232 THE CIRCULATION OF THE BLOOD 



ture of both kinds of vasomotor fibers is the sciatic. If the hind limb of 

 a dog is placed in a plethysmograph and simultaneously a record of the 

 mean arterial blood pressure taken, it will be found on cutting the sciatic 

 nerve that the volume of the limb increases, whereas the blood pressure 

 remains practically constant. Before placing the limb in the plethysmo- 

 graph, the muscles must of course be paralyzed by means of curare; 

 otherwise muscular contractions would confuse the result. If the 

 peripheral end of the cut nerve is now stimulated, vasoconstriction will 

 readily be observed. So far, then, the results demonstrate the presence 

 of vasoconstrictor nerve fibers alone. 



To demonstrate the presence of vasodilators a different procedure is 

 necessary. This is based on the following facts: (1) The vasodilator 

 nerve fibers degenerate more slowly than the vasoconstrictor; (2) they 

 are less depressed in their excitability by cooling the nerve; and (3) they 

 are more sensitive to weak slow faradic stimulation than the vasocon- 

 strictor fibers. Accordingly, if we cut the sciatic nerve two or three 

 days before the actual experiment, and then, while observing the volume 

 of the limb, proceed to stimulate the half-degenerated nerve with feeble 

 electric stimuli of slow frequency we shall usually observe a dilatation 

 of the limb instead of constriction; and even if we cool a stretch of a 

 freshly cut nerve before applying the stimulus, the same result will 

 often be obtained. 



The Origin of Vasomotor Nerve Fibers 



Having seen how the presence of vasomotor fibers may be detected in 

 peripheral nerves, we must now proceed to trace them back to their 

 origin from the central nervous system. The method for doing this con- 

 sists, in general, in observing the effect on the blood vessels produced by 

 cutting or stimulating the various nerve roots through which the fibers 

 might pass on their way to the nerve trunks. As a result of such obser- 

 vations it has been found that all of the vasoconstrictor fibers emanate 

 from the spinal cord in the region between the level of the second thoracic 

 and that of the second or third lumbar spinal roots, but from nowhere 

 else in the cerebrospinal axis. Section of the spinal cord below the level 

 of the second lumbar spinal roots produces no change in the volume of 

 the hind limb, provided the muscles be thoroughly curarized, nor does 

 stimulation of the lower end of the cut spinal cord have any effect. If 

 the last two thoracic or the first two lumbar spinal roots are stimulated, 

 however, evidence of vasoconstriction will be obtained. 



The restriction of the origin of vasoconstrictor fibers to the above- 

 mentioned regions of the spinal cord indicates that in proceeding to 

 the mixed nerve trunks they must travel along special nerve paths. 



