A HISTORY OF SCIENCE 



vivisections of lower animals and reptiles, soon demon- 

 strated beyond question the fact that the veins do 

 carry the return blood. " But this, in particular, can 

 be shown clearer than daylight," says Harvey. "The 

 vena cava enters the heart at an inferior portion, while 

 the artery passes out above. Now if the vena cava 

 be taken up with forceps or the thumb and finger, 

 and the course of the blood intercepted for some 

 distance below the heart, you will at once see it 

 almost emptied between the fingers and the heart, the 

 blood being exhausted by the heart's pulsation, the 

 heart at the same time becoming much paler even in its 

 dilatation, smaller in size, owing to the deficiency of 

 blood, and at length languid in pulsation, as if about 

 to die. On the other hand, when you release the vein 

 the heart immediately regains its color and dimensions. 

 After that, if you leave the vein free and tie and 

 compress the arteries at some distance from the heart, 

 you will see, on the contrary, their included portion 

 grow excessively turgid, the heart becoming so beyond 

 measure, assuming a dark-red color, even to lividity, 

 and at length so overloaded with blood -as to seem 

 in danger of suffocation; but when the obstruction 

 is removed it returns to its normal condition, in size, 

 color, and movement." 4 



This conclusive demonstration that the veins return 

 the blood to the heart must have been most im- 

 pressive to Harvey, who had been taught to believe 

 that the blood current in the veins pursued an oppo- 

 site course, and must have tended to shake his faith in 

 all existing doctrines of the day. 



His next step was the natural one of demonstrating 



T 74 



