THE CIRCULATION OF THE BLOOD AND LYMPH 153 



striking as are the effects of experimental stimulation of the 

 vagus trunk or the nervi accelerantes in their course, it is only 

 under exceptional circumstances that the efferent nerve-fibres, 

 at any rate before they have entered the heart, can be directly 

 excited in the intact body. In certain cases the pressure of a 

 tumour or an aneurism on the nerve-trunks, or, in the case of 

 the accelerators, the progress of a pathological change in the 

 sympathetic ganglia through which the fibres pass, has been 

 thought to bring about by direct stimulation a slowing or a 

 quickening of the pulse. In some individuals the vagus may be 

 excited by compressing it against the vertebral column or against 

 a bony tumour in the neck. 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 impulses are constantly passing out 

 from the medulla, for section of both vagi causes almost invari- 

 ably 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 accelerantes, 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 evi- 

 dence that, in the complete absence of stimulation from without, 

 the activity of the centre would languish, and perhaps be ulti- 

 mately 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. 



