468 RESPIRATION OF THE TISSUES. [Book ii. 



which are undoubtedly oxidized in the body is so small that it 

 may be neglected in the present considerations. If grape-sugar 

 be added to blood, or to a solution of haemoglobin, the mixture 

 may be kept for a long time at the temperature of the body, 

 without undergoing oxidation. Even within the body an even 

 slight excess of sugar in the blood over a certain percentage 

 wholly escapes oxidation, and is discharged unchanged. 



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

 of muscle, we drew attention (§ 58) 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 neces- 

 sary for the life of the muscle : when venous instead of arterial 

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

 bility speedily disappears, and unless fresh oxygen be admin- 

 istered the muscle soon dies. The muscle may however, during 

 the interval in which irritability is still retained after the sup- 

 ply 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 irrita- 

 bility, is not necessary for that explosive decomposition which 

 develops a contraction. A frog's muscle will continue to con- 

 tract 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 prop- 

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

 to which it is exposed, of storing it up in its own substance in 

 such a condition that it cannot be removed by simple diminished 

 pressure (so that the pressure of oxygen in the muscular sub- 

 stance may be considered as always nil), and yet has not entered 

 into any distinct combination which we can speak of as an oxi- 

 dation, but is still available for such a purpose. On the other 

 hand the muscular substance is always undergoing a decomposi- 

 tion of such a kind that carbonic acid is set free, sometimes, as 

 when the muscle is at rest, in small, sometimes, as during a con- 

 traction, in large quantities. The oxygen present in this car- 

 bonic acid, as an oxidation product, comes from the previously 

 existing store of which we have just spoken. The oxygen taken 

 in by the muscle, whatever be its exact condition immediately 

 upon its entrance into the muscular substance, sooner or later 

 enters into a combination, or perhaps we should rather say, 



