782 PHYSIOLOGY 



propounded whether the greater part of the substances, which enter the 

 blood plasma from the gut, pass between the cells or through the cells. Water 

 could of course pass in either way. Most of the inorganic salts such as 

 sodium chloride, as well as the very important constituents of the food, 

 the sugars, are insoluble in lipoids and would have to pass between the 

 cells. When the question is investigated by the use of dyestuffs, soluble 

 or insoluble in lipoids, it is found that the lipoid-soluble dyestuffs, such 

 as neutral red or toluidin blue, pass into the cells, whereas the dyestuffs 

 which are insoluble in such substances pass into the intercellular spaces. 

 Too much stress however must not be laid on these experiments. All these 

 dyestuffs are abnormal so far as the body is concerned. We cannot imagine 

 that, at any time in the course of evolution of the properties of the intestinal 

 epithelium, the cells were ever presented with or had to discriminate between 

 different dyestuffs. The fact that absorption of these dyestuffs is deter- 

 mined by the physical conditions of the cell membrane is no proof that the 

 absorption of the normal food constituents is determined in the same way. 

 In fact, it is quite legitimate to assume that the lipoid membrane or limiting 

 layer round every cell has as its main office, not the regulation of the access 

 of foodstuffs to the cell, but its protection from any of the foodstuffs which 

 it does not require for its metabolism. If it were not for such a membrane 

 the assimilation of a salt would be determined entirely by its concentration 

 in the immediate surroundings of the cell, whereas we know that assimila- 

 tion by any living organism, whether uni- or multi-cellular, is regulated 

 in the first place by the activity of the organism itself. According to this 

 activity and the needs thereby induced, the uptake of food material may 

 be large or small whatever its concentration in the surrounding medium. 

 It would indeed be strange that the whole absorbing surface of the intestine 

 should be covered by a membrane, of which the greater part was useless 

 for the absorption of the common foodstuffs, as would be the case if these 

 could only penetrate the membrane by the narrow chinks between the 

 cells. It seems more probable that the absorption of the different food- 

 stuffs, and probably also of the normal salts of the body, is effected by 

 the cells themselves, in accordance with their nutritional needs, and this 

 view is strengthened when we come to examine into the absorption even 

 of normal saline solutions. If 50 c.c. of normal sodium chloride solution 

 be introduced into a loop of intestine, it is absorbed steadily, so that at 

 the end of an hour not more than about 2.0 c.c. may be recoverable. The 

 absolute amounts absorbed differ in various experiments, but are fairly 

 uniform for repeated observations on one and the same animal. The 

 absorption of such a solution could be ascribed to the osmotic pressure of 

 the colloids in the blood plasma or lymph within the spaces of the villi. 

 If, instead of using isotonic solutions, hypertonic solutions are employed, 

 e. g. a 2 or 3 per cent. NaCl solution, absorption still takes place, but may 

 be preceded by an interval in which there is an actual increase of the fluid 

 contained in the gut. Here again we might ascribe the absorption to the 

 physical factors present, were it not that absorption is found to commence 



