44 THE HUMAN MECHANISM 



1. Physiology of the salivary glands; working glands and 

 resting glands. The function of the salivary glands is the 

 secretion or manufacture of saliva for use in the mouth, and 

 one of the first things we notice about this act of secretion 

 is that it is not constant but intermittent. Most organs have 

 periods of activity, or work, followed by periods of inactivity, 

 or rest, and these glands are no exception. Physiologists 

 frequently speak of " working glands " and " resting glands." 

 We all know that our own salivary glands work more effec- 

 tively at some times than at others. The mouth " waters " 

 at the sight of food; when we are in the dentist's chair the 

 flow of saliva often seems excessive, and at other times our 

 mouths are " parched " or " dry." 



2. The chemical composition of saliva. The saliva is some- 

 times thin and flows readily, while at other times it is thick 

 and viscous, or glairy. This difference is caused by the fact 

 that the amount of water in it varies under different con- 

 ditions. At all times, however, it is a fluid which consists 

 of water containing certain solids in solution. The amount 

 of these solids varies from five to ten parts in a thousand of 

 saliva, and they consist chiefly of three groups of compounds. 

 The first is mucin, familiar to us as the chief constituent of 

 the phlegm or mucus discharged from the nose and throat, 

 and giving to the fluid its viscous character; the second 

 group consists of substances known as enzymes, those in the 

 saliva having the power of changing starch to sugar; these 

 we shall study in detail in the chapters on digestion; the 

 third group consists of mineral or inorganic salts, of which 

 ordinary table salt, or sodium chloride, is the most important. 

 As we shall see, the salts and water are derived directly from 

 the blood, while the mucin and enzymes are manufactured 

 by the gland. 



3. Blood supply of the working gland. Whenever a gland is 

 actively working there is an increased flow of blood through 

 it. For this reason the resting gland is slightly pink, while 



