NERVE MECHANISM OF HEART. 283 



within the cranium ; and (d) reflexly by the stimulation of many 

 afferent nerves, particularly the sympathetic and those bearing 

 impulses from the abdominal viscera to the medulla, as well as 

 the ordinary sensory nerves, or by the afferent fibres of the oppo- 

 site vagus. 



Muscarine produces diastolic standstill of the heart by exciting 

 the local inhibitory ganglia or vagus terminals. Atropin causes 

 quickening of the heart's action by paralyzing the endings of the 

 vagus, and also those intrinsic mechanisms which are supposed to 

 have an inhibitory effect. Nicotine produces at first a slowing of 

 the heart by stimulating the inhibitory tone of the vagus. This 

 is soon followed by exhaustion of the terminal fibres and a con- 

 sequent quickening of the heart beat. Large doses of curare 

 paralyze the inhibitory fibres. Digitalis excites the vagus centre 

 in the medulla, and thereby reduces the rapidity of the heart's 

 beat. 



THE ACCELERATOR NERVES. 



After the possibility of increase of blood pressure has been 

 removed by section of the splanchnic nerves, and the tonic inhi- 

 bition of the vagi has been cut off, it has been found that stimu- 

 lation of the cervical portion of the spinal cord causes quicken- 

 ing of the heart beat. In the cervical portion of the spinal cord 

 nerve channels must then exist which are capable of stimulating 

 the muscle fibres of the heart, so as to cause it to beat more 

 quickly. These accelerator fibres pass through the communica- 

 ting branches from the cord to the last cervical or first dorsal 

 sympathetic ganglion, 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 consider- 

 able time after the stimulus is removed. Stimulation of the ac- 

 celerator 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. 



