CH. XXII.] FORMATION OF LYMPH 319 



between these two fluids, setting up osmotic interchanges through the 

 wall of the blood-vessel. 



The accurate meaning of these terms is explained in the section in small print 

 at the end of this chapter. 



If the lymph is produced by a simple act of filtration, then the 

 amount of lymph must rise and sink with the value of D d ; D 

 representing the capillary blood-pressure, and d the pressure in the 

 tissue spaces. > 



In support of this mechanical theory, various workers in Ludwig's 

 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 filtration 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 ; f 

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

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

 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. 



Heidenhain is also the inventor of the term lymphagogues \ 

 (literally, lymph drivers). These are substances like curare, which 1 

 have a specific action in causing an increased lymph flow. Heiden- \ 

 hain considers the majority of these act by stimulating the endothelial * 

 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 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 capil- 

 laries. Heidenhain took the arterial pressure in his experiments as 

 the measure of the capillary pressure; Starling points out, very 



