140 HUMAN PHYSIOLOGY. 



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, they gradually increase 

 in size as the young female approaches puberty. 



The gland presents at its convexity a small prominence of skin (the nipple) 

 which is surrounded by a circular area of pigmented skin (the areola). The 

 gland proper is covered by a layer of adipose tissue anteriorly and is attached 

 posteriorly to the pectoral muscles by a meshwork of fibrous tissue. During 

 utero-gestation the mammary glands become larger, firmer, and more lobu- 

 lated; the areola darkens and the veins become more prominent. At the 

 period of lactation the gland is the seat of active histologic and physiologic 

 changes, correlated with the production of milk. At the close of lactation 

 the glands diminish in size, undergo involution, and gradually return to 

 their original non-secreting condition. 



Structure of the Mammary Gland. The mammary gland consists of 

 an aggregation of some fifteen or twenty lobes, each of which is surrounded 

 by a framework of fibrous tissue. The lobe is provided with an excretory 

 duct, which, as it approaches the base of the nipple, expands to form a sinus 

 or reservoir, beyond which it opens by a narrowed orifice on the surface of 

 the nipple. On tracing the duct into a lobe, it is found to divide and sub- 

 divide, and finally to terminate in lobules or acini. Each acinus consists of a 

 basement membrane, lined by low polyhedral cells. Externally it is sur- 

 rounded by connective tissue supporting blood-vessels, lymphatics and nerves. 



MILK. 



Milk is an opaque, bluish-white fluid, almost inodorous, of a sweet taste, 

 an alkaline reaction, and a specific gravity of 1025 to 1040. When examined 

 microscopically it is seen to consist of a clear fluid (the milk-plasma), holding 

 in suspension an enormous number of small, highly refractive oil-globules, 

 which measure, on an average, jo&^ f an mcn m diameter. Each globule 

 is supposed by some observers to be surrounded by a thin, albuminous 

 envelope, which enables it to maintain the discrete form. The quantity of 

 milk secreted daily by the human female averages about two and a half 

 pints. The milk of all the mammalia consists of all the different classes of 

 nutritive principles, though in varying proportions. The relative proportions 

 in which these constituents exist are shown in the following table of analyses: 



