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 partial inflam mation or malformation 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 long ceased to be a matter of wonder how such bodies should become completely imbedded in the substance of the tusk, sometimes without any visible aperture, or how a leaden bullet may have become lodged in the solid centre of a very large tusk without having been flattened. Such a ball, aimed at the head of an elephant, may penetrate 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 was there spent, the ball would gravitate to the opposite and lower side of the pulp-cavity, as indicated in fig. 592.1- The presence of the foreign body exciting inflammation of the pulp, an irregular course of calcification ensues, which results in the disposition around the ball of a certain thickness of osteo-dentine. The pulp then resuming its healthy state and functions, coats the inner surface of the osteo-dentine in closing the ball, together with the rest of the conical cavity into which that mass projects, with layers of normal ivory.* The portion of the cement-forming capsule the base of the tusk, and the part of the which were perforated by the ball in its passage, are soon replaced by the active reparative power of these highly vascular bodies. The hole formed by the ball in the base of the tusk is then more or less completely filled up by a thick coat of cement from with out and of osteo-dentine from within. Traces of such a cicatrix closing the entrance have been more than once noticed : and Blumen bach deduced, therefrom, a property in the elephant's tusk to pour out bony matter in order to heal such wounds. The reparation is however effected by the calcification of the reproduced parts of the capsule and pidp.
By the continued progress of growth, the ball so inclosed is carried forwards, in the course indicated by the arrow in fig. 592., to the middle of the solidified exserted part of the tusk, as in the example in Blurnenbach's collection which he considered so curious. Should the ball have penetrated the base of the tusk of a young elephant, it may be carried forwards by the uninterrupted growth of the tusk until that base has become the apex, and be finally exposed and discharged by the con tinual abrasion to which the apex of the tusk is subjected. Yet none of these phenomena prove the absolute non-vascularity of the tusk, but only the low degree of its vascularity. Blood circulates, slowly no doubt, through the minute vascular canals which are con tinued 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 plasmatic colourless fluid by which its low vitality is maintained.
Developnzent.—The matrix of the tusk con sists of a large conical pulp, which is renewed quicker than it is converted, and thus is not only preserved, but grows, up to a certain period of the animal's life : it is lodged in the cavity at the base of the tusk ; this base is surrounded by the remains of the capsule, a soft vascular membrane of moderate thickness, which is confluent with the border of the base of the pulp, where it receives its principal vessels.
I bad the tusk and pulp of the great ele phant at the Zoological Gardens longitudinally divided, soon after the death of that animal in the summer of Although the pulp could be easily detached front the inner surface of the pulp-cavity, it was not without a certain resistance, and when the edges of the 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.* As the learned professor who has contributed the article " PACHYDER MATA " adopts Cuvier's description of the formation of the teeth of the elephant by deposition and transudation of the tissues from free surfaces of the formative organs, I have the more valued the rare opportunity of testing and confirming, by examination of the recent animal, the account of the processes of conversion of those organs into the dental tissues, which I gave in my " Odontography." Each molar of the elephant is formed in the interior of a membranous sac—the capsule, the form of which partakes of that of the future tooth, being cubical in the first molar, oblong in the last, and rhomboidal in most of the intermediate teeth ; but always decreasing in vertical extent towards its posterior end, and closed at all points, save where it is penetrated by vessels and nerves. It is lodged in an osseous cavity of the same form as itself, and usually in part suspended freely in the maxillary bone ; the bony case being destined to form part of the socket of the tooth. The exterior of the membranous capsule is simple and vascular, as shown at in. 5, fig. 592. ; its internal surface gives attachment to numerous folds or processes, as in most other Ungulate animals.
The dentinal pulp rises from the bottom of the capsule, or that part which lines the deepest part of the alveolus, in the form of transverse parallel plates extending towards that part of the capsule ready to escape from the socket. These plates adhere only to the bottom of the capsule ; their opposite extremity is free from all adhesion. This summit is thinner than the base ; it might be termed the edge of the plate: but it is notched, or divided into many digital processes. The tissue of these digitated plates is identical with that of the dentinal pulp of simple Mammalian teeth ; it becomes also highly vascular at the parts where the formation of the dentine is in active progress.