DISCOVERY OF THE CIRCULATION. 31 



into the arteries. Exposing the heart and great vessels in these animals, he 

 applied a ligature to the veins, which had the effect of cutting off the supply 

 from the heart so that it became pale and flaccid ; and by removing the 

 ligature the blood could be seen flowing into the organ. When, on the 

 contrary, a ligature was applied to the artery, the heart became unusually 

 distended, which continued so long as the obstruction remained. When 

 the ligature was removed, the heart soon returned to its normal condition. 

 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. 



By these simple experiments, the chain of evidence establishing the fact 

 of the circulation of the blood was completed. Truly it 'is said that here 

 began an epoch in the study of physiology; for then scientific observers 

 began to emancipate themselves from the ideas of the ancients, which had 

 controlled opinions for two centuries, and to study Nature for themselves by 

 means of experiments. 



Although Harvey described so perfectly the course of the blood and left 

 no doubt as to the communication between the arteries and veins, it was left 

 to others to actually see the blood in movement and follow it from one sys- 

 tem of vessels to the other. In 1661, Malpighi saw the blood circulating in 

 the vessels of the lung of a living frog, examining it with magnifying glasses ; 

 and a little later, Leeuwenhoek saw the circulation in the wing of a bat. 

 These observations completed the discovery of the circulation. 



In man and in the warm-blooded animals, the organism requires blood 

 that has been oxygenated in the lungs, and to meet this demand fully, the 

 circulatory system is divided into pulmonic and systemic. The heart is 

 double, having a right side and a left side, which are entirely distinct from 

 each other. The right heart receives the blood as it is brought from the gen- 

 eral system by the veins and sends it to the lungs ; the left heart receives the 

 blood from the lungs and sends it to the general system. It must be borne in 

 mind, however, that although the two sides of the heart are distinct from 

 each other, their action is simultaneous ; and in studying the motions of the 

 heart, it will be found that the blood is sent simultaneously from the right 

 side to the lungs and from the left side to the system. It will not be neces- 

 sary, therefore, to separate the two circulations in the study of their mechan- 

 ism ; for the simultaneous action of both sides of the heart renders it .possi- 

 ble to study its action as a single organ, and the constitution and operations 

 of the two kinds of vessels do not present any material differences. 



For convenience of study, the circulatory system may be divided into 

 heart and vessels, the latter being of three kinds : the arteries, which carry 

 blood from the heart to the general system; the capillaries, which distribute 

 the blood more or less abundantly in different parts of the general system ; 

 and the veins, which return the blood from the general system to the heart. 



