234 



THE CIRCULATION OF THE BLOOD 



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 



FIG. 202. Arm Plethysmograph. Apparatus for measuring the change in volume in 

 the arm due to variation in the blood supply. The arm is enclosed in a glass cylinder 

 which is completely filled with fluid, the opening through which the arm is inserted being 

 closed by a rubber sleeve, A . The cavity of the glass cylinder communicates through the 

 tube, F, G, with the test-tube, M, which is supported in the jar, P. Any variation in 

 volume in the arm will cause water to flow out or into the test-tube, M , which is lowered 

 as the tube fills, and raised as it empties. The rise and fall of the test tube, M, is com- 

 municated over the pulley, L, to the writing pen, N, which records the movements on the 

 smoked cylinder. Kymograph not shown. (Mosso.) 



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- 

 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 



