SECRETIONS. 71 



The serous membranes are the pleura, peritoneum, pericardium, synovial 

 sacs, etc. 



The serous ftiiids are of a pale amber color, somewhat viscid, alkaline, 

 coagulable by heat, and resemble the serum of the blood; their amount 

 is but small; the pleural varies from 4 to 7 drachms; the peritoneal from 

 I to 4 ounces ; the pericardial from I to 3 drachms. 



The synovial fluid is colorless, alkaline, and extremely viscid, from the 

 presence of synovine. 



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 situations 

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

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

 ment 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 tveo essentially different processes 

 are concerned : 



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

 ingredients of the secreted 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 



