7 34 SCIENCE PROGRESS 



salt is formed in the capillaries as not due merely to " fading 

 in contact with alkaline tissues." 1 



The change, if it must still be called "fading," is really 

 reduction by living tissues acting on the blue ferric salt in an 

 alkaline medium, the green or white ferrous salt being produced. 



My observations show that we can be more explicit as to 

 the fading, and assign it to its cause — intra vitam reduction. 

 No doubt the salts of the tissues constitute the requisite 

 alkalinity of the medium in which the reduction takes place, 

 and hence it is that in the (acetic) acid medium recommended 

 by Rawitz 2 the reduction is very much less complete : this is, 

 of course, what for histological purposes is desired. 



Reducing power is not absent from the blood, although its 

 energy is far below that possessed by tissues. It is demon- 

 strated in those vessels from which blood has not been washed 

 out by salt solution prior to injection, the mixture of the blood 

 and blue gelatine being greenish and not purple. If into 

 defibrinated blood in vitro a little warm, liquid blue gelatine 

 be poured, a green liquid is observed to be the result ; and if 

 into excess of liquid gelatine a few drops of blood be let fall 

 or a few flecks of blood-clot dropped, the blood in both con- 

 ditions will be seen to have become green. Either the living 

 leucocytes or the " reducing substances " (Pfliiger) in blood, 

 or both conjointly, are responsible for this reduction of the 

 Prussian blue ; but this change of colour to green cannot be 

 attributed to the action of the inorganic salts of the blood. 



The red of blood and the blue of the soluble Prussian blue, 

 if physical factors alone co-operated, would produce a purple, 

 reddish or bluish according as the colour of the blood or 

 the gelatine prevailed ; but not a green, which indicates that a 

 chemical factor has been at work. 



In blood-vessels, other than capillaries from which all the 

 blood has been previously washed out, the blue colour of the 

 gelatine is unaltered, whereas in the capillaries the substance 

 is of the palest green or is colourless. This is precisely what 

 we should have expected, since blood normally and blue gelatine 

 artificially is being brought within the sphere of the biochemical 

 (reducing) activity of the living cells only in the capillaries. 

 The thick-walled vessels, in that they merely convey blood to 



1 Mann, Physiological Histology, Oxford, 1902, p. 160. 



2 I did. p. 160. 



