THE CONTROL OF THE CIRCULATION 249 



t 



to the circulation of blood were it not for the valves and the forces 

 which move the blood beyond them (page 214). 



In erect animals the part of the circulation in which blood might stag- 

 nate as a result of gravity is the splanchnic area. Were such stagna- 

 tion to occur, the blood would not be returned to the right heart, so 

 that the arteries would not receive sufficient blood to maintain an ade- 

 quate circulation, particularly in the vessels of the brain. 



Simple experiments devised by Leonard Hill 19 ' 2S illustrate these prin- 

 ciples. When a snake, for example, is pinned out on a long piece of 

 wood and an opening made opposite the heart, this organ can be seen 

 to fill adequately with blood as long as the animal is maintained in the 

 horizontal position. When placed vertically, however, the heart be- 

 comes bloodless. If now the tail end of the animal is placed in a cylinder 

 of water so as to overcome the effect of gravity, the heart will be seen 



Fig. 76. Aortic blood pressure showing the effect of posture: A, vertical, head-up; B, hori- 

 zontal; C, vertical, head-down; D, horizontal. (L.H.) 



to fill again with blood. Evidently in such an animal there is no mechan- 

 ism to compensate for gravity. 



If a domestic rabbit with a large pendulous abdomen is held in the 

 vertical tail-down position, stagnation of blood in the splanchnic ves- 

 sels occurs to such an extent that in from fifteen to twenty minutes the 

 animal dies from cerebral anemia. If an abdominal binder is first of all 

 applied, the vertical position will not have the same consequences. This 

 experiment illustrates clearly the possible evil effects that gravity may 

 produce in animals in which no mechanism exists to compensate for it. 



Placing an animal such as a dog under light ether anesthesia in the 

 vertical tail-down position produces an immediate fall in arterial blood 



