THE VITAL PROCESSES 



relaxing of its walls. During contraction the cavities are 

 closed and the blood is forced out of them. During relax- 

 ation the cavities open and are refilled. The valves direct 

 the flow of the blood, being so arranged as to keep it 

 moving always in the same direction (Fig. 17). 



The heart, however, is not a single or a simple pump, 

 but consists in reality of four pumps which correspond to 

 its different cavities. These connect with each other and 

 with the blood vessels over the body in such a manner 

 that each aids in the general movement of the blood. 



Work of Auricles and Ventricles Compared. In the work 

 of the heart the two auricles contract at the same time 

 their contraction being followed imme- 

 diately by the contraction of both ven- 

 tricles. After the contraction of the 

 ventricles comes a period of rest, or 

 relaxation, about equal in time to the 

 period of contraction of both the auri- 

 cles and the ventricles. 1 On account 

 of the work which they perform, the 

 auricles have been called the " feed 

 pumps " of the heart ; and the ventri- 

 cles, the " force pumps." 2 It is the 

 function of the auricles to collect the blood from the veins, 

 to let this run slowly into the ventricles when both the 



moves a fluid by varying the size of a cavity is a pump, it is seen that not only 

 the heart, but the chest in breathing and also the mouth in sucking a liquid through 

 a tube, are pumps in principle. The ordinary syringe bulb illustrates the class of 

 pumps to which the heart belongs. (See Practical Work.) 



1 The contraction of the heart is known as the systole and its relaxation as the 

 diastole. The systole plus the diastole forms the so-called "cardiac cycle'' (Fig. 

 18). This consists of (i) the contraction of the auricles, (2) the contraction of the 

 ventricles, and (3) the period of rest. The heart systole includes the contraction 

 of both the auricles and the ventricles. 



2 Martin, The Human Body. 



FIG. 1 8. Diagram 

 illustrating the " cardiac 

 cycle." 



