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 ridding 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 Oj is continually supplied. In this, Og 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 
res'piration 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, COj and H 2 O. 
When O 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 
