246 THE HUMAN BODY. 



1616, demonstrated that the movement of the blood was a 

 continuous circulation as we now know it, and so laid the 

 foundation of modern Physiology. In his time, however, 

 the capillary vessels had not been discovered, so that al- 

 though 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 buck by an 

 artery. (2) The anatomical arrangement of the valves of 

 the heart and of the veins shows that the blood can only 

 flow /row 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 experiment can be made on the veins on the back 

 of the hand of any thin person, especially if the vessel* 

 be first gorged by holding the hand in a dependent posi- 

 tion for a few seconds. Select then a vein which runs 

 for an inch or so without branching, place one 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 will 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 capilla- 

 ries. (6) In the lower animals direct observation with the 

 microscope shows the steady flow of blood from the arte- 

 ries through the capillaries to the veins, but never in the 

 opposite direction. 



