MECHANISM OF ABSORPTION 433 



organs, an action which we may perhaps describe as making use 

 of purely physical processes, but not mastered by them, the possi- 

 bility must be admitted that in the cells of endothelial type which 

 line the serous cavities, the lymphatics, the bloodvessels, the alveoli 

 of the lungs, and the Bowman's capsules of the kidney (p. 484), the 

 element of secretion may be less marked, and more overshadowed 

 by the physical factors. And it may very plausibly be urged that 

 changes of considerable physiological complexity can only be 

 wrought on substances that have to pass through a cell of con- 

 siderable depth, while a mere film of protoplasm suffices for, and 

 indeed favours, mechanical filtration and diffusion. We have 

 already seen (p. 262), in the case of the lungs, that whatever the 

 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. 492), 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 circum- 

 stance 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 absorp- 

 tion, just as strychnine, when injected under the skin i.e., into 



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