Horses' teeth: a treatise on their mode of development, anatomy, microscopy, pathology, and dentistry

Front matter

UNITED STATES OF AMERICA.

HORSES' TEETH:

A TREATISE ON THEIR

Preface

THE favorable reception of the first edition of this work by both press and public and my desire to encourage the study of Veterinary Science and Comparative Anatomy are the chief reasons for a Kevised Edition. The improvements consist in an Appendix, numerous Illustrations, a new Index, and the correction of errors in and the addition of fresh matter to the text and vocabulary. .

I am indebted to Mr. Jacob L. Wortman of Philadelphia for the able article on fossil horses in the Appendix, and to Prof. E. D. Cope, editor of The American iSaturaUst, for a careful revision and improvement of it. Some of the reference notes, however, are my own.

It was not my intention originally to make the book an exponent of the Doctrine of Evolution. The discussion of the subject, however, is justifiable, for a work that does not embrace all the facts science furnishes is unworthy of the age, and to shirk the responsibility of the discussion^because the subject is unpopular is cowardly. The fact that fossil horses' teeth are inseparably connected Avith those of the modern horse renders their consideration unavoidable. Further, in addition to being one of the most important factors Paleontology has thus far furnished in ly PBEFACE.

elucidating the subject of Evolution, they give increased scope and importance to the book itself. Truly the late Dr. John W. Draper was right when, at a mere glance, he said : " The subject (horses' teeth) is so suggestive !"

So far as Evolution is concerned, I can only repeat what I said in the first Preface, namely, that it denotes improvement, and that Nature's laws are immutable, and to oppose them is as foolish as to beat the head against a stone wall.

Again, as said in the first Preface, I think I can say now from experience that Special Works, on account of the thoroughness with which they are usually prepared, are growing in public favor (an opinion in which so able a journal as The Syracuse (N. Y.) Standard concurs), and that while General Works have* their advantages, thoroughness of detail is not usually among them.

New York, September, 1883.

Contents

PAOK

INTRODUCTION — Fundamental Principles of Dental Science 7

Chapter I. Tooth-germs (odontogeny)

Periods at which the Germs are visible in the Fetus.— Dentine and Enamel Germs.— A Cement Germ in the Foal. — The Horse's Upper Grinders said to be developed from Five Germs, the Lower from Four.— Similar development of the Human Teeth.— Monsieur Magitot's Researches 31

CHAPTER n.

THE TEMPORARY DENTITION. •

Twelve Incisors and Twelve Molars. — Why the Incisors are calle " Nippers."— The Treatment of Foals Affects Teething.— Roots of Milk Teeth Absorbed by the Permanent.— The Tushes. 47

Chapter III. The Permanent Dentition

Distinction between Premolars and Molars.— The Bow-like Incisors.— Contrasts between the Upper and Lower Grinders, and the Rows formed by them. — The Incisors saved from Friction. — Horses' Teeth compared with those of other Animals. — Measurements. — Time's Changes. — Growth during Life 53

Chapter IV. The Canine Teeth Or Tushes

Practically Useless.— Diff^^ rent in their Nature from the other Teeth.— Were they formerly Weapons of Offense and Defense 5— Views of Messrs. Darwin. Hunter. Bell, Youatt, and Winter.— Their time of Cutting the most Critical Period of the Horse's Life 75

Chapter V. The Remnant Teeth

Usually regarded as Phenomenon?. — The Name. — Traced to the Fossil Horses, in which (in the Pliocene Period) they " Cea'^erl to be Functionally Developed.'"— Nature's Metamorphoses — "The Aeenciea which are at work in Modeling Animal and Vegetable Forms." — Why Remnant Teeth are often as it were, Prematurely Lost.— -Fossil Horses and a Fossil Toothed-Bird 94

Tl CONTENTS.

CHAPTER VI. DENTAL CYSTS AND SUPERNUMERARY TEETH. page

Teeth grovvins: in various parts of the Body.— Some Cysts more Prolific than others, Producing a Second, if not a Third, " Dentition."— Reports and Theories of Scientific Men.— Cases of Third Dentition in Human Beings 115

CHAPTER VII. horses' teeth under the microscope.

The Dentinal Tabes, Enamel Fibers, and Cemental Canals Described and Contrasted 130

Chapter VIII. The Pathology Of The Teeth

Importance of the Subject.— Caries caused by Inflamed Pulps, Blows, Virus, and Morbid Diathesis.— Supernumerary Teeth and other Derangements.— Trephining the Sinuses. -Gutta Percha as a Filling.— C'leaniug the Teeth.— A Diseased Fossil Tooth 136

Chapter IX. The Dentistry Of The Teeth

Reports of Cases Tr^^ated by Various Surgeons.— Gutta Percha as a Filling for Trephined Sinuses. — Teeth Pressing against the Palate. —Passing a Probe through a Decayed Tooth. — Death of a Horse from Swallowing a Diseased Tooth 176

Chapter X. Fractured Jaws

How Caused, and how to Distinguish an Abrasion of the Gums from a Fracture of the Bone. — Replacing an Eye, Amputating part of a Lower Jaw, taking a Fractured Tooth and Bones out through the Nostril, &c 194

Chapter XI. The Teeth As Indicators Of Age

Their various ways of Indicating Age.— The ' Mark's" Twofold Use.— The Dentinal Star.— Marks with too much Cement. — Tricks of the Trade.— Crib-biting.— Signs of Age Independent of the Teeth 203

Chapter XII. The Trigeminus Or Fifth Pair Of Nerves

Its Nature and the Relation it bears to the Teeth.— Its Course in the Horse and in Man 816

APPENDIX.— Recent Dif=coveries of Fossil Horses.— Views of an Evolutionist.— Original Home of the Horse.— Elephant Tooth-Germs.— Filling Children's Teeth 257

INDEX 279

II^TEODUCTIOK

The following matter, which is designed to give at least a synopsis of the fundamental principles of dental science, is compiled from the works of the best known odontologists. It is somewliat heterogeneous in its make-up, and is, moreover, considering that it is an Introduction to a special work, anomalous, being rather an adjunct to than an explanation of the work itself. Its lack of coherency and the few repetitions, the ine^'itable concomitants of all compilations, are offset by the interest of its historical records and the scope and clearness of its thoughts and deductions. AVhile it does not treat specially of horses' teeth, it is just as applicable to them as to human teeth, or to those of any of the other animals mentioned. It is believed that the student of dental science will find the matter as useful as it is interesting.

In his work entitled " The Anatomy of Vertebrates " (vol. i, pp. 357-8), Prof. Richard Owen says:

" A tooth is a hard body attached to the mouth or beginning of the alimentary canal, partially exposed, when developed. Calcified teeth are peculiar to the vertebrates, and may be defined as bodies primarily, if not permanently, distinct from the skeleton, consisting

Vlil INTRODUCTION.

of a cellular and tubular basis of animal matter, containing earthy particles, a fluid, and a vascular pulp.

*^In general, the earth is present in such quantity as to render the tooth harder than bone, in which case the animal basis is gelatinous, as in other hard parts where a great proportion of earth is combined with animal matter. In a very few instances, among the vertebrate animals, the hardening material exists in a much smaller proportion, and the animal basis is albuminous ; the teeth here agree, in both chemical and physical qualities, with bone.

'^I propose to call the substance which forms the main part of all teeth dentine.* The second tissue, which is the most exterior in situation, is the cement. The third tissue, which, when present, is situated between the dentine and cement, is the enamel.

" Dentine consists of an organized animal basis and of earthy particles. The basis is disposed in the form

* In a reference note in the Introduction to his " Odontography," Prof. Owen says: "Besides the advantaj^e of a substantive for an unquestionably distinct tissue under all its modifications in tlie animal kingdom, the term dentine may be inflected adjectively, and the properties of this tissue described without the necessity of periphrasis. Thus we may speak of the ' dentinal' pulp, 'dentinal' tubes or cells, as distinct from the corresponding properties of the other constituents of a tooth. The term 'dental' will retain its ordinary sense, as relating to the entire tooth or system of teeth."

I^ote.— The particular paragraph to which the above note refers is from Prof. Owen's " Odontography." " The Anatomy of Vertebrates " having been written about twenty-five years subsequent to the " Odontography," and therefore reflecting the Professor's riper thoughts, the extracts made from it were substituted for very similar matter in the " Odontography."

TtJBES WITH KOURISHIKG, COLORLESS FLUID. IX

of compartments or cells, and extremely minute tubes. The earthy particles have a twofold arrangement, being either blended with the animal matter of the interspaces and parietes of the tubes, or contained in a minute granular state in their cavities. The density of the dentine arises principally from the proportion of earth in the first of these states of combination. The tubes contain, near the formative pulp, filamentary processes of that part, and convey a colorless fluid, probably transuded * plasma.' They thus relate not only to the mechanical conditions of the tooth, but to the vitality and nutrition of the dentine. This tissue has few or no canals large enough to admit capillary vessels with the red particles of blood, and it has been therefore called ^unvascular dentine.'

" Cement always closely corresponds in texture with the osseous tissue of the same animal ; and whenever it occurs of difierent thickness, as upon the teeth of the horse, sloth, or ruminant, it is also traversed, like bone, by vascular canals. Wiien the osseous tissue is excavated, as in dentigerous vertebrates above fishes, by minute radiated cells, forming, with their contents, the ^ corpuscles of Purkinje,' these are likewise present, of similar size and form, in the cement, and are its chief characteristic as a constituent of the tooth. The hardening material of the cement is partly segregated and combined with the parietes of the radiated cells and canals, and is partly contained in disgregated granules in the cells, which are thus rendered white and opaque, viewed by reflected light. The relative density of the dentine and cement varies according to the proportion of the earthy material, and chiefly of that part which is combined with the animal matter in the walls of the cavities, as compared with the size

X INTEODUCTIOK.

and number of the cavities themselves. In the complex grinders of the elephant, the masked boar, and the copybara, the cement, which forms nearly half the mass of the tooth, wears down sooner than the dentine. '' The enamel is the hardest constituent of a tooth, and, consequently, the hardest of animal tissues; but it consists, like the other dental substances, of earthy matter arranged by organic forces in an animal matrix. Here, however, the earth is mainly contained in the canals of the animal membrane, and, in mammals and reptiles, completely fills those canals, which are comparatively wide, whilst their parietes are of extreme tenuity. The hardening salts of the enamel are not only present in far greater proportion than in the dentine and cement, but, in some animals, are peculiarly distinguished by the presence of the fluate of lime."

Again Prof. Qwen says (" Anat-of Vert." vol. 1, pp. 359-60) :

^' Teeth vary in number, size, form, structure, modifications of tissue, position, and mode of attachment in different animals. They are principally adapted for seizing, tearing, dividing, pounding, or grinding the food. In some animals they are modified to serve as weapons of offense and defense; in others, as aids in locomotion, means of anchorage, instruments for uprooting or cutting down trees, or for transport and working of building materials. They are characteristic of age and sex, and in man they have secondary relations subservient to beauty and to speech.

"Teeth are always most intimately related to the food and habits of the animal, and are therefore highly

MOST DURABLE OF ANIMAL SUBSTANCES. xi

interesting to the physiologist. They form for the same reason most important guides for the naturalist in the classification of animals; and their value, as zoological characters, is enhanced by the facility with which, from their position, they can be examined in living or recent animals. The durability of their tissues renders them not less available to the paleontologist in the determination of the nature and affinities of extinct species, of whose organization they are often the sole remains discoverable in the deposits of former periods of the earth's history."

Prof. A. Chauveau says (" Comparative Anatomy of the Domesticated Animals ") :

" Identical in all our domesticated animals by their general disposition, mode of development, and structure, in their external conformation the teeth present notable differences, the study of which offers the greatest interest to the naturalist. For it is on the form of its teeth that an animal depends for its mode of alimentation ; it is the regime, in its turn, which dominates the instincts, and commands the diverse modifications in the apparatus of the economy ; and there results from this law of harmony so striking a correlation between the arrangement of the teeth and the conformation of the other organs, that an anatomist may truly say, ^Give me the tooth of an animal, and I will tell you its habits and structure.'"

In a letter which I wro^e to Prof. Theodore Gill, of the Smiths'^nian Institution, Washington, D. C, I asked what there was about teeth that enabled naturalists to tell so much by them. In reply he said:

xii INTRODUCTION.

" The teeth are quite constant in the same type, are generally appreciably modified according to family, are the most readily preserved in a fossil state, and are in direct relation with the economy of the animal. Hence they furnish the best indications of the relations of the animal to which they belonged, especially in cases where the t}^e was not very different from an existing one. In the case of the older and more aberrant types, however, the indications furnished by the dentition should be accepted with great caution."

In the Introduction to his " Odontography " Prof. Owen gives, besides his own and other men's views, a history "of the leading steps to the present knowledge" of dental science (that is, up to 1844), of which the following are extracts :

" As regards the teeth, the principle of chief import to the physiologist arises out of the fact, which has been established by microscopic investigations, that the earthy particles of dentine are not confusedly blended with the animal basis, and the substance arranged in superimposed layers, but that these particles are built up with the animal basis as a cement, in the form of tubes or hollow columns, in the predetermined arrangement of which there may be discerned the same relation to the acquisition of strength and power of resistance in the due direction, as in the disposition of the columns and beams of a work of human architecture.

" Whoever attentively observes a polished section or a fractured surface of a human tooth may learn, even with the naked eye, that the silky and iridescent luster reflected from it in certain directions is due to the presence of a fine fibrous structure.

EARLY MICROSCOPICAL DISCOVERIES. Xlll

" Malpighi,* in whose works may be detected the germs of many important anatomical truths that have subsequently been matured and established, says the teeth consist of two parts, of which the internal bony layers (dentine) seem to be composed of fibrous and, as it were, tendinous capillaments reticularly interwoven.

"Leeuwenhoekjf having applied his microscopical observations to the structure of the teeth, discovered that the apparent fibers were really tubes, and he communicated a brief but succinct account of his discovery to the Royal Society of London, which was published, together with a figure of the tubes, in No. 140 of their Transactions. This figure of the dentinal tubes, with additional observations, again appeared in the Latin edition of Leeuwenhoek's works, published at Leyden in 1730. The dentine of the human teeth, and also that of young hogs, is described as being 'formed of tubuli spreading from the cavity in the center to the circumference.' He computed that he saw a hundred and twenty of the tubuli within the forty-fifth part of an inch. He was aware also of the peculiar substance now termed the cement, or cmista petrosa, which enters into the composition of the teeth of the horse and the ox.

" These discoveries may be said to have appeared before their time. The contemporaries of Leeuwen-

* An Italian physician ; born in 1628 ; died in 1604. He was the first to apply the newly-invented microscope in the study of anatomy.

X A Dutch naturalist and manufacturer of optical instruments. His microscopes were said to be the best in Europe. Besides his dental discoveries, he discovered the red globules of the blood, the infusorial animaVules, and that of the spermatozoa. Bom in Delft October 24, 1632 ; died there Augnst 26, 1723.

XIV INTKODUCTIOK.

hoek were not prepared to appreciate them ; besides they could neither repeat nor confirm them, for his means of observation were pecuHarlj his own; and hence it has happened that, with the exception of the learned Portal,* they have either escaped notice, or have been designedly rejected by all anatomists until the time of the confirmation of their exactness and truth by Purkinje in 1835."

Continuing the subject, Prof. Owen further says of the three constituent parts of teeth — dentine, enamel, and cement — beginning with

THE de:n'tine.

"Purkinje states that the dentine consists, not of superimposed layers, but of fibers arranged in a homogeneous intermediate tissue, parallel with one another, and perpendicular to the surface of the tooth, running in a somewhat wavy course from the internal to the external surface, and he believed these fibers to be really tubular, because on bringing ink into contact with them, it was drawn in as if by capillary attraction.

" On the publication of this discovery, it was immediately put to the test by Prof. Miiller, by whom the tubular structure of the dentine was not only confirmed, but the nature and one of the offices of the tubes were determined. He observed that the white color of a tooth was confined to these tubes, which were imbedded in a semi transparent substance, and he found that the whiteness and opacity of the tubes were removed by acids. On breaking a thin lamella of a tooth transversely with regard to its fibers, and examining the edge of the fracture, Miiller perceived tubes pro-

WHAT THE TUBES CONTAIN". XV

jecting here and there from the surfaces. They were white and opaque, stiff, straight, and apparently not flexible. This appearance is well represented in the old figure by Leeuwenhoek. If the lamellae had been previously acted upon by acid, the projecting tubes were flexible and transparent, and often very long. Hence Miiller inferred that the tubes have distinct walls, consisting of an animal tissue, and that, besides containing earthy matter in their interior, their tissue is, in the natural state, impregnated with calcareous salts."*

THE CEMENT.

" The organized structure and microscopic character of the cement were first determined by Purkinje and Faenkel, and the acquisition of these facts led to the detection of the tissue in the simple teeth of man and carnivorous animals. The cement is most conspicuous where it invests the root of the tooth, and increases m thickness as it approaches the apex of the root. The animal constituent of this part of the cement had been recognized by Berzelius as a distinct investment of the dentine long before the tissue of which it formed the hasis was clearly recognized in simple teeth. Berzelius describes the cemental membrane as being less consistent than the animal basis of the dentine, but resisting

* If Lord Bacon's theory is correct, the probability is that these tubes contain something besides earthy matter and calcareous salts, to wit, spirit. In "Novum Organ um" he says (B. Montagu, vol. xiv, p. 417): "All things abhor a solution of their continuity, but yet in proportion to their rarity. The more rare the bodies be, the more they suffer themselves to be thrust into small and narrow passages ; for water will go into a passage which dust will not go into, air which water will not go into, and flame and spirit which air will not go into."

XVI INTEODUCTION".

longer the solvent action of boiling water, and retaining some fine particles of the earthy phosphates when all such earth had been extracted from the dentinal tissue. Cuvier also states that the cement is dissolved with more difficulty in acid than the other dental tissues. Retzius,* however, states that the earth is sooner extracted by acid from the cement than from the dentine of the teeth of the horse.

"In recent mammalian cement the radiated cells, like the dentinal tubes, owe their whiteness and opacity to the earth which they contain. According to Retzius, * numerous tubes radiate from the cells, which, being dilated at their point of beginning, give the cells the appearance of an irregular star. These tubes form numerous combinations with each other, partly direct and partly by means of fine branches of Toi-(ro"tli ^^ -g-g^J-Q-o-th of an inch in diameter. The cells vary in size. The average size of the Purkinjean cells in human cement is yeVir^^^ ^^ ^^ inch. In sections made transversely to the axis of the tooth, it is clearly seen that these cells are arranged in parallel or concentric striae, of which some are more clearly and others more faintly visible, as if the cement were deposited in fine and coherent layers.' The layer of cement is found in

* Prof Retzius, of the University of Stockholm, informs us that he had been led by the iridescence of the fractured surface of the substance of a tooth to conceive that that appearance was due, as in the crystalline lens, to a fine fibrous structure, and that he communicated his opinions as to the regular arranpfement of these fibers to some of his colleagues in 1834 In 1835, having obtained a powerful microscope, he began a series of more exact researches on the intimate structure of the teeth in man and the lower animals, which he communicated to the Hoyal Academy of Sciences at Stockholm on January 18, 1836, being then unacquainted with the discoveries of Purkinje. — Owen,

EXOSTOSIS OF THE ROOT. xvii

the deciduous teeth, but is relatively thinner, and the Purkinjean cells are more irregular.

''^u growing teeth, with roots not fully formed, the cement is so thin that the Purkinjean cells are not visible. It looks like a fine membrane, and has been described as the periosteum of the roots, which are wholly composed of it ; but it increases in thickness with the age of the tooth, and is the seat and origin of what are called exostoses of the roots.' These gi'owths are subject to the formation of abscesses, and all the morbid actions of true bone.

"It is the presence of this osseous substance which renders intelligible many well-known experiments of which human teeth have been the subjects, such as their transplantation and adhesion into the combs of cocks, and the establishment of a vascular connection between the tooth and the comb.

"Under every modification the cement is the most highly organized and most vascular of the dental tissues, and its chief use is to form the bond of vital union between the denser and commonly un vascular constituents of the tooth and the bone in which the tooth is implanted. In a few reptiles (now extinct), and in the herbivorous mammalia, the cement not only invests the exterior of the teeth, but penetrates their substance in vertical folds, varying in number, form, extent, thickness, and degree of complexity, and contributing to maintain that inequality of the grinding surface of the tooth which is essential to its function as an instrument for the comminution of vegetable substances." *

* Cement Mistaken for Tartar (Odontot/ithosX— Surgeon E. Mayhew says (" The Horse's Mouth," &c.): "Within the alveolar cavity, the crusta petrosa, which becomes of con-

XVm IlfTRODUCTION.

THE EI^AMEL.

"The higher an animal is placed in the scale of organization, the more distinct and characteristic are not onl}^ the various organs of the body, but the different tissues which enter into their composition. This law is well exemplified in the teeth, although in the comparison of these organs we are necessarily limited to the range of a single primary group of animals. We have seen, for example, that the dentine is scarcely distinguishable from the tissue of the skeleton in the majority of fishes; but that its peculiarly dense, unvascular, and resisting structure, which is the exceptionable condition in fishes, is its prevalent character in the teeth of the higher vertebrates.

"So likewise with the enamel. This substance, which under all its conditions bears a close analogy with the dentine, is hardly distinguishable from that tissue in the teeth of many fishes. The fine calcigerous* tubes are present in both substances, and undergo similar subdivisions, the directions only of the trunks

siderable thickness around tlie root, is of a yellowish-white color ; but where, as on the crown of the tooth, it is exposed to the chemical action of food and air, it presents a darker aspect, and resembles an accumulation of tartar, for which indeed it has been mistaken. It fills up the infundibula of the grinders and lines those of the incisors. It is pierced by all the vessels which nourish the teeth."

The editor of "The Veterinarian" (1849), in a "review" of Mr. Mayhew's work, says: "Both English and French veterinary writers have mistaken the crusta petrosa for tartar, not being aware of its existence inside as well as outside of the tooth."

* This word is peculiar to if not originated by Prof. Owen. It is synonymous with the word Calciferous (limy).

and branches being reversed, agreeably with the contrary course of their respective developments. The proportion of animal matter is also greater in the enamel of the teeth of fishes than in the higher vertebrata, and the proportion of the calcareous salts incorporated with the animal constituent of the walls of the tubes is greater as compared with the subcrystalline part deposited in the tubular cavities.

'' The enamel may be distinguished, independently of its microscopic and structural characters, by its ghstening, subtransparent substance, which is white or bluish-white by reflected light, but of a gray-brown color when viewed, under the microscope, by transmitted light. * * * The enamel of the molar tooth of a calf, which has just begun to appear above the gum, and which can readily be detached from the dentine, especially near the beginning of the roots, is resolvable into apparently fine prismatic fibers. If these fibers be separately treated with dilute muriatic acid, and the residue examined with a moderate magnifying power, in distilled water, or, better, in dilute alcohol, portions of more or less perfect membranous sheaths or tubes will be discerned, which inclosed the eiii'thy matter of the minute prism, and served as the mold in which it was deposited.

"Prof. Retzius, who obtained a small portion of organic or animal substance from the enamel-fibers of an incompletely-formed tooth of a horse, conjectured that it was a deposition of that fluid which originally surrounds the loose enamel-fibers, and that 'in proportion as these fibers are pressed tighter together, and additional fibers are wedged between them, the organic deposition is forced away.'

"Retzius accurately describes the enamel-fibers of

XX INTRODUCTIOiT.

the horse as presenting the form of angular needles, about -g-fiVo^^ ^f ^^^ ^'^^^^ ^^ diameter, which are traversed by minute and close-set transverse striae over the whole or a part of the fiber; and he conjectures that if the enamel-fiber be a mass of the calcareous salts, surrounded by an organic capsule, that the striae may then belong to the capsule, and not to the enamel-fiber. The later researches of Dr. Schwann add to the probability of this conjecture ; and the absence of the Wiinute striae in the enamel of fossil mammalian teeth, at least in the examples which I have submitted to microscopic investigation, may depend upon the destruction of the original organic constituent of the enamel.

"The enamel-fibers are directed at nearly right angles to the surface of the dentine, and their central or inner extremities rest in slight but regular depressions on the periphery of the coronal dentine. Thus in the hnmaii tooth, the fibers which constitute the masticating surface are perpendicular, or nearly so, to that surface, while those at the lower part of the crown are transverse, and consequently have a position best adapted for resisting the pressure of the contiguous teeth, and for meeting the direction in which external forces are most likely to impinge upon the exposed crown of the tooth. The strength of the enamel-fibers is further increased by the graceful, wavy curves in which they are disposed. These curves are in some places parallel, in others opposed. Their concavities are commonly turned toward each other, where the shorter fibers, which do not reach the exterior of the enamel, abut by their gradually attenuated peripheral extremities upon the longer fibers. Other shorter fibers extend from the outer surface of the enamel toAvard

ENAMEL LINES PARALLEL AND WAVY. Xxi

the dentine, and are wedged into the interspaces of the longer fibers. In the teeth of fishes, the calcigerous tubes or fibers of the enamel, which ramify and subdivide like those of the dentine, have their trunks turned in the opposite direction, or toward the periphery of the tooth. So likewise in liuman teeth the analogous condition may be discerned in the slightly augmented diameter of the enamel-fibers at their peripheral as compared with their central extremities. When the extremities of the human enamel-fibers are examined with a magnifying power of 300 linear dimensions, by reflected light, they are seen to be coadapted, like the cells of a honey-comb, and, like these, to be, for the most part, hexagonal.