participates in this revolving course, advancing forward with the teeth. The partition between the tooth in use and its successor is perforated near the middle; and, in its progress forward, that part next the grinding surface is first absorbed; the rest disappearing with the absorption of the roots of the preceding grinder.

There are few examples of organs that manifest a more striking adaptation of a complex structure to the exigencies of the animal endowed with it, than the grinding teeth of the elephant. We perceive, for example, that the jaw is not encumbered with the whole weight of the massive tooth at once, but that it is formed by degrees as it is required; the division of the crown into a number of successive plates, and the subdivision of these into cylindrical processes, presenting the conditions most favourable to progressive formation. The fore and most abraded part of the tooth is fitted for the first coarse crushing of the branches of a tree the transverse enamel ridges of the succeeding part of the tooth divide it into smaller fragments, and the posterior islands and tubercles of enamel pound it to the pulp fit for deglutition. The structure and progressive development of the tooth not only give to the elephant's grinder the advantage of the uneven surface which adapts the millstone for its office, but, at the same time, secure the constant presence of the most efficient arrangement for the finer comminution of the food, at the part of the mouth which is nearest the fauces.

The central part of the tusk especially near the base of such as have reached their full size, is occupied by a slender cylindrical tract of modified ivory, perforated by a few vascular canals, which is continued to the apex of the tusk. It is not uncommon to find processes of osteo-dentine or imperfect bone-like ivory, projecting in a stalactitic form into the interior of the pulp-cavity, apparently the consequence of the partial inflammation of the vascular pulp.

The musket-balls and other foreign bodies which are occasionally found in ivory, are immediately surrounded by osteo-dentine in greater or less quantity. It has often been a matter of wonder. how such bodies should become completely imbedded in the substance of the tusk, sometimes without any visible aperture, or how leaden bullets may have become lodged in the solid centre of a very large tusk without having been flattened. The explanation is as follows:-A musket-ball, aimed at the head of an elephant, may penetrate, at a, fig. 289, the thin bony socket and the thinner ivory parietes of the wide conical pulp-cavity occupying the inserted base of the tusk; if the projectile force be there spent, the ball will gravitate to the opposite and lower side of the pulp

cavity, as indicated in fig. 289, b. The hole a is soon healed and filled up by ossification of the periosteum of the socket, and of the pulp next the thin wall of ivory which has been perforated. The ball sinks below the level of this cicatrix, and the presence of the foreign body exciting inflammation of the pulp, an irregular course of calcification ensues, which results in the deposition around the ball of a certain thickness of osteo-dentine. The pulp then resuming its healthy function, coats the surface of the osteodentine inclosing the ball, together with the rest of the conical cavity into which that mass projects, with layers of normal ivory.

By the continued progress of growth, the ball so inclosed is carried forward, in the course indicated by the arrow in fig. 289, to the middle of the solidified exserted part of the tusk, c. Should the ball have penetrated the base of the tusk of a young elephant, it may be carried on, by the uninterrupted growth and wear of the tusk, until that base has become the apex, and be finally exposed and discharged by the continual abrasion to which the apex of the tusk is subjected.

Yet none of these phenomena prove the absolute nonvascularity of the tusk, but only the low degree of its vascularity. Blood circulates, slowly no doubt, through the prolongations of the pulp into the minute vascular canals which are continued through the centre of the ivory to the very apex of the tusk: and it is from this source that the fine tubular structure of the ivory obtains the correspondingly minute villi carrying the plasmatic colourless fluid by which its low vitality is maintained.1

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The modification of dentine called 'ivory,' is characterised partly by the minute size of the tubes, which, at their origin from the pulp cavity, do not exceed 150th of an inch in diameter, in their close arrangement at intervals scarcely exceeding the breadth of a single tube, and, above all, on their strong and almost angular gyrations, which are much greater than the secondary curvatures of the tubes of ordinary dentine.

The dentinal tubes of ivory, as they radiate from the pulp-cavity, incline obliquely towards the pointed end of the tusk, and de

I had the tusk and pulp of an elephant at the Zoological Gardens longitudinally divided, soon after the death of that animal in the summer of 1847. Although the pulp could be easily detached from the inner surface of the pulp-cavity, it was not without a certain resistance; and when the edges of a co-adapted pulp and tooth were examined by a strong lens, the filamentary processes from the outer surface of the pulp could be seen stretching as they were withdrawn from the dentinal tubes before they broke. They are so minute that, to the naked eye, the detached surface of the pulp seems to be entire, and Cuvier was thus deceived in concluding that there was no organic connection between the pulp and the ivory. cxxxix. Ed. 1834, tom. i, p. 535.

scribe two slight primary curves, the first convex towards that end, the second and shorter one concave: these curves in narrow sections from near the open base of the tusk are almost obscured by the strong angular parallel secondary gyrations. The tubes divide dichotomously, at acute angles, and gradually decrease in size as they approach the periphery of the tusk.

The characteristic appearance of decussating curved striæ, with oblique rhomboidal spaces, so conspicuous on transverse sections or fractures of ivory, is due to the refraction of light caused by the parallel secondary gyrations of the tubes above described. The strong contour lines observed in longitudinal sections of ivory, parallel with the cone of the pulp-cavity, and which are circular and concentric when viewed in transverse slices of the tusk, are commonly caused by strata of minute opaque cellules, which are unusually numerous in the interspaces of the tubes throughout the substance of the ivory, and by their very great abundance and larger size in the peripheral layers of cement. The decomposition of the fossil tusks into superimposed conical layers takes place along the strata of the opaque cellules, and directly across the course of the gyrating dentinal tubes.

By the minuteness and close arrangement of the tubes, and especially by their strongly undulating secondary curves, a tougher and more elastic tissue is produced than results from their disposition in ordinary dentine; and the modification which distinguishes ivory' is doubtless essential to the due degree of coherence of so large a mass as the elephant's tusk, projecting so far from the supporting socket; and to be frequently applied in dealing hard blows and thrusts.

§ 222. Homologies of Teeth. In Histology tissues differ according to the kinds and degrees of force which they exercise in the living body: some, the nervous and muscular, e.g. are 'active; others, with lower endowments of elasticity, adhesiveness, hardness, &c., may be called 'passive,' and the classes of these tissues are less definite and distinct. In considering the homology of a tooth, in reference to its class of tissue, our view of it must not be restricted to its ordinary conditions in mammalia, where a central pulp-canal radiates a single system of dentinal tubes like the lacunal tubes from a Haversian canal, but should be extended to those less specialised states of tooth in which the body of dentine is traversed by several pulp-canals, either dichotomising, as in the molar of Orycteropus, vol. i. p. 369, fig. 247, or ramifying throughout the dentine, as in the laniariform tooth of Lamna (vol. i. p. 364, fig. 241).

A vascular matrix buds out in the shark from the membrane covering the jaw, as in the deer from that covering the cranium, and the blood-vessels, ramifying through such matrix, convey the phosphate of lime which hardens it; each ultimate ramification that radiates a system of dentinal tubes in the shark's tooth, corresponds with the same ramification of the artery radiating lacunal tubes in the matrix of the deer's antler.

After the tooth of the shark has been worn by the uses for which it was calcified, it is shed like the antler, and is succeeded by another. There is merely a difference in the place of succession, the new tooth rising close to, but not, as in the antler, directly under, the base of the old.

But the basis from which the matrix of both tooth and antler

grows is homologically the same. In both instances the gum, or corium, is pushed out by the growing matrix: in the deer it forms the velvet' which peels away from the ossified matrix, in the shark it is hardened into the enamel-like layer covering the matrix.

These are the differences that can be predicated in reference to the histological homology of the parts in question, and the shark's tooth answers to the deer's antler, plus the outer enamel-like covering, in mode of development, structure, growth, shedding, and succession. They correspond, alike, with osseous texture; and, under a less genus, with the parts of the dermo-skeleton.

But the tooth of a shark is homologous with that of a porpoise; therefore, teeth are referable to the dermo- or entero-skeletal parts of the osseous system.

Descending to the special homologies, we find that the idea of a recognition of answerable teeth in different animals has prevailed, more or less vaguely, in Anatomy, from an early period of the science.

When incisors,' 'canines,' and 'molars' were predicated of the dentition in different species, homologous teeth were recognised so far as the characters of those classes of teeth were defined and understood.

The Cuviers went a step further, and distinguished the molar teeth into 'false' and 'true,' into 'carnassial' and 'tubercular.' De Blainville pointed out a particular tooth by the name of 'principal,' which he believed himself able to trace from species to species.2

The first step in this inquiry is the elimination of those classes of Vertebrata and orders of Mammalia in which homology cannot be predicated of individual teeth. This limits the work to the group of mammals here termed Diphyodonts.'

' cxx". and cxxı".

The line Blainville' runs through that tooth in fig. 293.

Only in the Mammalian orders with two sets of teeth do those organs acquire fixed individual characters, supporting the application of special denominations; and this individualisation of the teeth is significative of the high grade of organisation of the animals manifesting it.

Originally, indeed, the name incisors,'' laniaries' or 'canines,' 'molars,' 'tuberculars,' were given to the teeth in Man and certain Mammals, as in Reptiles, in reference merely to the shape and offices so indicated; but names of teeth can now be used as arbitrary signs, in a more fixed and determinate sense. In some Carnivora, e.g., the front teeth have tuberculate summits, adapted for nipping and bruising, while the principal back teeth are shaped for cutting, and work upon each other like the blades of scissors. The front teeth in the Elephant project from the upper jaw in the form, size, and direction of long pointed horns. In short, shape and size are the least constant of dental characters; and the homologous teeth are determined, like other parts, by their relative position, by their connections, and by their development.

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Those teeth which are implanted in the premaxillary bones, and in the corresponding part of the lower jaw, are called 'incisors,' whatever be their shape or size. The tooth in the maxillary bone, which is situated at, or near to, the suture with the premaxillary, is the canine,' as is also that tooth in the lower jaw which, in opposing it, passes in front of its crown when the mouth is closed. The other teeth of the first set are the deciduous molars; the teeth which displace and succeed them vertically are the premolars ;' the more posterior teeth, which are not displaced by vertical successors, are the molars,' properly so called.

The premolars must displace deciduous molars in order to rise into place; the molars are a continuation, backward, of the primary or milk' series. It will be observed in fig. 294 that the last deciduous molar, d 4, has the same relative superiority of size to d 3 and d 2 which m 3 bears to m and m 1; and that the crowns of p 3 and p 4 are of a more simple form than those of the milk-teeth which they are to succeed: this, however, is not a constant character (see fig. 287, Hyrax). Teeth of each of the kinds arbitrarily termed incisors,' canines,' 'false molars,' and 'molars,' have received other special names, having reference to certain peculiarities of form or other property. The premolars in the human subject have been called bicuspids.' The last upper premolar and the first true molar in the Carnivora are

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