562 THE PHYSIOLOGY OF REPRODUCTION 



The Composition and Properties op Milk 



Milt 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 millimetres 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 r028 to 1"034.^ When 

 the cream is sldmmed off the specific gravity of couise 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.^ Human milk and cow's milk have been 

 most fully investigated, and it wiU suffice in this place to 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 1-0 



Fats 3-5 35 



Carbohydrates ..... 4'5 6'5 



Salts 0-7 0-2 



100-0 100-0 



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, pro- 

 ducing the weU-known clotting or curding of milk, when the 

 caseinogen is converted into whey albumen and insoluble casein. 

 Lactoglobuhn and lactalbumen are only present in small 

 quantities. 



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

 scribed, are olein, pahnatin, and stearin, with small quantities 

 of butyrin, capronin, and other fats of lower composition. 

 Lecithin and cholesterin are also present in small amounts, at 



• Halliburton, " The Chemical Constituents of the Body and Food," 

 Sohafer's Text-book of Physiology, vol. i., Edinburgh, 1898. 

 ^ See Halliburton, loc. cit., and Schafer. 



