320 LYMPH AND LYMPHATIC GLANDS [CH. XXII. 



justly, that this is incorrect, as there is between the arteries and the 

 capillaries the peripheral resistance in the arterioles. 



2. The permeability of the capillary wall. This varies enormously 

 in different regions; it is greatest in the liver, so that an intra- 

 capillary pressure which would cause lymph to flow here is without 

 effect on the production of lymph in the limbs. 



The flow of lymph may therefore be increased in two ways : 



1. By increasing the intracapillary pressure. This may be done 

 locally by ligaturing the veins of an organ ; or generally by injecting 

 a large amount of fluid into the circulation, or by the injection of 

 such substances as sugar and salt (Heidenhain's first class of 

 lymphagogues) into the blood. These attract water from the tissues 

 into the blood, and thus increase the volume of the circulating fluid 

 and raise the intracapillary pressure. 



2. By increasing the permeability of the capillary wall by injuring 

 its vitality. This may be done locally by scalding a part; or 

 generally, by injecting certain poisonous substances, such as peptone, 

 leech extract, decoction of mussels, etc. (Heidenhain's second class 

 of lymphagogues). These act chiefly on the liver capillaries ; curare 

 acts chiefly on the limb capillaries. 



In the light of our present knowledge on this question, it is 

 impossible to pronounce any absolutely positive opinion. But facts 

 appear to me to be accumulating which tell in favour of the secretion 

 theory. If the endothelial wall were a non-living membrane, physical 

 processes would obviously explain all the phenomena of lymph forma- 

 tion. But we must recognise that the endothelial cells are alive, 

 and that like other cells they are capable of a selective action which 

 may mask or counteract or assist the purely physical processes. If 

 the action of poisons was simply to injure the vessel wall and increase 

 its permeability, the amount of lymph should be proportional to the 

 intensity of the injury; but this is not found to be the case. 

 Heidenhain no doubt went too far when he attributed lymph forma- 

 tion almost exclusively to endotheliaJ activity ; and Starling has gone 

 too far in the other direction. My own opinion is that lymph 

 formation is mainly influenced by the physical conditions present, 

 for the action of such thin cells as those of the capillary wall cannot 

 be sufficiently great to entirely counteract these conditions ; at the 

 same time it is impossible to deny that there is some such action 

 as may be described by the terms " selective " or " secretory." The 

 question is closely related to that of absorption from the alimen- 

 tary canal, and we shall find in studying that subject that there 

 is a similar difference of opinion, and that recently published 

 researches confirm the theory of selective activity of the absorptive 

 epithelium. 



