262 THE CIRCULATION OF THE BLOOD 



limb of the cannula with the mercury manometer, and the end limb with the 

 pressure bottle. When the apparatus is ready anesthetize a mammal (dog, 

 cat, or rabbit), and bind it down to the animal-holder. Let one operator 

 attend strictly and at all times to the anesthetic, for the animal must not under any 

 condition recover consciousness during the experiment. 



Expose the carotid artery in the neck, as described in Experiment 1 2 above, 

 arrange it with ligatures for inserting the cannula, expose the vagus nerve 

 with the same care, and throw ligatures around it for convenience in lifting 

 it out of its bed. . Make in the carotid a V-shaped cut directed toward the 

 heart, insert and ligate the cannula as previously described. Before begin- 

 ning the experiment one should see that all the tubes are filled with the anti- 

 coagulating liquid and that the manometer is under pressure from 100 to 

 150 mm. mercury. When all is ready start the kymograph, ink the record- 

 ing pens, see that they are recording properly and that the adjustments are se- 

 cured, remove the bulldog forceps from the artery, and the pressure record 

 begins. 



1. Take a tracing of the normal arterial pressure and heart rhythm with 

 the recording paper moving at the rate of o . 5 cm. per second. 



2. Stimulate the vagus nerve with a mild-strength induction current. 

 If this stimulus is strong enough to produce change in blood pressure or in- 

 hibitions of the heart-rate, then allow sufficient time following the stimulus 

 for the blood pressure to return to the previous normal. Observing these 

 rules, vary the intensity of the stimulus from that which produces no ap- 

 parent effect to that which produces complete inhibition of the heart. Vary 

 the time of the stimulus from i to 10 seconds, using different strengths. 



3. 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 which will produce marked 

 effects on the heart rate and on the blood pressure. Expose the sciatic nerve, 

 or any other general nerve trunk, 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. 



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

 the signal marker. Do not disturb the animal or record until stable equi- 

 librium is again reached. 



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

 it with an electric current of the strength which previously just produced 

 inhibition. Repeat the experiment on the proximal end of the divided vagus. 

 The stimulation of the proximal end of the vagus produces no direct effect 

 on the heart rate when both vagi are cut (see 7 below), but does produce 

 profound changes on the blood pressure owing to vaso- motor effects. 



