366 RESPIRATION. 



In all these cases the great question which comes up for our consideration 

 is this : Does the oxygen pass from the blood into the tissues, and does the 

 oxidation take place in the tissues, giving rise to carbonic acid, which passes 

 in turn away from the tissues into the blood ? or do certain oxidizable redu- 

 cing 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. 



We may add that the oxidative power which the blood itself removed 

 from the body is able to exert on substances which are undoubtedly oxidized 

 in the body is so small that it may be neglected in the present considera- 

 tions. 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 with- 

 out undergoing oxidation. Even within the body a slight excess of sugar 

 in the blood over a certain percentage wholly escapes oxidation, and is dis- 

 charged unchanged. Many easily oxidized substances, such as pyrogallic 

 acid, pass largely through the blood of a living body and are discharged in 

 the urine without being oxidized ; though perhaps in some of these cases, 

 what appears to be an absence of oxidation is really an oxidation followed 

 by a subsequent equivalent reduction taking place in the urine or elsewhere. 

 The organic acids, such as citric, even in combination with alkaline bases, 

 are only partially oxidized ; when administered as acids, and not as salts, 

 they are hardly oxidized at all. It is, of course, quite possible that the 

 changes which the blood undergoes when shed might interfere with its 

 oxidative action, and hence the fact that shed blood has little or no oxidiz- 

 ing power, is not a satisfactory proof that the unchanged blood within the 

 living vessels may not have such a power. But did oxidation take place 

 largely in the blood itself, one would expect even highly diffusible substances 

 to be oxidized in their transit ; whereas if we suppose the oxidation to take 

 place in the tissues, it becomes intelligible why such diffusible substances as 

 those which the tissues in general refuse to take up largely should readily 

 pass unchanged from the blood through the excreting organs. 



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

 drew attention ( 61) 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 im- 

 possible 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 bloodvessels of a muscle, the irritability speedily disappears, 

 and unless fresh oxygen be administered 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 ex- 

 plosive decomposition which develops 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 hydrogen, both 

 from itself and from the medium in which it is placed. 



