24 VETERINARY PHYSIOLOGY 



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

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

 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-seereting 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 outline is 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 

 uiucin-forming cells. During the so-called resting state of 

 the gland, the cells are building up zymogen. When the 

 gland is active, the cells throw off the material they have 

 accumulated, and it undergoes a change to zyinin. 



(C) Excreting Epithelium does not manufacture materials of 

 use in the animal economy, but passes substances out of the 

 body. Such epithelium is seen in the kidneys, sweat glands, 

 sebaceous glands, mammary glands, and perhaps in the liver. 

 The cells are composed of a granular protoplasm, in which the 

 presence of the material to be excreted either in its fully 

 elaborated condition, or in process of preparation, may fre- 

 quently be demonstrated e.g. fat globules, iron containing 



