THEORIES OF LUDWIG AND HEIDENHAIN. 291 



hind-limbs we know that, in an animal at rest on the table, there is no 

 lymph now at all. Hence the sources of the lymph are confined to the 

 trunk. We can, moreover, exclude the thorax and its contents, since 

 ligature of the thoracic duct just above the diaphragm absolutely stops 

 the lymph now. Therefore, when dealing with the lymph now from the 

 thoracic duct, we deal only with the lymph coming from the abdominal 

 viscera. As I shall show presently, the abdominal viscera, so far as their 

 lymph is concerned, may be divided into two groups (1) the viscera 

 drained by the portal vein, and (2) the liver. 



Influence of venous obstruction. In testing the filtration hypothesis 

 on the lymph flow, we have to investigate whether the flow is always 

 proportional to the difference between the intra- and extracapillary 

 pressures. We may regard the extracapillary pressure as not varying 

 to any large extent, so that we have to see what effect is produced 

 on the lymph by variations in the intracapillary pressure in the 

 intestines and the liver. The simplest experiments on the subject 

 are those in which some large vessel is obstructed. Speaking generally, 

 we may say that obstruction of a large vein raises the pressure in 

 the capillaries immediately behind it, whereas obstruction of an 

 artery will diminish the pressure immediately in front of it. If, 

 for instance, we ligature the portal vein, the arterial pressure is very 

 little affected, while the pressure in the vein behind the ligature 

 rises enormously. In consequence of this, there is a large rise of 

 pressure in the capillaries of the intestines and spleen, so that the 

 spleen swells and the intestines become black from venous congestion, 

 haemorrhages being produced into their mucous membrane. The effect 

 of this ligature on the lymph flow from the thoracic duct is to increase 

 it four or five times. The lymph also becomes bloody and its total 

 solids are diminished. The diminution in solids is due solely to a 

 diminution in proteids, the salts remaining the same as before ; so that 

 we have here an increased capillary pressure, causing an increased trans- 

 udation of lymph containing a diminished percentage of proteid a result 

 which is also obtained when proteids are filtered with pressure through 

 dead animal membranes. The presence of red blood corpuscles in the 

 lymph is not a necessary consequence of a rise of pressure in the 

 portal vein. If a less excessive rise of pressure be produced by 

 ligaturing the vein, not at its entry into the liver but just below the 

 pancreatico- duodenal vein, thus leaving a circuitous route for the blood 

 to the liver through the anastomoses of this branch, an increased flow 

 of lymph is produced, containing less proteids than normal lymph, 

 but which may be quite free from red blood corpuscles. 



Still more striking is the effect produced by Heidenhain's experi- 

 ment of obstructing the vena cava just above the diaphragm (i.e. 

 between the opening of the hepatic veins and the heart). The lymph 

 is increased from ten to twenty fold, and it is found that the lymph 

 obtained after the obstruction is free from red blood corpuscles and is 

 more concentrated than normal lymph. Thus, in one experiment of 

 this description, the lymph flow rose from 3 c.c. in the ten minutes 

 preceding the obstruction to 25 c.c. in the ten minutes after the vein was 

 occluded. At the same time the percentage of solids in the lymph rose 

 from 4*8 per cent, before, to 6 '6 per cent, after the obstruction. 



What is the cause of this increased lymph flow and why is it more 

 concentrated ? To answer these questions we must find out first, the 



