COMPOSITION AND FORMATION OF LYMPH. 475 



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 diffusion. tension of calcium salts in the tissue 

 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 compara- 

 tively 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.* Accord- 

 ing to this observer, the lymph formed in the liver is normal^ more 

 concentrated than that of the rest of the body. The occlusion of 

 the vena cava causes a marked rise in the capillary pressure in the 

 liver, and most of the increased lymph-flow under these circum- 

 stances comes from the liver; hence the greater concentration. 

 The results of this experiment, therefore, do not antagonize the 

 filtration-and-diffusion theory. 



3. Heidenhain discovered that extracts of various substances, 

 which he designated as "lymphagogues of the first class," cause a 

 marked increase in the flow of lymph from the thoracic duct, the 

 lymph being more concentrated than normal, and the increased flow 

 continuing for a long period. Nevertheless, these substances cause 

 little, if any, increase in general arterial pressure; in fact, if injected 

 in sufficient quantity they produce usually a fall of arterial pressure. 

 The substances belonging to this class comprise such things as pep- 

 tone, egg-albumin, extracts of liver and intestine, and especially 

 extracts of the muscles of crabs, crayfish, mussels, and leeches. 

 Heidenhain supposed that these extracts contain an organic 

 substance which acts as a specific stimulus to the endothelial cells 

 of the capillaries and increases their secretory action. The results 

 of the action of these substances has been differently explained by 



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



