THE CIRCULATION. 



23. Passage of the Blood through the Heart. 



Let us now trace the course of the blood through the 

 several cavities of 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 dilatation : 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 un- 

 deviating course of heart-currents. 



24. The mechanism which enforces and regulates it, is 

 as simple as it is beautiful. Each ventricle has two open- 

 ings, an inlet and an outlet, each of which is guarded by 

 strong curtains, or valves. These valves open freely to 

 admit the blood entering from the right, but close inflex- 

 ibly 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 dilate 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. 



25. Frequency of the Heart's Action. The alter- 

 nation of contraction and dilation constitutes the heart- 



23. Course of the blood through the heart ? Course of heart-currents ? 



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 ? 



