LYMPH AND TISSUE FLUIDS 1135 



about 6 per cent, of solid matters, the proteins consisting of fibrinogen, 

 paraglobulin, and serum albumen. The salts are similar to those of 

 the liquor sanguinis, and are present in the same proportions. 



THE PRODUCTION OF LYMPH 



Many physiologists have thought that, in the transudation of the 

 fluid which forms the lymph, there is an active intervention on the 

 part of the endothelial cells forming the capillary wall, and that lymph 

 is therefore to be regarded as a true secretion. A careful investigation 

 of the known experimental facts has failed to show that the endo- 

 thelial cells act otherwise than passively, as filtering membranes of 

 variable permeability. The factors which are responsible for the 

 transudation of lymph may be divided into two classes mechanical 

 and chemical, the former depending largely on the pressure of the 

 blood in the vessels, and the latter chiefly on the metabolism of the 

 cells outside the vessels. 



According to the views here laid down, the formation of lymph 

 may be compared to a process of filtration. If this be correct 

 the amount of lymph formed in any given capillary area must be 

 dependent on the difference of pressure between the blood in the 

 vessels and the fluid in the extravascular tissue spaces. This latter 

 pressure is normally extremely low, so that in attempting to test 

 the truth of this view we must try the effects of altering the pressure 

 inside the vessels, in the expectation of finding that the lymph pro- 

 duction will rise and fall as the capillary pressure is increased or 

 diminished. On attempting to carry out such experiments in different 

 parts of the body, we have to recognise another factor besides the 

 capillary pressure, viz. the permeability of the vessel-wall. Whereas 

 the capillary walls in the limbs and connective tissues generally 

 present a very considerable resistance to the filtration of lymph 

 through them, and keep back the larger portion of the proteins of the 

 blood-plasma, the intestinal capillaries are much more permeable, 

 giving at moderate capillary pressures a continual flow of lymph and 

 separating off only a small proportion of the proteins. It is in the 

 liver, however, that we find the greatest permeability. Here a very 

 small pressure suffices to produce a great transudation of lymph, 

 containing practically the same amount of protein as the blood-plasma 

 from which it is formed. 



The ease with which fluid passes out from the capillaries of the liver is probably 

 due to the fact that these vessels, unlike most other capillaries of the body, have 

 not a complete endothelial lining. Thus it is impossible to display a continuous 

 endothelial lining by means of silver nitrate. The cells surrounding the capil- 

 laries are large and branched, and possess marked phagocytic powers, so that 

 after an injection of carmine granules or bacteria into the blood-stream these 

 bodies a^p found in quantity within the cells. Owing to the incompleteness of 



