no SEVENTEENTH CENTURY. FT. IIL 



ing upwards by the bandage. When he tied an artery and 

 a vein in the arm the same thing happened ; the blood 

 in the artery was flowing towards the hand, while in the 

 vein it was flowing from the hand towards the heart. 



This led Harvey to suspect that the blood is always 

 making a continuous journey round and round, first out of 

 the heart through the arteries to all parts of the body, and 

 then back through the veins to the heart again. And now 

 the use of the little valves became evident. While the 

 blood flows, as it should do, towards the heart, they lie open 

 and offer it no resistance, but directly anything drives it in 

 the wrong direction they close at once, and prevent it from 

 flowing backwards. The throbbing of the arteries was also 

 explained by this theory, for the blood being pumped into 

 them by a regular movement of the heart, they swell at each 

 rush of blood, and contract again before the next, and so 

 rise and fall in exact time with the beating of the heart. 



Harvey also found that Caesalpinus and his contempo- 

 raries had been right in suspecting that the blood makes a 

 small circuit from the heart through the lungs and back 

 again. We will try to understand all this with the assist- 

 ance of a diagram, which, however, you must remember is 

 only to help you, and not a real drawing of the parts. 

 Starting from the left lower chamber a of the heart, the 

 blood is pumped out of the left top corner of this chamber 

 into an artery in the direction of the arrow i. This artery 

 soon divides into two branches, one going downwards by 

 the arrow 2 to the lower part of the body, the other upwards 

 by the arrow 2' to the arms and neck ; and, after flowing 

 into the different parts of the body, the blood in the lower 

 artery returns by the lower vein, 5, 6, 7, while the blood of 

 the upper artery is returning by the upper vein 4', and both 

 streams pour into the right upper chamber of the heart, A 



