442 BLOOD AND LYMPH. 



filtration alone does not suffice to explain the composition of the 

 lymph under all circumstances. At present two divergent views 

 are held upon the subject. According to some physiologists, all 

 the facts known with regard to the composition of lymph may be 

 satisfactorily explained if we suppose that this liquid is formed 

 from blood-plasma by the combined action of the physical processes 

 of filtration, diffusion, and osmosis. According to others, it is 

 believed that, in addition to filtration and diffusion, it is necessary 

 to assume an active secretory process on the part of the endothe- 

 lial cells composing the capillary walls. The actual condition of our 

 knowledge of the subject can be presented most easily by briefly 

 stating some of the objections that have been raised by Heidenhain* 

 to a pure filtration-and-diffusion theory, and indicating how these 

 objections have been met. 



1. Heidenhain shows by simple calculations that an impossible 

 formation of lymph would be required, upon the filtration theory, 

 to supply the chemical needs of the organs in various organic and in- 

 organic constituents. Thus, to take an illustration that has been 

 much discussed, one kilogram of cows' milk contains 1.7 gms. CaO 

 and the entire milk of twenty-four hours would contain, in round 

 numbers, 42.5 gms. CaO. Since the lymph contains normally 

 about 0.18 part of CaO per thousand, it would require 236 liters of 

 lymph per day to supply the necessary CaO to the mammary glands. 

 Heidenhain himself suggests that the difficulty in this case may be 

 met by assuming active diffusion processes in connection with 

 filtration. If, for instance, in the case cited, we suppose that the 

 calcium of the lymph is quickly combined by the tissues of the mam- 

 mary gland, then the tension of calcium salts in the lymph will be 

 kept at zero, and an active diffusion of calcium into the lymph will 

 occur so long as the gland is secreting. In other words, the gland 

 will receive its calcium by much the same process as it receives its 

 oxygen, and will get its daily supply from a comparatively small 

 bulk of lymph. Strictly speaking, therefore, the difficulty we are 

 dealing with here shows only the insufficiency of a pure filtration 

 theory. It seems possible that filtration and diffusion together 

 would suffice to supply the organs, so far at least as the diffusible 

 substances are concerned. 



2. Heidenhain found that occlusion of the inferior vena cava 

 causes not only an increase in the flow of lymph as might be ex- 

 pected, on the filtration theory, from the consequent rise of pressure 

 in the capillary regions but also an increased concentration in the 

 percentage of protein in the lymph. This latter fact has been 

 satisfactorily explained by the experiments of Starling.f Accord- 



* "Archiv f. die gesammte Physiologic," 49, 209, 1891. 

 t "Journal of Physiology," 16, 234, 1894. 



