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: hut 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 amido-acids, and a substance known as 
antipeptone, but long before these products are finally reached, soluble 
bodies are formed which can be shown to he 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 tw^o 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 lates 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. — 2 1 
