HORSE. 577 



A transverse section of the incisor of a young Horse or Ass 

 taken across the part marked a in fig. 1 1 , shows a long oval island 

 of vascular cement in the centre, bounded by a border of enamel, 

 with an irregular crenate 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-cavity, and by the irregularly 

 tortuous dentinal tubes which, with the canals, indicate the last 

 converted 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 the section of the enamel itself, and shows the 

 extremities of the tubes cut transversely across, which tubes, as 

 before observed, were at this part directed chiefly in the axis of 

 the incisor towards the working surface of the crown. The tubes 

 in the outer tract of dentine, inclining more towards the sides of the 

 tooth, are more obliquely divided and at the ends of the section they 

 are seen lengthwise elegantly diverging towards 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 dentinal tubes proceeding from the residuary pulp- 

 tract make strong and irregular curvatures, diverging to include 

 the divided arese of the vascular canals, and in the outer layer 

 at one side oF the section, they describe strong zig-zag curves at 

 the middle of the outer division of the dentine. 



The diameter of the dentinal tubes at their central and larger 

 ends is pretty regularly, about -^t\v of an inch; at the middle 

 of their course ^th of an inch ; thence decreasing, and very rapidly 

 after the terminal bifurcations commence. 



The dentinal tubes are separated from one another by intervals 

 varying between once and twice the thickness of the tubes ; in some 

 parts of the dentine of the incisor they are more closely crowded 

 together, especially near their origin from the pulp-cavity. The 

 secondary gyrations of the dentinal tubes describe a curve about -i^,th 

 of an inch in length; these subside in the slender terminations 



p p 



