504 PHYSIOLOGY CHAP. 



odour, and with a peculiar, sweetish taste. Its specific gravity 

 varies from 1-Q25 to 1-034. In the fresh state it gives the same 

 reaction as cow's or goat's milk, i.e. slightly alkaline or arnphoteric, 

 while the milk of carnivora is faintly acid. If left to stand, a 

 yellowish layer, the cream, forms on the surface. In time it turns 

 sour or clots, by lactic fermentation. 



The percentage composition of human milk is very variable. 

 From the average of a great number of analyses carried out by 

 Pfeiffer and Leed, it contains 10'8-13 per cent solid constituents. 

 These are organic and inorganic. The organic are represented by 

 the three principal groups of food-stuffs proteins, fats, and carbo- 

 hydrates. 



The proteins are present to an amount of l'6-2'5 per cent. 

 The chief protein is caseinogen, a phosphorated substance belonging 

 to the nucleo-proteins, which has the property of clotting when 

 acted on by chyrnosin or rennin (Chapter II. p. 118). There is 

 further a lact-albumin which closely resembles serum-albumin, 

 as well as other proteins, e.g. lacto-globulin, lacto-mucin, etc. 



Some authors claim also to have detected a small amount of 

 protease and peptone in human milk, which has been disputed by 

 Dogiel and Hofmeister. If milk is filtered at high pressure 

 through a porous filter, the caseinogen is held back, while the 

 albumin passes, and can be precipitated by heating the filtrate. 

 It is not definitely known whether the skin which forms on the 

 surface of milk on boiling really represents protein contained in 

 the milk. It forms again as often as the first skin is removed. 

 It is probably an albumin or globulin formed during heating by 

 alteration of the caseinogen. 



The fat of milk forms a perfect emulsion, the droplets as seen 

 under the microscope being 2-5 //, in diameter, the so-called milk 

 globules. The stability of this natural emulsion depends on the 

 fact that each fat-globule is surrounded by a layer of caseinogen 

 solution which prevents the drops from fusing. If milk is treated 

 with caustic soda, and then with ether, the caseinogen is precipi- 

 tated, and the fat runs into large drops. 



When milk is allowed to stand, the fat-globules, which are 

 lighter, mount to the surface and form the cream, and if this be 

 skimmed off the milk loses a large amount of its butter. The 

 opacity of milk is due to the fatty globules. When the cream is 

 removed, the milk that remains has a higher specific gravity than 

 the natural milk. By centrifuging, milk can be almost entirely 

 deprived of its fat and becomes semi-transparent. 



Milk-fat is a mixture of all the animal fats. Palmitin, stearin, 

 and olein preponderate ; but they are always accompanied by 

 small quantities of butyrin and other glycerides. Generally 

 speaking, it may be said that the fat of milk resembles that of 

 adipose tissue ; its composition varies like the latter in different 



