CIRCULATING SYSTEM. 355 



sumption of oxygen diminishes, the body begins to ex- 

 perience fatigue. With the nocturnal animals, this ar- 

 rangement is probably reversed, to correspond with their 

 season of activity. 



The influence of temperature in increasing or diminish- 

 ing th0 consumption of oxygen is likewise considerable. 

 CRAWFORD found that a Guinea-pig confined in air at the 

 temperature of 55, consumed double the quantity which 

 it did, when placed in air at 104. He likewise found, in 

 such cases, that the venous blood, when the body was ex- 

 posed to a high temperature, had not its usual dark colour; 

 but, by its florid hue, indicated that no change had taken 

 place in its constitution, in the course of circulation. When 

 the temperature of warm-blooded animals is greatly aug- 

 mented, exertion becomes laborious, a great degree of las- 

 situde is speedily induced, and a condition of the system is 

 produced, similar to that which follows great muscular ef- 

 forts. In the cold-blooded animals, on the other hand, 

 whose exertions are so much under the influence of tem- 

 perature, that they become torpid when cooled below a 

 certain degree, the effect of a moderate degree of heat will 

 be to increase muscular action, and a corresponding con- 

 sumption of oxygen. The experiments of SPALLANZANI 

 and others, are in conformity with such suppositions. 



Upon a review of the different circumstances which have 

 been stated, as influencing the consumption of oxygen, it 

 appears obvious, that it keeps pace with the degree of mus- 

 cular action, and is dependent upon it, consequently, a 

 state of increased consumption is always followed by an 

 equally great decrease, in the same manner as activity is 

 followed by fatigue. Yawning and drowsiness indicate 

 muscular exhaustion, and they likewise indicate a decreased 

 consumption of oxygen. 



This consumption of oxygen is the index of the quanti- 

 ty of carbon which is thrown out by the system. In man, 



