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 the consumption of oxygen is likewise considerable. 
Crawrorp 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 im 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. 
72 
