104 MANUAL OF PHYSIOLOGY. 



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 

 domestic 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- 



FIG. 43. 



&yrSflf?a?wft 



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



cells (a). 



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

 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 



