44 ABSORPTION 



the lymph spaces of areolar tissue is taken up by the blood and 

 does not appear in the lymph. 



But even if we admit that substances can pass, by physical pro- 

 cesses alone, from serous cavities into the blood, and from the blood 

 into serous cavities, this has little bearing upon the question of 

 intestinal absorption. For we can hardly put anything into the 

 peritoneal cavity which is not foreign to it. It was never intended 

 to come into contact with the hundred and one solutions, extracts, 

 suspensions, and what not, which the industrious experimenter has 

 offered to its unsophisticated endothelium. It cannot possibly have 

 developed any high degree of ' selective ' power. In the intestine 

 everything is different. The mucosa is adapted to come into 

 contact with an immense variety of materials, all kinds of food- 

 substances mingled with many kinds of refuse, the products of 

 the action of numerous digestive ferments, and of a vigorous and 

 varied bacterial flora. All these it has to sift and try. It cannot fail 

 to have properties which suggest a severe and searching selection. 



The difference between a serous cavity and the intestine is well 

 illustrated by the following experiment, in which the changes in the 

 composition of a hypotonic (3 per cent.) solution of dextrose introduced 

 into the peritoneal sac and into a loop of intestine respectively were 

 compared (Cohnheim). 



Here the water and sugar are both taken up from the intestine and 

 the peritoneal cavity; but while the sugar concentration in the serous 

 sac falls markedly, as ought to be the case if the sugar is diffusing into 

 the blood along the slope of concentration, the percentage of sugar in 

 the intestine actually increases. Still more striking is the fact that 

 sodium chloride accumulates in the peritoneal liquid in a concentration 

 obviously tending to equality with that of the blood, as would happen 

 if the peritoneal lining were a dead diffusion membrane. On the other 

 hand, practically no sodium chloride passes into the lumen of the gut. 



Closely connected with the question of absorption from and 

 secretion (or transudation) into the serous cavities is the question 

 of the factors concerned in the formation of the lymph (which will 

 be considered in the next chapter), even although recent researches 

 throw grave doubt on the common view that these sacs are merely 

 expanded lymph spaces, and indicate that the liquid found in them 

 has a different origin from lymph. 



