28o RESPIRATION 



and secondly by the equally well-known accelerator impulses 

 passing to the heart through sympathetic branches. Increased 

 liberation of inhibitory impulses has been found to be a direct re- 

 sult of rise of arterial blood pressure (so that the inhibition tends to 

 prevent an excessive rise of arterial pressure and consequent fa- 

 tigue of the heart or over-distention of arteries), but is certainly 

 also a result of rise in oxygen pressure and diminution in CO2 

 pressure in the blood passing through the brain. An increase of 

 arterial blood pressure will, therefore, owing to the increased 

 rate of circulation, slow the heart. When the arterial blood pres- 

 sure is normal there is a considerable amount of vagus inhibition, 

 so that on section of the vagi the heartbeats quicken. It appears 

 also that this tonic nervous inhibition of the heart is itself reflexly 

 inhibited, either directly or indirectly, by increase of pressure on 

 the great veins opening into the heart. This was recently shown by 

 Bainbridge,^^ who found that, even if the accelerator nerves are 

 cut, increase in venous pressure causes marked quickening of the 

 heartbeats provided that the vagi are still intact. He showed that 

 any considerable increase in venous pressure causes quickening 

 of the heartbeat, and that the quickening depends upon the in- 

 tegrity of the vagus nerves. Part, at any rate, of this effect is due 

 to inhibition of the tonic inhibitory action of efferent vagus fibers. 

 Another part is probably due to reflex excitation of accelerator 

 nerves, but on this point the evidence was not so clear. The action 

 of the heart is not subject to direct voluntary control, but the ef- 

 fects of emotional stimuli on the rate of heartbeat are well known 

 and very evident. 



There is no necessary connection between rate of heartbeat and 

 circulation rate. This has been shown by various experiments, but 

 most strikingly by the experiments of Starling and his pupils on 

 the bodies of animals in which an artificial circulation through 

 the heart and lungs alone had been established, the physiological 

 connections with central nervous system and rest of the body being 

 cut off. In such a "heart-lung preparation" the rate of heartbeat 

 remains steady for long periods if the temperature is kept steady 

 and artificial respiration is maintained; but the flow of blood can 

 be varied within very wide limits by simply varying the rate at 

 which blood is supplied to the right side of the heart. Thus Pat- 

 terson and Starling found that with a pulse rate which was steady 

 at 144 the circulation rate in a heart-lung preparation from the 



" Bainbridge, Journ. of Physiol., L, p. 65, 19 15. 



