INTERNAL OR TISSUE RESPIRATION 265 



oxygen is less. In the lymph, the pressure is kept low by the avidity 

 of the tissues with which it is in contact, and possibly by the exis- 

 tence in it of oxidizable substances which have come from the tissues. 

 In the tissues there is no partial pressure at all, because the oxygen 

 that reaches them is at once stowed away in some compound in 

 which it has lost the properties of free oxygen. ) 



Assuming, then, that at least a great part of the oxidation and^ 

 consequent production of carbon dioxide goes on in the tissues, let 

 us follow the steps of the process, as far as we can, in the light of our 

 knowledge of the respiration of muscle. 



Respiration of Muscle. It is a remarkable fact that an excised 

 frog's muscle is capable of going on yielding carbon dioxide for a 

 long time, in the entire absence of oxygen, in a chamber, for instance, 

 filled with nitrogen or other indifferent gas. Not only so, but it 



Fig. 123. Fatigue of a Pair of Sartorius Muscles (Fletcher). A, in an atmosphere of 

 oxygen; B, in an atmosphere of nitrogen. A is partially restored by a rest of 

 five minutes. 



can be made to contract many times in this oxygen-free atmosphere, 

 although it loses its power of contraction sooner than in oxygen, 

 and does not show the same capacity for recuperation during an 

 interval of rest. In mammals the muscles can also be made to 

 contract repeatedly when the dissociable oxygen has, as far as pos- 

 sible, been got rid of from the blood by asphyxiating the animal, and 

 to give off a correspondingly large quantity of carbon dioxide, 

 although they lose their contractibility much more rapidly than the 

 muscles of the frog. This has usually been interpreted as meaning 

 that the carbon dioxide does not arise, so to speak, on the spot, from 

 the immediate union of carbon and oxygen, but that a stock of 

 it is taken up by the muscle, and stored in some compound or 

 compounds, which are broken down during contraction, and more 

 slowly during rest, carbon dioxide in both cases being one of 

 the end-products. In a normal muscle with intact circulation, 



