136 



THE CIRCULATION OF THE BLOOD 



Heart. To provide the power required to drive the 

 blood through the arteries, its pumping organ, the hearty 

 is a strong hollow body, whose thick walls are made up 

 entirely of muscular tissue. In order to keep separate 

 the blood in the two circulatory systems, the heart is 

 divided into two parts with no direct means of com- 

 munication between them. In each part are two cham- 

 bers, an auricle and a ventricle, separated from each 

 other by swinging doors, the valves, which by opening 

 only in one direction prevent the blood from flowing 

 backward. 



Heart muscle. The muscle which produces the power 

 to push the blood forward, is arranged so as to form the 



Aorta.. 



Cavity of auricle 

 Auriculo-ventncular valve 



Chordae tendinae 

 Cavity of ventricle 

 ry muscle 



FIG. 78. Diagram of heart during relaxation and contraction: A, auricle con- 

 tracting to fill ventricle ; B, auricle filling, ventricle emptying into aorta. 



walls and main partitions of the heart. The pumping ac- 

 tion consists in the contraction of the muscular walls 

 of both chambers when they are full of blood, until 

 they become so small that the blood is forced out through 

 the open valves. The valves then close and thereby pre- 



