338 THE HUMAN BODY 



force of the heart's beat, this gives us another useful method of 

 studying the distribution of the nerves concerned. For example, 

 the splanchnic nerves are branches which spring from the thoracic 

 portion of the sympathetic chain and pass through the diaphragm 

 to end in the solar plexus from which nerves pass to the arteries 

 of most of the abdominal viscera. The region whose blood-vessels 

 are innervated by these nerves is often spoken of as the splanchnic 

 region. When the splanchnic nerves are cut on both sides arterial 

 pressure falls enormously, from say 120 millimeters of mercury in 

 the carotid of a dog to 15 or 20 millimeters, most of the blood of 

 the Body lying almost stagnant in the dilated blood-vessels of 

 the abdomen. On the other hand, stimulation of the splanchnic 

 nerves so diminishes the paths open for the circulation of the 

 blood as to increase general blood-pressure enormously. 



The skin and the abdominal organs seem to be the predomi- 

 nant localities of distribution of the vasoconstrictor nerves: other 

 parts have them, but not in quantity sufficient to bring about 

 any great general change in the blood-flow. 



The Vasoconstrictor Center. This, one of the most important 

 of the " vital" centers of the medulla, has not been identified 

 anatomically with any particular group of nerve-cells, but its 

 location is quite sharply defined physiologically. There is a small 

 region of the medulla, known as the " vital knot, " whose destruc- 

 tion is promptly fatal to the life of the organism. This region 

 includes, in addition to at least one other "center," the vaso- 

 constrictor center. From this center there is a constant outflow 

 of impulses to all those arterioles of the Body whose muscles 

 contain vasoconstrictor nerve-endings. This constant stream of 

 constrictor impulses is the chief factor in the maintenance of so- 

 called vasomotor tone, a condition of continuous moderate con- 

 striction of the arterioles by which general arterial pressure is kept 

 at the proper level. 



It is probable that the vasoconstrictor center consists physi- 

 ologically of a number of associated centers which may act as a 

 unit or separately. These "partial" centers are in connection 

 with restricted vasomotor areas, and thus are enabled to bring 

 about local vasomotor effects. 



The Control of the Vasoconstrictor Center. This center, like 

 the other "vital" centers of the medulla, is kept in activity re- 



