THE NERVOUS REGULATION OF THE HEART 167 



fibres pass, has been thought to bring about by direct stimulation 

 a slowing or a quickening of the pulse. In some individuals the 

 vagus has been excited by compressing it against a bony tumour 

 in the neck; and by compressing the nerve against the vertebral 

 column it is possible to cause inhibition in many normal persons, 

 although it ought to be stated that the experiment is not free from 

 danger. But it is from the cardio-inhibitory and cardio-augmentor^ 

 centres in the medulla oblongata that the impulses which regulate 

 the activity of the heart are normally discharged. Inhibitory im- 

 pulses are constantly passing out from the medulla, for section of 

 both vagi causes almost invariably an increase in the rate of the 

 heart, at least in mammals, although the increase is less conspicuous 

 in animals like the rabbit, whose normal pulse-rate is high, than in 

 animals like the dog, whose pulse-rate is comparatively low. Section 

 of one vagus usually causes only a comparatively slight increase, for 

 the other is able of itself to control the heart. It is not certainly 

 known whether the augmentor centre in like manner discharges a 

 continuous stream of impulses, or is only roused to occasional activity 

 by special stimuli. For the results of section of the nervi acceler- 

 antes, or the extirpation of the inferior cervical and stellate ganglia, 

 are dubious and conflicting. But if it does exert a tonic influence 

 on the heart, this is feebler than the tone of the inhibitory centre. 

 As to the nature of this inhibitory tone, and the manner in which it 

 is maintained, we know but little. It may be that the chemical 

 changes in the nerve-cells of the inhibitory centre lead of themselves 

 to the discharge of impulses along the inhibitory nerves. But there 

 is some evidence that, in the complete absence of stimulation from 

 without, the activity of the centre would languish, and perhaps be 

 ultimately extinguished. For when the greater number of the 

 afferent impulses have been cut off from the medulla oblongata by 

 a transverse section carried through its lower border, division of the 

 vagi produces little effect on the rate of the heart. Also, when the 

 upper cervical cord and the brain are resuscitated after a period of 

 anaemia, the return of cardio-inhibitory tone is tardy in comparison 

 with the return of the truly automatic function of respiration, and 

 does not seem to precede the opening up of the afferent paths to the 

 cardio-inhibitory centre. Indeed, reflex inhibition may be produced 

 at a time when the inhibitory centre has regained none of its tone. 

 The suggestion is that the normal tone of the centre is largely 

 dependent upon reflex impulses. Be this as it may, we know that 

 the activity of the inhibitory centre is profoundly influenced and 

 that both in the direction of an increase and of a diminution by 

 impulses that fall into it through afferent nerves and by stimuli 

 directly applied to it. And we may assume that the same is true 

 of the augmentor centre. The common statement that stimulation 

 of the central end of one vagus, the other being intact, produces 



