500 



MAMMALIA. 



the pulp cavity (odontoblasts). These processes are continued 

 in the dentinal tubes of the dentine and only extend into the 

 enamel in rare cases. 



The teeth consist of 

 dentine, which is covered 

 by enamel on the crown 

 and by a thin layer of 

 cement, which has the 

 structure of bone, on the 

 root (Fig. 264). Occa- 

 sionally cement is found 

 outside the enamel on 

 the crowns (see below). 

 In a few cases, e.g. most 

 edentates, the tusks of 

 elephants, enamel is en- 

 tirely or almost entirely 

 absent. The dentition 

 of mammals is almost 

 always heterodont, i.e. it 

 consists of diiferent kinds 

 of teeth in different parts 

 of the jaws. There are 

 commonly four kinds of 

 teeth in each jaw. There 

 are : the incisors in front ; 

 they are usually chisel- 

 shaped and in the upper 

 jaw are the only teeth 

 placed in the premax- 

 illae : next to the incisors 

 and often separated from 

 them by a gap, called 

 diastema, come the can- 

 ines ; these are never more 

 than one on each side in 

 each jaw ; they are al- 

 most always strong conical j^rojecting teeth and are developed 

 in (or behind) the premaxillo-maxillary suture : the canines are 

 followed by the grinding teeth or back-teeth, which are almost 



¥l(i. 264. — Diagrammatic sections of various forms 

 of teeth. / Incisor (tusk) of elepliant with pulp 

 cavity persistently open at the base. // Human 

 incisor during development with tooth imperfectly 

 formed and pulp cavity widely open at base. 

 /// Completely formed human incisor with pulp 

 cavity contracted to a small aperture at the apex 

 of the root. IV Human molar with broad crown 

 and two roots. V Molar of the ox with long 

 tubercles or as it is sometimes described, with the 

 enamel covering the crown deeply folded and the 

 depressions filled in with cement ; the surface is 

 worn by use ; in the unworn tooth the enamel would 

 be continuous at the tops of the ridges. In all 

 the figures, the enamel is black, the pulp white, 

 the dentine represented by horizontal lines, and 

 the cement by dots (after Flower and Lydekker). 



i 



