CHAPTER XVII. 

 SECRETION. 



Secretion. Secretion is a term applied to a process by which complex 

 fluids are formed from the constituents of the lymph which is separated 

 from the blood-stream by the activities of the endothelial cells of the capil- 

 lary wall, as the blood flows through the capillary blood-vessels. In this 

 process the endothelial cell is aided by the physical forces, diffusion, osmosis, 

 and nitration. This separated or secreted material may be utilized in several 

 ways: 



1. For the repair of the tissues, for growth, for the liberation of energy. 



2. For the elaboration or production by specialized organs of a variety of 



complex fluids of widely different application. The fluids thus formed 

 are utilized for the most part to meet some special need of the body. 

 All such fluids are termed secretions. 



All secretions are products of the activities of epithelial cells covering 

 a flat, or lining a more or less complexly involuted, membrane which in 

 each instance may be termed a secretor organ. As the fluids for the most 

 part are poured out. on the surface of the body, they have been termed 

 external secretions: e.g., mucus, saliva, gastric juice, milk, sebaceous matter, 

 etc. Within recent years it has been demonstrated that the epithelium of 

 certain organs, for example, of those which do not possess a duct, such as 

 the thyroid, adrenals, hypophysis, etc., also produces certain specific con- 

 stituents which are however returned to the blood, and which in some un- 

 known but yet favorable way influence the general nutrition. To such 

 products of these organs the term internal secretions has been given. 



The blood, in addition to its nutritive constituents, contains a number of 

 principles, derived from the tissues, which are to be regarded as waste pro- 

 ducts, the outcome of the katabolic activity of the tissues and apparently 

 of no further use to the body. If retained, they would seriously if not 

 fatally interfere with the normal physiologic activities of the different tissues. 

 They are therefore removed by specialized organs after their separation from 

 the blood-stream. The waste products in solution thus removed are not 

 capable of being utilized for any specific purpose, and are therefore termed 

 excretions: e.g., urine, perspiration, etc. Excretion, also, is performed 

 by the activities of epithelial cells aided by the physical forces of diffusion, 

 osmosis and filtration; and though a distinction is made between the two 

 classes of fluids, no sharp line can be drawn between the cell processes which 

 take place in secretor and excretor organs. 

 All secretor organs may be divided into 



1. Epithelial. 



2. Reticular and vascular, the latter term indicating merely their relation 



to blood-vessels. 



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