GLANDULAR EPnHLLIA 



17 



swallowing. In mammals it also contains ferments or enzymes, 

 chiefly ptyalin, which is capable of transforming starch into soluble 

 materials, though the digestive action is probably not exercised to 

 a great extent. Pancreatic secretion, on the other hand, has little 

 or no mechanical action, but its enzymes are of the greatest im- 

 portance in digestion. The pancreas is mainly an exocrine gland 

 of compound acinous structure (Fig. 5). Imbedded among the 

 acini, however, are numerous small solid groups of endocrine cells, 

 the islets of Langerhans, which produce the important hormone, 

 insulin. 



Fig. 5. Part of a section of the pancreas of the rabbit. X 130. Two 

 islets of Langerhans appear in the upper part of the field and a smaller, 

 less conspicuous one near the bottom. The ancini composing the greater 

 (exocrine) portion of the gland are well shown and an artery and a vein 

 also appear at the bottom. 



The foregoing are all secretory glands. The term gland, how- 

 ever, is also applied to excretory organs, such as the kidney, which 

 remove from the body, with litt]e or no alteration, substances 

 brought to them by the circulatory system. This process contrasts 

 with secretion, in which the epithelial cells manufacture new pro- 

 ducts out of the materials brought to them. There are also cyto- 

 genic glands, which produce living cells, namely the ovaries, testes, 

 and lymph glands. 



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