HIPPOPOTAMUS. 569 



by sub -parallel cemental tubuli, about ^th of an inch in diameter, 

 the branches of which anastomose freely with the tubes radiating 

 from the cells. 



The osteo-dentine forms a pretty thick layer in the centre of 

 the lobes of the crown, and of the fangs of old molars. The tortuosity 

 of the fine tubes in this substance is extreme and they are very 

 irregular. 



In the ordinary-sized tusks the fine-tubed dentine, which forms 

 the concentric-lined ivory, continues with little or no alteration of 

 texture from the periphery to the pulp-cavity : but in very large 

 and old tusks the apex of that cavity contains osteo-dentine, and 

 this tissue is abundantly developed when the normal function of the 

 dentinal pulp is disturbed by injury or disease. I have been favoured 

 by Dr. Malcolmson, with a very remarkable example, in the inferior 

 canine tusk of the Hippopotamus (PI. 142, figs. 1 & 2), of the 

 subserviency of the osteo-dentine in the reparation of a complete 

 fracture. The injury was indicated externally by a sudden trans- 

 verse constriction of the tusk at xx, with an interruption in the 

 enamel at that part, and irregular deposits of dentine both there 

 and at the adjoining concavity of the tusk. A longitudinal section 

 of the tusk showed the pulp- cavity obliterated at the fractured 

 part and for some distance below it, towards the base of the tusk, 

 by a mass of osteo-dentine, deposited principally in the form of 

 nodules closely impacted together, their convex sides projecting 

 into the re-established pulp-cavity next the base : the general dispo- 

 sition of the osteo-dentine being very like that in the centre of the 

 tooth of the Cachalot. The remains of the pulp-cavity in the pro- 

 truded part or crown of the tusk were unusually conspicuous in the 

 form of a narrow canal near the concave side of the tusk, and 

 opening, like a fistula, upon that surface just beyond the fracture, 

 another irregular slender canal extended transversely through part 

 of the uniting substance and opened upon the concave side of the 

 tusk, just below the preceding. From these appearances it may be 

 concluded that the tusk, either by the action of a shot, or other 

 violence, had been snapped across its implanted and hollow base 

 with probably also fracture and injury to the prominent socket ; 

 but that the broken portions being held together by their adhesion 



