320 LYMPH AND LYMPHATIC GLANDS [CH. XXIII. 



laboratory showed that increased capillary pressure due to obstruction 

 of the venous outflow increases the amount of lymph formed ; and 

 that diminution of the pressure in the lymph spaces, by squeezing 

 out the lymph previously contained in them, leads to an increase in 

 the transudation. 



On the other hand, there were some facts which could not be well 

 explained by the nitration theory, among which may be mentioned 

 the action of curare in causing an increase of lymph flow. 



Heidenhain was the first to fully recognise that the laws of 

 filtration and osmosis as applied to dead membranes may be con- 

 siderably modified when the membranes are composed of living cells ; 

 and he considered that the formation of lymph is due to the selective 

 or secretory activity of the endothelial walls of the capillaries. This 

 so-called vital action of the endothelial cells is seen in the fact that 

 after the injection of sugar into the blood, in a short time the per- 

 centage of sugar in the lymph becomes higher than that in the 

 blood. There must, therefore, be some activity of the endothelial 

 cells in picking out the sugar from the blood and passing it on to 

 the lymph. The excess of chlorides in lymph is also in favour of 

 the same view. 



Heidenhain was the inventor of the term lymphagogues (literally, 

 lymph drivers). These are substances like curare, which have a 

 specific action in causing an increased lymph flow. Heidenhain 

 considered that the majority of these act by stimulating the endo- 

 thelial cells to activity. This conclusion, however, has been subjected 

 to much criticism. In this country the question has been taken up 

 by Starling, who has shown that the influence of vital action is 

 not so marked as Heidenhain supposed it to be, but that most of 

 the phenomena in connection with lymph formation can be explained 

 by the simpler mechanical theory. Starling considers that the 

 amount of lymph produced in any part depends on two factors : 



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

 laries. Heidenhain took the arterial pressure in his experiments 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 unknown 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. Liver lymph is also 

 richer in protein than lymph from 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 



