594 THE PHYSIOLOGY OF REPRODUCTION 



THE COMPOSITION AND PROPERTIES OF MILK 



Milk is essentially an emulsion, its white appearance being caused 

 by the reflection of the innumerable fat globules which it contains in 

 suspension. These globules, which are from '0015 to '005 millimetre 

 in diameter, tend to float chiefly at the top, where they help to form 

 the cream, or that part of the fluid which is richest in fatty 

 constituents. The specific gravity of both human and cow's milk is 

 from about 1*028 to i'034. 1 When the cream is skimmed off the 

 specific gravity of course rises. 



It is not proposed in the present work to deal more than very 

 briefly with the composition and properties of milk in different 

 animals. 2 Human milk and cow's milk have been most fully investi- 

 gated, and it will suffice in this place o give a short account of their 

 respective constituents. 



The average composition of cow's milk as compared with human 



milk is as follows : 



Cow's. Human. 



Water- - 88'3 88'8 



Proteins 3'0 TO 



Fats - 3'5 3'5 



Carbohydrates 4'5 65 



Salts - --- 07 0'2 



lOO'O 100-0 



The proteins of milk are caseinogen, lactalbumen, and lacto- 

 globulin. Of these caseinogen is the most important. This is the 

 substance which is acted on by the ferment of rennet, producing the 

 well-known clotting or curding of milk, when the caseinogen is 

 converted into whey albumen and insoluble casein. Lactoglobulin 

 and lactalbumen are only present in small quantities. 



The fats of milk, which occur in small globules as just described, 

 are olein, palmatin, and stearin, with small quantities of butyrin, 

 capronin,and other fats of lower composition. Lecithin and cholesterin 

 are also present in small amounts, at any rate in cow's milk. The 

 percentage of volatile fatty acids is less in human than in cow's milk. 



Lactose or milk-sugar is the carbohydrate of milk, but other 

 carbohydrates (animal gum, dextrin, etc.) have also been stated to 

 occur. Lactose under the influence of certain micro-organisms 

 becomes converted into lactic acid, which causes the souring of 

 milk. 



Milk is rich in calcium and potassium salts (especially in calcium 

 phosphate), but magnesium, sodium, and other salts (chiefly chlorides) 

 are also present in small quantities. The amount of iron in human 



1 Halliburton, "The Chemical Constituents of the Body and Food," Sharpey 

 Schafer's Text-Book of Physiology, vol. i., Edinburgh, 1898. 



2 See Halliburton, loc. cit., and Sharpey Schafer. 



