ABSORPTION 273 



from tissues to bloodvessel. Even as regards absorption, which 

 it is difficult to divorce from lymph formation, osmotic pressure 

 does not help as much as might be expected. Experimentally 

 it can be shown that there is no definite relation between the rate 

 at which the sugars are absorbed and the osmotic pressure they 

 exert. Serum, isotonic with blood plasma, may be absorbed 

 from a loop of intestine, when the blood pressure in the capil- 

 laries of the intestine is greater than the pressure within the 

 bowel. Evidently in this case osmotic pressure cannot have 

 been exercised, while absorption by filtration is negatived by the 

 higher pressure in the capillaries. Salt solution isotonic with 

 blood plasma may also be readily absorbed by the bloodvessels 

 in the peritoneal cavity. In all these cases something is occur- 

 ring which is opposed to what might have been expected on a 

 purely physical basis. But perhaps few things are more remark- 

 able in this respect, or more difficult to explain, than the effect 

 of the injection of dextrose, which causes a post-mortem floiv of 

 lymph for as long as an hour after the circulation has ceased. 

 Whether the question be considered from its physical or secretory 

 aspect, this post-mortem flow is inexplicable. 



Indiffusible substances such as protein are believed to exert but 

 little osmotic pressure, and some consider none whatever. The 

 large size of the protein molecule, and the small number present 

 in such concentration (as 6 or 7 per cent.), as is represented by 

 blood serum, explains why they exert little or no pressure. 

 The passage of protein through the capillary wall to the lymph 

 vessel cannot be satisfactorily explained by osmotic pressure ; a 

 filtration has consequently been assumed to be the agent at work, 

 as the blood in the capillaries is at a higher pressure than the 

 lymph in their capillaries. 



Starling, whose name is so closely identified with the investiga- 

 tion of lymph production from its physical aspect, has urged 

 the permeability of the capillary wall as an important factor. 

 His observations show that the normal undamaged capillary of the 

 limbs and connective tissue offers a very considerable resistance 

 to the filtration of lymph, and keeps back a large portion of the 

 proteins of the blood plasma ; on the other hand, the intestinal 

 capillaries, and especially the capillaries of the liver, are very 

 permeable ; a very small capillary pressure in the latter suffices 

 to produce a large transudation of lymph containing as much 

 protein as the plasma itself. A capillary of a limb normally 

 impermeable may by injury be at once converted into a permeable 

 capillary, within which the slightest increase in pressure brings 

 about lymph production. 



Starling records the remarkable fact that no lymph can be 

 obtained from a resting limb, though active or passive movements 



18 



