THE EFFECT OF CHANGE OF POSTURE. 



93 



circulation is then at an end. This is so, because the great abdominal 

 veins bag out under the hydrostatic pressure ; in them the whole of the 

 blood collects, for it can pass rapidly through the dilated arterioles ; 

 there is no mechanism left for filling the heart ; thus the heart, empty 

 of blood, continues to beat to no purpose. 



If the abdominal wall be compressed with the hand, the capacity of the 

 veins and splanchnic area is reduced, the right heart is once more filled with 

 blood, the arterial pressure rises, and the circulation is renewed. On 

 taking off the hand, the heart once more empties, the arterial pressure falls, 

 and the circulation ceases. When the animal is returned to the horizontal 

 position, the influence of gravity is abolished, and the circulation im- 

 mediately becomes re-established. Such experiments prove that in the 

 normal animal there are two chief compensatory mechanisms by which 

 the hydrostatic effect of gravity is overcome, namely, the vasomotor 

 mechanism of the arterioles and the respiratory muscles. It is necessary 

 to examine these separately, and estimate the relative power of each. 



The vasomotor tone of the great splanchnic area can ^be easily 

 abolished, and without affection of the respiratory mechanism, by section 

 of the splanchnic 

 nerves ; that is to say, 

 if these nerves are 

 reached by a lumbar 

 incision, and all in- 

 terference with the 

 thorax or abdominal 

 wall is avoided. 



The effect of sec- 

 tion of the splanchnic 

 nerves is shown in 

 Fig. 63. The arterial 

 pressure in the verti- 

 cal feet-down posture 

 falls very consider- 

 ably, but neverthe- 

 less the circulation 

 remains efficient, on 

 account of the action 

 of the respiratory 

 muscles. A form of 

 respiration is evoked 

 which consists of 

 thoracic inspiratory 

 aspirations, combined 

 with powerful expiratory abdominal compressions. Thereby the diastolic 

 filling of the heart is maintained, and the velocity of flow through the 

 splanchnic capillaries checked. On dividing the abdominal wall by a 

 crucial incision, the support of the abdominal muscles is withdrawn, the 

 splanchnic vessels dilate, and the pressure falls to a further extent. 

 Finally, on suddenly opening the thorax, the pressure falls to zero, and 

 the circulation ceases. 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 muscles can compensate 

 for the influence of gravity when the vasomotor mechanism is paralysed. 



FIG. 63. Aortic pressure. Morphinised dog. Hill and 

 Barnard. 



A, Vertical feet-down position. Splanchnic nerves divided. 



B, Effect of compressing abdomen. 



C, Abdominal wall divided. 



D, Thorax opened. 



