NITROGENOUS COMPOUNDS 77 



no sulphur at all, and they contain less carbon than the typical 

 compounds. The percentage of nitrogen is higher, owing to 

 the preponderance of arginin in their composition. The last- 

 mentioned substance forms from 50 to 80 per cent, of the 

 primary dissociation products of the protamines. 



Sclero-proteins. Until comparatively recent times it was 

 considered doubtful whether these substances should be 

 regarded as true proteins or merely as allied bodies. The 

 term " albuminoids " was exclusively applied to them by some 

 authors to emphasise this difference. They occur chiefly in 

 the external coverings, supporting structures and fibrous tissues 

 of animal bodies, e.g. skin, hair, cartilage, bones, tendons, etc. 

 In the native condition they are all very hard or tough, and 

 insoluble. They are, therefore, more difficult to investigate 

 than the more readily soluble compounds previously con- 

 sidered. It has been found, however, that they give the same 

 colour reactions, form similar salts with acids and bases, are 

 resolved by ferments and by hydrolytic treatment into pro- 

 teoses, peptones, and, finally, into amino-acids. In short, the 

 differences between the sclero-proteins and the other members 

 of the group are mainly superficial. 



Collagen and keratin are amongst the most important 

 sclero-proteins ; and these differ so widely in point of composi- 

 tion and properties that it is doubtful whether they should 

 really be included in the same group. For example, the 

 former contains no sulphur, whereas the latter contains about 

 5 per cent, of that ingredient. 



Collagen is the name given to the substance which forms 

 the ground matrix of bones, cartilage, and fibrous tissues 

 (tendons, etc.). It can be obtained by exposing these to 

 tryptic digestion, by which the other albuminoids are rendered 

 soluble ; the collagen is left unchanged. On prolonged boil- 

 ing with water, or more rapidly in the presence of acids, it 

 undergoes a change probably a process of hydra tion and is 

 converted into gelatine, sometimes called glutin, or, in its 

 impure state, glue. 



Gelatine is soluble in hot water, but insoluble in cold, and 

 solutions containing more than about one per cent, of the 



