THE HEART. 121 



culus papillaris. Thus the contraction of the mttsculi papillares 

 not only prevents the cusps from being flung into the auricle, 

 but keeps their edges in apposition. These muscles contract 

 at the same time as the ventricular wall with which their 

 fibres are continuous, and the contraction of the ventricle 

 pushes the blood against the backs of the valves, so as to 

 bring them quite together, block up completely the passage 

 into the auricle, and leave the blood no other aperture of 

 exit save into the artery. 



88. If we apply our ear to any one's chest, we find that the 

 heart makes some noise in its action, that there is a perpetual 

 " pit-pat, pit-pat," or recurrence of two successive sounds ; 

 first a slightly prolonged sound, then, a moment afterwards, 

 another short and clear, and after that a longer interval 

 before the first sound is repeated. The first sound will be 

 found to occur at the same moment as the beat against the 

 chest, and nearly at the same moment as the pulse at the 

 wrist; it is synchronous also with the systole or contrac- 

 tion of the ventricles, and is caused by the vibration of the 

 auriculo-ventricular valves when suddenly closed by the 

 pressure of the blood on them. It continues audible in an 

 animal when the chest has been laid open, so does the second. 

 The second sound is caused by the closure of the arterial 

 valves, as has been experimentally proved by introducing a 

 pair of needles, one into the pulmonary artery, and the other 

 into the aorta, so as to prevent the valves shutting, and 

 observing that at once the sound ceases. 



The beating of the heart against the chest is called the 

 impulse, and is caused by the apex, which is at all times in 

 contact with the wall of the chest, being pressed against it 

 in the ventricular contraction by jerking upwards, forwards, 

 and to the left. This movement is probably caused by the 

 arch of the aorta being thrown into a rigid and more ex- 

 panded curve when filled with blood, while it is fixed in its 

 position behind ; but it has been suggested that it results 

 from the disposition of the muscular fibres of the ventricles. 



89. The frequency of the heart's pulsations varies in health 

 with different circumstances, but principally according to age. 

 In infancy the beats are 120 or more per minute; in early 

 life they quickly dimmish in frequency ; in the adult they 



