442 TEXT-BOOK OF PHYSIOLOGY. 



During utero-gestation the mammary glands become larger, firmer, 

 and more tabulated; the areola darkens and the blood-vessels, especially 

 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 these activities cease, 

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

 former non-secreting condition. 



Structure of the Mammary Gland. Each mammary gland consists 

 of an aggregation of some 15 or 20 irregular pyramidal lobes, each of which 

 is surrounded by a framework of fibrous tissue. This tissue affords support 

 for blood-vessels, lymph-vessels, and nerves. Each lobe is provided with a 

 single excretory duct, the lactiferous duct, which as it approaches the areola 

 expands into a fusiform ampulla or reservoir. At the base of the nipple 

 the ampullae contract to form some 15 or 20 narrow ducts, which, ascending 

 the nipple, open by constricted orifices 0.5 mm. in diameter on its apex 

 (Fig. 204). 



On tracing the lactiferous duct into a lobe, it is found to divide and 

 subdivide into a number of branches, which pass into smaller masses the 

 lobules. The lobule in turn is composed of a large number of tubular acini 

 or alveoli, the final terminations of the lobular ducts. Each acinus consists 

 of a basement membrane lined by a single layer of low cuboidal epithelial 

 cells (Fig. 205). Externally the acinus is surrounded by blood-vessels, 

 nerves, and lymphatics. 



MILK. 



Milk as obtained during active lactation is an opaque bluish- white fluid, 

 almost inodorous, with a sweet taste, an alkaline reaction, and a specific 

 gravity of from 1.025 to 1.040. 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 the average 

 about To"iroir of an mcn m diameter. It has been asserted by some observers 

 that each globule is surrounded by a thin proteid envelope which enables it 

 to maintain the discrete form. This, however, is at present disbelieved. 



The quantity of milk secreted daily by the human female averages about 

 1200 c.c. 



Chemic analysis has shown that the milk of all the mammalia consists 

 of all the different classes of nutritive principles, though in different propor- 

 tions, which are necessary to the growth and development of the body. The 

 only exception appears to be an insufficient amount of iron for the formation 

 of the coloring-matter of the blood, the hemoglobin. 



Caseinogen is the chief protein constituent of milk. Associated with it, 

 however, are two other proteins, lactalbumin and lactoglobulin, both of 

 which are present in but small quantity. When milk is treated with acetic 

 acid, sodium chlorid, or magnesium sulphate to saturation, the caseinogen 

 is precipitated as such, and after the removal of the fat with which it is entan- 

 gled may be collected by appropriate chemic methods. On the addition of 

 rennet, an alcoholic extract of the mucous membrane of the calf's stomach, 

 which contains the enzyme rennin or pexin, the caseinogen undergoes a 



