558 UNGULATES. 



The enamel-fibres are smaller than those of the teeth of the 

 Carnivora : and the cement is much thicker upon the roots of the 

 teeth of the Hog, and can be more readily traced over the enamelled 

 crown, especially in hollows of the irregular grinding surface, and 

 upon those parts of the dentine of the tusks which are not covered 

 by enamel. Here, and upon the roots of the teeth, the Purkingian 

 cells were developed, and of a full oval figure, about a^,th of an 

 inch in long diameter : the cemental tubuli, communicating with 

 these cells on the one hand, and with the terminal plexuses of the 

 dentinal tubuli on the other, traverse the thicker layers of cement 

 in horizontal and parallel directions, resembling in this jespect the 

 dentinal tubes. 



The complex molar of the Phacochere yields a beautiful demon- 

 stration of the microscopic characters of the several tissues of a 

 tooth. 



The dentine, as we have seen, is broken up into detached, long, 

 and slender prisms. In a longitudinal section of one of these 

 denticles, the calcigerous tubes ascend vertically through the central 

 third part of the prism, and in the lateral parts gently diverge 

 or bend away towards the periphery ; and when they are close to 

 the enamel increase their outward curve and the width of their 

 clear interspaces, bifurcating there once or twice : several of the 

 terminal branches dilate into opake cells close to the enamel ; the 

 secondary undulations of the tubes are minute but well marked, 

 especially in the ends of the tubes. One or two medullary canals, 

 with a thin coat of osteo-dentine, extend along the centre of the 

 prism. In transverse sections the ends of the tubes, cut across, 

 occupy all but the marginal part : they rarely shew interspaces 

 wider than the breadth of a tube, and are in some places even closer 

 together. The peripheral terminations of the tubes, appear to 

 be more strongly bent than when seen in longitudinal sections. 

 The tubes are smaller, more numerous, and send off much fewer 

 lateral ramuli than in the dentition of the Horse. 



In the middle part of the transverse section the dentinal cells 

 are sub-elliptic, with a clear outline, and include about twenty 

 of the dentinal tubes ; towards the periphery they are indicated by 



