278 MANUAL OF PHYSIOLOGY. 



These accelerator fibres pass through the communicating branches 

 from the cord to the last cervical or first dorsal sympathetic gan- 

 glion, and thence to the heart. Stimulation of the ganglia or the 

 branches passing thence to the heart quickens its beat. The effect 

 of stimulus applied to these nerves does not begin to show itself 

 until a comparatively long time after it has been applied, and the 

 acceleratory effort continues for a considerable time after the 

 stimulus is removed. Stimulation of the accelerator fibres has 

 little effect on the tonic inhibition of the vagus, which takes place 

 equally well whether the accelerators are stimulated or not, while 

 the action of the accelerators is totally suspended so long as the 

 vagus is being stimulated. 



AFFERENT CARDIAC NERVES. 



Besides these nerve channels bearing impulses to the heart, 

 others pass from the heart to the medulla, probably having their 

 origin in the inner lining of the heart, which is known to be the 

 part most sensitive to stimulus. 



These fibres appear to be of two kinds, one of which affects the 

 cardio-inhibitory centre and diminishes the pulse rate; the other 

 affects the vaso-inhibitory centre and lowers the blood pressure. 

 Increase of the intra-ventricular pressure stimulates both these 

 sets of fibres, and thus we see that over-filling of the heart from 

 increase of blood pressure, etc., causes retardation of its beat, and 

 an equilibrium is thus established between the general blood 

 pressure and the force of the heart beat. 



