CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 549 



capillaries supply the lymph which helps to fill up the labyrinth 

 of the reticulum and the lacteal chamber. But to a much greater 

 extent than elsewhere (cf. 302) 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 base- 

 ment 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 intestine 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 

 term in reference to the well-known passage of such substances 

 through thin membranes or porous partitions. When a strong 

 solution of sugar or of common salt is separated by a thin mem- 

 brane (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 solution to the 

 stronger; if, to begin with, simple water be substituted 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 physical one since it is not accom- 

 panied, necessarily, by any chemical change in the diffusing 

 substance, ncr 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 chemical 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 associated 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 solution 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 substances from 

 the interior of the intestine does not take place according to the 



