86 GENERAL BIOLOGY 



a source of their activity, without the presence of an 

 activating substance, also formed in the pancreas 

 and transported to the muscles in the blood. It 

 seems likely that the majority of enzymes are thus 

 compounded of a zymogen, secreted in the cells in 

 the form of granules, and an activator with which 

 it must unite before becoming capable of its specific 

 action. In the stomach of warm-blooded animals 

 the activator of the pepsin is hydrochloric acid, 

 which is only excreted under the stimulus of the 

 presence of food. But there seems to be also an 

 antipepsin formed, which neutralizes the enzyme 

 and prevents self-digestion. 



Internal Secretions and Hormones. - - The cells 

 and tissues of an organism, either singly or grouped 

 in glands, produce a variety of secretions, such as 

 starch, cellulose, egg-yolk, silica crystals, lime, 

 mucus, zymogen granules, etc. These products 

 are usually visible, and, being often extruded from 

 the cells in which they are formed, they are spoken 

 of as external secretions. By means of experi- 

 ment it has been demonstrated that in addition to 

 the external secretion of certain organs there is 

 also an ^eternal secretion, so called, which, instead 

 of collecting in ducts and being thus transported 

 away from its source of origin, passes directly into 

 the blood stream. Many " ductless " glands, such as 

 the thyroid, have a large blood supply, which takes 

 up such an internal secretion, but other glands, such 

 as the pancreas, in addition to the evident external 



