CH. XX.] INNERVATION OF THE HEART 247 



tambour to the cardiometer ; when the heart expands, the air will move in the 

 reverse direction. These movements are written by the end of the lever of the 

 tambour, and variations in the excursions of this lever correspond with variations 

 in the amount of blood expelled from or drawn into the heart with systole and 

 diastole respectively. By calibrating the instrument the actual volume of the 

 blood expelled can be ascertained. 



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 ; from these cells fresh fibres are distributed 

 among the muscular fibres. In addition to these nerves, which are 

 efferent, we have to mention : 



3. 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, b, and c ; there is a good 

 deal of blending of the rootlets before they ultimately emerge from 

 the skull, but the a group corresponds fairly well with the fibres of 

 the glossopharyngeal, b with those of the vagus, and c with those of 

 the spinal accessory. 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-inhibitory. 

 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 so exercised varies 

 a good deal in different animals. The most potent artificial stimulus 



