214 



THE CIRCULATION OF THE BLOOD 



Vaso-motor Tone. Vaso-constrictor changes are constantly occur- 

 ring in the blood-vessels of the organs of the body, a fact that has been 

 abundantly demonstrated by the plethysmographic experiments just men- 

 tioned. Direct inspection of the ear of an albino rabbit will show that the 

 arteries, and veins as well, are now full and large and red, and the interspaces 

 filled with blood, and now pale and constricted, and the interspaces apparent- 

 ly bloodless. If the cervical sympathetic is cut as in Bernard's experiment, 

 then the ear vessels remain dilated, that is, they lose their tone, showing 

 that the condition is dependent primarily on the constant discharges cf nerve 

 impulses from the nervous system. It is said that 

 the vessels regain their tone after a time when the 

 nerves are cut. The regained power may be ascribed 

 to the muscle fibers themselves. 



Vaso-constrictor Center. When the tonic in- 

 fluence exerted by the nerve-fibers on the arterioles 

 is traced back into the central nervous system, it is 

 found to be associated with the activity of certain 

 groups of nerve-cells, or centers, which are called 

 the vasp-constrictor centers. This determination is 

 made in part by the method of sectioning. A lesion 

 of the cerebro-spinal axis below the corpora quad- 

 rigemina is followed by partial or complete general 

 dilatation of the blood-vessels and great fall of blood 

 pressure. This is due to the isolation of the vaso- 

 constrictor center, which lies in the floor of the fourth 

 ventricle, a millimeter or two caudal to the corpora 

 quadrigemina, 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 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. They are 

 more often observed in mammalian blood-pressure 



experiments after prolonged 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 conditions they do not act independently of the 

 medullary center; but when the function of the latter has been interrupted 



FIG. 204. Diagram 

 Showing the Paths of the 

 Vaso- constrictor Fibers 

 along the Cervical Sympa- 

 thetic and the Abdominal 

 Splanchnic. Aur, Artery 

 of ear; G.Cs, superior 

 cervical ganglion; An. V, 

 annulus of Vieussens; G.St. 

 stellate ganglion; D.I, 

 D.I I, D.V, thoracic spinal 

 nerves; Abd. Spl, abdomi- 

 nal splanchnic. The arrows 

 indicate the direction of 

 vaso-constrictor impulse. 



