THE WORK OF ORGANS AND CELLS 47 



in the copious stream of water and salts which during secre- 

 tion passes through from the blood and lymph to the duct. 

 If this were so, it would be possible to dissolve from the 

 gland a substance exhibiting in general the same properties 

 as the secretion itself. But this is not generally the case. 

 Extracts of fresh glands commonly fail to exhibit the char- 

 acteristic properties of normal secretions, although these ex- 

 tracts may often be changed by chemical means into the 

 elements of the secretion. We are therefore compelled to 

 believe that the activity of a gland means something more 

 than the mere discharge of previously stored substances ; 

 that is to say, the material of the granules in the resting 

 cells is not simply set free when the gland secretes, but is 

 at the same time chemically changed. In the digestive juices, 

 for example, we have active substances called enzymes, which, 

 it has been shown, are derived from other substances, called 

 zymogens, in the gland cells. The chemical change from the 

 one into the other is as essential to the process of secretion 

 as is the visible flow from the duct. 



These facts then present to us the picture of the cell as 

 the working or physiological unit, as we have already seen 

 that it is the anatomical unit of the gland (p. 32). The 

 work of the gland is the sum of the work of its constituent 

 cells. During the period of rest these cells manufacture 

 from the blood zymogens or other substances which they 

 store away in the form of granules within their cytoplasm. 

 When they are stimulated by the nervous impulse a chemical 

 change takes place in them, the zymogens are changed to 

 enzymes and other substances, and these, together with the 

 water, salts, etc., derived from the blood, form the secretion. 



7. Physiology of muscular contraction. At first sight mus- 

 cles and glands seem to differ in action or function no less 

 than in form and structure. No two acts are apparently 

 more unlike than lifting a weight by the muscles of the 

 arm and the secretion of saliva by the salivary glands. But 



