i7o SCIENCE PROGRESS 



The considerations so far presented will show that our 

 knowledge of assimilative processes is yet far from complete. 

 They will show also, however, how far we have travelled from 

 the original position of Liebig. Instead of the diet proteid 

 marching intact into the tissue cell, it seems probable that the 

 animal body treats the proteid presented to it as the plant treats 

 its reserve proteid, that is, as a convenient store of amino-acids, 

 on the liberation of which depends the selective utilisation 

 which is so necessary. 



If our knowledge of the assimilation of the nitrogenous 

 foodstuffs is yet very far from complete, we know less about 

 what happens to proteids in the tissues themselves. In spite 

 ■of some difference of opinion on the point, recent research seems 

 to me to emphasise more than ever the view that when food 

 supply and oxygen supply are normal, the nitrogenous basis 

 of living tissues exhibits marked stability. At the same time, 

 however, it is, of course, certain that degradation of the tissue 

 proteids is always going on. Upon what lines and as the 

 result of what processes does this occur? 



The most enlightening research carried out in this connection 

 during recent years has concerned itself with what is called 

 " autolysis " (auto-digestion) of the tissues. When an organ 

 removed from the body is placed under antiseptic conditions, 

 chemical processes within it do not cease. Breakdown continues 

 or is exaggerated [Salkowski]. If we chose aseptic rather than 

 antiseptic conditions, these processes of the surviving life of the 

 tissue are yet much more in evidence. The antiseptic, meant 

 to prevent the intrusion of bacteria, also interferes with the 

 activity of the normal agents acting in the tissue, but when 

 we avoid contamination by aseptic removal of the organ to 

 be studied, the survival processes are found to be rapid and 

 important [Jacoby, Magnus-Levy, and many others]. 



Among other changes it is found that during such survival 

 life the proteids present in the organ concerned undergo diges- 

 tion and yield products for the most part identical with those 

 formed by ordinary digestion in the bowel. The tissue is said 

 to undergo autolysis. We recognise, indeed, that every cell has 

 the power of producing within itself proteoclastic (as well as 

 many other) enzymes. 



It is difficult to avoid the belief that such intracellular 

 ferments play their parts in the normal physiological life of 



