ACTION OF THE HEART. 333 



with blood in these vessels, which, supported by the semi- 

 lunar valves, had attained a state of momentary repose. The 

 impact between this fluid in motion, and that in a state of 



rest, gives rise to the sound The second sound 



occurs during diastole, and in its mechanism closely resembles 

 the first. The blood having been driven with much force 

 into the aorta and pulmonary artery, a portion of it recoils, 

 but is checked in its rapid descent towards the heart by the 

 semilunar valves. The sound is caused by the concussion 

 thus induced, the force of which is, however, by no means 

 sustained by the valves alone, for they are thoroughly sup- 

 ported by the ventricles and their contents. This is obvious, 

 since there can be no approach to a vacuum in the heart. 

 The valves are regarded as separating media, which do not 

 themselves sustain the force of the descending blood." 



During contraction the heart becomes firm, like any other 

 muscle in the body; it also elongates, and rotates on its own 

 axis from left to right. It is the protrusion or elongation of 

 the heart, coupled with the turn to the left, which causes 

 the impulse or thrust against the side of the chest. 



The sounds of the heart and impulse are observed to recur 

 with the greatest regularity, and are termed rythmic. There 

 is a period of rest between each complete action of the heart, 

 a pause which occupies nearly one-fourth the time that the 

 heart requires to complete its contractions and movement. 

 Thus the heart, which is supposed never to rest, from the birth 

 of an animal to its death, is during a certain period inactive, 

 and its structure rests after each effort. 



CIRCULATION IN THE BLOOD-VESSELS. 



The arteries recoil to a certain extent on the blood, which 

 is propelled into them by the heart. They are dilated and 



