THE BLOOD AND THE LYMPH 273 



in nature. Heidenhain's experiments of obstructing the vena cava 

 and the abdominal aorta respectively prove that capillary blood -pressure 

 greatly influences the quantity of lymph produced. In the former case 

 the lymph from the trunk was increased many times; in the latter case 

 is was decreased in proportion to the obstruction in the capillary blood- 

 flow. It is blood-pressure and blood-flow then that influence the 

 filtrative and diffusive elements in the production of lymph. 



How osmosis is determined by the molecular and ionic concentration 

 of the plasma and the lymph has been discussed elsewhere (page 221). 

 The conditions are, however, not those of dead animal membranes and 

 of glass vessels, but of living endothelium and epithelium under intricate 

 control. How far these differences alter the osmosis we do not know, 

 but perhaps not much. The "membranes" in the organism are so 

 minute and the forces in detail so slight that there is little probability 

 of learning with present methods much more than is now known as to the 

 details of the osmotic relations of these two similar liquids. Perhaps the 

 unicellular or other simple animals may be made to tell us more than has 

 yet been told in this direction, and here research is much demanded. 



The blood-capillary wall's permeability is partly the same as the last 

 condition, for the wall constitutes one of the membranes through which 

 osmosis must take place, and its permeability in small part determines 

 the osmosis. The main element, however, in the permeability is like 

 secretive function in that the protoplasm of the membranes concerned 

 may be altered, and so apparently that it can hasten or retard, or choose 

 this or that, according to local or to organic needs. Thus, research 

 by Galeotti shows that whereas the permeability of serous membranes 

 is not influenced by their death, that of epithelium and endothelium is 

 greatly altered when the protoplasm dies. Whether the nervous system 

 brings about the normal condition is as yet undecided, but it may do so 

 by the fibrils which some observers (e. g., Sihler) have claimed surround 

 the capillaries. The influence exerted by the tissue-metabolism and tissue- 

 needs on lymph-production is direct and controlling, for it is to serve the 

 tissues that the lymph exists. 



THE PHYSICAL CONSTITUTION OF LYMPH is simple, there being present 

 a very variable plasma, analyses of which have been already given 

 (on page 255), the lymphocytes (small mononu clear leukocytes) de- 

 scribed on page 266, and the thrombocytes. The specific gravity of 

 lymph varies widely, between 1020 and 1050 perhaps. In color it is 

 watery, opalescent, or milky even after a fat-containing meal, the 

 partial opacity being given by a varying proportion of emulsion composed 

 of particles of fat much finer than those of milk even. In taste it is 

 like blood-plasma, and in odor nearly so. Owing to the small amount 

 of fibrin which forms in clotting lymph (0.05 per cent.) and to the prac- 

 tical absence of erythrocytes, its coagulum is much less firm than that 

 of blood. Red corpuscles are usually found in lymph, sometimes enough 

 to give it a pinkish tinge, but these must be considered as present by a 

 physiological sort of accident only. The leukocytes in the lymph are 

 18 



