THE NERVOUS REGULATION OF THE HEART 1097 



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 



FIG. 454. Blood-pressure curve from rabbit showing effect of excitation of central 

 end of depressor nerve (mercurial manometer). (BAYLISS.) 



superior laryngeal branch of the vagus, and runs parallel with the vagus 

 to the cardiac plexus (Fig. 453). It is purely afferent, stimulation of its 

 peripheral end causing no effect. On stimulating its central end fall of 

 blood pressure (Fig. 454) 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 is due to universal dilata- 

 tion of the blood-vessels of the body, the greater part, however, being 

 played by the splanchnic area. This nerve is probably brought into 

 action whenever the pressure in the aorta is so high as to constitute 

 a serious check to the expulsive action of the heart. It is stated that 

 under these conditions a current of action may be detected in the 

 trunk of the depressor nerve, and that if both depressor nerves be 

 cut when the aortic pressure is high the blood pressure rises still 

 higher. It presents a means by which the heart can be relieved of 



