32 CIRCULATION OF THE BLOOD 



veins and is sent out into the arteries. Exposing the heart and the 

 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 other hand, 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 observations the chain of experimental evi- 

 dence establishing the fact of the circulation of the blood was com- 

 pleted. 



Although Harvey accurately described the course of the blood 

 and left no doubt as to the communication between the arteries and 

 veins, it remained for others actually to see the blood in movement and 

 follow it from one system 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 general 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 at the same time from the right side 

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

 necessary, therefore, to separate the two circulations in the study of 

 their mechanism ; for the simultaneous action of both sides of the 

 heart renders it possible 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. 



There are three kinds of bloodvessels : arteries, which carry blood 

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

 blood in the parts ; and veins, which return the blood from the general 

 system to the heart. 



