CH. XXL] INNERVATION OF THE HEART 249 



Innervation of the Heart. 



The nerves of the heart, which under normal circumstances 

 control its movements, are : 



1. Cardiac branches of the vagus (inhibitory fibres). 



2. Cardiac branches of the sympathetic (augmentor and acceler- 

 ator fibres). 



These pass to the heart and terminate in certain collections of 

 ganglion cells in its wall (terminal ganglia) ; from these cells post- 

 ganglionic fibres are distributed among the muscular fibres. In 

 addition to these nerves, which are efferent, we have to mention : 



The sensory or afferent nerves of the heart, the best known of 

 which is called the depressor nerve. This nerve, starting from the 

 cardiac tissue, joins the vagus trunk; it passes to the bulb, especially 

 to the vaso-motor centre. We shall therefore postpone its study 

 until we are considering the vaso-motor nerves. 



The Vagus. The ninth, tenth, and eleventh cranial nerves arise 

 close together from the grey matter in the floor of the fourth ventricle, 

 and leave the bulb by a number of rootlets. These rootlets are 

 divided by Grossmann into three groups, a, &, and c ; there is a good 

 deal of blending of the rootlets before they ultimately emerge from 

 the skull, but the a (the uppermost) group corresponds fairly 

 well with the fibres of the glossopharyngeal, b with those of 

 the vagus, and c (the lowermost) with those of the spinal acces- 

 sory. The rootlets of the tenth nerve pass through two ganglia 

 called respectively the jugular ganglion, and the ganglion trunci 

 vagi. The fibres of the spinal accessory nerve which join the 

 vagus are chiefly motor, especially to the larynx, but some go to the 

 heart. The vagus gives off branches to many organs, pharynx, larynx, 

 heart, lungs, oesophagus, and various abdominal organs. We have, 

 however, in this place only to deal with its cardiac fibres. It has 

 been known since the experiments of the Brothers Weber in 1845 

 that stimulation of one or both vagi produces slowing or stoppage of 

 the beats of the heart. It has since been shown that in all vertebrate 

 animals, this is the normal result of vagus stimulation ; the pheno- 

 menon is called inhibition, and the nerve-fibres cardio-irihibitory. 

 Section of one vagus produces slight acceleration of the heart ; this 

 result is better marked when both vagi are divided. This shows that 

 the restraining influence of the vagus is being continuously exercised ; 

 it is, however, found that the amount of vagus control varies a 

 good deal in different animals. The most potent artificial stimulus 

 which can be applied to the vagus nerve to produce inhibition of the 

 heart is a rapidly interrupted induction current ; severe mechanical 

 stimuli have a slight effect, but chemical and thermal stimuli are 

 ineffective. 



