CH. xiv. THE CIRCULATION OF THE BLOOD. 109 



had indeed suggested that blood from the heart flowed 

 through the lungs (or the pan we breathe with), and came 

 back again to the heart ; and Csesalpinus had even noticed 

 that if you tie up a vein it swelis on the side of the bandage 

 away from the heart ; but the notions of all these men 

 were very vague and unsatisfactory. 



The subject remained quite obscure till, while Harvey 

 was studying at Padua, his master Fabricius discovered that 

 many of our veins have curious valves inside them, made 

 by the folding of the lining of the vein. These valves, which 

 are just like little transparent pockets, lie open towards the 

 heart so long as the blood is flowing in that direction ; but 

 if you press on a vein in your arm for instance and 

 force the blood away from the heart towards the fingers, the 

 valves close at once, and the vein swells up because the 

 blood cannot flow on. 



Fabricius thought that the use of these valves was merely 

 to prevent the blood escaping too quickly into the branches 

 of the vein ; but this explanation did not satisfy Harvey, and 

 he determined to try to discover which way the blood 

 moved in the different vessels which held it. In order to 

 do this he laid bare the artery of a living animal, say in its 

 leg, and tied it round' tight, so that the blood could not flow 

 past the bandage. He found that the artery became very 

 full of blood and throbbed strongly above the place where he 

 had bound it, but in the lower part of the leg it did not 

 throb at all. This proved to him that the blood in the 

 artery was flowing from the heart to the leg of the animal, 

 and was stopped on its way down by the bandage. He then 

 tied up a vein in the same, way, and this time the swelling 

 was in the lower part of the leg, below where the vein was 

 tied. Therefore it was clear that the blood in the vein wag 

 flowing fiom the leg to the heart, and was stopped from flow- 



