DISCOVERY OF THE CIRCULATION. 



33 



These experiments are models of simplicity and pertinence. First, he showed that a 

 ligature tightly applied to a limb prevented the blood from entering the artery and 

 arrested pulsation. The ligature then relaxed and applied with moderate tightness so 

 as to compress only the superficial veins allowed the blood to pass into the part by the 

 arteries, but prevented its return by the veins, which consequently became excessively 

 congested. The ligature being removed, the veins soon emptied themselves and the 

 member regained its ordinary appearance. He observed the " knots " in the veins of the 

 arm when a ligature was applied, as for phlebotomy, and showed that the space be- 

 tween these knots, which are formed by the valves, could be emptied of blood by press- 

 ing toward the heart, and would not fill itself while the finger was kept at the lower 

 extremity. It was impossible, by pressure with the fingers, to force the blood back 

 through one of the valves. 



FIG. 9. Harvey's observations on the flow of blood in the veins. (Harvey.) 



By such simple, yet irresistibly conclusive experiments was completed the chain of 

 evidence establishing the fact of the circulation of the blood. Truly it is said that here 

 commenced an epoch in the study of physiology ; for then the scientific world began to 

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