ix THE SKIN AND CUTANEOUS GLANDS 505 



animals, particularly in the proportion in which the olein, palmitin, 

 and stearin are present. Human milk contains twice as much 

 olein as it does palmitin and stearin ; in cow's milk the three 

 glycerides are present in approximately equal amounts. In human 

 milk the fat content varies from 2'5-4'3 per cent, while in cow's 

 milk it may amount to 6 per cent. 



The carbohydrates of milk are lactose or milk sugar, which is a 

 disaccharide, and splits on absorption of water into two mono- 

 saccharides, dextrose and (jalactose. By means of an enzyme which 

 is apparently pre-formed in the milk or forms in it on standing, or 

 by the bacteria from the air, lactose undergoes lactic acid ferment- 

 ation, by which milk turns sour and clots, the lactic acid which 

 is formed neutralising the alkali which holds the caseinogen in 

 solution. Lactose does not ferment like glucose on merely adding 

 yeast. But there are special bacteria which produce an alcoholic 

 fermentation with formation of a small amount of lactic acid. 

 This is utilised for the preparation of kumis from the milk of 

 mares, and kefir from that of cows, which are much used in Russia. 

 According to bacteriologists these fermented milks are due to the 

 symbiotic action of two different microbes, a Uastomycete (saccharo- 

 mycete) which produces alcoholic fermentation, and a schizomycete 

 which causes the lactic fermentation of lactose (Gorini). 



Lactose is more plentiful in human milk than in cow's milk : 

 in the former it oscillates between 5'5 and 6'9 per cent, while in 

 cow's milk the average is 4'8 per cent (Konig). Another difference 

 between the two kinds of milk is that human milk is richer in 

 lecithin, and poorer in mineral constituents, especially in lime and 

 phosphoric acid it contains about J the lime and -J the phosphoric 

 acid of cow's milk. 



Of the inorganic salts it is remarkable that potash and 

 phosphoric acid preponderate over sodium or chlorine, as is also 

 the case in the blood corpuscles and the muscles. 



A copious literature in regard to the constituents and 

 physiological properties of milk has recently sprung up ; it is 

 chiefly concerned with the study of milk by new methods learned 

 from physical chemistry and bacteriology. Milk serum has been 

 investigated on the same lines as the various blood serums with 

 regard to its bactericidal, haemolytic, and other properties. No 

 final or even general conclusions have been arrived at, but the 

 specific character of each kind of milk has been demonstrated, 

 confirming the old adage, that " mother's milk is a treasure 

 impossible to replace " (Seiffert). 



This rapid survey of the chemical composition of milk brings 

 out the fundamental fact that this secretion is a chemical 

 elaboration by the secretory cells of the mammary gland, and not 

 a mere transudate of the blood or lymph circulating through it, 

 as was formerly held. Caseinogen is rarely present in the blood ; 



