250 



THE CIRCULATION OF THE BLOOD 



pressure, as shown in the tracing (Fig. 76), followed by a certain de- 

 gree of compensation even while the animal is still in the erect position. 

 The extent to which this compensation occurs varies with the depth of 

 the anesthesia. If the experiment is repeated after administering a large 

 dose of chloroform, not only will the initial fall be much greater, but 

 subsequent compensation will be practically absent. The application of 

 these facts in the operating room will be self-evident. 



Leonard Hill has shown that three factors are involved in the com- 

 pensating mechanism: (1) the tonicity of the abdominal musculature; 



Fig. 77. Tracing to show the effect of gravity on the arterial blood pressure. At A, the 

 animal was placed in the vertical position; at B, the abdomen was compressed; at C, a crucial 

 incision was made in the abdomen; at D, the pleural cavity was opened; at F, the animal was 

 returned to the horizontal position. (From Leonard Hill.) 



Fig. 78. The effect of gravity on the aortic pressure after division of the spinal cord in the 

 upper dorsal region. By placing the animal in the vertical feet-down posture, the pressure fell 

 almost to zero, but on returning it to the horizontal posture, the circulation was restored. (From 

 Leonard Hill.) 



(2) the tone of the splanchnic blood vessels; (3) the pumping action of 

 the respiratory movements. The importance of the first-mentioned fac- 

 tor can be readily shown by making a crucial incision of the abdom- 

 inal walls in an animal in the erect position (Fig. 77), and that of 

 the second factor by cutting the great splanchnic nerves, or the spinal 

 cord. After such an operation, even while in the horizontal position, as 

 we have seen, the blood pressure falls to a considerable extent. If the 



