576 UNGULATES. 



211. Microscopic Structure. — The body of the long molar teeth of 

 the Horse consists of columns of fine-tubed unvascular dentine 

 (PI. 137, a), coated by enamel (&) which descends in deep folds into 

 the substance of these teeth; the enamel is covered by cement (c), 

 thickest in the interspaces of the inflected enamel-folds and upon the 

 crowns of the molars, where it is permeated by vascular canals, 

 thinnest on the crowns of the canines and incisors. At the roots of 

 these teeth, and on those developed from the worn down molars, 

 the dentine is immediately invested by cement. 



In a vertical section of the incisor, as in PI. 136, fig. 11, the pulp- 

 cavity, contracting as it approaches the vertical enamel-fold, divides 

 near the end of that fold, and extends a little way between it and 

 the periphery of the incisor, or leaves a few medullary canals and 

 a modified thin tract of irregularly formed dentine, between the 

 reflected and the outer coat of enamel but rather nearer the former. 

 Above this tract, near the summit of the crown, the dentinal tubes 

 proceed in a nearly vertical direction, with a gentle sigmoid primary 

 flexure, where they diverge from the perpendicular ; lower down 

 the dentinal tubes diverge in opposite directions, curving from the 

 remains of the pulp-fissure towards the outer and the inner enamel ; 

 and are described by Retzius as being bent in the form of the Greek 

 £ (1) ; but the course of two distinct series of dentinal tubes, and 

 not that of a single tube is illustrated by this comparison ; when the 

 pulp -cavity becomes single and central, as at the lower half of the 

 tooth, the tubes diverge to the periphery, with one principal primary 

 curve, convex towards the crown. Each tube is bent in minute 

 secondary gyrations to within a short distance of its peripheral 

 termination, where it is much diminished in size, and is dichoto- 

 mously branched. The tubes at their commencement from the 

 upper calcified tracts of the pulp-cavity, which usually retain some 

 remnants of that vascular receptacle in the form of medullary canals, 

 are strongly and irregularly flexuous, before they fall into the 

 ordinary primary curves. Those tubes proceeding towards the inner 

 reflected fold of enamel, are more vertical than the tubes going 

 to the periphery, 



(1) Loc. cit. p. 27. 



