236 



THE CIRCULATION OF THE BLOOD 



|Aur. 



is made in part by the method of sectioning. A lesion of the cerebro-spinal 

 axis below the corpora quadrigemina is followed by partial or complete 

 general dilatation of the blood vessels and a great fall of blood pressure. 

 This is due to the isolation of the vaso-constrictor center, w r hich lies in the 

 floor of the fourth ventricle, a millimeter or two caudal to the corpora quad- 

 rigemina, and extends longitudinally over an area of 

 about three millimeters. Owsjannikow has shown 

 that the center is composed of two halves, each 

 half lying in the lateral column to the side of the 

 median line. This center is in constant action dur- 

 ing 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 varia- 

 tions in the blood pressure known as Traube-Hering 

 waves. These waves are more often 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-establishment 

 of the lost vascular tone. If these centers be de- 

 stroyed 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 sympathetic 

 constrictor centers or more probably to a funda- 

 mental property of the muscles themselves. This 

 experiment w r as 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 operation, was later re-established. 



Vaso-constrictor Reflexes. Under normal conditions the medullary 

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

 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. 



FIG. 204. Diagram 

 Showing the Paths of the 

 Vaso-constrictor Fibers 

 along the Cervical Sym- 

 pathetic and the Ab- 

 dominal Splanchnic. 

 Aur. Artery of ear; G. 

 Cs, superior cervical 

 ganglion; An. V, an- 

 nulus of Vieussens; G. 

 St, stellate ganglion; D. 

 I, D.II, D.V, thoracic 

 spinal nerves; Abd. Spl, 

 abdominal splanchnic. 

 The arrows indicate the 

 direction of vaso-con- 

 strictor impulse. 



