1114 



PHYSIOLOGY 



Krogh suggests that this enormous increase in the number of patent capil- 

 laries may be brought about to meet requirements of the muscle other than 

 those for oxygen. These observations afford further support for the view 

 already put forward that the capillaries do not play a merely passive role 

 in the circulation, but by active dilatation or constriction are largely respon- 

 sible for determining the actual blood supply to each tissue in accord with 

 its metabolic requirements. 



Under normal circumstances a blood corpuscle never stays long enough 

 in the proximity of the tissues to lose its whole store of oxygen. If however 

 the further supply of oxygen to the blood be prevented, as in asphyxia, 

 the last traces of oxygen disappear from the blood. The enormous avidity 

 of the tissues for oxygen under these circumstances is shown by the following 

 experiment (Ehrlich). If a saturated solution of methylene blue be injected 



FIG. 511. Curves showing the rate at which arterial blood is reduced on bubbling 

 through a gas free from oxygea, and the effect on the rate of the presence of 

 . CO 2 and of lactic acid. Ordinates = percentage saturation of oxyhaemoglobin. 

 Abscissa? time in minutes. (MATHISON.) 



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. These pheno- 

 mena are due to the production in the tissues of reducing bodies, whose 

 avidity for oxygen is so great that they are 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 



