62 PHYSIOLOGY 



This pigment changes from purple to scarlet as this 

 occurs. Oxygen thus continually drawn from the envi- 

 ronment is offered through the circulation of the blood 

 to every organ in the body. Each organ takes it thence, 

 and takes the more the greater its activity. The mere 

 colour of the blood reveals whether it holds much oxygen 

 or little. So great is the avidity of the organs for 

 oxygen that they act as chemical reducers. Methylene 

 blue loses its colour under the activity of reducing sub- 

 stances. If an animal into whose circulation methylene 

 blue has been introduced be sacrificed, none of the 

 pigment appears on first examination of the organs to 

 have reached a number of them. Only after exposure 

 to the air do they become blue. The pigment had 

 reached them, but so great was their avidity for oxygen 

 that it existed in them in its colourless reduced condition, 

 until their surface was exposed directly to the air. 



What is the meaning of this avidity for oxygen ? The 

 tie is close between the body's intake of oxygen and its 

 warmth. The body is built of substances chemically 

 unstable, and it is essential for life that they be so. 

 They break down, and in doing so liberate energy 

 engaged within them until then in so-called potential 

 form. They are complex and they break down variously. 

 They crumble gradatim. Every downward step in their 

 decomposition liberates heat that contributes to the 

 temperature of the body. Manifold substances arise 

 from them in this degradation, and these vary in 

 different organs and probably at different times. But 

 the end-results of the decomposition are practically in- 

 variable. The end-products are relatively simple. It 

 was one of these that John Mayow had before him 

 in his heavy irrespirable gas, given out alike by the 

 animal in living and by the flame in burning carbon 



