VASO-CONSTRICTOR NERVES 213 



a second set of nerves, also discovered by Claude Bernard, which produce 

 exactly the opposite influence, i.e., dilatation. These nerves are called vaso- 

 dilator nerves. 



Mall has also shown that veins, at least the portal vein, possess a vaso- 

 motor nerve supply as well as arteries. 



Vaso- constrictor Nerves. The presence of vaso-constrictor nerves 

 can be shown in several different ways, of which the most convincing is that 

 of direct inspection. If a vascular membrane, like the web of the frog's 

 foot or the bat's wing, be adjusted on the stage of a microscope for direct 

 inspection, and the smaller arterioles are under observation, then upon the 

 stimulation of the general nerve supplying the part these arterioles will sharply 

 decrease in size. In fact the vaso-constriction is often so great as com- 



FIG. 203. Plethysmogram of the Hind Limb of a Cat, showing Vaso-constriction upon Stimu- 

 lating the Sciatic 64 times per second. To be read from right to left. (Bowditch and Warren.) 



pletely to occlude the vessel. Very soon after the stimulation the vessel 

 again dilates to its normal size. 



The presence and course of the vaso-constrictor nerve supply to the 

 organs of the body have been demonstrated not by direct inspection, but 

 by the use of various forms of the plethysmograph. A plethysmograph is an 

 instrument designed to measure the variations in the volume of an organ. 

 If the finger, the whole hand, the spleen, or the kidney be placed in such an 

 instrument and the proper steps be taken to record the volume changes, it 

 will be found that the volume of the enclosed organ is constantly changing 

 with every variation of the blood pressure. If the nerves to the organ are 

 stimulated by the usual rapidly interrupted induction current, for example, 

 the splanchnics to the kidney, then there is a decrease in the volume of the 

 organ. This decrease takes place even when there is a simultaneous in- 

 crease of the arterial blood pressure, a result that can be explained only on 

 the assumption of vascular decrease in the organ. The decrease in the flow 

 of blood to the specific organ can be induced only by a great decrease in the 

 size of the arterioles produced by contractions of the circular muscles of 

 their walls. 



