248 A MANUAL OF PHYSIOLOGY 



Many of the older experiments made by method (2) are 

 too inexact to yield more than qualitative results, and the 

 same is true of some of the researches with the more primi- 

 tive and imperfect methods of artificial circulation. The 

 mere difference of colour between the venous and arterial 

 blood of a muscle, or other active organ, is sufficient to 

 show that oxygen is taken up and carbon dioxide given out 

 by it to the blood. This is the case in muscles at rest, 

 and even in muscles with artificial circulation after they have 

 become inexcitable. 



In active muscles more oxygen is used up and more 

 carbon dioxide produced than in the resting state. Chauveau 

 and Kaufmann, in their experiments on one of the muscles 

 used by the horse in feeding, found that the consumption of 

 oxygen and the production of carbon dioxide might be three 

 times as great in activity as in rest. 



In the submaxillary salivary gland there was also an 

 increase of carbon dioxide during activity, but not propor- 

 tionally so great as in muscle. In the active brain it is not 

 easy to demonstrate any increase at all (Hill). 



For excised mammalian muscles (hind-limbs of dog), as 

 has been said, the respiratory quotient increases when the 

 temperature is reduced. As the temperature is raised, the 

 opposite effect is observed. Stimulation of the muscle causes 

 a rise in both oxygen consumption and carbon dioxide pro- 

 duction, but proportionally more in the former, and the 

 respiratory quotient diminishes. When the excised muscle 

 begins to deteriorate in the course of some hours, the con- 

 sumption of oxygen falls off more quickly than the produc- 

 tion of carbon dioxide. 



All this goes to show that the two processes are to a great 

 extent independent of each other. At the higher tempera- 

 tures, during muscular contraction, and when the vitality of 

 the muscle is still but little impaired, the conditions are 

 relatively favourable to the chemical changes in which 

 oxygen is combined. Low temperature, rest, and diminished 

 vitality, are relatively favourable to the splitting up of sub- 

 stances that yield carbon dioxide. But it must be remem- 



