* DIGESTIVE SYSTEM. $91 
motion in the jaw, there are, likewise, im 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 hke 
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, 
the cheeks render essential service, by bringing back the 
food which falis 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 fiuid, termed 
saliva or spittle. This fluid is secreted by almost the whole 
ternal 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. Xx 
