RA TE OF ENZYMIC A CTION. 3 2 1 



the osmotic pressure of the dissolved products. This water may, of 

 course, be removed by subsequent evaporation at a low temperature, to 

 avoid injuring the ferment, and again dialysing; but practically the 

 diffusive power of the usual products of digestion is so low as to render 

 a process of alternate dialysis and evaporation a tedious and almost im- 

 possible method of freeing the solution completely of the products of 

 digestion. This action of the accumulated products of digestion renders 

 all digestive experiments carried out in glass essentially different from 

 those which go on within the alimentary canal, where the products of 

 digestion are removed as fast as they are formed. Not only must the 

 natural process run more quickly, but there is no reason for assuming 

 that it will even run qualitatively along the same lines. To take as an 

 example the tryptic digestion of proteids. There are formed, as we shall 

 see later, as end products, certain ainido-acids, and a substance known as 

 antipeptone, but long before these products are finally reached, soluble 

 bodies are formed which can be shown to be capable of absorption and 

 assimilation by the epithelial cells lining the intestine. 



Digestion experiments in vitro teach us the effects of digestion alone, 

 sundered from its constant companion in the natural process absorption ; 

 and no perfect method has hitherto been devised whereby the effects of 

 these two processes working in conjunction can be demonstrated. In 

 the animal body the pure effect of digestion and absorption cannot be 

 observed by studying the chemical composition of the intestinal contents 

 and that of the contents of the channels of absorption, because the pro- 

 ducts of digestion are not merely absorbed by the lining cells, but are 

 profoundly modified by them in the process. Nor can the combined 

 effect of digestion and absorption be studied in perfection by any known 

 method of digestion and dialysis, because no artificial dialyser bears any 

 but a very remote resemblance to the living intestine. A dialyser of 

 parchment paper not only removes diffusible substances with infinite 

 slowness compared with the intestinal epithelium, 1 but it also acts on 

 purely physical laws, diffusion taking place at rates directly proportional 

 to the diffusion coefficients of the substances involved ; while the living 

 epithelium takes up with great avidity soluble substances which do not 

 diffuse at all, and absolutely refuses passage to other very diffusible sub- 

 stances, such as soluble salts of iron. That is to say, absorption by the 

 cell is selective, being governed, indeed, by fixed and definite laws, pro- 

 bably purely physical and chemical at bottom, but profoundly modified 

 by the action of living protoplasm. 2 



The effects of removal of products of digestion by dialysis has been studied 

 by Sheridan Lea, 3 in the case of starch digestion by ptyalin, and proteid 

 digestion by trypsin. The rapidity of dialysis was increased by mechanically 

 raising and lowering the dialysing tube, and the rate of digestion and nature 

 of products formed were compared with those in an exactly similar experiment 

 arranged in a glass vessel. It was found (1) that the speed of digestion was 

 in all cases increased, and (2) that before the process came to a standstill 

 much more conversion took place than it was possible to attain to in glass, 

 although complete conversion never took place in either case ; these differences 

 were in every case more marked when concentrated solutions of the material 

 to be digested were used, showing that the slower digestion and earlier stoppage 



1 Heidenhain, Arch. f. d. gcs. PhysioL, Bonn, 1888, Suppl. Heft, Bd. xliii. S. 60. 



2 For a further consideration of this subject, see " Proteid Absorption," p. 430. 



3 Journ. PhysioL, Cambridge and London, 1890, vol. xi. p. 226. 



VOL. I. 21 



