LYMPH FORMATION 601 



and extremities reaching the neck capillaries by way of the cir- 

 culation. He further showed that the greatest lymph flow was 

 produced when a horse ate during compression of the jugular veins, 

 a result which he explained by the accumulation of metabolic 

 products leading to increased secretion of lymph. 



These experiments are obviously inconclusive and open to other 

 interpretations. Observations on the carotid pressure give no in- 

 formation about capillary pressure ; during exercise of the trunk 

 and legs it is extremely unlikely that the neck was completely at 

 rest, and no account was taken of the influence of increased 

 respiratory movements causing increased lymph flow along the 

 cervical trunk ; compression of the jugular veins will increase the 

 capillary pressure of the part. 



Hamburger has, however, stated later that during exercise of the 

 body with the neck at rest there is a fall in pressure not only in 

 the carotid but also in the jugular, and yet the lymph flow is 

 increased three to five times. It is impossible to explain this 

 lymph flow by filtration ; but until we know that in this experiment 

 the muscles of the head and neck are not doing work, it is 

 impossible to say that the lymph flow is not caused by increased 

 tissue activity. 



(3) Another important observation made by Hamburger against 

 a pure filtration theory of lymph formation was the length of time 

 a lymph flow could continue after death, a phenomenon which will 

 be considered later and shown to be due probably to osmosis. 



When we consider Heidenhain's and Hamburger's experiments 

 as a whole, we may certainly conclude that they fail to bring 

 forward any evidence which necessitates a belief in a secretory 

 activity of the capillary wall. Their experiments are capable of a 

 simpler physical explanation, which has been summed up by 

 Starling as follows : " The formation of lymph and its composition, 

 apart from the changes brought about by diffusion and osmosis 

 between it and the tissues it bathes, depend entirely on two 

 factors (1) The permeability of the capillary wall ; (2) The intra- 

 capillary blood pressure." But, whilst we may admit that the 

 explanation of these and similar experiments is probably to be 

 found in the alteration of mechanical factors, it does not follow 

 that alterations in the same factors bring about the normal varia- 

 tions in lymph formation. For these experiments introduce 

 changes so gross that they might well overwhelm and mask a normal 



