VASO-CONSTRICTOR REFLEXES 



247 



|Aor, 



VKC 



two halves, each half lying in the lateral column to the side of the median 

 line. This center is in constant action during life, and its discharges are 

 responsible for the vascular tone described in the previous paragraph. 

 The vaso-constrictor center varies in its activity, sometimes producing 

 wave-like contractions with relaxations of the arterial walls, producing 

 variations in the blood-pressure known as Traube- 

 Hering waves. These waves are more of ten observed 

 in mammalian blood-pressure experiments after pro- 

 longed operations, when the center may be supposed 

 to be itself in a weakened condition. 



Secondary vaso-motor centers are present in the 

 spinal cord as proven by Goltz. Under normal con- 

 ditions they do not act independently of the medul- 

 lary center; but when the function of the latter has 

 been interrupted 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-establish- 

 ment 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 possible sympa- 

 thetic constrictor centers or more probably to a 

 fundamental property 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 along the Cervical Sym- 

 of the hind limbs, lost as a result of the operation, 5 SpUnchnic. 

 was later re-established. Aur. Artery of ear; G. 



Vaso-constrictor Reflexes. Under normal con- gangiion^^w. F, au- 

 ditions the medullary center responds to afferent nulus of Vieussens; G. 

 stimuli by vaso-motor reflexes. The secondary vaso- j ' ^jf pV thoracic 

 motor centers in the spinal cord, when removed from spinal nerves; Abd. Spl, 

 the influence of the bulbar center, can and do ^he^rows indicate the 

 respond to afferent impulses by similar vaso-motor direction of vaso-con- 

 strictor impulse, 

 action. 



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

 ceed 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 cere- 

 brum may cause vaso-dilatation, as in blushing, or vaso-constriction, as in 

 the pallor of fear or of anger. 



FIG. 204. Diagram 



