xiv THE LYMPH 521 



The composition of the lymph also changed, since it became turbid 

 and whitish, not from increase in the fats and leucocytes, but 

 owing to a kind of partial precipitation of the proteins. This 

 turbidity does not always persist ; sometimes it ceases after 15 to 

 30 minutes. In any case the percentage content of solids in the 

 lymph increases, even after it has become clear. Lastly, the lymph, 

 while of greater density, becomes less coagulable during the 

 occlusion of the aorta. On repeating this experiment Starling 

 saw .that the pressure in the inferior vena cava is not altered, and 

 may even rise slightly under the conditions described, while that in 

 the aortic system is greatly reduced. The lymph that continues 

 to flow after the occlusion of the aorta can therefore only come 

 from the lymphatics of the liver. Indeed, on ligaturing the latter, 

 he found that the entire now of lymph from the thoracic duct was 

 arrested. 



According to Heidenhain these phenomena cannot all be 

 interpreted on the mechanical theory of nitration ; according to 

 Starling, on the other hand, since they demonstrate that the 

 increase of lymph flow is invariably associated with a correspond- 

 ing increase of pressure in certain capillary regions, they are 

 cogent arguments for the importance of nitration in the formation 

 of lymph. The various changes in constitution and concentration 

 presented by the lymph from the different regions have still, 

 however, to be explained. 



It will be observed that in all the experiments referred to, the 

 increase of pressure in the blood capillaries is due to a block in 

 the venous circulation, which is accompanied either by abnormal 

 retardation or by venous stasis. This fact never occurs under 

 physiological conditions. It may be conjectured that the walls of 

 the blood-vessels are altered by the long stagnation of the venous 

 blood, that they become more permeable, more sensitive to changes 

 in pressor effects, and permit an abnormal filtration of lymph, to 

 which they do not lend themselves under normal conditions. We 

 cannot, therefore, from these facts deduce a physiological theory of 

 the normal formation of lymph by a process of simple filtration. 

 Physiologically, capillary pressure only varies in consequence of 

 slow oscillations in tone of the small arteries, which are provided 

 with strong muscles. When these dilate, capillary pressure rises, 

 because, owing to diminished resistance, a larger amount of the 

 impulsive force of the heart is transmitted to the capillaries ; but, 

 in addition to the rise of pressure, the velocity of circulation 

 through the capillary network rises also, so that its walls are 

 bathed in a blood that undergoes rapid and constant renewal. In 

 order to establish the significance of filtration in the formation of 

 lymph under physiological conditions, it must also be shown that a 

 simple rise in arterial and capillary pressure, with unimpeded 

 venous flow, constantly produces increase in the lymphatic 



