200 



DIGESTION. 



where completely covered. The cement contains true bone-lacunae and canaliculi, and, in 

 very old teeth, a few Haversian canals, except near the neck, where the layer is very thin. 

 It is closely adherent to the dentine and to the periosteum lining the alveolar cavities. 



Pulp- Cavity. In the interior of each tooth, extending from the apex of the root or 

 roots into the crown, is the pulp-cavity, which contains a collection of minute blood- 

 vessels and nervous filaments, held together by longitudinal fibres of white fibrous tis- 

 sue. This is the only portion of the tooth endowed with sensibility. Its blood-vessels 

 and nerves penetrate by a little orifice at the extremity of the root. 



The dentine and enamel of the teeth must be regarded as perfected structures ; for, 

 when the second or permanent teeth are lost, they are never reproduced, and when these 

 parts are invaded by wear or by decay, they are incapable of regeneration. The integrity 



of the pulp, even, is not necessary to the 

 stability of the teeth ; for examples are 

 numerous in which the pulp loses its 

 vitality from various causes, and yet the 

 tooth remains and is as serviceable as 

 ever, being only discolored by the decom- 

 position of the structures in the pulp- 

 cavity, which can neither escape nor 

 become absorbed. 



The descriptive anatomy of the teeth 

 in the human subject shows how well 

 calculated they are to perform their va- 

 ried functions, and how admirably they 

 are adapted to a diet composed of articles 

 derived from both the animal and the 

 vegetable kingdom. The thirty-two per- 

 manent teeth are divided as follows : 



1. Eight incisors, four in each jaw, 

 called the central and lateral incisors. 



2. Four canines, or cuspidati, two in 

 each jaw, just back of the incisors. The 

 upper canines are sometimes called the 

 eye-teeth, and the lower canines, the 

 stomach-teeth. 



3. Eight bicuspid the small, or false 

 molars just back of the canines ; four 

 in each jaw. 



4. Twelve molars, or multicuspid, 

 situated just back of the bicuspid; six 

 in each jaw. 



The incisors are wedge-shaped, flat- 

 tened antero-posteriorly, and bevelled 

 at the expense of the posterior face, giv- 

 ing them a sharp, cutting edge, which is 

 sometimes perfectly straight but is gen- 



FIG. 49. Tooth of the cat, in situ. fWaldeyer.) 

 1, enamel; 2, dentine; 3, cement; 4, periosteum of the alveo- 

 lar cavity ; 5, lower jaw ; 6, pulp-cavity. 



erally more or less rounded. The upper 

 incisors are generally larger and strong- 

 er than the lower. In the upper jaw the central incisors are larger than the lateral ; 

 while in the lower jaw the lateral incisors are larger than the central. Each of the incisors 

 has but a single root. The special function of the incisor teeth is to divide the food as it 

 is taken into the mouth. The permanent incisors make their appearance from the sev- 

 enth to the eighth year. 



