292 SECRETION 



spaces and the contained fluid is practically lymph. 

 It diminishes friction when the organs they enclose 

 rub against one another. 



Synovial membranes secrete a fluid resembling lymph, 

 but it also possesses a protein a mucin-like sub- 

 stance, which renders it viscid. Synovial membranes 

 prevent friction between adjacent surfaces of bone 

 entering into the formation of joints. 



The other secretions of the body, as the aqueous 

 humor of the eye, gastric secretions, etc., will be 

 described under the physiology of the parts. 



THE SECRETING GLANDS 



These are a group of cells given off as a pouch from 

 the mucous membrane or skin, and communicating 

 with the lining membrane or surface the secretion 

 is to act upon by means of an open passageway, 

 called a duct. Their epithelial cells vary in their 

 structure and function dependent on their location. 

 The epithelial cells of the secretory glands are sur- 

 rounded by a net-work of blood capillaries, lymph 

 vessels, and nerves; the nerves are in direct connec- 

 tion with the epithelial cells and bloodvessels. 



How these epithelial cells absorb from the lymph 

 and blood plasma their essential constituents of the 

 secretions and change them into their different chemic 

 and physiologic fluids is not definitely known, except 

 that they are the result of metabolic processes going 

 on within the cells. 



All secretory glands are controlled by nerve centres 

 situated in the central nerve system. Normal 

 secretions of glands are brought about by a reflex 

 action. In all reflexes there must be a sensitive 

 surface to receive the impression (skin, mucous mem- 

 brane, etc.), an afferent nerve (one which transmits 

 the impression to the centres in the brain), an efferent, 

 nerve (one which transmits the return stimulus to 



