132 HORSES' TEETH UX0ER THE MICROSCOPE. 



nate edge next the cement, and an even edge next the 

 dentine, which is here clearly seen to be divided into 

 an inner and an outer tract by an irregular series of 

 the vascular canals continued from the summit of the 

 pulp-cavic}', and by the irregularly tortuous dentinal 

 tubes, which, with the canals, indicate the last con 

 verted remnant of the pulp in this part of the crown. 

 The inner tract of dentine next the island of enamel 

 is well defined, and a little broader than tlie secretion 

 of the enamel itself, and shows the extremities of the 

 tubes cut transversely, which, as before observed, were 

 at this point directed chiefly in the axis of the incisor 

 toward the working surface of the crown. The tubes 

 in the outer tract of dentine, inclining more toward 

 the sides of the tooth, are more obliquely divided, and 

 at the ends of the section they are seen lengthwise, ele- 

 gantly diverging toward the sides of the section: This 

 tract of dentine is bounded externally by a layer of 

 enamel, one-sixth part thicker than that forming the 

 central island; and the enamel is coated by an outer 

 layer of cement, of its own thickness at the sides, but 

 thinning off at the two ends of the section. The den- 

 tinal tubes proceeding from the residuary pulp-tract 

 make strong and irregular curvatures, diverging to 

 incluiie the divided areas of the vascular canals, and 

 in the outer layer, at one side of the section, they de- 

 scribe strong zigzag curves at the middle ofthe outer 

 division of the dentine. 



"The diameter of the dentinal tubes at their central 

 and larger ends is pretty regular, about go^op th of an 

 inch ; at the middle of their course, -g-oV^^h of an inch, 

 thence decreasing, and very rapidly, after the terminal 

 bifurcations begin. The tubes are separated from one 

 another by intervals varying between once and twice 



