2;o RESPIRATION 



When this increased utilization reaches its physiological limit, 

 further increase in the oxygen consumption per minute can only 

 be obtained by increase in the rate of heartbeat. 



The mixed venous blood returning to the heart comes from 

 various parts of the body; but during muscular exertion a very 

 greatly increased proportion must come from the muscles. Now 

 there is evidence from a series of experiments by Leonard Hill 

 and Nabarro that the venous blood returning from the muscles 

 contains even during rest far less oxygen and more CO 2 than at 

 any rate the venous blood returning from the brain. 9 Without 

 obstructing the vessels they collected venous blood returning from 

 muscles through the deep femoral vein, and from the brain 

 through the torcular Herophili in the dog. The following table 

 shows the average of about eight determinations in each case. 



It will be seen ( I ) that during rest the blood lost three and one- 

 half times as much of its charge of oxygen in the muscles as in 

 the brain; (2) that during the intense activity of a tonic or clonic 

 fit (produced by absinthe) the percentage loss of oxygen by the 

 blood was only slightly increased in either the brain or the muscles. 

 The animals were anaesthetized with morphia or chloroform, so 

 it is possible that the circulation was less active than in normal 

 animals; but the difference between the brain circulation and 

 that through muscles is none the less striking. 



In the light of these experiments we can see what is presumably 

 happening as regards the mixed venous blood during muscular 



9 Leonard Hill and Nabarro, Journ. of Physiol., XVIII, p. 218, 1895. 



