40 CIRCULATION OF THE BLOOD 



the ventricles, their contraction produces a twisting of the point upon 

 itself. An untwisting occurs during the ventricular diastole. 



If the heart is grasped with the hand during its action, it is observed 

 that the systole is attended with a palpable hardening. Like any other 

 muscle, the heart is sensibly hardened during contraction. 



The projection of the point of the heart forward during the systole is 

 not due to elongation of the ventricles. When the ventricles contract, 

 their transverse diameter is slightly diminished and the antero-posterior 

 diameter is correspondingly increased. The ventricles themselves are 

 sensibly shortened during their systole. 



Cardiac Cycle. A cardiac cycle includes the alternate contraction 

 and dilatation of its several cavities. It is to be remembered that the 

 two sides of the heart act together. Dividing the cycle into eighths and 

 beginning with the contraction of the auricles, the following represents 

 a complete revolution : one-eighth, the auricles contract and the ven- 

 tricles dilate ; three-eighths, the ventricles contract and the auricles are 

 passive; four-eights, the auricles dilate and the ventricles are passive. 

 If the condition of the walls of the several cavities is now compared with 

 the condition of the valves, it will be found that the auriculo-ventricular 

 valves are closed during the three-eighths of the cycle occupied in con- 

 traction of the ventricles, and the semilunar valves are open ; and that the 

 semilunar valves are closed during the remaining five-eighths of the cycle, 

 while the auriculo-ventricular valves are open. It is possible, also, to 

 connect the action of the valves with the condition of the walls of the 

 different cavities, and the direction of the blood-currents with what are 

 known as the heart-sounds. 



Sounds of the Heart. The first sound of the heart accompanies con- 

 traction of the ventricles. It is heard at its maximum of intensity over 

 the apex in the fifth intercostal space and is conducted upward toward 

 the base. This is a compound sound. Its elements are, vibration of the 

 mitral and tricuspid valvular curtains at the time of their closure, a sound 

 due to muscular contraction and an impression conveyed to the ear from 

 the impulse of the heart against the walls of the chest. The first sound 

 is relatively low in pitch, prolonged and " booming." Its low pitch is 

 due to the length of the free borders of the valvular curtains as 

 compared with the semilunar valves : the prolongation of the sound is 

 attributed to the prolongation of the ventricular contraction during 

 three-eighths of the cardiac cycle ; the impulsion element comes from 

 the striking of the apex against the thoracic walls. 



The second sound which follows the first practically without an inter- 

 val is relatively high in pitch and is shorter than the first sound. This 

 is a simple sound and is due to the vibration of the valves during their 



