568 UNGULATES. 



^ 



other at equal distances of about 3-Joth of an inch, throughout their 

 whole course, the waves of adjoining tubes being parallel along 

 planes which are parallel with the outer conical surface of the 

 exposed end of the tusk before it is worn. Each bend of the tube 

 appears to equal the thickness of the layers into which the dentine 

 of the tusk is resolved in decomposition, and which form the fine 

 concentric lines upon the cut or broken transverse sections of the 

 ivory. The tubes gradually diminish in size as they approach the 

 periphery of the dentine ; they give off minute branches ; and in 

 some places these dilate into very minute corpuscles, along the lines 

 of the concentric rings of the transverse sections, and the contour 

 lines of the longitudinal sections of the tusk. 



The dentinal tubes at their commencement from the pulp-cavity 

 of the molar teeth give a diameter of ^^th of an inch ; and their 

 dichotomous bifurcations are more conspicuous than in the tusks. 



The fibres of the enamel of the tusk have the usual disposition 

 vertical to the plane of the dentine, but are little curved, and have 

 no transverse strise ; most of these fibres are hexagonal and extend 

 through the entire thickness of the enamel : near the base of the 

 tusk they may be detached singly or separately from the part where 

 they bad been last laid on. In the thicker enamel of the molars 

 the fibres are more wavy in their course, and I could perceive none 

 that extended through the entire thickness of the section ; but new 

 ones every where arise by small ends wedged into the intervals of 

 those next the dentine, the larger ends of the fibres being always 

 towards the periphery. The cement which covers thickly those 

 parts of the incisive and canine tusks which are bare of enamel, 

 also extends in a very thin layer upon that substance, from which 

 it is soon worn in the teeth in use. The coronal cement of the 

 molar teeth forms a thicker layer, especially in the angles of the 

 folds, but it is by no means so abundant as in the teeth of the corres- 

 ponding folds of the molar teeth of the Ruminants. 



In the parts of the crown where the cement is thickest, and 

 upon the fangs, a few vascular canals are present : the radiated 

 cells are most numerous at the periphery of the cement, generally 

 affecting an oval form, arranged in concentric layers, with the 

 long axis in the direction of the layer, these layers being crossed 



