11 THE CARDIO-IXHIBITORY CENTRE 75 



beat, while the other, the accelerator, conveys impulses 

 which make it beat faster. Since experiments have showTi 

 that the mechanism just described exists equally in the 

 mammalian heart, we may at once apply these striking 

 results t^ the human heart. It is, in fact, recorded of a 

 certain well-known physiologist, that having a small hard 

 tumour in his neck, in close proximity to the vagus nerve, 

 he could press the vagus against this tumour and by 

 thus stimulating it mechanically cause a stoppage of his 

 own heart-beat. 



The heart, then, is controlled by two kinds of antagon- 

 istic influences, analogous to those previously described 

 as controlling the muscular walls of the arteries. More- 

 over both the cardiac nerves are connected with the 

 central nervous system, the one coming from the spinal 

 bulb, the other from the spinal cord, so that the in- 

 fluences they convey to the heart must, as in the case of 

 the vaso-raotor nerves, originate in the central nervous 

 system. We saw, however (p. 69), that the impulses 

 carried by the vaso-motor nerves are generated in a very 

 specially localised part of the spinal bulb, and the inter- 

 esting question at o:ice arises : Is there a similarly local- 

 ised centre in which the impulses which modify the beat 

 of the heart take their origin ? The answer to this ques- 

 tion is in the affirmative, for experiment shows tliat the 

 impulses which, travelling along the vagus, can stop or, 

 as the physiologist savs "inhibit," the heart's beat, are 

 generated in a limited part of the spinal bulb, in close 

 proximity to the vaso-motor centre. This part is there- 

 fore known as the cardio-inhibitory centre. There 

 are reasons for supposing that this centre, like the vaso- 

 motor centre, is continually at work sending out impulses 

 to the heart along the vagus, which check its activity, so 

 that in many animals the heart beats more quickly after 

 the vagus nerves are cut. 



The cardio-inhibitory centre may, like the vaso-motor 



