144 A MANUAL Of PHYSIOLOGY 



the augmentor nerves the power of causing destructive (katabolic) 

 changes, apart from mechanical effects. But all that we really know 

 is that electrical changes and chemical changes can both be evoked 

 in living tissues. We are quite ignorant of the relation between 

 the two. 



The Normal Excitation of the Cardiac Nervous Mechanism. 

 We have now to inquire how this elaborate nervous 

 mechanism is normally set into action. And we may say at 

 once that, striking as are the effects of experimental stimula- 

 tion 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 im- 

 pulses which regulate the activity of the heart are normally 

 discharged. Inhibitory impulses seem to be 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 

 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, 



