156 HUMAN PHYSIOLOGY. 



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 mam- 

 mary glands become larger, firmer, and more lobulated ; 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 grad- 

 ually return to their original non-secreting condition. 



Structure of the Mammary Gland. The mammary gland con- 

 sists 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 trac- 

 ing the duct into a lobe, it is found to divide and subdivide, and 

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

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

 surrounded 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, T o1y^ ^ an ^ ncn * n 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 : 



