600 CIRCULATION OF BLOOD AND LYMPH. 



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 preganghonic 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 and 

 superior mesenteric gangha, where they mainly end, and their 

 path is continued by the postganglionic or sympathetic fibers 

 arising from these ganglia. 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 begin- 

 ning, 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 



