654 THE MECHANISM OF ABSORPTION 



is possibly more so. MacCallurn injected 1000 c.c. of a '9 per 

 cent. NaCl solution into a rabbit and found a large secretion of 

 fluid containing '25 per cent, dextrose in the gut. Reid injected 

 about 100 c.c. of a 5 per cent, solution of NaCl into a dog and 

 found that the absorption of water was not increased and the 

 absorption of dextrose was diminished. It would be difficult to 

 say that these two results have no connection with each other, 

 and that the explanation of the failure of an increased concentra- 

 tion of NaCl in the blood to increase correspondingly the absorp- 

 tion of water is not to be found in MacCallum's experiments. 



In the other experiments of this series Reid dealt with the 

 absorption of substances in solution, and sought an answer to the 

 question whether the rate at which a substance in solution is 

 absorbed is determined by the relative partial pressures of the 

 substance on the two sides of the intestinal epithelium. 

 Heidenhain showed that when either a 1 per cent, or a '3 per cent, 

 solution of NaCl is being absorbed, the concentration of NaCl 

 in the solution becomes rapidly altered until it is equal to the 

 concentration of NaCl in the blood, and that when that point 

 of equilibrium has been reached, water and salt are absorbed 

 at the same rate. Hamburger and Hober consider that the 

 solution during concentration becomes isotonic with the blood 

 before the substance and solvent are absorbed at the same rate. 

 The difference of opinion between these observers is immaterial 

 in this connection. It is clear that the whole phenomenon can 

 be explained by diffusion and osmosis ; the alteration of the 

 concentration of the solution up to the point of equilibrium 

 could be accounted for by the rapid diffusion into the blood 

 of either solvent or substance in solution ; and the final absorption 

 of the solution as a whole could be accounted for by the osmotic 

 pressure of substances in the blood to which the intestinal 

 epithelium is impermeable. Reid found, like Cohnheim, that 

 the point of equilibrium for a solution of dextrose in water was 

 a concentration of about 4 per cent., and that if a stronger or 

 weaker solution was used, its concentration alters until this 

 point of equilibrium was reached. Thus, if a 2 per cent, solution 

 of dextrose in water is placed in the intestine, it becomes a 

 4 per cent, solution within half-an-hour. He further found that 

 if 2 per cent, of dextrose was dissolved in a - 6 per cent. NaCl 

 solution, the concentration of dextrose remained unaltered and 



