CHAP, v.] NUTRITION. hi 



shewn in a succeeding section, that proteid food increases, and 

 fatty food diminishes, the metabolism of the body ; and we have 

 already discussed the manner in which proteid material may give 

 rise to fat. A bitch fed on meat for a given period ; gave off more 

 fat in her milk than she could possibly have taken in her food, and 

 that too while she was gaining in weight, so that she could not 

 have supplied the mammary gland with fat at the expense of 

 fat previously existing in her body; she apparently obtained it 

 ultimately from the proteids of her food. In the 'ripening* of 

 cheese we have a similar conversion of proteids into fat, though 

 this appears to be effected by the agency of certain fungi. 

 We have also indications that the casein is, like the fat, formed in 

 the cells of gland, and not simply separated from the blood. When 

 the action of the cell is imperfect, as at the beginning or end of 

 lactation, the albumin in milk is in excess of the casein; but as 

 long as the cell possesses its proper activity the formation of casein 

 becomes prominent. When milk is kept at 35 C. out of the body 

 the casein is said to be increased at the expense of the albumin, 

 but the substance thus formed out of albumin is probably not real 

 casein but ordinary alkali-albumin, produced by the action of the 

 alkalis of the milk on the albumin. It has been suggested 

 that the casein may be formed by a splitting up of albumin by 

 some fermentative process, and a ferment capable of effecting this 

 is said to have been isolated. That the milk-sugar also is formed in 

 and by the protoplasm of the cell, is indicated by the facts that 

 it is found in no other part of the body, and that its presence in 

 milk is not dependent on carbohydrate food, for it is main- 

 tained in abundance in the milk of carnivora when these are fed 

 exclusively on meat, as free as possible from any kind of sugar or 

 glycogen. We thus have evidence in the mammary gland of the 

 formation, by the direct metabolic activity of the secreting cell, of 

 the representatives of the three great classes of food-stuffs, proteids, 

 fats and carbohydrates, out of the comprehensive substance proto- 

 plasm. And what we see taking place in the ^oammary cell is 

 probably a picture of what is going on in all protoplasmic bodies. 

 If the fat of the milk were not ejected from the mammary cell, the 

 mammary gland would become a mass of adipose tissue, especially 

 if, by a slight change in the metabolism, the production of fat were 

 exalted at the expense of the production of casein or milk-sugar. If, 

 again, by a similar slight change the milk-sugar were accumulated 

 rather than the fat or proteid, we should have a result which, by an 

 easy step, would bring us to glycogenic tissue. And, lastly, if the 

 proteid accumulation were greater than the fatty, or the saccharine, 

 these being earned off in some way or other, we should have an 

 image of the nutrition of an ordinary nitrogenous tissue. 



That both the secretion and ejection of milk are under the 

 control of the nervous system is shewn by common experience, 

 but the exact nervous mechanism has not yet been fully worked 



