THE BLOOD. 83 



action of oxygen, while the effect of the application of car- 

 bonic acid is to cause them to swell up, and assume a more 

 or less globular figure. 



On this fact Henle reasons thus : accompanying these 

 changes of form there are alterations in the state of aggrega- 

 tion of the colouring matter of the corpuscles; thus, in 

 oxygen, or in any saline solutions, the plasma remains clear 

 and colourless, the blood discs being flattened, and the colour- 

 ing matter contained within them condensed, while in carbonic 

 acid or water the plasma becomes coloured by the escape of 

 a portion of the hematine from the corpuscles, which swell 

 up, and assume a form approaching more or less the globular. 



Now the difference in colour between venous and arterial 

 blood Henle maintains may be accounted for by reference 

 to the form of the corpuscles, and the consequent condition 

 of the particles of the colouring matter. 



And it is also by reference to the state of the colouring 

 matter that Henle accounts for the fact that blood which 

 has once acquired a very dark colour is thereby rendered 

 incapable of re-assuming the bright hue of arterial blood 

 on the application of oxygen or saline solutions, because, he 

 says, that the pigment which had escaped into the plasma, 

 under the influence of the carbonic acid, cannot be made to 

 enter into the corpuscles again, when by means of oxygen 

 they are again flattened. 



The colour of the blood, then, according to Henle, depends 

 upon the single fact of the form of the corpuscle, and that 

 this colour is so much the more bright as these are flat. 



Finally, in support of his theory, Henle refers to changes 

 of colour presented by certain inorganic substances from an 

 alteration in the state of aggregation of its constituent par- 

 ticles : thus it is well known that the ioduret of mercury 

 recently sublimed is yellow ; in cooling its colour passes to 

 scarlet, and pressure determines this change in an instanta- 

 neous manner. 



Such is Henle's mechanical theory of the changes of colour 

 experienced by the blood on the addition of reagents, a theory 

 which, however ingenious it may be, I deem to be insufficient 



H 4 



