METHODS OF PHYSIOLOGICAL RESEARCH 27 



mechanical work, and this can be changed by means of a dynamo 

 into electricity and be made to serve finally for the production of 

 the electric light. Thus we perform daily the remarkable experi- 

 ment of re-transforming, after millions of years, into its original 

 form, the kinetic energy of the sun's rays which the plants of 

 the Carboniferous age employed for storing up carbon, and thus 

 illuminating our nights with the radiance of the sun that shone 

 upon the surface of the earth in immemorial times (Cf. Bunge). 



The application of the law of the conservation of energy to the 

 energetics of organisms was attempted by Robert Mayer, and 

 has since been undertaken many times. By the calorimetric 

 researches also of Dulong, Helmholtz, Rosenthal, Rubner, and 

 others, the proof has been afforded experimentally that this law 

 is as true in living nature as in lifeless. But our knowledge is 

 extremely scanty concerning the mode of action of energy in the 

 various performances of the body, concerning the transformations 

 undergone by the energy in its path through the living substance. 

 In this respect plant physiology, which is indebted especially to 

 the striking researches of Pfeffer upon the energetics of the 

 plant-cell for important discoveries and suggestions, is relatively 

 farther advanced than animal physiology. In this subject of the 

 energetics of living substance the future offers a wide field of 

 labour, which is full of reward. 



The second of the great discoveries, which also has yielded chiefly 

 to plant physiology its most important results, but has not yet 

 been employed at its full value in the science of the physiology of 

 animals, is the fact that organisms are composed of cells. The be- 

 ginnings of the cell-theory are to be found in botanical studies. 

 The microscopists of the seventeenth and eighteenth centuries, es- 

 pecially Malpighi, Treviranus,Mohl,and Meyen found that plants are 

 composed of small microscopic chambers, or cells, and elongated 

 tubes which have liquid contents. The elongated tubes soon 

 proved themselves to be structures that arise from series of cells 

 by a dissolution of the transverse walls. Brown found next a 

 more solid nucleus as a wide-spread structure in the liquid cell- 

 contents. But Schleiden first put into general form the idea 

 that all plants are composed of cells, and he distinguished as an 

 essential constituent of the cell-contents, besides the cell-sap and 

 nucleus, the semi-liquid motile plant-slime, which was termed by 

 Mohl protoplasm. In the meantime the wide occurrence of cells 

 in the animal kingdom had become recognised, and, soon after 

 Schleiden, Schwann founded the cell-theory for the animal kingdom 

 by showing that animals are composed of cells or cell-products, 

 and in their development progress from forms that contain only a 

 few similar cells. Later, embryology established the fact that in 

 general all organisms are developed from a single cell, the egg- 

 cell, into a cell-community which may become large and powerful, 



