IX ANIMAL TISSUES.^HARRIS. 261 



decoloration was not due to ionized hydrogen at the kathode, 

 for when the cortical excitability had disappeared, the re- 

 duction of the pigment at a stimulated spot could no longer 

 be obtained. 



These workers therefore recognized the simultaneous 

 activity of two processes oxidation and reduction, the precise 

 colour at any moment being the result of the relative pre- 

 dominance of the one process over the other. Frequenth' 

 they found that oxidation prevailed over reduction. 



In 1896 I(-')*found that living tissues of cat and rabbit, 

 — kidney, liver, heart, glands — -reduced the blue pot- 

 assio-ferric ferrocyanide in the Prussian blue and gelatine 

 injection mixture to the green or white leuco state of the di- 

 potassio-ferrous ferrocyanide which on exposure to air slowly, 

 or bj' treatment with hydrogen dioxide rapidly, became blue 

 again. 



The pigment was reduced only in the washed out smaller 

 vessels and capillaries; in presence of blood not washed out of 

 the larger vessels, the Prussian blue remained unreduced. 

 The colour of the blood was therefore a purple. 



In 1899 the term" reductase" as indicating a tissue- 

 ferment, capable of effecting reduction processes seems to 

 have been first used bj^ Abelous and Gerard. (^) 



Pozzi-EscoT('*)in 1902 published the results of work onthe 

 reducing action of vegetable and animal tissues on solutions 

 of indigo, litmus and Prussian blue out of contact with air. 

 He confirmed Rey-Pailhade in finding that the tissues could 

 form hydrogen sulphide from sulphur and could reduce 

 potassium iodide when out of contact with air. 



He held that a reductase might be suspected when a 

 living tissue decomposes H2 O2. but does not affect a mixture 

 of guaiacum and H2 O2. 



*At this date I had seen only Ehrlieh's paper on oxygen avidity. 



