26 HUMAN PHYSIOLOGY 



the gland, takes up nourishing matter and forms this mucin- 

 yielding substance. During the active state of the gland, 

 the mucin-yielder is changed to mucin, and is extruded from 

 the cells into the lumen of the gland. 



Mucin is a substance of great importance in the animal 

 economy. When precipitated and freed from water it is 

 white and amorphous. On the addition of water it swells 

 up and forms a glairy mass. In the presence of alkalies, it 

 forms a more or less viscous solution, and from this solution 

 it is precipitated by acetic acid. In composition it is a pro- 

 teid linked to a molecule allied to the sugars, glucosamine 

 C 6 H 11 NH 2 5 , and is therefore called a glyco-proteid. When 

 boiled with an acid it yields sugar. (See Chemical Physiology, 

 p. 6.) 



In adult life the great function of mucin is to give to 

 certain secretions a slimy character which renders them of 

 value as lubricants. 



(B) Zymin-secreting Epithelium. Another form of secret- 

 ing epithelium of great importance is that which forms the 

 various juices which act upon the food to digest it. These 

 juices owe their activity to the presence of enzymes or zymins. 



A zymin-forming gland after a prolonged period of rest 

 shows cells closely packed together, so that it is difficult to 

 make out their borders. The protoplasm is loaded with 

 granules which are much smaller than those seen in the 

 mucin-forming cells, and which do not swell up in the same 

 way, under the action of reagents. The nucleus is often 

 obscured by the presence of these granules. 



When the gland has been actively secreting, the granules 

 become fewer in number, and are confined to the free ex- 

 tremity of the cell; they are obviously passing out. The 

 cell becomes smaller, and its outlines are more distinct and 

 the nucleus more apparent. 



The granules which fill the cells are not composed of the 

 active enzyme. If extracts of the living cells be made, they 

 are inert, and it is only after the granules have left the cell, 

 or are in the process of leaving, that they become active. 

 Hence, the granules are said to be composed of zymin- 

 forming substance or zymogen. 



The series of changes are parallel to those described in the 



