MILK. 73 



diameter; as they pass into the substance of the gland, each duct divides 

 into a number of branches, which are distributed to distinct lobules and 

 terminate in the acini. 



An acinus is made up of a number of vesicles composed of a homoge- 

 neous membrane, lined by pavement epithelium. The gland vesicles are 

 held together by white, fibrous tissue, which unites the lobules into lobes. 



MILK. 



Milk has a pale blue color, is almost inodorous, of a sweetish taste, an 

 alkaline reaction, and a specific gravity varying from 1.025 to 1-046. 

 Examined microscopically it is seen to contain an immense number of 

 globules, measuring the 1Q ^ g ^ of an inch in diameter, suspended in a clear 

 fluid ; these are the milk globules, formed of a small mass of oily matter 

 covered by a layer of albumin. 



The quantity of milk secreted by the human female in 24 hours, during 

 the period of lactation, is about two to three pints ; the quantity removed by 

 the infant from a full breast at one time being about two ounces. 



COMPOSITION OF MILK. 



Water, 890.00 



Proteids, including casein and serum albumin, . . . 35.00 



Fatty matter (butter), 25.00 



Sugar (lactose) with extractives, 48.00 



Salts, 2.00 



Casein is the nutritive principle of milk, and constitutes its most important 

 ingredient. It is held in solution by an alkali, but upon the addition of an 

 acid it undergoes coagulation, passing into a semi-solid form. The presence 

 of lactic acid, resulting from a transformation of milk sugar, causes spon- 

 taneous coagulation to take place. 



The Fatty matter is more or less solid at ordinary temperature, and con- 

 sists of margarine and oleine ; when subjected to the churning process the 

 globules run together and form a coherent mass, the butter. 



When milk is allowed to stand for a varying length of time, the fat 

 globules rise to the surface, forming a layer more or less thick r the cream. 



Milk sugar or lactose is an important ingredient in the food of the young 

 child ; it is readily transformed into lactic acid in the presence of nitrogen- 

 ized ferments. 



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