en. xxix.] SECRETING GLANDS. 4 6 7 



CHAPTER XXIX. 



SECIIKTISG GLANDS. 



BEFORE passing on to the action of the digestive secretions on 

 foods, it will be well to discuss some of the general aspects of the 

 question, and the varieties of glands by means of which these 

 substances are formed. 



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 serving some ulterior office in the economy, and 

 those which are discharged 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 secretions, as a rule, consist of substances which do not pre- 

 exist in the same form in the blood, but require special cells and 

 a process of elaboration for their formation, e.g., the liver cells for 

 the formation of bile, the mammary gland-cells for the formation 

 of milk. The excretions, on the other hand, commonly consist of 

 substances which exist ready-formed in the blood, and are merely 

 abstracted therefrom. If from any cause, such as extensive 

 disease or extirpation of an excretory organ, the separation of an 

 excretion is prevented, and an accumulation of it in the blood 

 ensues, it frequently escapes through other organs, and may be 

 detected in various fluids of the body. An instance of this is seen 

 after the kidneys have been removed. Urea then accumulates in 

 the blood. But this is never the case with secretions ; for after 

 the removal of the special organ by which each of them is manu- 

 factured, the secretion is no longer formed. 



The circumstances of their formation, and their final destina- 

 tion, 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. And since 

 the differences of the two processes of separation, corresponding 

 with those in the several purposes and destinations of the fluids, 

 are not yet ascertained, it will be sufficient to speak in general 

 terms of the process. 



Kvery secreting apparatus consists essentially of a layer of 

 secreting cells arranged round a central cavity ; they take from 



n H 2 



