CHAPTER VII 

 SECRETIONS 



1. SECRETIONS IN GENERAL 



SECRETIONS are of various significance for the animal 

 economy. Some serve to remove from the body the waste 

 products of metabolism (e.g. secretion of urine) ; some furnish 

 the fluids necessary for the digestion and absorption of foods ; 

 again, there are the secretions of milk-glands, the food for 

 the infant; the secretion of the sebaceous glands, a protec- 

 tive covering for the skin ; and the sweat secretions which 

 regulate the temperature of the organism. 



Secretions are produced by the gland-cells, i.e. by modi- 

 fied epithelial cells. They are found: 



(a) As isolated cells between other epithelial cells. 



In this group belong the secreting epithelial cells of the mucous 

 membrane (goblet cells), cylindrical cells which, when empty of 

 secretion, contain granular protoplasm and oval nuclei; this 

 granular protoplasm during the formation of secretions changes to 

 a clear mass, the unchanged protoplasm and the nuclei withdraw- 

 ing to the bottom of the cell. The clear mass then leaves the 

 cell, is deposited on the free surface and constitutes the secretion. 

 In the goblet cells the formation and the pouring out of the 

 secretion take place simultaneously; finally the whole cell empties 

 itself and dies. 



(&) As congregated in the glands. 



The glands are invaginations of the skin or mucous mem- 

 brane of various forms, some in the form of tubes (tubular), 

 some in the form of sacs (acinous), branched or unbranched. 



The wall of the gland duct forms a layer of cells which is 

 supported by a membrana propria and surrounded by capil- 



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