900 



TEETH. 



of dentine and cement. The primary pulp, 

 which first appears as a papilla rising from the 



of the same form and diameter throughout, 

 except in the immature animal, when it widens 



Fig. 57-i. 



ree surface of the alveolar gum, is the part of 

 the matrix which by its calcification consti- 

 tutes the dentine ; it sinks into a cell and 

 becomes surrounded by a closed capsule in 

 every mammiferous species, at an early stage 

 of the formation of the tooth ; and, as the 

 cement is the result of the ossification of the 

 capsule, every tooth must be covered by a 

 layer of that substance. In those teeth which 

 possess enamel, the mould or pulp of that 

 constituent is developed from the capsule 

 covering the coronal part of the dentimil pulp. 

 In the simple teeth the secondary or enamel 

 pulp covers the crown like a cap ; in the 

 complex teeth it sends processes into depres- 

 sions of the crown, which vary in depth, 

 breadth, direction, and number in the nu- 

 merous groups of the herbivorous and om- 

 nivorous quadrupeds. The clentinal pulp, 

 thus penetrated, offers corresponding com- 

 plications 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 ce- 

 ment ; the cement, like the capsule which 

 formed it, being the outermost substance, and 

 the enamel being interposed between it and 

 the dentine. The dental matrix presents the 

 most extensive interdigitation of the dentinal 

 and enamel pulps in the Capybara and Ele- 

 phant. The processes of formation and cal- 

 cification of the several constituents of Mam- 

 malian teeth will be found described in the 

 Introduction to my " Odontography " * and 

 in the article TOOTH. 



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 extri- 

 cates itself by exciting the absorbent process, 

 whilst the ceil is deepened by the same pro- 

 cess and by the growth of the jaw into an 

 alveolus for the root of tne tooth. Where 

 the formative parts of the tootn are repro- 

 duced indefinitely to repair by their progres- 

 sive calcification the waste to which the work- 

 ing surface of the crown of the tooth has been 

 subject, the alveolus is of unusual depth, and 



* P. xli. 



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 more or less ra- 

 pidly 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 socket which 

 results from the development of two or more 

 fangs is peculiar to animals of the class Mam- 

 malia.* 



In the formation of a single fang the ac- 

 tivity of the reproductive process becomes 

 enfeebled at the circumference, and is pro- 

 gressively contracted within narrower limits 

 in relation to a single centre, until it ceases 

 at the completion of the apex of the fang ; 

 which, though for a long time perforated for 

 the admission of the vessels and nerves to the 

 interior of the tooth, is, in many cases, finally 

 closed by the ossification of the remaining 

 part of the capsule. 



When a tooth is destined to be implanted 

 by two or more fangs, the reproduction of 

 the pulp is restricted to two or more parts of 

 the base of the coronal portion of the pulp, 

 around the centre of which parts the sphere 

 of its reproductive activity is progressively 

 contracted. The intervening parts of the 

 base of the coronal pulp adhere to the cap- 

 sule, which is simultaneously calcified with 

 them, covering those parts of the base of the 

 crown of the tooth with a layer of cement. 

 The ossification of the surrounding jaw being 

 governed by the changes in the soft, but 

 highly organised, dental matrix, fills up the 



* On the strength of this generalisation, I have 

 established the Mammalian nature of the huge 

 extinct animal called Basilosaurus by Dr. Harlan, 

 and have advocated the claims of the diminutive 

 Amphitherium and Phascolotherium of the oolitic 

 slate of Stoneslield to be admitted into the same 

 high class, against the objections raised by M. de 

 Blainville. See " Comptes Rendus de 1'Acad. des 

 Sciences," Oct. 22, 1838. The bifid base of the 

 teeth of certain sharks not being implanted in a 

 socket, forms no true exception to the rule enunciated 

 in the text. See Geological Transactions, 2d series, 

 vol. vi. p. 66. 



