THE CONTROL OF THE CIRCULATION 245 



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. 76. Aortic blood pressure, showing the effect of posture: A, vertical, head-up; B, hori 

 zontal; C, vertical, head-down; D, horizontal. (L.H.) 



(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 



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.) 



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 

 animal is now placed in the vertical tail-down position, however, it falls 



