1022 



PHYSIOLOGY 



this region; but it seems probable that the activity of these cells is sub- 

 ordinate to impulses arriving at them from the common meeting-place 

 of visceral impulses, viz. the medulla. 



The most important of the afferent nerves, which affect reflexly the 

 action of the heart, are the nerves coming from the heart itself and the aorta. 

 In the mammalian ventricle, nerve fibres can be seen running over the 

 surface of the ventricle which are entirely afferent, stimulation of their 

 peripheral ends causing no effect on the heart beat. Stimulation of their 

 central ends may cause one of four conditions : 



(a) Slowing of the heart. 



(6) Rise of blood pressure from constriction of the splanchnic area. 



(c) Fall of blood pressure by dilatation of the arterioles of the body. 



Sup-, lar. n. 



Depressor 



S.C.G. 



--Symp. 



- -Vagus 



Sup-. Cerv. Gang. 

 Depressor 

 -Cerv. symp.n. 



Vago. symp. 



RABBIT DOG 



FIG. 461. Diagrams of the connections of the depressor nerve in the rabbit and dog, 

 according to Cyon. It will be noticed that in the latter animal the depressor nerve 

 runs in the vagus trunk, together with the sympathetic nerve, for the greater part 

 of its course. 



(d) Reflex movements. The heart does not seem to be provided with the 

 nerves of ordinary or tactile sensibility. There is no doubt however that 

 under abnormal circumstances impulses arising in the heart can give rise to 

 sensations of pain, which are referred not so much to the heart as to the 

 surface of the body over the left side of the chest and left arm, in the region 

 of the distribution of the cutaneous branches of the second and third dorsal 

 roots. 



An important afferent nerve coming from the heart, or rather from the 

 beginning of the aorta, is the depressor nerve. In the rabbit this rises by 

 two roots, one from the trunk and the other from the superior laryngeal 

 branch of the vagus, and runs parallel with the vagus to the cardiac plexus 

 (Fig. 461). It is purely afferent, stimulation of its peripheral end causing no 

 effect. On stimulating its central end, fall of blood pressure (Fig. 462) and 

 reflex slowing of the heart are produced, the latter effect being abolished by 

 section of both vagi. It has been shown by Bayliss that the depressor effect 



