302 THE CIRCULATION 



traction. Whatever be the vaso-motor mechanism in the artery, the two 

 sorts of movement and the normal usual tonus which is the balance 

 between them are facts easily observed. See Fig. 162 for a variant of 

 ordinary vaso-motor apparatus found in an amphibian. 



The vaso-constrictor centers and nerves have been known longer than 

 the nervous apparatus of vaso-dilatation, but their exact courses are still 

 not completely understood. There seems to be in the floor of the fourth 

 ventricle of the brain and just above the point of the calamus scriptorius 

 a nerve "center" whose severance from the cord by transverse incision 

 of the medulla below it occasions vaso-dilatation. This is brought about 

 evidently by removing from the arteries concerned the tonic and continued 

 vaso-constrictor stimulus. This cluster of nerve-cells, double probably, 

 part on each side, dominates vaso-motion. Its stimulation causes con- 

 striction. Influences from it pass downward to the grey horn and thence 

 by spinal neurones to centers of more local influence situated mostly 

 in the dorsal sympathetic ganglia; thence the influence passes outward 

 in the post-ganglionic fibers of the sympathetic. As there is no good 

 evidence of a general vaso-dilator center, this clump of cells and fibers in 

 an area of the medulla a few millimeters square may be considered both 

 the vaso-motor and the vaso-constrictor center, the latter term being 

 preferable as somewhat more specific. Stimulation with electricity of 

 this little region in the medulla or of the ends of fibers exposed by trans- 

 verse section just below it causes a marked rise of blood-pressure which 

 is due to general vaso-constricton. The nature of the normal stimulus 

 is not understood, but it is probably in part chemical and resident in the 

 blood flowing through it. Besides being influenced like the respiratory 

 center by changes in the oxygen and carbon dioxide content of the blood, 

 the vaso-constrictor center is promptly stimulated to action by a decrease 

 in the amount of blood flowing through it. Its chief work is to keep 

 the blood-supply up to the normal standard by increasing the blood- 

 pressure when necessary. This is readily accomplished by vaso-con- 

 striction. On the other hand, when more blood than is normal is passing 

 through this center its action of constriction lessens and the blood-pres- 

 sure consequently falls. Besides these efferent neural means there is 

 probably a more or less elaborate system of afferent fibers for regulating 

 the blood-supply of the various parts of the body depressors acting 

 reflexly on the constrictor center or on the sympathetic ganglia. By 

 being thus affected by the amount and perhaps by the pressure of the 

 blood flowing through them and through the arteries, the double center 

 in the medulla and subsidiary centers in the cord below it are able to 

 control the general blood-supply. 



These assistant centers probably have specialized duties for main- 

 taining the pressure in various areas, large or small, of the organism. 

 The impulses connecting them with the main center seem to pass down 

 the anterior lateral columns, some crossing over meanwhile. The 

 precise locations of these subsidiary centers have not been determined, 

 but they are probably in the anterior horns and perhaps in part in the 



