CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 421 



than elsewhere (cf. 244) this current of transudation from 

 within the capillary to without is accompanied by a reverse 

 current from without to within. The diffusible substances in 

 question pass from the intestine through the layer of epithelium 

 cells, through the attenuated reticular lymph-space between the 

 basement membrane and the capillary wall, and through the 

 capillary wall into the blood current. Their passage consists 

 of two stages ; that through the epithelium cells from the intes- 

 tine to the lymph-space, and that from the lymph-space into the 

 blood vessels. These two stages may be expected to differ, 

 seeing that the structures concerned are different ; but we may 

 at first consider them as one, and speak of the passage from 

 the intestine into the blood as a single 'event. 



In speaking of these substances as diffusible we are using 

 the terms in reference to the well-known passage of such sub- 

 stances through thin membranes or porous partitions. When 

 a strong solution of sugar or of common salt is separated by a 

 thin membrane (vegetable parchment, dead urinary bladder, 

 dead intestine, &c.) from a weak solution of sugar or of salt, 

 the sugar or salt passes with a certain rapidity from the stronger 

 to the weaker solution, and water passes from the weaker solu- 

 tion to the stronger ; if, to begin with, simple water be substi- 

 tuted for the weaker solution the effect is at first still more 

 striking. Peptone passes in the same manner but as we have 

 seen much more slowly. The process is spoken of as a physi- 

 cal one since it is not accompanied, necessarily, by any chemical 

 change in the diffusing substance, nor is there any necessary 

 change in the membrane or partition. The rate at which a 

 substance diffuses, and the total amount of diffusion which can 

 take place, are determined by certain qualities of the substance 

 (which we may call physical though they depend on the chem- 

 ical nature of the substance) in relation to certain qualities of 

 the membrane ; thus two salts may diffuse through the same 

 membrane at different rates, with different rates in the associ- 

 ated current of water, the osmotic current as it is called, from 

 the weaker to the stronger solution ; and the same substance 

 may pass at different rates through different membranes. By 

 a number of observations, in which various substances in solu- 

 tion and several known membranes or partitions have been 

 employed, a certain number of " laws of diffusion " have been 

 established. 



Now if by the statement that diffusible substances pass by 

 diffusion into the blood-capillaries of the intestine we are led 

 to expect that the passage takes place exactly according to the 

 laws established by observations on ordinary membranes we 

 should be led into error ; for the disappearance of these sub- 

 stances from the interior of the intestine does not take place 

 according to the laws which regulate their disappearance from 



