CHAP, ii.] RESPIRATION 349 



the blood ? or do certain oxidizable reducing substances pass from 

 the tissues into the blood, and there become oxidized into carbonic 

 acid and other products, so that the chief oxidation takes place in 

 the blood itself? 



There are, it is true, reducing oxidizable substances in the 

 blood, but these are small in amount, and the quantity of carbonic 

 acid to which they give rise when the blood containing them is 

 agitated with air or oxygen, is so small as scarcely to exceed the 

 errors of observation. 



On the other hand, it will be remembered that in speaking of 

 muscle, we drew attention to the fact that a frog's muscle removed 

 from the body (and the same is true of the muscles of other animals) 

 contains no free oxygen whatever; none can be obtained from 

 it by the mercurial air-pump. Yet such a muscle will not only 

 when at rest go on producing and discharging a certain quantity, 

 but also when it contracts evolve a very considerable quantity, of 

 carbonic acid. Moreover this discharge of carbonic acid will go on 

 for a certain time in muscles under circumstances in which it is 

 impossible for them to obtain oxygen from without. Oxygen, it is 

 true, is necessary for the life of the muscle : when venous instead 

 of arterial blood is sent through the blood-vessels of a muscle, the 

 irritability speedily disappears, and unless fresh oxygen be ad- 

 ministered the muscle soon dies. The muscle may however, 

 during the interval in which irritability is still retained after the 

 supply of oxygen has been cut off, continue to contract vigorously. 

 The supply of oxygen, though necessary for the maintenance of 

 irritability, is not necessary for the manifestation of that irritability, 

 is not necessary for that explosive decomposition which developes 

 a contraction. A frog's muscle will continue to contract and to 

 produce carbonic acid in an atmosphere of hydrogen or nitrogen, 

 that is in the total absence of free oxygen both from itself and 

 from the medium in which it is placed. 



Thus on the one hand the muscle seems to have the property 

 of taking up and fixing in some way or other the oxygen to which 

 it is exposed, of converting it into intra-molecular oxygen, in which 

 condition it cannot be removed by simple diminished pressure, so 

 that the tension of oxygen in the muscular substance may be con- 

 sidered as always nil ; while on the other hand the muscular sub- 

 stance is always undergoing a decomposition of such a kind that 

 carbonic acid is set free, sometimes, as when the muscle is at rest, 

 in small, sometimes, as during a contraction, in large quantities. 

 But if the oxygen tension of the muscular tissue be thus always 

 nil, the oxygen of the blood-corpuscles, in which it is at a com- 

 paratively high tension, will be always passing over, through the 

 plasma, through the capillary walls, the lymph spaces and the 

 sarcolemma, into the muscular substance, and as soon as it arrives 

 there will be hidden away as intra-molecular oxygen, leaving the 

 oxygen tension of the muscular substance once more nil. Con- 



