LUDWIG AND MODERN PHYSIOLOGY. 19 



Those who have any acquaintance with the structure of 

 the animal body will know that there exists in the higher 

 animals, in addition to the system of veins by which the 

 blood is brought back from all parts to the heart, another 

 less considerable system of branched tubes, the lymphatics, 

 by which, if one may so express it, the leakage of the blood- 

 vessels is collected. Now, without inquiring into the why 

 of this system, Ludwig and his pupils made and continued 

 for many years elaborate investigations which were for long 

 the chief sources of our knowledge, their general result 

 being that the efficient cause of the movement of the lymph, 

 like that of the blood, was mechanical. At the Berlin Con- 

 gress in 1890 new observations by Professor Heidenhain of 

 Breslau made it appear that under certain conditions the 

 process of lymph formation does not go on in strict accord- 

 ance with the physical laws by which leakage through 

 membranes is regulated, the experimental results being of 

 so unequivocal a kind that, even had they not been con- 

 firmed, they must have been received without hesitation. 

 How is such a case as this to be met? The "Neovitalists " 

 answer promptly by reminding us that there are cells, i.e., 

 living individuals, placed at the inlets of the system of 

 drainage without which it would not work, that these let in 

 less or more liquid according to circumstances, and that in 

 doing so they act in obedience, not to physical laws, but to 

 vital ones — to internal laws which are special to themselves. 



Now, it is perfectly true that living cells, like working 

 bees, are both the architects of the hive and the sources of 

 its activity, but if we ask how honey is made it is no answer 

 to say that the bees make it. We do not require to be told 

 that cells have to do with the making of lymph as with 

 every process in the animal organism, but what we want to 

 know is how they work, and to this we shall never get an 

 answer so long as we content ourselves with merely ex- 

 plaining one unknown thing by another. The action of 

 cells must be explained, if at all, by the same method of 

 comparison with physical or chemical analogues that we 

 employ in the investigation of organs. 



Since 1890 the problem of lymph formation has been 



