COMPOSITION OF MILK. 381 



The relative quantity of the" several ingredients of milk varies 

 with the kind of diet used. A vegetable diet increases the per- 

 centage of sugar, but diminishes that of the other constituents, and 

 also the general quantity of milk. A rich meat diet increases 

 both the general quantity and the percentage of fats and proteids. 



The quantity of milk secreted in the twenty-four hours is 

 extremely variable in different individuals and under different 

 circumstances in the same individual ; the average in general 

 being about two pints. 



The amount of the different materials in milk varies under the 

 following rules. The proportion of albumin increases as the milk 

 sugar decreases, and the fat remains the same as the period of 

 lactation advances. The portions of milk last drawn are much 

 richer in fats than that which is first taken from the gland. In 

 the evening the milk is richer in fat than in the morning. The 

 general amount of solid constituents falls, up to the age of thirty 

 years, then gains slightly until thirty-five, after which age the 

 milk becomes decidedly thinner. These points should be borne 

 in mind in the selection of a wet-nurse. 



Mode of secretion. Although the blood contains albumins, fats, 

 etc., very similar to those which form the solid parts of the milk, 

 we have good reason for thinking that the constituents of milk are 

 not merely extracted from the blood, but that the manufacture 

 of this highly valuable secretion is due to the activity of the 

 protoplasm of the gland cells, which construct the various ingre- 

 dients out of their substance. 



It has been suggested, as a simple explanation of the formation 

 of milk, that the cells undergo fatty degeneration, and the secre- 

 tion is then only the debris of the degenerated cells. 



Some facts support this view. In the first place the ingredients 

 one finds in milk are suggestive of, though not indentical with, 

 the chemical materials which can be obtained from protoplasm 

 by chemical disintegration, rather than of any group of substances 

 found in the blood. Further we find that the so-called colostrum 

 corpuscles, which appear to be secreting cells filled with fat parti- 

 cles, are thrown off from the gland in the early stages of the 

 secretion, and appear in numbers in the milk. 



