STRUCTURE. 303 



* engine-turning,' upon the surface of transverse or oblique sec- 

 tions or fractures of the tusk, which is characteristic of true 



* ivory.' 



In the incisors of certain Rodents, and in the molars of the Sloths 

 and Megatherioids, or the large extinct phyllophagous Bruta, more 

 or less of the dentine is modified by the persistence of certain tracts 

 of the pulp-cavity, forming vascular or medullary canals; the cha- 

 racters of this " vascular dentine" will be described in the sections 

 devoted to the dentition of the species possessing it. 



The dentine of the long and slender prismatic denticles which 

 are aggregated to form the compound tooth of the Orycterope(l) is 

 unvascular, and is characterized chiefly by the frequent division and 

 wide angles at which the branches of the bifurcating calcigerous 

 tubes diverge, before resolving themselves into their minute wavy 

 terminations. 



The cement, which, with the dentine, is present in all teeth 

 of mammalian animals, is characterized, except where it forms an 

 extremely thin layer, by the radiated calcigerous cells. (2) These 

 are usually arranged in lines or layers parallel with the surface of 

 the ceemental coat and with each other; they average a diameter 

 of fj-th of an inch in Man ; they are rather smaller in the true 

 Carnivora ; proportionally larger in some of the Pachyderms ; but 

 present their most minute size in some of the Ruminants, as the 

 Giraffe and Ox. The radiated forms of these cells arises from the 

 numerous minute tubes continued from or opening in them, and 

 which are analogous to the more parallel calcigerous tubes of the 

 dentine, with which they communicate. 



In the thick radical cement of the Seal's teeth, and in both the 

 radical and coronal cement of the teeth of most Herbivora, very con- 

 spicuous vascular canals are present in addition to the calcigerous 

 tubes and cells. These canals in the cement of the molars of the 

 Megatherium are numerous and constant, take a course parallel with 

 each other and transverse to the outer plane of the cement, and 

 anastomose by loops close to the dentine, towards which the con- 

 vexity of the loops is directed. (3) 



(1) PI. 78. (2) Corpuscula Purkingii, (3) PI. 84, b, b. 



