DISCOVERY OF THE CIRCULATION. 175 



centuries, and study Nature for themselves by means of 

 experiments. 



Though Harvey described so perfectly the course of the 

 blood, and left not a shadow of doubt as to the communica- 

 tion between the arteries and veins, it was left to others to 

 actually 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, 

 in examining it with magnifying glasses ; and a little later, 

 Leeuwenhoek saw the circulation in the wing of the bat. 

 The great discovery was then completed. 



Enough has been said in the preceding historical sketch 

 to give a general idea of the course of the great nutritive 

 fluid, and the natural anatomical and physiological divisions 

 of the circulatory system. There is a constant flow from the 

 central organ to all the tissues and organs of the body, and a 

 constant return of the blood after it has passed througli 

 these parts. But before the blood, which has thus been 

 brought back, is fit to return again to the system, it must pass 

 through the lungs and undergo the changes which constitute 

 the process of Respiration. In some animals, like fishes, the 

 same force sends the blood through the gills, and from them 

 through the system. In others, like the reptiles, a mixture 

 of aerated and non-aerated blood takes place in the heart, 

 and the general system never receives blood that has been 

 fully arterialized. But in man and all warm-blooded ani- 

 mals, the organism demands blood that has been fully purified 

 and oxygenated by its passage through the lungs, and here 

 we find the first great and complete divisions of the circula- 

 tion into the pulmonary and systemic, or, as they have been 

 called, the lesser and greater circulation. The heart in this 

 instance is double; having a right and left side which are 

 entirely distinct from each other. The right heart receives 

 the blood as it is brought from the system by the veins, and 

 sends it to the lungs ; the left heart receives the blood from 



