SECRETION OF SALIVA, AND ITS USES. 177 
bourhood of tbe mouth; of these there are three pair in Man, 
two beneath the tongue (fig. 107), and one in the cheek, each 
pouring-in its secretion by a separate canal. The salivary fluid 
is principally composed of water, in which a small quantity 
of animal matter and some saline substances (chiefly common 
salt) are dissolved; the whole amount of these, however, is 
not more than 1 part in 100. The secretion of saliva is not 
constantly going on; but the fluid is formed as it is wanted. 
The stimulus by which the gland is set in action may be simply 
the motion of the jaws ; thus, on first waking in the morning, 
the mouth is usually dry, but it is soon rendered moist by the 
movements w T hich take place in speaking. The contact of 
solid substances with the membrane lining the mouth appears 
also to excite the flow; hence dryness of the mouth may 
often be remedied for a time, when no water is at hand, 
by taking a pebble into its interior, and moving this from 
side to side. There are certain substances, however, whose 
presence in the mouth has a special influence in provoking 
an increased secretion of saliva; and every one knows, too, 
that the simple idea of savoury food will excite an increased 
flow, making the “ mouth water ” as it is popularly termed. 
These are instances of the power of the nervous system, 
through which such impressions are conveyed, over the act of 
secretion. 
191. In the case of farinaceous or starchy food, the admix¬ 
ture of saliva occasions the commencement of that chemical 
change in which its digestion consists, namely, its conversion 
into sugar; but in general, the benefit derived from this pro¬ 
cess of insalivation is just that which is obtained by the 
chemist, when he bruises in a mortar, with a small quantity 
of fluid, the substances he is about to dissolve in a larger 
amount of the same. If the preliminary operations of masti¬ 
cation and insalivation be neglected, the stomach has to do the 
whole of the work of preparation, as well as to accomplish 
the digestion; thus more is thrown upon it than it is adapted 
to bear; it becomes over-worked, and manifests its fatigue by 
not being able to discharge even its own proper duty. Thus 
the digestive function is seriously impaired, and the general 
health becomes deranged in consequence. A malady of this 
kind is very prevalent in the United States; and is almost 
universally attributed by medical men, in part at least, to the 
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