TEETH OF UNGULATA. 365 



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 osteo- 

 dentine 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 ex- 

 posed 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 



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 T y^ o~o* n °^ an ^ ncn * n 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- 



1 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 or- 

 ganic connection between the pulp and the ivory, cxxxix. Ed. 1834, torn, i, p. 535. 



