302 STRUCTURE. 



in it, I have termed the * vascular dentine,' forms the body or axis 

 of the tooth in the Sloth-tribe,(l) and is present in smaller pro- 

 portion in the centre of the teeth of the Armadillos. The teeth 

 of the Orycteropus consist of congeries of long and slender pris- 

 matic columnar denticles, each consisting of a body of dentine, 

 with a coating of cement, by which they are united together to 

 form a composite tooth, as in some of the Cartilaginous Fishes. 



129. Structure. — In most Mammals the body of the tooth 

 consists of a gelatinous animal basis and calcareous earth, com- 

 bined and arranged according to the plan which characterizes the 

 tissue called " unvascular dentine. "(2) The compartments of the 

 basal substance, which I have called ' calcigerous' or ' dentinal cells,' 

 and which contain the hardening salts in their densest state, are 

 sub-circular (PL 95), or sub-hexagonal (PI. 70). The calcigerous 

 and nutrient tubes varying from ^^ to apoo^h of an inch in diameter, 

 and placed with intervals equal to from two to six of their own dia- 

 meters, proceed, at first with strong, and then with gentle curves from 

 the pulp-cavity to the outer surface of the dentine, their general direc- 

 tion being always more or less nearly at right angles to that surface : 

 those tubes which proceed from the apex, and from the basal third- 

 part of the pulp-cavity are the straightest. They are nearly parallel 

 to one another, both in their general course and curvatures ; but, as 

 the outer surface of the tooth exceeds the inner one in extent, the 

 tubes slightly diverge in their course and divide, decreasing in diame- 

 ter to their peripheral extremities, and rapidly so near their termina- 

 tion where they become irregularly fiexuous and often interlaced. The 

 dichotomizing calcigerous tubes send off from their sides much more 

 minute branches, which quickly divide and sub-divide in the inter- 

 spaces of the trunks, and penetrate the dentinal cells. In the unvas- 

 cular dentine of all MammaUan teeth the tubes present the " primary 

 curvatures," and " secondary gyrations" described in the Introduc- 

 tion. (3) These curvatures are strongest and least regular in the den- 

 tine of the Elephant's grinders. In that of the tusks of the Elephant 

 and Mastodon, the form, extent, and parallelism of the secondary 

 curvatures of the dentinal tubes, cause the pecuhar appearance of 

 the decussation of curved lines, like the ornamental work called 



(1) PI. 82, fig. 1. (2) Introductior, p. iii, & iv. (3) P. xvi. 



