CH. xxi.j EFFECT OF GRAVITY ON THE CIRCULATION. 2Q3 



arterial pressure from any other cause will produce a rise of 

 capillary pressure. 



3. By narrowing the veins leading from the capillary area ; 

 complete closure of the veins may quadruple the capillary pressure. 

 This leads secondarily to an increased formation of lymph (dropsy); 

 as when a tumour presses on the veins coming from the legs. 



4. Any circumstance that leads to increased pressure in the 

 veins will act similarly ; this is illustrated by the effects pro- 

 duced by gravity on the circulation, as in alterations of posture. 



Capillary pressure is decreased by the opposite conditions. 



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 

 h,iu_nnir 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 

 riivtilation 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 over- 

 come the effects of gravity. If, however, 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 hydro- 

 static effect of gravitation, and the heart-cavities once more fill 

 with blood during every diastole. Another experiment, originally 

 performed by Salathe, can be demonstrated 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 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 : 



