CH. XXXI.] 



SECRETING GLANDS 



495 



pour their secretion into it by ducts, are the salivary glands, 

 liver, and pancreas, and will be described in the chapters dealing 

 with those organs. 



Before passing on to a study of the digestive secretions on foods, 

 we may consider some general questions relating to secreting organs. 



It is the function of gland-cells to produce by the metabolism of 

 their protoplasm certain substances called secretions. These materials 

 are of two kinds, viz., those which are employed for the purpose of 



Fio. 344. A. Villus of sheep. B. Villi of man, (Slightly altered from Teichmann.) 



serving some useful office in the economy, and those which are dis- 

 charged from the body as useless or injurious. In the former case 

 the separated materials are termed secretions ; in the latter they are 

 termed excretions. 



The circumstances of their formation, and their final destination, 

 are, however, the only particulars in which secretions and excretions 

 can be distinguished ; for, in general, the structure of the parts 

 engaged in eliminating excretions is as complex as that of the parts 

 concerned in the formation of secretions. It will, therefore, be 

 sufficient to speak in general terms of the process. 



