1186 PHYSIOLOGY 



following experiment (Ehrlich). If a saturated solution of methylene 

 blue be injected into the circulation of a living animal and the animal 

 be killed ten minutes later, it is found on first opening the body that 

 most of the organs present their natural colour, although the blood 

 is a dark blue colour. On exposure to the atmosphere all the organs 

 acquire a vivid blue colour. The avidity of the tissues for oxygen has 

 been so great that they have been able to decompose the methylene- 

 blue molecule, with the formation of a colourless reduction- product, 

 which on exposure to the air undergoes oxidation again and re-forms 

 methylene blue. If the tissues are able to effect the reduction of a 

 comparatively stable body like methylene blue, it is easy to understand 

 their power of reducing oxyhsemoglobin, which is so unstable that it 

 is decomposed by simple physical means such as exposure to a vacuum. 



It was long debated whether the chief processes of oxidation 

 took place in the blood or in the tissues. Our experiences with muscle 

 would alone serve to convince us that, in some tissues at any rate, pro- 

 cesses of oxidation take place, and the methylene-blue experiment 

 shows that these processes of oxidation are intense in all the chief 

 organs of the body. It has been found moreover that it is possible to 

 keep a frog alive after substituting normal saline solution for his blood, 

 if he be placed in absolutely pure oxygen, and that in this case indeed 

 the metabolism of the animal goes on as actively as before. As the 

 frog has no blood, it is evident that its metabolic processes, consisting of 

 the taking up of oxygen and the giving out of carbon dioxide, must 

 have their seat in the tissues. 



As a result of the oxidative changes in the tissues carbon dioxide 

 is produced, and the tension of this gas in the tissues therefore rises. As 

 Barcroft has pointed out, in cold-blooded animals the dissociation of oxy- 

 haemoglobin with the setting free of oxygen must be largely conditioned 

 by the rise of carbon dioxide tension in the tissues, since at the normal 

 temperature of these animals the evolution of oxygen from hemoglobin 

 is extremely slow. The alteration in reaction of the blood caused by 

 a rise in C0 2 tension or by the presence of small amounts of lactic acid, 

 markedly quickens the rate at which oxyhaemoglobin gives up its 

 oxygen, as is shown in Fig. 497. The carbon dioxide tension in the 

 tissues may be approximately measured by taking the tension of this 

 gas in fluids such as the bile or urine. Here it may amount to 8 or 10 

 per cent, of an atmosphere, and since the carbon dioxide in venous 

 blood is rarely above 6 per cent, of an atmosphere, there is a descending 

 scale of tensions from tissue to blood, just as there is an ascending scale 

 in the case of oxygen. This gas therefore passes from the tissues through 

 the lymph into the blood by a simple process of diffusion. 



The carbon dioxide carried by the blood is, like the oxygen, chiefly 

 in a state of chemical combination. From dogs' venous blood we 



