MAMMARY GLANDS. 155 



which in different glands possess the capability of elaborating ele- 

 ments characteristic of their secretions. 



In the production of the secretion two essentially different proc- 

 esses are concerned: 



1. Chemic. The formation and elaboration of the characteristic 

 organic ingredients of the secreted fluids e. g., pepsin, pancrea,tin 

 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 dis- 

 charged by the gland ducts. The discharge of the secretions 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 muscu- 

 lar 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 irritation, mental emotion, etc. 



MAMMARY GLANDS. 



The mammary glands, which secrete the milk, are two more or 

 less hemispheric organs, situated in the human female on the 

 anterior surface of the thorax. Though rudimentary in childhood, 



