928 



TEETH. 



outer layer of cement is relatively thicker 

 than in the tusks of the Mammoth or in those 

 of the Indian elephant. The general cha- 

 racter of the microscopic structure of the 

 ivory of the Mastodon's tusk is the same as 

 that of the elephant. The peripheral ex- 

 tremities of the dentinal tubes are, in some 

 parts of the tusk, straighten than in the rest 

 of their course ; the straighten extremities 

 were those which were first formed in the 

 calcification of the peripheral part of the pulp, 

 and this first-formed ivory is accordingly, in 

 such parts, more like the ordinary dentine, 

 and is analogous to the thin peripheral cap of 

 such substance in the teeth of the Sloth and 

 of some fishes. 



The pulp soon, however, becomes subject 

 to that modification of the calcifying processes 

 by which the more tortuous disposition of 

 the tubuli and the more frequent interposition 

 of opaque cellules are produced ; mollifications 

 which, in establishing the characters of ivory, 

 present a step in the transition from true 

 dentine to osteo-dentine. 



By the minuteness and close anrangement 

 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. 



The central part of the tusk, especially 

 near the base of such as have reached their 

 full size, is occupied by a slenden 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 

 Jig. 592.f The presence of the foreign body 



* Ilaller seems to have been the first to notice 

 these irregular internal deposits in the pulp-cavity 

 of the elephant's tusk. Elementa Physiologic, 

 torn. viii. p. 519. 



t Camper, " Description Anatomiquc d'un Ele'- 



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 

 surrounding the base of the tu.sk, and the part 

 of the pulp, 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 pulp. 



By the continued progress of growth, the 

 ball so inclosed is carried forwards, in the 

 course indicated by the arrow \nfig. 592., to 

 the middle of the solidified exserted part of 

 the tusk, as in the example in Blumenbach'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. 



Development. 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. 



phant Male," fol. p. 54. Cuvier, Annalcs du Mu- 

 se'um, torn. viii. (1806) p. 115. 



* Cuvier, " Annales du Museum," torn. viii. p. 115, 

 1806, " Sur les defenses des elephans, la structure, 

 I'accroissement, les caraeteres distinctifs de 1'ivoire, 

 et sur les maladies," first clearly stated that the ball 

 or foreign body in the tusk of the elephant was 

 immediately surrounded by a substance different 

 from the regular ivory. The great anatomist ob- 

 serves, " To ute la portion d'ivoire en dehors de la 

 balle est semblable au reste ; il n'y a que ce qui 

 1'entoure immcdiatement qui soit irre'gulier." 



