THE VASOMOTOR NERVES. 613 



but passing upward or downward in the sympathetic chain 

 return, to some other spinal nerve as postganglionic fibers. In this 

 way, therefore, it happens that the various intercostal nerves and 

 the nerves of the brachial and sciatic plexus contain vasoconstrictor 

 fibers as postganglionic or sympathetic fibers. On the other hand, 

 the vasoconstrictor fibers destined for the great vascular region of 

 the intestines and other abdominal viscera, after reaching the sym- 

 pathetic chain by way of the white rami as preganglionic fibers, do 

 not return to the spinal nerves by the gray rami. They leave the 

 sympathetic chain, still as preganglionic fibers, in the branches of the 

 splanchnic nerves and through them pass to the celiac ganglion, 

 where they mainly end, and their path is continued by the post- 

 ganglionic or sympathetic fibers arising from this ganglion. More 

 specific information concerning the origin of the vasomotor fibers 

 to the different organs is given in condensed form farther on. It is 

 quite important in the beginning, however, to obtain a clear general 

 conception of the paths taken by the constrictor fibers from their 

 origin in the spinal cord to their termination, on the one hand, 

 in the vessels of the skin, or, on the other, in the vessels of the 

 abdominal and pelvic viscera. 



The Tonic Activity of the Vasoconstrictor Fibers. A very 

 important fact regarding the vasoconstrictor nerve fibers is that 

 they are constantly in action to a greater or less extent. This 

 fact is demonstrated by the simple experiment of cutting them. 

 If the sympathetic nerve in the neck is cut in the rabbit the blood- 

 vessels of the ear become dilated. If the splanchnic nerves on 

 the two sides are cut the intestinal region becomes congested, 

 and the effect in this case is so great that the general arterial pressure 

 falls to a very low point. From these and numerous similar ex- 

 periments we may conclude that normally the arteries that is, 

 the arterioles are kept in a condition of tone by impulses received 

 through the vasoconstrictor fibers. Cut these nerves and the arte- 

 ries lose their tone and dilate, with the 'result that, the peripheral 

 resistance being diminished, the lateral pressure falls on the arterial 

 side and rises on the venous side. The relatively enormous effect 

 upon aortic pressure caused by paralysis of the tone of the arteries 

 in the splanchnic area shows that under normal conditions the 

 peripheral resistance in this great area plays a predominant part 

 in the maintenance of normal arterial pressure, and by the same 

 reasoning variations in tone in the arteries of this region must 

 play a very large part in the regulation of arterial pressure. 



The Vasoconstrictor Center. As stated in the last two para- 

 graphs, the vasoconstrictor fibers emerge from the cord over a 

 definite region, and they exhibit constant tonic activity. It has 

 been shown, moreover, that if the cord be cut anywhere in the 



