296 SECRETION 



ence to a popular prejudice apparently well founded the child usu- 

 ally is taken from the breast so 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 

 characters of ordinary cow's milk are sufficiently familiar. Human 

 milk is neither so white nor so opaque as cow's milk, having ordinarily 

 a slightly bluish tinge. After secretion has become fully established, 

 milk possesses no viscidity and is nearly opaque. It is almost inodor- 

 ous, 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 that of cow's milk. A short time after its discharge from the 

 gland, the reaction of milk becomes first amphoteric and afterward 

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

 than in the milk of most of the inferior anim'als. 



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

 is subject to considerable variation, the minimum of eighty-nine obser- 

 vations being 1025, and the maximum, 1046 (Vernois and Becquerel). 

 The observations 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. 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 liquid is heated 

 in a vacuum or in an atmosphere of carbon dioxide or of hydrogen. 



When milk is coagulated by any substance acting on the casein or 

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

 casein with most of the fatty particles, and a nearly clear, greenish yel- 

 low 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 fre- 

 quently is coagulated during a thunder storm, which probably is due to 

 electric action on the ions that enter into the constitution of the casein 

 molecules. 



On being allowed to stand for a short time, milk separates, without 

 coagulating, into two tolerably distinct portions. A large proportion of 

 the globules rises to the top, forming a yellowish white and very opaque 

 liquid, called cream, leaving the lower portion poorer in globules and 

 of a decidedly bluish tint. In healthy milk the stratum of cream forms 

 one-fifth to one-third of the entire mass of the milk. In the human 

 subject the skim-milk is not white and opaque, but it is nearly as trans- 

 parent as whey. The specific gravity of the cream from milk of the 



