PROPERTIES AND COMPOSITION OF THE MILK. 371 



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. Authors have not noted any marked and constant 

 variations in the quantity of milk in females of different ages. 



Properties and Composition of the Milk. 



The general appearance and characters of ordinary cow's milk are sufficiently famil- 

 iar and may serve as a standard for comparison with the milk of the human female. 

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

 bluish tinge. The milk of different healthy women presents some variation in this 

 regard. After the secretion has become fully established, the fluid possesses no visicidty 

 and is nearly opaque. It is almost inodorous, of a peculiar soft and sweetish taste, 

 and, when perfectly fresh, 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 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, according to Vernois and Becquerel, is 

 1032 ; although this is subject to considerable variation, the minimum of eighty-nine 

 observations being 1025, and the maximum, 1046. 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, which is probably due to the combined action of heat and the 

 atmosphere upon the caseine. Although a small quantity of albumen 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 an atmosphere of carbonic acid or of hydrogen, or in a 

 vacuum. 



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 more rapidly in warm than in cold weather. It is a curious fact that fresh milk is 

 frequently coagulated during a thunder-storm, a phenomenon which has never been sat- 

 isfactorily explained. 



On being allowed to stand for a short time, the 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 fluid, called cream, leaving the lower portion 

 poorer in globules and of a decidedly bluish tint. In healthy milk, the stratum of cream 

 forms from 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 transparent as the whey. A 

 very good method of testing the richness of milk is by the use of little graduated glasses, 

 called lactometers, by which we can measure the thickness of the layer of cream. The 

 specific gravity of the cream from milk of the average specific gravity of 1032 is about 

 1024. The specific gravity of skim-milk is about 1034. 



Microscopical Characters of tJfe Milk. If a drop of milk be examined with a magni- 

 fying power of from three hundred to six hundred diameters, the cause of its opacity 

 will be apparent. It contains an immense number of minute globules, of great refractive 

 power, held in suspension in a clear fluid. These are known under the name of milk- 

 globules and are composed of margarine, oleine, and a fatty matter, peculiar to milk, 

 called butyrine. In human milk the particles are perfectly spherical ; but in cow's milk 

 they are often polyhedric from mutual compression. This difference is due to the softer 

 consistence of the butter in human milk, the globules containing a much larger propor- 

 tion of oleine ; and, if cow's milk be warmed, the particles also assume a spherical form. 



