THE EFFECT OF CHANGE OF POSTURE 



197 



his belt before entering into action. Similarly, men and women with 

 lax abdominal wall and toneless muscles take refuge in the wearing of 

 abdominal belts. To maintain a vigorous circulation and digestion, 

 it is necessary to exercise the muscles daily, particularly those of the 

 abdomen. 



The question may be studied experimentally by passing cannulse 

 down the external jugular vein and carotid artery into respectively 

 the superior vena cava and aorta of a dog, anaesthetized, and placed 

 upon a specially constructed animal table, which is made to turn 

 round an axis passing through the body at the level of the cannulse. 

 Upon turning the table, any alteration in the level of fluid in the 

 manometer tubing is thus avoided. The effect of changes of posture 

 are then truly recorded. On placing the animal in the feet-down 

 posture, the arterial and venous pressures immediately fall. The 

 venous pressure remains down until the horizontal posture is once 

 more assumed. The arterial pressure rapidly rises again to normal 



FIG. 101. AORTIC PRESSURE. (L. H.) 



A, Vertical feet-down position ; B, C, effect of abdominal comj ression ; 

 D, horizontal position. 



(FD, Fig. 100), and often it may be found to rise above normal. 

 'The respiratory undulations are frequently intensified while the animal 

 is in the feet-down posture. If left long in the feet-down position, 

 the compensatory mechanism gradually fails and the arterial pres- 

 sure falls (Fig. 100). 



If the spinal cord be divided at the level of the first dorsal vertebra, 

 the influence of the bulbar centres on the parts below the section is 

 removed. Abdominal and intercostal respiration is paralyzed, and 

 the breathing becomes purely diaphragmatic. The tone of the great 

 splanchnic area of arterioles is lost, the tone of the abdominal wall 

 is abolished, and thus the capacity of the abdominal vessels is 

 greatly increased. The total effect on the animal, when lying in 

 the horizontal posture, is a considerable fall of arterial pressure, 

 and a marked diminution of the respiratory undulations of pressure. 

 If the animal be now dropped into the vertical feet-down posture, 

 the arterial pressure falls rapidly, and may reach zero ; the circulation 

 is then at an end. This is so because the great abdominal veins sag 

 out under the hydrostatic pressure. In them the whole of the blood 



