34 



CIRCULATION OF THE BLOOD. 



emancipate itself from the ideas of the ancients, which had held despotic sway 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 not a shadow 

 of 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 system of vessels to the otheiv 

 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 circula- 

 tion in the wing of a 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 

 through 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 

 incident to 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 animals, 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 com- 

 plete divisions of the circulation into the pulmonary and systemic, or, as they have been 

 called, the lesser and greater circulation. The heart in this instance is double ; hav- 

 ing 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 the lungs 

 and sends it to the system. It mast 

 be borne in mind, however, that al- 

 though the two sides of the heart 

 are distinct from each other, their 

 action is simultaneous ; and, in 

 studying the motions of this or- 

 gan, we shall find 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 necessary, therefore, to sep- 

 arate the two circulations in our 

 study of their mechanism ; for the 

 simultaneous action of both sides 

 of the heart enables us to study 

 its functions 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 system ; the capillaries, which distribute the blood more or less abun- 

 dantly in different parts of the system ; and the veins, which return the blood from 

 the system to the heart. The functions of each of these divisions may be considered 

 separately. 



FIG. 10. Diagram of the four carities of the heart. (Bernard.) 

 od, right auricle ; cZ, right ventricle ; og, left auricle , <vg, left 

 ventricle. The arrows indicate the course of the blood. 



