736 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 dye-stuffs, soluble or insoluble in 

 lipoids, it is found that the lipoid- soluble dye-stuffs, such as neutral red or 

 toluidin blue, pass into the cells, whereas the dye-stuffs which are insoluble 

 in such substances pass into the intercellular spaces. Too much stress, how- 

 ever, must not be laid on these experiments. All these dye-stuffs are ab- 

 normal 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 

 dye-stuffs. The fact that absorption of these dye-stuffs is determined 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 food-stuffs 

 to the cell, but its protection from any of the food-stuffs which it does not 

 require for its metabolism. If it were not for such a membrane the assimila- 

 tion of a salt would be determined entirely by its concentration in the imme- 

 diate surroundings of the cell, whereas we know that the assimilation of 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 food-stuffs, 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 

 20 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 in- 

 crease 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 before the fluid in the gut has attained isotonicity with 



