312 CIRCULATION. 



circulated in the capillaries is forced into the venous radicles 

 in a steady stream. As the supply to the capillaries of differ- 

 ent parts is regulated by the action of the small arteries, and 

 as this supply is subject to great variations, there must neces- 

 sarily be corresponding variations in the intensity of the 

 current in the veins, and the quantity of blood which these 

 vessels receive. As we should anticipate, then, the venous 

 circulation is subject to very great variations arising from ir- 

 regularity in the supply of blood, aside from any action of 

 the vessels themselves, or any external disturbing influences. 

 A great variation in the venous current is observed in the 

 veins which collect the blood from the intestinal canal. 

 During the intervals of digestion, these vessels carry a com- 

 paratively small quantity of blood; but during digestion, 

 they are laden with the fluids received by absorption, and the 

 quantity is immensely increased. 



It often happens that a vein becomes obstructed from 

 some cause which is entirely physiological, as the action of 

 muscles. The immense number of veins, as compared with 

 the arteries, and their free communications with each other, 

 provide that the current, under these circumstances, is sim- 

 ply diverted, passing to the heart by another channel. When 

 any part of the venous system is distended, the vessels react 

 on the blood, and exert a certain influence on the current, 

 always pressing it toward the heart, for the valves oppose the 

 flow in the opposite direction. 



The intermittent action of the heart, which pervades the 

 whole arterial system, is generally absorbed, as it were, in 

 the passage of the blood through the capillaries ; but when 

 the arterioles of any part are very much relaxed, the impulse 

 of the central organ may extend to the veins. Bernard has 

 shown this in the most striking manner, in his well-known 

 experiments on the circulation in the glands. 1 When the 

 glands are in physiological activity, the quantity of blood 



1 BERNARD, Liquides de V Organisme, Paris, tome i., p. 301 ; and Journal de 

 I Anatomic ct de la Physiologic, Septembre, 1864, p. 507 et seq. 



