THE BIOCHEMISTRY OF RESPIRATION 261 



but if minced animal tissue be added, the rate of indophenol 

 formation is accelerated ten- to a hundred-fold. By extracting 

 the indophenol with alcohol and estimating its amount colouri- 

 metrically, a measure is obtained of the oxidase content of a 

 tissue. 1 The writer found that in practically all the mammals 

 and birds examined the heart muscle is richest in indophenol 

 oxidase. Next in order come kidney, brain, liver, and lung. 

 The oxidasic power seems, as a rule, to vary with the magnitude 

 of the oxidation processes occurring in the tissues. Thus the 

 grey matter of the brain contains at least three times more 

 oxidase than the white matter, and the renal cortex ten times 

 more than the renal medulla. The oxidase content of muscles 

 is extremely variable. The pectoral muscles of vigorous flying 

 birds are nearly as rich in oxidase as the heart muscle, whilst 

 their leg muscles contain less than a third as much. The tame 

 duck, on the other hand, has much more oxidase in its leg 

 muscles than in its pectoral muscles. In mammals, red muscles 

 such as the diaphragm and tongue, which are in a constant state 

 of activity, are very much richer in oxidase than the white 

 muscles. Taking the organism as a whole, its oxidasic power 

 varies with the magnitude of its respiratory exchanges. For 

 instance, the C0 2 output of the canary and sparrow is about 

 eight times larger than that of the goose, whilst the oxidase 

 content of their tissues is twice as large. The starling, pigeon, 

 and duck have an intermediate C0 2 output and intermediate 

 oxidase content. Of mammals, the harvest mouse has the 

 greatest respiratory exchange and greatest oxidase content of 

 its tissues, whilst next in order in respect of both qualities 

 come the common mouse, the rat, and then larger mammals 

 such as the rabbit, cat, dog, and ox. 



At the same time it must be admitted that the apparent 

 oxidase content of a tissue does not vary in absolute measure 

 with its gaseous metabolism. One reason of this is that the 

 oxidase reaction which is taken as a measure of the oxidase 

 content of the tissues is to some extent interfered with by 

 the reducing substances which are always present. These 

 substances tend to absorb the available oxygen or abstract it 

 from the indophenol blue formed, so that some tissues, such 

 as the white muscles of certain animals, may fail to oxidise 

 the indophenol reagent completely, though undoubtedly they 



1 Vernon, Jonrn. Physiol. 42, p. 402, 191 1 ; 43, p. 96, 191 1. 

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