NERVOUS CONTROL OF THE CIRCULATION 179 



246. Regulation of Blood Pressure in the Brain. The 



regulation of blood pressure in the brain is by means of 

 the inhibitory nerves. Excitement in the brain increases 

 blood pressure there, and that pressure gives rise to 

 inhibitory impulses by which the heart's action is re- 

 strained and danger to the brain is averted. Or, the 

 inhibition may be a reflex impulse originating in the heart 

 itself and sent up to the inhibitory center by the afferent 

 fibers of the vagus, and the heart may be thus enabled to 

 regulate its action according to its own necessities. 



247. The Vasomotor Nervous System is that which regu- 

 lates muscular action in the blood vessels. It belongs to 

 the sympathetic system. The muscles of arteries and 

 veins are composed of plain muscular fibers, and the 

 nerves belonging to them appear to end in fine plexuses 

 round the fibers. Two sets of nerves for the blood vessels 

 have been made out, called the vasoconstrictor and the 

 vasodilator, whose influences correspond to the influences 

 of the accelerator and inhibitory fibers of the cardiac 

 nerves. It should be noted that in the vertebrate ani- 

 mals no inhibitory nerve fibers exist in the nerves supply- 

 ing the voluntary muscles, while the involuntary muscles 

 usually have both accelerator and inhibitory fibers. 



248. The Vasoconstrictor Nerves have been traced to the 

 ganglia of the sympathetic chain and thence to the ante- 

 rior horns of the spinal cord. From there fibers pass up 

 to the vasomotor center in the gray matter of the medulla 

 oblongata. 



249. Vasodilator Nerves accompany the vasoconstrictor 

 for a part of their course, and finally reach the same center 

 in the medulla. They carry inhibitory impulses, that is, 

 their action checks the contraction of the muscle fibers 

 in the blood vessels and permits them to dilate. 



