4 oo TEXT-BOOK OF PHYSIOLOGY. 



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 lobu- 

 lar ducts. Each acinus consists of a basement membrane lined by a 

 single layer of low cuboidal epithelial cells (Fig. 182). 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 micro- 

 scopically, it is seen to consist of a clear fluid, the milk plasma, hold- 

 ing in suspension an enormous number of small, highly refractive 

 oil-globules, which measure on the average about -^-^-^ of an inch 

 in 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. 



vJThe 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 proportions, which are necessary to the growth and devel- 

 opment of the body. The only exception appears to be an insuffi- 

 cient amount of iron for the formation of the coloring-matter of the 

 blood, the hemoglobin. 



(Jfl^geinoprpp is the chief proteid constituent of milk. Associated 

 withir>however, are two other proteids, lactalbumin and lac^oglob- 

 ulin, 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 re- 

 moval of the fat with which it is entangled may be collected by ap- 

 propriate chemic methods. On the addition of rennet prepared from 

 the mucous membrane of the calf's stomach, which contains the 

 enzyme rennin or pexin, the caseinogen undergoes cleavage into 

 an insoluble proteid, casein or tyrein, and a small quantity of a new 

 soluble proteid. To this process the term coagulation has been given. 

 The presence of calcium phosphate appears to be essential to this 

 process, inasmuch as it does not take place if the milk be completely 

 decalcified by the addition of potassium oxalate. After coagulation, 

 the more or less solid mass of milk separates into a liquid portion, 

 the serum, and a solid portion, the coagulum. The former, generally 

 termed whey, consists of water, salts, lactalbumin, sugar; the latter, 

 the curd, consists of the casein and entangled fat. Boiling the milk 



