ABSORPTION 407 



cavities is the question of the factors concerned in the formation of 

 the lymph, even although recent researches throw grave doubt on 

 the common view that these sacs are merely expanded lymph 

 spaces, and indicate that the liquid found in them has a different 

 origin from lymph. We ought to distinguish the lymph as we col- 

 lect it from the great lymphatic trunks, not only from the liquids 

 of the serous cavities, but still more sharply from the liquid which 

 fills the multitudinous clefts and spaces of the tissues. It is now 

 pretty definitely established that the tissue spaces do not com- 

 municate by actual passages with the lymphatic vessels, but that 

 the latter form everywhere a closed system like the blood-vascular 

 system, the lymph capillaries merely lying in the tissue spaces 

 (W. G. McCallum, etc.). This conception entails a radical change 

 in the current views of lymph production. If the lymphatics form 

 a closed system, the lymph cannot be actual tissue fluid, but only 

 tissue fluid modified by its passage through the walls of the 

 lymph capillaries, just as tissue fluid is not actual blood-serum, 

 but serum modified by its passage through the walls of the 

 blood capillaries. 



Although it is customary to speak of the lymph obtained from 

 the lymphatic vessels as if it were perfectly homogeneous, there 

 is no experimental ground for supposing that the lymph from 

 different tracts, or the tissue liquid in contact with the cells 

 of different organs, or even the tissue liquid in contact with one 

 and the same cell at different parts of its periphery, has a uniform 

 composition, or even a uniform molecular concentration. There 

 are indeed certain general considerations which show that this 

 cannot be so. 



The teaching of Ludwig, that lymph is formed by the filtration, 

 and in a minor degree, diffusion, of the constituents of blood- 

 plasma through the walls of the capillaries into the tissue spaces, 

 was based on such facts as the increase in the tissue liquid of a 

 limb or organ which occurs when the exit of blood from it by the 

 veins is hindered, or when the quantity of the circulating liquid 

 is increased by the injection of blood or salt solution. It was 

 first seriously called in question by Heidenhain, who advanced 

 the theory that lymph is secreted by the endothelium of the blood 

 capillaries. One of Heidenhain's strongest arguments in favour of 

 his secretion theory was the existence of substances which, when 

 injected into the blood, increased the flow of lymph from the 

 thoracic duct of the dog without affecting appreciably the 

 arterial pressure. He divided these so-called lymphagogues into 

 two classes : (i) substances like peptone, extracts of the head 

 and liver of the leech, extract of crayfish muscle, egg-albumin, 

 etc., which cause not only an increase in the rate of flow, but an 

 increase in the specific gravity and total solids of the lymph ; 



