CH. XXII.] EFFECT OF GRAVITY 279 



will act similarly; this is illustrated by the effects produced by 

 gravity on the circulation, as in alterations of posture. 



Capillary pressure is decreased by the opposite conditions. 



Capillary pressure is much more influenced by changes in the 

 venous pressure, than by changes in the arterial pressure, since there 

 is between the arteries and capillaries the variable and usually un- 

 known peripheral resistance of the arterioles. 



Effect of gravity on the circulation. The main effect of gravity is 

 that the veins are filled with blood in the part which is placed down. 

 Thus, if an animal is placed suddenly with its legs hanging down, less 

 blood will go to the heart, and the blood-pressure in the arteries will 

 fall temporarily in consequence. This hydrostatic effect of gravity is 

 soon overcome by an increased constriction of the vessels of the 

 splanchnic area, when the vaso-motor mechanism is working normally. 

 The efficient action of the " respiratory pump " is also of importance 

 in counteracting gravity. 



A very striking illustration of the effect of gravity on the circula- 

 tion can be demonstrated on the eel. The animal is anaesthetised, 

 and a small window is made in the body wall to expose the heart. 

 If the animal is then suspended tail downwards, the beating heart is 

 seen to be empty of blood ; all the blood accumulates in the tail and 

 lower part of the body ; the animal has no " respiratory pump," such 

 as a mammal possesses, to overcome the effects of gravity. If, how- 

 ever, the animal, still with its tail downwards, be suspended in a 

 tall vessel of water, the pressure of the water outside its body 

 enables it to overcome the hydrostatic effect of gravitation, and 

 the heart-cavities once more fill with blood during every diastole. 

 Another experiment was originally performed by Salathe on a 

 " hutch " rabbit. If the animal is held by the ears with its legs 

 hanging down, it soon becomes unconscious, and if left in that position 

 for about half an hour it will die. This is due to anaemia of the 

 brain ; the blood accumulates in the very pendulous abdomen which 

 such domesticated animals acquire, and the vaso-motor mechanism of 

 the splanchnic area is deficient in tone, and cannot be set into such 

 vigorous action as is necessary to overcome the bad effects of gravity. 

 Consciousness is, however, soon restored if the animal is placed in a 

 horizontal position, or if while it is still hanging vertically the abdomen 

 is squeezed or bandaged. A wild rabbit, on the other hand, suffers no 

 inconvenience from a vertical position ; it is a more healthy animal in 

 every respect; its abdomen is not pendulous, and its vaso-motor 

 power is intact. 



We shall, a few pages later, be considering the methods by which 

 blood-pressure may be estimated in man. The effects of gravity on 

 the pressure in various parts can be well shown by alterations of 

 posture. This is an important practical question, especially during 



