MILK. 



95 



Milk varies both in the amount of solids in solution, and fat, 

 according to the age and general condition of the animal, period 

 of lactation, time of day, etc. 



Since human milk is much poorer in proteid, fat, and salts (see 

 Table), and richer in sugar, than that of the cow and other do- 

 mestic animals, it is necessary to dilute the latter with water, and 

 add sugar, when it is substituted for human milk in feeding infants. 



The great value of milk as nutriment depends upon the fact 

 that it contains every class of food-stuff, viz., proteids, fat, carbo- 

 hydrates, salts, and water, in the proportion demanded by the 



FIG. 43. 



Microscopic appearance of milk in the early stage of lactation, showing 

 colostrum cells (a). 



economy ; the salts in milk being those required for building up 

 the bones of the infant, viz., phosphates and carbonates of lime, etc. 



The normal variations in these proportions are not very great, 

 but as artificial modifications of the percentage of water are com- 

 mon, a knowledge of the method of testing the purity of milk is 

 necessary. 



Milk Tests. The specific gravity of milk gives an easy measure 

 of the solids in solution, but unfortunately it gives no estimate of 

 the amount of fat suspended in the emulsion. Therefore, to test 

 milk adequately two methods must be employed : one to estimate 



