384 MANUAL OF PHYSIOLOGY. 



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 ingredients out of their substance. 



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

 of milk, that the cells undergo fatty degeneration, anji 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 identical 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 par- 

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

 secretion, and appear in numbers in the milk. 



But these colostrum corpuscles soon cease to be thrown off in 

 the secretion, and the saccules of the glands during active lacta- 

 tion do not contain any sign of the debris of cast-off cells, or any 



