68 PHYSIOLOGY CHAP. 



Every assimilatory or anabolic process results in an accumu- 

 lation of energy, and necessarily implies a source of kinetic, which 

 can be transformed into potential, energy. Each dissimilatory or 

 katabolic process, on the other hand, results in a dispersion of 

 energy, and presupposes a store of potential, to be transformed 

 into kinetic, energy. This is why the two opposite processes are 

 simultaneous, or constantly and rapidly alternating, during life, 

 while the two together constitute metabolism, which as we have 

 seen is the physiological basis of all the phenomena of life. 



Since in green plants anabolic largely predominate over 

 katabolic processes, the energy which they develop by oxidation is 

 inadequate for the synthetic formation of their highly complex 

 organic substances, and the intervention of the energy derived 

 from the sun's rays becomes necessary. 



In animals, on the contrary, in which katabolic processes 

 largely predominate, the energy which they develop by the 

 oxidation of organic substances is not only enough to : yield 

 mechanical work, and to keep the temperature of the body above 

 that of the environment, but also suffices to secure the anabolic 

 processes, or new organic syntheses, by elaboration of the food- 

 stuffs drawn from plants. 



The destruction of the organic molecules by the katabolic pro- 

 cesses does not take place all at once, so as immediately to turn 

 combustible substances into final products ; but it is effected 

 gradually and successively, the more complex being converted 

 into other less complex molecules, and these into the end-products 

 rejected by the body. 



The presence of oxygen is not essential to all these regressive 

 metamorphoses. In the absence of free oxygen, protoplasm is able 

 for a certain time to obtain oxygen from the combinations in 

 which it is held loosely or firmly, and thus to develop kinetic 

 energy. The great plasmodia of the Myxomycetes, e.g., if placed in 

 a medium deprived of oxygen, will continue their movements for 

 three hours ; ciliated epithelia can live even longer without oxygen 

 (Engelmann) ; excised frog's muscle placed in an atmosphere of pure 

 hydrogen will give off carbonic acid for many hours before it 

 becomes inexcitable (Hermann). Many organisms of the lowest 

 orders, particularly in the numerous groups of bacteria, have the 

 faculty of living permanently without oxygen. Pasteur, who was 

 the first to call attention to this most important phenomenon, gave 

 the name of anaerobic to the organisms which live in the absence of 

 oxygen, in contradistinction from the aerobic, which can only 

 live in presence of this gas. According to Tarozzi (1905),' the 

 incapacity of anaerobic bacteria to develop in culture media in the 

 presence of oxygen, is due not to a toxic action of the oxygen on 

 these microbes (as has been stated by many authors) but rather to 

 chemical modifications of the proteins in the broth used for the 



