174 CIRCULATION. 



merous experiments upon living animals ; exposing the heart 

 of cold-blooded animals, in which the movements are com- 

 paratively slow; studying also the action of this organ in 

 warm-blooded animals, after its movements had become 

 enfeebled. As we shall see when we come to describe the 

 movements of the heart, nothing can exceed the simplicity 

 and accuracy of the descriptions of Harvey, which are uni- 

 versally acknowledged to be correct in almost every par- 

 ticular. 



Harvey completed his description of the circulation, by 

 experiments showing the course of the blood in the arteries 

 and veins, and the uses of the valves of the veins. 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 tight- 

 ness 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. 1 



He observed the "knots" in the veins of the arm when a 

 ligature is applied, as for phlebotomy, and showed that the 

 space between these knots, which are formed by the valves, 

 could be emptied of blood by pressing 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. 3 



By such simple, yet irresistibly conclusive experiments, 

 was completed the chain of evidence establishing the 

 fact of the circulation of the blood. Truly is it said that 

 here commenced an epoch in the study of physiology ; for 

 then the scientific world began to emancipate itself from the 

 ideas of the ancients, which had held despotic sway for two 



1 Op. eit. t p. 55 et seq. 2 Ibid., p. 65. 



