PHYSIOLOGICAL CHEMISTRY 307 



cipitated by full saturation with magnesium sulphate and half 

 saturation of ammonium sulphate. 



Caseinogen is not coagulated by heat (see table, p. 312). 



THE SCLERO-PROTEINS. 



This comprises the group of proteins formerly termed albuminoids. 

 They are obtained mainly from " the hard " or supporting structures 

 of the body. 



Collagen, the precursor of gelatin, forms the chief constituent of 

 white fibrous tissue and of the organic substance of bone. It also 

 exists in cartilage, where, however, it is mixed with several other 

 bodies (see under mucinoids, p. 308). 



Preparation of Collagen. A piece of tendon is macerated overnight 

 in 1 % caustic alkali to remove other proteins, and then washed with 

 water till alkali free. The resulting mass is collagen. Place a piece of 

 this in a flask and boil it for ten minutes with water which is rendered 

 faintly acid with acetic acid. By this treatment, the collagen is trans- 

 formed into gelatin and, on cooling the solution, it gelatinises. 



Gelatin. This is really the hydride of collagen, the boiling with 

 acidulated water in the above experiment having caused the collagen to 

 take up a molecule of water. Conversely, the gelatin can be recon- 

 verted into collagen by heating it to 130 C., whereby it loses water. 



EXPERIMENT VIII. Divide a solution of gelatin in lukewarm water 

 into five portions, to which apply the following tests: (1) the Biuret 

 reaction : a violet colour is produced. (2), the xantho-proteic reaction : 

 only a slight colouration is produced. (3) the Millon's test : only a 

 slight reddening of the precipitate occurs on boiling. (4) the glyoxylic 

 test : absent or very faint. (5) half saturation with Am 2 S0 4 : salted 

 out. 



The reason why the second, third and fourth tests are not very 

 distinct, is because gelatin does not yield aromatic bodies on decomposi- 

 tion, and both these tests depend on the presence of aromatic bodies. 

 Some varieties of gelatin give these reactions more distinctly than 

 others, but absolutely pure gelatin is said not to give them at all, so 

 that their presence is held to depend on native protein in the gelatin. 



The other sclero-proteins are unimportant. They are Keratin, which 

 occurs in the skin and its appendages and in the medullary sheaths of 

 nerves ; it is remarkable for the large percentage of sulphur which it 

 contains ; Elastin, which is found in elastic fibres, and contains a very 

 small percentage of sulphur, but a considerable amount of aromatic 

 bodies. 



All these sclero-proteins except keratin yield glycin as their chief 



