198 A TEXTBOOK OF PHYSIOLOGY 



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

 is no mechanism left for falling the heart. Thus the heart, empty of" 

 blood, continues to beat to no purpose. If the abdominal wall 

 be compressed with the hand (B, C, Fig. 101), 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 immediately becomes re-established. 

 The effect of compression of the abdomen in the horizontal position 

 is also evident (B, C, Fig. 102) but slight. In the feet-up position 

 the aortic pressure rises under the influence of gravity, returning 

 to .normal if compensation takes place or when the horizontal posi- 

 tion is resumed (Fig. 102). Such experiments prove that in the 

 anaesthetized animal there are two chief compensatory mechanisms 

 by which the hydrostatic effect of gravity is overcome namely, the 

 vaso-motor mechanism of the arterioles and the respiratory pump.- 



FIG. 102. AOBTIC PRESSURE. (L. H.) 



M. Horizontal position; B, C, abdomen compressed; D, vertical feet-up position; 

 E, F, abdomen compressed; G, horizontal. 



It is necessary to examine these separately, and estimate the relative 

 power of each. 



The vaso-motor tone of the great splanchnic area can be easih r 

 abolished, 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 interference with the thorax 

 or abdominal wall is avoided. 



As the result of section of the splanchnic nerves in the vertical feet- 

 down posture, the arterial pressure falls very considerably; but, 

 nevertheless, the circulation may remain efficient, on account of the 

 action of the respiratory pump. A form of respiration may be 

 evoked which consists of thoracic inspiratory aspirations, combined 

 with powerful 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 a crucial incision, the support of the abdominal muscles is withdrawn, 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,. 



