66 HUMAN PHYSIOLOGY. 



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 nervous centres in the medulla oblongata influence secretion, (i) by 

 increasing or diminishing the amount of blood entering a gland; (2) by 

 exerting a direct influence upon the secreting cells themselves, the centres 

 being excited by reflex irritation, mental emotion, etc. 



MAMMARY GLANDS. 



The Mammary Glands secrete the milk, and undergo at different 

 periods of life remarkable changes in structure. Though rudimentary in 

 childhood, they gradually increase in size as the young female approaches 

 puberty. 



The gland presents, at its convexity, a small prominence of skin, the 

 nipple, surrounded by an areola of a deeper tint. It is covered anteriorly 

 by a layer of adipose tissue and posteriorly by a fibrous structure which 

 attaches it loosely to the pectoralis muscle. 



During utero-gestation the mammae become large, firm, well-developed 

 and lobulated ; the areola becomes darker and the veins more prominent. 

 In the intervals of lactation the glands gradually shrink in size to their 

 original condition, undergo involution, and become non-secreting organs. 



Structure of the Mammae. The mamma is a conglomerate gland, 

 consisting of a number of lobes, from 15 to 20 in number, each of which is 

 subdivided into lobules made up of gland vesicles or acini. The ducts 

 which convey the secretion to the exterior, the lactiferous ducts, open by 

 15 to 20 orifices upon the surface of the nipple, at the base of which they 

 are dilated to form little reservoirs in which the milk collects during the 

 periods of active secretion. . 



The walls of the lacteal duct consist of white, fibrous tissue, and non- 

 striated muscular fibres, lined by short columnar cells, which disappear 

 during active lactation. The ducts measure about the j 1 ^ of an inch in 

 diameter ; as they pass into the substance of the gland, each duct divides 

 into a number of branches, which are distributed to distinct lobules and 

 terminate in the acini. 



An acinus is made up of a number of vesicles composed of a homoge- 

 neous membrane, lined by pavement epithelium. The gland vesicles are 

 held together by white, fibrous tissue, which unites the lobules into lobes. 



