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TEXT-BOOK OF PHYSIOLOGY 



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

 cells (Fig. 216). Externally the acinus is surrounded by blood-vessels, 

 nerves, and lymphatics. 



MILK 



Physical Properties. 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, holding in 

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

 which measure on the average about ToVo" f an mc ^ i n diameter. It has 

 been asserted by some observers that each globule is surrounded by a thin 

 protein envelope which enables it to maintain the discrete form. This, how- 

 ever, is at present disbelieved. 



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

 1200 c.c. 



Chemic Composition. The chemic analysis of milk shows that it con- 

 sists of all the different classes of nutritive principles, which are necessary 

 to the growth and development of the body. The only exception appears 

 to be an insufficient amount of iron (3 or 4 milligrams per 1000 grams of 

 milk) for the formation of the coloring matter of the blood, the hemoglobin. 

 This is provided, however, by the liver in which the iron accumulates during 

 intra-uterine life. According to Bunge the liver of a rabbit newly born con- 

 tains as much as 18.2 milligrams per 100 grams of body-weight, while at 

 the end of twenty-four days it contains only 3.2 milligrams per 100 grams of 

 body-weight. The composition, however, varies not only in different classes 

 of mammals but in the same animal at different times and under different 

 physiologic conditions. Average compositions of milk of certain animals are 

 shown in the following table: 



THE COMPOSITION OF MILK 



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 

 conversion into an insoluble protein, casein or tyrein. 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 



