RODENTS. 



399 



cribing a larger segment of a smaller circle, the lower ones a smaller 

 segment of a larger circle ; these are the longest incisors and usually 

 have their alveoli extended below or on the inner side of those of the 

 molars to the back part of the lower jaw.(l) Like the molars of the 

 Megatherium and other teeth of unlimited growth, the implanted part 

 of the long and large incisors retains the form and size of the exposed 

 part or crown, to the widely open base, which contains a long conical 

 persistent dentinal pulp, and is surrounded by the capsule in a 

 progressive state of ossification as it approaches the crown, an 

 enamel pulp being attached to the inner side of that part of the 

 capsule which covers the convex surface of the curved incisor. 

 The matrix is here noticed in connexion with the tooth, because 

 it is always found in full development and activity, to the time 

 of the Rodent's death. The calcification of the dentinal pulp, 

 the deposition of the earthy salts in the cells of the enamel 

 pulp, and the ossification of the capsule, proceed contempora- 

 neously ; fresh materials being added to the base of the vascular 

 matrix as its several constituents are progressively converted into 

 the dental tissues in the more advanced part of the socket. The 

 tooth thence projecting consists of a body of compact dentine, 

 sometimes with a few short medullary canals continued into it 

 from the persistent pulp-cavity, with a plate of enamel laid upon 

 its anterior surface, and a general investment of cement, which is 

 very thin upon the enamel, but less thin, in some Rodents, upon 

 the posterior and lateral parts of the incisor. The substances of 

 the incisor diminish in hardness from the front to the back part 

 of the tooth ; the enamel consisting of two layers, of which the 

 anterior and external is denser than the posterior layer, and the pos- 

 terior half of the dentine being by a modified number and arrange- 

 ment of the calcigerous tubes less dense than the anterior half. (2) 



The abrasion resulting from the reciprocal action of the upper 

 and lower incisors produces accordingly an oblique surface, sloping 

 from a sharp anterior margin formed by the dense enamel, like that 

 which slopes from the sharp edge formed by the plate of hard 



(1) PI. 104, fig. 1, i. (2) PL 106. 



