134 HOUSES' TEETH UNDER >THE MICROSCOPE. 



The clear substance forming the walls of the canals is 

 arranged in concentric layers, the thickness of the 

 walls being about equal or rather less than the area of 

 the canal. The radiated cells, generally of a full oval, 

 sometimes of an angular form, are chiefly dispersed in 

 the interspaces of the vascular canals, and with their 

 long axis parallel with the plane of the layers of the 

 coats. The finer system of tubes radiating from the 

 cells, and corresponding by minute branches from the 

 vascular canals, freely intercommunicate. In the 

 peripheral cement of the incisors examined by me, I 

 found no vascular canals, but only the radiated cells, 

 and the fine tubuli which I have called 'cemental,' 

 and which traverse the cement at right angles to its 

 plane, and communicate with the tubes radiating from 

 the cells. These are more usually elliptical than in 

 the thicker central cement, their long axis being par- 

 allel with the borders of the cement. They are most 

 abundant next the enamel, and rarely encroach upon 

 the clear peripheral border of the cement. The exte- 

 rior coronal cement of the molars (Plate 137, c), is as 

 richly permeated by vascular canals (v #), as is the 

 central cement of the incisor. 



"The enamel-fibers of the horse's incisor are very 

 slender, not exceeding twice the diameter of the denti- 

 nal tubes. They extend, with a single sigmoid curve, 

 through the entire thickness of the layer, contiguous 

 fibers curving in opposite directions. The peripheral 

 border, or that next the cement, is everywhere indented 

 with hemispherical pits from jfoth to 2 1 6 tb of an 

 inch in diameter, from four to six of the radiated cells 

 of the cement being often clustered together in the 

 larger depressions. The inner or dentinal border is 

 nearly even and straight; here are seen the short 



