CH. xxii.] FORMATION OF LYMPH. 319 



of vital action is not so marked as Heidenhain supposes it to be, 

 but that most of the phenomena in connection with lymph 

 formation can be explained by the simpler mechanical theory. 

 Starling's views may be briefly stated as follows : 



The amount of lymph produced in any part depends on two 

 factors : 



1. The pressure at which the blood is flowing through the 

 capillaries. Heidenhain took the arterial pressure in his experi- 

 ments as the measure of the capillary pressure ; Starling points 

 out, very 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 intracapillary 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 lymphagogucs) 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, <fec. (Heidenhain's 

 second class of lymphagogues). These act chiefly on the livor 

 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 formation. 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 formation almost 



