CH. xiv. THE CIRCULATION OF THE BLOOD. in 



Columbus, and the botanist Caesalpinus, who all lived in 

 the sixteenth century, had indeed suggested that blood from 

 the heart flowed through the lungs (or the part we breathe 

 with), and came back again to the heart ; and Csesalpinus 

 had even noticed that if you tie up a vein it swells 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, bctow where the vein was tied. 



