THE CIRCULATION OF THE BLOOD 335 



discovered, so that although he was quite certain that the blood 

 got somehow from the final branches of the aorta to the radicles 

 of the venous system, he did not exactly know how. 



The proofs of the course of the circulation are at present quite 

 conclusive, and may be summed up as follows: (1) Blood injected 

 into an artery in the dead Body will return by a vein; but injected 

 into a vein will not pass back by an artery. (2) The anatomical 

 arrangement of the valves of the heartland of the veins shows 

 that the blood can only flow from the heart, through the arteries 

 and back to the heart by the veins. (3) A cut artery spurts from 

 the end next the heart, a cut vein bleeds most from the end 

 farthest from the heart. (4) A portion of a vein when emptied 

 fills only from the end farthest from the heart. This observation 

 can be made on the veins on the back of the hand of any thin 

 person, especially if the vessels be first gorged by holding the 

 hand in a dependent position for a few seconds. Select then a 

 vein which runs for an inch or so without branching, place a finger 

 on its distal end, and then empty it up to its next branch (where 

 valves usually exist) by compressing it from below up. The ves- 

 sel will then be found to remain empty as long as the finger is kept 

 on its lower end, but to fill immediately when- it is removed; 

 which proves that the valves prevent any filling of the vein from 

 its heart-end backwards. (5) If a bandage be placed around the 

 arm, so as to close the superficial veins, but not tight enough to 

 occlude the deeper-seated arteries, the veins on the distal side 

 of the bandage will become gorged and those on its proximal side 

 empty, showing again that the veins only receive blood from their 

 ends turned towards the capillaries. (6) In the lower animals 

 direct observation with the microscope shows the steady flow 

 of blood from the arteries through the capillaries to the veins, but 

 never in the opposite direction. 



