VASO-CONSTRICTOR REFLEXES 215 



by section of the cord, then after a few days the spinal cells below the section 

 take on central functions and bring about a re-establishment of the lost 

 vascular tone. If these centers be destroyed by the destruction of the cord, 

 then the tone of the vessels immediately disappears but is regained after the 

 lapse of a much longer time. This can be ascribed to the presence of possi- 

 ble sympathetic constrictor centers or more probably to a fundamental prop- 

 erty of the muscles themselves. This experiment was carried out by Goltz 

 and Oswald, who found that after destruction of the lower part of the spinal 

 cord, the tone of the vessels of the hind limbs, lost as a result of the opera- 

 tion, was later re-established. 



Vaso- constrictor Reflexes. Under normal conditions the medul- 

 lary center responds to afferent stimuli by vaso-motor reflexes. The second- 

 ary vaso-motor centers in the spinal cord, when removed from the influence 

 of the bulbar center, can and do respond to afferent impulses by similar 

 vaso-motor action. 



The afferent impulses which excite reflex vaso-motor action may proceed 

 from the terminations of sensory nerves in general, or possibly from the 

 blood-vessels themselves, and the constriction which follows generally occurs 

 in the area whence the impulses arise. Yet the reflex may appear elsewhere. 

 Impulses proceeding to the vaso-motor center from the cerebrum may cause 

 vaso-dilatation, as in blushing, or vaso-constriction, as in the pallor of fear 

 or of anger. 



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 upward 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. 



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 produces an inhibition of the tonic activity 

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

 blood-vessels, thus relieving the heart from the overstrain of 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 depres- 



