422 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK 11. 



one side of an ordinary diffusion septum. This can be ascer- 

 tained by introducing solutions of the substances, of various 

 strength, into a loop of intestine, isolated in the living animal 

 by the method described in 211, and watching their disap- 

 pearance by analysis of the contents of the loop. For instance, 

 sodium sulphate passes through an ordinary diffusion septum 

 with a rapidity rather greater than that of dextrose, whereas 

 dextrose disappears from the intestine distinctly more rapidly 

 than sodium sulphate , peptone which diffuses very slowly 

 indeed through an ordinary diffusion septum disappears rap- 

 idly (though not so rapidly as dextrose) from the intestine , 

 and when the details of the disappearance from the intestine of 

 weak solutions of two salts which diffuse through an ordinary 

 membrane at different rates, which have as it is said different 

 osmotic equivalents, are studied, these details are quite differ- 

 ent from those of ordinary diffusion. The more the matter is 

 studied the more decidedly apparent becomes the difference be- 

 tween ordinary diffusion and the absorption of diffusible sub- 

 stances from the intestine. 



Two opposite processes are carried on by the wall of the 

 alimentary canal : on the one hand material is transferred 

 from the blood stream to the inside of the canal in the form 

 of the several digestible juices, and on the other hand di- 

 gested material is transferred from the inside of the canal to 

 the blood stream. The former process we without hesitation 

 regard as the work of the epithelium cells forming the lining 

 of the canal, whether the cells lie as in the gastric and Lieber- 

 kiihn's glands in the thickness of the wall of the canal, or as in 

 the pancreas are removed to some distance from it ; we call the 

 process 'secretion.' And the evidence goes increasingly to 

 show that the other process is also the work of epithelium cells, 

 that the two processes are in the main alike save that the cur- 

 rent resulting from the activity of the cells is in opposite direc- 

 tions in the two cases. We might in fact venture to speak of 

 absorption from the canal as an inverted secretion. And we 

 may regard as wholly secondary the fact that in the small intes- 

 tine the cells of Lieberkiihn appear at least to be chiefly de- 

 voted to ordinary secretion, and those of the villi to the inverted 

 secretion. We may further consider the conversion of food 

 into diffusible substances as in the main a means by which the 

 material of the food enters more readily into the substance of 

 the epithelial cell and so is placed more easily within its grasp ; 

 and we have seen the material, having thus entered, appears in 

 certain cases to undergo an immediate change, the maltose being 

 converted into dextrose, and the peptone into some other pro- 

 teid, diffusibility being in the latter case lost. 



Such a view, however, of absorption as a kind of secretion 

 must in the first instance be confined to the first step of the 



