-II THE CIRCULATION OP THE BLOOD 



sure (Loven reflex). Similarly when the centra] end of one of the sen- 

 sory roots of the leg of a dog is stimulated, there is a rise in arterial 

 blood pressure and an increase in the volume of the limb. 



THE INFLUENCE OF GRAVITY ON THE CIRCULATION 



If the arterial blood pressure is measured in the arm and leg in a man 

 standing erect, a difference corresponding to the hydrostatic effect of 

 gravity will be found between the two readings. In comparison with 

 the high pressure normally existing in the arteries, this difference is. 

 however, of little significance. On the other hand, in the veins, Avhere 

 the average pressure is low, gravity would cause serious embarrassment 

 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 

 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 lighl ether anesthesia in the 

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

 pressure, as shown in the tracing (Fig. 76), followed by a certain de- 

 gree of compensation even while the animal is si ill in the erect position. 

 The extenl to which this compensation occurs varies with the depth of 

 the anesthesia. If the experiment is repeated after administering a large 

 dose of chloroform, not only will the initial fall he much greater, but 



