SECRETION OF SALIVA, AND ITS USES. 177 



bourhood of the 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 which 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 



N 



