CHEMICAL BASIS OF THE ANIMAL BODY. 1209 



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 to the milk of 

 other animals. Nevertheless human milk shews, apart from 

 the difference of composition (see 407), certain differences 

 from cow's milk, which are due to a distinct but characteristic 

 difference in the reactions of the casein contained in each. 

 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 acid, yields a very imperfect precipitate which is finely 

 flocculent, almost granular as compared with the compact and 

 coarsely flocculent precipitate yielded by cow's milk. (3) The 

 casein in human milk is, as already stated, very incompletely 

 precipitated by the addition of acids and can only be completely 

 precipitated by saturation with magnesium sulphate. (4) Ca- 

 sein from human milk is less soluble in water than is that of 

 the cow. 



Some very recent researches seem to shew that casein from 

 human milk may not be a nucleo-albumin. 



TJie nucleo-albumin of bile. 



Attention has already been drawn to the fact that the 

 'mucin' of bile is in reality a nucleo-albumin (see p. 1198). 

 It is best prepared as follows. Bile is mixed with five volumes 

 of absolute alcohol and centrifugalized. The precipitate thus 

 obtained is then dissolved in water and the above process 

 repeated two or three times, leaving the substance for as short 

 a period as possible in contact with the alcohol. 



Recent research has shown that the ' mucin ' of normal 

 urine is in reality chiefly a nucleo-albumin, although of course 

 true mucin is present in large amount in catarrhal affections of 

 the bladder. According to some observers the vitellin of egg- 

 yolk is a nucleo-albumin, but this is a matter of some doubt. 



Nucleo-proteids. 



The nucleoalbumins have been regarded as compounds of pseudo- 

 or para-nuclein with proteids. Very recently substances have been 

 prepared which in their general reactions as to precipitability, etc., 

 closely resemble the nucleo-albumins and leave a residue of nuclein 

 on digestion with pepsin, but when they are decomposed by boiling 

 with acids, members of the xanthine series are obtained. From this 

 it appears that they are compounds of proteid with nuclein, not with 

 pseudonuclein, and it is proposed to mark this difference by apply- 

 ing to them the name nucleo-proteid. They have so far been ob- 

 tained chiefly from nuclei and cell protoplasm, but one is said to 

 be present in serum. Some substances with the general reactions of a 



