VI ABSORPTION FROM THE INTESTINES 267 



soluble and highly diffusible. Now we know that if such 

 a solution is separated by a thin membrane from a solution 

 of ordinary non-diffusible proteins, there would be a rapid 

 transmission of the diffusible substances through and 

 across the membrane. The conditions necessary for such 

 a process are evidently present in the intestines where 

 the solution in its interior is separated by what is practi- 

 cally a thin membrane from the (albuminous) blood in the 

 capillaries just below tlie epithelial cells. It is thus very 

 tempting to suppose that the absorption of amino-acids 

 and sugars (also of salts) is the result of their diffusibility 

 and of the conditions to which they are exposed. And 

 indeed loithin certain limits this view is correct. But it 

 does not by any means explain the whole process. For 

 if substances of differing diffusibilities be placed in the 

 intestines it is not found that the most diffusible sub- 

 stance is necessarily absorbed the fastest. In fact we 

 find that the details of the absorption are in many ways 

 so peculiar that we must again, as in the case of the fats, 

 look to the living epithelial cells of the villi as determining 

 and completely controlling the process, which is thus 

 partly physical but chiefly due to the special activity of 

 cells. 



The fats pass, as already stated, into the lacteals and 

 thence through the lymphatic vessels and thoracic duct 

 into the blood. Amino-acids and sugar, on the other 

 hand, appear to be taken up by the capillary blood-vessels 

 of the villus, so that very little if any of them gets to 

 the lacteal radicle. From the capillaries of the villi the 

 amino-acids and sugar are then carried along the portal 

 vein to the liver, where they probably undergo some 

 further change. So that while the fat reaches the blood 

 very little changed, the cleavage products of the proteins 

 and the sugars though also taking a roundabout course, 

 viz., by the liver, are probably altered before they are 

 thrown into the general blood-stream ; for the portal 

 blood in which they are carried is acted upon by the 



