THE ARTERIAL BLOOD-PRESSURE 275 



c. Test the sensitiveness of the left vagus. 



d. Allow the vagus to fall back in its warm bed and stimulate the skin 

 of the animal at some sensory surface, say the lips, the ear, or the foot. 

 By varying the intensity of the stimulus, a strength will be found which 

 will produce no reflexes of the voluntary muscles, but will produce marked 

 effects on the heart rate and on the blood-pressure. 



Expose the sciatic nerve, or any other nerve trunk containing afferent 

 or sensory fibers, cut it, and stimulate the central end for five seconds. 

 With a proper strength of stimulus a greater effect is produced on the 

 heart and on the blood-pressure than by stimulating a small spot of skin. 

 This stimulus will also reflexly accelerate respiration. 



e. Cut the right vagus nerve and mark the exact time on the tracing by 

 the signal marker. Do not disturb the animal of the record until stable 

 equilibrium is again reached. 



/. Now lift up the distal end of the divided right vagus, and stimulate 

 it with the strength which previously just produced inhibition. Repeat 

 the experiment on the proximal end of the divided vagus. The reflex 

 effects are still threefold, cardiac, vasomotor and respiratory. The stimu- 

 lation of the proximal end of the vagus produces effects on the heart rate 

 when one vagus is still intact. See Experiment / below. 



g. After 10 to 15 seconds cut the left vagus, marking the time of cutting 

 on the tracing with the same care as before. As soon as the vagus nerves 

 are cut, the heart-rate will be observed to increase sharply and the blood- 

 pressure to rise. The respirations also change in rate and depth, a fact 

 which can be noted directly and by its influence on the blood-pressure 

 tracing. 



h. Lift up the distal end of the left vagus, and stimulate it with an 

 electric current of the strength which previously just produced inhibition. 

 Stimulate the proximal end of the divided vagus. The stimulation 

 produces no direct inhibitory effect on the heart rate when both vagi are 

 cut, but does produce profound changes in the blood-pressure owing to 

 vaso-motor reflexes. 



Occasionally an animal will be found in which one or both vagi are 

 comparatively inactive. 



k. If the rabbit is used, stimulate the depressor nerve, which produces 

 a marked fall in blood-pressure from reflex effects, explain. 



/. Repeat the stimulation of the central end of the sciatic as described 

 in /, now that the vagus nerves are cut. The stimulation of this nerve 

 no longer produces decrease in the heart-rate, but occasionally an accelera- 

 tion. The blood-pressure is influenced as before, showing that the vaso- 

 motor centers are reflexly stimulated. 



m. When you have finished the outline of experiments, give an excess 

 of ether to kill the animal and continue the record until it is dead. The 



