DIGESTIVE SYSTEM. 32 



motion in the jaw, there are, likewise, in many cases, co- 

 existing forms of teeth. In the jaw, with the vertical mo- 

 tion, the teeth are sharp or pointed on the summits, and 

 they divide the food by the edges passing each other like 

 scissars, or by the mutual opposition of the unequal sur- 

 faces. Where the jaw has an extensive motion, the sur- 

 faces of the teeth are flattened and grooved, and well adapt- 

 ed for triturating grain or herbs. 



While the food is thus undergoing the process of masti- 

 cation, it is necessary to change its position, in order to 

 place all the parts under the action of the cutting surfaces 

 of the teeth. In the accomplishment of this purpose, 

 thje cheeks render essential service, by bringing back the 

 food which falls on the outside of the teeth. The tongue, 

 however, is the instrument which chiefly serves to regulate 

 the motions of the food in the mouth. Its delicate sense 

 of touch and taste, fit it for ascertaining the changes which 

 are taking place ; while its flexibility enables it to turn the 

 substances in every direction, and give to the whole the re- 

 quisite preparation. Here we can discern three coexisting 

 organs, where the food requires to be chewed : the jaws must 

 perform certain motions ; teeth are requisite ; and a flexible 

 tongue. Where chewing is not performed, the tongue nei- 

 ther possesses the same flexibility of motion, nor the same 

 delicacy of feeling. 



The food, while subjected to the process of mastication, 

 is, at the same time, mixed with a peculiar fluid, termed 

 saliva or spittle. This fluid is secreted by almost the whole 

 internal surface of the mouth, and from the parotid, sub- 

 maxillary and sublingual glands. Its chemical nature, 

 as a whole, has been already noticed ; but it remains 

 to be ascertained, whether all these glands secrete the 

 same kind of fluid, or each a liquor peculiar to itself. On 

 VOL. i, x 



