36 VETERINARY PHYSIOLOGY 



with an acid it yields a sugar (see Chemical Physi- 

 ology). 



(B) Zymin-secreting Epithelium. — Another kind of secret- 

 ing epitheHum forms the various juices which act upon the 

 food to digest it. These juices owe their activity to the 

 presence of enzymes or zyniins (p. 7). 



A zymin-forming gland, after a prolonged period of rest, 

 shows cells closely packed together, so that it is ditiicult 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 

 extremity 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, the}" 

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

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

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

 ing substance or zymogen. 



The series of changes are parallel to those described in 

 the mucin-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 

 thev have accumulated, and it undergoes a chano-e to 

 zymin. 



(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 and 

 sweat glands, and probably in the liver. The cells are 

 composed of a granular protoplasm, in which the presence 

 of the material to be excreted either in its fally elaborated 

 condition, or in process of preparation, may frequently be 

 demonstrated — e.g. iron-containing particles. These cells 

 do not merely take up material from the blood and pass 



