BLOOD PRESSURE IN A MAMMAL AND ITS REGULATION 239 



of the skin of the finger at that level. Vary the experiment by measuring 

 the pressure with the finger held at the level of the top of the head; with the 

 finger held as low as possible; held at the level of the heart. Tabulate the 

 measurements. The capillary blood pressure at the level of the heart is 

 usually from 40 to 50 mm. of mercury. 



19. The Arterial Blood Pressure in a Mammal and Its Nervous 

 Regulation. After the student has measured the arterial blood pres- 

 sure by Hale's method, described above, he is in a position to study the 

 variations and coordinations in the blood circulatory apparatus. The re- 

 cording apparatus consists of writing pens, seconds time marker, signal marker, 

 blood-pressure manometer preferably Ludwig's mercury manometer, and 

 a continuous paper kymograph preferably Ludwig's weight-driven form 

 for a continuous record of the arterial blood pressure. Connect the cannula 

 with the mercury manometer which is provided with a pressure bottle. 

 Use a cannula of the form shown in figure 185, connecting the side 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 12 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 recording 

 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 0.5 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 



