252 THE HUMAN BODY. 



now know it, and so laid the foundation of modern Physi- 

 ology. In his time, however, the capillary vessels had not 

 been 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 

 heart and 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 per- 

 son, 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 vessel will then be found to remain empty as 

 long as the finger is kept on its lower end, but to fill im- 

 mediately 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. 



