SECRETION. 139 



Mucus is a pale, semitransparent, alkaline fluid, containing epithelial cells 

 and leukocytes. It is composed, chemically, of water, an albuminous prin- 

 ciple (mucin), and mineral salts; the principal varieties are nasal, bronchial, 

 vaginal, and urinary. 



Secreting glands are formed of the same elements as the secreting mem- 

 branes, but instead of presenting flat surfaces, are involuted, forming tubules, 

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

 pound 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 secretion two essentially different processes 

 are concerned: 



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

 ingredients of the secreted fluids e. g., pepsin, pancreatin take 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 to the gland during its repose. 



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

 washing out the characteristic ingredients, which are discharged by the gland 

 ducts. The discharge of the secretion is intermittent; they are retained in 

 the glands until they receive the appropriate stimulus, when they pass into 

 the larger ducts by the vis a tergo, and are then discharged by the contraction 

 of the muscular walls of the ducts. 



The activity of glandular secretion is hastened by an increase in the blood- 

 pressure and retarded by a diminution. 



The nerve centers in the medulla oblongata influence secretion: 



1. By increasing or diminishing the amount of blood entering a gland. 



2. By exerting a direct influence upon the secreting cells themselves, the 

 centers being excited by reflex stimulation, mental emotion, etc. 



