DENTAL SYSTEM OF MAMMALIA. 267 



The teeth in Mammals, as in the fore^oin^ classes, are formed 



~ o * 



by superaddition of the hardening salts to pre-existing moulds of 

 animal pulp or membrane, organised so as to insure the arrange- 

 ment of the earthy particles according to that pattern which cha- 

 racterises each constituent texture of the tooth, together with a 



' O 



course of vitalising plasma through its tissue. 



The complexity of the primordial basis, or ' matrix,' corre- 

 sponds, therefore, with that of the fully-formed tooth, and is 

 least remarkable in those conical teeth which consist only of 

 dentine and cement. The primary pulp, fig. 129, i* 9 which 

 first appears as a papilla rising from the free surface of the 

 alveolar gum, is the part of the matrix which, by its calcification, 

 constitutes the dentine. In simple teeth, the secondary, or 

 enamel pulp, covers the dentinal pulp like a cap; in complex 

 teeth it sends processes into depressions of the coronal part of the 

 dentinal pulp, which vary in depth, breadth, direction, and 

 number, in the different groups of the herbivorous and omni- 

 vorous quadrupeds. The dentinal pulp, thus penetrated, offers 

 corresponding complications of form ; and, as the capsule follows 

 the enamel pulp in all its folds and processes, the external cavities 

 or interspaces of the dentine become occupied by enamel and 

 cement the cement, like the capsule which formed it, being the 

 outermost substance, fig. 237, c, and the enamel, ib. e, being in- 

 terposed between it and the dentine, ib. d. The dental matrix 

 presents the most extensive interdigitation of the dentinal and 

 enamel pulps in the Wart-hog, Capybara, and Elephant. 



The matrix of the mammalian tooth sinks into a furrow, and 

 soon becomes inclosed in a cell in the substance of the jaw-bone, 

 from which the crown of the growing tooth extricates itself by 

 exciting the absorbent process, whilst the cell is deepened by the 

 same process, and by the growth of the jaw, into an alveolus for 

 the root of the tooth. Where the formative parts of the tooth 

 are reproduced indefinitely, to repair, by their progressive calcifi- 

 cation, the waste to which the working surface of the crown of 

 the tooth has been subject, the alveolus is of unusual depth, and 

 of the same form and diameter throughout, figs. 215 and 216, 

 except in the immature animal, when it widens to its bottom or 

 base. In teeth of limited growth, the dentinal pulp is reproduced 

 in progressively decreasing quantity after the completion of the 

 exterior wall of the crown, and forms, by its calcification, one or 

 more roots or fangs, which taper to their free extremity. The 

 alveolus is closely moulded upon the implanted part of the tooth ; 

 and it is worthy of special remark, that the complicated form of 



