Horses' teeth : a treatise on their mode of development, physiological relations, anatomy, microscopical character, pathology, and dentistry : based on the works of well-known odontologists and veterinary surgeons : to which is added a vocabulary of the medical and technical words used

Preface

LIKE all Special Works, the present volume will i encounter more or less public prejudice. However, the thoroughness with which Special Works and Articles are usually prepared has somewhat changed public sentiment, and the tendency of the times appears to be in their favor. General Works have their advantages, but thoroughness of detail is not usually among them. It may also encounter prejudice on account of the doctrine of Evolution which pervades its pages; but this should not be, for it refers almost exclusively to the Horse, and, besides, indicates Progress. Nature's laws are immutable, and to oppose them is as foolish as to beat the head against a stone wall. Whatever may be said of the original thoughts in the book, the compiled matter is from the works of well-known authors, some of which are also Special Works. Further, many of the definitions in the Vocabulary are from Special Cyclopedic Articles.

The w^ork is the result of less than twelve months of research and study, which is some excuse for the small proportion of original matter. While it is not exhaustive, it is believed that a good foundation has been laid whereon to build, and that it will meet the present requirements of the Veterinary Profession, and may also be useful to both Horsemen and Farmers.

ly PKEFACE.

In order to make the work acceptable to all classes of readers, much of the selected matter has been edited and more or less condensed, a vocabulary of most of the technical words used and parenthetical explanations have been made, and an attempt has been made to relieve the monotony of dry anatomy by inserting reference notes and headings that are at least apropos.

As I neither understand Veterinary nor Human Surgery, an explanation, if not an apology, is required, for as a rule the trained surgeon only should exercise the prerogative of an Instructor.

At the request of Stephen J. Cox, Esq., I consented to become the Editor of a book whereof Mr. Charles D. House was to have been author. Mr. House furnished the facts for which he has received credit, and then abandoned the project. Whether I acted wisely or not in completing the work myself, and whether Mr. Cox does or does not deserve thanks for inducing me to begin it, is for the reader to decide.

I am indebted to the Astor Library for the use of many books ; also to the Cooper Union ; to Prof. J. M. Heard, of the New York College of Veterinary Surgeons, for the use of books and for valuable suggestions; also to Prof. F. G. Fairfield of the same College for the interest he took in the parts relating to microscopy, and to Surgeons J. S. and C. C. Cattanach for the use of books and anatomical specimens. To Prof. John W. Draper, of the University of the City of New York, wlio perceived at a glance that the subject is "so sugg-estive;" Dr. George Thurber, of the Orange Judd Company, and W. B. Smith, Esq., of the Authors' Publishing Co., I am under obligation for encouragement to persevere in my labors.

W. H. C.

Contents

PAGE

INTRODUCTION '^

Chapter I. Tooth-germs (odontogeny)

Periods at wliicli the Germs are visible in the Fetus.— Den- , tine and Enamel Germs.— A Cement Germ in the Foal. | —The Horse's Upper Grinders said to be developed from Five Germs, th3 Lower from Four. -Similar development of the Human Teeth 31

Chapter II. The Temporary Dentition

Twelve Incisors and Twelve Molars.— Why the Incisors are called " Nippers."— The Treatment of Foals Affects Teething.— Temporary Teeth Absorbed by Permanent. 47

Chapter III. The Permanent Dentition

Distinction between Premolars and Molars.--The Bow-like Incisors.— Contrasts between Upper and Lower Grmders and Rows formed bv them.— The Incisors saved from Friction.— Hors3s' teeth compared with those of other Animals.— Measaremeuts.— Time's Changes, &c. 53

Chapter IV. The Canine Teeth Or Tushes

Practically Useless.— Diff 3 rent in their Nature from other Teeth — Wers they formerly Weapons of Offense and Defense?— Views of Profs. Darwin, Hanter, Bell, Youatt, and Winter.— Time of Cutting a Critical Period. . 75

CHAPTER Y.

THE REMNANT TEETH. •

Usually rea:arded as Phenomenons.— The Name.— Traced to Fossil Hors-s. in which (in the Pliocene Period) they Ceased to be Fnnt^tionally Developed.— Nature's Metamorphoses.—" The Agencies which are at work in Modeling- Animal and Veqretable Forms."- Why they are often, as it were, Prematurely Lost.— Fossil Horses. . . 94

Vi COjnTENTS.

PAGE

Chapter VI. Dental- Cysts And Supernumeraky Teeth

Teeth jrrowing in various parts of the Body. — Some very- Prolific Cysts, Froduciiig a Second, it not a Third, •'Dentition." — Rexjorts and Theories oi Scientific Men. 115

CHAPTER VIL

HORSES' TEETH UNDER THE MICROSCOPE.

The Dentinal Tubes, Enamel Fibers, and Cemental Canals

Descriijed and Contrasted 130

CHAPTER Vni.

Importance of the Subject. — ^Caries caused by Inflamed 1

Pulps, Blov/s, Virus, and Morbid Diathesis. — Supernumerary Teeth and other Derangements. — Trephining the Sinuses. — Gutta-Percha as a Filling. — Cleaning the Teeth.— A Diseased Fossil Tooth 136

CHAPTER IX.

THE DENTISTRY OF THE TEETH.

Eeports of Oases Treated by Various Surgeons. — Gutta-percha as a Filling for Trephined Sinuses. — Teeth Pressing against the Palate. — Death of a Horse from Swallo^ving a Diseased Tooth 175

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 Fractur£d 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. T. . . 203

CHAPTER XIL

THE TRIGEMINUS OR FIFTH PAIR OF NERVES.

Its Nature and thn Relation it bears to the Teeth. — Its

Course in the Horse and in Man 216

INDEX 257

INTRODUOTIOF.

The following matter, which is designed to give at least a s3aiopsis of the fundamental principles of dental science, is compiled from the works of the best known odontologists. It is somewhat 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 inevitable concomitants of all compilations, are offset by the interest of its historical records and the scope and clearness of its thoughts and deductions. While 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

Viii INTRODUCTION'.

oi' 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 liis " Odontography," Prof. Owen says: "Besides the advantage of a substantive for an unquestionably distinct tissue under all its modifications in the 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 constitueiits of a tooth. The term 'dental' will retain its ordinary sense, as relating to the entire tooth or system of teeth."

Wote.— 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."

II^TRODUCTIOK. 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 different thickness, as upon the teeth of the horse, sloth, or ruminant, it is also traversed, like bone, by vascular canals. When 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 IKTHODUCTIOK.

and number of the cavities themselves. In the complex grinders of the elei^hant, the masked boar, and the copybara, the cement, which forms nearly half the mass of the tooth, wears dov>m sooner than the dentine. " The enamel is the hardest constitnent of a tooth, and, consequently, the hardest of animal tissues; but it consists, Uke the other dental substances, of earthy matter arrans^ed 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. Owen says ("Anat. of Vert." vol. i, pp. 359-60) :

*"' Teeth vary in num'ber, 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

INTRODUCTIOiT. Xl

interesting to the ph3^siologist. They form for the same reason most important guides for tlie naturalist .in the classilication of animals; and their A'alue, as zoological characters, is enhanced by the facihty 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 theT)omesticated xinimals") :

"Identical inxill our domesticated animals by their general disposition, mode of development, and structure, in their external conformation the teetli present notable diffarences, 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 wrote to Prof. Theodore Gill, of the Smithsonian 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 type was not very different from an existing one. In the case of the older and more aberrant types, liowever, 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 cbief 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 Vv^ith 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.

" Malpighi,* in whose works may be detected the gei'ins 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.

"LeeuwTnhoek,t 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 tlie 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 yonng 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 tlie forty-fifth part of an inch. He was awara also of the peculiar substance now termed the cement, or crusta jwtrosa, which enters mto 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 animalcules, and that of the spermatozoa. Born in Delft October 34, 1632 ; died there Au^st 26, 1723.

Xiv IHTRODtJCTIOK.

hoek were not prepared to appreciate them ; besides they could neither repeat nor confirm them, for his means of observation were pccuHarly his own; and hence it has happened that, with tlie 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 DENTIIy^E.

'•'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 denthie 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 semitransparent 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-

INTRODUCTION". XV

jecting here and there from the surfaces. They were white and opaque, stiff, straight, and apparently not flexible. Tliis appearance is well represented in the old figure by Leeuwenhock. 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 cem!ent 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 cemen fc is most conspicuous where it invests the fang of the tooth, and increases m thickness as it approaches the apex of the fang. 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 basis was clearly recognized in simple teeth. Berzelius describes the cemen tal membrane as being less consistent than the animal basis of the dentine, but resistmg

* If Lord Bacon's theory is correct, the probability is that these tubes contain something besides earthy matter and calcareous saltr,, to wit, spirit. In " Novum Organum " lie says (B. Montagu, vol.xiv, p. 417): "All things abhor a solution ot their continuity, but vet in proportion to their rarity. The more rare the bodies be, the more they suffer themselves to be thrust into small and narrow passatjes; for water will go into a passage which dust will not ^o into, air whix^h water vAM not go into, and flams and spirit which air will not go into."

Xvi IKTRODUCTIOK".

longer the solvent action of boiling water, and retaining some fine particles of the earthy phosphates when all such earth liad 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 Eetzius, * 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 t¥o"oo"^^^ ^^ -g-g-i-oo-th of an inch in diameter. The cells vary in size. The average size of the Purkinjean cells in human cement is jeVo^^^ c>f an inch. In sections made transversely to the axis of the tooth, it is clearly seen that tiiese 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 tlie University of Stockholm, informs us that he had heen led by the iridescence of the fractured surface of the substanco of a tontli to conceive that that appearance was du3, as in the crystalline lens, to a fine fibrous structure, and that he communicated his opinions as to the ren^ular arranofement 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 Royal Academy of Sciences at Stockholm on January 13, 1836, being then unacquainted with the discoveries of Purkinje. — Owen.

INTRODUCTION". Xvii

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

'-'In growing teeth, with fangs 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 fangs; but it increases in thickness with the age of the tooih, and is the seat and origin of what are called exostoses of the fang, which are wholly composed of it.' These growths 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, aud its chief use is to form the bond of vital union between the denser and commonly unvascular 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 whicli is essential to its function as an instrument for the comminution of vegetable substances." *

* Cement Mistaken fou Taetah (ODONTOT/iTnosV — Surgeon E. Mayliew says ("The Horse's Mouth," &c.): "Within the alveolar cavity, the ci'usla petrosa, which becomes of cou*

Xvm IKTRODUCTIOIT.

THE EK"AMEL.

"The higlier an animal is placed in the scale of organization, tiie more distinct and characteristic are not only 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 distinguisha])le 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 ftround the fang", 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 accmnulation of tartar, for which indeed it has been mistaken. It fills up the infimdibula of the grinders and lines those of the incisors. It is pierced by all the vessels wliich nourish the teeth." ! The editor of ''The Veterinrtrian " (18i9), 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 Ccdciferous (limy).

INTRODUCTION". XIX

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 nshes than in the higher vertebrala, and the proportion of tlie 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 fangs, 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 alcOliol, portions of more or less perfect membranous sheaths or tubes will be discerned, which inclosed the earthy matter of the minute prism, and served as the mold in which it was deposited.

"Prof. Ketzius, 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 whicli 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 INTRODUCTION".

the horse as presenting the form of angular needles, about ^i^Vo^^^ <^f ^^^ ii^ch in 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 stride 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 minute 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 ansfles 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 human 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 toward

II^TRODUCTION". 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 human 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. AVhen 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.