276 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXL 



the heart. For both reasons, then, the venous pressure will fall, but 

 that fall is limited, as pointed out above, to such an increase only as 

 the heart is capable of overcoming successfully. 

 Capillary pressure is increased by 



1. Dilatation of the arterioles; the blood-pressure of the large 

 arteries is then more readily propagated into them. 



2. The size of the arterioles remaining the same, increase of 

 arterial pressure from any other cause will produce a rise of capillary 

 pressure. 



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

 plete 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 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-cavi- 

 ties once more fill with blood during every diastole. Another experi- 

 ment 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 is due to anaemia of the 



