THE GENERAL COMPOSITION OF MILK 7 



kept for milking are usually pregnant during the later months of 

 lactation. 



Cows' milk, therefore, does not appear to undergo any loss of 

 nutritive value towards the end of lactation, but in women there 

 is a tendency for this to occur. 



Milk in Relation to Diet. Much work has been done upon 

 this subject, though many of the statements which have been 

 made with regard to it are not based upon fact. 



Fundamentally the question is one of the mode of action of the 

 mammary gland, and must be considered in relation to the quality 

 and quantity of the food, both upon the total amount and upon 

 the individual constituents of the milk. 



1. Quantity of Food. When the total amount of food taken 

 falls below the minimal daily requirements of the organism, either 

 as a whole or in any one of the necessary constituents, the milk 

 tends rather to decrease in total amount, than to show a deficit 

 of that particular constituent which is supplied in insufficient 

 quantity in the diet. 



This is illustrated by the work of Woll, who found that the 

 milk of cows which were receiving too short an allowance of food 

 decreased in amount. 



Fingerling found that goats receiving a diet in which there was 

 too little lime and phosphorus gave less milk until a sufficient 

 amount of these substances was restored. 



Any factor acting disadvantageously on the general health or 

 economy of the organism tends to produce a fall in the amount of 

 milk given. This applies also to the human species. 



2. Addition of Special Substances to the Food. Numerous 

 attempts have been made to increase special constituents of the 

 milk by giving additional amounts of these constituents in the 

 dietary, both of cows and of women. 



One or two observers have claimed a partial success, but the 

 most careful observations show that no appreciable increase in 

 any one constituent can be obtained by its addition to the food, 

 even in large quantities. This has been attempted for fat and 

 sugar, and for combinations of lime, iron, and phosphorus. The 

 experiments are considered fully in the chapters dealing with these 

 constituents. 



The amount of fat produced varies greatly with the same cow 

 at different times. 



Variations in the Fat Content. The fat content is low in 

 the milk obtained when the gland is full. As milking proceeds, 

 the fat content rises and is highest in the end-milk, or strippings. 

 In estimating the total percentage of fat in either human or cows' 

 milk it is essential that a sample of the milk given in the whole 

 period of milking should be taken. 



The fat content also varies within the twenty-four hours, being 

 apparently highest towards mid-day. 



