ABSORPTION 367 



it. When the cells that line the intestine are injured or destroyed, 

 or subjected to the action of certain poisons, absorption from it is 

 diminished or abolished. And in their normal state they do not take 

 up indiscriminately all kinds of diffusible substances, nor absorb those 

 which they do take up in the direct ratio of their diffusibility, nor 

 do they reject everything which does not diffuse. Albumin, for 

 example, which does not pass through dead animal membranes, is to 

 a certain extent taken up from a loop of intestine without change. 

 Cane-sugar and dextrose are absorbed more rapidly than their velocity 

 of diffusion would indicate, when compared with inorganic salts. 

 And it has been shown that the water, organic and inorganic solids 

 of the serum of an animal are absorbed from a loop of its intestine 

 when the pressure in the capillaries of the intestinal wall is con- 

 siderably greater than in the cavity of the gut. Since the serum in 

 the intestine and the plasma in the capillaries must be isotonic, and 

 practically identical in chemical composition, the absorption cannot 

 be due to ordinary osmosis or diffusion. Nor can it be due to filtra- 

 tion, since the slope of pressure is from the capillaries to the lumen 

 of the gut (Waymouth Reid). It is therefore extremely difficult to 

 reconcile this experiment with any purely physical theory of absorp- 

 tion. 



But if it be true that the action of the columnar epithelium of the 

 intestinal mucous membrane is governed by a secretive and selective 

 power that makes use of purely physical processes, but is not 

 mastered by them, the possibility 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. 397), the element of secretion is 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 

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

 indeed favours, mechanical filtration and diffusion. We have already 

 seen (p. 242), 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. 407), 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. 

 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). 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 strychnia, 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 



