THE EFFECT OF CHANGE OF POSTURE 199 



and the circulation ceases (Fig. 103). By compression of the abdomen, 

 or by a return to the horizontal posture, the circulation can be once 

 more renewed. 



This experiment shows that the respiratory pump can compensate 

 for the influence of gravity when the vaso -motor mechanism is 

 paralyzed. The respiratory pump can be paralyzed by itself and 

 without damage to the vaso-motor mechan'sm by the injection of 

 curari. The power of the heart may then be sufficient by itself to 

 maintain the circulation in the feet-down position, so long as the 

 capacity of the abdominal vessels is kept under control by the vaso- 

 motor nerves. 



The effect on the circulation of rendering the intrathoracic pressure 



FIG. 103. AORTIC PRESSURE: MORPHINIZED DOG. (L. H.) 



A, Vertical feet-down position, splanchnic nerves divided ; B, effect of compressing 

 abdomen ; C, abdominal wall divided ; D, thorax opened. 



positive e.g., by compression of the thorax is that the blood stag- 

 nates in the abdomen, and the circulation ceases, whenever, by any 

 means, the thoracic pressure is rendered sufficiently positive to over- 

 come the venous pressure produced by the driving power of the heart 

 find the compressive action of the abdominal wall. Owing to the 

 influence of gravity, this state of affairs is brought about more easily 

 in the vertical feet-down position than in the horizontal posture. 

 Compensation for the positive intrathoracic pressure is supplied by 

 firm compression of the abdomen; the heart then fills, and the 

 arterial pressure regains its normal level. 



Measurements of arterial pressure likewise reveal the effect of 

 gravity upon the circulation in man. In a normal man standing up- 



