THE MECHANISM OF TISSUE RESPIRATION 163 
form, and that this store of combined oxygen confers upon the 
protoplasm its peculiar properties of instability and excitability. 
Further, it is supposed that once the store is exhausted, excita- 
bility is lost, or, conversely, that the persistence of excitability 
in living tissues after total deprivation of oxygen implies the 
existence within them of intramolecular oxygen. This hypo- 
thesis of intramolecular oxygen has been generally accepted, 
for it offers a reasonable explanation not only of the above- 
recorded facts concerning CO, production, but of many others. 
If the capacity of an organism or tissue for CO, production 
be a measure of its capacity for storing up intramolecular 
oxygen, then it follows that this capacity varies between very 
wide limits—e,g. as 100 to 1,200 or more. However, we know 
that at least some of the CO,, and probably a great deal of it, 
arises in other ways than by intramolecular oxidation. The 
blood and lymph of mammals, and presumably also of the frog, 
contain 40 to 50 per cent. of their volume of loosely bound CO, 
Now we know that two-thirds or more of the total weight of an 
animal consists of water, only a small part of which is present 
in the blood and lymphatic vessels. Most of it is found within 
the cells, and this intracellular fluid, for aught we know to the 
contrary, may contain as much loosely bound CO, as the lymph 
of the lymphatics. If this were the case, then the total body fluids 
would contain about 300 c.c. of loosely bound CO, per kilo- 
gram, and if only two-thirds of it were liberated in frogs kept 
in an atmosphere of nitrogen, it would be unnecessary to assume 
that any of it was formed from intramolecular oxygen. Unfor- 
tunately no determinations have been made of the acidity or 
alkalinity of the blood and tissues of the frogs at the end of the 
experiments, or of the CO, set free from them on acidification, 
as compared with that from the blood and tissues of normal 
frogs. As we shall see later on, there can be very little doubt 
that acids are formed in the tissues under the conditions of the 
experiment, though we cannot say in what amount. The dis- 
charge of CO, by an organism kept in absence of oxygen is 
therefore no unequivocal proof of the existence of intramolecular 
oxygen in the tissues, though the estimations by the writer of 
the CO, discharged from mammalian kidneys on perfusion with 
oxygenless saline seem valid, and. it was proved that in their 
case perfusion with dilute lactic acid provoked no liberation 
of CO, 
