TUBES DICHOTOMOUSLY BRANCHED. 131 



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 they diverge in opposite 

 directions, curving from the remains of the pulp- 

 fissure toward the outer and the inner enamel, and are 

 described by Ketzius as being in the form of the Greek 

 e; but the course of two distinct series of dentinal 

 tubes, and not of a single tube, is illustrated by this 

 comparison. When the pulp-cavity comes single and 

 central, as at the lower half of the tooth, the tubes 

 diverge to the periphery, with one principal primary 

 curve, convex toward the crown. Each tube is bent 

 in minute secondary gyrations to within a short dis- 

 tance of its peripheral termination, where it is much 

 diminished in size, and is dichotomously branched. 

 The tubes at their beginning form the upper calcified 

 tracts of the pulp-cavity, which usually retain some 

 remnants of that vascular receptacle in the form of 

 medullary canals, and are strongly and irregularly 

 flexuous before they fall into the ordinary primary 

 curves. These tubes, proceeding toward the inner 

 reflected folds of enamel, are more vertical than the 

 tubes going to the periphery. 



" A transverse section of the incisor of a young horse 

 or ass, taken across the part marked a in Fig. 11, shows 

 a long oval island of vascular cement in the center, 

 bounded by a border of enamel, with an irregular ere- 



