SECRETION. 65 



The function of serous fluids is to moisten the opposing surfaces, so as 

 to prevent friction during the play of the viscera. 



The mucous membranes are soft and velvety in character, and line the 

 cavities and passages leading to the exterior of the body, e.g., the gastro- 

 intestinal, pulmonary and genito-urinary. They consist of a primary 

 basement membrane covered with epithelial cells, which, in some situa- 

 tions, are tessellated, in others, columnar. 



Mucus is a pale, semi-transparent, alkaline fluid, containing epithelial 

 cells and leucocytes. It is composed, chemically, of water, an albuminous 

 principle, mucosine, and mineral salts ; the principal varieties are nasal, 

 bronchial, vaginal and urinary. 



Secreting Glands are formed of the same elements as the secreting 

 membranes ; but instead of presenting flat surfaces, are involuted, forming 

 tubules, which may be simple follicles, e.g., mucous, uterine or intestinal ; 

 or compound follicles, e.g., gastric glands, mammary glands; or racemose 

 glands, e.g., salivary glands and pancreas. They are composed of a 

 basement membrane, enveloped by a plexus of blood vessels, and are 

 lined by epithelial and true secreting cells, which in different glands 

 possess the capability of elaborating elements characteristic of their 

 secretions. 



In the production of the secretions two essentially different processes 

 are concerned : 



1. Chemical. The formation and elaboration of the characteristic organic 

 ingredients of the secreting fluids, e.g., pepsin, pancreatin, takes place 

 during the intervals of glandular activity, as a part of the general function 

 of nutrition. They are formed by the cells lining the glands, and can often 

 be seen in their interior with the aid of the microscope, e.g., bile in the 

 liver cells, fat in the cells of the mammary gland. 



2. Physical. Consisting of a transudation of water and mineral salts 

 from the blood into the interior of the gland. 



During the intervals of glandular activity, only that amount of blood 

 passes through the gland sufficient for proper nutrition ; when the gland 

 begins to secrete, under the influence of an appropriate stimulus, the blood 

 vessels dilate and the quantity of blood becomes greatly increased beyond 

 that flowing through the gland during its repose. 



Under these conditions a transudation of water and salts takes place, 

 washing out the characteristic ingredients, which are discharged by the 

 gland ducts. The discharge of the secretions is intermittent ; they are 

 retained in the glands until they receive the appropriate stimulus, when 



