THE MAMMARY GLANDS. 67 



lates the gland to greater effort and increases the production of the 

 gland. Thus, the amount of urea is increased by an increase of 

 nitrogenous waste, as by exercise, or by an increase of nitrogenous 

 matter in the blood, as by a full meal of animal food. 



(c) The nervous system exerts an active influence upon secretion. 

 This is usually reflex in character, and frequently is active through 

 the vaso-motor nerves, as when the salivary and gastric secretions 

 are increased by the sight or smell of food, as well as by its actual 

 administration. There is also a so-called trophic influence of the 

 nervous system, which directly affects the secreting power of a 

 gland : this is especially well shown in the case of the chorda 

 tympani in its relation to the secretion of the submaxillary 

 gland. 



Describe the correlation of the secretions. 



There often seems to be a relation in the amount of one secre- 

 tion to that of some of the others ; thus in a diseased condition of 

 the intestinal mucous membrane, with increased secretion, there 

 will often be an increase of the bile secreted, and perhaps a 

 diminution of other secretions, as salivary or urinary. And, again, 

 unusual perspiration is followed by decrease in the activity of the 

 kidneys. 



THE MAMMARY GLANDS. 

 Describe the mammary glands. 



They are large glands which are made up of several distinct 

 lobes. Each lobe has its duct, which leads to the nipple, and 

 there are about twenty such lobes and ducts. The lobes are 

 subdivided, and the small lobes, lobules, or alveoli are made up 

 of the terminal tubules of the duct, which lie in a mesh of 

 fibrous alveolar tissue containing considerable fat in its reticu- 

 lum. The ultimate divisions of the duct (alveoli) are lined with 

 columnar epithelial cells, as are the ducts and their branches, but 

 the epithelium of the ducts becomes flat (squamous) near the nip- 

 ple. The main ducts (lactiferous ducts) are sacculated, and during 

 lactation the secretion of the alveoli collects in them, and is drawn 

 from them through their small orifices in the nipple. The flow of 

 milk is also aided by the presence of a small amount of unstriped 

 muscular fibre in the wall of the ducts. 



The mammae are abundantly supplied with blood-vessels, lym- 



