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 C0 2 production, but of many others. 

 If the capacity of an organism or tissue for C0 2 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 C0 2 , 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 C0 2 

 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 C0 2 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 C0 2 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 C0 2 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 C0 2 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 C0 2 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 C0 2 . 



