PROPERTIES AND COMPOSITION OF MILK. 333 



consideration the variations in the quantity of milk secreted by different 

 women, it may be assumed that the daily production is between two and three 

 pints (950 and 1,420 grammes). 



Certain conditions of the female are capable of materially influencing the 

 quantity of milk secreted. It is evident that the secretion is usually some- 

 what increased within the first few months of lactation, when the progressive 

 development of the child demands an increase in the quantity of nourish- 

 ment. If the menstrual function become re-established during lactation, the 

 milk usually is diminished in quantity during the periods, but sometimes it 

 is not affected, either in its quantity or composition. Should the female 

 become pregnant, there generally is a great diminution in the quantity of 

 milk, and that which it secreted is ordinarily regarded as possessing little 

 nutritive power. In obedience to a popular prejudice, apparently well founded, 

 the child is usually taken from the breast as soon as pregnancy is recognized. 

 No marked and constant variations have been observed in the quantity of 

 milk in females of different ages. 



Properties and Composition of Milk. The general appearance and char- 

 acters of ordinary cow's milk are sufficiently familiar. Human milk is nei- 

 ther so white nor so opaque as cow's milk, having ordinarily a slightly 

 bluish tinge. After the secretion has become fully established, the fluid 

 possesses no viscidity and is nearly opaque. It is almost inodorous, of a 

 peculiar soft and sweetish taste, and when perfectly fresh it has a decidedly 

 alkaline reaction. The taste of human milk is sweeter than x that of cow,'s 

 milk. A short time after its discharge from the gland, the reaction of milk 

 becomes faintly acid; but this change takes place more slowly in human 

 milk than in the milk of most of the inferior animals. 



The average specific gravity of human milk is 1032 ; although this is 

 subject to considerable variation, the minimum of eighty-nine observations 

 being 1025, and the maximum, 1046 (Vernois and Becquerel). The observa- 

 tions of most physiological chemists have shown that this average is nearly 

 correct. 



Milk is not coagulated by heat, even after prolonged boiling; but a 

 thin pellicle then forms on the surface, which is probably due to the com- 

 bined action of heat and the atmosphere upon the caseine. Although a 

 small quantity of albumin exists in the milk, this does not coagulate on 

 the surface by the action of the heat, for the scum does not form when the 

 fluid is heated in a vacuum or in an atmosphere of carbon dioxide or of 

 hydrogen. 



When the milk is coagulated by any substance acting upon the caseine 

 or when it coagulates spontaneously it separates into a curd, composed of 

 caseine with most of the fatty particles, and a nearly clear, greenish-yellow 

 serum, called whey. This separation occurs spontaneously at a variable time 

 after the discharge of the milk, taking place much sooner in warm than in 

 cold weather. It is a curious fact that fresh milk frequently is coagulated 

 during a thunder-storm, a phenomenon which has never been satisfactorily 

 explained. 



23 



