CHAPTER XXIX 



SECRETING GLANDS 



BEFORE passing on to the action of the digestive secretions on foods, 

 it will be well to discuss 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 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 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 extirpa- 

 tion 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 manufactured, the secretion is no longer formed. 



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. 



Every secreting apparatus consists essentially of a layer of secret- 

 ing cells arranged round a central cavity ; they take from the lymph 



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