406 A MANUAL OF PHYSIOLOGY 



complete explanation may be of the gaseous exchange which takes 

 place through the alveolar membrane, physical diffusion undoubtedly 

 plays an important part. We shall see, too (p. 466), that in the 

 case of the kidney the endothelium of the Bowman's capsule, 

 although by no means devoid of selective power, does seem to have 

 allotted to it a simpler task than falls to the share of the ' rodded ' 

 epithelium . 



Absorption from the Peritoneal Cavity. Further, it has been stated 

 that interchange between blood-serum, circulated artificially in the 

 vessels of dogs and rabbits which have been dead for hours, and 

 liquids introduced into the peritoneal cavity, is essentially the same 

 as in the living animal, and can be explained on purely physical 

 principles (Hamburger). But there is one experiment, at any rate, 

 which is certainly difficult so to explain viz., the absorption from the 

 peritoneal cavity of sodium chloride solution isotonic with the blood- 

 serum, an absorption which goes on with considerable rapidity. 

 Starling has supposed that this is due to the circumstance that the 

 proteins of the serum exert osmotic pressure, the peritoneal membrane 

 being almost or altogether impermeable for them in comparison to 

 its permeability for the salt solutions. In consequence, water passes 

 into the bloodvessels from the peritoneal cavity. The solution thus 

 becomes more concentrated as regards sodium chloride, some of 

 which accordingly enters the blood by diffusion, and so on. But 

 even isotonic serum is absorbed from the peritoneal cavity, and it 

 seems to savour of special pleading to suggest, as has been done, 

 that this takes place through the lymphatics, and not at all through 

 the bloodvessels. 



Up to a certain point an increase in the intraperitoneal pressure 

 favours absorption, but beyond this it hinders it by interfering with 

 the circulation. The removal of a portion of the fluid in this con- 

 dition facilitates the absorption of the rest a fact which has long 

 been applied in the operation of tapping. Ligation of the thoracic 

 duct has little effect on the fate of liquids injected into serous cavities, 

 since the bloodvessels play the chief part in their absorption, just as 

 strychnine, when injected under the skin i.e., into 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. 



Formation of Lymph. Closely connected with the question of 

 absorption from and secretion (or transudation) into the serous 



