MILK. 



Milk physiologically considered stands as a secretion 

 in the strictest sense of the word, since its organic constitu- 

 ents are specific products of the activity of the cells of the 

 mammary gland. Chemically it presents a perfect emul- 

 sion of fat whose menstruum holds in solution three proteins 

 (lactalbumin, lactglobulin, and caseinogen), lactose, inor- 

 ganic salts (chiefly Ca), gases, and traces of creatinine, leci- 

 thin, cholesterol, urea, and citric acid. In the comparison 

 of the composition of human and cow's milk, the chief point 

 to be noted is the relatively low percentage of total solids, 

 protein and fat, and high percentage of lactose in human 

 as against cow's milk. Fresh milk possesses either an ampho- 

 teric or alkaline reaction, but with phenolphthalein it also 

 has a certain acidity. Upon standing, it develops a definite 

 acid reaction by the change of the lactose into lactic acid. 

 This throws the caseinogen out of solution and the milk is 

 said to be sour. Fresh milk does not coagulate by heat, but 

 forms a scum supposed to consist of some form of caseinogen 

 and calcium salts. 



The specific gravity of human and cow's milk is approxi- 

 mately the same (1.028-1.034). It is increased by the re- 

 moval of the fat (cream), which has a specific gravity lower 

 than that of water. 



General Reactions. 



(a) Examine under the microscope fresh milk, skimmed 

 milk, and colostrum. What are the differences? 



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