THE CIRCULATION 157 



the heart. In the first place, the venous blood, rendered dark 

 and impure by contact with the changing tissues of the body, 

 returns to the right heart by the veins. It enters and fills the 

 right auricle during its expansion ; the auricle then contracts 

 and fills the right ventricle. Almost instantly, the ventricle 

 contracts forcibly and hurries the blood along the great artery 

 of the lungs, to be purified in those organs. Secondly, having 

 completed the circuit of the lungs, the pure and bright arterial 

 blood enters the left auricle. This now contracts and fills the 

 left ventricle, which cavity, in its turn, contracts and sends the 

 blood forth on its journey again through the system. This 

 general direction from right to left is the uniform and undevi- 

 ating course of heart-currents. 



24. The mechanism which compels this regularity is as sim- 

 ple as it is beautiful. Each ventricle has two openings, an 

 inlet and an outlet, each of which is guarded by strong cur- 

 tains, or valves. These valves open freely to admit the blood 

 entering from the right, but close inflexibly against its return. 

 Thus, when the auricle contracts, the inlet valve opens ; but as 

 soon as the ventricle begins to contract, it closes promptly. 

 The contents are then, so to speak, cornered, and have but one 

 avenue of escape that through the outlet valve into the 

 arteries beyond. As soon as the ventricle begins to expand 

 again, this valve shuts tightly and obstructs the passage. The 

 closing of these valves occasions the two heart-sounds, which 

 we hear at the front of the chest. (Figs. 38, 39, 40, and 41.) 



25. Frequency of the Heart's Action. The alternation of 

 contraction and expansion constitutes the heart-beats. These 

 follow each other not only with great regularity, but with 

 great rapidity. The average number in an adult man is about 

 seventy-two in a minute. But the heart is a susceptible 

 organ, and many circumstances affect its rate of action. Heat, 

 exercise, and food increase its action ; cold, fasting, and sleep 

 diminish it. Posture, too, has a curious influence ; for if while 



24. Openings of the ventricles ? How guarded ? How do the valves operate ? The 

 consequence ? Heart-sounds ? 



25. Heart-beats ? The heart as a susceptible organ ? Heat, exercise, etc. ? Posture ? 



