406 PHYSIOLOGY 



by rapid respiration. This is in harmony with numberless observa- 

 tions in animals, in which the work can be increased at will, when a 

 corresponding increase in the products of respiration, the consumption 

 of nutritive materials, and the evolution of heat is readily shown. It is 

 perhaps better to consider all those phenomena of respiration as its re- 

 sults, the decomposition of the protoplasm being the primary and essen- 

 tial feature. Indeed the phenomena of respiration may all be directed 

 to lidding the body of the products of an inevitable decomposition of 

 the unstable proteins of the living protoplasm. 



Role of oxygen. — When energy is released from chemical compounds, 

 the more completely they are decomposed the more energy is liberated, 

 as a rule. In anaerobic respiration the decomposition does not go so far 

 as in aerobic, for the resulting substances are not so simple, and probably 

 therefore the energy released is far less. The fact that growth either does 

 not occur at all, or is very limited, when oxygen is cut off from plants 

 accustomed to it, also indicates this. Herein, indeed, appears the prob- 

 able role of oxygen in respiration. It seems to be necessary not to com- 

 bine with carbon compounds, but, by combining with and so removing 

 substances whose presence interferes with the usual reactions, to enable 

 the respiratory processes to go on to completion. 



The common idea is that oxygen combines directly with carbon and so causes 

 " combustion." But chemical studies of the combustion of certain gases show- 

 that it does not do this, even at high temperatures. Water vapor, which yields H 

 and OH ions by dissociation, furnishes the necessary OH ions for facilitating the 

 decomposition of the carbon compounds, and this decomposition does not proceed 

 at all in the absence of water, not even in pure oxygen. The oxygen does combine, 

 however, with hydrogen to regenerate water, so that a small quantity of water serves, 

 provided O2 is continually supplied. In this, Oo behaves somewhat as the depolar- 

 izer does in a galvanic battery, wherein its function is that of an oxidizing agent to 

 convert into water the hydrogen that otherwise would accumulate on the cathode 

 and stop the chemical action. Undoubtedly other "depolarizers" than oxygen 

 are present in the cells ; and in some organisms the long continuance of anaerobic 

 respiration without serious harm may be thus explicable. The presence of oxidiz- 

 ing enzymes, also, may be essential to the fixation of oxygen. 



End products. — When, therefore, O is supplied, the end products of 

 decomposition are in large part the most stable ones, C0 2 and H 2 0. 

 When 2 is not available, these are less prominent, while ethyl and higher 

 alcohols, organic acids, aromatic compounds, hydrogen, etc., are the 

 more abundant end products. In the one case certain parts of the pro- 

 toplasm break into simpler and simpler compounds; in the other the 



