CH. XX.] 



REFLEX INHIBITION 



251 



JugularGanglion 



Ganglion 

 trunci 

 Vagi 



Vagus 



early that no nerves have yet grown to the heart, these drugs have 

 little or no effect. (Pickering.) 



Reflex Inhibition. Thus there is no doubt that the vagi nerves 

 are simply the media of an inhibitory or restraining influence over 

 the action of the heart, which is conveyed through them from the 

 centre in the medulla oblongata, which is always in operation. The 

 restraining influence of the 

 centre in the medulla may> 

 be reflexly increased by 

 stimulation of many afferent 

 nerves, particularly those 

 from the nasal mucous 

 membrane, the larynx, and 

 the lungs. A blow on the 

 abdomen causes inhibition 

 and fainting; a blow on the 

 larynx, even a moderate one, 

 will kill. There is no com- 

 parison between the ease 

 with which stimulation of 

 the laryngeal or pulmonary 

 fibres produces inhibition, as 

 compared to the difficulty of 

 obtaining inhibition from the 

 alimentary tract. Chloro- 

 form vapour* and tobacco 

 smoke in some people and 

 animals, by acting on the 

 terminations of the vagi or 

 their branches in the respi- 

 ratory system, may also pro- 

 duce reflex inhibition of the 

 heart. Some very remark- 

 able facts concerning the 

 readiness by which reflex 

 inhibition of the fish's heart 

 may be produced were made 

 out by M'William; any 

 slight irritation of the tail, gills, mucous membrane of mouth and 

 pharynx, or of the parietal peritoneum, causes the heart to stop 

 beating. 



In connection with the subject of reflex inhibition, it may be 



* Embley, however, considers that the usual cause of sudden death during 

 the early stages of chloroform narcosis is the direct action of the drug on the vagus 

 centre. In animals, cutting the vagi immediately sets the heart going again. 



Cardiac 

 branches 



Cervical 

 'Sympathetic. 



^nferior Cervical 

 Ganglion 



Subclauian 

 Artery 



Annulus 



Ganglion Stellatum 



Ant. root 

 d.Thoracic 

 Nerve 



Post, root 



Third 

 Thoracic 

 Ganglion 



FIG. 258.- Heart nerves of mammal. (Diagrammatic.) 



