50 GENERAL PHYSIOLOGY 



characterised physiology since the death of Johannes Miiller, and 

 to which attention has already been directed, namely, the entire 

 lack of a comparative physiology. The science has not yet 

 entered upon this important heritage from Miiller, our greatest 

 master. How few objects of research the physiology of to-day 

 possesses — the dog, the rabbit, the guinea-pig, the frog, and a few 

 other higher animals. How little known are the many splendid 

 objects offered to the observant eye by the enormous number of 

 lower animals. And it is precisely among these objects that there 

 are to be found such as are fitted in a surprising degree for the 

 cell-physiological solution of elementary physiological questions. 



It is quite true that if one attempts to treat the problems of 

 digestion, resorption, and motion, from the standpoint of cell- 

 physiology on men or on the higher animals solely, he will soon 

 run against more or less serious technical difficulties in the investi- 

 gation of the living gland-cell, the intestinal epithelium-cell, and 

 the muscle-cell. Nevertheless, the admirable investigations of 

 Heidenhain upon secretion, the formation of lymph and resorption, 

 have shown what result the cell-physiological method has been 

 able to achieve even here. Such systematic histological experi- 

 ments, in which the living cell, while its connection with the bodj' 

 is still intact, is put under definite conditions, and the final results 

 are then investigated after the sudden death of the animal for the 

 purpose of drawing conclusions regarding the events that take place 

 during life under the corresponding conditions, will still without 

 doubt yield much of value. In the tissue-cells the conditions are 

 relatively favourable for chemical investigation ; at least in many 

 cases chemistry is capable of investigating metabolism in large 

 living cell-complexes, and drawing from them conclusions re- 

 garding the life of the individual cells. In fact, we are indebted 

 to this phase of chemistry for very decided light upon animal 

 metabolism. But, naturally, in the animal body little oppor- 

 tunity is afforded for employing pure tissues, i.e., complexes of 

 similar cells, as objects of research, and the uncertainty of the 

 significance of the results increases enormously in proportion to the 

 morphological complication of the object. Moreover, investigations 

 on tissue-cells are limited by the fact that frequently, at least in 

 warm-blooded animals, the tissues offer serious obstacles to the 

 employment of methods such, e.g., as that of microscopic experiment 

 during normal life. The free-living cells in the organism, such as the 

 white blood-corpuscles, offer considerably fewer difficulties in this 

 respect ; and thus it has come about that in very recent times we 

 have obtained very detailed knowledge of the vital phenomena of 

 leucocytes, especially through the labours of Metschnikoff, Massart 

 Leber, Buchner, and many others. 



If, however, the comparative-physiological standpoint, which 

 Johannes Miiller always defended energetically, be adopted, an 



