RESPIRATION 261 



oxygen, it would still be oxygen in transit from the capillaries 

 towards places where the partial pressure of oxygen is less. In 

 the lymph, the pressure is kept lo\v by the avidity of the tissues 

 with which it is in contact, and possibly by the existence 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 ex- 

 cised frog's muscle is capable of going on producing carbon 



FIG. no. 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. 



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 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 mam- 

 mals the muscles can also be made to contract repeatedly when 

 the dissociable oxygen has, as far as possible, been got rid of 

 from the blood by asphyxiating the animal, and to produce 

 a correspondingly large quantity of carbon dioxide, although 

 they lose their contractility much more rapidly than the muscles 

 of the frog. This leads us to the important conclusion that the 

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

 the immediate union of carbon and oxygen. Oxygen is essential 

 to muscular life and action. But a stock of it is apparently 



