CHEMICAL BASIS OP THE ANIMAL BODY. 23 



In addition to these, according to the older views, milk, even when 

 quite fresh, frequently contained traces of a proteid which since it 

 yielded the biuret reaction was usually spoken of as a peptone, and was 

 by some observers called ' lactoprotein ' \ It was stated to increase in 

 amount in the milk on standing for some time and more especially if 

 warmed to 40, and to be considerably increased during the clotting 

 induced by rennin 2 . Recent researches have however shewn that 

 perfectly fresh milk contains no substance which yields a biuret 

 reaction, its presence being due to its formation during the processes 

 employed in its separation 3 . If the milk undergoes an acid (lactic) 

 fermentation a substance may now be obtained from it which yields a 

 biuret reaction but is not a true peptone but a primary albumose. 



When milk is kept for some time at a temperature above 50 and 

 below its boiling point, a firm skin is formed over its surface composed 

 largely of casein 4 . Its formation is not to be regarded as being specially 

 characteristic of milk, for pure casein dissolved in dilute alkalis exhibits 

 the same phenomenon, as also do alkali-albumin, chondrin, gelatin and 

 the nitrate from 1 p.c. starch when it is concentrated on a water-bath. 

 Its formation is probably due to the rate of evaporation from the surface 

 of the milk being more rapid than the fluid diffusion into the upper 

 layer 5 ; and in accordance with this it is found that its appearance is 

 considerably facilitated by blowing a rapid stream of air or any in- 

 different gas, such as carbonic oxide, over the surface of the warmed 

 milk. 



Our knowledge of the chemical properties of casein as already 

 described is based entirely upon researches carried out upon the 

 milk of cows. There is no reason to suppose that all that has been 

 said does not apply equally well to the milk of other animals. Never- 

 theless human milk shews, apart from the difference of composition 

 (see 513), certain differences from cow's milk, which are due to a 

 distinct but characteristic difference in the reactions of the casein 

 contained in each 6 . This is shewn by the following facts. (1) Human 

 milk clots less firmly than cow's milk and sometimes not at all with 

 rennin. (2) The casein in human milk, on the addition of acetic 



1 Hammarsten, Maly's Bericht. Bd. vi. (1876), S. 13. Palm (Eussian), Ibid. Bd. 

 xvi. (1886), S. 143. For other references see Halliburton, loc. cit. p. 459. 



2 Hoppe-Seyler, Handbuch d. phys.-path. chem. Anal. 1883, S. 480. 



3 Neumister, Zt.f. Biol. Bd. xxiv. (1888), S. 280. 



4 Sembritzky, Pfliiger's Arch. Bd. xxxvn. (1885), S. 460. See also Maly's Ber. 

 Bd. xvn. (1887), S. 157. 



5 Hoppe-Seyler, Virchow's Arch. Bd. xvn. (1859), S. 420. 



6 Simon, Animal Chemistry (Sydenham Soc.), Vol. n. 1846, p. 53. Also in "Die 

 Frauenmilch u. s. w." Berlin 1838. Biedert, Virchow's Arch. Bd. LX. (1874), S. 352. 

 Biel, see Abst. in Maly's Ber. Bd. iv. (1874), S. 166. Langgaard, Virchow's Arch. 

 Bd. LXV. (1875), S. 352. 



