9 2 



THE MECHANISM OF THE CIRCULATION. 



the animal-board being constructed to turn round an axis passing 

 through the body at the level of the intravascular openings of the 

 cannula?. 1 Then the effect of posture on the venous and arterial pres- 

 sures is recorded. 2 On placing the animal in the feet-down posture the 

 arterial 3 and venous pressures immediately fall. The venous pressure 

 remains down until the horizontal posture is once more assumed 

 (Fig. 61). The arterial pressure rapidly rises again to normal, and often 



it may be found to rise above 

 normal. The respiratory un- 

 dulations are frequently in- 

 tensified while the animal is in 

 the feet-down posture. 



if the spinal cord be di- 

 vided at the level of the first 

 dorsal vertebra, the influence 

 of the bulbar centres on the 

 parts below the section is re- 

 moved. Abdominal and inter- 

 costal respiration is paralysed, 

 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, 



Fig. 61. — Aortic pressure. A, feet-down; 

 B, horizontal posture. — Hill. 



Fig. 62. — Aortic pressure and intracranial pressure. Spinal cord divided at first 

 dorsal vertebra. Effect of feet-down posture. — Hill. 



when lying in the horizontal posture, is a considerable fall of arterial pres- 

 sure, 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 (Fig. 62) : the 



1 Any alteration in the level of the extravascular column of fluid in the manometer 

 tubing is thus avoided. 



- Hill, Journ. Physio!., Cambridge and London, 1895, vol. xviii. p. 15 ; Hill and 

 Barnard, ibid., 1897, vol. xxi. p. 323; Hill, "Cerebral Circulation," London, 1896. 



3 This fall had been previously observed by Cybulski, St. Petersb. Med. Wchnschr., 

 1878, S. 93 ; and Friedmann, Med. Jahrb., Wien, 1882, S. 225. 



