104 



MANUAL OF PHYSIOLOGY. 



For further details concerning the muscle tissue the student 

 must turn to the chapter on that subject. Here, however, it may 

 not be out of place to describe briefly the special character of 

 the muscles found in the wall of the digestive tube and their 

 general arrangement. 



Mastication. In man, the introduction of food into the mouth 

 is generally accomplished by artificial means, so that the biting 

 teeth (incisors) and the tearing teeth (canines) are comparatively 

 little used for obtaining a suitable morsel of food (Fig. 46). In 



FIG. 46. 



FIG. 47. 



FIG. 46. Transverse section of the Canine Tooth of a man. (a) Enamel ; 

 (b) Dentine; (c) Pulp cavity ; (d] Crusta petrosa. (Cadiat.) 



FIG. 47. Structural elements of the Enamel of Tooth. A. Prisms cut 

 across showing the hexagonal section. B. Isolated prisms. (Kolliker.) 



the mouth the all-important act of chewing or mastication is ac- 

 complished by means of the motions of the lower jaw, the tongue, 

 and the cheeks. This process of breaking up the solid parts of 

 the food ought to be continued until all hard substances are 

 ground into a soft pulp. 



Structure of the Teeth. The exposed part of the teeth is cov- 

 ered by a dense substance of flinty hardness called enamel, which 

 is developed from the epithelium, and consists of hexagonal 

 prisms set on end, which are really modified epithelial cells but 

 only contain about 2 per cent, of animal matter (Fig. 47). The 

 bulk of the tooth is made up of dentine, a substance like bone in 



