458 HUMAN DENTITION. 



the earliest of the adult teeth that are formed, rightly observes, " The 

 teeth between these two points make a quicker progress than those 

 behind. "(I) In the Quadrumana the progress is slower, the second 

 molar preceding the bicuspids, and the last molar, the canines. 

 We may readily suppose that the larger grinders are sooner required 

 by the frugivorous Orang, than by the higher omnivorous Bimanous 

 species ; but the more immediate condition of the earlier deve- 

 lopment of the canines and premolars in Man is their smaller relative 

 size. 



176. Microscopic Structure. — The progressive steps in the ac- 

 quisition of the knowledge of the intimate structure of the Human 

 teeth have been pointed out in the * Introduction,' pp. v — xvi. The 

 body of the tooth consists of hard or unvascular dentine (PI. 122, d) ; 

 the exserted part or crown is invested by enamel (ib. e) ; the whole 

 tooth is coated by cement, (ib. c), but this attains the thickness re- 

 quisite for the development of its characteristic radiated cells only 

 upon the fang or fangs. Every tooth has an internal cavity (PI. 

 122, v), which contains the remains of the vascular dentinal pulp, 

 and is thence termed ' pulp-cavity' : it is progressively diminished in 

 size as the dentine is completed ; remains widest at the base of the 

 crown, and contracts as it descends along the fang or fangs, dividing 

 according to the number of these ; and, when they are connate and 

 form apparently a simple root, the pulp-cavity indicates by its division 

 into linear fissures, continued from the coronal dilatation, the number 

 of fangs which compose such undivided root. After the pulp-cavity 

 has been reduced by the completion of the dentine, it is further di- 

 minished by the conversion of part of the residuary pulp into a layer 

 of osseo-dentine (ib. fig. 4 a, 5 & 8 o). The cement, which is thickest 

 at the end of the fangs, sometimes penetrates the pulp-cavity, and 

 blending with the osseo-dentine closes the aperture, except where 

 two or more minute canals perforate it for the passage of capillary 

 blood-vessels, and nerve-filaments. 



The pulp-cavity of the front incisor, when exposed by a lon- 

 gitudinal section through the breadth or transverse axis of the 

 tooth, (ib. fig. 9) presents the form of a long and narrow in- 

 verted cone ; the base being concave and next the crown, the apex 



U) Op. cit. p. 82. 



