26 PHYSIOLOGY CHAP. 



and more particularly Fischer and his school, has led within the 

 last few years to important results. While these do not as yet 

 account fully for all the different chemical units which build 

 up the complex protein molecule, they represent a great advance 

 in this direction. A brief review of this work, which has 

 profoundly modified most of the theories previously held by 

 physiologists, is essential. 



The analytical method is invariably employed in investigating 

 the chemical structure of highly complex bodies. The complex 

 substance must be decomposed and split up into its simpler 

 constituents, i.e. into the units of which it is built up. For 

 proteins, hydrolytic cleavage is the method of artificial decom- 

 position that gives the best results, i.e. decomposition with 

 absorption of molecules of water. This hydrolytic cleavage or 

 hydrolysis of proteins may take place by the prolonged action 



(a) Of mineral acids, by boiling the protein with concentrated 

 hydrochloric acid or 25 per cent sulphuric acid for twelve to fifteen 

 hours (method proposed by Fischer, and generally used in his 

 laboratory) ; 



(6) Of alkalies ; and 

 (c) Of proteoly tic ferments. 



The most important result of all the researches in to 1 hydrolytic 

 cleavage up to the present time is that even the most unlike 

 proteins have, among themselves, a very similar constitution, 

 judging from the end products. These are invariably the same, 

 no matter what process of hydrolytic decomposition is employed. 

 It was formerly believed that one essential difference only existed 

 between hydrolysis by the proteoly tic ferments and that by acids 

 and alkalies': the disintegrating action of the ferments was 

 supposed to be more gradual, since before reaching the final 

 products of cleavage, which no longer yield the biuret reaction, 

 those intermediate cleavage products were obtained which are 

 known by the name of proteoses and peptones (of which we shall 

 treat fully in the physiology of Digestion). These products were 

 supposed not to appear in the cleavage effected by strong acids 

 and bases, but complex products with similar properties have now 

 been isolated and studied by Fischer and Abderhalden. Some of 

 the final products of cleavage are still unknown ; most of them, 

 however, have been isolated and identified. They are the organic 

 ' compounds known as amino-acids, or organic acids, in the molecule 

 of which an amino-group (NH 2 ) is substituted for one or more 

 atoms of hydrogen ; our knowledge of the various amino-acids that 

 arise from proteins by cleavage is mainly due to Fischer, who has 

 devised new methods for their isolation and recognition. The 

 number and variety of the amino-acids at present isolated is shown 

 in the following table of Abderhalden : 



