248 



THE CIRCULATION OF THE BLOOD 



Afferent influence upon the vaso-motor centers is well shown by the 

 action of the depressor nerve, the existence of which was demonstrated by 

 Cyon and Ludwig. The depressor is a small afferent nerve which passes 

 up to the medulla from the heart, in which it takes its origin. It runs up- 

 ward in the sheath of the vagus or in the superior laryngeal branch of the 

 vagus or as an independent branch, as in the rabbit, communicating by 

 filaments with the inferior cervical ganglion as it proceeds from the heart. 

 If, in a rabbit, this nerve be divided and the central end stimulated during 

 a blood-pressure observation, a remarkable fall of blood-pressure takes 

 place, figure 205. 



FIG. 205. Blood-pressure Record (lower) and Respiratory Record (upper) Obtained 

 from a Dog upon Stimulating the Central End of the Divided Vagus, Both Vagi being Cut. 

 The marked fall in blood -pressure is due to the effect of stimulating the depressor fibers 

 contained in the vagus trunk of the dog. (New figure by Dooley and Dandy.) 



The cause of the fall of blood pressure is found to proceed primarily 

 from the dilatation of the vascular district within the abdomen supplied by 

 the splanchnic nerves, in consequence of which the vessels hold a much 

 larger quantity of blood than usual. The engorgement of the splanchnic 

 area very greatly diminishes the amount of blood in the vessels elsewhere, 

 and so materially diminishes the blood-pressure. The function of the de- 

 pressor nerve is that of conveying to the vaso-motor center afferent nerve 

 impulses from the heart, which produce an inhibition of the tonic activity 

 of the vaso-motor center and, therefore, a diminution of the tension in the 

 blood vessels. This diminishes the overstrain on the heart in propelling 

 blood into the already too full or too tense arteries. It has been shown by 

 Porter and Beyer that the fall in blood-pressure, following stimulation of the 

 depressor nerve, will still occur, even when the abdominal vaso- constriction 

 is kept constant by a simultaneous stimulation of the splanchnics. It is 

 therefore evident that the inhibitory effect of depressor- nerve stimulation is 

 a general one and not confined to the splanchnic area alone. 



