dentine 
 is very evident in the Labridse, as all around the first-formed 
 dentine is a projection which forms the groove into which 
 the enamel fits, as in the Gadidse. The tubular enamel 
 cannot therefore be mistaken for dentine. From the above 
 description it is seen that the glandular tissue has not been 
 produced by any invagination of the epithelial cells of the 
 early enamel organ, as might perhaps have been assumed, 
 but it invades the enamel organ from without. In sections 
 which have been cut in such a manner as to include the 
 mucous membrane of the mouth in the same vertical plane, 
 the connexion of this tissue in the enamel organ with the 
 layers of gland substance immediately beneath the mucous 
 glands on the surface of either the mouth or pharynx is very 
 clearly shown, and many sections demonstrate that this 
 connexion is maintained during the whole period of the 
 deposition of the enamel when this structure has once 
 appeared. 
 
 In the two other species of Labrus examined there are 
 certain modifications of structure which although differing 
 in arrangement would serve the same purpose. 
 
 In Halichceres and in Pseudolabrus no connexion with 
 the glandular tissue of the mucous membrane can be 
 detected as in Tautoga, but the secreting tubes, as we have 
 called them, are much convoluted and alternate with the 
 vascular tubes as in Sargus. In Halichmres the blood-vessels 
 have more the appearance of very thin-walled sinuses, which 
 are of very large proportions compared with the tubes (fig. 108 ). 
 The secreting tubes form loops around the sinuses, and from 
 their lower margins bundles of minute processes extend to 
 the stroma, in which masses of calcospherites are also visible. 
 Fig. 109, from a specimen in which the enamel was retained 
 in situ, shows this arrangement very completely. 
 
 In both Halichceres and Pseudolabrus cellular bodies are 
 
DEVELOPMENT OF THE ENAMEL 
 
 191 
 
 FIG. 108. Part of enamel organ of Halichceres (Labridae) showing secret- 
 ing tubes and blood channels, t. Tubes ; b. blood-vessels. ( x 500. ) 
 
 FIG. 109. Portions of calcified enamel in stroma of enamel organ of 
 a Wrasse (Halichceres pocecilopterus). e. Enamel. ( x 150.) 
 
192 MICROSCOPIC ANATOMY OF THE TEETH 
 
 seen within the tubes ; these take stains deeply and would, 
 appear to be the elements which separate the lime salts 
 from the blood in the vascular tubes and sinuses, and take 
 the place of the more definite glandular structures seen in 
 Tautoga. 
 
 For further details on the subject the reader may be 
 referred to the original paper (16 d). 
 
 In the paper before alluded to (56) Mr. Thornton Carter refers to the 
 author's description of the enamel organs of Sargus and Labrus, and says 
 his own material lends no support to these views. He did not examine 
 the species described by the author, and however anomalous it may 
 appear there are apparently considerable differences in the mode of 
 development of the enamel within the same family of fishes. There 
 can, we venture to say, be no doubt about the appearances in the enamel 
 organs of Tautoga in Sargus ovis and in the two Japanese fish ; while in 
 another fish belonging to the Labridse, the freshly-preserved head of which 
 was sent him from America, the author could find no evidence of any such 
 change in the enamel organ, which showed, in all the germs examined, 
 the external epithelium and ameloblasts and no penetration by blood- 
 vessels. The whole fish not having been received, it was not possible to 
 identify the species. Mr. Carter says that in Pagellus centrodontus, one of 
 the Sparidse, and in Labrus bergylta, which he examined, although he found 
 the vascular canals in the enamel organ described by the author, he did not 
 find the tubes or gland-like bodies, but persistent ameloblasts. Mr. Carter 
 says the author's figures show no cytological details, but a reference to 
 the photographs will show the presence of cells, although these have no 
 resemblance to ameloblasts or the cells of the external epithelium, but are 
 arranged as are the cells of a secreting gland in definite relations to the 
 channel or lumen which they surround. Their minute round nuclei are 
 also characteristic of glandular cells (see figs. 101 and 103). 
 
 It is scarcely necessary to repeat that the specimens of Tautoga and 
 Sargus were freshly preserved, and the appearances are not due to imper- 
 fect preservation. Whether these cells which surround the central lumen 
 are derived from ameloblasts may be a doubtful point, but in the very 
 large series of slides prepared by the author there was no indication of 
 this, neither was there any trace of the cells of the external epithelium, 
 as the photographs plainly show. Mr. Carter suggests that these views 
 are based on those of C. S. Tomes with respect to the Gadidse, but the 
 observations above recorded were quite independent of these and followed 
 on an attempt to trace the process in Sargus with freshly fixed material, 
 as Tomes considered this to be similar to what he described in the Gadidse. 
 The author was enabled to show that the enamel organ in both the Sparidse 
 and Labridse is penetrated by blood-vessels ; this penetration, as shown 
 both by Mr. Carter and by C. S. Tomes, does not occur in the Hake. 
 To clear up this question much further investigation is required, as the 
 
DEVELOPMENT OF THE ENAMEL 193 
 
 development of the enamel in osseous fish has been but very imperfectly 
 studied, and it is only by examining a large series of specimens of the 
 various families that any systematically arranged conclusions can be 
 arrived at ; but that very remarkable variations in the structure and 
 arrangement of the enamel organ do exist we appear to have very complete 
 evidence. The remarkable fact that the circulating blood is brought 
 into intimate relations with the cells of the enamel organ within its sub- 
 stance certainly points to a mode of development of the enamel which seems 
 to have no parallel in other vertebrates. 
 
 In the Elasmobranchii, according to Tomes, both the Summary, 
 epiblastic epithelial cells and those of the mesodermic 
 dentine papilla take part in the formation of the enamel, 
 the enamel being calcified by the ameloblast cells in a matrix 
 which is furnished by the dentine papilla. In the osseous 
 fish the investigation of only a few families has been 
 thoroughly carried out, but while in many the enamel organ 
 is both in structure and function similar to that of the 
 higher vertebrata, in those which possess a more or less 
 tubular enamel a very different structure of the enamel 
 organ is found. Although at first, as in the Gadidae, in 
 the Sparidae and Labridae the enamel organ is composed of 
 a true layer of ameloblasts and an external epithelium, only 
 a very narrow zone of enamel is laid down by the amelo- 
 blasts, which soon disappear, a delicate stroma taking their 
 place, and within this stroma is differentiated an enamel 
 organ of an altogether different structure, consisting in the 
 case of Sargus and Labrus of an apparently definite secreting 
 apparatus, the lime salts being elaborated by cells which 
 are arranged between and around vascular tubes which 
 penetrate the enamel organ from the capsule. The calcifying 
 substance is conveyed by special channels to the stroma, 
 which occupies the interval between this structure and the 
 forming enamel and in which calcification takes place. In 
 Tautoga the secreting structure takes the form of tubular gland- 
 like structures regularly disposed within the enamel organ. 
 
 In the Gadidae no such definite tubular structure has been 
 described, but C. S. Tomes was the first to draw attention 
 to these changes in the enamel organ. No blood-vessels 
 are present in the enamel organ of the Gadidae, and according 
 to Tomes there are no cells to which can be assigned the 
 function of separating the lime salts. 
 
194 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Further investigation is necessary to determine if this 
 conversion of the first-formed enamel organ into a secretory 
 structure is peculiar to the enamels which contain tubes 
 from the outer surface, but the fact that the first-formed 
 layer of enamel in all the fish examined contains no tubes 
 and is laid down under the influence of normal ameloblasts. 
 would tend to indicate that the various modifications of 
 this secretory structure are peculiar to those forms which 
 possess tubular enamel. 
 
 From the evolutionary standpoint the method of calcifica- 
 tion above described presents a curious problem. In 
 Elasmobranchs we see the first evidence of the formation 
 of enamel, which takes place by the agency of true amelo- 
 blasts, arranged as a well-defined internal epithelium of the 
 enamel organ as in mammalia, and it is also present in the 
 placoid scales of the skin of Elasmobranchs. While this mode 
 of calcification of the enamel appears to be exactly similar 
 to that of the higher vertebrata, in many Sparidse and 
 Labridse, although the deposit of enamel begins in this 
 manner, only a very small amount of the tissue is deposited 
 under these conditions, and the secreting structure before 
 described takes up the functions of the ameloblasts. 
 
 The attention of the author was drawn, by J. T. Carter 
 to a paper by N. R. Harrington (11), in which he describes 
 the microscopic structure of the lime- secreting glands of the 
 earthworm (Lumbricus). In these glands there is a regular 
 arrangement of sinuses or vascular tubes with secreting 
 cells between them, but there are no separate definite 
 channels for the conveyance of the secretion as in the fish, 
 but it passes back between the laminae to be discharged into 
 the oesophagus. The author has been unable to trace any 
 other structure of a similar nature adapted to this purpose, 
 either among the vertebrata or in vertebrata. 
 
 A comparison might also be made with one of the ductless 
 glands as far as structure is concerned, although it is not 
 engaged in the secretion of lime salts. Dahlgren and 
 Kepner (7) figure a section through the infundibular gland 
 of the flounder (Pseudopleuronectes americanus) . This shows 
 that the secreting cells are arranged along sinusoids filled 
 with blood ; each sac -like invagination of the secreting cells is 
 
DEVELOPMENT OF THE ENAMEL 195 
 
 interposed between, two sinusoids, and the secreted substance 
 is discharged into the lumen between the rows of secreting 
 cells. We thus see among the ductless glands a similar 
 arrangement to that found in the fish above described, 
 although this secretes some substance, the nature of which 
 is not known, into the brain cavity fluids. 
 
 REFERENCES 
 
 1. Andrews, R. (a) ' Formation of Enamel.' Intern. Dental Journal, 1891. 
 (b) Trans. Columbrian Dent. Congress, 1894. 
 
 2. Arnell. ' Zur Kenntniss der zahnbildenden Gewebe.' Retzius' Bio- 
 
 logisch. Untersuchungen, II., 1882. 
 
 3. Bowerbank. ' The Structure of the Shells of Molluscous and Conchi- 
 
 ferous Animals.' Trans. Microscop. Soc., 1869, i. 123. 
 
 4. Carpenter, W. B. (a) ' On the Microscopic Structure of Shells.' Brit. 
 
 Assoc. Eept. (1844-7), 1844, pp. 12 ff. ; 1847, pp. 93-134. 
 (6) The Microscope, 1875. 
 
 5. Carter, Thornton, (a) ' The Cytomorphosis of the Marsupial Enamel 
 
 Organ,' &c. Phil. Trans. Roy. Soc., Ser. B, ccviii. 271-305. 
 (b) ' On the Cytomorphosis of the Enamel Organ in the Hake.' Quar. 
 Journ. Micr. Sci., vol. Ixiii, pt. 3, Dec. 1918, pp. 387-400. 
 
 6. Caush, D. ' Is there Uncalcified Tissue in the Enamel ? ' Dental 
 
 Cosmos, Feb. 1905, xlvii. 
 
 7. Dahlgren and Kepner, W. A. A Text-book of the Principles of Animal 
 
 Histology, 1908, p. 40. 
 
 8. v. Ebner, V. ' Strittige Fragen liber den Bau des Zahnschmelzes.' 
 
 Sitzungsber. d. k. Akad., Wien, 1890, Bd. xcix, pp. 57-104. 
 
 9. Eve, F. ' On Cystic and Encysted Solid Tumours of the Jaw, with 
 
 Observations on the Structure of the Enamel Organ.' Trans. 
 Odontol. Soc. Great Brit., 1885, xviii. 39-61. 
 
 10. Graham. Researches, ed. Angus Smith, 1877, also Phil. Trans. Roy. 
 
 Soc., 1850 and 1861, cli. 183. 
 
 11. Harrington, N. R. ' The Calciferous Glands of the Earthworm/ 
 
 Journ. of Morphology, SuppL, vol. xv (1889), p. 106. 
 
 12. Harting, P. Sur la production artificielle de quelques formations 
 
 calcaires organiques. Amsterdam, 1872. 
 
 13. Hertwig, O. (a) ' Ueber den Bau und die Entwickelung der Placoid- 
 
 schuppenund Zahne.' Jenaische Zeitschr.f. Naturw., 1874, Bd. viii, 
 pp. 331-404. 
 
 (b) ' Ueber das Zahnsystem der Amphibien u. seine Bedeutung fur die 
 
 Genese des Skeletts der Mundhohle.' Archivfur Micr. Anat., 187 1, 
 Bd. xi, Suppl., pp. 1-208. 
 
 (c) Lehrbuch der Entwickelungsgeschichte. Jena, 1888. 
 
 (d) ' Ueber das Hautskelett der Fische.' Morphologische Jahrb., 1876, 
 Bd. ii, pp. 328-95 ; Bd. v, pp. 1-21 (part ii) ; Bd. vii, pp. 1-41 
 (part iii). 
 
 O 2 
 
196 MICROSCOPIC ANATOMY OF THE TEETH 
 
 14. Herrissant. ' Nouvelles recherches sur la formation des dents et sur 
 
 celle des gencives.' Hist, de VAcad. R. des Sciences, Paris, 1754 
 memoires, p. 433. 
 
 15. Leduc, S. The Mechanism of Life, 1911, p. 45. 
 
 16. Mummery, J. H. (a) ' On the Process of Calcification in Enamel 
 
 and Dentine.' Phil. Trans. Roy. Soc., Ser. B, ccv. 95-113. 
 
 (b) ' Comparative Studies in Calcification.' Proc. Roy. Soc. of Med., 
 
 1915, vol. ix (Odontological Section), pp. 6-31. 
 
 (c) ' On the Nature of the Tubes in Marsupial Enamel and its Bearing 
 
 on Enamel Development.' Phil. Trans. Roy. Soc., Ser. B, ccv. 
 295-313, 1914. 
 
 (d) ' On the Structure and Development of the Tubular Enamel of the 
 
 Sparidse and Labridae.' Phil. Trans. Roy. Soc., Ser. B, ccviii. 251-69. 
 
 17. Paul, F. ' Some Points of Interest in Dental Histology: The Enamel 
 
 Organ.' Dental Record, 1896, xvi. 493-517. 
 
 18. Poulton,E.B. ' True Teeth and the Horny Plates of Omithorhynchus.' 
 
 Quar. Journ. Micr. Sci., 1888, vol. xxix, N.S., pp. 9-48. 
 
 19. Philip, J. C. Physical Chemistry : its Bearing on Biology and Medicine. 
 
 London, 1910. 
 
 20. Pauli, W., and Samec, M. ' Ueber Loslichkeitsbeeinflussung von Elek- 
 
 trolyten durch Eiweisskorper.' Biochem. Zeitschr., xvii. 235, 1910. 
 
 21. Rose, C. ' Contributions to the Histogeny and Histology of Bony and 
 
 Dental Tissues.' Dental Cosmos, Nov. and Dec. 1893. 
 
 22. Rainey, G. The Mode of Formation of Shells of Animals, d-c., by a 
 
 Process of Molecular Coalescence. London, 1838. 
 
 23. Spee, Graf v. ' Ueber die ersten Vorgange der Ablagerung des Zahn- 
 
 schmelzes.' Anat. Anzeig., 1887. 
 
 24. Tims, H. W. Marett, and Hopewell Smith, A. ' Tooth-germs in the 
 
 Wallaby, Macropus billiardieri.' Proc. Zool. Soc., London, 1911, 
 pt. iv, pp. 926-42. 
 
 25. Thompson, D'Arcy W. Growth and Form. Camb., 1917. 
 
 26. Tomes, C. S. (a) ' Upon the Structure and Development of the Enamel of 
 
 Elasmobranch Fishes.' Phil. Trans. Roy. Soc., Ser. B, cxc. 443 (1898), 
 
 (b) ' Upon the Development of the Enamel in certain Osseous Fish.' 
 Phil Trans. Roy. Soc., Ser. B, cxciii. 42 (1900). 
 
 (c) Dental Anatomy, 7th ed. 
 
 (d) ' On the Development of Marsupial and other Tubular Enamels, &c.' 
 Phil. Trans. Roy. Sos., 1898, clxxxix. 107-22. 
 
 27. Traube. ' Reichert's and Du Bois Raymond's Archives (1867). 
 
 Gesammelte Abhandlungen, Berlin, 1899, p. 213. 
 
 28. Underwood, A. (a) Journ. Brit. Dental Assoc., xix. 470. 
 
 (b) Underwood and Wellings. Trans. Int. Med. Congress, 1913, Sec- 
 tion of Stomatology. 
 
 29. Woodhead, G. Sims. ' On Inflammation in Bone.' Trans. Odontol. 
 
 Soc. Great Brit., 1892-3, xxv. 30-53. 
 
 30. Williams, Leon. ' On the Formation and Structure of Dental Enamel.' 
 
 Denial Cosmos, 1896. 
 
CHAPTER IV 
 THE DENTAL PULP 
 
 DEPARTING from the more usual method of treating the 
 subject, it may be better to consider the formation and 
 structure of the pulp before proceeding to a description 
 of the dentine, as it is the formative organ of the dentine 
 and in intimate relation with it in the completed tooth. 
 The dental pulp does not disappear as a complete organ 
 during the functional life of the tooth, as does the enamel 
 organ after the completion of the enamel. The pulp, with 
 its cells, nerves, and blood-vessels, maintains the nutrition of 
 the dentine during the whole period of its functional activity. 
 
 The main supporting structure of the pulp is the connective Con 
 tissue. This is of the myxomatous or gelatinous variety, 
 similar to the Wharton's jelly of the umbilical cord, although 
 also containing ordinary fibrous connective tissue. 
 
 As stated by Schafer (SO), the manner in which these 
 connective-tissue fibres arise is by no means clear, and two 
 distinct and opposed views are held by histologists upon the 
 subject. 
 
 The pulp of the tooth in early stages of its development 
 is chiefly made up of embryonic mesodermic cells, and 
 those which give rise to connective tissue are known by the 
 name of mesenchyme cells (Hertwig). These cells possess 
 very large nuclei and are surrounded by a very small amount 
 of cytoplasm. They are furnished with many protoplasmic 
 prolongations or branches, and lie in an albuminoid or 
 mucoid ground substance of a jelly-like consistency. 
 
 According to some histologists, as Waldeyer (33) and 
 Fleming (7), the fibres are processes of the cells, but it is 
 considered by Kolliker, Ranvier (25), and others, that the 
 fibres arise within the intercellular substance. F. Mall (19) 
 and Haidenhain (9) hold that the ground substance is as 
 much a portion of the cell network or ' syncytium ' as the 
 substance of the cell itself, and that the fibres arise in this, 
 which they term the exoplasm, distinguishing it from the 
 
198 MICROSCOPIC ANATOMY OF THE TEETH 
 
 endoplasm or cytoplasm of the cell. Apart from the 
 processes of the cell, there would appear to be a distinct 
 development of fibres within the exoplasm, as seen in the 
 so-called Wharton's jelly of the umbilical cord, which, as 
 previously stated, has a strong histological resemblance to 
 the tissue of the developing tooth-pulp. 
 
 That these large cells of the tooth- pulp have long inter- 
 communicating processes is very evident in preparations 
 of actively growing pulps (figs. 110 and 111). These photo- 
 
 FIG. 110. Human tooth-pulp. Open-ended premolar (pyridin silvei). 
 Large connective tissue cells and processes. ( x 450. ) 
 
 graphs were taken from an erupted human premolar in 
 which the root portion was still in an early stage of develop- 
 ment. The preparations were stained by the pyridin silver 
 method, and the pulp is seen to be crowded with large 
 cells of very various sizes and shapes, which are provided 
 with long connecting processes interlacing in the pulp. 
 Their nuclei are very conspicuous and the fibrillation is seen 
 to pass across the interior of the cell. This is very evident 
 in the part of the preparation from which fig. 110 was taken, 
 but the strong yellow colour of the section prevents this 
 fibrillation being distinctly brought out without over- 
 exposure of the rest of the image. There is evidence in these 
 
THE DENTAL PULP 
 
 199 
 
 3 
 
 f 
 i 
 
 ' / I 
 
 :-' - 1 
 
 ^w-^/P 
 
 35J 
 
 
 ' 
 
 FIG. 111. Similar to fig. 110. ( x 450.) 
 
 ,-4 
 
 FIG. 112. Human tooth-pulp at forming root. Cajal 
 silver nitrate. (x!50.) 
 
200 MICROSCOPIC ANATOMY OF THE TEETH 
 
 preparations of a finer fibrillation between the larger cells 
 and their processes, which may perhaps be the development 
 of fibres within the exoplasm as described by Mall. If this 
 be the case it seems possible that the connective -tissue fibres 
 of the pulp are formed in both ways by prolongation from 
 the cell protoplasm or endoplasm and within the ground 
 substance or exoplasm. Fig. 112, however, from a similar 
 
 z 
 
 
 FIG. 113. Fibres of Von Korff in tooth germ of Cat. o. Odontoblasts ; 
 /. fan-like expansion of fibre bundles ; g. corkscrew -like fibres ; z, 
 odontogenic zone ; c. commencing calcification ; a. ameloblasts. 
 (From illustration to his paper.) 
 
 pulp prepared by Ramon y Cajal's silver nitrate process, 
 would seem to indicate that the fibres in this early stage are 
 all processes of cells, and the finer fibrillation seen in figs. 110 
 and 111 appears to arise from minute cells. In the dentine 
 papilla in the early stages of the developing tooth germ in 
 the embryo, bundles of connective tissue have been described 
 by Von Korff (fig. 113), and the delicate connective- tissue 
 fibres which enter into the formation of the dentine at all 
 
THE DENTAL PULP 
 
 201 
 
 FIG. 114. Macropus. Pulp in early stage before commencement of 
 calcification. No differentiation of odontoblasts. s. Stratum intermedium 
 of enamel organ ; a. ameloblasts ; p. pulp tissue. ( x 400. ) 
 
 
 
 
 p 
 
 
 
 FIG. 115. Rounded cells at the margin of the pulp. a. Ameloblasts ; 
 p. pulp. (x500.) 
 
202 MICROSCOPIC ANATOMY OF THE TEETH 
 
 stages of its development and constitute the foundation 
 fibres of its matrix, are in many cases seen to arise from 
 cells, as will be described later. Before the commencement 
 of calcification there is no distinct differentiation of a peri- 
 pheral layer of cells so characteristic of the functionally 
 active pulp, but rounded cells are soon seen accumulating 
 at the periphery (figs. 114 and 115) with large nuclei and 
 a very small amount of cytoplasm, so that the cells appear 
 to consist almost wholly of nuclei. 
 
 FIG. 116. Macropus. Rounded cells forming a definite layer at the 
 margin, p. Pulp ; o. first indication of odontoblast layer ; a. ameloblasts ; 
 s. stratum intermedium ; r. stellate reticulum. ( x 150.) 
 
 In fig. 116 short processes can be detected prolonged from 
 the cytoplasm of the cells, but no definite dentinal fibril has 
 appeared. In a later stage, shown in fig. 117, the outer cells 
 form a definite layer, and their prolongations extend as 
 the dentinal fibril traversing the first deposited colloidal 
 substance in which calcification later takes place. It is 
 seen in this illustration that the dentinal fibril is a broad 
 expansion of the cell substance and not the narrow process 
 often shown in shrunken preparations. 
 
 The early stages of development of the cells of the human 
 
THE DENTAL PULP 
 
 203 
 
 FIG. 117. Differentiation of odontoblasts. No calcification commenced. 
 o. Odontoblasts ; od. odontogenic zone : a. ameloblasts. (x500.) 
 
 FIG. 118. Developing cells of pulp. c. Developing 
 capillary vessel. ( x 500.) 
 
204 MICROSCOPIC ANATOMY OF THE TEETH 
 
 pulp are well described and figured by Dr. Paul in a paper 
 contributed to the Odontological Society in 1899 (23). 
 
 The development of the cells of the dental pulp is further 
 shown in figs. 118 and 119. 
 
 The rounded cells are thus seen to become differentiated 
 into a definite layer of more or less cylindrical cells, the 
 odontoblasts, which surround the pulp and form the mem- 
 brana eboris of Waldeyer. These cells are larger and more 
 fully developed at the coronal portion of the tooth-pulp, and 
 smaller and less conspicuous in the fully formed root. 
 
 FIG. 119. Large irregularly-shaped cells at pulp margin, a. Ameloblasts ; 
 p. pulp; 5. stratum intermedium. (x500.) 
 
 Each odontoblast has a large round or oval nucleus and 
 the cell protoplasm, or cytoplasm, is finely granular. On 
 its dentine aspect it is somewhat flattened, and the dentinal 
 fibril, which is a continuation of its cytoplasm, slightly 
 tapers to its entrance into the dentinal tube (figs. 120 
 and 121). It has been described as not possessing a cell 
 wall, but this is highly improbable, and we must assume 
 tha/b the cell wall is of such extreme tenuity that it is not 
 observable in ordinary preparations. As, however, it has 
 beon shown by Hanazawa (10) that an outer layer to the 
 dentinal fibril is distinctly stainable by hsematoxylin within 
 
THE DENTAL PULP 
 
 205 
 
 FIG. 120. Section of tooth of Macropus cut without decalcification. 
 d. Calcified dentine ; o. odontogenic zone ; od. odontoblasts ; p. pulp. 
 (x450.) 
 
 od- 
 
 FIG. 121. Large odontoblasts. Macropus. Section cut without decalcifica- 
 tion. od. Odontoblasts ; o. odontogenic zone ; d. dentine. (x650.) 
 
206 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the dentinal tube, it appears that this must necessarily be 
 an extension of the cell membrane of the odontoblast. 
 
 Each odontoblast is furnished with this protoplasmic 
 prolongation into the dentinal tube and with a delicate 
 process at its pulp extremity which passes into the connective 
 tissue ; this process is, however, often difficult to detect. 
 A lateral process of the cell has also been described con- 
 necting the cells to one another at the dentine margin. 
 Professor Paul describes these lateral processes as ' collars ' 
 consisting ' of a delicate network of pulp fibrils ', and does 
 not consider them to be any portion of the cell itself (23). 
 He says they are only visible in developing teeth and entirely 
 
 FIG. 122. Human adult premolar showing transverse 
 processes ot odontoblasts. ( x 75.) 
 
 absent in adult pulps. In fig. 122, however, a photograph 
 from a fully formed functional premolar tooth shows 
 lateral processes connecting the odontoblasts very clearly, 
 and these certainly appear to form portions of the cell. 
 This was photographed from a specimen of the author's 
 stained with iron and tannin, and, the 'same appearance is 
 to be seen in. many sections. 
 
 In fig. 123 a curious condition of the dentine is represented. 
 The photograph was from a carious tooth, and the dentinal 
 tubes are seen to be connected to a transverse tube with 
 which they form anastomoses ; the tubes are filled with 
 micro-organisms. This same condition was seen in a healthy 
 developing tooth in several parts of the circumference near 
 the pulp margin. It is very difficult to account for this 
 condition. A comparison of figs. 122 and 123 might suggest 
 
THE DENTAL PULP 
 
 207 
 
 that these connecting lateral processes of the odontoblasts 
 become sometimes involved in the calcification and persist 
 
 
 FIG. 123. Human molar. Transverse anastomosing 
 branches of dentinal tubes. ( x 350. 
 
 FIG. 124. Termination of tubes within the dentine, (x 350.) 
 
 as horizontal tubes in the matrix. These transverse tubes 
 within the dentine are, however, very rarely met with, and 
 the author has only found them in the instances above 
 mentioned. 
 
208 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Sometimes more than one dentinal fibril arises from 
 a single odontoblast, but in many cases, especially where 
 shrivelled odontoblasts are represented, these appearances 
 are deceptive, several cells with their processes being seen 
 groupad together. 
 
 A dentinal tube is sometimes seen to terminate in a clear 
 space in the matrix between the surrounding tubes, and end 
 in numerous fine terminal branches (fig. 124). 
 
 Vessels of the Pulp 
 
 The blood-vessels of the tooth-pulp enter the apical 
 foramen in one or more arterial branches. They traverse 
 the pulp in company with the nerve trunks (figs. 125 and 127), 
 branch frequently, and ultimately form a vascular plexus 
 beneath the odontoblast layer. While, however, the capil- 
 lary vessels which arise from them for the most part form 
 loops in this situation, many pass across the odontoblast 
 region and are se3n in contact with the forming dentine in 
 developing teeth. 
 
 According to Guido Fischer (6) the capillary vessels in 
 transverse section are seen in two forms one in which there 
 is a distinct adventitia, and smaller ones in which only the 
 endothelial coat is visible. The veins are apparent as open 
 spaces in the connective tissue of the pulp, with very delicate 
 walls. The vessels are in intimate relation with the nerve 
 trunks in the pulp, the latter often partially surrounding the 
 blood-vessels (fig. 126). 
 
 Lymphatics. It has usually been considered that the dental 
 pulp contains no lymphatics, but recent researches appear 
 to have demonstrated that a lymphatic system is present. 
 G. Fischer, writing in 1909 (6), says that lymph vessels, 
 although found around the roots of teeth, are not present 
 in the pulp, but he describes peri vascular lymph spaces as 
 being present, especially around the odontoblasts. Dewey and 
 Noyes, in a recent study of the lymphatic vessels of the dental 
 pulp, describe the work of Schweitzer on this subject and 
 consider that the recent date of this investigation is the 
 reason why it is not sufficiently known, and the statement that 
 there are no lymph vessels in the dental pulp still maintains its 
 
THE DENTAL PULP 
 
 209 
 
 FIG. 125. Nerves and blood-vessels of human tooth-pulp. 
 Blood corpuscles in situ. ( x 800. ) 
 
 FIG. 126. Arterial and venous capillaries in human pulp. Nerve 
 
 bundles partially surrounding blood-vessels. (x450.) 
 
 P 
 
 JIUMMlt'RY 
 
210 MICROSCOPIC ANATOMY OF THE TEETH 
 
 position in the text-books. Schweitzer in 1907 and 1909 (31) 
 published the results of his experiments on animals, and 
 was able to demonstrate the presence of lymph vessels in the 
 pulp ; this discovery was confirmed by Bartels in 1909 (1), 
 and also by Testut (32), who says, referring to Schweitzer, 
 ' This author has clearly demonstrated the presence of true 
 lymph vessels in the dental pulp'. The earlier observers, 
 as Korner and Halle, found that although they could not 
 find lymph vessels in the pulps of the animals experimented 
 
 FIG. 127. Blood-vessels and nerves of the pulp. ( x 600. ) 
 
 upon, single particles of the insoluble Prussian blue employed, 
 painted on the pulp at its upper end, were found as far as 
 the apex, but these results were not sufficiently convincing 
 even to establish the statement that although no lymph 
 vessels could be demonstrated, the pulp had the power of 
 resorption, and, as was suggested by Morgan, these particles 
 might be carried by wandering cells (20). 
 
 Dewey and Noyes repeated the experiments of the previous 
 investigators, and made use of both the methods employed 
 by them : that of Korner, Starr, and Ollendorf of injecting 
 Prussian blue directly into the pulp, and Schweitzer's method 
 of injecting the lymph vessels in the gums. These authors 
 
THE DENTAL PULP 211 
 
 were able to confirm Schweitzer's results by both methods, and 
 say, ' There is every evidence to support the correctness of 
 Schweitzer's statement that in the upper jaw the lymphatics 
 leaving the dental pulps course within the bony portion 
 of the maxilla and emerge through the infra-orbital and 
 other foramina into the subcutaneous tissue ; those of the 
 lower jaw enter the inferior dental canal, where they run 
 along with the blood-vessels and nerves, and thence pass 
 into the subcutaneous tissue. From either jaw lymph 
 vessels enter the sub maxillary and deep cervical glands.' 
 
 The Nerves of the Pulp 
 
 The nerves of the pulp and teeth are derived from the 
 fifth cranial nerve. The upper teeth are supplied by the 
 superior maxillary nerve, which arises from the Gasserian 
 ganglion, and the branches of this nerve which are distributed 
 to the teeth are the anterior, middle, and posterior superior 
 dental nerves. 
 
 The posterior superior dental nerve arises from the trunk 
 of the superior maxillary nerve before it enters the infra - 
 orbital groove, and divides into two branches which enter 
 the posterior dental canal and supply the upper molar teeth 
 and the mucous membrane of the antrum, also contributing 
 small filaments to the mucous membrane of the gum and 
 cheek. 
 
 The middle superior dental nerve arises at the back of the 
 orbital foramen, passes along a special canal in the anterior 
 wall of the antrum, and supplies the premolar teeth. 
 
 The anterior superior dental nerve arises near the infra - 
 orbital foramen and is the largest branch of the superior 
 dental nerve. It traverses the canal in the front wall of the 
 antrum and supplies the canine and incisor teeth. It has 
 a nasal branch to the inferior meatus and to the floor of the 
 nasal fossae. 
 
 The inferior maxillary nerve is the largest division of the 
 fifth nerve and consists of two portions, the larger one 
 arising from the Gasserian ganglion, and the smaller con- 
 sisting of the motor root of the fifth nerve, which unites with 
 the larger branch after passing through the foramen ovale 
 of the sphenoid bone. Beneath the external pterygoid 
 
 p 2 
 
212 MICROSCOPIC ANATOMY OF THE TEETH 
 
 muscle this nerve divides into two branches, a small anterior 
 branch chiefly consisting of motor fibres and distributed to 
 the temporal, masseter, and external pterygoid muscles, 
 and a buccal branch, which is its only sensory portion. 
 The posterior, the larger branch of the inferior maxillary 
 nerve, consists chiefly of sensory fibres and divides into the 
 auric ulo-temporal, lingual, and inferior dental nerves. 
 
 The inferior dental nerve passes beneath the external 
 pterygoid muscle to the outer side of the lingual nerve and 
 enters the inferior dental canal, whence it supplies branches 
 to the molar and premolar teeth and the canine. At the 
 mental foramen it issues from the canal and divides into the 
 incisor branch distributed to the lower incisors, and the 
 mental branch passing to the face in the mental region. 
 
 Several bundles of medullated fibres enter the pulp in 
 company with the blood-vessels, and maintain an intimate 
 association with them throughout their course. The main 
 bundles divide and subdivide and give off numerous branches 
 to the periphery of the pulp, but the larger nerve trunks 
 pursue a more or less direct course to the crown portion of 
 the pulp, where they lose their medullary sheath and the 
 axis cylinders spread out to their ultimate distribution. 
 Many preparations show that in company with these 
 medullated fibres are other nerve fibres which do not stain 
 with osmic acid, and probably are derived from the sym- 
 pathetic system and possess trophic functions. The passage 
 of the larger medullated bundles to the crown of the pulp 
 before dividing is accounted for by the much larger area 
 of dentine to be supplied at the crown portion of the tooth 
 than at that of its lateral margins. These larger bundles 
 of nerve fibres can often be seen at a definite point in the 
 pulp to lose their medullary sheath and neurolemma, and 
 give rise to a brush or fan-shaped expansion of delicate 
 fibres (fig. 128). These are the fibres of the axis cylinder of 
 the nerve, and the neurofibrils constituting them pass into 
 a plexus beneath the odontoblast region the plexus of 
 Raschkow. 
 
 At the lower margin of the odontoblast layer, as shown 
 by the author in a recent paper, 1 the delicate nerve fibres 
 1 Communicated to the Royal Society in May 1918. 
 
THE DENTAL PULP 213 
 
 of the plexus form synaptic connexions with nerve-end cells 
 which are present in a distinct row among the odontoblast 
 cells on their pulp aspect (fig. 129). 
 
 These cells are mostly stellate in form and associated 
 in groups. Each cell is provided with a distal process or 
 axon, which is unbranched and passes direct to the dentine, 
 where it enters the dentinal tube in company with the dentinal 
 fibril but forms no connexion with it. Other processes or 
 ' dendrons ' are given off from the peripheral portions of 
 
 FIG. 128. Human pulp. Ground section (Weil process: iron and 
 tannin), b. Blood-vessel. Axis cylinder of medullated nerve expanding 
 in a brush -like form at crown of pulp. ( x 600.) 
 
 the ' end cell ' (as it may for convenience be called), which 
 are branched, and pass to the odontoblast cells to form 
 a delicate network around them and communicate with the 
 dendrons of neighbouring ' end cells '. The dendrons from 
 one ' end cell ' do not become directly continuous with those 
 from neighbouring ' end cells ', but their communications 
 with these as with the fibres of the deep plexus (or plexus of 
 Raschkow) are synaptic (see fig. 129) ; for it has been shown 
 that throughout the nervous system each nerve with its 
 nerve-end body forms a separate and distinct neuron, and the 
 impulses which pass from cell to cell are passed across 
 
214 MICROSCOPIC ANATOMY OF THE TEETH 
 
 minute spaces which exist between the terminal branches 
 of the dendrons of communicating cells. . This is known as 
 the neuron theory, and has been confirmed by many observa- 
 tions and experiments. 
 
 Although able to show in 1912 (22 a) that neurofibrils enter 
 
 -d 
 
 n 
 
 FIG. 1 29. Human premolar. Nerve cells and their processes in the pulp. 
 d. Dentine ; ' s. separation between pulp and dentine crossed by axon 
 processes and dentinal fibril: n. nerve-end cells; p. pulp. (x800.) 
 
 the dentinal tubes and are distributed to the dentine, it was 
 long before the author was able to establish the correct mode 
 of distribution of the nerves of the pulp. In former prepara- 
 tions he had noticed as a very puzzling fact that the nerve 
 fibres which passed up to the dentine from the deep plexus 
 were very much larger and thicker than those which entered 
 into the plexus beneath the odontoblasts, and attributed this 
 
THE DENTAL PULP 215 
 
 to the collection or drawing together of these fine fibres of 
 the plexus into larger strands where they enter the odonto- 
 blast layer (fig. 130). As shown above, however, it is seen 
 that these larger strands passing to the dentine are processes 
 of the 'end cells'. 
 
 The bundles of neurofibrils vary very much in size, and 
 even in the deep plexus many much larger strands are seen 
 passing across it to the * end cell ' layer. At the crown of the 
 pulp these strands of neurofibrils are so much larger that 
 
 ^ 
 
 FIG. 130. Neurofibrils entering the dentine at the 
 cornu of the pulp. ( x 800. ) 
 
 it appears in specimens in which the ' end cells ' are not 
 stained that they pass direct from the medullated fibres 
 to the dentine, but when the ' end cells ' are fully stained 
 it is seen that none of these strands pass direct to the dentine 
 but they all enter the ' end cell ' before their final distribu- 
 tion. These bodies have so long eluded observation because 
 they remain transparent and invisible even in preparations 
 in which the nerve fibres are very completely demonstrated. 
 Further experiment, however, showed that in these 
 cases the gold had not been thoroughly reduced, and unless 
 the reduction is very complete, the pulp showing a deep 
 
216 MICROSCOPIC ANATOMY OF THE TEETH 
 
 purple coloration, the ' end cells ' are not visible. In these 
 sections the ' end cells ' are a deep black and the odontoblast 
 cells and their processes are only very faintly stained a pur- 
 plish brown, but the dentinal fibril is in many parts distinctly 
 traceable into the dentinal tube and the beaded black nerve 
 fibre is seen passing in with it (fig. 129). 
 
 The photographs from which the accompanying figures 
 were taken were mostly from parts of the sections where 
 the pulp was slightly separated from the dentine, as these 
 
 FIG. 131. Nerve-end cells with their axon processes passing to the den- 
 tine (d). The beaded fibre on the left under strong tension. ( x 800.) 
 
 show more clearly the relations of the nerves to the odonto- 
 blasts and their processes the fibrils. They also show clear 
 evidence of the great extensibility of the neurofibrils, and 
 exhibit with great distinctness their characteristic beading. 
 It can be seen in fig. 130 that when the pulp is in contact 
 with the dentine, the axon of the ' end cell ' has a wavy or 
 undulating course, but when the pulp is partially pulled 
 away the fibres become straightened out, and if the separa- 
 tion is very wide they are further pulled out of the tubules 
 of the dentine and are seen as strands of great tenuity. 
 These eventually part asunder as seen in fig. 131, but they 
 
THE DENTAL PULP 
 
 217 
 
 undergo a very great amount of stretching before they 
 finally give way. 
 
 The nerve fibre in this respect shows a very great difference 
 from the connective-tissue fibre, which is possessed of a very 
 slight amount of extensibility. Many preparations were ex- 
 amined with a high power of the microscope to ascertain if 
 any of the nerve fibres in the deep plexus are distributed to 
 the odontoblast layer without the intervention of the 'end 
 cells ', but there was no evidence of this in sections where 
 
 
 FIG. 132. Nerve-end cells showing lateral communications, n. Nerve 
 fibre ; /. dentinal fibre. ( x 1,200.) 
 
 the ' end cells ' were distinctly stained. Sometimes one of 
 the larger * end cells ' is seen with fine processes radiating 
 from it on all sides ; but these are scarce, and in most places 
 the beaded delicate fibres which form a network around the 
 odontoblast cells are distinctly seen to be given off from 
 lateral processes of the * end cells '. Sometimes the processes 
 to the dentine appear to be wound together in spirals, 
 and there is often an appearance of fibrillation in the cell 
 body. In many places clear nuclei are seen in the cells. 
 
 There is great variation in the size of these cells, but 
 probably many which appear to be single are really groups 
 
218 MICROSCOPIC ANATOMY OF THE TEETH 
 
 of cells (figs. 132 and 133). Small enlargements are seen 
 upon the nerve fibres distributed to the odontoblasts, but 
 these definite enlargements are not seen upon the axon 
 processes, which only exhibit the irregular beading character- 
 istic of neurofibrils. 
 
 Huber.(12), Guido Fischer (6), and others, looked upon 
 these enlargements as constituting the end bodies of the 
 nerve fibres of the pulp. They are very abundantly seen in 
 fish and reptiles, as shown by Retzius (26). 
 
 It is these fibres with the nodes or enlargements which 
 the author first described as forming the marginal plexus 
 from which the processes to the dentinal tubes arose. 
 
 FIG. 133. Nerve-end cells in the pulp. ( x 1,000.) 
 
 What was formerly described, however, as the marginal 
 plexus is evidently a portion of the network of fine nerve 
 fibres which envelop the odontoblasts and reach up to the 
 surface of the dentine, this network being derived as described 
 above from the dendrons of the ' end cells '. 
 
 The fibres distributed to the odontoblasts form no con- 
 nexions with them, but closely invest and surround 
 them. The passage of the axon fibres into the dentinal 
 tube is very clearly demonstrated, and can be traced in 
 suitably stained preparations as a winding black fibril 
 within the tube. Nodes or enlargements are often seen upon 
 the fibrils within the tubes, and here and there very delicate 
 branches are given off from these nodes to the finer branches 
 of the dentinal tubes (fig. 135). In fig. 136 are seen the nerve 
 
THE DENTAL PULP 
 
 219 
 
 fibres within the tubes just beneath the granular layer of 
 Tomes under the cement. At the enamel junction they 
 spread out in fine arborization s within the terminal branches 
 of the dentinal tubes, and some enter the interprismatic 
 
 FIG. 134. The network of nerve FIG. 135. Neurofibrils in dentinal 
 fibrils enveloping the odontoblasts. tubes near pulp, showing nodes or 
 (x800.) enlargements. (x800.) 
 
 FIG. 136. Neurofibrils in the dentinal tubes near the 
 cement margin. ( x 700.) 
 
 spaces of the enamel and terminate in small bulbous enlarge- 
 ments. In the very fine terminal branches of the dentinal 
 tubes at the cement margin, only a very delicate uniform 
 dotting can be seen in these gold preparations, and this 
 appearance is also seen in the canaliculi of the cement with 
 
220 MICROSCOPIC ANATOMY OF THE TEETH 
 
 which these fine branches are continuous. But while these 
 appearances would seem to indicate that there is a direct 
 nerve communication between the pulp and the cement, we 
 cannot definitely prove this until the fibre in this situation 
 is seen to be clearly continuous, since in all metallic im- 
 pregnations deceptive appearances may arise from deposits. 
 It is, however, noticeable that in these fine divisions of the 
 tubes the dotting is uniform and does not show the variation 
 in size of the particles which so often occurs in imperfectly 
 reduced preparations. 
 
 Where the so-called spindles are present in the enamel, 
 especially at the apices of the cusps, fine winding fibres are 
 sometimes seen within them which do not appear with 
 ordinary staining reagents, but these spindles do not appear 
 in the author's preparations to contain definite nerve-end 
 organs as described by Homer (28). It is impossible, how- 
 ever, to speak with certainty on this point, and the spindles 
 being comparatively infrequent a great many teeth should 
 be examined to decide the question. From the preparations 
 already made, however, one would be inclined to consider 
 the penetration of these bodies by nerve fibres more as due 
 to the fact that nerve fibres in the tubes would be likely 
 to penetrate into any open space with which the tubes 
 communicate than as constituting definite nerve-end organs. 
 
 The refraction in these spindles greatly interferes with 
 accurate observation even in thin sections. It is considered 
 by many, as previously stated, that the spindles are inter- 
 prismatic spaces which have remained uncalcified, and the 
 author's own observations on the enamel of marsupials 
 appear to confirm this interpretation of their nature. 
 
 In fig. 137 the mode of distribution of the nerves of the 
 pulp and dentine is represented diagrammatically. 
 
 The mode of distribution of the nerves of the pulp above 
 described appears to be peculiar to the teeth, as such 
 a mode of termination of sensory fibres in ' end cells ' is 
 not met with in other organs. It would appear to show the 
 interposition of a peripheral sensory neuron in the course 
 of the distribution of a sensory nerve, a condition not seen 
 elsewhere in the body. Of the sensory nature of at all 
 events the majority of these nerve fibres there can 
 
THE DENTAL PULP 
 
 221 
 
 scarcely be any doubt. Dentine is an extremely sensitive 
 structure. Healthy dentine, when fractured without 
 exposure of the pulp, is usually very sensitive ; it is also 
 a common clinical observation that in coning a living tooth 
 for crowning, when the enamel has been removed, the 
 grinding of the dentine causes acute pain. 
 
 In the excavation of a carious tooth, although little 
 
 FIG. 137. Diagram. Scheme of distribution of the nerves of the pulp. 
 m. Axis cylinder of medullated nerve ; x. medullary sheath ; p. nerve 
 plexus ; s. synaptic terminations ; n. nerve-end cell ; dn. dendrons ; 
 a. axon ; o. odontoblasts ; d. dentine ; /. dentinal fibril. 
 
 pain is felt on removal of the superficial layers, when the 
 lowest layer is raised from the healthy dentine beneath the 
 pain is acute. From these and other observations we must 
 conclude that sensory nerve fibres are distributed to the 
 dentine, but the question arises whether all the nerves .of 
 the pulp are made up of sensory fibres. We know also that 
 the surface of an exposed pulp is very sensitive, so we must 
 suppose that sensory fibres are distributed to the odonto- 
 blast layer. We should, therefore, imagine that both sensory 
 
222 MICROSCOPIC ANATOMY OF THE TEETH 
 
 and trophic fibres would be distributed to the pulp and also 
 to the dentine which contains within the tube a protoplasmic 
 prolongation of the odontoblast cell, the maintenance of the 
 functions of which would probably require a nervous supply. 
 
 Historical Review 
 
 The question of the innervation of the dentine has long 
 been a matter of much controversy, the majority of observers 
 holding that sensation was conducted by the dentinal fibril, 
 while others thought it probable that true nerve fibres were 
 supplied to the dentine. 
 
 For a long period it was considered that dentine is not 
 sensitive, and even such an eminent authority as John Hunter 
 was of this opinion. He says (13) : ' We may presume 
 that the bony substance' (of the teeth) 'itself is not capable 
 of conveying sensations to the mind, because it is worn 
 down in mastication and occasionally worked on by operators 
 in living bodies without giving any sensation of pain in the 
 part itself.' Duval showed, in a paper which he read before 
 the Academy of Medicine in Paris in 1831, that there is 
 sensation in the dentine, and was of opinion that it is 
 chiefly manifested just beneath the enamel (5). 
 
 Salter, in his classical work on Dental Pathology, writing 
 in 1874, says : ' The nerves of the tooth-pulp form loops 
 towards the periphery which may be readily demonstrated 
 by the action of caustic alkali, and from these, according to 
 Boll, large numbers of very minute fibrils proceed outwards 
 between the ivory cells and their tubular prolongations. 
 It is highly probable that these are the nervous elements 
 distributed to the dentine, but whether they pass into the 
 intertubular substance, or, fastening upon the tube walls, 
 are so piloted into the ivory structure, is quite uncertain. 
 It is, however, highly improbable that they pierce the wall 
 of the ivory cell and occupy the axes of the tubes ' (29). 
 
 Boll's observations were made in 1868 (2), on the teeth 
 of Rodents, which he treated with a very weak solution of 
 chromic acid (~ to ~ P er cent.). His preparations showed 
 fine fibres in great abundance, which passed between the 
 odontoblasts, and where the dentinal fibril was pulled out 
 
THE DENTAL PULP 223 
 
 from the dentine he could also trace them between these 
 fibrils. From these observations he concluded that the nerve 
 fibres entered the tubes of the dentine. Boll was not 
 successful in actually tracing them into the hard substance, 
 but he appears to have been the first observer who had 
 succeeded in tracing them so far in their course from pulp 
 to dentine. 
 
 Kolliker (1867), speaking of the nerves of the pulp, says : 
 ' As regards their endings, one sees, here and there, loop- 
 like curves of the fibres, but it is beyond doubt that these 
 are not the last endings.' 
 
 Professor Klein, in 1883 (15), says : * Numerous medul- 
 lated nerve fibres, forming plexuses, are met with in the pulp 
 tissue ; on the outer surface of the pulp they become 
 non-medullated fibres, and probably ascend in the dentinal 
 tubes.' 
 
 Several histologists have attempted to account for sensa- 
 tion in the hard substance of the tooth apart from the 
 actual presence of nerves, considering that the soft material 
 occupying the dentinal tubule conducted sensation to the 
 nerves of the pulp. .The principal supporters of this view 
 have, been Hope well Smith and Dr. Aitchison Robertson, 
 although they differ in their views as to the actual anatomical 
 path by which sensation is transmitted. 
 
 Hopewell Smith (11) has long held the view that the 
 function of the odontoblast cell is that of a nerve-end organ, 
 sensation being conveyed through the dentinal fibril, which 
 is a protoplasmic prolongation of this cell. He was able to 
 show, by means of teased-out preparations from the pulp, 
 that the nerve fibres when stained with methylene blue, 
 took on the characteristic varicose appearance, and were 
 seen in great abundance immediately around the odonto- 
 blasts. He was unable to trace fibres into the cells, although 
 they enclosed them in a fine network. 
 
 Hopewell Smith, while allowing the objection that the 
 epiblastic nerve fibres are connected with cells derived from 
 the mesoblast, says : ' Accepting the statement of Schafer 
 (that all nerve fibres and nerve cells are originally derived 
 from the neural or neuro-sensory epiblast) one is led to the 
 conclusion that odontoblasts cannot possibly be, from 
 
224 MICROSCOPIC ANATOMY OF THE 'TEETH 
 
 the developmental point of view, ganglion cells in which 
 sensory, or tactile, or trophic influences arise de novo. 
 But it is no argument against the idea that they serve as 
 sensation transmitters.' In the last edition of his Histology, 
 1919, this author says : 'As the result of his researches the 
 author has the strongest conviction that these fibres ter- 
 minate in a basketwork of varicose fibres embracing and 
 often closely attached to the cell walls of the individual 
 odontoblasts.' 
 
 In the absence of evidence of the passage of nerves into 
 the dentine we can understand this to be a possible, although 
 an improbable, explanation of the mode in which the 
 dentine transmits sensory impulses, but we do not think 
 there has ever been any proof that such mode of transmission 
 occurs in the mammalian tooth. Weil of Munich held 
 a similar view and says : ' As things stand at present 
 nothing lies in the way of the theory that each delicate 
 fibril of the basal layer of the membrana eboris is a means 
 of connexion between the nervous system and the odonto- 
 blasts, and that the latter formations may be regarded 
 as nerve endings,' but he goes on to say that it is only an 
 hypothesis that time may prove to be true, and must limit 
 himself to saying that nerve fibres or groups of such cannot 
 be found beyond the cortical layer of the pulp. 
 
 Magitot in 1 879 ( 1 8) described certain ramified cells beneath 
 the odontoblasts which he said were in continuity by their 
 processes with the nerve fibres on one hand and the basal 
 prolongations of the odontoblasts on the other, thus ' forming 
 a direct chain of sensation '. The existence of such an 
 arrangement has, however, never been corroborated by other 
 observers. 
 
 In 1891 Aitchison Robertson contributed a paper to 
 the Royal Society of Edinburgh in which he asserted his 
 conviction ' that the central processes of the odontoblasts 
 become continuous with nerve fibres ' (27). 
 
 ' The central process seems to become the axis cylinder 
 of a nerve fibre which gradually acquires a primitive sheath 
 in which the medullary substance slowly accumulates till an 
 ordinary medullated nerve fibre results.' He would thus 
 look upon the odontoblasts and dentinal fibrils as the 
 
THE DENTAL PULP 225 
 
 terminal organs of the nerve fibres, and says : * We may 
 regard the odontoblast and its peripheral process as an end 
 organ, which, if not itself sensitive, at once transmits sensory 
 impulses to the nerve with which it is connected.' 
 
 He would thus agree with Hopewell Smith's views, but 
 goes further than that author in describing a direct con- 
 tinuation of the pulp process of the odontoblast into a true 
 nerve fibre. 
 
 One is aware that there is some analogy to this mode 
 of termination in the endings of the olfactory nerves. From 
 the olfactory cell with its peripheral termination in the 
 olfactory hairs a nerve process proceeds towards the 
 olfactory bulb, but in this case the olfactory cell is an 
 epiblastic cell and the nerve fibre apparently represents 
 the axon of a true nerve cell. The cell and the nerve fibre 
 are formed from the same layer of the blastoderm. 
 
 Carl Huber, in 1898 (12), by the use of methylene blue, 
 traced nerve fibres to the plexus beneath the odontoblasts, 
 and from that to what he considered their terminations at 
 the inner border of the dentine. In this he corroborated 
 the researches of Retzius (26), who in 1894 traced nerve 
 fibres in the pulp of the mouse to the inner surface of the 
 dentine ; but while Huber does not believe in the possi- 
 bility of their entering the dentine, Retzius appears to be 
 more doubtful on this point, for he says : * In vertical 
 sections the fibres, like a string of tiny beads, stretch 
 between the odontoblasts to the surface, and run a little 
 way tangentially. In a tangential section they can be 
 partially traced under the dentine.' 
 
 Retzius has figured and described the nerve distribution in 
 the teeth of reptiles and fish, and figures them as terminating 
 within the tooth-pulp. 
 
 Morgenstern (21) in 1892 and 1895 published his views 
 on the distribution of the nerves to the hard tissues of the 
 teeth. He made use of Golgi's method and described 
 medullated fibres entering the dentine. He considered that 
 they occupied tubes distinct from the dentinal tubules 
 and also traversed the dentinal tubules, and terminated 
 either at the dentine- enamel margin or in the substance 
 of the enamel, in the knob -shaped enlargements seen 
 
 MUMMERY Q 
 
226 MICROSCOPIC ANATOMY OF THE TEETH 
 
 often at the crown of the tooth projecting into the enamel 
 (the spindles). 
 
 We should be very unwilling to translate the thick 
 black lines often obtained in the dentine in Golgi prepara- 
 tions as medullated nerves ; they appear more in the nature 
 of irregular deposits of the chromate of silver, and these 
 appearances probably led this observer to the extraordinary 
 conclusion at which he arrived. 
 
 Professor Homer of Strassburg (28) has been for several 
 years engaged in an investigation of the nerves of the 
 teeth. There is little doubt he succeeded in tracing nerve 
 fibres into the dentine, but he was unable to carry conviction 
 with the few preparations he procured, and his statement 
 that the nerve fibres enter the dentinal fibril (which he 
 considers to be a tube) has not been substantiated. He 
 considered that there was a normal termination of the nerves 
 to be found in the spindles at the enamel margin. 
 
 Pont, in 1900 (24), compared the odontoblasts to peripheral 
 neurons (nerve cells with their processes), which occur in 
 other peripheral organs such as the retina, and considered 
 that the nerve fibres, without forming any direct anatomical 
 communication with the odontoblast, formed synapses, that 
 is, that the nerves which envelop the cell transmit sensation 
 from the cell to the afferent nerve, actual anatomical con- 
 nexion not being necessary for the transmission of impulses 
 from the peripheral cells. The same objection would hold 
 good here that applies to Hopewell Smith's view, that we 
 should not expect a synaptic any more than a direct com- 
 munication between a cell formed from the mesodermic 
 layer and a true nerve fibre of ectodermic origin. It is now 
 generally considered that independent neurons never form 
 direct communications, but that all such between individual 
 nerve cells and their processes with neighbouring cells are 
 synaptic. 
 
 Since the publication of the author's paper in 1912 
 the late Professor Dependorf of Leipzig has published the 
 results of his researches on the same subject (3). He 
 employed many different staining and impregnation methods, 
 usually staining small pieces of teeth in bulk, and he corrobo- 
 rated the author's observation that the nerve fibres of the 
 
THE DENTAL PULP 227 
 
 pulp enter the dentinal tubes. He also described the small 
 nodes or enlargements on the fibres. Certain statements of 
 this author as to the passage of nerve fibres across the 
 tubes on the surface of the sectioned dentine and figured 
 as quite independent of the tubes, are difficult to under- 
 stand. He says : ' From the dentigenous substance and 
 in the dentine they ' [the neurofibrils] ' are distributed in the 
 area of the intercellular substance, and take a course indepen- 
 dent of Tomes' fibres, forming a wide-meshed network. 
 Neurofibrillse are also found in the area of Tomes' fibres 
 within the dentinal tubuli. Hence the nerve fibres in the 
 dentine have two courses : one within the basic inter- 
 cellular substance, the other within the dentinal tubuli, 
 and there may be a possible connexion between these fibres. 
 Possibly their function differs, one group governing sensa- 
 tion, the other, nutrition.' A paper on the innervation of 
 the dentine was also published by Fritsch in 1914 (8). 
 
 The present author's own investigations date back to 
 1891, when he made use of various stains in the attempt 
 to demonstrate the distribution of the nerves of the pulp. 
 Pulps treated with weak chromic acid by the method of 
 Boll showed a multitude of fine fibres close to the dentine, 
 but they could not be traced further. 
 
 The iron and tannin method gave much better results, 
 and showed very clearly the connexion of these fine fibres 
 with the medullated fibres of the pulp, 1 and in a few cases 
 the actual passage of the fibre into the dentinal tube, but 
 the demonstration was not sufficiently convincing to be 
 made use of. 
 
 The intra-vitam method with methylene blue was carried 
 out on two occasions once with Professor Miller in Berlin, 
 and was again kindly conducted for the author by Professor 
 Starling at University College. 
 
 These preparations showed an abundance of neurofibrils 
 proceeding to the dentine, but it was impossible to obtain 
 sections, as the colour cannot be retained during decalcifica- 
 tion. The recent results have been obtained with a modi- 
 fication of the Beckwith gold chloride method, the specimens, 
 
 1 Tomes, C. & A Manual of Dental Anatomy, 7th ed., p. 79. 
 Q 2 
 
228 MICROSCOPIC ANATOMY OF THE TEETH 
 
 impregnated with a very weak solution of gold chloride 
 (1 in 5,000), being reduced in alkalis. 
 
 As shown by the author in several communications (22), 
 this method, as first employed, demonstrated nerve fibres 
 in connexion with the medullated fibres of the pulp passing 
 
 !, W 
 
 o z 6 
 
 FIG. 138. Pulp of human premolar prepared by the Weil process. 
 Stained borax carmine, o. Odontoblasts ; z. odontogenic zone ; b. blood- 
 vessels ; n. nerve trunks ; w. basal layer of Weil. ( x 50.) 
 
 in extraordinary abundance to the dentine and entering the 
 tubes ; it also revealed an intricate plexus of neurofibrils 
 between and around the odontoblasts. The ' end cells ', how- 
 ever, were not shown in these preparations, and only appeared 
 when the modified method, above described, was adopted. 
 The basal layer of Weil. Professor Weil (34) of Munich 
 described a layer or space which lies immediately beneath 
 the odontoblast layer (fig. 138), and which is especially 
 
THE DENTAL PULP 229 
 
 visible in preparations which have been made by the Koch- 
 Weil balsam process, but is also seen in many decalcified 
 sections. 
 
 With low powers of the microscope no distinct structure 
 is visible in this area, but with higher powers it is seen 
 to be occupied by an interlacement of delicate fibres. Weil 
 compared this layer to a basement membrane, looking upon 
 the odontoblasts as being analogous to a layer of epithelial 
 cells, but there is very little evidence to support any such 
 view, and the actual structure of this layer can, we think, 
 be quite clearly demonstrated. 
 
 The denser connective tissue of the pulp extends in this 
 position as an interlacement of delicate connective-tissue 
 fibres which pass between the odontoblasts to the dentine, 
 and there become incorporated in the matrix, but the same 
 area is also occupied by the plexus of fine nerve fibres 
 beneath the odontoblasts which communicate with nerve 
 ' end cells ' within the odontoblast layer. It is also crossed 
 by the pulp processes of the odontoblasts, which are usually 
 delicate and not very evident in many preparations. In 
 order to show that this so-called layer of Weil was not 
 a clear space, but occupied by fibres which do not stain by 
 ordinary methods, the author removed the balsam from 
 a Weil preparation which showed the area very distinctly, 
 by soaking it in chloroform. The section was then restained 
 with gold, reduced by the Beckwith process ; the clear 
 layer was no longer visible, being entirely occupied by 
 a dense plexus of delicate nerve fibres forming the deep 
 plexus, or plexus of Raschkow, the delicate connective-tissue 
 fibres remaining unstained. 
 
 The plexus of Raschkow is much more evident in some 
 parts of the pulp than in others, and this is also noticeable 
 with regard to the basal layer. It is not seen at the root 
 end of the tooth where the nerve plexus is also absent, and 
 at the coronal portion where the nerve fibres from the divid- 
 ing medullated trunks pass more directly to the odontoblast 
 layer this area described by Weil cannot be detected. 
 
 Von Ebner and Rose doubted its existence as a distinct 
 structure and considered it was artificially produced, but 
 that this rarified area of the pulp tissue is present in many 
 
230 MICROSCOPIC ANATOMY OF THE TEETH 
 
 sections we think there can be no doubt, and in properly 
 prepared Weil sections there is no evidence of any stretching 
 of the pulp tissue or disturbance of any of the tissues ; in 
 fact, they are seen in their definite relations to one another 
 probably more perfectly than by any other process (22 d). 
 In 1908 Mr. Law (17) read a paper before the Odonto- 
 logical section of the Royal Society of Medicine in which 
 he described his investigations with Bethe's methylene blue 
 process. He found large fibres running up to the dentine 
 in the pulp, but the few sections made use of did not show 
 any connexion with nerve fibres in the pulp, and it could 
 not therefore be clearly demonstrated that they were nerve 
 fibres. He stated that in these preparations he could find 
 no trace of the deep plexus, or plexus of Raschkow. 
 
 Dr. Dentz of Utrecht found in some sections of developing 
 human teeth a row of pear-shaped bodies near the outer 
 margin of the dentine. These bodies had very much the 
 appearance of nerve-end organs ; they each contained 
 several large bodies resembling nuclei, and the area of den- 
 tine surrounding the outer club-shaped end showed a certain 
 amount of modified dentinal tissue with few or no tubes. 
 These bodies apparently represent some abnormality in 
 development, as being of very large size (^th inch), they 
 must have been conspicuous in other preparations had they 
 been constantly present in deciduous teeth. No nerve fibres 
 could be traced into them, and it is difficult to account for 
 their occurrence in these preparations. These bodies are 
 shown in figs. 139 and 140 under a high and low power, and 
 were photographed from the specimen kindly lent to the 
 author by Dr. Dentz. 
 
 Persistent Pulps. Persistent pulps are found in teeth 
 which are subjected to constant and severe attrition, as the 
 incisors of Rodents. The pulp is not completely enclosed 
 within a bony cavity as in the majority of teeth, but remains 
 open at the base, and is continually growing during the life 
 of the animal, new hard tissue being deposited by the 
 formative cells of the pulp as the tip of the tooth becomes 
 worn down in use. The nerve trunk to the pulp runs 
 backward beneath the tooth and then bends abruptly and 
 enters the pulp, showing that the tooth is continually 
 
THE DENTAL PULP 
 
 231 
 
 growing in a backward direction from its original position 
 in the jaw. Remarkable effects sometimes result from this 
 
 
 
 a . 
 
 FIG. 139. Dr. Dentz's specimen showing position of corpuscle-like 
 bodies (a) shown in fig. 140. ( x 50.) 
 
 FIG. 140, Dr. Dentz's specimen. Curious corpuscle-like body in dentine 
 of a temporary tooth. ( X 250.) 
 
 continuity of growth. Many instances are on record where, 
 from fracture or displacement of the upper incisor of 
 a rabbit, the lower tooth, being no longer worn down at 
 its cutting edge, continues to grow forward, and following 
 
232 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the curve of the tooth sometimes turns directly backwards, 
 penetrating the skull and brain. 
 
 The persistent growth of the great incisor teeth forming 
 the tusks of the elephant sometimes gives rise to remarkable 
 conditions. Several cases have been recorded where an 
 elephant has been struck by a javelin or loaded spear-head 
 which has penetrated the pulp at the free-growing end, 
 and the vitality of the tissues has been so great that the 
 death of the pulp has not followed, but the spear carried 
 forward by the growing tooth has been completely surrounded 
 
 FIG. 141. Skull of Rat (Mus decumamis). Dislocation and overgrowth 
 of incisors. The right upper incisor penetrates the skull. (From a specimen 
 lent to the author by Mr. Montagu Hopson.) 
 
 by the subsequently deposited ivory and has become 
 immovably fixed in the tusk. Such a case is shown in the 
 photograph (fig. 142). The head of the javelin and a portion 
 of the iron shaft are embedded solidly in the ivory. This 
 specimen was obtained from the ivory worker by a merchant 
 in the City, who could not be persuaded to part with it 
 but lent it to the author to be photographed. There are 
 two somewhat similar specimens in the Hunterian Museum 
 at the Royal College of Surgeons. The effects of the con- 
 tinuous growth of teeth with persistent pulps is also shown in 
 fig. 141, the skull of the rat previously referred to on p. 107. 
 Other examples of teeth with persistent pulp are found 
 in the incisor of the Dugong, the large upper canines of the 
 
THE DENTAL PULP 
 
 233 
 
 Walrus, and in many marsupials as in the lower incisors 
 of the Kangaroo the procumbent incisors of the Hypsi- 
 prymninae, and both the incisors and molars of the rodent- 
 like Wombat. In the Wombat also a layer of cement 
 is continued over the surface of the incisors covering the 
 enamel, while in Rodents the cement is not continued 
 over the surface of the enamel. In these scalpriform incisors 
 a sharp edge of enamel is maintained by the unequal wear 
 of the tissues of the tooth, the dentine and cement wearing 
 
 FIG. 142. Spear-head embedded in tusk of Elephant. 
 
 down much more quickly than the hard enamel which 
 projects as a sharp cutting edge. 
 
 In the fish, the rostral teeth of the Saw-fish (Pristis) grow 
 from persistent pulps, and they are also implanted in sockets. 
 
 There are certain peculiarities in the shape and extent 
 of the pulp cavities in the teeth of ancient forms of man 
 recently discovered. 
 
 In the Heidelberg mandible the pulp cavities are very 
 much larger in proportion to the hard tissues than in 
 modern man. Especially in the Krapina teeth, as shown in 
 Professor Keith's skiagrams, is this noticeable, the pulps 
 
234 MICROSCOPIC ANATOMY OF THE TEETH 
 
 being very large and extending deeply into the root (14). 
 The meaning of this large pulp cavity and the special 
 purpose it served are somewhat difficult to understand. 
 
 REFERENCES 
 
 1. Bartels. Das Lymphgefdss- System. Jena, 1909. 
 
 2. Boll, Franz. ' Untersuchungen fiber die Zahnpulpa.' Archivf. Micr. 
 
 Anat., 1868, vol. iv, p. 73. 
 
 3. Dependorf . ' Beitrage zur Kenntniss der Innervimng der menschlichen 
 
 Zahnpulpa und des Dentins.' Deutsche Monatschr. f. Zahnheil- 
 kunde, 1913, vol. xxxi, p. 689. 
 
 4. Dewey, K., and Noyes, F. B. 'A Study of the Lymphatic Vessels 
 
 of the Dental Pulp.' Dental Cosmos, vol. lix, 1917, pp. 436-44. 
 
 5. Duval, J. R. Observations pratiques sur la sensibilite des substances 
 
 dures des dents. Paris, 1833. 
 
 6. Fischer, G. Bau und Entwickelung der Mundhohle des Menschen, 1910. 
 
 7. Fleming. ' Virchow's Festschrift, 1891.' Archivf iir Anatomie, 1897. 
 
 8. Fritsch, C. ' Untersuchungen iiber den Bau und die Innervimng des 
 
 Dentins.' Archivf. Micr. Anat., 1914, pp. 307-20. 
 
 9. Haidenhain, M. Plasma und Zelle, 1907. 
 
 10. Hanazawa, K. 'A Study of the Minute Structure of Dentine, especially 
 
 of the Relation between the Dentinal Tubules and Fibrils.' Dental 
 Cosmos, Feb. 1917 and March 1917, vol. lix. 
 
 11. Hopewell Smith, A. (a) Histology and Patho-histology of the Teeth, 
 
 1st ed., p. 170 and appendix. 
 (b) Normal and Pathological Histology of the Mouth, 1919, vol. i. 
 
 12. Huber, C. ' The Innervation of the Tooth-pulp.' Dental Cosmos, 
 
 1898, vol. xl, p. 803. 
 
 13. Hunter, J. John Hunter's Works. Palmer's edition, 1835, ii, p. 50. 
 
 14. Keith, A. Proc. Roy. Soc. Med. (Odontological Section), 1913, vol. vi, 
 
 p. 105. 
 
 14 a. Keith, A., and Knowles, F. H. S. 'A Description of Teeth of 
 Paleolithic Man from Jersey.' Journ. Anat. and Phys., 1912, 
 vol. 46, pp. 12-27. 
 
 15. Klein, E. Elements of Histology, 1883, p. 174. 
 
 16. Kolliker. Gewebelehre, 1889. 
 
 17. Law, W. J. Proc. Roy. Soc. Med. (Odontological Section), 1908, p. 45. 
 
 18. Magitot et Legros. ' Morphologic du follicule dentaire chez les 
 
 Mammiferes.' Journ. de VAnat. et de la Phys., Paris, 1879, vol. xv, 
 pp. 248-93. 
 
 19. Mall, F. American Journal of Anatomy, vol. i, 1901-2. 
 
 20. Morgan, G. ' Lymph Glands in relation to the Teeth and Gums.' 
 
 Brit. Dent. Journ., 1903, vol. xxiv, p. 521-8. 
 
 21. Morgenstern. ' Ueber das Vorkommen von Nerven in den harten 
 
 Zahnsubstanzen.' Deutsche Monatschr. f. Zahnheilkunde, 1892, 
 p. 436, and 1895, p. 111. 
 
THE DENTAL PULP 235 
 
 22. Mummery, J. H. (a) ' On the Distribution of the Nerves of the Dental 
 
 Pulp.' Phil Trans. Roy. Soc., 1912, vol. ccii, pp. 337-49. 
 (6) The Nerve Supply of the Dentine.' Proc. Roy. Soc. Med. (Odonto- 
 logical Section), 1912, vol. v, pp. 166-88. 
 
 (c) ' The Innervation of Dentine.' Dental Cosmos, March 1916. 
 
 (d) ' Basal Layer of Weil.' Journ. Brit. Dent. Assoc., 1892, vol. xiii, 
 pp. 777-81. 
 
 23. Paul. ' A Contribution to the Histological Study of Dentine.' Trans. 
 
 Odontol. Soc. Great Brit., 1902, p. 25. 
 
 24. Pont. ' La Cataphorese en art dentaire et plus specialement dans les 
 
 cas de dentine hypersensible.' Trans. Third Int. Dent. Congress, 
 Paris, 1900, p. 232. 
 
 25. Ranvier. Traite technique d'Histologie, 1889. 
 
 26. Retzius, G. Biologische Untersuchungen, Neue Folge, iv, v, and vi, 
 
 1892, 1893, and 1894. 
 
 27. Robertson, Aitchison. ' On the Relations of Nerves to Odontoblasts 
 
 and on the Growth of Dentine.' Trans. Roy. Soc. of Edinburgh, 
 vol. xxxvi, p. 323. 
 
 28. Romer, O. JV 'erven im Zahnbein : Zahnhistologische Studie: Freiburg, 
 
 1899. 
 
 29. Salter, James. Dental Pathology and Surgery, 1874. 
 
 30. Schafer, E. A. ' Microscopic Anatomy.' Vol. ii, pt. i, of Quairi's 
 
 Anatomy, 1912. 
 
 31. Schweitzer. ' Ueber die Lymphgefasse des Zahnfleisches u. der Zahne 
 
 beim Menschen u. bei Saugethieren.' Archivf. Micr. Anat., 1907, 
 p. 807 ; 1909, p. 27. 
 
 32. Testut. Traite d'Anatomie humaine, 1912, p. 58. 
 
 33. Waldeyer. Sitzungsberichte d. Pr. Akad., 1895. 
 
 34. Weil, W. A. Zur Histologie der Zahnpulpa, Leipzig, 1887, p. 55. 
 
, CHAPTER IV 
 DENTINE 
 
 THE bulk of the human tooth, as well as that of the 
 teeth of other mammalia and many vertebrates, consists 
 of dentine, and although found throughout the animal 
 kingdom in many different forms, this tissue has certain 
 common characteristics which clearly separate it from 
 the other hard tissues of the teeth. In distinction from 
 enamel it has a collagen basis, and when subjected to the 
 action of acids this gelatinous substance retains the form of 
 the calcified dentine. In this respect it resembles bone, 
 but there are differences in structure in all the different 
 varieties of dentine which distinguish it as a special tissue 
 peculiar to the teeth and dermal appendages. 
 
 The form of dentine found in the human tooth, and the 
 teeth of mammalia generally, is the so-called tubular 
 dentine, the organic matrix being permeated by a series 
 of parallel tubes with fine branches which everywhere 
 penetrate it. 
 
 This tubular dentine has been called by Tomes ' ortho- 
 dentine ' to distinguish it from other varieties which might 
 also be described as tubular. 
 
 The classification of dentines adopted by Tomes is as 
 follows : 1 
 
 Orthodentine. 
 
 Plicidentine. 
 
 Vasodentine. 
 
 Osteodentine. 
 Rose, however, has suggested a different classification : 
 
 f Vasodentine. 
 Orthodentme including | vitrodentine . 
 
 Trabecular dentine. 
 Bone dentine. 
 
DENTINE 237 
 
 Rose includes under his vitrodentine the outer layer of the 
 teeth of Selachia, which Tomes considers to be enamel. There 
 are certain objections to Rose's classification which are con- 
 sidered by Tomes in a paper on the subject in 1898 (226), 
 and as his classification of dentines appears to be the simplest 
 and least open to objections, it has been adopted in the present 
 work. As Tomes points out, the term-' trabecular dentine ' 
 is perhaps a better descriptive term than ' osteodentine ', 
 and we may conveniently describe this variety as osteo- or 
 trabecular dentine. 
 
 Orthodentine (tubular dentine). To the naked eye typical 
 orthodentine, as seen in human teeth, is a yellowish white 
 semi-translucent substance covered by enamel in that por- 
 tion of the tooth which is raised above the gum margin, 
 and in contact externally with the overlying cement at 
 the neck and root of the tooth. 
 
 If a tooth is placed in an acid solution, such as a 3 to 5 per 
 cent, solution of nitric or hydrochloric acid, the lime salts are 
 removed and the collagen basis substance alone remains and 
 still exhibits the general structure and appearance of the calci- 
 fied dentine ; it is soft and elastic and can be penetrated by 
 a needle. The resulting substance is collagen, which is 
 converted into gelatine by boiling, thus having the same 
 characteristics as bone. 
 
 When both bone and dentine are treated with strong 
 acids a residue remains which consists of elastin, and is 
 probably present in the dentine in the Neumann's sheaths 
 of the tubules. 
 
 Chemical Composition. As in enamel, the salt which greatly 
 predominates in dentine is calcium phosphate, the other 
 salts present being calcium carbonate and magnesium phos- 
 phate, and according to Von Bibra (2) traces of calcium 
 fluoride . The same author also gives the percentage of animal 
 matter as 27-61, while the percentage of organic matter in 
 enamel according to recent analysis is from 1 to 2. 
 
 Tomes, however, considers that the amount of organic 
 matter in dentine has been over-estimated, and would place 
 it at about 19 per cent., as he considered that the water 
 retained in combination with dentine, even when dried at 
 212 F., amounts to about 8 per cent, of the whole, and he 
 
238 MICROSCOPIC ANATOMY OF THE TEETH 
 
 would therefore consider the following to be a much nearer 
 approach to the actual proportions in dentine. 
 
 Fresh human dentine : 
 
 Free water (which can be dried out at 212 F.) . 10 
 Dry dentine ....... 90 
 
 100 
 Dried human dentine : 
 
 Organic matter . . . . . .19-6 
 
 Combined water . . . . 8-4 
 
 Salts 72 
 
 100 
 Von Bibra in one analysis gives : 
 
 Organic matter (tooth cartilage) . . . 27-61 
 
 Fat . . 0-40 
 
 Calcium phosphate and fluoride . . . 66-72 
 
 Calcium carbonate ...... 3-36 
 
 Magnesium phosphate ..... 1-08 
 
 Other salts 0-83 
 
 100 
 
 In these analyses the large amount of organic matter 
 present in dentine compared with that in enamel is especially 
 to be noticed, and the great preponderance of phosphate 
 over carbonate of calcium in both dentine and enamel. 
 
 Black (3) undertook a careful investigation to determine 
 the percentage of lime salts in the dentine of teeth of 
 apparently different density, and came to the conclusion 
 that there is a scarcely appreciable difference between the 
 percentages of lime salts in all the teeth examined. 
 
 He stated ' that neither the density, nor the percentage 
 of lime salts, nor the strength, is in any degree a factor in 
 predisposing the teeth to caries or hindering its inception 
 or progress '. It was pointed out, however, by Professor 
 W. D. Miller that the resisting power might not depend so 
 much on the percentage of lime salts in the teeth as upon the 
 molecular conditions present ; the degree of stability of 
 the compounds formed with the organic material of the 
 tooth. Although the amount of the salts in the tissues may 
 
DENTINE 239 
 
 be approximately the same, the manner in which they are 
 bound together in the dental tissues may vary considerably. 
 The Dentine Matrix. This is a clear, more or less homo- 
 geneous looking substance traversed by the dentinal tubes. 
 In young teeth, where the deposition of the dentine is still 
 proceeding, the denser part of the tissue is bordered by a clear 
 layer between it and the odontoblasts in which the tubes are 
 clearly seen but not so sharply defined (fig. 143) ; this is the 
 odontogenic zone (the pradentine of Continental authors), 
 and is occupied by tissue on the borderland of calcification 
 in which the salts are not yet fully deposited. The calcified 
 
 p 
 
 FIG. 143. Human tooth near open end. d. Dentine ; o odontogenic 
 zone ; od. odontoblasts ; p. pulp. ( x 150.) 
 
 dentine has a festooned outline towards this zone, made up 
 of rounded bodies, the contours of which are formed by 
 the calcospherites or spherical masses of lime salts which 
 build up the fully calcified substance of the dentine, as will 
 be more fully described in treating of its development. 
 Delicate fibres derived from the connective tissue of the pulp 
 are present in the matrix and form its fibrillar basis, the 
 existence of which is usually completely veiled by the dense 
 calcification ; but it appears occasionally in parts acted upon 
 by the acids of caries, the slow action of the weak acid 
 gradually undoing, as it were, the process of development 
 and revealing the foundation substance. At the borders 
 of carious cavities a very fine lamination is sometimes 
 
240 MICROSCOPIC ANATOMY OF THE TEETH 
 
 seen more or less parallel with the dentine surface 
 (fig. 144), and in young, growing teeth treated with silver 
 nitrate by the Cajal process these parallel lines are seen to 
 be concentrically arranged. This part of the subject, 
 however, will be further considered under the development 
 of the dentine. 
 
 The Granular Layer of Tomes. Within the marginal portion 
 of the dentine and in contact with the cement, although 
 sometimes separated from it by a clear layer, is seen a line 
 
 FIG. 144. Adult human molar affected by caries. Action of acids of caries, 
 showing striation and contours as in figs. 178 and 179. ( x600.) 
 
 of dark contours, angular or crescentic in form, which has 
 a granular appearance under the microscope, and owing 
 to this appearance was named ' the granular layer of 
 Tomes ', Sir John Tomes having been the first to describe it. 
 These small spaces are usually confined to the dentine 
 beneath the cement, but are occasionally, although rarely in 
 man, found under the enamel. They appear to be a normal 
 portion of the structure of the dentine and are found in 
 all teeth ; the tubes of the dentine communicate with them 
 by their fine terminal branches. The rounded contours of 
 the calcifying substance are seen to surround them, and 
 
DENTINE 241 
 
 they are in this sense interglobular spaces, but they are not 
 apparently due, as are the interglobular spaces presently 
 to be described, to a want of complete coalescence of the 
 calcospherites but are normal structures surrounded by the 
 calcifying bodies, for the tubes enter them and do not pass 
 uninterruptedly across them as in the large interglobular 
 spaces beneath the enamel. These bodies probably afford 
 a channel of communication between the pulp and the 
 canaliculi of the cement ; they are in fact seen to do so in 
 
 FIG. 145. Human molar. Communication of dentinal tubes 
 with spaces of granular layer. ( X250.) 
 
 many instances (fig. 145). We should then look upon this 
 layer as part of the normal structure of dentine. They 
 are found in many Mammalia, and are very conspicuous 
 in the dentine of the Cetacea, where they are arranged in 
 concentric rows. In many teeth other much larger inter- 
 globular spaces are seen, generally situated beneath and not 
 far from the enamel in the substance of the first-formed 
 dentine (fig. 146). They are evidently formed by the want of 
 coalescence of the calcospherites which form the calcified 
 portion of the matrix. In teeth from rickety subjects, in 
 whom the calcification of the bones and teeth is very 
 
242 MICROSCOPIC ANATOMY OF THE TEETH 
 
 imperfect, these interglobular spaces are very abundant, and 
 are usually associated with very conspicuous defects in the 
 enamel. The contents of the interglobular spaces are usually 
 granular, but sometimes appear quite clear. It was noticed 
 by Tomes that in caries of the dentine the tubes filled 
 with micro-organisms are more expanded or dilated within 
 the interglobular spaces, as they probably meet with less 
 resistance than in the calcified dentine. In advanced caries 
 beneath the enamel the interglobular spaces break down 
 
 FIG. 146. Interglobular spaces in dentine. ( x 300.) 
 
 and are often seen to be crowded with micro-organisms. The 
 spaces are probably occupied by the uncalcified ground 
 substance of the dentine as first stated by Kolliker. 
 
 The fact that the tubes of the dentine communicate with 
 the spaces of the granular layer, while in the larger spaces 
 they form no such communication, would apparently point 
 to the explanation that the spaces of the granular layer 
 represent a normal and functional structure, while the 
 larger spaces are due to a defect in calcification. 
 
 A curious condition which the author has met with 
 
DENTINE 243 
 
 several times in dentine is shown in fig. 123, previously 
 referred to, where a transverse dentinal tube anastomosing 
 with several normally-directed longitudinal ones is seen. 
 ft is very difficult to account for this condition, but in 
 one unerupted human premolar a similar arrangement of 
 the tubes was seen in many places around the circumference 
 of the pulp. 
 
 The Dentinal Tubes. As seen in a drv section, the tubules Fine 
 
 "U "U * 
 
 of the dentine arise by open ends at the periphery of the O f ^ e " 
 pulp cavity and gradually become narrower as they extend tubes. 
 
 FIG. 147. Branching of dentinal tubes (silver pyridin). ( x 150.) 
 
 throughout its substance, terminating at the cement and 
 enamel margins in very fine subdivisions or branches. These 
 fine branches are, however, not confined to the terminations 
 of the tubes, but suitable stains show their presence in all 
 parts of the dentine (see figs. 147-151). Referring to these 
 fine ramifications of the dentinal tubules, Rose says (19) that 
 at the edge of the dentine the ramifications represent a true 
 dichotomous branching of the dentinal tubes, and that 
 this occurs but rarely in the portions of the dentine nearer 
 to the pulp ; in the preparations from which the accompany- 
 ing illustrations are taken such a termination of the tubes 
 
 R 2 
 
244 MICROSCOPIC ANATOMY OF THE TEETH 
 
 is, however, frequently seen. He considers that this dicho- 
 tomous branching (fig. 124) represents the ' real branching 
 of the protoplasmic dentinal fibres formed by the fusion 
 together of the dentine processes of the young odontoblasts 
 so as to form a single process ', and that they have no 
 similarity to the very delicate connecting branches between 
 the dentinal tubules (shown in figs. 147, &c.) ; these 
 he considers are not lateral outgrowths of the dentinal 
 fibril, but are the remains of the uncalcified ground substance 
 
 _ FIG. 148. Branching of dentinal tubes (pyridin and silver). ( x250.) 
 
 or matrix of the dentine. A glance at the illustrations in 
 the present work will, we think, afford convincing proof 
 that these fine branches are connected with the dentinal 
 tubes and emanate from them. In the specimens shown 
 in Rose's figures the dentinal tubes only show short stunted 
 branches, the Golgi method of staining not having brought 
 out the finer extensions. 1 He further says, ' We look in vain 
 for these connecting branches in sections of decalcified dentine, 
 though the true terminal branches of the dentinal tubules 
 are all the more distinct '. The specimens here shown are, 
 
 1 As stated elsewhere, the author considers that the method of Golgi, 
 excellent as it is for many tissues, is not applicable to the study of dentine. 
 
DENTINE 245 
 
 however, all from decalcified teeth except fig. 151, which was 
 taken from a ground section. In ground sections the author 
 has found that the finer branches cannot be brought out in 
 the same preparation in all parts of the dentine as they can 
 in the decalcified sections. Rose states that in transverse 
 sections the branches cannot be seen to communicate with 
 the dentinal fibril, but figs. 149 and 150, where the tubes 
 are seen cut transversely, distinctly show that they can be 
 seen to do so. As the stain penetrates these ramifications 
 
 FIG. 149. Branching of dentinal tubes (silver pyridin). ( x250. 
 
 uninterruptedly there seems very strong evidence that they 
 contain fine subdivisions of the protoplasmic fibril. It is 
 difficult to explain the origin of these fine divisions, but it 
 was supposed by Von Ebner and Kolliker that they arise 
 secondarily by an outgrowth from the dentinal fibril. 
 Waldeyer considered that they were the connecting bridges 
 between the odontoblasts, but Rose says he has never 
 succeeded in demonstrating these connecting bridges, and 
 that ' nowhere in the animal kingdom has such a phenomenon 
 been observed as a process of a connective-tissue cell giving 
 off numerous lateral branches in the form of a feather ' 
 Despite these objections, and whatever may be the 
 explanation of the origin and formation of these delicate 
 
246 MICROSCOPIC ANATOMY OF THE TEETH 
 
 offsets from the dentinal tubes, the evidence that they 
 exist and arise from the tubes is, we think, not capable 
 of contradiction. In fig. 124 the fine terminations 
 are seen at the cement margin of the dentine, and in 
 the photograph of the ground section (fig. 151) they are seen 
 just beneath the enamel. The decalcified sections afford 
 very beautiful microscopic objects, especially under a low 
 power, the ground substance being everywhere traversed 
 by these delicate ramifying processes passing in every 
 
 FIG. 150. Branching of dentinal tubes. Transverse section 
 (silver pyridin). ( X 250.) 
 
 direction and seen in transverse as well as longitudinal 
 
 section. In fig. 149 there is an appearance of very minute 
 
 thorny projections bordering these fine branches. Both 
 
 the ground and decalcified sections were made from teeth 
 
 in which the soft parts had been retained and fixed in formol. 
 
 Primary The tubes do not pass in a straight course from the pulp 
 
 secondar ^ ^ e margin, but in an undulatory manner, and the curves 
 
 curva- or undulations are seen to be both short and abrupt and 
 
 longer and more open. The long undulations are called the 
 
 ' primary curvatures ' and the shorter ones the ' secondary 
 
 curvatures '. This appearance of curvature is due to the 
 
DENTINE 
 
 247 
 
 more or less spiral course of the tubes, but the degree of 
 curvature varies very greatly in different teeth and in 
 different parts of the same tooth. This arrangement of 
 the tubes gives rise to an appearance in the dentine described 
 as Schreger's lines. These lines are parallel with the surface Schreger 
 contour of the dentine, and are due to the primary curva- 
 tures of the tubes coinciding with one another in the 
 thickness of the dentine. 
 
 The contour lines of Owen also describe markings due Contour 
 
 lines of 
 Owen. 
 
 '( R 
 
 
 FIG. 151. Branching of dentinal tubes just beneath the enamel (silver 
 pyridin). Ground section. </. Interglobular space. (x250.) 
 
 to the same cause, and others due to the presence of rows of 
 interglobular spaces which occur in great abundance in the 
 teeth of Cetacea and in the ivory of the Elephant's tusk. 
 
 In some places tubes are seen to terminate in branches 
 within the substance of the dentine (figs. 124 and 147). 
 
 In transverse sections of dry dentine the tubes appear 
 as round holes within the matrix with a strongly-marked 
 surrounding border giving them the appearance of rings 
 (fig. 152). This appearance, however, is somewhat deceptive, 
 even very thin sections having an appreciable thickness. 
 The appearance may be due to this fact, and the double 
 
248 MICROSCOPIC ANATOMY OF THE TEETH 
 
 contour may not indicate a distinct wall to the tube. 1 
 The existence of such wall to the tube is denied by some 
 observers, and considerable controversy has arisen as to 
 the exact structure, and the relations of the dentinal tube, 
 matrix, and dentinal fibril within the tube. Most authorities, 
 however, consider there is a distinct lining substance to the 
 tube the sheath of Neumann (16). This is considered to 
 be of the nature of a tissue on the borderland of calcification ; 
 it is indestructible by strong acids and remains, after thorough 
 
 FIG. 152. Human premolar. Transverse section (silver pyridin). 
 Showing stained sheath of Neumann. t (x800.) 
 
 destruction of the collagen basis, as a confused mass of 
 fibres consisting of elastin (C. S. Tomes). 
 
 The den- In living dentine the tubes are occupied by a soft fibril, 
 ajjjj * the dentinal fibril, which is a prolongation of the substance 
 sheath of o f the odontoblast cell in the pulp. The dentinal fibril was 
 * first described by Sir John Tomes (21). It is accompanied 
 within the tube by one or more neurofibrils prolonged from 
 the nerve-end cells in the pulp, which course with it through- 
 out the dentine. The neurofibrils are not visible in dentine 
 when stained by ordinary methods, but are only revealed by 
 the employment of special methods of staining. As stated 
 above, various opinions have been held as to the exact 
 
 1 In caries of the dentine an extraordinary amount of thickening o- 
 the tube wall is often seen. 
 
DENTINE 249 
 
 relations of the dentinal fibril to the tube of the dentine. 
 Romer considers that there is no definite wall to the dentinal 
 tube, which is merely an interval or channel in the matrix : 
 he thus does not acknowledge the existence of the sheath 
 of Neumann as a definite structure. He considers the so- 
 called Neumann's sheath to be a part of the soft fibril, 
 and that in the preparations in which it is claimed that the 
 sheath is stained, what is really stained is the outer surface 
 of the soft dentinal fibril. He shows a figure in the illustra- 
 tions to his paper on the subject in which, in transverse 
 section, the fibril has fallen out, and there is a perforation 
 in the dentine with no wall to it. Judging by the illustration 
 it would seem that in this instance the fibril only is stained, 
 and the sheath not, as the author has several preparations 
 which show an outer ring enclosing another dark-stained 
 ring which is the border of the dentinal fibril. 
 
 In these preparations stained by a modification of the 
 silver nitrate method of Ramon y Cajal, the sheath of Neu- 
 mann is stained a deep black, and the dark-stained fibril 
 is seen lying within the ring, an appearance which cannot 
 be explained on the assumption that the supposed sheath 
 is the outer border of the stained fibril (fig. 152). In this 
 photograph it is to be seen that at the thinned- off margin 
 of the section the segments of circles are visible, surrounded 
 by a thin black line, and at the margin of these segments 
 this thin line is seen to project from the dentine, indicating 
 that it is a firm substance not connected with the fibril and 
 firmer than the surrounding matrix substance. 
 
 A very elaborate examination into the minute structure 
 of dentine has lately been undertaken by Professor Hana- 
 zawa of Tokyo (11). He made use of very many different 
 reagents and methods of staining, and concludes that the 
 wall of the dental tubule has no special resistance to acids 
 or alkalis, and cannot be isolated, and it is unreasonable to 
 call it a sheath, and that the fibrillar substance isolated by 
 mechanical and chemical methods is the dentinal fibril and 
 not the sheath of Neumann. A strong argument, however, 
 for the view that this residue consists of Neumann's sheath 
 is found in the fact mentioned by Tomes, that a tooth 
 section boiled in a caustic alkali or allowed to putrefy still 
 
250 MICROSCOPIC ANATOMY OF THE TEETH 
 
 shows the dentinal sheaths, but, as stated by Kolliker, the 
 fibrils cannot be seen. Hoppe (10) and others have also 
 demonstrated it in fossil teeth, where it is quite certain that 
 no protoplasmic material can be present. 
 
 Romer considers that the dentinal fibril or process of 
 the odontoblast is tubular and contains a fluid or semifluid 
 substance, but his description of this hollow fibril as the 
 dentinal tubule is, we think, singularly unfortunate and leads 
 to much confusion. The nerve fibres which he describes as 
 entering the dentine he thus considers run within the tubular 
 dentinal fibril. For the history of these different views and 
 the full literature on the subject the reader may be referred to 
 Summary, the original paper (18). From this and other observations on 
 the structure of the tubes and fibril the following would 
 appear to be the best-established views of these structures. 
 The odontoblast cell is provided with a process, the dentinal 
 fibril, which enters the dentinal tube. This process is 
 a prolongation or extension of the protoplasm of the cell, 
 and carries with it into the dentine an extension of the 
 delicate cell membrane which invests the odontoblast cell. 
 This enclosing membrane stains, according to Hanazawa, with 
 hsematoxylin, but the fibril cannot, we think, be accurately 
 described as a tube, but simply as a protoplasmic process 
 with a slightly denser, stainable outer layer. The fibril 
 does not, judging from appearances, entirely fill the tube, 
 but there is a slight space around it along which fluids 
 can pass. 
 
 The lining of the dentinal tube along which this fibril 
 passes would appear to be of a distinctly different consistence 
 to the rest of the matrix, and it colours with basic stains 
 and with silver nitrate. This lining, usually described as the 
 sheath of Neumann (and not considered to exist by Romer), 
 is shown by Hanazawa to be present, but he says it is not 
 unusually resistant to acids, thus differing from Neumann, 
 Tomes, and others. 
 
 The substance which is isolated after the dentine matrix 
 has been destroyed by strong acids he considers to consist 
 of the dentinal fibrils themselves, which he says show more 
 resistance to acids than the walls of the tubes. He describes 
 the fibril as nearly filling the dentinal tube, only a very slight 
 
DENTINE 251 
 
 interval being perceptible between it and the tube wall, 
 and that acids do not cause contraction or shrinkage of the 
 fibril. He also describes slight enlargements of the fibril at 
 the points where processes are given off to the finer divisions 
 of the tubes. 
 
 The dentinal fibril contributes delicate protoplasmic off- 
 sets to the branches of the dentinal tubes which also 
 contain the finer subdivisions of the neurofibrils. 
 
 FIG. 153. Calcified deposits in pulp, showing incorporation of connective- 
 tissue fibres and cells. Compare with fig. 206 showing calcification within 
 connective tissue of the follicle. ( X 150.) 
 
 Secondary Dentine. Erratic deposits of calcified sub- 
 stance are occasionally found in the pulp, the cells of the 
 pulp taking on a calcifying function. This deposit may 
 appear as detached masses or as an incremental deposit 
 upon the inner surface of the already formed dentine. The 
 latter form, called by Hope well Smith adventitious dentine, 
 is frequently found to be formed as a protection against the 
 approach of caries. 
 
 The detached masses (fig. 153) are round, oval, or irregular 
 in form ; they sometimes appear to be tubular, but the 
 arrangement of the apparent tubes is not uniform as in 
 
252 MICROSCOPIC ANATOMY OF THE TEETH 
 
 normal dentine, but they are collected into bunches or 
 extend as radiating irregular prolongations. This deposit 
 may apparently take place as a normal physiological process, 
 but is usually a pathological one, more or less associated 
 with degenerative changes. We know that calcification may 
 take place in any degenerating tissue deprived of its natural 
 blood supply, as occurs in many tissues and organs under 
 such conditions. 
 
 Secondary deposits in the pulp cavity are met with 
 
 FIG. 154. Pulp stone in Elephant's incisor showing incorporation 
 of connective tissue. ( x 350.) 
 
 in great frequency in other Mammalia, and may partake 
 of the characters of osteo- or vaso-dentine. In the Elephant, 
 where the pulp cavity of the tusk has been injured, 
 the deposit of the secondary nodules is an attempt at repair 
 by the active cells of the pulp, and results in the formation 
 of numerous rounded, often concentrically marked bodies, 
 which frequently are seen to be fused into a larger mass 
 (fig. 154). 
 
 The author has shown (15 a) that in the Elephant these 
 masses of calcified matter in the pulp are calcified within 
 a very evident connective-tissue stroma. 
 
DENTINE 253 
 
 The odontoblasts in old age atrophy and disappear, and 
 sometimes the whole pulp is occupied by dense connective 
 tissue in which no cells, blood-vessels, or nerves are to 
 be seen. The pulp may also undergo fatty degeneration. 
 
 Vascular canals are occasionally seen in human dentine, 
 especially in the root portion of the tooth. 
 
 Plicidentine. The ultimate structure of plicidentine is 
 similar to that of ordinary orthodentine, but the pulp 
 chamber, instead of being of a simple form and occupying the 
 
 FIG. 155. Transverse section of plicidentine tooth of Zygobatis, 
 showing intercommunicating tubes of the columns radiating from the 
 pulp cavities. (250.) 
 
 centre of the tooth, is folded upon itself, each portion of 
 the pulp having its separate system of radiating tubes, the 
 intricate foldings so produced forming a very complicated 
 pattern in many of these teeth (fig. 155). There may be a 
 central pulp cavity which sends out offshoots to the different 
 portions of the folded dentine, forming a star-shaped figure 
 as seen in Lepidosteus, or as in Lepidosteus spatula there 
 may be no definite central pulp cavity but the position 
 of this central portion is occupied by numerous small 
 radiating systems of tubes around small ingrowths of the 
 pulp. Around this central system are long processes of 
 
254 MICROSCOPIC ANATOMY OF THE TEETH 
 
 dentine in which the tubes are given off from the foldings 
 of the pulp, as Tomes says, ' like a paddle-wheel '. 
 
 These radiating portions are sometimes branched and 
 subdivided, showing a still more complicated pattern, and 
 the effect of a convoluted tubular system is given as in the 
 fossil Labyrinthodon (fig. 56). 
 
 As shown by Tomes there are two systems on which 
 plicidentine is formed : one in which the foldings may be 
 described as lateral, as in the lizard Varanus, in the 
 bony pike, Lepidosteus, and in Labyrinthodon, and another 
 
 FIG. ] 56. Transverse section of tooth of Labyrinthodon showing highly 
 convoluted plicidentine. Reduced from Owen's Odmitography. 
 
 in which parallel vertical* columns of the pulp give rise to 
 radiating dentinal systems around them, these systems 
 lying side by side and the terminations of the tubes of one 
 system often communicating with those from neighbouring 
 columns (figs. 157 and 158). 
 
 This latter structure is seen in the Rays, Myliobates, and 
 Zygobates (fig. 155), andin the rostrum of the saw-fish (Pristis) 
 and in the teeth of the Cape Ant-eater (Orycteropus) . In the 
 lower part of the tooth in these animals the pulp is more 
 or less fused by the union of the pulps of the separate 
 denticles. ' Instead of being regarded as a plicidentine 
 such a tooth might be said to be built up of a series of 
 small, parallel, fused denticles, or exceedingly broadened 
 and fused cusps.' * 
 
 1 C. S. Tomes, Dental Anatomy, 7th ed., p. 89. 
 
DENTINE 
 
 2f>5 
 
 FIG. 157. Tooth of a tertiary fossil fish (Aetobatis) showing the plici- 
 dentine in longitudinal section. From a specimen lent to the author by 
 Mr. J. Humphreys. 
 
 FIG. 158. Longitudinal section of plicidentine from the fossil eocene fish 
 Edaphodon. From Mr. Humphrey's collection, p. Pulp cavity. 
 
256 MICROSCOPIC ANATOMY OF THE TEETH 
 
 While we may look upon plicidentine as orthodentine in 
 which the arrangement of the tubes is so modified as to 
 produce a number of different and complicated patterns, 
 the next variety of dentine which we shall consider has an 
 altogether different structure. 
 
 Vasodentine. Vasodentine differs from orthodentine 
 mainly in the fact that it contains no dentinal tubes, but 
 their place is taken by a system of blood channels containing 
 blood-vessels in connexion with the vessels of the tooth- 
 pulp. These blood-vessels have a very definite and regular 
 arrangement in typical vasodentines, blood corpuscles are 
 seen within them, and the blood in life circulates through 
 them. 
 
 ' The arrangement of the vascular canals is regular and 
 striking, , reminding one of the appearance of the vessels 
 in an intestinal villus ; in fact, an intestinal villus petrified, 
 whilst its capillary network remained pervious, and red 
 blood continued to circulate through it, would form no 
 bad representation of a typical vasodentine tooth.' 1 
 
 Perhaps the most typical vasodentine is seen in the tooth 
 of the Hake (Merluccius) (fig. 159 and Plate VI). 
 
 A strongly-marked lamination of the matrix parallel to the 
 surface of the pulp cavity is very evident in these teeth, and 
 that this is a structural condition is shown by the breaking 
 up of sections of the Hake's tooth into parallel laminae. 
 Thorn-like projections from the vascular tubes along the 
 lines of the laminse are also evident in many places (fig. 159). 
 
 The outer layer of the tooth in the Hake contains no 
 blood channels and shows a faint indication of lamination. 
 This layer is described by Rose as vitrodentine, but as this 
 part of the dentine does not differ in structure or develop- 
 ment from the matrix material which intervenes between 
 the vascular tubes, Tomes does not consider that a distinctive 
 name should be applied to it. 
 
 The vascular canals which do not enter this layer form 
 loops at its inner boundary which sometimes communicate 
 with a single peripheral channel in this situation (fig. 181). 
 
 In the flounder, at and near the tip of the conical tooth, the 
 dentine is of the true orthodentine variety ; but lower down 
 1 C. S. Tomes, Dental Anatomy, 7th ed., p. 91. 
 
DENTINE 257 
 
 scattered vascular tubes appear, and in the lower portion 
 of the tooth there are no dentinal tubes, but a typical vaso- 
 dentine has taken their place. From this, as Tomes says, 1 
 ' it may be learnt that hard dentine and vasodentine are 
 not fundamentally dissimilar, and that they may pass into 
 one another by imperceptible gradations, so that it cannot 
 be said exactly at what point the name of vasodentine is to 
 be given to it '. The teeth of the extinct Megatherium show 
 a regular system of vascular canals on the inner portion of 
 
 
 FIG. 159. Vasodentine of Hake (Gadus Merluccius) showing thorn-like 
 projections and longitudinal striation of the matrix. ( X 150.) 
 
 the dentine terminating at a definite line, the rest of the 
 tissue being made up of tubular or ortho- dentine. The 
 outer part of the dentine of the Manatee shows forms which 
 have the general appearance of vascular loops, but con- 
 siderably modified by the encroachment of calcification 
 upon them, the rounded contours of the margins suggesting 
 a resemblance to elongated interglobular spaces (Tomes). 
 
 From the evolutionary standpoint the above observations 
 
 are of considerable interest ; it is an undecided question 
 
 whether the ancestral form of dentine was of the tubular 
 
 or vasodentine variety. They seem to exist side by side 
 
 1 C. S. Tomes, Dental Anatomy, 7th ed. ; p. 94. 
 
 MUMMERY g 
 
258 MICROSCOPIC ANATOMY OF THE TEETH 
 
 in very early forms, but in the higher Mammalia the tubular 
 or ortho- dentine seems to have supplanted in most cases the 
 probably more primitive vasodentine. 
 
 In Sargus ovis loops are seen within the dentine which 
 appear to be the remains of vascular tubes, and may indicate 
 that the dentine of Sargus was derived from a vasodentine. 
 Similar loops at the base of the tooth in Sargus are evidently, 
 in the author's preparations from freshly-fixed material, in 
 communication with the pulp cavity, while isolated crescentic 
 forms are seen deeply in the dentine. In ground preparations 
 
 FIG. 160. Vascular network in centre of incisor tooth of Sargus 
 ovis. e. Enamel ; d. dentine. Ground section. (x!50.) 
 
 of similarly preserved incisor teeth of Sargus, what certainly 
 appear to be vascular canals are seen passing out from the 
 narrow prolongation of the pulp cavity in the crown and 
 forming a branched network which reaches up to the enamel 
 (fig. 160). This was found in all the teeth examined, when 
 they were so cut as to expose the central part of the axis 
 of the tooth, and this branching system appears to be 
 confined to this area and to extend in a direction parallel 
 to the flattened surfaces of the. tooth. Again, in Scarus, 
 loops having every appearance of being vascular tubes are 
 seen in abundance in the dentine, and at the enamel margin 
 they distinctly project into the enamel (fig. 161). 
 
 In the teeth of the Tasmanian Devil (Sarcopkilus ur sinus) 
 of which a good preparation, preserved in formol, was 
 
DENTINE 259 
 
 given to the author by Dr. Mackenzie, not only scattered 
 crescentic loops are found in the deeper parts of the dentine, 
 but in several places around the circumference of the pulp 
 channels are seen in the dentine communicating with the 
 pulp, and blood-vessels enter them, the endothelial cells 
 and blood corpuscles being evident within the canals in 
 the dentine. The blood-vessels can be traced along the pulp 
 and seen to cross the odontoblast layer and enter the 
 tubular canals, which penetrate some little distance into the 
 dentine. As, however, these vascular canals pursue a very 
 
 e 
 
 FIG. 161. Vascular loops in tooth of Pseudoscarus. e. Enamel ; 
 d. dentine. Ground section. (x!50.) 
 
 twisted course, more deeply in the dentine only portions 
 of them are seen, cut across (fig. 162). 
 
 In the Cynomys (Prairie Marmot) (fig. 163) the author 
 found the dentine to be permeated by vascular canals in 
 great abundance. He has not been able to find any record 
 of the previous observation of this condition in Cynomys, 
 but it is very evident in ground sections of the incisor and 
 can scarcely have failed to be detected. Short vascular 
 canals in the dentine of many rodents are described by 
 Owen (17) and J. Tomes (21 b). 
 
 From these observations we see that the most permanent 
 and highly developed form of dentine would appear to be 
 tubular or ortho- dentine, and that in the course of develop- 
 
 S 2 
 
260 MICROSCOPIC ANATOMY OF THE TEETH 
 
 ment to higher forms the vasodentine structure has gradu- 
 ally disappeared, only existing here and there as vestiges ; 
 but in Sarcophilus and Cynomys there would appear to 
 be a more complete indication of reversion to the original 
 structure. 
 
 A similar argument applies to the tubular enamel of 
 marsupials, where we see a reduction of the tube system 
 even in the same class of the mammalia, until in the Wombat 
 it has ceased to be apparent, but in Hymx, Dipus, and Sorex 
 
 FIG. 162. Dentine of Sarcophilus ursinus. Capillary 
 vessels entering dentine. ( x 350.) 
 
 among the higher mammalia we find a more or less complete 
 reversion to the original tubular enamel, and in man and 
 other mammals a slight penetration by tubes shows that all 
 traces of such a condition have not been finally lost. 
 
 Osteodentine or Trabecular Dentine. Osteodentine is very 
 nearly allied to bone in structure. There is no distinct 
 pulp cavity, but the interior of the tooth is traversed by 
 bony trabeculae, the interspaces being occupied by medullary 
 canals and blood-vessels which may be considered to take 
 the place of the pulp. The large medullary channels at the 
 base of the tooth divide and ramify like the branches of 
 
DENTINE 261 
 
 a tree, spreading out on every side towards the circum- 
 ference. In some sharks, and in the Pike, a more regular 
 arrangement of these medullary spaces is seen at the 
 periphery, giving this layer more the appearance of ortho- 
 dentine ; but the tubes do not enter at their bases into a pulp 
 cavity, but are continuous with the main branches of the 
 medullary system in the centre of the tooth as in Lamna 
 (fig. 164). 
 If treated with alcoholic fuchsin (allowed to penetrate by 
 
 FIG. 163. Dentine of incisor of Cynomys (Prairie Marmot) 
 showing vascular loops. ( x 350.) 
 
 capillary attraction) the structure of an osteodentine tooth 
 is beautifully brought out, the stain penetrating into 
 the very finest divisions of the branching processes, which 
 resemble the canaliculi of bone (fig. 165). 
 
 In some examples of osteodentine the resemblance of the 
 tissue to bone is much more marked than in the specimens 
 shown in the illustrations, and traces of lamination are seen. 
 ' The similarity of the channels of the pulp in osteodentine 
 to Haversian canals in bone is in some respects close ; so 
 similar, that when teeth consisting of osteodentine become, 
 as in many fish they do, anchylosed to the subjacent bone, 
 
262 MICROSCOPIC ANATOMY OF THE TEETH 
 
 it becomes impossible to say at what point the dentine ends 
 and the bone commences ; and this difficulty is intensified 
 by the fact that the bone of many fishes lacks lacunae and 
 is almost exactly like dentine.' l 
 
 The Development and Calcification of Dentine. As pre- 
 viously stated, dentine is developed from the mesodermic 
 dentine papilla, which is a condensation of the cells of the 
 mesoderm beneath the previously-formed ectodermic enamel- 
 forming cells. As shown by Von Brunn (4), in several orders 
 
 FIG. 164. Osteodentine tooth of Lamna cornubica (Porbeagle Shark). 
 Osteodentine ; central tubes radiating from medullary channels, e. Enamel. 
 Fuchsin stain by capillary attraction. ( x 50.) 
 
 of the Mammalia the appearance of epithelial cells always 
 precedes and appears to determine the formation and limits 
 of growth of the dentine. Von Brunn and Von Ebner (6) 
 were unable to substantiate the existence of this determining 
 epithelial organ in the formation of the dentine of the root 
 in man, but as explained in the chapter on ' The Tooth 
 Follicle and its Connexions ', the author has found a similar 
 condition in human teeth. He has shown that this epithelial 
 sheath proceeds from cells of the follicle which are not 
 differentiated to form an enamel organ. This, however, 
 1 Tomes's Dental Anatomy, 7th ed., p. 100. 
 
DENTINE 263 
 
 will be more fully considered when treating of the * Sheath 
 of Hertwig '. 
 
 In teeth in which enamel is present, however, the forma- 
 tion of an enamel organ always precedes the differentiation 
 of the cells of the papilla to form a dentine organ, although 
 calcification in Mammalia commences first at the summit of 
 the dentine papilla. In some fish the calcification of the 
 enamel precedes that of the dentine, and in marsupials, 
 although the process of calcification commences in the 
 
 I 
 
 FIG. 165. Enamel and osteodentine of Heterodontus (Cestracion). 
 Fuchsin stain by capillary attraction. ( x 50.) 
 
 dentine, a very much larger proportion of enamel is soon 
 laid down above it ; this enamel, however, is not at this 
 stage fully calcified. 
 
 When the dentine papilla is first seen to form a definite 
 dentine organ or tooth-pulp, and is enclosed above and at 
 the sides by the prolongation of the enamel organ, there is 
 no differentiation of the connective-tissue cells to form a 
 distinct peripheral layer beneath the enamel organ, but they 
 are uniformly distributed throughout the pulp. 
 
 Very soon, however, larger rounded cells are seen accumu- 
 lating at the margins of the pulp. These cells have large 
 nuclei and a very small amount of cytoplasm, and many have 
 short truncated prolongations, but there is no appearance 
 
264 MICROSCOPIC ANATOMY OF THE TEETH 
 
 of a distinct process as shown in fig. 1 15, &c. The connective 
 tissue of the pulp is seen passing between these cells, which 
 lie in its meshes. The rounded cells at the circumference 
 soon assume a pyriform or subcylindrical form, and a long 
 process extends from their upper end which develops into 
 the dentinal fibril (fig. 166). These cells are described as 
 odontoblasts. The first appearance of the dentine occurs 
 in the form of a clear layer of a semi-transparent substance 
 which borders the pulp and is in contact with the enamel 
 
 FIG. 166. Macropus. First commencement of calcification of the dentine 
 at the coronal part of the pulp. a. Ameloblasts ; o. odontoblasts ; c. com- 
 mencing calcification of the dentine ; b. odontogenic zone. ( x 650.) 
 
 organ cells externally and with the now distinctly differ- 
 entiated odontoblasts internally, and their dentinal processes 
 can be seen passing across this area. A thin line of calcification 
 commences with rounded contours towards the pulp, and is 
 separated from the odontoblasts by the clear layer above 
 described. 
 
 Two different views have been held as to the mode of 
 formation and calcification of dentine. These may be called 
 the ' conversion ' and ' secretion ' theories. For a long time 
 the view most widely received and embodied in all the 
 principal text-books was the conversion theory of Waldeyer, 
 
DENTINE 265 
 
 Boll, and others, and upheld by C. S. Tomes in the earlier 
 editions of his Dental Anatomy, where he says : ' The 
 dentine is, I believe, formed by the direct conversion of 
 the odontoblast cells just as the enamel is by the enamel 
 cells, and is derived from them and from them alone.' 
 Schwann also looked upon the dentine as being probably 
 the ossified substance of the pulp, and Waldeyer, considering 
 the process of ossification to be identical with that of 
 ordinary bone, held that the dentinal fibrils are the central 
 remains of the odontoblasts, while their peripheral portions 
 become basis substance. 
 
 The other view, that of secretion, was held by John Hunter, 
 who says, ' The ossified part of a tooth would seem to have 
 much the same connexion with the pulp as a snail has with 
 its shell '. 
 
 Kolliker, Lent, Hertz, and Baume looked upon the forma- 
 tion of dentine as a secretion process. Baume says : ' The 
 odontoblasts secrete a material which calcifies, rather than 
 that they themselves are converted.' 
 
 Tomes, in the later editions of his book, considers that 
 the dentine is calcified by a process of secretion, and follow- 
 ing the previous investigations of Von Ebner and others, 
 the conversion theory of the formation of dentine has been 
 to a great extent abandoned, this conversion theory being 
 that the odontoblast cell becomes actually converted into 
 dentine matrix, its centre remaining uncalcified as the soft 
 fibril, and the rest of the cell forming in different degrees 
 of calcification the Neumann's sheath and the matrix. 
 The view held by the author and by numerous histologists 
 at the present day is that the cells of the pulp secrete 
 a material which calcifies, they themselves not entering 
 into the calcified substance but receding farther and farther 
 into the pulp as calcification advances, and the fibril becomes 
 more and more elongated. 
 
 The Dentine Matrix. Professor von Ebner, in his paper 
 in the Handbuch der Zahnheilkunde, 1891, described the 
 resemblances of dentine to bone : he showed that in decal- 
 cified dentine (treated with hydrochloric acid in a salt 
 solution) a fibrillar structure can be detected, and that by 
 tearing the decalcified dentine the fibrillse could be some- 
 
266 MICROSCOPIC ANATOMY OF THE TEETH 
 
 times isolated. He describes these fibrillse as being very 
 fine, scarcely more than 5 \j, thick, and as showing the same 
 characteristics as the glue-giving connective -tissue fibres, 
 but they are not arranged in lamellae as in bone. The fibres 
 cross each other and mostly in planes perpendicular to the 
 dentinal tubules. Von Ebner considered that these fibres 
 were due to a change in the upper and peripheral portion 
 of the odontoblast cell, and according to Rose's explanation 
 of this view, in some cases ' the peripheral ends of the 
 odontoblasts are first of all changed into a homogeneous 
 layer of non-granular protoplasm in which the fibrils 
 become visible only later'. 1 This author also says 'the 
 dentinal fibrils are those remains of the cell bodies which 
 are left when the odontoblasts are changed into gelatine- 
 yielding fibrils, and which retain their protoplasmic struc- 
 ture '. According to this view there would be a gradual 
 using up of the peripheral portion of the odontoblast cell 
 as calcification advances, the layer of uncalcified dentine 
 in the odontogenic zone being formed ' by the change of 
 the peripheral portion of the odontoblasts into gelatine- 
 yielding fibrils. The dentine calcifies by the deposition of 
 salts of lime between the fibrils.' In fig. 167 delicate fibres 
 are seen crossing the odontoblasts and entering the odonto- 
 genic zone which certainly appear to be fine fibres from 
 the pulp, and in fig. 168, from the tooth of a calf treated with 
 chromic acid, fibres can be seen crossing the odontoblasts 
 and entering the odontogenic zone. 
 
 The author has but lately found further corroboration of 
 his view that connective-tissue fibres prolonged from the 
 tissue of the pulp enter the dentine and form its fibrillar 
 basis as explained on p. 268. 
 
 It appears possible that a portion of this fibrillar founda- 
 tion substance, forming the odontogenic zone, may be laid 
 down by the odontoblasts as Von Ebner describes ; but many 
 
 1 Rose, in his paper above referred to, says : ' As Partsch correctly 
 remarks., Mummery has examined pathologically changed human teeth 
 or unfavourably preserved preparations.' This assumption of Professor 
 Partsch endorsed by Rose is made without any evidence, he never 
 having seen the preparations or made any inquiry concerning them. 
 Such a method of criticism needs no further comment. 
 
DENTINE 
 
 267 
 
 z 
 
 -P 
 
 FIG. 167. Human premolar. Weil process, showing the fine connec- 
 tive tissue investing the odontoblasts. d. Dentine : z. odontogenic zone : 
 o. odontoblasts; p. pulp. (x250.) 
 
 P 
 
 FIG. 168. Tooth of Calf. Ground section, by the Weil process, showing 
 fibres between and around odontoblasts becoming incorporated in odonto- 
 genic zone. d. Dentine ; z. odontogenic zone ; o, odontoblasts ; p. pulp. 
 (x400.) 
 
268 MICROSCOPIC ANATOMY OF THE TEETH 
 
 preparations of the author's suggest that this zone is also 
 penetrated by delicate connective-tissue fibres from the 
 pulp (15 a). The opinions of such an authority as Professor 
 von Ebner, who has studied this subject for many years, must 
 always be received with the greatest respect. 
 
 Klein (12 a) held that the network of reticular tissue in 
 the substance of the odontoblasts is the reticular basis of 
 the dentine matrix, which would be thus an intracellular 
 substance. 
 
 The author in a paper published in 1892 (15 a) showed that 
 connective-tissue fibres from the pulp pass into the forming 
 dentine, and considered that the fibres seen in its substance 
 are the incorporated connective -tissue fibres of the pulp 
 which thus form a meshwork or foundation in which calci- 
 fication takes place. The fibres are intercellular and not 
 intracellular as considered by Klein. 
 
 As described in this paper, in preparations cut by the 
 Koch- Weil process and consequently not decalcified, processes 
 were seen springing from the dentine and blending with the 
 connective tissue of the pulp all around the margin of the 
 pulp cavity. These processes have the appearance of 
 connective-tissue bundles partially impregnated with lime 
 salts. 
 
 At the inner margin of the dentine they are seen to 
 spring from its substance in a direction more or less parallel 
 to the surface, these horizontal bundles of fibres blending 
 together into larger bundles at right angles to the surface 
 of the dentine, much as the spreading roots of a tree coalesce 
 to form its trunk. These bundles, the high refractive index 
 of which suggests their partial calcification, are plainly 
 seen to be continuous with the general connective tissue of 
 the pulp. One is reminded, in looking at these larger pro- 
 cesses, of the similar appearances in the formation of bone 
 in membrane, where spiculae are seen shooting out in advance 
 of the calcified substance (figs. 169 and 170). 
 
 At the apex or coronal portion of the pulp cavity these 
 processes are more slender, form wide open loops, and can be 
 traced for some distance into the pulp. 
 
 In sections cut somewhat obliquely (not in the same plane 
 as the odontoblasts) there is an appearance of small deeply- 
 
DENTINE 
 
 269 
 
 FIG. 169. Inner margin of pulp of human tooth. Large 
 bundles incorporated in dentine. ( X 350.) 
 
 FIG. 170. Similar to fig. 171. (x350.) 
 
270 MICROSCOPIC ANATOMY OF THE TEETH 
 
 stained cells or cell nuclei crowded upon and following the 
 course of the bundles of connective-tissue fibres above 
 described. 
 
 These cells are distinctly smaller than the odontoblasts, 
 so that one would conclude that among the odontoblasts 
 are other cells which play an important part in dentine 
 development but are not arranged in a definite layer. 
 The author has since shown that these cells are not destitute 
 
 FIG. 171. Human premolar. Margin of pulp cavity. a. opposite 
 a stellate connective-tissue cell sending processes into the forming dentine. 
 (x,350.) 
 
 of processes as stated in his paper, but a differential connec- 
 tive-tissue stain shows long processes proceeding from them 
 and a very definite amount of cytoplasm around the nucleus 
 (fig. 171). 
 
 In sections cut in the plane of the odontoblasts, where 
 these larger fibre bundles are not present, a distinct reticulum 
 of fine connective-tissue fibres can be seen passing between 
 and enveloping the odontoblasts, and by careful focusing 
 they can be seen to be gathered into bundles and incor- 
 porated with the matrix substance out of which they appear 
 to spring. The small elongated and irregular-shaped 
 connective-tissue cells are seen mingled with the odonto- 
 
DENTINE 271 
 
 blasts. It is difficult to trace these fine fibres within the 
 clear odontogenic zone, but in the tooth of the calf kept for 
 a long time in chromic acid they are very distinctly visible 
 (fig. 168). These observations were very clearly corroborated 
 by the examination of sections of the incisor teeth of a rat 
 prepared by the Weil process. A very strong connective 
 tissue is seen in the pulp, and an open-meshed reticulum 
 of connective-tissue fibres at the margin of the dentine 
 surrounded by small cells similar in appearance to those 
 of the main substance of the pulp. In these specimens 
 the fibres can be clearly traced into the formed dentine. 
 
 It has been recently stated * that the author has receded from his view 
 of the part taken by the odontoblasts in the calcification of the dentine, 
 as expressed in his paper in 1892 (15 a), and it is made to appear that he 
 did not then look upon the odontoblasts as the calcifying cells ; but in 
 that paper he speaks of the dentine as a tissue ' which, according to the 
 view of secretion here maintained, is a material elaborated by the odontoblasts 
 and other cells, upon a connective-tissue foundation '. He never considered 
 the odontoblasts to have any but a calcifying function, but considered 
 that other small cells found in the pulp also took some part in the produc- 
 tion of the matrix, and recent observations, as shown in fig. 171, have con- 
 firmed him in that conclusion. Nowhere in the paper referred to or else- 
 where has he denied the principal part taken by the odontoblasts in the 
 calcifying process, and never at any time has he considered that sensation 
 was conducted by the dentinal fibril, as this criticism might appear to 
 suggest. 
 
 In the rat the fibrous strands are very distinct within 
 the formed dentine, forming a band of a slightly darker 
 appearance than the rest of the dentine and fading away 
 in the deeper parts of the tissue approaching the enamel 
 (fig. 172). 
 
 The incorporation of connective-tissue fibres in the 
 forming dentine of the Elephant is very evident (fig. 173), 
 and, as described in considering the development of vaso- 
 dentine, a very abundant connective tissue is incorporated 
 in the tooth of the Hake, showing that in fish, as well as in 
 several orders of the Mammalia and in man, connective tissue 
 forms a framework or basis to the dentine (fig. 181). 
 
 Von Korff in 1905 published a paper (13) in which he 
 
 1 A. Hopewell Smith, Normal and Pathological Histology of the Mouth, 
 vol. i, p. 290 (1919). 
 
272 MICROSCOPIC ANATOMY OF THE TEETH 
 
 showed that in early stages of the development of dentine 
 there was a penetration of pulp fibres between the odonto- 
 blasts, and argued that this indicated that the dentine was 
 not a product of the odontoblasts but was formed by the 
 connective tissue of the pulp. The fibres described by Von 
 Korff in the early development of the dentine are arranged 
 in a corkscrew-like form, terminating in the forming dentine 
 in a fan-shaped expansion of fibres (fig. 174). He claimed in 
 this paper that he was the first to describe connective 
 tissue in the dentine, but as Von Ebner points out, he had 
 
 FIG. 172. Pulp and dentine of Rat showing incorporation of connective 
 tissue in dentine, (x 175.) 
 
 himself described it thirty years before and, as he states, it 
 had also been demonstrated by Gebhart, Ramon y Cajal 
 (1888), and Mummery (1892). 
 
 Guido Fischer (7), writing in 1910, says that while he 
 was at first inclined to agree with Von Korff s theory, he 
 cannot accept this author's conclusions. While pulp fibres 
 are evident in developing dentine, as had been pointed out 
 by previous authors, he does not agree to this interpretation 
 of them as dentine producers, and agrees with Von Ebner 
 that the collagen fibres of the dentine substance are arranged 
 at right angles to the dentinal tubes, that they are not 
 gelatine yielding, and do not show double refraction. 
 
FIG. 173. Incorporation of connective tissue of pulp in ivory 
 of Elephant's tusk. ( x 100.) 
 
 FIG. 174. Fibres of Von Korff in tooth germ of Cat. o. Odontoblasts ; 
 /. fan-like expansion of fibre bundles ; g. corkscrew-like fibres ; z. 
 odontogenic zone ; c. commencing calcification ; a. ameloblasts. 
 (From illustration to his paper.) 
 
274 MICROSCOPIC ANATOMY OF THE TEETH 
 
 There is, we think, no doubt that bundles of connective- 
 tissue fibres, which pass into the dentine as Von Korff 
 describes, are to be seen in developing teeth, but, as Von 
 Ebner says, they do not course parallel to the dentinal 
 tubes as claimed by Von Korff, but pass within the dentine 
 generally at right angles to the tubes. These corkscrew-like 
 bundles of connective tissue are occasionally seen in the 
 pulps of fully erupted teeth (fig. 175). 
 
 Studnicka adduces, as an example of the share of the pulp 
 in dentine formation, the fibre bundles occasionally observed 
 in growing dentine, which are identical in appearance with 
 
 P 
 
 FIG. 175. Corkscrew-like fibres in human permanent tooth. 
 d. Dentine ; p. pulp. ( x 250.) 
 
 those described by Von Korff in early stages of development, 
 and which are more to be compared with the Sharpey fibres 
 in bone. Von Ebner considers that the fact that in the later 
 stages of dentine formation gelatine yielding bundles of 
 connective tissue incorporated in the dentine are to be seen, 
 which, as he says, he and the present author independently 
 observed, has no direct bearing upon the typical first 
 development of the dentine. He also looks upon these 
 gelatine-containing bundles as analogous to the penetrating 
 fibres of Sharpey. 
 
 The fibres described by Von Korff in developing teeth 
 are not gelatine-yielding fibres and do not show double 
 refraction, and we must conclude they are to be looked upon 
 
DENTINE 275 
 
 as matrix or foundation fibres in which the lime salts are 
 deposited by the agency of the odontoblasts. 1 
 
 That connective-tissue fibres from the pulp in young 
 teeth with uncompleted roots are sometimes seen passing 
 in a radiating manner into the dentine is shown in fig. 171. 
 The specimen from which this photograph was taken was 
 stained with Van Giesen stain, which brings out connective 
 tissue very conspicuously. The fibres can be seen passing 
 from the pulp and spreading out into the dentine ; deeper 
 within its substance they pass, not parallel, but more or less 
 at right angles to the dentinal tubes. The appearance in 
 these sections of small cells apparently in intimate relation 
 with these fibres when the section passes in this direction 
 is a little puzzling, as the odontoblasts are not visible as 
 a distinct layer where these rows of cells are evident, although 
 their nuclei can be seen in places lying between them. 
 This appearance was illustrated in the author's paper of 
 1892, where he described the cells as being destitute of pro- 
 cesses ; a more selective connective -tissue stain has, however, 
 since shown that a distinct cytoplasm surrounds these 
 nuclei, and that processes of the cell are produced from it 
 extending on every side (fig. 171 at a). In the later stages 
 of dentine formation, when these penetrating fibres are seen 
 in the dentine, they appear to be partially impregnated 
 with lime salts in advance of the general line of calcification, 
 and the question arises whether these smaller cells take any 
 part in the calcification of the matrix. 
 
 In the paper above referred to the author expressed the 
 opinion that ' These cells secrete a material which calcifies 
 along the line of the odontogenic fibres ', but it is very 
 difficult to decide if this is the case or not. 
 
 1 In the new edition of his Histology (1919) Professor Hopewell Smith 
 says : ' It is probable that these connective-tissue fibres are considered 
 by Howard Mummery to be the terminations of the myelinic nerve fibres 
 of the pulp.' It is difficult to understand how any histologist could make 
 such a mistake. V. Korff described these fibres only in young developing 
 teeth at the very first commencement of calcification, and although a few 
 are now and then to be met with in older pulps they are easily distinguished 
 (see fig. 175). The nerve fibres shown by the author arise from unmistakable 
 medullated nerve trunks, and moreover the fibres of V. Korff take the stain 
 very differently and much more faintly in gold preparations. 
 
276 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Von Ebner says : ' Without doubt the odontoblasts are the 
 essential and sole dentine formers in the growth of typical 
 normal dentine ; they are the gelatine-yielding cells as well 
 as the lime-salt producers ; under special conditions other 
 cells of the pulp can, however, lay down a dentine-like 
 substance.' l 
 
 That calcification can take place in the pulp independently 
 of the odontoblasts is seen in the formation of pulp-stones 
 and irregular calcific deposit, and is consistent with what is 
 shown of calcification in degenerations of connective tissues 
 elsewhere. 
 
 A comparison of fig. 177, where the connective-tissue fibres 
 of the periodontal membrane are entering the forming 
 cement, with figs. 169 and 171 showing the incorporation 
 of connective-tissue bundles in the pulp with the dentine, 
 is interesting as showing the great similarity of the appear- 
 ances in the two tissues. 
 
 To recapitulate : 
 
 Summary. (1) In the early development of the dentine, connective- 
 tissue fibres from the pulp pass in bundles into the forming 
 dentine in a more or less radiating manner, but they do not 
 course, within the dentine, in a direction parallel to that of 
 the tubes, but transversely to them. This incorporation 
 of the connective-tissue fibres is not immediately connected 
 with the calcification of the matrix. The fibres at this stage 
 are not lime-containing and do not show double refraction, 
 but they form an organic foundation or scaffolding in which 
 calcification takes place. 
 
 (2) It is not only in the earliest stages of dentine formation 
 that this meshwork of foundation tissue is laid down, but 
 it also occurs throughout the active growth of the dentine, 
 although in the later stage the fibre bundles have not the 
 regular arrangement described by Von Korff in the early 
 stages of development. Bundles of connective tissue are, 
 however, evident, which are becoming incorporated with the 
 forming dentine in later stages and appear to be analogous 
 to the penetrating fibres of Sharpey in bone. 
 
 (3) In teeth in use, in which the first formation of root and 
 crown is completed, but the cells at the circumference of the 
 
 1 In a letter. 
 
DENTINE 
 
 277 
 
 FIG. 176. Adult human molar affected by caries. Action of aoids of caries, 
 showing striation and contours as in figs. 178 and 179. ( x 600.) 
 
 Fro. 177. Sharpey's fibres penetrating forming cement ; from the same 
 tooth as that from which figs. 170 and 171 were taken. ( x 350.) 
 
278 MICROSCOPIC ANATOMY OF THE TEETH 
 
 pulp are still depositing the dentine, delicate connective- 
 tissue fibres from the pulp are continuously being incor- 
 porated with the forming dentine. 
 
 There seems to be every reason to believe that the principal 
 active agents in the separation and deposition of the lime 
 salts are the odontoblasts, and that they shed out their 
 product into the organic framework laid down to receive it, 
 and according to Von Ebner also contribute a delicate 
 fibrillar foundation to the matrix substance. 
 
 Calcification of the Dentine. In young growing teeth there 
 is a portion of the matrix bordering on the pulp forming 
 a marginal band between the calcified dentine and the 
 odontoblast cells. This layer, the odontogenic zone, shown 
 in figs. 167 and 168, appears to consist of the collagenous 
 basis substance of the dentinal matrix in which the deposit 
 of the lime salts takes place. The uncalcified portion of 
 the matrix, in teeth that have not been treated with acids, 
 unlike the calcified part, takes the stain readily, and the 
 advancing calcification is seen encroaching upon it in the 
 form of rounded masses of the lime salts, or calcospherites, 
 some of these calcospherites lying free in the surrounding 
 uncalcified material (fig. 143). The spherites in this ad- 
 vancing layer are seen to be quite clear and to exhibit no 
 radial or concentric markings in the calcified preparations. 
 
 When a young tooth is decalcified, the rounded contours 
 of the calcifying border have exactly the same appearance 
 as in the calcified tooth ; they appear structureless and 
 consist of the calcoglobulin basis of the spherites, which 
 takes stains readily. The interglobular spaces in imper- 
 fectly-developed dentine show also very clearly the advance 
 and coalescence of these bodies in the basis substance. 
 
 In some sections prepared by Ramon y CajaPs silver- 
 nitrate method, and which were taken from an unerupted 
 human premolar, a further stage in the consolidation of 
 the matrix is revealed which appears to throw a strong light 
 on the mode of calcification of dentine, and to make clear 
 the meaning of certain appearances in the adult tissue 
 which had not received a satisfactory explanation. 
 
 At the borders of carious cavities, a fine striation, very 
 much like muscle striation, is seen in the dentine in some 
 
DENTINE 279 
 
 cases, where the action of the secreted acid has been 
 in considerable advance of the invading micro-organisms. 
 An examination of the silver-stained developing tooth shows 
 that this appearance is due to structural conditions which 
 have been revealed by the action of the acid (fig. 176). 
 
 As previously stated, the calcospherites seen at the 
 margins of human dentine appear clear and structureless, 
 but it was pointed out by Rainey, referring to the calcifica- 
 tion of the clear calcospherites in shell, that ' as the develop- 
 
 FIG. 178. Calcoglobulin contours in forming dentine. 
 Uneiupted premolar. ( x 700.) 
 
 ment progresses the globules lose their bright and structure- 
 less character and begin to present laminae and radiating 
 lines just as the artificial calculi do, the lines being more 
 distinct when the globules are suffering disintegration '. 
 Figs. 178 and 179 show that this is the case in the dentine. 
 The decalcified calcospherites are seen to have first coalesced 
 into larger bodies and then to have become laminated, 
 showing strongly-marked concentric striae. The dentine is 
 stained a deep brown by the Cajal process and is everywhere 
 traversed by these contours. The circular bodies are of 
 very various sizes, and the outermost striae composing the 
 
280 MICROSCOPIC ANATOMY OF THE TEETH 
 
 rings are drawn out into laminae which pass more or less 
 parallel to the surface of the dentine. Fig. 179 shows the 
 drawing out of the marginal striae, which remain equidistant 
 from one another. It is seen in this figure that the dentine, 
 in a thin section, splits along these lines of lamination, the 
 splitting not only following the extended laminae but also 
 the contours of the round bodies themselves, where they have 
 not been drawn out in this manner. It will be noticed that 
 the lines remain absolutely parallel and never join with 
 
 FIG. 179. Calcoglobulin contours in forming dentine. 
 Unerupted premolar. ( x 600.) 
 
 one another in the same layer. This form of lamination 
 of the dentine would thus appear to have nothing to do 
 with physiological lines of growth, but to be due to a purely 
 physical cause, the extension of the elements of the globular 
 bodies in parallel lines. In teeth in a further stage of develop- 
 ment the rounded contours are to a very great extent lost, 
 and only the parallel striae remain as evidence of the original 
 structure. 
 
 In the completed dentine the striae are hidden by the 
 dense calcification, but as shown above, occasionally 
 revealed in caries by the action of the bacterial acid products 
 
DENTINE 281 
 
 upon the matrix. In the finished dentine of the Wombat's 
 incisor this lamination is particularly well marked and 
 conspicuous. That this lamination or stratification of 
 the dentine is due to the structure of the calcospherites is, 
 we think, very evident, and is the result of the rhythmic 
 or periodic deposit of the lime salts in the colloid. The 
 phenomena of the production of Liesegang's rings (fig. 180) 
 gives strong evidence of this, showing that the phenomena 
 of diffusion are periodic. As Leduc says (14) : ' The growth 
 of an osmotic production shows itself not as a continuous 
 process but periodically.' D'Arcy Thompson (20) also refers 
 
 FIG. 180. Liesegang's rings. Stratification by periodic diffusion. (Pro- 
 duced by diffusion from a drop of silver nitrate in a solution of gelatine 
 to which a drop of sodium arsenite has been added. ) From Leduc. 
 
 to this when he says : ' Among these various phenomena, 
 the concentric striation observed in the calcospherite has 
 acquired a special interest and importance. It is part of 
 a phenomenon now widely known and recognized as an 
 important factor in colloid chemistry under the name of 
 Liesegang's rings.' These rings are formed when a salt, 
 such as bichromate of potash, is placed on gelatine poured 
 upon a glass plate, the diffusion of the salt in the colloidal 
 gelatine solution showing a rhythmic deposit, leading to the 
 formation of concentric rings, and it is this rhythmic deposit 
 which occurs in the formation of the calcospherite in the 
 colloid. As Leduc says, all the phenomena of life are periodic, 1 
 although Mr. Carter states, referring to Leon Williams's 
 
 1 For a discussion of periodicity and rhythmic deposit see Leduc 's 
 Mechanism of Life, chap, vi, p. 67, and D'Arcy W. Thompson's Growth and 
 Form, pp. 427-31. 
 
282 MICROSCOPIC ANATOMY OF THE TEETH 
 
 work on enamel, ' There is no support for any theory which 
 endeavours to explain the appearances found in formed or 
 forming enamel as being due to intermittent rhythmical 
 secretion from the cells ' (5) . 
 
 Mr. F. J. Bennett, in a paper read before the Odonto- 
 logical Society in 1888, described the appearances produced 
 by the action of glycerine on dentine in which laminae were 
 brought into view, but the structure of the calcospherites 
 was not apparent in these preparations (1). 
 
 From these observations it would appear that the stages 
 in the calcification of dentine are : 
 
 Summary. First, the appearance of the small, clear, circular bodies 
 which form in a colloidal matrix by the coalescence of 
 minute particles, as seen in artificial experiments. 
 
 Secondly, the coalescence of these clear bodies, which 
 becoming incorporated in the basis substance of the dentine 
 are still more completely fusod together, lose their structure- 
 less character, and exhibit concentric lines. 
 
 Thirdly, the coalesced calcospherites undergo disintegra- 
 tion, their concentric elements being spread out into the 
 laminae of the dentine and the lime salts becoming equally 
 diffused in tho calcified matrix. 
 
 Fig. 80 shows calcospherites formed in Harting's albumin 
 experiment referred to on p. 140. It is seen that the 
 concentric markings on the calcified artificially-produced 
 spherite are precisely similar to those in the calcoglobulin 
 substance of the decalcified dentine. It is noticeable that 
 while in the calcification of enamel we meet with spherites 
 having radial striae, these are not seen in dentine, where all 
 the component cpherites are seen to be of the concentrically- 
 arranged variety. 
 
 As to the exact mode and channels of secretion of the 
 lime salts in dentine we have still but little accurate know- 
 ledge. We know that the deposit takes place, not in direct 
 contact with the odontoblasts but gradually advances from 
 above upon the odontogenic zone ; this would suggest 
 that the lime salts pass by dialysis from the tubes of the 
 dentine, containing the protoplasmic prolongation of the 
 secreting odontoblast cell, into the dentinal matrix, and the 
 minute subdivisions of the tubes with their protoplasmic 
 
DENTINE 283 
 
 contents would afford a very efficient means of distribution 
 of the calcifying substance within the matrix. It was 
 pointed out by Erwin Hohl (9), that in longitudinal section, 
 while the sheath of Neumann is very evident, with suitable 
 staining, in the calcified portion of the dentine, it is not to be 
 seen in the odontogenic zone, where the tubes appear to 
 have no definite walls. This observation is confirmed by the 
 author's silver- stained sections. Hohl considers that this fact 
 points to ' the dependence of the sheath of Neumann on 
 calcified dentine substance ' . 
 
 Whether this sheath is concerned in the calcifying process 
 and serves the purpose of a dialysing membrane might be 
 considered, but if this lining sheath to the tubules really 
 exists or not is still considered by some a matter of con- 
 troversy ; its presence would not, however, appear to be 
 necessary to account for this mode of impregnation of the 
 matrix, for such dialysis could take place through the outer 
 limiting membrane of the dentinal fibril into the matrix 
 substance, and, as stated above, its permeation by the 
 minute subdivisions of the tubes would much facilitate the 
 process. 
 
 It is difficult to explain the deposit of the lime salts at 
 a distance from the odontoblast cell itself, unless we consider 
 that the dentinal fibril which is an extension of the cell takes 
 an active part in the process, as suggested. 
 
 The Calcification of Vasodentine. Vasodentine is de- 
 veloped around the walls of a central pulp cavity which is 
 traversed by large and abundant blood-vessels, these vessels 
 becoming incorporated in the dentine as calcification pro- 
 ceeds, and not receding deeper into the pulp as they do in 
 Mammalia. 
 
 As C. S. Tomes says : * In the calcification of the formative 
 pulp into vasodentine, this recession of its vessels does not 
 take place ; the whole vascular network of the papilla 
 remains, and continues to carry blood circulating through it, 
 even after calcification has crept up to and around it.' In 
 a vasodentine pulp the connective tissue is very abundant, 
 and forms a definite layer of fibres around its circumference. 
 These fibres had originally been looked upon as odonto- 
 blasts ; they occupy the position of the odontoblast cells in 
 
284 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the formation of orthodentine, and are arranged very much 
 in the same manner, but a careful examination of a thin 
 section shows that there are no nuclei, and that the tissue 
 composing this layer is made up of fibres and not cells 
 (fig. 181). 
 
 Rounded or polygonal cells are, however, to be seen lying 
 among them in places which have a strong resemblance to 
 osteoblasts. The absence of odontoblasts would appear to 
 be consistent with the absence of dentinal tubes in vaso- 
 
 FIG. 181. Connective-tissue fibres lining pulp cavity of Hake 
 (Merluccius). Ground Weil section (unstained). ( x 150.) 
 
 dentine, as the typical odontoblast is provided with a process, 
 the dentinal fibril, which is not found in vasodentine. In 
 teeth in which orthodentine is found in one portion of the 
 tooth, while the rest is composed of vasodentine, as in the 
 flounder, odontoblasts are seen in the area of the pulp tissue 
 beneath the tubular dentine, although absent farther down, 
 where vasodentine has taken its place. Tomes had at first 
 considered that this layer around the pulp consisted of 
 elongated odontoblasts, but after a reconsideration of his 
 own and the author's preparations, he agreed that there 
 was no evidence of this, and that it really is composed of 
 
DENTINE 285 
 
 specially arranged connective-tissue fibres. Thin sections of 
 freshly-fixed preparations of the teeth of the Hake, which 
 had not been decalcified and were prepared by the Weil 
 process, confirmed the author's conclusions, as neither these 
 nor the earlier preparations showed any nuclei in the layer, 
 and its connexion with the connective tissue of the pulp was 
 quite evident. As further evidence of the nature of this 
 bordering layer, the teeth of the Hake often break up in 
 the direction of the bordering fibres, and this splitting 
 appears to be continuous with them. 1 
 
 The Calcification of Osteodentine. From the great simi- 
 larity of osteodentine to bone we should expect to find 
 a similar mode of development to that of membrane bone. 
 
 In osteodentine there is no separate pulp cavity, but 
 medullary spaces traversed by trabeculoe of bony substance. 
 The spiculse or trabeculse are clothed with cells in every 
 way resembling osteoblasts ; they are continuous with 
 bundles of connective-tissue fibres, and at the junction of 
 the tooth with the bone of attachment these trabeculae 
 become incorporated with the bone tissue of which they 
 form a part. The development is in every respect similar 
 to that of bone in membrane, and the connective-tissue 
 bundles become incorporated in the calcified tissue as 
 Sharpey's fibres do in bone. 
 
 It is thus seen that orthodentine, vasodentine, and osteo- 
 dentine are all calcified on a connective-tissue foundation, 
 and as Tomes says : ' The development of the several 
 varieties of dentine which seem to run into one another 
 structurally by almost imperceptible gradations comes into 
 line and so seems more intelligible.' 
 
 1 As pointed out on p. 256, the teeth of the Hake are often seen to break 
 up in a transverse direction as well, following the transverse lamination 
 of the dentine. 
 
 REFERENCES 
 
 1. Bennett, F. J. ' On certain Points connected with the Structure of 
 
 Dentine.' Trans. Odontol. Soc. Great Brit., vol. xxi, p. 6, 1889. 
 
 2. v. Bibra. See ref. on p. 117. 
 
 3. Black, G. V. ' An Investigation of the Physical Characters of the 
 
 Human Teeth in Relation to their Diseases.' Dental Cosmos, May 
 1895, vol. xxxvii, p. 353 et seq. 
 
286 MICROSCOPIC ANATOMY OF THE TEETH 
 
 4. v. Brunn. ' Ueber die Ausdehnung des Schmelzorganes in seiner 
 
 Bedeutung f. die Zahnbildung.' Archiv f. Mikr. Anat., 1887, 
 Bd. xxix, pp. 367-83. 
 
 5. Carter, Thornton. ' The Cytomorphosis of the Marsupial Enamel 
 
 Organ,' &c. Phil. Trans. Roy. Soc., vol. ccviii, p. 288. 
 
 6. v. Ebner, V. Handbuch der Zahnheilkunde, p. 253. 
 
 7. Fischer, G. Ban u. Entwickelung der Mundhohle des Menschen. 1910. 
 
 8. Hunter, John. John Hunter's Works, Palmer's Edition, 1835. 
 
 9. Hohl, Erwin. ' Beitrag zur Histologie der Pulpa und des Dentins.' 
 
 Archiv f. Anat., 1896. 
 
 10. Hoppe, F. ' Untersuchungen iiber die Constitution des Zahnschmelzes.' 
 
 Virchow's Archiv f. Pathol. Anat., Berlin, 1862, Bd. xxiv, pp. 13-32. 
 
 11. Hanazawa, K. ' A Study on the Minute Structures of Human Dentine.' 
 
 Trans. Panama Pacific Dental Congress, 1915, p. 80. and Dental 
 Cosmos, Feb. 1917 and March 1917, vol. lix. 
 
 12. Klein, E. (a) Elements of Histology. 
 
 (b) Klein and Noble Smith. Atlas of Histology. London, 1879. 
 
 13. v. Korff. ' Die Entwicklung der Zahnbeingrundsubstanz der Sauge- 
 
 thiere.' Archiv f. Mikr. Anat., xlvii. 1. 
 
 14. Leduc, S. Mechanism of Life, p. 68. 
 
 15. Mummery, J. H. (a) ' Some Points in the Structure and Develop- 
 
 ment of Dentine.' Phil. Trans. Roy. Soc., London, 1892, Ser. B., 
 vol. clxxxii, pp. 527-45. 
 
 (b) ' On the Process of Calcification in Enamel and Dentine.' Phil. 
 Trans. Roy. Soc., vol. ccv, pp. 95-113, 1914. 
 
 16. Neumann. Ein Beitrag zur Kenntniss des normalen Zahnbeins und 
 
 Knochengewebes. Leipzig, 1863. 
 
 17. Owen, R. Odontography, p. 406. 
 
 18. Romer, O. Zahnhistologische Studie, 1899. 
 
 19. Rose, C. ' Contributions to the Histogeny and Histology of Bony and 
 
 Dental Tissues.' Dental Cosmos, Nov. and Dec. 1893. 
 
 20. Thompson, D'Arcy W. Growth and Form, p. 427. 
 
 21. Tomes, J. (a) ' On the Presence of Fibrils of Soft Tissue in the Dentinal 
 
 Tubes.' Phil. Trans. Roy. Soc., 1856. 
 
 (b) ' On the Structure of the Dental Tissues of the Order Rodentia.' 
 Phil. Trans. Roy. Soc., 1850, pp. 529-68. 
 
 22. Tomes, C. S. (a) A Manual of Dental Anatomy, 7th ed., p. 83. 
 
 (b) * Upon Rose's proposed Classification of the Forms of Dentine.' 
 Anat. Anzeig., Bd. xiv, No. 13, 1898. 
 
CHAPTER VI 
 CEMENT 
 
 THIS tissue, both by its structure, its chemical composi 
 tion, and its mode of development, is seen to be simply 
 a slightly modified form of bone. This, the. ' crusta petrosa ' 
 of early authors, is usually termed cement by the general 
 anatomist ; but cementum is the term which has been 
 usually applied to it by dental histologists, although the 
 French have retained the word ' cement J . 1 
 
 Cement in man is confined to the roots of the teeth, 
 where it forms a continuous investment of the dentine, but 
 in many compound teeth it forms the cementing substance 
 between the plates, as in the Elephant and the Capybara, 
 and in Ungulates before the teeth come into use it forms 
 a complete investment of the crown. 
 
 As previously pointed out and as shown in the compound 
 teeth above referred to, a sharp masticating surface to the 
 tooth is maintained by the unequal wear of the different 
 tissues. The cement, being the least resistant, is worn 
 down more readily than the dentine, and the latter more 
 easily than the hard enamel, which by projecting above the 
 other tissues affords a rough sharpened surface for the 
 purposes of mastication. 
 
 At the neck of the human tooth the cement is usually 
 considered to terminate at the point of contact with the 
 enamel, but there are considerable variations in the relations 
 of the two tissues in this situation in normal teeth. This 
 question was especially studied by J. Choquet, who from the 
 
 1 In the present work the author has ventured to adopt the word 
 'cement' for this tissue. The Committee on nomenclature of the 
 Anatomical Society have confirmed the use of the word cement, and 
 it is the term in use in all the text-books of general anatomy. It would 
 appear very desirable that one word only should be employed to describe 
 the same tissue, and for these reasons the author has thought it advisable 
 to use the word cement instead of cementum, although the latter has 
 been in use for a long time in English and American works on dental 
 anatomy 
 
288 MICROSCOPIC ANATOMY OF THE TEETH 
 
 examination of a large number of teeth found four different 
 conditions present : 
 
 1. The enamel overlaps the cement. 
 
 2. The cement overlaps the enamel. 
 
 3. The two tissues terminate in direct contact. 
 
 4. There is a solution of continuity and an exposed surface 
 of dentine between the enamel and cement (3). 
 
 In the first case, where the enamel overlaps the cement 
 there is a marked difference in the percentage of cases 
 between the teeth of young subjects and those of adults. 
 In the young teeth this arrangement of the tissues was 
 found in 57 per cent., and in adults in 12-5 per cent. In 
 the second case, where the cement was seen to overlap 
 the enamel, the difference between the teeth at different 
 ages was more marked, this condition not being observed 
 in a single case in young teeth ; but in adults 62-5 per cent, 
 of teeth examined showed this overlap. These results are 
 easily understood on considering the process of development. 
 The formation of the cement proceeding after the enamel 
 is fully laid down, the osteoblasts continuing to deposit 
 cement add somewhat to the thickness of the tissue at 
 the neck of the tooth, and an overlap is easily comprehended, 
 and would be only found in adult teeth. 
 
 The conditions 3 and 4 exist according to this investiga- 
 tion in exactly similar proportions in adult teeth. In some 
 few cases a large overlap of cement with numerous lacunae is 
 seen, as shown in fig. 182. This is probably a pathological 
 condition, and is of rare occurrence, produced by an over- 
 activity of the cells of the follicle wall, probably due to 
 some chronic irritation of the tissues surrounding the tooth ; 
 but it is to be noticed that this deposit of cement takes 
 place outside the Nasmyth's membrane, which has been 
 clearly proved to be of an epithelial nature and derived 
 from the ectodermic enamel organ. 
 
 The basis substance or matrix of the cement appears 
 almost structureless or very faintly granular, and its collagen 
 foundation retains its form and structure after decalcifica- 
 tion by acids. Like bone, cement has a lamellar structure 
 and encloses lacunae and canaliculi, the lacunae enclosing 
 a nucleated cell (an included osteoblast), whose processes 
 
CEMENT 289 
 
 are prolonged into the canaliculi (figs. 183-186). The lacu- 
 rial cells are very clearly seen in specimens prepared by the 
 Weil process, which have been previously stained with borax 
 carmine. 
 
 The lacunal cells in bone were first described by Virchow. 
 They entirely fill the lacuna in the fresh state and send 
 processes along the canaliculi. The lacunal cells in cement 
 are identical in essential structure with those of bone, 
 enclosing a large readily stained nucleus with one or two 
 
 FIG. 182. Large overlap of cement, human molar. 
 d. Dentine ; e. enamel ; c. cement. ( x 50. ) 
 
 nucleoli. In several instances the lacuna with its contained 
 cell and processes has been isolated from the bone, the 
 walls of the space having resisted the decalcifying acid, as 
 the Neumann's sheaths do in dentine; and fig. 183, from 
 Schafer's Microscopic Anatomy, shows such a separated 
 lacuna with its contained cell and processes. As this author 
 says, ' It can scarcely be doubted that the protoplasm of 
 the nucleated corpuscle takes an important share in the 
 nutritive process in bone, and very probably serves both 
 to modify the nutritive fluid supplied from the blood and 
 to further its distribution through the lacunar and canali- 
 
 MUMMERY 
 
290 MICROSCOPIC ANATOMY OF THE TEETH 
 
 cular system of the bony tissue' (7). These remarks will 
 apply with equal force to cement. 
 
 It is claimed by Walkhoff and by Hope well Smith (4) that 
 normal cement does not contain lacunae, but while these 
 are rarely found in the thin layer at and near the neck of 
 the tooth, there seems little doubt they are abundantly 
 present in that of the root, and in the development of the 
 cement at the growing root tip they are distinctly seen 
 becoming included in the forming tissue. It would appear 
 that if all cement containing lacunae is to be looked upon 
 as pathological, there would be very few teeth that could 
 be said to possess normal cement. In fig. 190 lacunae are 
 seen in the thin layer on the outside of the root where 
 
 FIG. 183. Bone cell isolated (Schafer after Joseph), a. Proper wall of 
 lacuna shown at a part where the corpuscle has shrunk away from it ; 
 c. cell ; n. nucleus. 
 
 the deposition of this tissue has apparently been quite 
 normal. 
 
 The cement at the neck of the tooth is clear and 
 translucent, and seldom shows any trace of lamination ; but 
 in the root portion of the tissue the lamellae are very con- 
 spicuous, and represent lines of incremental growth as in bone. 
 
 The processes of the lacunal cells communicate with one 
 another and with the fine terminations of the dentinal tubes 
 in the granular layer of the dentine. They thus, where 
 present, form a chain of communication of protoplasmic 
 material between the periodontal membrane and the pulp, as 
 is clearly shown in figs. 184-186 from a tooth of a marsupial, 
 and in a human molar in fig. 187. Those who deny the 
 existence of lacunae in normal cement would not admit 
 that such intercommunication exists. 
 
 As in bone, the perforating fibres of Sharpey are seen to 
 
CEMENT 
 
 291 
 
 FIG. 184. Tooth of Bettongia, showing communication of dentinal 
 fibrils with periodontal membrane. ( x 250.) 
 
 FIG. 185. Lacunal cells within the lacunas of cement (Bettongia), 
 showing communication of dentinal fibrils with canaliculi. The lacunal 
 cell is seen to occupy the whole lacuna. Two nucleoli are seen within the 
 dark-stained nucleus. ( x 350.) 
 
 U 2 
 
292 MICROSCOPIC ANATOMY OF THE TEETH 
 
 enter the substance of the cement from the periodontal 
 membrane and penetrate it in more or less parallel lines. 
 These fibres are prolongations of the connective-tissue 
 bundles, and serve to attach the membrane to the tooth. 
 It is undecided if they are calcined or not. In caries of 
 cement the micro-organisms penetrate along the lines 
 of these fibres exactly as they do along the canals of the 
 dentine, which would lead us to suppose that they are not 
 
 FIG. 186. Similar preparation to fig. 185. ( x 350.) 
 
 so fully impregnated with lime salts as the surrounding 
 matrix. 
 
 Haversian canals are said to be occasionally seen in the 
 cement of human teeth, but are of rare occurrence, and when 
 present are generally found in the thick portion between the 
 roots of the molars. The lamellae are arranged concentrically 
 to the canal, as in bone. Vascular canals are also occasionally 
 seen, which do not exhibit the structure of Haversian canals, 
 but simply appear as channels or perforations in the sub- 
 stance of the tissue. 
 
 In the specimen figured (fig. 192) there were numerous 
 canals in the dentine of the root, and each of these is seen 
 to be surrounded by a layer of cement containing lacunae. 
 
CEMENT 293 
 
 The cement appears to be in contact with the granular 
 layer of the dentine (fig. 184), but a clear layer often, but 
 not always, intervenes. It is sometimes described as 
 a structureless layer of dentine, sometimes as cement. 
 In figs. 184, 185, and 186, the granular layer is seen in 
 immediate contact with the cement, and the fine terminal 
 branches of the dentinal tubules are continuous with the 
 canaliculi. This condition is also seen in many of the 
 
 Y 
 
 Fia. 187. Communication of dentinal tubes with cells of granular layer 
 and with canaliculi of the cement, c. Cement ; d. dentine ; g. granular 
 layer. (Human molar. ) ( x 150.) 
 
 author's preparations of human teeth. The majority of 
 the canaliculi are directed towards the outer surface (figs. 
 187, 188, and 189. Many appearances suggest that the outer 
 layer consists of the first-formed cement, but it is very 
 difficult to speak with any certainty on the point (see figs. 
 186 and 190). 
 
 The lacunae and their canaliculi are scattered or arranged 
 in rows ; they are much more irregular in form than the 
 lacunae of bone, and often have a tufted appearance, and 
 the canaliculi are frequently seen in the roots of teeth 
 extending across several lamellae and of great length. In 
 irregularly deposited cement some of the lacunae have no 
 apparent canaliculi ; these are the bodies which Hope well 
 
294 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Smith calls abrachiate lacunae. The incremental lines in 
 cement follow the contour of the root and form the 
 laminse in thick cement ; they are, as in bone, the indica- 
 tion of the incremental deposit of the calcined tissue, and 
 have been known as ' the incremental lines of Salter '. 
 
 Development of Cement. In the cement of human teeth, 
 which is confined to the roots, the process of ossification is 
 exactly similar to that of bone in membrane. In ungulates 
 and animals possessing coronal cement, a cement organ has 
 
 FIG. 188. Feathery canaliculi in cement in root of 
 human molar. ( x 150.) 
 
 been described by Robin and Magitot (6). They say : 
 ' The coronary cement is produced by the mode of ossifica- 
 tion, called ossification by substitution, or by the ossification 
 of preceding cartilage of the same form, for which is sub- 
 stituted a corresponding osseous layer.' The details of the 
 process of ossification given by these authors do not, how- 
 ever, exactly correspond to those of the ossification of bone 
 in cartilage generally accepted. They describe the invasion 
 of the fibre-cartilaginous basis substance by points or spots 
 of ossification which form small plates produced into pro- 
 longations or trabeculee which arise from their periphery, 
 
CEMENT 
 
 295 
 
 FIG. 189. Similar to fig. 188. Laminae of the cement 
 (human). ( x 150.) 
 
 d 
 
 FIG. 190. Dentine cement and bone in situ. Lacunae are visible in 
 the cement, b. Bone; c. cement ; p. periodontal membrane ; d. dentine. 
 
 (x80.) 
 
296 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the vessels having completely disappeared from the cartilage 
 in the region of ossification. The subsequent removal of 
 this first-formed bone and the re-deposit of bony matter in 
 the interior of the cartilage are however, not described by 
 them in considering this mode of calcification of the cement. 
 In the radicular cement of human teeth, where the 
 process is similar to that seen in membrane bones, the 
 connective tissue of the follicle with its rich vascular supply 
 invests the forming root ; but a layer of epithelial cells, 
 extending from the epithelial elements in the coronary 
 portion of the follicle, and known as the sheath of Hertwig, 
 extends downwards along the margin of the dentine as far 
 as it is formed, and as described on p. 320 is the form- 
 determining organ of the dentine as first shown by Von 
 Brunn (2) . The dentine is laid down beneath this epithelial 
 sheath, which always intervenes between it and the con- 
 nective tissue of the periodontal membrane during the active, 
 growth of the root. Where no cement has begun to 
 form, as can be seen at the tip of the forming root, the 
 epithelial layer is in contact with the dentine. As soon as 
 the development of the cement commences, bundles of 
 connective tissue and osteoblasts in every respect similar to 
 those of bone pass between the epithelial cells of the sheath 
 .from the surrounding connective tissue of the follicle, 
 separating the epithelial masses composing the sheath from 
 one another, and the fibrous bundles become firmly attached 
 to the dentine. The osteoblasts can be seen in sections to 
 occupy little spaces or divisions between the connective- 
 tissue bundles, and as in bone, some of them become included 
 within the forming tissue, and remain as the lacunal cells 
 of the finished cement (figs. 191 and 193). 
 
 As was first pointed out by Kolliker, the calcareous sub- 
 stance is deposited in little flakes or plates which afterwards 
 coalesce (fig. 194). The connective-tissue bundles become 
 incorporated in the forming cement, and form the Sharpey's 
 fibres as in bone (fig. 193, &c.). 
 
 The osteoblasts are probably modified connective-tissue 
 cells, but they are considered by some authorities to be 
 leucocytes derived from the circulating blood ; they are 
 very abundant and lie in the first-deposited cement 
 
CEMENT 
 
 297 
 
 FIG. 191. Cement and periodontal membrane in an adult tooth 
 g. Granular layer of dentine ; c. cement with Sharpey's fibres ; p. perio- 
 dontal membrane and osteoblasts. ( x 700.) 
 
 FIG. 192. Vascular canals in dentine surrounded by cement. 
 Root of human molar tooth. ( x 150.) 
 
298 MICROSCOPIC ANATOMY OF THE TEETH 
 
 between the little flakes above described, their processes 
 passing between them. These are the more faintly stained 
 cells seen between the flakes in fig. 194. 
 
 Schafer (Microscopic Anatomy) says : ' Osteoblasts are 
 probably specially modified connective-tissue corpuscles, 
 perhaps of the nature of plasma cells ; but after being 
 included in the lacunae they may undoubtedly be regarded 
 as homologous with the lamellar cells of connective tissue. 
 
 d 
 
 FIG. 193. The penetrating fibres of Sharpey in forming cement, osteo- 
 blasts lying between them. s. Remains of Hertwig's epithelial sheath ; 
 d. dentine ; c. cement. ( x 450.) 
 
 It is not probable that they are produced from leucocytes, 
 as suggested by Kassander ' (5). 
 
 Calcification. Two different views have been held as to the 
 mode of calcification in bone and cement, one being that the 
 osteoblasts are actually converted into bony substance, the 
 other that they secrete the calcifying material. 
 
 A similar controversy has been held over the mode of 
 deposition of enamel and dentine, but the view more 
 generally held with regard to the process occurring in bone 
 is in harmony with that chiefly received in reference to the 
 other tissues of the tooth, that it is formed by the secretion 
 of a material which calcifies and not by an actual conversion 
 
CEMENT 299 
 
 of the cell substance into bone. This view was upheld by 
 Gegenbaur and Kolliker, while Waldeyer (9) and others 
 maintain that there is a ' direct conversion of the protoplasm 
 of some of the osteoblasts into bony tissue '. Schafer, in 
 supporting the secretion theory, points out that there is no 
 indication of cell areas in the formed tissue, and no half- 
 calcified osteoblasts are to be seen. 
 
 As is seen in fig. 191, which shows the penetrating fibres 
 in the cement in a Weil preparation from a tooth freshly 
 
 FIG. 194. Forming cement showing that it is deposited in flakes. The 
 faintly stained cells between the flakes are the osteoblasts. ( x 450.) 
 
 fixed in sublimate, the Sharpey's fibres appear as channels 
 in the substance of the tissue communicating with the 
 exterior and in close contact on the inner side with the 
 granular layer of the dentine. If these run, as they appear 
 to do, in actual channels in the cement, they may serve 
 as a means of communication between the fibrils of the 
 dentine and the periodontal membrane, and in the absence 
 of lacunae with their canaliculi, may serve to keep up this 
 communication. In caries in the cement the micro- 
 organisms penetrate the tissue along the lines of these fibres 
 exactly as they do the tubes of the dentine. 
 
 Absorption. Absorption occurs in the temporary teeth 
 as a normal physiological process, and is the agency by which 
 
300 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the deciduous teeth are removed to make room for their 
 permanent successors. In the permanent teeth the process 
 is chiefly a pathological one, although the moulding of the 
 forming roots appears to be by an alternation of absorption 
 and deposition as in the formation of bone. 
 
 In the temporary teeth, absorption commences in the 
 cement, but not necessarily at the nearest point to the 
 erupting permanent tooth, and is not due to pressure from 
 beneath. The cement and dentine show semilunar indenta- 
 
 FIG. 195. Absorption of temporary tooth. Weil process. (x!50. ) 
 
 tions, the lacunae or foveolse of Howship, and if the tooth 
 is not previously shed, absorption will proceed to the 
 excavation of the enamel (figs. 195 and 196). 
 
 These excavations are occupied by large multinucleated 
 cells, the osteoclasts, which are the active agents in absorp- 
 tion both in bone and teeth. By what means they produce 
 this result is not determined. It is considered probable 
 that they secrete an acid which has a solvent effect upon 
 the lime salts, but this has never been definitely proved. It 
 has also been suggested that they cause absorption by the 
 protrusion of amoebiform processes into the hard tissues, 
 but the view more generally held is that they secrete some 
 
CEMENT 
 
 301 
 
 solvent agent, probably of an acid nature. The osteoclasts 
 are usually multinucleated or giant cells, and vary very 
 much in size and in the number of their nuclei. It is con- 
 sidered by many that both osteoclasts and osteoblasts are 
 modified connective-tissue cells and are interchangeable, 
 osteoblasts becoming converted into osteoclasts and vice 
 versa. Another view of their origin is that they result from 
 the fusion of leucocytes, but the former is the view more 
 generally held. Hope well Smith states, speaking of the 
 
 FIG. 196. Absorption of a temporary tooth. Ho wahip's lacunae 
 and osteoblasts. Weil process. Stained carmine. ( x 250. ) 
 
 absorption of permanent teeth, that osteoclasts do not exist 
 ' in the innermost zone of the root membrane ' ; he denies 
 the presence of osteoclasts in the absorption of the dentine 
 and cement of permanent teeth, and also considers that 
 true Howship's lacunae are not found on the tooth side of the 
 membrane (4). It is, however, very evident in many 
 instances that Howship's lacunae and osteoclasts are present 
 in both dentine and cement which is undergoing absorption 
 (fig. 197), and they show no recognizable difference to those 
 seen in absorbing bone or temporary teeth. 
 
 Cement being but a modified form of bone, it is reason- 
 
302 MICROSCOPIC ANATOMY OF THE TEETH 
 
 able to expect that the same process of absorption accom- 
 plished by the same agents would be found in the teeth. 
 Osteoclasts have been shown to be present in the absorption 
 of the dentine of implanted teeth, by Wilkinson, and were 
 also present within the lacunae of Howship in the absorption 
 of the implanted tooth in a dog, in Scheff s implantation 
 experiment. 
 
 In the roots of teeth which have been the subjects of 
 slight pathological changes, alternations of absorption and 
 
 FIG. 197. Absorption of dentine in permanent molar. Howship's 
 lacunae and osteoblasts. Weil process. (x!50. ) 
 
 deposition are seen to have taken place, the semilunar 
 excavations in both bone and dentine being occupied by 
 a deposit of cement, sometimes containing ordinary branched 
 lacunae and canaliculi and sometimes small lacunae destitute 
 of processes. In an abscessed tooth large portions of the 
 root are often seen to be excavated and absorbed by 
 the osteoclasts (fig. 198). 
 
 As previously stated, both in the bone and in the per- 
 manent teeth alternations of absorption and deposition are 
 seen, in the permanent teeth generally as the result of 
 pathological conditions, but in the formation of the roots 
 of permanent teeth this alternation can often be detected. 
 
CEMENT 
 
 303 
 
 A moulding or shaping process appears to take place, the 
 osteoblasts apparently laying down more tissue than is 
 eventually required and the osteoclasts removing the super- 
 fluous material. This double action of the formative cells 
 of the cement can be well seen in fig. 216, where osteo- 
 blasts are laying down the tissue, and large giant cells are 
 absorbing it on their inner side. 
 
 Many authorities, especially the French histologists, deny 
 the existence of a distinct absorbent organ, and look upon 
 
 FIG. 198. Absorption of dentine and re-deposit of cemental 
 tissue. Weil process. (x!50.) 
 
 the absorption of the temporary teeth as a physiological 
 process of rarefying osteitis, in which absorption and deposi- 
 tion alternate. This view is held by Redier, Malassez, and 
 Galippe, and was embodied in a paper contributed to the 
 International Dental Congress of 1910 by J. Choquet, who 
 came to the same conclusion from his own researches (3). 
 As Professor Sims Woodhead states (10), * Rarefying osteitis 
 must be looked upon as a process rather than a distinct 
 disease ' ; and again on p. 477 : 'In rarefying osteitis there 
 is increased absorption of bone accompanied by a corre- 
 sponding new formation.' 
 
 That such a process does occur in temporary teeth is 
 
304 MICROSCOPIC ANATOMY OF THE TEETH 
 
 evidenced by the fact that alternations of removal and 
 deposition of tissue are often seen in these teeth during the 
 process of absorption, and this is also evident in the absorp- 
 tion of the roots of permanent teeth. As previously stated, 
 osteoblasts and osteoclasts appear to be interchangeable, 
 the same cells sometimes performing one function, some- 
 times another ; the multinucleated osteoclasts being simply 
 modified osteoblasts. 
 
 In a recent paper (8) Dr. Eugene Talbot says : ' When 
 once the absorption of cementum has occurred, it is rarely if 
 ever reproduced.' The tissue which fills up the absorbed 
 areas in many teeth which have undergone chronic absorp- 
 tion certainly appears to be an irregular deposit of cement, 
 which in many of the author's specimens is laminated. 
 
 Black also says : ' These absorptions ... are afterwards 
 repaired by the deposits of cementum, and the lamellae 
 of cementum subsequently la-id down are seen to pass over 
 them without any material disturbance.' (1). 
 
 One would imagine that in the very recent absorptions 
 produced in Talbot's experiments on dogs sufficient time 
 had not been allowed for the process of re -deposition to take 
 place. 
 
 REFERENCES 
 
 1. Black, G. V. The Periosteum and Peridental Membrane. 
 
 2. v. Brunn, A. See references to Chapter VIII. 
 
 3. Choquet, J. (a) ' Notes sur les rapports anatomiques entre I'Email et 
 
 le Cement.' UOdontologie, Feb. 1899. 
 
 (6) ' fitudes sur la resorption des racines des dents temporaires,' &c. 
 Trans. Fifth Int. Dent. Congress, Berlin, 1910. 
 
 4. Hopewell Smith, A. Normal and Pathological Histology of the Teeth, 
 
 1919, p. 84. 
 
 5. Kassander. Anat. Anzeig., 18, 1900. 
 
 6. Robin et Magitot. Genese et developpement des follicules dentaires.' 
 
 Journ. de I 'Anat. et de la Physiol., vol. iv, 1861. 
 
 7. Schafer, E. A. Microscopic Anatomy, 1912, p. 148. 
 
 8. Talbot, Eugene. ' Bone Absorption around the Roots of Teeth.' 
 
 Dental Cosmos, May 1919. 
 
 9. Waldeyer. Archiv f. Milcr. Anat., i. 1865. 
 
 10. Woodhead, G. Sims. Practical Pathology, Edin., 1892, p. 474. 
 
CHAPTER VII 
 THE PERIODONTAL MEMBRANE 
 
 THE periodontal membrane, alveolodental periosteum or 
 pericementum, surrounds the implanted part of the tooth 
 in man, intervening between the cement of the root and 
 the bone of the alveolus. It is derived from the connective 
 tissue of the tooth-sac or outer portion of the follicle, and 
 consists of connective-tissue fibres and cells with nerves and 
 blood-vessels, but no elastic tissue. 
 
 It is held by Malassez (2) that the term alveolodental 
 periosteum commonly employed in describing this membrane 
 is ' as false from the physiological point of view as from the 
 anatomical '. He considers it shows none of the charac- 
 teristics of an enveloping membrane, but solid fibrous 
 fasciculi, which he considers form a kind of circular ligament. 
 He further says : * Reviewing the evidence of comparative 
 anatomy, we see that in many animals the teeth are not 
 enclosed in alveoli, but they are seen to be simply included 
 in the mucous membrane of the gum, but are also attached 
 to the maxilla by solid ligament ous fasciculi, the analogue 
 of the so-called alveolodental periosteum.' 
 
 Ranvier l also stated that ' there exists between the tooth 
 and its alveolus no separable membrane such as is present 
 around the long bones ', and looks upon the alveolar cavity 
 as ' nothing but an enlarged medullary space, communicating 
 with neighbouring spaces '. 
 
 The white fibrous connective-tissue bundles, of which this 
 membrane is chiefly composed, pass from tooth to bone in 
 a more or less transverse direction, and they become blended 
 with the tissue of the gum at the neck of the tooth. These 
 bundles are attached to the cement by strong fibrous bands 
 which pass into its substance (figs. 191, 193, and 199) as 
 Sharpey's fibres, and they penetrate the bone of the alveolus 
 in a similar manner. The direction of the connective-tissue 
 strands or bundles varies considerably in different parts. 
 1 Ranvier, Unpublished Lectures at the College of France. 
 
306 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Near the end of the root they are chiefly oblique, passing 
 upwards and outwards to their attachment to the bone, but 
 are crossed by bundles of fibres passing in the opposite 
 direction and interlacing with them. It is thus seen that 
 there is no separate membrane connected with the tooth 
 and another with the alveolus, but its fibres are continuous 
 from one side to the other, although they are coarser on the 
 outer side of the membrane and finer on the side of the 
 cement ; but there is no distinct arrangement in layers, 
 
 FIG. 199. Periodontal membrane showing oblique direction 
 of fibres and their intercrossing. ( x 450.) 
 
 the finer fibres passing insensibly into the coarser ones. 
 The oblique direction of the fibres and their intercrossing 
 with others passing in an opposite direction are well shown 
 in fig. 199. 
 
 The general arrangement of the fibres is such as to swing 
 or suspend the tooth within the socket and allow of a certain 
 amount of movement, preventing undue pressure on the 
 nerves and blood-vessels of the alveolus. 
 
 Blood-vessels are abundant in the periodontal membrane 
 midway between the bone and the tooth, and numerous 
 capillaries are seen near the cement, and it is richly 
 supplied with nerves. 
 
THE PERIODONTAL MEMBRANE 307 
 
 Both the blood-vessels and nerves of the membrane are 
 chiefly derived from the main trunks which are supplied to 
 the tooth-pulp. 
 
 The final terminations of the neurofibrils bordering the 
 cement have not hitherto been satisfactorily demon- 
 strated. 
 
 Dr. Black says that after destruction of the nerves enter- 
 ing the apical foramen, such as occurs in alveolar abscess, 
 the membrane still remains sensitive, and considers ' it 
 follows, therefore, that the nerves entering the membrane 
 through the walls of the alveolus are sufficient for the 
 maintenance of the sensory functions '. Black describes 
 nerve bundles entering the wall of the alveolus by way of 
 the Haversian canals, but he does not actually demonstrate 
 any such nerve supply, and although nerve fibres enter the 
 bone in company with the arteries we do not think it has 
 ever been shown that they have any distribution without the 
 bone, but are supplied to the coats of the arteries. 
 
 The same author in his work on the Periosteum and Peridental 
 Membrane devotes a chapter to the description of lymphatics 
 in the membrane, but there can, we think, be little doubt 
 that the structures which he describes as lymphatics, and 
 which he compares to the Peyer's patches of the small 
 intestine, are not glands but the epithelial remains of the 
 sheath of Hertwig, present in all teeth. 
 
 The remains of the epithelial sheath are seen as isolated 
 collections of epithelial cells lying in a more or less complete 
 row at a little distance from the cement. 
 
 The continuous sheath which extends downwards during 
 the formation of the root becomes cut up by the invading 
 fibrous bundles, and in many places disappears ; but where 
 the membrane is not very dense, remains as rounded or 
 elongated collections of cells which have a strong resem- 
 blance to glandular tissue. 
 
 The limiting membrane which Black describes appears to 
 be the result of the union of the cell walls of contiguous cells, 
 and is very marked in the network of epithelial cells forming 
 the sheath of Hertwig beneath the forming roots of the 
 teeth, as described on another page. 
 
 Fusiform connective-tissue cells are everywhere present 
 
 X 2 
 
308 MICROSCOPIC ANATOMY OF THE TEETH 
 
 between and among the fibrous elements of the membrane, 
 and the osteoblasts, probably derived from them, are in 
 contact with the bone and the cement. Calcified bodies are 
 often found in the peripdontal membrane, the so-called 
 ' epithelial pearls ', which appear to be due to calcification 
 in the cell nests or epithelial remnants (fig. 200). 
 
 Large multinucleated cells, the osteoclasts, are often 
 seen in the situation where absorption of the cement is 
 in progress. Under the influence of these different cells, 
 
 FIG. 200. A so-called epithelial pearl in the periodontal 
 membrane, c. Cement ; d. dentine. ( x 150.) 
 
 absorption and re-deposition of cement is often seen to be 
 taking place at the surface of the tissue as above described. 
 The periodontal membrane is derived from the connec- 
 tive tissue of the capsule, which, as described in the chapter 
 on development, extends around the dentine papilla ; the 
 connective-tissue fibres which it contains penetrate the 
 cement which is deposited around them, and they thus 
 become involved in the calcified tissue, as are the penetrating 
 fibres of Sharpey in bone, with which they are strictly 
 analogous. As in bone, it is doubtful if they are fully calcified, 
 and the occurrence of caries in the cement where the micro- 
 
THE PERIODONTAL MEMBRANE 309 
 
 organisms pass along the course of the Sharpey's fibres 
 suggests that they are not calcified, the organisms penetrating 
 channels in the cement occupied by the fibres exactly as 
 they do the tubes of the dentine. 
 
 It can be easily understood from its mode of development 
 that the blood-vessels and nerves which are supplied to that 
 portion of the papilla which afterwards becomes the dental 
 pulp are common to both pulp and membrane, some branches 
 passing to the pulp, others to the membrane. It is evident 
 that the principal vascular supply comes from the large 
 vessels at the apex of the root, but these also form com- 
 munications with the vessels of the gum and alveolus. 
 
 The rich supply of both blood-vessels and nerves to the 
 membrane would fully account for its great sensibility in 
 inflammatory conditions. If lymphatics are present, as 
 must be expected, in the periodontal membrane, it is not 
 at present known whether they form a perivascular network 
 or exist as distinct lymphatic vessels, but there is, we think, 
 every evidence that the lymphatic system described by 
 Black is in reality the sheath of Hertwig, the remarkable 
 resemblance of these regularly arranged cells to a tubular 
 system easily lending itself to such an interpretation. 
 
 The Gum 
 
 The gum is the name given to that portion of the mucous 
 membrane of the mouth which surrounds the teeth. It is 
 continuous with, and identical in structure with, the mucous 
 membrane of the rest of the oral cavity, but is slightly 
 denser and is blended with the periosteum of the alveolar 
 bone and with the periodontal membrane. 
 
 The microscopic structure is characteristic of all mucous 
 membranes. The surface layer is made up of flattened 
 epithelial cells, beneath which, as in the epidermis, are 
 found the stratum corneum, stratum lucidum, stratum 
 granulosum, and the cylindrical cells of the Malpighian 
 layer. The flattened epithelial cells of the external layer 
 are continually being shed from the surface, and their 
 characteristic forms are seen in all preparations of the fluids 
 of the mouth along with the mouth bacteria shown in all 
 
310 MICROSCOPIC ANATOMY OF THE TEETH 
 
 cover-glass preparations. The gum tissue is firmly bound 
 down to the bone beneath, its firm fibrous tissue being 
 blended with that of the periosteum, with which it is con- 
 tinuous. Numerous large papillae, single or compound, cover 
 its surface, and involved in the gum tissue near its outer 
 margin cell nests or more or less open spaces (epithelial coils) 
 are found, probably the bodies which were considered by 
 Serres to be glands. The structure and probable function 
 of these bodies will be considered in treating of the tooth 
 follicle and its connexions, but there is little doubt they are 
 derived from the epithelial cells of the dental lamina or 
 tooth-band. 
 
 The gum is very poorly supplied with nerves, as its well- 
 known lack of sensibility would indicate, but it has an 
 abundant vascular supply. 
 
 Simple mucous glands are abundantly found in the gum 
 tissue. 
 
 REFERENCES 
 
 1. Black, G. V. Periosteum and Peridental Membrane, chap. x. 
 
 2. Malassez, L. ' Sur 1'existence d'amas epitheliaux autour de la racine 
 
 des dents,' etc., with appendix on the so-called Alveolar Periosteum. 
 Archives de PhysioL, 1885, Ser. 3, vol. v, pp. 129-48. 
 
CHAPTER VIII 
 THE TOOTH FOLLICLE AND ITS CONNEXIONS 
 
 IN the present chapter it is proposed to consider the 
 histology of the permanent tooth follicle and the sheath of 
 Hertwig, which is intimately connected with it. 
 
 The Follicle. As shown in Chapter I, the tooth is enclosed 
 in a sac within the bony crypt of the jaw. This sac is com- 
 posed chiefly of connective tissue derived from the outer 
 layer of the mesodermic dentine papilla, and surrounds the 
 whole tooth, including the enamel germ. This extension 
 of the papilla is generally described as an upgrowth from 
 its margins, which is continued until it meets over the con- 
 tained tooth germ, thus completely enclosing it. Whether, 
 however, such upgrowth really -occurs is somewhat doubtful, 
 and Tomes considers it is more probable that the tissue 
 in which the dentine organ is formed has become ' more 
 pronounced ', that the follicle is in fact formed by a con- 
 densation of the connective tissue in the neighbourhood of 
 the tooth germ. 
 
 The bony crypt of the temporary" teeth does not com- 
 pletely surround the tooth, but is open at the top, and is 
 more comparable to a deep groove than a closed crypt. 
 The follicle can be easily separated from the bony crypt 
 and from its attachment to the tissue of the gum, and 
 removed as a separate sac, the mucous membrane of the 
 gum remaining undisturbed, as explained in the chapter 
 on Development, p. 11. 
 
 The dental follicle of the temporary teeth has been very 
 fully described by Magitot (8) in collaboration with Robin 
 and with Legros, (6) and the epithelial elements were first 
 fully investigated by these authors and by Malassez, the latter 
 having specially studied the remnants of the tooth-band in 
 connexion with the occurrence of epithelial tumours in the 
 depth of the jaws. 
 
 Malassez (7) showed that independently of the epithelial 
 process, which develops into the enamel organ, the tooth-band 
 
312 MICROSCOPIC ANATOMY OF THE TEETH 
 
 produces other buds, and that there is a growth or pro- 
 liferation of these buds within the follicle. 
 
 He divides these epithelial products into three principal 
 groups, including those found in the connective tissue of 
 the capsule between the follicle and the surface : 
 
 1. A superficial group attached to the deep surface of the 
 epithelium of the gum. 
 
 2. An intermediary group situated between the mucous 
 membrane and the follicle. 
 
 3. A deep group connected with the enamel organ. 
 Under the first group he includes products composed of 
 
 cells of the Malpighian type and other club-like collections 
 and strands of cells which have a cylindrical form. 
 
 In this layer are found the epithelial bodies spoken of by 
 the French authors as ' globes epidermiques '. These have 
 been further studied by Warwick James in a paper on 
 the eruption of the teeth (5), who gives them the more 
 descriptive term of ' epithelial coils '. These coils appear 
 to originate from the club -like cell accumulations which 
 bear a strong resemblance to the cell nests in epithelioma, 
 and can be seen in various stages of transition in the epithelial 
 coils. 
 
 In the later condition of the epithelial coil it is difficult 
 to trace in its structure any indication of the epithelial 
 cells, and it has more the appearance of being made up of 
 concentric layers of a delicate connective tissue, but the 
 stages of development shown in figs. 202, 203, 204, make its 
 epithelial nature quite evident. These epithelial coils open 
 out more and more until at last, near the surface of the 
 gum, the concentric striae have disappeared, and they merely 
 remain a's wide spaces in the connective tissue and open on 
 the gum surface. 
 
 Warwick James considers they take an important part 
 in the eruption of the temporary teeth, causing the tissues 
 to give way and open out to form a channel for the 
 erupting tooth. A similar function is assigned by Malassez 
 to the epithelial strands found in the gubernaculum of the 
 permanent teeth (see p. 21). It is probably these bodies 
 in the gum which Serres described as glands. 
 
 The intermediary group is situated between the mucous 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 313 
 
 membrane and the dental follicle, and the cellular masses 
 are seen to form a kind of band or irregular network (see 
 
 FIG 201. Elongated mass of epithelial cells in follicle. ( x 350.) 
 
 FIG. 202. Cell nest in follicle. Early stages of epithelial 
 coil. ( x 350.) 
 
 fig. 201) ; these strands communicate with the prolongations 
 of the mucous membrane, and also with cells deeper placed 
 
314 MICROSCOPIC ANATOMY OF THE TEETH 
 
 and within the follicle. Epithelial coils are sometimes 
 present among the cells of this group. 
 
 The third group of epithelial products, which is found 
 within the follicle itself, is chiefly derived from the enamel 
 organ and the buds which separate from it, but is also in 
 connexion with the cells of the intermediary group. 
 
 It was shown both by Robin and Magitot and by Malassez, 
 that the external epithelium of the enamel organ does not 
 form a continuous investment, but in many places shows 
 
 FIG. 203. Epithelial coil. (x!50.) 
 
 openings or intervals penetrated by blood-vessels, which 
 thus come to lie upon the outer cells of the stellate reticulum 
 and in direct contact with them. 
 
 The external epithelium is also seen to give off conspicuous 
 buds from its outer surface (fig. 205). It is thus seen that 
 the epithelial cells present in the follicle of the temporary 
 tooth and in the tissue which intervenes between this and 
 the mucous membrane of the gum are abundant, and, far 
 from degenerating, are proliferating in this situation. 
 
 The broad band of epithelium which forms the neck of 
 the enamel organ, derived from the tooth-band, becomes 
 broken up as previously described (Chap. I), and the con- 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 315 
 
 FIG. 204. Epithelial coil (globe epidermique). (x 150.) 
 
 to 
 
 FIG. 205. Budding of the external epithelium (human tooth), s. Stellate 
 reticulum ; e external epithelium ; b. blood-vessels. (Photograph from a 
 preparation by Mr. W. James.) ( x 150.) 
 
316 MICROSCOPIC ANATOMY OF THE TEETH 
 
 necting bridges with the neighbouring teeth and with the 
 tooth-band become broken up, separated, and partially 
 absorbed ; but in many places these cells do not disappear 
 but proliferate within the connective tissue, and are found 
 in the different forms described by Malassez and Magitot. 
 Malassez described these cell proliferations in connexion 
 with the formation of tumours in the gum and periodontal 
 membrane, and did not fully recognize their true histological 
 and physiological significance ; but he was the first to show 
 that not only in the coronal part of the follicle but in the 
 periodontal membrane surrounding the root of the tooth 
 such epithelial remains are found. 
 
 As will be shown later, the epithelial masses in the 
 periodontal membrane are also derived from the cells of 
 the upper part of the follicle, and although seen in fully 
 formed teeth only as separated groups of cells, are really 
 the remains of a continuous epithelial layer known as the 
 sheath of Hertwig, as was first pointed out by Von Brunn. 
 
 The follicle of the permanent teeth just prior to eruption 
 does not appear to have been hitherto very fully described, 
 chiefly owing to the difficulty of obtaining suitable material 
 for the investigation. As Warwick James says : ' Previous 
 investigations into the histology of these remains have been 
 almost entirely restricted to foetal tissues ; after birth they 
 have been considered chiefly from the standpoint of patho- 
 logy ; ' and he further says : ' The authors ' (referred to 
 above) ' seem to imply that the primary connexion with the 
 epithelial tooth-band is lost, and that a secondary connexion 
 is established between the buds of the epithelium which 
 extend throughout the follicle.' Warwick James considers 
 that ' the proliferation and formation of buds indicates 
 a continuous growth, which continues with variable activity 
 until the eruption of the teeth takes place '. This process 
 certainly appears to occur in the follicle of the permanent 
 teeth, although Legros and Magitot state that ' all end by 
 being absorbed and disappearing ', and other authors, pro- 
 bably referring to this statement, have concluded that 
 nothing remains in the follicle prior to eruption but con- 
 nective tissue and tiny masses of epithelium, the so-called 
 glands of Serres. 
 
PLATE V, 
 
 Molar tooth in the follicle semi -diagrammatic, c. Connective tissue of the 
 capsule ; e. position of enamel ; d. dentine ; cm. cementum ; h. Hertwig's 
 sheath ; ft. continuation of Hertwig's sheath ; //-. inflections of Hertwig's 
 sheath around the forming roots ; . partially detached layers of cells forming 
 Nasmyth's membrane ; eo. termination of enamel organ at neck of tooth ; 
 z. odontogenic zone ; p. pulp ; o. odontoblasts. (The epithelial elements are 
 coloured red. x 8.) 
 
 To face p. 316. 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 317 
 
 It is clearly seen in the author's preparations of the 
 permanent tooth follicle that, far from this being the case, 
 the epithelium at this late stage is present in great abun- 
 dance, and Warwick James's conclusion with regard to the 
 temporary teeth is no doubt also true as regards the per- 
 manent, that ' the epithelium is produced continuously up 
 to the period of eruption ', and that while some cells are 
 proliferating, others are degenerating, those immediately 
 over the enamel organ appearing to be undergoing degenera- 
 tive changes, while those in the centre of the follicle would 
 seem to be in a normal, active condition. 
 
 There are probably some differences in the arrange- 
 ment of the epithelial tissue within the follicle of the 
 permanent teeth, as they are more completely enclosed 
 within the bony crypt than are the temporary teeth, 
 and thus further cut off from the oral epithelium and its 
 extensions. 
 
 The author had an excellent opportunity of studying the 
 histology of the follicle of the permanent tooth in a specimen 
 kindly given to him by his friend Mr. Dolamore. This, a lower 
 second molar entirely enclosed in the follicle and freshly 
 preserved in formol, enabled him to procure a series of sections 
 by the freezing method, including the whole width of the tooth, 
 and the connexion of the follicle with the tooth at the neck 
 was fortunately maintained, the enclosed area representing 
 the calcified enamel (which had been decalcified in formic 
 acid). These preparations of the follicle are represented by 
 the drawing on Plate V, which, though semi-diagrammatic, 
 accurately shows the relations and connexions of the tissues 
 as seen in the sections. 
 
 Apart from the epithelial cells, two rather remarkable 
 appearances are met with in the follicle. Calcified masses 
 of irregular shape are seen in the centre (fig. 206). These 
 are within the connective tissue and not in contact with 
 epithelium. The calcification encloses small rounded or 
 fusiform cells which do not resemble osteoblasts, but appear 
 to be the unaltered connective-tissue cells of the surrounding 
 tissue. These small masses are exactly similar in structure 
 to the calcified bodies in the centre of the pulp in many teeth 
 (see fig. 153). They do not appear to show the structure of 
 
318 MICROSCOPIC ANATOMY OF THE TEETH 
 
 cement, and are always well separated from the inner 
 epithelial layers. 
 
 The other noticeable appearance is the presence of 
 a fenestrated membrane-like expansion which is seen only 
 opposite the sulcus between the cusps of the molar tooth 
 (fig. 207). It is difficult to decide what this membrane repre- 
 sents ; if it forms a portion of the inner layer of Nasmyth's 
 membrane it is difficult to account for its fenestrated nature, 
 as the clear layer in preparations of the membrane is never 
 
 FIG. 206, Calcified masses within the follicle at 
 its centre. ( x 50.) 
 
 fenestrated but entire. The openings in the membrane are 
 of very unequal sizes, and the whole structure appears to 
 be of a stouter and tougher nature than the thin fenestrated 
 membrane which can be raised from the enamel surface, 
 especially in marsupials. 
 
 At the point of junction with the tooth, the follicle is 
 narrow and composed of strong connective tissue. Epithelial 
 cells are present in this narrow part of the follicle, and can 
 be traced upwards to its coronal portion, where they are in 
 great abundance, and can be seen in all stages of growth 
 and degeneration. 
 
 It is noticeable that the epithelial masses appear to have 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 319 
 
 no connexion with the connecting band or bridges ; they 
 appear to form, as Warwick James suggests they do in the tem- 
 porary tooth follicle, a secondary system established by the 
 anastomosis of the buds, and in the permanent tooth follicle 
 they are apparently cut off from connexion with the oral 
 epithelium. 
 
 Figs. 208, 209, 210 show the broad masses of epithelium 
 at the inner margin of the follicle, the positions of which 
 are indicated in the diagram. 
 
 FIG. 207. Fenestrated membrane at the centre of the follicle margin 
 at the point where a prolongation of the cells passes between the enamel 
 cusps. ( x 750.) 
 
 It is seen that while some of these cells, especially those 
 in contact with the enamel organ, are flattened and arranged 
 in many layers, deeper in, a network of epithelial cells is 
 present which show no apparent signs of degeneration. In 
 fig. 209 it can be seen that rounded cell accumulations are 
 present, resembling cell nests, and in many places show 
 multiple nuclei. They are apparently undergoing amitotic 
 division in this situation, and it has been shown that 
 this form of cell division in which the nucleus divides with 
 or without the division of the cell body is generally found 
 in stratified epithelia when undergoing degenerative changes 
 such as keratinization. 1 In this case there is no division of 
 the cell body but merely of the nucleus, and in many cells 
 1 Schafer, E. A., Microscopic Anatomy, p. 96. 
 
320 MICROSCOPIC ANATOMY OF THE TEETH 
 
 the author has been able to detect as many as eight nuclei 
 in an individual cell, as shown in the description of Nasmyth's 
 membrane. 
 
 The most striking point in these preparations is the great 
 abundance of epithelium, and this in a stage when the 
 enamel is completed and the tooth about to erupt ; they 
 also show that Nasmyth's membrane is attached at this 
 stage to the inner margin of the follicle, although in most 
 places its connexion is very slight and it is easily detached. 
 
 FIG. 208. Mass of epithelium at inner margin of follicle. Some 
 portions of last-formed enamel still attached. ( x 150.) 
 
 This point is further considered in describing Nasmyth's 
 membrane. 
 
 The Sheath of Hertwig. Although it was long ago shown 
 that an epithelial investment of the whole tooth is found in 
 mammalian teeth, the real importance and significance of 
 this discovery has scarcely been sufficiently recognized. 
 
 Not only is the enamel laid down by epithelial cells, but 
 the growing dentine, although a mesodermic product, is 
 also under the influence of an epithelial organ, the ' sheath 
 of Hertwig ', and, as will presently be shown, it would appear 
 that the conclusion of Von Brunn that where there is ' no 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 321 
 
 epithelial sheath there are no odontoblasts and no dentine 
 formation ' is probably correct, and is also true for the 
 teeth of man, and it is demonstrated that the forming tooth 
 
 FIG. 209. Epithelium at inner margin of follicle. Cell nests 
 and multiple nuclei. ( x 150.) 
 
 FIG. 210. Inner margin of follicle. Flattened epithelial 
 cells at lateral margin. ( x 150.) 
 
322 MICROSCOPIC ANATOMY OF THE TEETH 
 
 is surrounded by an investing open-meshed network of 
 epithelium which extends from the upper or coronal part 
 of the follicle, enveloping the forming roots on their outer 
 aspect. 
 
 It was shown by O. Hertwig (4) that in certain amphibia 
 the teeth are more or less surrounded by an epithelial sheath or 
 investment, and Von Brunn (2), carrying the investigation 
 farther, showed that in many mammalian orders such an 
 epithelial sheath exists ; in fact in all the specimens examined, 
 which included those of Rodents, Ungulates, and Carnivora. 
 He concluded, from his observations, that the epithelial 
 sheath which surrounds the roots of the forming teeth really 
 determines the deposition and limits of the dentine, and as 
 it were moulds the dentine of the roots, confining it within 
 certain limits and preventing its extension into the surround- 
 ing connective tissue. He says that where there is no 
 Hertwig's sheath there are no odontoblasts and no dentine, 
 that it is in fact the moulding or determining organ of the 
 dentine of the root. He was unable to find this sheath in 
 human teeth, and although its presence in Mammalia 
 seemed to indicate that it must be present also in man, the 
 material at his disposal did not show it. 
 
 The preparations of human teeth made by Professor 
 Von Ebner also failed to reveal it, but both he and Von Brunn 
 concluded that it must be present, and would be seen if 
 suitable preparations could be made (3). 
 
 It was considered by Von Brunn, in common with all those 
 who have described this sheath in Mammalia, that it was 
 produced by a downward growth of the inner and outer 
 layers of the enamel organ, the function of which, they 
 considered, was not only the formation of enamel but also 
 that of determining the limitation of the growth of the 
 dentine. 
 
 The author's preparations of the follicle and tooth in 
 position above described would appear to show, however, that 
 on this point the previously mentioned authorities were 
 mistaken. It is seen that the enamel organ is not prolonged 
 beyond the point where the follicle is attached to the tooth 
 at its neck, but that the enamel organ terminates where the 
 enamel terminates. This is seen not only in all the sections 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 323 
 
 from this one specimen but in several other less perfect 
 preparations of the follicle procured, and an investigation 
 of a Rodent developing tooth for comparison confirms the 
 conclusion arrived at. In the mouse, which was one of the 
 animals examined by Von Brunn, it is seen that the enamel 
 organ terminates at the point of junction of the follicle 
 with the tooth, and is clearly not continued downwards on 
 to the roots (fig. 211). 
 
 P 
 
 FIG. 211. Developing tooth of Mouse, a. Termination of enamel organ 
 at the point where the enamel terminates : the space occupied by the 
 enamel, of which some portions have escaped complete decalcification, is 
 contracted by disturbance of the section in cutting ; d. dentine ; p. pulp. 
 
 (x400.) 
 
 It would appear that the great difficulty of obtaining 
 sections showing the attachment of the follicle with dis- 
 tinctness has led to this misinterpretation of the conditions. 
 Especially in the tooth germs of the lower animals with 
 their irregular crowns it is very difficult to be sure of the 
 exact relations of the follicle to the tooth, but in the simpler 
 teeth of man it is very clearly visible in complete longi- 
 tudinal sections. 
 
 Y 2 
 
324 MICROSCOPIC ANATOMY OF THE TEETH 
 
 We thus see that the sheath of Hertwig is not a downward 
 extension of the cells of the enamel organ, and we have to 
 look elsewhere for its origin. 
 
 In figs. 212 and 213 the strand of cells composing the 
 sheath can be seen passing to the outside of the enamel at 
 the point of junction of the follicle with the tooth, and the 
 termination of the cell layers of the enamel organ is distinctly 
 
 e.o 
 
 FIG. 212. The junction of the follicle with the tooth at the neck. 
 e.o. Enamel oigan ; h. Her twig's sheath passing to the outer side of enamel 
 organ ; o. osteoblasts and thin layer of cement ; d. dentine. ( x 150.) 
 
 visible. It is seen that the sheath of Hertwig has no connexion 
 whatever at this point with the enamel organ, but is com- 
 pletely shut off from it within the connective tissue of the 
 follicle. 
 
 The strands of epithelial cells of which the sheath is com- 
 ' posed pass to the coronal part of the follicle, and appear 
 to be derived from the cell elements which are there present 
 in such abundance. 
 
 While these cell extensions are plainly seen at the neck 
 of the tooth passing to the outer side of the terminal portion 
 of the enamel organ, they cannot be traced very far into the 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 325 
 
 tissues of the follicle above, as these tissues are very much 
 narrowed just above the line of junction (see fig. 210 and 
 Plate V) with the tooth, and the nests of epithelial cells are 
 much compressed and obscured by the dense bands of con- 
 nective tissue ; also it must be remembered that in the 
 upper part of the follicle these epithelial cells have ceased their 
 functional activity and are probably undergoing absorp- 
 tion, while lower down, around the root, they are active and 
 
 e.o 
 
 FIG. 213. Junction of follicle and tooth under higher magnification 
 from a similar preparation, h. Hert wig's sheath ; e.o. enamel organ ; 
 c. cement ; d. dentine. ( x 400. ) 
 
 fully developed during the formation of the dentine and 
 cement of the growing root. 
 
 With regard to the actual sources of these cells, they are 
 probably derived from proliferations of the tooth-band 
 within the follicle, and possibly also from the buddings or 
 proliferations of the external epithelium of the enamel organ 
 with which the epithelial cells of the follicle form con- 
 nexions. See fig. 205. 
 
 In young teeth with forming roots., the sheath of Hertwig 
 is seen passing down the side of the root parallel to its 
 surface but not in contact with it, and is a more or less 
 
326 MICROSCOPIC ANATOMY OP THE TEETH 
 
 continuous band, often arranged as a distinct network, so 
 that the tooth at this stage is enclosed in an epithelial net 
 with open meshes. The sheath is continued to the forming 
 root tip, and as it nears this point it approaches more closely 
 to the dentine ; the network arrangement is not seen, but 
 two distinct layers of epithelial cells in close contact, which 
 at the tip of the root lie upon the dentine and turn round 
 it to extend into the connective tissue of the forming pulp 
 in a long curling band, the termination of this band bc-ing 
 
 FIG. 214. Hertwig's sheath surrounding forming root. Eoot about half 
 formed. The sheath is seen to be in contact with the dentine at the 
 lower end (h 2 ) ; the osteoblasts and depositing cement are seen to the inner 
 side of h l ; h 3 . the curling band of Hertwig's sheath separating the pulp (p) 
 from the connective tissue of the follicle. ( x 150.) 
 
 formed by the reflection of the two layers of cells, which form 
 a close loop. A similar loop is continued from the opposite 
 side of the root tip, but these two loops do not meet, but 
 leave a space between them occupied by the developing pulp 
 with its entering nerves and blood-vessels. It can be seen in 
 figs. 214, 215, and 216 that the odontoblasts and the form- 
 ing dentine are completely cut off from the surrounding 
 connective tissue of the follicle by this epithelial band. An 
 examination of the forming root at different stages of growth 
 shows that this epithelial inflection in man gradually shortens 
 
FIG. 215. Root tip at later stage. Shortening of the epithelial sheath. 
 o. Osteoblasts ; h 1 . Hertwig's sheath ; h 2 . Hertwig's sheath in contact with 
 the dentine ; d. dentine ; p. pulp, (x 150.) 
 
 FIG. 216. Root tip at still later stage. Involution of Hertwig's sheath 
 much shortened, o. Osteoblasts and osteoclasts in forming cement ; 
 h 1 . Hertwig's sheath in contact with dentine ; h 2 . termination of sheath 
 in pulp (p.). (x!50.) 
 
328 MICROSCOPIC ANATOMY OF THE TEETH 
 
 as the development of the root progresses, until when the 
 apical foramen is completely formed it only just reaches the 
 tip of the root (fig. 217). 
 
 h~ 
 
 FIG. 217. From a specimen in which the root was nearly completed. 
 The epithelial band (h.) only just turns round the root tip and does not 
 enter the pulp. h. Hertwig's sheath. 
 
 FTG. 218. Hertwig's sheath (s.) around root 
 of molar. ( x 50.) 
 
 There is thus every evidence that in man also the epithelial 
 sheath is the determining or moulding organ of the dentine 
 of the root, as claimed for the lower Mammalia by Von Brunn. 
 
 Between the roots of a molar tooth not only is the sheath 
 seen in the neighbourhood of the dentine, but a band of 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 329 
 
 a 
 
 i$>1i 
 
 FIG. 219. Epithelial sheath beneath root in nearly completed 
 tooth. Network of epithelial cells. ( x 250. ) 
 
 FIG. 220. Bands of epithelial cells of Hertwig's sheath 
 beneath root of growing tooth. ( x 250.) 
 
 epithelium passes directly across beneath the two root tips. 
 In this interval between the roots the sheath is often seen 
 to form a very dense network, and scattered epithelial 
 
330 MICROSCOPIC ANATOMY OF THE TEETH 
 
 cells are also found in the neighbouring connective tissue 
 (fig- 218). 
 
 The network in this situation is well shown in fig. 219, 
 and fig. 220 shows parallel strands of epithelium at a farther 
 distance from the root end. 
 
 In one instance a large mass of epithelium resembling the 
 accumulations in the coronal part of the follicle was seen 
 within the connective tissue below the roots. The larger 
 strands forming the network often appear to have a limiting 
 
 h 
 
 os 
 
 FIG. 221. The breaking up of the network of Hertwig's sheath by in- 
 vading connective-tissue fibres of periodontal membrane, os. Osteoblasts 
 and forming cement ; h. remains of Hertwig's sheath. ( x 250. ) 
 
 membrane as if they were enclosed in a tube. This appear- 
 ance, however, is probably due to union of the cell walls of 
 contiguous cells. 
 
 If the sheath is traced upwards from the root tip it is 
 seen to become less and less a continuous band, being cut 
 up and separated into isolated groups of cells by the invad- 
 ing connective-tissue bundles of the periodontal membrane, 
 which pass between them to become attached to the forming 
 cement as Sharpey's penetrating fibres ; these with the 
 osteoblasts lie to the inner side of the sheath and continue 
 the formation of the cement (fig. 221). Many of these 
 
TOOTH FOLLICLE AND ITS CONNEXIONS 331 
 
 isolated groups of epithelial cells disappear, but in most 
 adult teeth a few can be seen in the periodontal membrane 
 at the side of the tooth, and in many cases they are met 
 with in great abundance. 
 
 Summary 
 
 It has thus been shown that the enamel organ proper is 
 not continued downwards to form the Hertwig's sheath, 
 that the enamel organ terminates at the point of termination 
 of the enamel, and that the sheath is a prolongation of other 
 epithelial cells in the follicle which lie to its outer side. It 
 is shown that in man, as in other Mammalia, there is every 
 evidence that it moulds and determines the dentine of the 
 root, and is always present where dentine is laid down. 
 
 As it is shown that the sheath is not the continuation of 
 the enamel organ, it may be considered that two separate 
 epithelial organs are formed from the tooth-band and the 
 cells derived from it the enamel organ, which is especially 
 differentiated to form enamel, and the epithelial sheath, 
 which is the form- determining organ of the dentine. 
 
 The enamel which covers the exposed part of the tooth 
 is an epithelial product derived from the ectoderm ; the 
 dentine and cement are products of the mesoderm ; but the 
 whole tooth is surrounded at different stages of its growth 
 by an ectodermic structure, the sheath of Hertwig. 
 
 It can, therefore, consistently be maintained that the for- 
 mation of the whole tooth depends upon the proliferation 
 of the ectodermic epithelial elements. The two structures 
 would have a common origin, but while one presides over 
 the formation of the enamel, the other is developed for the 
 determination and limitation of the growth of the dentine, and 
 as the enamel organ atrophies when the enamel is completed, 
 and only persists as the cornified cells of Nasmyth's membrane, 
 so the epithelial sheath becomes absorbed after the complete 
 deposition of the dentine, and only a few epithelial cells 
 remain as the epithelial debris of Malassez. 
 
 It has been stated by several authors, especially by 
 C. S. Tomes (9), that in the Edentata an enamel organ is 
 present, although no enamel is formed, and in a paper 
 published in 1876 he showed the presence of enamel organs 
 
332 MICROSCOPIC ANATOMY OF THE TEETH 
 
 to be universal and quite independent of any after-formation 
 of enamel. The question arises as the result of the observa- 
 tions above recorded, if this epithelial organ in the 
 Edentates can be properly called the 'enamel organ '. 
 
 It is an arrangement of epithelial cells which no doubt 
 bears a strong resemblance to an enamel organ, but the 
 function of these cells is not that of enamel production ; and 
 as it has been shown that in all Mammalia examined and 
 in some other Vertebrates (by Hertwig) that an epithelial 
 sheath is present, we should conclude that it cannot be 
 absent in the Edentates, and is in them also the dentine 
 limiting organ, probably having no relation to the formation 
 of enamel either as a functional or vestigial characteristic. 
 
 A paper on the enamel organ of the Edentates was 
 published by Ballowitz in 1892 (1). 
 
 REFERENCES 
 
 1. Ballowitz, E. ' Das Schmelzorgan des Edentaten, seine Ausbildung im 
 
 Embryo und die Persistenz seines Keimrandes bei dem erwachsenen 
 Thiere.' Archivf. Mikr. Anat., 1892, Bd. xl, pp. 133-57. 
 
 2. v. Brunn, A. (a) ' Ueber die Ausdehnung des Schmelzorganes in seiner 
 
 Bedeutung fiir die Zahnbildung.' Archiv f. Mikr. Anat., 1887, 
 Bd. xxix, pp. 367-83. 
 
 (fe) ' Beitrag zur Kermtniss der Zahnentwickelung.' Archivf. Mikr. Anat., 
 1891, Bd. xxxviii, pp. 142-56. 
 
 3. v. Ebner, V. Handbuch der Zahnheilkunde, Wien, 1890, pp. 252-4. 
 
 4. Hertwig, O. ' Ueber das Zahnsystem der Amphibien und seine Bedeu- 
 
 tung fiir die Genese des Skeletts der Mundhohle.' Archivf. Mikr. 
 Anat., Bd. xi, Suppl., 1874. 
 
 5. James, W. W. 'A Preliminary Note on the Eruption of the Teeth.' 
 
 Proc. Roy. Soc. Med., 1909. 
 
 6. Legros et Magitot. ' Origine et formation des follicules dentaires chez 
 
 les Mammiferes.' Journ. de V Anat. et de la Physiol., vol. iv, 1873. 
 
 7. Malassez, L. ' Sur le role des debris epitheliaux paradentaires.' 
 
 Archives de Physiol, Paris, 1885, 3 e ser., v. 309-40, and .vi. 379- 
 449. 
 
 8. Robin et Magitot. ' Genese et developpement des follicules dentaires. ' 
 
 Journ. de Physiol., vol. iv, 1861. 
 
 9. Tomes, C. S. ' On the Development of the Teeth of the Newt, Slow- 
 
 worm, Frog, and Green Lizard.' Phil. Trans. Roy. Soc., 1876, vol. clxv, 
 pt. i, pp. 285-96. 
 
CHAPTER IX 
 NASMYTH'S MEMBRANE 
 
 NASMYTH'S membrane, or the enamel cuticle, is a very 
 delicate membrane which can be detached from the surface 
 of the enamel by the action of acids, but which is not other- 
 wise visible to the naked eye. 
 
 The nature of this acid-resisting membrane was for a long 
 time a matter of controversy. It was called by Nasmyth 
 the persistent dental capsule, but this designation does not 
 suitably describe its anatomical relations, as it has been 
 conclusively shown to be in its principal part an epithelial 
 product, and the connective tissue of the capsule appears 
 to have no part whatever in its formation. 1 The view that 
 Nasmyth's membrane consisted of cement was held by 
 Professor Owen, who says, speaking of human teeth (5), 
 the cement ' is thinnest upon the crown, and very gradually 
 increases in thickness as it approaches the end of the fang ; 
 it is only on the implanted part of the tooth that the radiated 
 cells which demonstrate the close analogy between cement 
 and bone exist ; elsewhere the clear basis of the cement 
 alone is present, and this is soon worn away from the 
 enamel of the crown '. It was also considered by Sir John 
 Tomes (9), and afterwards in the earlier editions of the Dental 
 Anatomy by C. S. Tomes, that this membrane represented 
 a thin layer of cement (10) corresponding to the layer of 
 cement covering the enamel in Ungulates. The chief 
 argument in favour of this view brought forward by these 
 authors was that encapsuled lacunae occurred in the fissures 
 of enamel in many instances, the very great overlapping 
 of the enamel by the cement which is occasionally, but 
 rarely seen in human teeth, and which would appear to be 
 of a pathological nature, lending further evidence in support 
 of this view. Professor Huxley (3) considered that Nasmyth's 
 
 1 A paper on ' The Presence of the Sheath of Hertwig in the Teeth of 
 Man, with Notes on the Follicle and Nasmyth's Membrane ', was com- 
 municated to the Royal Society by the author in 1918. 
 
334 MICROSCOPIC ANATOMY OF THE TEETH 
 
 membrane was the ' membrana preformativa ' between the 
 enamel organ and the forming enamel, and Lent (4) in 
 1853 was of the same opinion. Nasmyth was the first to 
 show that a membrane could be raised from the surface 
 of formed enamel which had not been exposed to wear. 
 
 Kolliker held that the membrane was a final product of 
 the enamel cells, while Waldeyer considered it was derived 
 from the external epithelium of the enamel organ. The true 
 histology of the membrane had, however, never been demon- 
 strated until Dr. Paul published his researches (6). He sepa- 
 rated the membrane from the tooth by immersing it in acids, 
 employing in his first experiments 5 per cent, nitric acid, 
 and stripped it from the enamel, staining with silver nitrate 
 or with eosin, but later found he obtained better results by 
 decalcifying in phloroglucin and staining in Ehrlich's 
 hsematoxylin. 
 
 These preparations demonstrated that Nasmyth's mem- 
 brane consists of two distinct layers an inner clear layer in 
 immediate contact with the enamel, and lying upon this 
 a layer or layers of epithelial cells which from their position 
 must have been derived from the enamel organ. 
 
 In former observations on this membrane it had been 
 considered that the polygonal markings observed upon the 
 clear layer of the membrane were the impressions of the 
 enamel prisms, but Dr. Paul points out that these markings 
 are very many times larger than those caused by the enamel 
 prisms, which can also be seen in the clear layer. 
 
 It can be seen, as shown in fig. 222, that the epithelial 
 cells are distinctly visible upon the upper surface of the 
 membrane, while the impressions of the enamel prisms from 
 beneath are also visible, and are seen in this figure in the 
 same field of the microscope as the cells and their impres- 
 sions. It needs no measurement to show the great dis- 
 proportion of size between the two sets of markings. The 
 preparations from which these photographs were taken were 
 made by the author from specimens prepared by a slightly 
 different method from that employed by Dr. Paul. The 
 membrane was floated off from the tooth in formic acid, 
 the detached fragments being well washed and stained with 
 Weigert's iron hsematoxylin. 
 
NASMYTH'S MEMBRANE 
 
 335 
 
 There are certain appearances in many of these specimens 
 not described by Dr. Paul in his original papers, and very 
 difficult to explain on the assumption that the epithelial 
 cells are those of the external epithelium of the enamel 
 organ only, as they have previously been considered to be. 
 These appearances can be, we think, clearly accounted for 
 if Nasmyth's membrane is viewed in connexion with the 
 follicle and studied in longitudinal section. 
 
 The clear layer is in direct contact with the enamel, and, 
 
 FIG. 222. Nasmyth's membrane, floated off the enamel in acid. 
 Cellular and clear layers. Clear layer showing impressions of enamel 
 prisms. (x250.) 
 
 as stated above, was considered by Kolliker to be a final 
 product of the enamel cells, although he apparently did not 
 recognize the double nature of the membrane. 
 
 Whether this be so or not it is difficult to say, but in the 
 longitudinal sections a clear layer is often seen on the under 
 side of the detached enamel organ cells, and refractile 
 particles of enamel which have escaped complete decalcifica- 
 tion are attached to it. This might suggest that the clear 
 pellicle represents a membrane corresponding to the ' mem- 
 brana pref ormativa ' of Huxley or the internal ameloblastic 
 membrane of Leon Williams. 
 
336 MICROSCOPIC ANATOMY OF THE TEETH 
 
 It can be seen in fig. 223 that the outlines of the epithelial 
 cells are impressed upon the clear layer in areas where the 
 cells themselves are absent, and are distinctly visible on the 
 upper surface of the clear membrane, while the impressions 
 of the enamel prisms from beneath are seen at the margins. 
 These impressions of the prisms are very distinct, and in 
 many parts are arranged in parallel lines or striae. There 
 seems no doubt that the impressions so arranged indicate 
 the structure of the enamel surface which produces them, 
 
 ; 1 
 
 FIG 223. Nasmyth's membrane allowing impressions of cells of cellular 
 layer on clear layer, c. Epithelial cells ; i. impressions of cells on clear 
 layer. (x250.) 
 
 showing that the enamel is not smooth, but is arranged in 
 a series of ridges apparently produced by the outcrop of the 
 enamel prisms (fig. 224). The impressions would appear to 
 be produced by the incremental lines of Pickerill, which he 
 considers with several other authors to give rise to the 
 appearance of the striae of Ketzius in enamel. 
 
 That such imbrication lines are present in enamel is 
 clearly shown by this moulding of the inner membrane upon 
 them. In some specimens, where the clear layer is seen 
 alone, having entirely separated from the cellular layer, 
 this stratification is very marked. It was shown by Pickerill 
 
NASMYTH'S MEMBRANE 337 
 
 that these imbrication lines are constantly present in the 
 enamel of healthy teeth, and he drew attention to the fact 
 that specimens of Nasmyth's membrane stained with silver 
 nitrate show a distinct striation which he considered corre- 
 sponds to the furrows between each imbrication line. 
 
 In fig. 224 the impressions of the individual prisms are 
 clearly visible along these lines, which evidently in this case 
 represent the summits of the ridges. 
 
 The Cellular Layers. In some parts of the floated pre- 
 parations a single layer only of epithelial cells is seen, but 
 
 Fro. 224. Floated preparation of Nasmyth's membrane showing 
 impressions of enamel prisms in parallel lines. ( x 250.) 
 
 in most places there are two or many layers. The cells vary 
 considerably in size, so much so that an average measure- 
 ment will not give any clear idea of their dimensions. They 
 are mostly polygonal in outline, separated from one another 
 by a distinct interval, as in epithelia elsewhere, and these 
 channels are crossed by bridges or processes. These processes 
 are in many cases very strongly marked, and in some parts, 
 at the margins, quite thick horny projections from the cell 
 are seen. According to several authors, this strongly 
 marked fibrillation of the cell is intimately associated with 
 the condition of keratinization (Schafer). 
 
 The cytoplasm of the cell body is clearly defined, and the 
 
338 MICROSCOPIC ANATOMY OF THE TEETH 
 
 nuclei exhibit the ordinary nuclear structure with one or 
 two strongly marked nucleoli. 
 
 As stated in describing the follicle, amitotic division of the 
 nucleus without division of the cell body is apparent in many 
 of these cells in the floated preparations, corresponding to 
 the same appearances in the cells of the follicle itself (fig. 225). 
 It is interesting to note that according to several authorities, 
 and as stated by Dahlgren and Kepner, ' In nearly all 
 stratified epithelia, especially in the higher vertebrates, the 
 nucleus divides by mitosis to increase the number of cells, 
 but it changes to amitosis without a division of the cell body 
 in the latter part of the cell's life. The probable object is 
 
 FIG. 225. Amitotic division in the cells of Nasmyth's membrane. 
 Floated preparation. Camera lucida drawing ( x 800.) 
 
 to enlarge the nuclear surface for increased metabolism, the 
 formation of keratin in this case ' (2). 
 
 Wilson says (12), ' Those cells that divide amitotically 
 are on the road to ruin/ and quotes Von Rath, who reached 
 the conclusion that * when once a cell has undergone amitotic 
 division it has received its death warrant it may indeed 
 continue for a time to divide by amitosis, but inevitably 
 perishes in the end '. 
 
 Certain areas of the membrane show accumulations of cells 
 arranged more or less concentrically, and apparently sur- 
 rounded by a delicate limiting membrane (fig. 226). Dense 
 concentric bodies are also seen in parts of the membrane form- 
 ing definite cell nests (fig. 227) ; these are very opaque and 
 stain badly, and apparently represent a late stage in the de- 
 generation and keratinization of the cells. Cells arranged in 
 
NASMYTH'S MEMBRANE 
 
 339 
 
 FIG. 226. Cell accumulations in a floated preparation 
 of Nasmyth's membrane, ( x 250. ) 
 
 FIG. 227. Cell nest in Nasmyth's membrane. Floated preparation. 
 (Compare with nest in follicle, fig. 202. ) ( x 250.) 
 
 an extended series giving very much the appearance of tubes 
 are also seen, and have every resemblance to similar cell 
 extensions in the follicle. Scattered among the cells in these 
 
 z 2 
 
340 MICROSCOPIC ANATOMY OF THE TEETH 
 
 preparations are elongated crescentic or thread-like bodies, 
 which appear to be degenerated nuclei as described by 
 Romer in altered epithelial cells. 
 
 In many parts of these preparations cells are seen which 
 are quite different in appearance. They are elongated, with 
 nuclei at their distal ends, and appear to be attached to the 
 clear membrane by broad expansions ; they have, in fact, 
 a strong resemblance to altered and shrivelled ameloblasts 
 (figs. 228 and 229). Owing to the method of preparation this 
 
 FIG. 228. Elongated cells in Xasmyth's membrane ( x 250.) 
 
 elongation of the cells cannot be due to stretching, and when 
 these specimens are compared with those of Nasmyth's mem- 
 brane seen in longitudinal section (figs. 230, 231, and 232), it 
 seems impossible to avoid the conclusion that these are cells 
 of the inner epithelium of 'the enamel organ which have 
 undergone keratinization. 
 
 Passing to an examination of these longitudinal sections, 
 it is seen that these appear to shed a new light on the nature 
 and extent of the cells included in the membrane. 
 
 There have been considerable differences of opinion with 
 regard to the persistence of the external epithelium in the 
 later stages of calcification, some authors, as Professor 
 Underwood (11), stating that it becomes blended with the 
 
NASMYTH'S MEMBRANE 
 
 341 
 
 FIG. 229. Nasmyth's membrane. Elongated cells on membrane 
 and impressions of enamel prisms on clear layer. ( x 250.) 
 
 FIG. 230. Separation of the layers of the enamel organ 
 from the follicle. ( x 250. ) 
 
 capsule, others, as Legros and Magitot, asserting that the 
 cells of the external epithelium of the enamel organ atrophy 
 before the completion of the calcification of the enamel. 
 
342 MICROSCOPIC ANATOMY OF THE TEETH 
 
 These preparations of the follicle show that both the 
 external and internal epithelium persist up to the period 
 immediately before eruption. The attachment to the cells 
 of the follicle is very slight, and in most of the preparations 
 large portions of Nasmyth's membrane are seen loose and 
 lying across the section, being attached by one or both 
 extremities to the follicle (see figs. 230, 231, and 232). 
 
 As shown in figs. 231 and 232, the internal epithelium, the 
 stratum intermedium, and flattened cells oi the external 
 
 FIG. 231. The enamel organ cells on the inner margin of the follicle, 
 showing commencing detachment. Farther along these layers are com- 
 pletely separated. Longitudinal section. ( x 400. ) 
 
 epithelium are all seen blended with the follicle, but showing 
 a tendency to detach in places. In fig. 231 the whole of the 
 enamel organ, with the exception of the stellate reticulum, 
 which has long since disappeared, is seen becoming detached 
 from the follicle. It is also seen in these figures that not only 
 these cells but others deeper within the follicle here and 
 there become detached with them. The attachment to the 
 enamel is probably more complete than that to the follicle, 
 but when the enamel has been decalcified without disturb- 
 ance of the neighbouring tissues, as in this case, the attach- 
 ment to the cells of the follicle is better maintained. 
 
NASMYTH'S MEMBRANE 
 
 343 
 
 This detachment of other cells than those constituting 
 the enamel organ will account for the presence, in the floated 
 preparations of the membrane, of the rounded cell masses 
 and the cell nests above described, and we must consider 
 that Nasmyth's membrane consists not only of the external 
 epithelium, but also, and more evidently, of the two inner- 
 most layers of the enamel organ, which have not disappeared 
 on the eruption of the tooth but remain attached to the 
 enamel. 
 
 FIG. 232. Partial detachment of Nasmyth's membrane from 
 follicle. Columnar cells, many vacuolated. ( x 250.) 
 
 Summary 
 
 In preparations of Nasmyth's membrane by separation 
 from the surface of the enamel by acids, cells are seen which 
 do not appear to have any connexion with the enamel organ, 
 such as the rounded bodies and the cell nests. 
 
 These preparations would appear to be explained by the 
 longitudinal sections of the tooth in the follicle. It is seen 
 that the attachment of the enamel organ with other altered 
 and degenerated cells in contact with it is very slight, and 
 these latter cells show a tendency to separate from the 
 follicle with those of the enamel organ. It is evident that 
 
344 MICROSCOPIC ANATOMY OF THE TEETH 
 
 far from the external epithelium of the enamel organ dis- 
 appearing, it persists with the two inner epithelial layers, so 
 that we cannot avoid the conclusion that the membrane 
 consists of the altered cornified cells of the whole enamel 
 organ with the exception of the stellate reticulum, and also 
 includes other altered and degenerated cells of the follicle. 
 
 The clear layer was considered by Kolliker to represent 
 a continuous structureless layer formed by the enamel cells 
 after their work is completed. 
 
 Another view of its nature may be that it is a persisting 
 ameloblastic membrane, which according to Leon Williams 
 and many others is definitely to be seen between the ends 
 of the ameloblasts and the forming enamel, and corresponds 
 to one position of the ' membrana preformativa ' of Huxley. 
 If the internal ameloblastic membrane had not previously 
 disappeared, it would certainly come to occupy the position 
 of the clear layer of Nasmyth's membrane over the com- 
 pleted enamel, favouring the conclusions of Huxley and 
 Leon Williams that enamel is calcified by the dialysis of 
 lime salts through a membrane. As this dialysing membrane 
 persists while the tooth remains within the follicle, the cells 
 of Nasmyth's membrane would probably still be active and 
 functional, as they are in contact with the blood-vessels of 
 the capsule, and they thus may contribute to the final 
 consolidation of the enamel, as the author has shown is in all 
 probability the case, in the teeth of Sargus. It has been sug- 
 gested that after the eruption of the tooth, when the cells are 
 undergoing keratinization, the whole membrane may serve 
 as a dialysing membrane separating the lime salts from 
 the saliva. Pickerill, who adopts this view, says : ' The 
 enamel, after the eruption of the tooth, is covered with 
 a dead membrane, Nasmyth's membrane ; it is immersed 
 in a fluid saliva, containing in solution lime salts and 
 organic material (mucin and albumin), and it has been 
 shown that fluids with solids in solution can pass into the 
 outer layers of enamel ' (in experiments conducted by 
 Pickerill on the permeability of enamel from without). 
 ' It would seem therefore in every way probable that 
 Nasmyth's membrane acts as an ordinary dialysing mem- 
 brane, through which crystalloids pass, but colloids do not. 
 
NASMYTH'S MEMBRANE 345 
 
 Therefore, other things being equal, and so long as the 
 lime salts, especially the calcium phosphate, remain in 
 solution, they must tend to pass through the membrane 
 and penetrate the enamel, and the mucin and albumin are 
 kept back. No doubt the process is very slow and gradual, 
 depending largely on the relative osmotic pressures on either 
 side of the membrane, but it must undoubtedly take 
 place ' (7). 
 
 These considerations are, however, more or less of a 
 speculative nature, and it is difficult to arrive at any definite 
 conclusions with regard to the functions of Nasmyth's 
 membrane ; still it would appear that, given a membrane 
 separating a colloid from a crystalloid, diffusion must, by 
 the laws of osmosis, take place through it. It has been 
 considered by some that Nasmyth's membrane acts as 
 a protective covering to the enamel, preserving it from the 
 action of acids, but it is difficult to conclude how far this 
 can be the case, for in healthy conditions any acid normally 
 present in the mouth would not be likely to have any in- 
 jurious effect upon the enamel, and we cannot consider that 
 Nasmyth's membrane has been evolved for the protection 
 of the teeth from a pathological process such as caries. 
 
 So long as it is a continuous membrane it would no doubt 
 serve the latter purpose, but when detached in places, would 
 in all probability rather favour the process than otherwise, 
 as bacteria would proliferate beneath the separated mem- 
 brane, which would retain their acid products in contact 
 with the enamel surface. 
 
 In view of the direct evidence of the cellular nature of 
 Nasmyth's membrane it is scarcely necessary to discuss at 
 any length the earlier views of its origin. Although not the 
 usual function of the follicle in its coronal portion to lay 
 down cement in teeth the crowns of which are not normally 
 covered with this tissue, it seems theoretically conceivable 
 that a kind of attempt to do so might occasionally result in 
 the deposition of a thin layer outside Nasmyth's membrane. 
 
 It seems, however, highly improbable, and we cannot but 
 think that, as has been suggested, the so-called encapsuled 
 lacunae may be isolated cells of the enamel organ, especially 
 as a thin layer of cement does not show lacunae in its 
 
346 MICROSCOPIC ANATOMY OF THE TEETH 
 
 normal situation on the roots of teeth in man. The presence 
 of the large cornified epithelial cells with projecting pro- 
 cesses occasionally seen in preparations of the membrane, 
 would lend support to this view. 
 
 The calcined isolated bodies, above described, sometimes 
 found in the follicle, are within the connective tissue and 
 well separated from the bordering epithelial layers. They do 
 not appear to have the characteristic structure of either bone 
 or cement, and resemble the similar erratic calcification 
 which sometimes occurs in the centre of the connective 
 tissue of the pulp. 
 
 In 1914 a series of papers was published at Milan by 
 Dr. Arturo Beretta of Bologna (1) on the enamel cuticle. 
 The author has not had an opportunity of seeing the original 
 paper, but only a review which appeared in the Dental 
 Cosmos (May 1915). This author concludes that Nasmyth's 
 membrane results from the transformation of the amelo- 
 blastic layer into the basal membrane (probably referring 
 to the clear layer) and of the upper ameloblastic epithelia 
 into areas of granular consistence, which may be called 
 cuticular epithelial remnants. He would thus agree partially 
 with Kolliker's view, so far as the inner transparent layer 
 is concerned ; but it is difficult to understand what is meant 
 by the areas of granular consistence, as preparations of the 
 membrane show a definite layer or layers of cells. He says 
 that the enamel cuticle remains throughout life, and with 
 the advance of age increases in thickness. This statement 
 is hardly in accordance with previous observations. 
 
 Eruption of the Teeth 
 
 The teeth, both of the temporary and the permanent set, 
 are formed deeply in the tissues of the jaw, but soon after 
 the crowns are fully formed take their positions on its 
 upper margin, and the crowns of the teeth are fully exposed 
 within the mouth. The exact method by which this erup- 
 tion of the teeth is brought about has been a matter of 
 much controversy. It seems quite evident that there are 
 several factors concerned in the process, and no one theory 
 
NASMYTH'S MEMBRANE 347 
 
 that has been brought forward is sufficient in itself to 
 account for the phenomenon. 
 
 In a recent paper by Warwick James and A. T. Pitts, 
 the authors consider that there are two chief factors con- 
 cerned in eruption (16). 
 
 (1) A process of advancement of the tooth in the tissues. 
 
 (2) A process of denudation by absorption of the tissues 
 overlying and surrounding the tooth. 
 
 In the process of advancement they consider that the 
 point of eruption is determined by the presence of the 
 epithelial columns connecting the oral epithelium with that 
 lining the tooth follicle. The advancement of the tooth is 
 partly due to unequal rates of growth between the various 
 tissues surrounding the tooth, and they consider that the 
 elongation of the root plays some part in eruption. 
 
 As pointed out by Tomes (17), the elongation of the root 
 is alone quite inadequate to produce the effect. Teeth with 
 stunted roots are frequently erupted, and a tooth with 
 fully completed root may remain within the jaw and erupt 
 late in life. 
 
 He compares eruption in man with that in reptiles, 
 showing that ' a force quite independent of increase in 
 length shifts the position of and " erupts " successive 
 teeth '. 
 
 Messrs. James and Pitts compare the eruptive process to 
 ' the opening of a book, the hinged portion being advanced, 
 pari passu, with the separation of the pages of the volume, 
 until it comes to occupy the same level as the free edges '. 
 This implies the movement of the tooth, the cause of which 
 still has to be accounted for ; for while the degeneration 
 and absorption of the cells of the follicle and capsule, and 
 especially the opening out of the epithelial coils (figs. 203 and 
 204) described on p. 312, would afford a path for the tooth 
 to the surface, it must still be moved forward in this path 
 to its final position in the jaw. 
 
 Blood pressure was considered by Constant (13) to be the 
 cause, of the movement of the tooth, the vascular tissues 
 beneath the tooth serving as the propulsive force. It seems 
 very probable that this is one of the factors in eruption, 
 but not sufficient in itself to account for the whole process. 
 
348 MICROSCOPIC ANATOMY OF THE TEETH 
 
 It was considered by A. Underwood that eruption may 
 be caused by the movement of the soft parts surrounding 
 the tooth, due to the growth and changes taking place in 
 the periodontal membrane, and comparable to some extent 
 with the movement of the mucous membrane carrying the 
 teeth over the jaw in the Sharks (18). 
 
 A somewhat similar explanation of the process of eruption 
 is given by Thornton Carter. 1 He describes a slight amount 
 of ossification of the margin of the cartilaginous jaw in the 
 Dog-fish, and says: * The crust of bone, which is of a transitory 
 nature, being constantly absorbed and deposited, is instru- 
 mental in causing a progressive movement of the sliding 
 membrane,' and further, * When a functional tooth is shed, 
 absorption of the underlying bone takes place, and also 
 absorption of the fibrous membrane at its outcrop. As 
 a natural consequence of the absorption of the underlying 
 bone and fibrous membrane there is a rapid proliferation 
 of cells at the margin of the cartilage with formation of 
 bony tissue over the same area. The newly formed bony 
 tissue operates on the fibres of the sliding membrane, which 
 is poor in cellular elements and ill-adapted for active growth, 
 and causes the membrane to move upwards and bear with 
 it the next successional tooth.' That the presence of this 
 bony deposit, however, is not essential to the process appears 
 to be indicated by the fact that it is not always found 
 in Elasmobranchs. The section of the edge of the jaw of 
 Lamna at the Natural History Museum does not show the 
 presence of any bone, and no bone is figured in Rldewood's 
 drawing of the eruption of the teeth in Carcharias in the 
 Cambridge Natural History. 
 
 Carter's conclusions regarding eruption of the teeth in 
 man are as follows : ' Thus we may conclude that in man 
 the cause of eruption, or at least an active factor in producing 
 eruption, is to be found in the disproportionate growth 
 occurring in the tissues forming the tooth and the tissues 
 surrounding the tooth.' 
 
 It seems impossible to deny that there is a forward move- 
 ment of the tooth in eruption, which is probably due to 
 
 1 Colyer's Dental Surgery and Pathology, 1919. 
 
NASMYTH'S MEMBRANE 349 
 
 many causes, such as the elongation of the roots, the growth 
 of the bone of the jaw, the development of the periodontal 
 membrane, and the blood pressure in the vascular tissues 
 around and beneath it. 
 
 This advancement of the tooth is therefore probably due 
 to several concomitant forces, and is one factor in eruption, 
 the other being the absorption and opening out of the 
 tissues overlying the tooth. 
 
 Warwick James, in a previous paper (15), showed very 
 clearly how such a path is prepared by the opening out of 
 the overlying tissues of the capsule as the tooth erupts. 
 He showed that the globes epidermiques or epithelial coils, 
 as he prefers to call them, which are due to degenerative 
 changes in the epithelial elements of the capsule, form very 
 wide spaces within the connective tissue over the tooth ; that 
 these increase in size until they appear as mere openings 
 in the connective tissue, all trace of their epithelial structure 
 being lost, and they eventually open out upon the surface ; 
 but this opening out takes place not by the advance of these 
 bodies, but by a separation or rarefaction of the tissue of the 
 capsule, which is gradually withdrawn on either side of the 
 erupting crown, yielding in the first place at the situation 
 of the epithelial coils. 
 
 The changes which take place in the epithelium of the 
 follicle and the formation of the epithelial coils have been 
 more fully considered in treating of the follicle (p. 311). 
 
 Guido Fischer (14), in his paper on the eruption of the teeth, 
 described the union of the outer and inner layers of the 
 enamel organs into one continuous epithelial layer which 
 just previous to eruption becomes blended with the epithe- 
 lium of the mouth. These observations were made on the 
 erupting teeth of the cat, and the illustrations to the paper 
 show this epithelial layer blended with the surface epithelium 
 on either side of the opening through which the erupting 
 tooth is advancing. 
 
 It is difficult to reconcile this description with what is 
 seen in the sections of the human follicle just before eruption, 
 described in Chapter VIII, for Fischer's figures do not show 
 the detachment of any layer or layers of cells to form 
 Nasmyth's membrane, and one would be more inclined to 
 
350 MICROSCOPIC ANATOMY OF THE TEETH 
 
 look upon this epithelial layer which he describes as con- 
 sisting of the cells of the deeper part of the follicle. 
 
 Nasmyth's membrane not being shown would suggest the 
 probability that the separating layers forming this membrane, 
 as shown in figs. 230 and 231, &c., have become completely 
 detached and lost in the preparations figured in the paper. 
 In several of the author's sections this has occurred, and 
 would be probably still more likely to happen with paraffin 
 sections treated with alcohol. 
 
 In this case, as suggested, it is the deeper epithelial cells 
 of the follicle which are connected with the surface epithelium 
 and not the cells of the enamel organ. 
 
 As it is considered by Fischer that the outer and inner 
 layers of the enamel organ become united and blended with 
 the surface epithelium, he cannot apparently agree with the 
 views of Von Brunn and Ballowitz that these layers are 
 continued downwards as the sheath of Hertwig, and in fact, 
 in his figures, the sheath of Hertwig is shown as discon- 
 tinuous at the neck of the tooth and is neither a prolongation 
 of the cells of the enamel organ nor of the deeper epithelial 
 cells of the follicle. 
 
 Teeth do not always erupt in a vertical direction as in man, 
 where the new tooth appears immediately beneath that 
 which is being shed. 
 
 In osseous fish the successional teeth usually appear at 
 the sides of the tooth in use, but in Sargus both the molars 
 and incisors erupt directly beneath the tooth which is shed, 
 and in the pharyngeal teeth of Labrus the same mode of 
 succession is seen. In the Sharks new teeth come into use 
 in successive rows, being carried forward by the movement 
 of the mucous membrane over the rounded cartilaginous 
 jaws. 
 
 Some teeth never erupt, but remain embedded in the jaws, 
 as in the female Narwhal, and many instances are recorded 
 in man where teeth have never erupted or have appeared 
 long after the normal period of eruption. 
 
 Although we have some knowledge of the phenomena 
 accompanying the process, the actual conditions necessary 
 and the forces which govern the process of the eruption of 
 the teeth are still but very imperfectly understood. 
 
NASMYTH'S MEMBRANE 351 
 
 REFERENCES 
 
 1. Beretta, A. ' The Enamel Cuticle Histological and Histogenetic 
 
 Researches.' La Stomatologia, Milan, No. 9, 1914. 
 
 2. Dahlgren and Kepner, W. A. A Text-book of the Principles of Animal 
 
 Histology, 1908. 
 
 3. Huxley, T. H. ' On the Development of the Teeth and on the Nature 
 
 and Import of Nasmyth's Persistent Capsule.' Quar. Journ. Micr. 
 Sci., No. 3, 1853. 
 
 4. v. Lent, E. ' Ueber die Entwicklung des Zahnbeins und des 
 
 Schmelzes.' Zeitsch.fur wissenschaft. Zoologie, Sechster Bd., p. 121, 
 1885. 
 
 5. Owen, R. Odontography, p. 466. 
 
 6. Paul, F. T. ' The Enamel Organ.' Dental Record, 1896, vol. xvi, 
 
 pp. 493-8. 
 
 7. Picked]], H. P. Prevention of Dental Caries, 2nd ed., pp. 135-6. 
 
 8. Schafer, E. A. ' Text-book of Microscopic Anatomy.' Quairts Anat., 
 
 vol. ii, pt. i, 1912, p. 96. 
 
 9. Tomes, J. A System of Dental Surgery, 1st ed., 1859, pp. 266-72. 
 
 10. Tomes, C. S. Dental Anatomy, 7th ed., p. 122. 
 
 11. Underwood, A. Aids to Dental Anatomy and Physiology, 3rd ed., p. 24. 
 
 12. Wilson, E. B. The Cell in Development and Inheritance. New York, 
 
 1896, p. 82-4. 
 
 13. Constant, T. E. ' The Eruption of the Teeth.' Troisieme Congres 
 
 Dentaire Intern., Paris, 1900, vol. ii, pp. 180-92. 
 
 14. Fischer, G. ' Beitrage zum Durchbruch der bleibenden Zahne u. zur 
 
 Resorption des Milchgebisses.' Anatomische Hefte, 1. Abtheilung, 
 116. Heft (38. Bd., H. 3), Wiesbaden, 1909. 
 
 15. James, W. ' A Preliminary Note on the Eruption of the Teeth.' 
 
 Proc. Eoy. Soc. of Med., 1909 (Odontological Section). 
 
 16. James, W. W., and Pitts, A. T. ' Some Notes of the Dates of Eruption 
 
 in 4,850 Children under 12.' Proc. Roy. Soc. Med. (Odontological 
 Section), vol. v, No. 5, pp. 80-101. 
 
 17. Tomes, C. S. Dental Anatomy, 7th ed., pp. 238-9. 
 
 18. Underwood, A. Aids to Dental Anatomy and Physiology, 3rd ed., p. 86. 
 
CHAPTER X 
 THE ATTACHMENT OF TEETH 
 
 WHILE the general study of the attachment of teeth may 
 be better considered in works on dental anatomy, the 
 microscopic structure of the teeth and bone and their con- 
 necting tissues concerned in the different modes of attach- 
 ment render it necessary to dwell somewhat on the different 
 methods by which the union of the teeth with the jaws is 
 brought about. 
 
 Our knowledge of the forms of attachment, especially in 
 fish, is chiefly due to C. S. Tomes, who was the first to 
 describe the existence of a special bone of attachment in 
 reptiles and fish. 
 
 The various modes of attachment of teeth may be classified 
 as follows : 
 
 1. Fibrous attachment. 
 
 2. Attachment by an elastic hinge. 
 
 3. Anchylosis. 
 
 4. Socketing or gomphosis. 
 
 (1) Attachment by Fibrous Membrane. This form is seen 
 in the Sharks and Rays. 
 
 The class Plagiostomi, to which these forms belong, is 
 characterized by a cartilaginous skeleton. The teeth of the 
 Sharks have no direct attachment to the jaw, but are 
 attached to the mucous membrane by strong fibrous bands 
 which envelop the spurs or processes at the base of the 
 tooth and firmly bind it down to the tough fibrous mem- 
 brane covering the jaws. A sliding movement of this fibrous 
 mucous membrane takes place over the jaws as the teeth 
 come into use, the membrane with its attached rows of 
 teeth rolling, as it were, over the rounded margin of the jaw 
 and thus bringing the functional rows of teeth successively 
 into an upright position. As the movement progresses the 
 teeth of the front row are cast off, or shed, and the next row 
 of teeth take their place. 
 
 That this forward movement of the whole fibrous mem- 
 
THE ATTACHMENT OF TEETH 353 
 
 brane, carrying the teeth with it, really occurs, was clearly 
 shown by the well-known specimen figured and described 
 by Professor Owen (2). In this case the jaw of a shark 
 (Galeus) was penetrated by the spine of a Sting-ray. The 
 spine entered the jaw in the region of the developing teeth 
 and the scar extended from this position to the front row 
 of the teeth in use, which showed a malformation, the result 
 of arrested development in the region of the injury. This 
 could not have occurred unless the whole of the mucous 
 membrane had been carried forward, sliding over the jaw 
 beneath (fig. 233). 
 
 m 
 ---c 
 
 FIG. 233. Part of the lower jaw of a Shark (Galeus) pierced by the barbed 
 caudal spine (s.) of a Sting-ray (Trygon), showing the effect of the wound 
 of the dental matrix on the teeth, which have advanced in their revolving 
 course over the jaw. m. Mucous membrane ; c. cartilaginous jaw ; a. in- 
 jured calcified teeth. 
 
 Various modifications of the method of attachment by 
 fibrous membrane are seen in fishes. In some the attach- 
 ment admits of a very slight amount of rocking movement, 
 and transitional forms between this and true hinging are 
 met with in great abundance. 
 
 As seen in the Eel (fig. 234), the teeth are situated on 
 little pedicles of bone, the bone of attachment, first described 
 by C. S. Tomes. This bone of attachment corresponds to 
 the alveolus of a socketed tooth ; it is developed for the 
 attachment of the tooth, and is removed or absorbed when 
 the tooth is shed. 
 
 It has not the regular microscopic structure of the bone 
 
354 MICROSCOPIC ANATOMY OF THE TEETH 
 
 of the jaw, with which it is continuous. Both in its lamina- 
 tion and in the irregular lacunae and other spaces which it 
 exhibits it has a coarser appearance than the bone beneath. 
 It is well developed in the teeth of Ophidia. The fusion of 
 the teeth with the bone of attachment is often so complete 
 that it is difficult to distinguish tooth from bone, especially 
 in osteodentine teeth. 
 
 (2) Attachment by an Elastic Hinge. A simple form of 
 
 FIG. 234. Teeth of Eel showing union to bone of attachment, and 
 enamel tips. (Photographed from a specimen lent to the author by 
 Sir Charles Tomes. ) e. Enamel tips ; 6. bone of attachment. 
 
 attachment by a hinge allowing of movement of the tooth 
 upon the bone of attachment is seen in the Echineis, the 
 sucking fish of the Shark, which attaches itself by a sucker 
 on the b?,ck of the head to the skin of the Shark, and although 
 also capable of free movement, obtains most of its nourish- 
 ment from the Crustacea and other organisms found in the 
 slimy substance on the shark's skin. 
 
 The sucker allows of free movement of the fish in a for- 
 ward direction, but it cannot be detached backwards by 
 any force applied from the front. Each tooth is attached 
 to a special bone of attachment, but is not anchylosed to it ; 
 
THE ATTACHMENT OF TEETH 
 
 355 
 
 it slides freely upon the bone, and exhibits a modified form 
 of hinge. The summit of this pedestal of bone is in the form 
 of a convex ring, slightly raised on one side, resembling the 
 socket of a ball-and-socket joint with the centre cut out, 
 leaving an elevated ring of bone. The base of the tooth is 
 accurately adapted to this surface, and slides easily upon it, 
 forming the convex side of the joint ; the opening for the 
 passage of the tissues of the pulp is large, to allow of the 
 
 FIG. 235. Hinged tooth from lower jaw of Echineis squalipeta. e. Enamel 
 tip ; a. part of capsule ; b. capsule containing fibres forming hinge ; c. bone 
 of attachment. 
 
 movements of the tooth taking place without injury to the 
 nerves and blood-vessels. A capsular ligament surrounds 
 the whole, giving a still greater likeness to a ball-and-socket 
 joint (fig. 235). 
 
 The capsule is strengthened anteriorly and posteriorly 
 by a fibrous band, there being a distinct depression on the 
 bone and also on the tooth, for the attachment of these 
 fibres. It will be seen that this arrangement would allow 
 of a sliding movement of the tooth on the bone of attachment 
 in every direction, but the ring of bone being slightly more 
 elevated on the anterior aspect, the motion is limited in 
 this direction more than in the opposite, and the tooth can 
 be bent over much more in the direction of its point than 
 in any other direction. The strong elastic ligament ous band 
 being stretched, no doubt serves to draw the tooth into 
 
 A a 2 
 
356 MICROSCOPIC ANATOMY OF THE TEETH 
 
 position again. The amount of bending down of which these 
 teeth are capable is not very great when compared with 
 such complete forms of hinging as those of the Hake and 
 the Pike, but when the minute teeth are examined under 
 a hand lens and pushed inwards with a needle, they are seen 
 to be considerably depressed and to recover their position 
 immediately upon removal of the pressure, while owing to 
 the shape of the bony pedestal they resist pressure in the 
 opposite direction. 
 
 The amount of movement is quite sufficient, associated 
 with the strong inward curve of the tooth, to make escape 
 very difficult or impossible for any small creatures captured 
 as prey, which would meet with no obstruction in passing 
 over the crowns of the teeth in entering the mouth. We 
 have thus, in Echineis, a stage in the transition from the 
 fixed teeth of the Shark to true hinged teeth, for here there 
 are elastic ligamentous bands in addition to the capsule 
 seen in the Eel, directing the motion of the tooth chiefly in 
 a particular direction, and the shape of the opposed sur- 
 faces, above described, would appear to be of especial value 
 in giving to this form of hinge as free a movement as possible. 
 
 For a further description of the teeth of Echineis, the 
 reader may be referred to the author's original paper (1). 
 Hake. From the foregoing description it is easier to comprehend 
 
 the structure of the more perfectly hinged teeth of the 
 Hake (Merluccius). 
 
 In this fish the outer row of teeth are anchylosed to the 
 jaw (fig. 236), but the inner row are hinged, and the hinging 
 is of a very complete nature, the tooth being able to be bent 
 inwards to an angle of about 45 and to recover its position 
 with a snap. 
 
 In fig. 237, which is a photograph of a ground section 
 of the hinged tooth of the Hake, and in Plate VI the tooth is 
 seen in its partially depressed position. The front portion 
 of the tooth is thickened at its base, and when returned to 
 its place, this thickened portion is received on an elevated 
 pad or buttress of bone. 
 
 In this preparation it was noticed that the tooth was 
 also attached to the inner margin of the bony pedestal. 
 In the published figures of the hinged teeth of the Hake 
 
PLATE VI. 
 
 Hinged tooth of the Hake (Merluccius) from a longitudinal section taken to 
 one side of the middle line. b. Bone of attachment ; /. uncalcified portion of 
 posterior hinge ; a. anterior hinge ; s. stiffened elastic part of posterior hinge ; 
 d. insertion of calcified tooth into posterior hinge ; e. triangular space occupied 
 by interlacing elastic (?) fibres ; i. insertion of outer portion of posterior hinge 
 into bone of attachment (b). (Drawn by the author from a specimen stained with 
 Mann's methyl -eosin.) x 50. 
 
 B. Diagrammatic representation of bone of attachment viewed from above, 
 the tooth being removed, h. The two halves of the anterior hinge coloured red ; 
 /. pedestal of bone ; r. opposite pulp cavity ; ph. the red dotted line represents 
 the attachment of the posterior hinge ; b. foramen for passage of blood vessels 
 to pulp. 
 
 To face p. 356. 
 
THE ATTACHMENT OF TEETH 357 
 
 b- 
 
 FIG. 236. Anchylosed tooth of Hake (Mcrluccius). 
 t. Tooth; b. bone of attachment. (x50.) 
 
 e 
 
 FIG. 237. Hinged tooth of Hake (Merluccius). Ground section to one 
 side of middle line. p. Thickened base of tooth ; a. anterior hinge ; b. bone 
 of attachment forming raised pedestal ; e. triangular space ; /. fibrous 
 part of posterior hinge. ( x 30.) 
 
358 MICROSCOPIC ANATOMY OF THE TEETH 
 
 this band is not shown, and this observation led the author 
 to examine these hinged teeth in fresh specimens and in 
 decalcified serial sections in order to make clear the mean- 
 ing of this appearance in the ground section. A fresh 
 specimen of this fish of unusually large size was examined, 
 and it was found that a probe could be passed through to 
 the pulp at the centre, but met with resistance to the right 
 and left of this position, showing that the tooth lies free 
 upon the bone only in the middle line of the pedestal. Teeth 
 were decalcified and examined in a series of sections from 
 one side of the tooth to the other. These preparations 
 showed that there is a band of fibrous tissue attached to 
 the tooth and bone on either side of the central opening. 
 On dividing the anterior bands in the fresh tooth, the tooth 
 resumed its position when depressed almost as well as when 
 these bands were entire, but it was unsteady on the pedestal, 
 and easily displaced laterally. Division of the posterior 
 hinge entirely prevented the return of the tooth to the up- 
 right position. The function of this anterior hinge would 
 thus appear to be the increase of the resiliency of the 
 whole structure and the prevention of lateral displace- 
 ment. In the figure in Tomes's Dental Anatomy, the section, 
 presumably a ground one, had been taken through the 
 centre of the longitudinal axis, and the anterior hinge con- 
 sequently is not shown. In Plate VI B a diagrammatic draw- 
 ing of the bone of attachment is shown, the tooth having 
 been removed. The upper part of this diagram represents 
 the elevated pad of bone or pedestal, and it is seen that the 
 pulp cavity is prolonged forward, forming a semicircular 
 hollow at the posterior margin of the pedestal. The two 
 bands which make up the anterior hinge reach from each 
 side only as far as this depression. The ridge of bone at r 
 is at a considerably lower level than the pedestal, and the 
 larger posterior hinge is attached to it, encircling the posterior 
 half of the tooth and bone. 
 
 The structure of the posterior or principal hinge is shown 
 in fig. 238, photographed from a decalcified section. The outer 
 border of the dentine on the inner side of the tooth is con- 
 tinued downwards to its attachment to the bone, as is well 
 shown at k in fig. 238 and in Plate VI. This portion of the 
 
THE ATTACHMENT OF TEETH 
 
 359 
 
 dentine is not calcified, but appears to be somewhat of the con- 
 sistence of whalebone, although as a mesoblastic product 
 having no other analogy with it ; acids have no effect upon it. 
 It contains no vascular canals, and at its upper portion at/ in 
 this figure is continuous with the dentine at the inner margin 
 of the pulp cavity, and in this portion a few scattered vascular 
 canals are seen, but it terminates at e, where the fibres of 
 the fibrous band d are inserted into it. These fibres again 
 
 FIG. 238. Section of decalcified tooth of Hake (Merluccius) taken to one 
 side of middle line, showing anterior hinge on one side (a.) stained with 
 Mann's methyl eosin. e. Triangular space ; d. insertion of calcified tooth 
 into fibrous portion of posterior hinge ; s. stiffened elastic part of posterior 
 hinge ; i. junction of fibrous part with s. ; b. bone of attachment ; k. in- 
 sertion of posterior hinge into bone. ( x 50. ) 
 
 unite at i with the outer band s. Between these two 
 portions of the hinge is an elongated space in which separated 
 strands of fibres are seen. That the two portions of the 
 hinge above described are of a different structure is shown 
 by their staining reactions. When sections are stained with 
 methyl eosin, the dentine and bone are stained red, but the 
 fibrous band/ in the plate is of a bright blue, taking the colour 
 in the same manner as uncalcified connective tissue ; the 
 posterior portion of the band s is coloured uniformly red. 
 
360 MICROSCOPIC ANATOMY OF THE TEETH 
 
 It would thus appear that the posterior hinge consists of two 
 different substances an outer stiffened portion which is 
 possessed of considerable rigidity but great elasticity, and 
 an inner fibrous portion made up of strands of fibres of an 
 elastic nature, which become relaxed when the tooth is 
 pushed backwards and straighten out when the pressure is 
 removed, and with the stiffer arched spring-like portion at 
 the back immediately return it to its position on the bone. 
 
 FIG. 239. The posterior hinge of the tooth of the Hake (Merluccius). 
 d. Calcified portion of the tooth inverted into the fibrous portion of the 
 hinge/. ; s. stiffened elastic part of posterior hinge ; p. pulp of the tooth. 
 
 (x 150.) 
 
 In fig. 239 the fibrous portion of the hinge is shown under 
 higher magnification and its attachment to the prolonged 
 portion of the vasodentine (d). These fibres, when teased 
 out in glycerine or water, curl very much in the same manner 
 as those of ordinary elastic tissue. 
 
 The anterior hinge, as shown in Plate VI B and fig. 238, a, is 
 divided into two portions, the tooth in the centre lying free 
 on the bony pedestal. 
 
 This hinge is made up of stout fibres, which are inserted 
 into the tooth above and the bone below. It is of consider- 
 able thickness and stains blue like the fibrous portion of the 
 
THE ATTACHMENT OF TEETH 361 
 
 posterior hinge (Plate VI A). Serial sections show that when 
 the central longitudinal axis is approached, this band is no 
 longer visible, and the thickened portion of the lower end of 
 the dentine lies free in the section, portions of the pulp only 
 being attached to it. 
 
 This apparatus is perfectly adapted to the capture of 
 active prey ; the small fish and other creatures which form 
 the food of the Hake depress the teeth on entering the 
 mouth, a very slight pressure only being necessary, and their 
 escape is prevented by the springing back of the teeth into 
 position. 
 
 The passage of the captured creature backwards in the 
 mouth is still further facilitated by the hinged teeth on the 
 vomer and pharyngeal bones. 
 
 Most of these predatory fish swallow their prey in a whole 
 and living condition, and their teeth are not available for 
 any process of mastication. 
 
 In other members of the Cod family a transitional form 
 of hinging is met with. In the Haddock (Gadus ceglefinus) Haddock, 
 the tooth rests upon a ring of what appears to be dentine, 
 which becomes blended below with the bone of attachment. 
 The upper part of the tooth is quite separated from this 
 lower portion so far as the continuity of the hard tissues is 
 concerned, but surrounded at the point of contact with a 
 fibrous band forming a kind of capsular ligament which is 
 attached to the circumference of the portions above and below 
 the flange. This evidently forms a hinge and allows (fig. 240) 
 of a limited amount of rocking movement, as can be seen in 
 several sections where teeth are seen to be inclined to one side, 
 the opposite side of the ligament being stretched. The pulp, 
 as pointed out by C. S. Tomes, is continued into the cavity 
 below the flange, and he describes odontoblasts in this 
 portion of the pulp, so that we must look upon the tissue 
 in this part of the tooth as a not very clearly defined dentine, 
 and as this author states, its implantation would indicate 
 a transition in the teeth of the Haddock to a socketed type. 
 
 The hinged teeth of the Angler (Lophius piscatorius) are Angler. 
 attached by radiating fibrous ligamentous bands to the bone, 
 the front of the tooth being free and resting on the bone, 
 but there is no definite differentiated elastic hinge as in the 
 
362 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Hake. The elasticity is, however, very complete, and the 
 tooth instantly returns to its position when the pressure is 
 removed. 
 
 Pike. In the Pike (Esox lucius) the marginal teeth are anchy- 
 
 losed and the palatal teeth are hinged, but the hinging is 
 obtained in a different manner. As pointed out by Tomes, 
 the osteodentine pulp is traversed by elastic bands or 
 trabeculae, which remain uncalcified, and are firmly attached 
 to the dentine at the margin of its dense peripheral portion 
 
 r h 
 
 FIG. 240. Tooth of Haddock (Gadus ceglefinus) showing 
 modified hinging at h. 
 
 9 
 
 and to the bone of attachment beneath, on which the tooth 
 rests. 
 
 The strong fibrous bands attaching the tooth to the bone 
 on its inner side are not elastic, and the elasticity resides 
 in the trabeculse within the pulp ; when pushed back the 
 tooth recovers its position by the contraction of these 
 bands. 
 
 The arrangement of the teeth of the Pike is eminently 
 adapted for the retention of living prey, the position of the 
 hinged teeth on the palatal bones directing the captured 
 prey in a longitudinal direction, and thus enabling it to 
 pass into the throat as described by Tomes. Every 
 
THE ATTACHMENT OF TEETH 363 
 
 angler for pike knows that the fish may often be held for 
 a considerable time and "even drawn out of the water when 
 not hooked, the retention of the living bait between the 
 teeth being so complete that it cannot be withdrawn. The 
 above-named author found examples of hinged attachment 
 in many specimens of deep-sea fish obtained on the Challenger 
 expedition. 
 
 (3) Anchylosis. This is the more common form of attach- 
 ment in fish and reptiles ; there is no intervening vascular 
 and fibrous membrane, and the union does not take place 
 directly with the bone of the jaw, but through the medium 
 of the bone of attachment. The fusion of the tooth with 
 the bone of attachment is usually very complete, so much so 
 that, as C. S. Tomes points out, in grinding a section of an 
 anchylosed tooth the bone of attachment often comes away 
 from the bone, while it is firmly attached to the tooth 
 (fig. 236). 
 
 A curious modification of this mode of attachment is 
 seen in the Mackerel, where the tooth is anchylosed to the 
 bone at its lateral margins but unattached below, being, as 
 it were, slung within the bone. 
 
 The author in an examination of two species of Wrasse 
 (Labrus) found a curious modification of anchylosis in these 
 fishes. The tooth when about to erupt has a large pulp 
 cavity with the usual dentine pulp consisting of connective 
 tissue and blood-vessels and a layer of large odontoblasts. 
 The pulp is extremely vascular and the vessels form a con- 
 tinuous network of loops within the odontoblast layer in 
 close contact with the tubular dentine. The pulp tissue is 
 seen to merge into bone beneath the open end of the rootless 
 tooth. When the tooth is erupted and the deposition of 
 the dentine is completed, the bone tissue invades the pulp 
 chamber and entirely fills it, the bone becoming continuous 
 with the dentine around the inner margin of the pulp, the 
 odontoblasts having entirely disappeared. 
 
 This is not the same condition as in the anchylosed 
 teeth of the Pike, where the tissue of the tooth is an osteo- 
 dentine, but an actual substitution of the dentine-forming 
 pulp by true bone (figs. 241 and 242). 
 
 Anchylosis is found in the teeth of the Python and the 
 
364 MICROSCOPIC ANATOMY OF THE TEETH 
 
 FIG. 241. Pharyngeal tooth of a Wrasse (Tautoga). Unerupted tooth 
 showing pulp and odontoblasts and large vascular supply, d. Dentine. 
 (X25.) 
 
 FIG. 242. Pharyngeal tooth of Wrasse (Tautoga), erupted. The dentine 
 pulp is completely filled with bone. d. Dentine ; p. bone tissue in pulp. 
 (x25.) 
 
THE ATTACHMENT OF TEETH 365 
 
 lizards, and in the poison fangs of the viperine snakes. 
 When the tooth is attached to the bone on the outer side, 
 as in Varanus, the attachment is described as pleurodont ; 
 when to the summit of the bone of attachment, as in the 
 Eel, it is spoken of as acrodont. 
 
 (4) Gomphosis or Socketing. This is the mode of attach- 
 ment seen in human and mammalian teeth generally. The 
 tooth is implanted in a cavity in the bone forming the 
 socket, and is separated from the bone by a fibrous and 
 vascular membrane or ligament, the periodontal membrane. 
 
 A separate socket is developed for each tooth in Mam- 
 malia, but in the Crocodile, as shown by Tomes, ' the suc- 
 cessive teeth come up and occupy a socket which is more or 
 less already in existence '. 
 
 In this form of attachment we have the intervention of 
 a vascular membrane, and the bone of attachment is repre- 
 sented by an alveolus, which is a special process of bone 
 developed to receive the tooth, and which becomes absorbed 
 and removed when it is shed. 1 
 
 The teeth of the rostrum of Pristis (the Saw-fish), which 
 consist of plicidentine and are of persistent growth, afford 
 an example of socketed teeth in fish, a very rare mode of 
 attachment in this class. 
 
 1 This special development of the alveolar process is well shown in the 
 Manatee and in the Sheep, the forming teeth at the back of the mandible 
 lying in a separate tube of bone as seen in two preparations in the 
 Odontological Section of the Hunterian Museum of the Royal College of 
 Surgeons. 
 
 REFERENCES 
 
 1. Mummery. J. H. ' On the Teeth of Echineis.' Trans. Odontol. Soc. 
 
 Great Brit., vol. xxxi, No. 3, p. 62. 
 
 2. Owen, R. OdontograpJiy, p. 39, Plate xxviii. 
 
 3. Tomes, C. S. ' Studies upon the Attachment of Teeth.' Trans. 
 
 Odontol. Soc. Great Britain, 1874-5, vol. vii, pp. 41-56. 
 
CHAPTER XI 
 HORNY TEETH 
 
 THE teeth of the Cyclostomata (Lampreys, &c.) are horny 
 cones ; they show no true calcification, but are made up of 
 cornified cells of the stratum corneum of the epidermis. 
 New horny cones are formed beneath the tooth in use, and 
 successively take its place when shed. 
 
 In the Lamprey (Petromyzori) these horny structures are 
 found at the sides of the mouth and on the tip of the tongue. 
 
 In the Hagfish (Bdellostoma) there is a single tooth in 
 the roof of the mouth and minute teeth upon the tongue. 
 This fish is partly parasitic in its habits, living on the mud 
 at the sea bottom, and often boring its way into the bodies 
 of large fish, especially the cod, to which it is often very 
 destructive, eating out the soft parts of its prey by means 
 of the rasping action of its tongue teeth. 
 
 Beard (2) has described in Bdellostoma the existence of 
 a true layer of odontoblasts and the formation of a cap of 
 imperfectly calcified dentine, and he considered that his pre- 
 parations showed a small cap of enamel upon this dentine. 
 Warren (8), however, denies the presence of calcified 
 enamel or dentine, and shows that a differential stain (picro- 
 nigrosin) indicates that there is no calcification, but the 
 tooth is entirely horny. There is simply a superficial 
 resemblance to calcified teeth, and no odontoblasts are 
 formed. He considers that the dentine cap described by 
 Beard is the succeeding horny tooth beneath that in use, 
 and that it arises in the same manner by the cornification 
 of epithelial cells. Professor Howes (4) in 1894 expressed 
 the opinion that the ' odontoblasts ' which Beard described 
 showed no calcification, and Ayres (1) in the same year 
 could find no trace of dentine or enamel 
 
 If Warren's views are accepted, and the evidence afforded 
 by his specimens would appear to be conclusive, these teeth 
 
HORNY TEETH 367 
 
 are in both Petromyzon and Bdellostoma horny structures 
 derived from the epithelium, and have no likeness or analogy 
 to calcified teeth. 
 
 It has been considered that the teeth of the Cyclostomes 
 are vestiges of the calcified teeth of former types, but it is 
 now more generally held that they represent a stage in tooth 
 evolution. 
 
 As Dr. Bridge says (3), ' The structure and development 
 of the teeth in the Cyclostomes lend no support to the view 
 that these teeth are degenerate calcified structures. With 
 greater probability they represent a stage in the evolution 
 of teeth and dermal spines, which has been succeeded by 
 a later stage in which calcification superseded cornification 
 as a method of hardening.' 
 
 There is, in the first place, a downgrowth of the epithelium 
 forming a kind of tooth follicle, and beneath this a small 
 mesodermic papilla. The first indication of a forming tooth 
 occurs above this papilla, and the epidermal cells become 
 flattened. The tooth penetrates the superimposed cells and 
 appears on the surface, the indication of another tooth 
 being already present in a cornified layer of cells beneath it, 
 as shown in the figure taken from Warren's paper (fig. 243). 
 He draws attention to the close resemblance which this struc- 
 ture, with the mesodermal papilla below, bears to a develop- 
 ing hair. The figure shows that the tooth arises within the 
 corium, and is not formed from cells derived from the 
 mesoblastic papilla. 
 
 The Teeth of Ornithorhynchus. Horny structures which 
 serve the functional purposes of teeth are seen in the Ornitho- 
 rhynchus ; but they are more correctly described as horny 
 plates, for, as will be seen, these are the plates in which the 
 true teeth of the animal are embedded, and are not developed, 
 as in the Cyclostomata, as independent dental structures. 
 For a long time these horny plates were considered to 
 represent the only functional teeth of Ornithorhynchus, until 
 in 1888 Professor Poulton(5) discovered true calcified teeth in 
 an embryo. From an examination of the available material 
 he concluded that the true teeth were beneath the horny 
 plates, but Oldfield Thomas in 1889 (7), from an examination 
 of other specimens, found that the calcified teeth were above 
 
368 MICROSCOPIC ANATOMY OF THE TEETH 
 
 and embedded in the horny plates, and persist for a con- 
 siderable portion of the life of the animal, and ' are only 
 shed after being worn down by friction with food and sand '. 
 Dr. Semon, who has studied the habits of the Ornitho- 
 rhynchus, says that it feeds chiefly on ' grubs, worms, snails, 
 and, most of all, mussels ', which are stowed into its cheek 
 pouches, ' the food being chewed and swallowed above the 
 surface as the animal drifts slowly along '. He considers 
 
 
 FIG. 243. Vertical section of developing tooth of Petromyzon marinus, 
 showing a successional tooth which is just beginning to cornify at its 
 apex beneath the functional tooth, d. Dermis ; dp. dental papilla ; 
 ep. epidermis lining buccal funnel ; ep l . epidermis which has formed the 
 horny functional tooth ht ; ep 2 . epidermis forming the horny cone of the 
 successional tooth ht 1 . (From Warren.) 
 
 that for cracking the hard shells of the mollusc upon which 
 it mainly feeds, the horny plates are preferable to brittle 
 teeth (6). 
 
 The eight or ten molar teeth which form the calcined 
 dentition of the animal are replaced by the horny structures, 
 which, developed from the epithelium of the mouth, are 
 produced around and under the true teeth, embedding them, 
 the short roots of the teeth passing through them to the 
 bone of the jaw. As Dr. Beddard says, ' The epithelium of 
 
HORNY TEETH 369 
 
 the mouth grows gradually under the calcified teeth, a 
 method of growth which has possibly something to do with 
 the shedding of the latter '. 
 
 The calcified teeth, which are all molars, consist of dentine 
 and enamel of somewhat imperfect structure, the dentine 
 being characterized by the immense number of interglobular 
 spaces present, which would appear to be an indication of 
 imperfect calcification, which is still more apparent in the 
 short roots ; these, as C. S. Tomes says, ' are of a softer, 
 coarser material than the crown, which itself is not of a high 
 type of dentine structure '. 
 
 In the Tadpole, prior to the commencement of the forma- 
 tion of true calcified teeth, there are horny plates upon the 
 jaws, and on the inner margins of the lips are numerous 
 horny projections, each one of which is, according to 
 C. S. Tomes, the product of a single epithelial cell ; these 
 little conical teeth, as well as the larger plates on the jaws, 
 are continually being shed and renewed from beneath. 
 
 In the Chelonia (Turtles and Tortoises) there are no 
 calcified teeth, but a horny casing covers both upper and 
 lower jaws, which is broad and crushing in the vegetable 
 feeders and elevated into a sharp ridge in the carnivorous 
 species, as in Cheloneimbricata (the Hawk's-bill turtle), where 
 this horny covering forms a hooked beak with a sharp 
 edge. 
 
 Horny plates are also present in the jaws of the Sirenia 
 (Manatee and Dugong), which are considered to be allied 
 to the Ungulates, although formerly classed as Cetaceans. 
 These horny plates in the lower jaw of the Dugong cover 
 the abortive rudimentary calcified teeth which never become 
 functional. 
 
 REFERENCES 
 
 1. Ayres. Biological Lectures at Wood's Hall, 1894. 
 
 2. Beard, J. (a) ' The Nature of the Teeth of Marsipobranch Fishes.' 
 
 Morph. Jahrb., 1889, Bd. iii, pp. 727-53. 
 
 (b) ' The Teeth of Myxinoid Fishes.' Anat. Anzeig., 1888, Bd. iii, 
 pp. 169-721. 
 
 3. Bridge, T. W. Cambridge Natural History, vol. vii, p. 248. 
 
 4. Howes. Nature, Nov. 1894. 
 
 MUMMERY 
 
370 MICROSCOPIC ANATOMY OF THE TEETH 
 
 5. Poulton, E. B. (a) ' True Teeth in the Young Ornithorhynchus para- 
 
 doxus.' Proc. Roy. Soc. Lond., 1888, vol. xliii, pp. 353-56. 
 (6) ' The True Teeth and the Horny Plates of Ornithorhynchus.- Quar. 
 Journ. Micr. Sci., 1889, vol. xxix, N.S., pp. 9-48. 
 
 6. Semon, Dr. In the Australian Bush. 
 
 7. Thomas. O. ' On the Dentition of Ornithorhynchus.' Proc. Roy. Soc. 
 
 Lond., 1889, vol. xlvi, pp. 126-31. 
 
 8. Warren, E. ' On the Teeth of Petromyzon and Myxine.' Quar. Journ. 
 
 Micr. Sci., 1902, vol. xlv, pp. 631-7. 
 
INDEX 
 
 Abrachiate lacunae, 294. 
 Absorption, 299. 
 
 in temporary teeth, 300. 
 
 in permanent teeth, 302. 
 Acrodont, 365. 
 Adsorption, 140. 
 Adventitious dentine, 251. 
 Aitchison Robertson, odon to blasts 
 
 and nerve fibres, 224. 
 Alternations of absorption and 
 
 deposition, 302. 
 Alveolodental periosteum, 305. 
 Ameghino on concrescence, 33. 
 Ameloblastic membranes, 132, 152. 
 
 differential staining of, 133. 
 Ameloblasts, 129. 
 
 Carter on Tomes' processes, 175. 
 
 clear bodies in, 151, 172. 
 
 communicating processes of, 133. 
 
 crescentic nuclei in, 130, 150. 
 
 cytoplasm of, 130. 
 
 in Elasmobranchs, 179, 193. 
 
 in GadidaB, 180, 193. 
 
 in Labridse, 188, 193. 
 
 in SparidsB, 193. 
 
 mitosis in, 150. 
 
 Tomes' processes of, 131, 153, 171. 
 Amelo-dentinal junction, 77. 
 Amitotic division in cells of 
 follicle, 319. 
 
 in cells of Nasmyth's membrane, 
 
 338. 
 Amphibia, epithelial sheath of 
 
 Hertwig, 322. 
 Analysis of dentine, 238. 
 Analysis of enamel, 47. 
 
 C. S. Tomes on, 48. 
 
 Lovatt Evans on, 49. 
 Anaptomorphus, 35. 
 Anchylosis of teeth, 352, 363. 
 Andrews, R., on fibrillar basis of 
 
 enamel, 157. 
 
 Angler, hinged teeth of, 361. 
 Ant-eater (Cape), 254. 
 Anterior superior dental nerve, 211. 
 Anthropoid apes, defects in enamel 
 
 of, 57. 
 
 Archaeopteryx, 3. 
 Arctomys, enamel of, 112. 
 
 Area, proportion to that of dentine, 
 
 164. 
 Arnell on globular bodies in amelo- 
 
 blasts, 152. 
 Arrhenius, 12. 
 Attachment of teeth, 352. 
 
 by anchylosis, 363. 
 
 hi Echineis, 354. 
 
 by elastic hinge, 352, 354. 
 
 by fibrous membrane, 352. 
 
 in Haddock, 361. 
 
 in Hake, 356. 
 
 by implantation in sockets, 365. 
 
 in Pike, 362. 
 
 Attachment, bone of, 5, 353. 
 Auriculo-temporal nerve, 212. 
 Axon of nerve-end cells, 213. 
 Aye-aye, rodent type of dentition, 
 116. 
 
 Ballowitz on sheath of Hertwig, 
 
 332. 
 
 Bartels on lymphatics of pulp, 210. 
 Basal layer of Weil, 229. 
 Bateson, 8. 
 Baume, 15, 265. 
 Bdellostoma, 4, 366. 
 Beading of neurofibrils, 216. 
 Beaver, incisors of, 3. 
 
 enamel of, 108. 
 Beckwith, gold process for nerves, 
 
 227. 
 Bennett, F. J., action of glycerine 
 
 on dentine, 282. 
 
 Beretta, A., on Nasmyth's mem- 
 brane, 346. 
 
 Bettongia, enamel of, 97. 
 Beust, Von, luchsin staining me- 
 thod, 82. 
 
 Bibra, Von, 47, 237. 
 Birds, teeth of, 3. 
 Black, G. V., on absorption in 
 
 cement, 304. 
 
 on lime salts in dentine, 238. 
 on lymphatics of periodontal 
 
 membrane, 307. 
 on mottled teeth, 71, 103. 
 on nerves of periodontal mem- 
 brane, 307. 
 
 b2 
 
372 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Blocks of calcified substance in 
 
 enamel, 55. 
 Blood-vessels in enamel organ, 184, 
 
 187, 193. 
 of pulp, 208. 
 Boar, tusks of, 3. 
 Boas, Von, on Scarus, 94. 
 Bolk on evolution of molar teeth,37. 
 Bone of attachment, 5, 353. 
 Bone dentine (Rose), 236. 
 
 and dentine, Von Ebner on, 265. 
 Born, system of modelling, 18. 
 Bowerbank on shell formation, 142. 
 Branching of dentinal tubes, 244. 
 
 of prisms, 52. 
 Bridge, T. W., on origin of horny 
 
 teeth, 367. 
 
 Bridges, connecting, in enamel, 65. 
 Brunn, Von, on epithelial sheath, 
 
 262. 
 Brush-like termination of nerves in 
 
 pulp, 212. 
 Budding of external epithelium, 
 
 123. 
 Bulbous ending to tubes in Bettongia, 
 
 98. 
 
 Cajal, Ramon y, on intercellular 
 bridges, 153. 
 
 silver nitrate process, 98. 
 Calcareous deposits due to physical 
 
 processes, 144. 
 Calcification, 134. 
 
 at birth, 30. 
 
 in fish, 177. 
 
 of dentine, 262, 278. 
 
 of enamel, 134. 
 
 Calcified masses in follicle, 317. 
 Calcium phosphate, influence of, 
 
 140, 168. 
 Calcoglobulin, 139. 
 
 in forming dentine, 278. 
 Calcospherites, 140. 
 
 coalescence of, 140. 
 
 in caries, 162. 
 
 in Crab, 141. 
 
 in dentine, 279. 
 
 in enamel, 159. 
 
 in Prawn, 141. 
 
 under osmotic membrane, 136. 
 Calf, connective tissue in forming 
 
 dentine of, 271. 
 Cape ant-eater, 254. 
 Capybara, cement in, 287. 
 
 enamel in, 112. 
 
 molar of, 106. 
 
 Carious dentine, lamination at 
 border of, 239. 
 
 Carpenter, W. B., on shell forma- 
 tion, 142. 
 Carter, T., on colloidal nature of 
 
 fibril, 174. 
 
 on development in Gadidae, 181. 
 on eruption, 348. 
 on marsupial enamel, 175. 
 on mitosis in ameloblasts, 150. 
 on outer ameloblastic membrane, 
 
 152. 
 on penetration of blood-vessels, 
 
 146. 
 
 on rhythmic deposit, 281. 
 Carnivora, partial penetration of 
 
 enamel by tubes, 105. 
 Caush, D., on staining of enamel, 
 
 163. 
 Cell accumulations in Nasmyth's 
 
 membrane, 338. 
 division in ameloblasts, 150. 
 of follicle, 319. 
 
 of pulp, differentiation of, 202. 
 nests in Nasmyth's membrane, 
 
 338. 
 
 theory, 10. 
 Cement, 287. 
 
 canaliculi of, 293. 
 Choquet on, 287. 
 connexion with dentine, 290. 
 conversion theory of develop- 
 ment of, 299. 
 development of coronary cement, 
 
 294. 
 
 deposit in flakes, 296. 
 Hope well Smith on lacunae, 290. 
 incremental lines of Salter, 294. 
 Kolliker on, 296. 
 lacuna of, 290. 
 lacunal cells in, 290. 
 lamination of, 290. 
 micro-organisms in, 292. 
 osteoblasts in, 296. 
 outer layer of, 293. 
 overlap of cement, 288. 
 in Ungulates, 287. 
 vascular canals in, 292. 
 Cement organ, 294. 
 Cement substance in enamel, 55. 
 Cementum, see Cement, 287. 
 Centrifugal calcification of enamel, 
 
 174. 
 Centripetal calcification of enamel, 
 
 156. 
 Cesiracion, penetration of enamel by 
 
 tubes, 84. 
 
 Cetacea, granular layer in, 241. 
 Cheiromys, dentition of, 116. 
 Chelonia, 369. 
 
INDEX 
 
 373 
 
 Chemical composition of dentine, 
 
 237. 
 
 of enamel, 47. 
 Choquet, I., on absorption, 303. 
 
 on cement at neck of tooth, 287. 
 Chromic acid, employed by Boll 
 
 for nerves, 222. 
 Cingulum, 33. 
 Coalescence of calcospherites, 140, 
 
 279. 
 
 Collagen, basis of dentine, 236. 
 Collars of F. Paul, 206. 
 Colloidal suspensions, 139. 
 Communication of dentinal tubes 
 
 with cement, 290. 
 Concavo-convex form of enamel 
 
 prisms, 69. 
 
 Concomitant variation, 9. 
 Concrescence theory, 33. 
 Conical stage in development, 31. 
 Connecting bridges, 16, 66. 
 Connective tissue in dentine, 239, 
 
 266, 268, 272. 
 
 in dentine of Elephant, 271. 
 cells in developing dentine, 270. 
 in pulp-stone of Elephant, 252. 
 of pulp, 266. 
 in vasodentine, 285. 
 Constant, T. E., on blood pressure 
 
 in eruption, 347. 
 Contour lines of Owen, 247. 
 Conversion theory of development, 
 
 144, 264. 
 Cope, E. D., on trituberculism, 11, 
 
 33. 
 
 Copper sulphate experiment, 135. 
 Creodonts, teeth of, 105. 
 Crescentic nuclei of ameloblasts, 
 
 130. 
 
 Crusta petrosa, 287. 
 Cryer, M., 182. 
 Cuscus, bulb-like ending to tubes, 
 
 98. 
 
 Cyclostomata, 366. 
 Cynomys, vascular canals in den- 
 tine of, 259. 
 
 Darwin, 7. 
 
 Dahlgren and Kepner on amitotic 
 
 division, 338 
 
 on infundibular gland, 194. 
 Decussation of enamel layers in 
 
 rodents, 108. 
 Degenerated nuclei in Nasmyth's 
 
 membrane, 340. 
 Delabarre, 23. 
 Dendrons of nerve-end cells, 213. 
 
 Dental lamina or tooth- band, 15. 
 Dentinal tubes in enamel, 82. 
 Dentine, 236. 
 
 action of glycerine on, 282. 
 
 adventitious, 251. 
 
 calcification of, 262, 278. 
 
 calcoglobulin contours in, 278. 
 
 of Cestracion, 263. 
 
 chemical composition of, 237. 
 
 classification of, 236. 
 
 coalescence of spherites in, 279. 
 
 communication with cement, 290. 
 
 connective tissue basis of, 239. 
 
 development of, 262. 
 
 direction of calcific deposit, 282. 
 
 disintegration of spherites in, 161. 
 
 fibril, 248. 
 
 fibrillar nature of matrix, 265. 
 
 fine branches of tubes in, 244. 
 
 of Flounder, 256. 
 
 germ, 21. 
 
 granular layer of, 240. 
 
 Hanazawa on, 249. 
 
 Hoppe on sheath of Neumann, 
 250. 
 
 interglobular spaces in, 242. 
 
 lamination of, 280. 
 
 of Lamna, 262. 
 
 matrix of, 239, 265. 
 
 odontogenic zone, 239. 
 
 orthodentine, 237. 
 
 osteodentine, 260. 
 
 papilla, 21, 263. 
 
 plicidentine, 253. 
 
 primary curvatures of tubes, 246. 
 
 resemblances to bone in, 268. 
 
 Romer on dentinal fibril, 250. 
 
 of Sarcophilus, 260. 
 
 Schreger's lines in, 247. 
 
 secondary curvatures of tubes, 
 246. 
 
 sheath of Neumann, 248. 
 
 striation in, 278. 
 
 striation in Wombat, 281. 
 
 trabecular dentine, 237, 260. 
 
 transverse sections of, 247. 
 
 vasodentine, 256. 
 Dentz on corpuscles in dentine, 
 
 230. 
 Dependorf on nerves of dentine, 
 
 226. 
 
 Dermal appendages, teeth con- 
 sidered as, 5. 
 
 Descent with modification, 7. 
 Development of cement, 294. 
 
 of dentine, 262. 
 
 of enamel, 119. 
 Dewar, Sir J., 12. 
 
374 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Dewey on lymphatics of pulp, 208. 
 Dialysis, Graham on, 137. 
 Diameter of enamel prisms, 55. 
 Differential staining in tooth of 
 
 Hake, 359. 
 Dilatations in marsupial enamel, 
 
 cause of, 166. 
 Diphyodont, 32. 
 Dipnoi, 177. 
 Dipus, enamel of, 84. 
 Dogfish, eruption of teeth in, 348. 
 Dominants, 7. 
 Dromotherium, 34. 
 Dursy, 21. 
 Duval on sensitiveness of dentine, 
 
 222. 
 
 Ebner, V. von, on basal layer of 
 Weil, 229. 
 
 on detached tubes in enamel, 116. 
 
 enamel of Petaurus, 115. 
 
 enamel tubes in Hare, 115. 
 
 fibres in dentine, 274. 
 
 fibrillar nature of dentine, 266. 
 
 fine branches of dentinal tubes, 
 245. 
 
 function of odontoblasts, 276. 
 
 globular bodies in ameloblasts, 
 152. 
 
 Hertwig's sheath, 262. 
 
 ingrowth of enamel tubes, 102. 
 
 Leon Williams's views, 163. 
 
 marsupial enamel, 173. 
 
 Nasmyth's membrane and inter- 
 prismatic substance, 163. 
 
 resemblances between bone and 
 dentine, 265. 
 
 staining of enamel, 56. 
 
 striae of Retzius, 74. 
 Echineis, attachment of teeth in, 
 
 354. 
 Edentates, Hertwig's sheath in, 
 
 331. 
 
 Eel, attachment of teeth in, 353. 
 Elasmobranchs, 84, 193. 
 Elastic hinge, 354. 
 Elephant, cement in, 287. 
 
 connective tissue in dentine, 271. 
 
 enamel prisms of, 66. 
 
 incisors, 3. 
 
 pulp stones in, 252. 
 
 spear-head in tusk of, 232. 
 Enamel, 46. 
 
 analysis of, 48, 49. 
 
 Bibra, Von, on, 47. 
 
 calcified disks of, 56. 
 
 calcium salts of, 47. 
 
 of Capybara, 46. 
 
 connecting bridges in, 56. 
 
 cross striae of, 55, 59. 
 
 crypt, 39. 
 
 defects in structure, 56, 58, 83. 
 
 of Elephant, 46. 
 
 Frankland's method for, 49. 
 
 germ, 119. 
 
 Harting on spherites in, 59. 
 
 Hertz on, 59. 
 
 Hoppe-Seyler on double salt in, 47. 
 
 hypoplastic, 76. 
 
 interprismatic substance, 71. 
 
 Lovatt Evans on analysis of, 48. 
 
 magnesium phosphate in, 47. 
 
 organic matter in, 48, 55. 
 
 penetration of, 82. 
 
 position of, 46. 
 
 radiating fibres in, 83. 
 
 Retzius, striae of, 72. 
 
 Schreger's lines, 77. 
 
 septum, 39. 
 
 spindles in, 78. 
 
 staining of, 58. 
 
 thorn -like processes in, 78. 
 Enamel prisms, 50. 
 
 arcade arrangement of, 60. 
 
 branching of, 53. 
 
 concavo-convex forms, 61. 
 
 course of, 64. 
 
 diameter of, 54. 
 
 Ebner, Von, on, 60. 
 
 of Elephant, 66, 69. 
 
 forms of, 60. 
 
 granular nature of, 55. 
 
 hexagonal forms, 52. 
 
 impressions of, 76, 337. 
 
 interlocking of, 66. 
 
 Leon Williams on, 55. 
 
 in Manatee, 52. 
 
 membrane-like expansions on, 70. 
 
 needle splitting of, 65. 
 
 of Phacochosrus, 53. 
 
 Pickerill on, 54. 
 
 Smreker on, 60. 
 
 supplementary, 53. 
 Encapsuled lacunae, 333. 
 End cells, 217. 
 Endoplasm, 200. 
 Entoconid, 36. 
 Epithelial coils, 24, 312. 
 
 inflection, 13. 
 
 pearls, 20, 308. 
 
 remnants, 331. 
 
 Erosion, calcospherites in, 163. 
 Eruption of teeth, Carter on, 348. 
 
 Constant on, 347. 
 
 Fischer on, 349. 
 
 W. James on, 347. 
 
INDEX 
 
 375 
 
 Eruptive stage of Goodsir, 13. 
 
 Eskimo, molar of, 35. 
 
 Evans, Lovatt, on chemical com- 
 position of enamel, 48. 
 
 Eve on stellate reticulum, 128. 
 
 Evolution of tubular dentine, 257. 
 
 Exoplasm, 200. 
 
 Experiment on decalcification of 
 enamel, 104. 
 
 Extensibility of neuron" brils, 217. 
 
 External epithelium, functions of, 
 
 121. 
 not continuous, 314. 
 
 Factors in eruption, 347. 
 
 Fan -shaped spreading of Tomes' 
 
 fibres, 157. 
 
 Fat soluble in food, 170. 
 Fenestrated membrane, 154, 319. 
 Fibril, dentinal, 248. 
 
 development of, 202. 
 Fibrillar basis of enamel, 104, 157. 
 
 of shell, 142. 
 Fibrillar nature of dentine matrix, 
 
 265. 
 Fibrous attachment of teeth, 352. 
 
 nature of Rat enamel, 114. 
 Fischer, G., on eruption, 349. 
 Fish, early calcification of enamel 
 in, 263. 
 
 Retzius on nerve terminations 
 in, 225. 
 
 tooth development in, 30. 
 Fixation, T. Carter on, 181. 
 Flakes, cement deposited in, 296. 
 Flounder, dentine of, 256. 
 Fluoride of calcium, 237. 
 Follicle of tooth, 311. 
 
 epithelium in, 320. 
 
 junction with tooth, 318. 
 
 origin of, 311. 
 
 Follicular stage (Goodsir), 13. 
 Foramen ovale, 211. 
 Forsyth Major, 36. 
 Foundation fibres of dentine, 239, 
 
 265, 266. 
 
 Frankland's method, 49. 
 Fritsch on innervation of dentine, 
 
 227. 
 Functions of stellate reticulum, 127. 
 
 of stratum intermedium, 147. 
 
 Gadidae, 84, 180. 
 
 Gadus ceglefinus, 181, 361. 
 
 Galeus, injury of jaw by spine, 353. 
 
 Galippe on absorption, 303. 
 
 Gasserian ganglion, 211. 
 
 Gebhart, 272. 
 
 Gegenbaur on osteoblasts, 299. 
 
 Gelatine- yielding fibres, 274. 
 
 Gels, 137. 
 
 Germ cells, 10. 
 
 Gidley, 37. 
 
 Gland-like enamel organ of Tautoga, 
 
 187. 
 
 Glands of Serres, 17, 23, 3lO. 
 Globes epidermiques, 24, 312. 
 Golgi method for dentine, 225, 244. 
 Gomphosis, 352, 365. 
 Goodsir on development, 13. 
 Graham on dialysis, 137. 
 Granular layer of dentine, 240. 
 Granularity of prisms at dentine 
 
 margin, 83. 
 Granules in ameloblasts, 151. 
 
 of enamel blocks, 57. 
 
 in enamel of Sargus, 92. 
 Gubernaculum, 23. 
 Gum, structure of, 309. 
 
 Haddock, modified hinge in, 361. 
 
 Hagfish, 366. 
 
 Haidenhain on connective tissue, 
 
 197. 
 
 Hair and teeth, relations of, 5. 
 Hake, hinged teeth of, 356, 358. 
 striation in dentine of, 285. 
 connective tissue in pulp, 271. 
 vasodentine of, 256. 
 Halichoeres, 186. 
 
 Halle on lymphatics of pulp, 210. 
 Hanazawa on dentine, 204, 249. 
 Haplodont, 34. 
 Hare, absence of pigment in enamel, 
 
 106. 
 
 Harrington, N. R., on lime- secret- 
 ing glands, 194. 
 Harting, P., on calcospherites, 139, 
 
 167. 
 
 on structure of prisms, 59. 
 Haversian canals in cement, 292. 
 Heidelberg mandible, pulps of teeth 
 
 in, 233. 
 Hertwig, O., on epithelial sheath, 
 
 296. 
 
 on placoid scales, 177. 
 Hertz on enamel striae, 59. 
 on secretion theory, 265. 
 on spindles of enamel, 78. 
 Heterodont, 32. 
 Hohl, E., on Neumann's sheath, 
 
 283. 
 
 Homodont, 32. 
 Honeycomb, 156. 
 
376 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Hopewell Smith on absence of 
 
 lacunae in cement, 290. 
 on absorption in permanent teeth, 
 
 301. 
 on blood-vessels in enamel organ, 
 
 121. 
 
 on dentine development, 271. 
 on myelinic fibres and connective 
 
 tissue, 275. 
 
 on nerves of pulp, 224. 
 on odontoblasts as sensation 
 
 transmitters, 223. 
 on tubes in Sargus, 88. 
 Hoppe on sheath of Neumann in 
 
 fossil teeth, 250. 
 Hoppe- Seyler on double salt in 
 
 teeth, 47, 168. 
 Hopson, M., on overgrowth of Rat 
 
 incisors, 107, 232. 
 Horny plates of Ornithorhynchus, 
 
 367. 
 
 Horny teeth, 366. 
 Howship's lacunae in absorption, 
 
 301. 
 Huber on nerve terminations, 218, 
 
 225. 
 Humphreys, J., on evolution of 
 
 teeth, 43. 
 
 Hunter, J., on secretion theory, 265. 
 on sensitiveness of dentine, 222. 
 Hutton, 12. 
 
 Huxley, T. H., on dental lamina, 14. 
 on evolution, 10. 
 on ' membrana preformativa ', 
 
 152-335. 
 
 on Nasmyth's membrane, 333. 
 Hydrochcerus, molar of, 106. 
 Hijenodon, 105. 
 Hypocone, 35. 
 Hypoconid, 36. 
 Hypoconulid, 36. 
 Hypoplastic enamel, 76. 
 
 Imbrication lines in Nasmyth's 
 membrane, 336. 
 
 Imperfections in enamel, 56. 
 
 Implantation of teeth, 5. 
 
 Impressions of cells on Nasmyth's 
 
 membrane, 336. 
 
 of prisms on Nasmyth's mem- 
 brane, 336. 
 
 Incremental lines in enamel, 75. 
 
 Independent calcification of inter- 
 prismatic substance, 161. 
 
 Inferior dental nerve, 212. 
 maxillary nerve, 211. 
 
 Inner ameloblastic membrane, 132. 
 
 Intercolumnar bridges, 66. 
 
 Interglobular spaces, 241. 
 Interlocking of tissues in Elephant, 
 
 78. 
 Internal epithelium, processes of 
 
 nuclei of, 130. 
 
 Involution of epithelial cells, 119. 
 Iron and tannic stain for lateral 
 
 processes of odontoblasts, 206. 
 for nerves, 227. 
 
 James, W. W., on cells of follicle, 
 
 316. 
 
 on epithelial coils, 24, 312. 
 on eruption, 347. 
 
 Kangaroo rats, persistent pulps in, 
 
 233. 
 
 spindles in enamel of, 81. 
 Kassander on osteoblasts, 298. 
 Keith on Krapina teeth, 233. 
 Kelvin, Lord, 11. 
 Keratinization of follicle cells, 319. 
 Klaatsch, 178. 
 Klein on intracellular fibrillse of 
 
 dentine, 268. 
 on termination of nerves of pulp, 
 
 223. 
 Kolliker on deposit of cement in 
 
 flakes, 296. 
 on development, 14. 
 on enamel tubes, 102. 
 on fine branches of dentinal 
 
 tubes, 245. 
 on inner layer of Nasmyth's 
 
 membrane, 334, 344. 
 on origin of connective tissue 
 
 fibres, 197. 
 on osteoblasts, 299. 
 on secretion theory, 265. 
 on termination of pulp nerves, 
 
 223. 
 Korff, Von, on connective tissue in 
 
 dentine, 200, 271, 274. 
 Korner on lymphatics of pulp, 210. 
 Krapina teeth, pulp cavities in, 233. 
 Kiikenthal on concrescence, 33, 42. 
 
 Labio-dental lamina, 15. 
 Labridae, penetration of enamel by 
 
 tubes in, 84, 93. 
 Labrus bergylta, 192. 
 Labrus, eruption in, 6. 
 Labyrinthodon, 254. 
 Lacunae of cement, 288. 
 Lacunal cells, 289. 
 Lamarck, 10. 
 Lamellae of cement, 288. 
 Laminae in dentine of Hake, 256. 
 
INDEX 
 
 377 
 
 Laminae in enamel, 159. 
 Lamination of dentine, 239, 280. 
 Lamna cornubica, enamel of, 84, 86. 
 Lamprey, 366. 
 Lankester, Sir E. R., 12. 
 Lateral enamel ledge of Bolk, 38. 
 Law, W. J., on blood-vessels in 
 enamel organ, 122. 
 
 on nerves of dentine, 230. 
 Layers of Nasmyth's membrane, 
 
 325. 
 Leche on dentine papilla, 21. 
 
 on development, 14. 
 
 on development in fish, 30. 
 Leduc on osmotic growths, 11. 
 
 on rhythmic periodic deposit, 281. 
 Lent on Nasmyth's membrane, 334. 
 
 on secretion theory, 265. 
 Lepidosteus, 253. 
 Leporidse, 108. 
 Leucocytes, as origin of osteoblasts, 
 
 298. 
 
 Liesegang's rings, 281. 
 Lime salts in dentine, Black on, 238. 
 Lingual nerve, 212. 
 Loeb on semi-permeable mem- 
 branes, 137. 
 Lophius, 361. 
 Lumbricus, lime- secreting glands in, 
 
 194. 
 
 Lymphatics of periodontal mem- 
 brane, 307. 
 
 of pulp, 208. 
 
 McKay on mottled teeth, 71. 
 Mackenzie, Dr., 259. 
 Mackerel, attachment in, 363. 
 Macropus, blood-vessels in enamel 
 organ, 122. 
 
 slow calcification in, 263. 
 
 staining of enamel in, 164. 
 
 teased preparations of, 66, 158. 
 Macropus billiardieri, vessels in, 
 
 121. 
 Magitot on cement organ, 294. 
 
 on dental follicle, 311. 
 
 on external epithelium, 122. 
 
 on nerve terminations, 224. 
 Malassez on absorption, 303. 
 
 on dental ligament, 305. 
 
 on epithelial debris, 311. 
 
 on external epithelium, 122. 
 
 on follicle, 312. 
 
 on gubernaculum, 23. 
 Mall on connective tissue fibres, 197. 
 Manatee, alveolus of, 365. 
 
 dentine of, 257. 
 
 enamel of, 52. 
 
 Marcusen, 14. 
 Marginal plexus, 218. 
 Marmot, Prairie, dentine of, 112. 
 Marsupial tubular enamel, 96. 
 
 ameloblastic membranes in, 152. 
 
 bending of tubes in, 97. 
 
 Bettongia, termination of tubes 
 in, 98. 
 
 calcospherites in, 159. 
 
 Carter, T., on, 174. 
 
 decalcification of, 104. 
 
 development of, 173. 
 
 dilatations in enamel of, 97, 
 165. 
 
 fibrillar foundation of, 104, 157. 
 
 honeycomb in, 156. 
 
 organic basis of, 103. 
 
 Paul on, 165. 
 
 staining of, 100. 
 
 teased preparations of, 66. 
 
 Tomes, C. S., on tubes of, 103. 
 on development of, 173. 
 
 Tomes, J., on tubes in, 97. 
 
 Tomes' processes of, 153. 
 
 transverse sections of, 100, 102. 
 
 transverse striation in, 158. 
 
 variations in amount of penetra- 
 tion by tubes, 97. 
 Matrix of cement, 288. 
 
 of dentine, 239, 265. 
 Medullary canals of osteodentine, 
 
 261. 
 
 Megatherium, dentine of, 257. 
 Mellanby, feeding experiments, 
 
 170. 
 
 ' Membrana preformativa ' of Hux- 
 ley, 335. 
 Membrane - like expansions in 
 
 enamel, 65. 
 
 Membranes in enamel organ, 132. 
 Mendel, researches of, 7. 
 Mental branch of inferior maxillary 
 
 nerve, 212. 
 
 Mesenchyme cells, 197. 
 Mesonyx, 105. 
 Metacone, 34. 
 Metaconid, 36. 
 
 Methylene blue intra-vitam stain- 
 ing, 227. 
 
 Middle superior dental nerve, 211. 
 Miller, W. D., on interlocking of 
 tissues in elephant, 78. 
 
 on lime salts in dentine, 238. 
 Molar, evolution of human, 32. 
 Monophyodont, 32. 
 Morgan on lymphatics of pulp, 210. 
 Morgenstern on nerves of pulp, 225. 
 Motor root of fifth nerve, 211. 
 
378 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Mouse, termination of enamel organ 
 
 in, 323. 
 
 Multitubercular theory, 32. 
 Muridae, enamel of, 113 . 
 Myliobates, 254. 
 Myxine, 4. 
 
 Narwhal, incisor of, 3, 350. 
 Nasmyth's membrane, 333. 
 as a dialysing membrane, 344. 
 attachment to follicle, 342,. 
 Biretta, A., on, 346. 
 cell accumulations in, 338. 
 cell nests in, 338. 
 cells involved in, 335. 
 cellular layer of, 337. 
 clear layer of, 335. 
 considered as cement, 333. 
 elongated cells in, 340. 
 impressions of cells on, 336. 
 
 of prisms on, 336. 
 inclusion of deep cells of follicle 
 
 in, 343. 
 
 keratinization of cells of, 337. 
 longitudinal sections of, 340. 
 methods of preparation of, 334. 
 Navel, enamel, of Bolk, 39. 
 Needle-like splitting of prisms, 65. 
 Neo-Lamarckians, 11. 
 Nerves of the pulp, 212. 
 accompanying blood-vessels, 208. 
 at cornu of pulp, 212. 
 axons of nerve cells, 213. 
 Beckwith method of staining, 
 
 227. 
 brush-like spreading of axis 
 
 cylinder, 212. 
 dendrons of end cells, 213. 
 end cells, 213, 218. 
 medullated nerves of pulp, 212. 
 
 of periodontal membrane, 306. 
 plexus of Raschkow, 213, 229, 
 
 230. 
 
 supply to odontoblasts, 218. 
 subdivisions of, in pulp, 212. 
 authors on, 222. 
 
 Aitchison Robertson, 223. 
 
 Boll, 222. 
 
 Dependorf, 226. 
 
 Duval, 222. 
 
 Fritseh, 227. 
 
 Hopewell Smith, 223. 
 
 Huber, 225. 
 
 Hunter, J., 222. 
 
 Klein, 223. 
 
 Kolliker, 223. 
 
 Law, 230. 
 
 Magitot, 224. 
 Morgenstern, 225. 
 Pont, 226. 
 Retzius, 225. 
 Romer, 226. 
 Salter, 222. 
 Weil, 224. 
 Neumann's sheath, 237, 248. 
 
 dependent on calcification, 283. 
 Neurofibrils around odontoblasts, 
 
 218. 
 
 beading of, 216. 
 extensibility of, 216. 
 in dentinal tubes, 218. 
 wavy course of, 216. 
 Neuron theory, 214, 220. 
 New Mexico mammals, 33. 
 Noyes on lymphatics of pulp, 208. 
 
 Odontoblasts, Aitchison Robertson 
 on, 224. 
 
 dentinal process of, 206. 
 
 development of, 202, 264. 
 
 functions of, 276. 
 
 Hopewell Smith on, 223. 
 
 lateral processes of, 206. 
 
 Paul on, 206. 
 
 pulp processes of, 206. 
 Odontogenic zone, 239, 264. 
 Olfactory nerve, 225. 
 Ollendorf on lymphatics of pulp, 
 
 210. 
 
 Organic basis of enamel, 103. 
 Origin of enamel spindles, 81. 
 
 of Hertwig's sheath, 322. 
 
 of species, 7. 
 
 Ornithorhynchus, teeth of, 4, 367. 
 Orthodentine, 237. 
 Orycteropus, 254. 
 Osborn on mammalian molar, 32. 
 
 on neo-Lamarckism, 11. 
 Osmotic membranes, 136. 
 Osseous fish, development in, 180. 
 Osteoblasts, origin of in forming 
 
 cement, 296. 
 Osteoclasts, 308. 
 
 in implanted teeth, 302. 
 Osteodentine, 260. 
 
 calcification of, 285. 
 
 in Heterodontus, 261, 263. 
 
 in Lamna, 262. 
 
 in Pike, 362. 
 Ostwald on colloids, 139. 
 Outer ameloblastic membrane, 132. 
 Outer layer of dentine in relation 
 to cement, 293. 
 
 of teeth in Selachia, 237. 
 
INDEX 
 
 37D 
 
 Oval bodies in ameloblasts, 151. 
 
 spherites in enamel, 160. 
 Overgrowth of Rodent incisors, 107. 
 Owen, R., contour lines of, 247. 
 
 on coronal cement, 333. 
 
 on penetration of Shark's jaw by 
 Sting- ray, 353. 
 
 on Rodent enamel, 108. 
 
 Pagellus, 192. 
 
 Pan genesis, 11. 
 
 Papillary stage of Goodsir, 13. 
 
 Paracone, 34. 
 
 Paul on collars of odontoblasts, 206. 
 
 on external epithelium, 121. 
 
 on functions of stellate reti- 
 culum, 127. 
 
 on inclusions of dentine matrix, 
 165. 
 
 on Nasmyth's membrane, 334. 
 
 on tubes of marsupial enamel, 
 
 103. 
 Pauli and Samec on solubility of 
 
 calcium salts, 169. 
 Pavement epithelium, 60. 
 Pea, Mendel's experiments on, 7. 
 Periodicity of deposit, 281. 
 Periodontal membrane, 305. 
 
 blood-vessels of, 306. 
 
 direction of fibres in, 305. 
 
 lymphatics of, 307. 
 
 nerves of, 306. 
 
 origin of, 308. 
 
 sensibility of, 309. 
 Peripheral sensory neuron, 220. 
 Persistence of cells of tooth-band, 
 316. 
 
 of external epithelium, 340. 
 Persistent pulps, 230. 
 Petaurus, Von Ebner on enamel of, 
 
 115. 
 
 Petromyzon, 366. 
 Phalanger, terminations of enamel 
 
 tubes, 98. 
 
 Philip on osmotic membranes, 138. 
 Physical phenomena in calcifica- 
 tion, 135. 
 Pigmentation in Retzius's striae, 74. 
 
 of Rodent enamel, 106. 
 Pike, hinging in tooth of, 362. 
 Pinna, shell- membrane in, 143. 
 Pitts, A. T., on eruption, 347. 
 Placoid scales, 145, 177. 
 
 stage of development, 31. 
 Plagiostomes, development, 178. 
 
 succession of teeth in, 352. 
 Pleurodont, 365. 
 Plicidentine, 253. 
 
 Polybuny, 36. 
 
 Polyphyodont, 32. 
 
 Pont on peripheral neurons, 226. 
 
 Porcupine, enamel of, 112. 
 
 Posterior superior dental nerve, 211. 
 
 Post- permanent teeth, 21. 
 
 Poulton on blood-vessels in enamel 
 
 organ, 122. 
 
 on teeth of Ornithorhynchus, 367. 
 Pradentine, 239. 
 Prairie marmot, 259. 
 Prawn, calcification in, 142. 
 Precipitation of phosphates, 169. 
 Pre-milk teeth, 20. 
 Preponderance of phosphates in 
 
 enamel and dentine, 167, 237. 
 Primary curvatures of dentinal 
 
 tubes, 246. 
 Primitive dental groove of Goodsir 
 
 13. 
 
 Primitive dental lamina, 15. 
 Prisms of enamel, 50. 
 Processes of connective- tissue cells, 
 
 198. 
 
 Proliferation of cells of follicle, 311. 
 Protocone, 34. 
 Protoconid, 36. 
 Protodont, 34. 
 Prussian blue for lymphatics of 
 
 pulp, 210. 
 Pseudolabrus, 186. 
 
 secreting organ in, 148. 
 Pseudoscarus, absorption of enamel 
 
 in, 94. 
 blood-vessels in enamel organ, 
 
 146. 
 
 maxillary teeth of, 96. 
 Pulp stones in Elephant, 252. 
 Python, anchylosis in, 363. 
 
 Rainey on disintegration of spher- 
 ites, 279. 
 
 on molecular coalescence, 138. 
 Ranvier on origin of connective 
 
 tissue, 197. 
 
 on periodontal membrane, 305. 
 Raschkow, plexus of, 212, 230. 
 Rat, connective tissue in dentine, 
 
 271. 
 
 course of prisms, 113. 
 fibrous nature of enamel, 114. 
 overgrowth of incisors, 232. 
 secreting organ in, 129. 
 Rath, Von, on amitotic division, 
 
 338. 
 
 Recessives, 8. 
 
 Redier on absorption, 303. 
 Reduction of organs, 9. 
 
380 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Relations of areas of enamel and 
 
 dentine, 180. 
 
 of nerves and vessels in pulp, 208. 
 Reptiles, tooth development in, 30. 
 
 nerve terminations in, 225. 
 Resorption of enamel tubes, 117. 
 Retzius, brown striae of, 72. 
 
 on nerve terminations, 218, 225. 
 on nerves in fish and reptiles, 218. 
 Robin and Magitot on cement 
 
 organ, 294. 
 
 on external epithelium, 122. 
 on structure of follicle, 314. 
 Rodent enamel, 106. 
 Rods of enamel, 55. 
 Romer, O., on degenerated nuclei, 
 
 340. 
 
 on nerves of dentine, 226. 
 on non-existence of Neumann's 
 
 sheath, 249. 
 
 on spindles of enamel, 80. 
 on structure of dentine, 250. 
 on branching of dentinal tubes, 
 
 243. 
 
 Rose, C., on basal layer of Weil, 229. 
 on classification of dentines, 236. 
 on concrescence, 42. 
 on development of teeth in 
 
 reptiles and fish, 30. 
 on incorporation of fibres from 
 
 odontoblasts in dentine, 266. 
 on stellate reticulum, 127, 146. 
 on surface membrane in Plagio- 
 
 stomes, 179. 
 
 on tooth-band in birds, 3. 
 Ryder, 11. 
 
 Salter, J., on termination of pulp 
 
 nerves, 222. 
 Sappey, 23. 
 
 Sarcophilus ursinus, 258. 
 Sargus, enamel organ of, 193. 
 
 blood-vessels in, 182. 
 
 eruption in, 350. 
 
 staining of tubes in enamel, 86. 
 
 vascular canals in, 94. 
 Sargus noct, 89, 186. 
 
 ovis, 87. 
 
 vulgaris, 90. 
 
 Sawfish, persistent pulp in, 233. 
 Schafer, Sir E. A., on connective 
 tissue, 197. 
 
 nutrition of enamel organ, 24. 
 
 osteoblasts, 298. 
 
 secretion theory in cement, 299. 
 Schreger's lines in dentine, 247. 
 
 in enamel, 77. 
 
 Schwann on conversion theory, 
 
 265. 
 Schweitzer on lymphatics of pulp, 
 
 208, 210. 
 
 Sciurus, enamel of, 111. 
 Scleroblasts, 177. 
 Scott on origin of premolar, 36. 
 Secondary curvatures of dentinal 
 
 tubes, 246. 
 
 Secondary dentine, 251. 
 Secreting organ in Sargus, 184. 
 
 in Tautoga, 187. 
 Secretion theory, 144, 265. 
 Segregation, 8. 
 Semon on habits of Ornitho- 
 
 rhynchus, 368. 
 
 Sensory and trophic fibres, 221. 
 Sensory nature of nerves of pulp, 
 
 220. 
 
 Sensory neuron, 220. 
 Serration of prisms in rat, 113. 
 Serres, glands of, 17, 310. 
 Sexual selection, 9. 
 Sharpey's fibres in cement, 290. 
 Sheath of Hertwig, 296, 320. 
 
 source of cells of, 325. 
 Sheath of Neumann, 237, 248. 
 Shell, calcification of, 139. 
 Smreker on enamel prisms, 60. 
 Socketing of teeth, 352. 
 Sols, 137. 
 Somatic cells, 10. 
 Sorex, tubular enamel of, 84. 
 Spaces at amelo- dentinal junction, 
 
 165. 
 Spee, Graf von, on globular bodies 
 
 in ameloblasts, 152. 
 Speech, teeth as aids to, 3. 
 Spindles in enamel, 78. 
 
 relation of nerves to, 220. 
 Staining of enamel, 58. 
 Starling, Professor, 227. 
 Starr on lymphatics of pulp, 210. 
 Stellate reticulum, 120. 
 
 absence in lower vertebrates, 127. 
 
 form of cells, 125. 
 
 Leon Williams on, 125. 
 
 part in calcification, 146. 
 
 Rose on, 127, 146. 
 Sterna Wilsoni, tooth germs in, 3. 
 Stratum intermedium, blending 
 with ameloblasts, 128. 
 
 nuclei of cells of, 128. 
 
 processes of cells of, 133. 
 Striation in dentine, 278. 
 
 of enamel prisms, 59. 
 Stroma in enamel organ of G.adida 1 , 
 180. 
 
INDEX 
 
 381 
 
 Studnicka on fibres in dentine, 274. 
 Successional teeth, formation of, 16. 
 Superior maxillary nerve, 211. 
 Supernumerary teeth, origin of, 20. 
 Surface tension, 139. 
 Symington, J., skiagram of develop- 
 ment, 24. 
 Synapses, 213, 226. 
 
 Tadpole, horny teeth of, 369. 
 
 Taeker on paracone, 37. 
 
 Talbot, E., on absorption in cement, 
 
 304. 
 
 Talon, 34. 
 Tarsius, 35. 
 Tasmanian devil, 258. 
 Tatusia, 32. 
 
 Tautoga onitis, 186, 193. 
 Teased preparations of marsupial 
 
 enamel, 62, 158. 
 Terebratula, membrane in, 143. 
 Terminations of nerves at cement 
 
 margin, 219, 246. 
 at enamel margin, 219. 
 in enamel spindles, 226. 
 of tubes in Macropus, 165. 
 Testut on lymphatics of pulp, 210. 
 Thomas, Oldfield, on teeth of 
 
 OrnitharJiynchus, 367. 
 Thompson, D'Arcy W., on adsorp- 
 tion, 140. 
 
 on calcareous deposits, 144. 
 on Liesegang's rings, 281. 
 on organic particles in space, 12. 
 on Pauli and Samec's views, 169. 
 Tims, W. Marett, on blood-vessels 
 
 in enamel organ, 121. 
 on cingulum, 33. 
 on monophyodonts, 32. 
 on pre-milk teeth, 21. 
 on teeth of Canidee, 37. 
 Tomes, Sir Charles, on ameloblastic 
 
 membranes, 133. 
 on bone of attachment, 5, 353. 
 on chemical composition of 
 
 enamel, 48. 
 
 on classification of dentines, 236. 
 on clumsy joint, 165. 
 on conversion theory, 265. 
 on crescentic nuclei, 130. 
 on development in Elasmo- 
 
 branchs, 178. 
 
 on development in Gadidse, 180. 
 on development in marsupials, 
 
 153. 
 
 on enamel of Selachia, 84. 
 on eruption of teeth, 347. 
 
 on incorporation of Tomes' pro- 
 cesses, 145. 
 
 on interprismatic substance, 71. 
 on Nasmyth's membrane, 333. 
 on nature of tubular enamel, 103, 
 
 173. 
 
 on origin of Rodents, 116. 
 on post-permanent teeth, 21. 
 on stellate reticulum, 128. 
 on teeth of Creodonts, 105. 
 on terminations of dentinal tubes, 
 
 165. 
 Tomes, Sir John, decalcification of 
 
 marsupial enamel, 104. 
 on dentinal fibril, 248. 
 on enamel of marsupials, 97. 
 on enamel of Rodents, 108. 
 on granular layer in Rodents, 240. 
 on serration of prisms in rat, 
 
 113. 
 Tomes' processes of ameloblasts, 
 
 131, 153. 
 Torrens, 7. 
 
 Trabecular dentine, 236, 260. 
 Transverse fibrillation of marsupial 
 
 enamel, 157. 
 sections of marsupial enamel, 
 
 100. 
 
 tubes in dentine, 206. 
 Traube's membranes of precipita- 
 tion, 135. 
 Triconodon, 34. 
 Trigon, 34. 
 Trigonid, 36. 
 Trituberculism, 33. 
 Tschermak, 7. 
 Tubes in enamel from without, 84, 
 
 96. 
 
 Tubular dentine, 236. 
 enamel, 84. 
 
 Underwood on cells of stellate 
 reticulum, 1^7. 
 
 on external epithelium, 340. 
 
 on spaces between ameloblasts, 
 131. 
 
 on spherites in erosion, 163. 
 
 on striae of Retzius, 74. 
 Unequal wear of tissues, 107. 
 Unerupted teeth of Sargus, 91. 
 Ungulates, cement in, 287. 
 
 Varanus, attachment in, 365. 
 Vascular canals in cement, 292. 
 
 in dentine, 253. 
 
 in dentine of Rodents, 259. 
 
382 MICROSCOPIC ANATOMY OF THE TEETH 
 
 Vascular loops in dentine of 
 Manatee, 257. 
 
 of Cynomys, 259. 
 
 of Sarcophilus, 258. 
 
 of Sargus, 258. 
 Vascular tubes in Labrus, 185. 
 
 in Sargus, 185, 193. 
 Vasodentine, 236, 256. 
 
 calcification of, 283. 
 
 connective tissue in, 285. 
 
 transition to orthodentine, 257. 
 Vertical succession of teeth, 90, 
 
 350. 
 Vries, De, on evolution, 7. 
 
 Waldeyer on connective tissue, 197. 
 on conversion theory, 264. 
 on Nasmyth's membrane, 334. 
 on spindles in enamel, 78. 
 Walkhoff on cement substance of 
 
 enamel, 71. 
 
 on imperfections in enamel, 58. 
 on Smreker's views, 61. 
 on spindles of enamel, 80. 
 Wallaby, blood-vessels in enamel 
 
 organ of, 121. 
 Walrus, tusks of, 3. 
 
 persistent pulps in, 233. 
 Wart-hog, enamel of, 54. 
 Water in combination in enamel, 
 
 237. 
 
 Wavy course of neuro fibrils, 216. 
 Wedl on blood-vessels in enamel 
 organs, 147. 
 
 Weil on basal layer, 228. 
 on nerves of pulp, 224. 
 Weissmann on acquired characters, 
 
 10. 
 Wellings on spaces between amelo- 
 
 blasts, 131. 
 
 on stellate reticulum, 127. 
 Wharton's jelly, 198. 
 Wilkinson on absorption in im 
 
 planted teeth, 302. 
 Williams, Leon, on enamel, 163, 
 
 173. 
 
 on fibrous enamel of Rat, 114. 
 on functions of stratum inter- 
 medium, 149. 
 on globular bodies in ameloblasts, 
 
 152. 
 on independent calcification of 
 
 prisms, 163. 
 
 on secreting organ in Rat, 129. 
 on striae of Retzius, 74. 
 Wilson on amitotic division, 338. 
 
 on the cell, 10. 
 
 Wing processes of prisms, 62. 
 Woodhead, G. Sims, on calcification, 
 
 168. 
 
 on rarefying osteitis, 303. 
 Woodward, S., on evolution of 
 
 molar, 37. 
 Wombat, absence of enamel tubes, 
 
 97. 
 
 persistent pulps in, 233. 
 striation of dentine in, 281. 
 
 Zsigmondy on striae of Retzius, 76. 
 
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 Medical Journal. 
 
 STOMATOLOGY IN GENERAL PRACTICE: 
 
 or Diseases of the Teeth and Mouth. By H. P. PICKERILL, 
 CH.B., M.D.S. (Birm.), L.D.S. (Eng.). Demy 8vo (8 x 5J). 
 Cloth. 65 illustrations. Pp. 280. Price 17s. net. 
 
 This work fills the gap between medicine and dentistry. It 
 includes the anatomy and physiology of the teeth, deformities of 
 the jaw and teeth, fractures of the jaw, oral tumours, &c. The 
 surgical treatment of the teeth is limited practically to the local 
 treatment of deep cavities, pericementitis and alveolar abscess. 
 The operation of extraction is described, but the filling of teeth 
 is merely mentioned. The technic of mechanical dentistry is 
 not entered into. The surgery of the jaw, and especially the 
 treatment of fractures, is fully described. 
 
 *The book is comprehensive, and reference is made to almost every 
 condition of disease which occurs in the mouth. 'British Medical Journal. 
 
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 UNIVERSITY OF CALIFORNIA LIBRARY