THE TEETH. 41 



sorted to being adapted to the nature of the food. It is to be understood 

 that these mechanical terms are not mere metaphors, they indicate the 

 actual nature of the apparatus. The object aimed at is to obtain the food 

 in such small portions, and in such a bruised or pulpy condition, that di- 

 gestion can be accomplished promptly. In man the number of _ 



, . r r . ^ ; . m T The teeth. 



temporary teeth is twenty, ten in each jaw. They are arranged 

 in three classes four incisors, two canines, and four molars for the up- 

 per and under jaw respectively. The permanent teeth, which are eventu- 

 ally substituted for these temporary ones, are thirty-two in number, class- 

 p . 1 ified for each jaw as four incisors, two ca- 



nines, four bicuspids, and six molars. 

 Their arrangement is exemplified in Fig. 

 1, representing the lower jaw, in which 

 i is the middle and lateral incisor, c the 

 canine, b the two bicuspids, and m the 

 three molars. 



The movements of the teeth, aided by 

 those of the tongue, accomplish a due 

 abrasion of the food, and simultaneously 



The human lower jaw. . . _. . . 



incorporate it with the saliva, llus is, 



therefore, a purely mechanical operation. It is analogous to Mechanical na _ 

 the methods to which chemists resort in their laboratories ture of mastica- 

 when they prepare solid materials for exposure to reagents. 



The mingling of food with saliva, or insalivation, effects a double ob- 

 ject. Coated over with a glairy juice, the bruised substance passes 

 along the resophageal tube into the stomach ; but there are also certain 

 chemical changes, which, commencing in the mouth, are of essential im- 

 portance to the completion of digestion. 



The stomach is an expansion of the alimentary canal between the 

 oesophagus and duodenum, of a conical figure, the base of D escr i pt i on of 

 which is to the left. It communicates with the oesophagus the human 

 by its cardiac orifice, and by its pyloric with the duodenum. 

 It consists of three coats or tunics the serous or peritoneal, which is 

 exterior ; the muscular, which is intermediate ; and the mucous, which is 

 interior. They are connected with each other by cellular tissue. The 

 fibres of the muscular coat run in three different directions, constituting 

 three layers ; the superficial ones are longitudinal, radiating from the oesoph- 

 agus over the surface of the organ ; those of the middle layer are circular, 

 or ring-like ; they are well developed about the middle of the stomach, 

 and by their contractions sometimes make it assume a divided appear- 

 ance, as though composed of two compartments. Toward the pylorus 

 they are also greatly re-enforced. The fibres of the third layer take, for 

 the most part, an oblique direction. The interior or mucous coat is some- 



