CIRCULATION OF THE BLOOD. 121 



the tricuspid and mitral valves open. With these events the cardiac cycle 

 is again completed. 



Sounds of the Heart. If the ear be placed over the cardiac region, two 

 distinct sounds are heard during each revolution of the heart, closely following 

 each other, and which differ in character. 



The sound coinciding with the systole in point of time the first sound 

 is prolonged and dull, and caused by the closure and vibration of the auric- 

 uloventricular valves, the contraction of the walls of the ventricles, and the 

 apex-beat; the second sound, occurring during the diastole, is short and sharp, 

 and caused by the closure of the semilunar valves. 



The frequency of the heart's action varies at different periods of life, 

 but in the adult male it beats about seventy-two times a minute. It is in- 

 fluenced by age, exercise, posture, digestion, etc. 



Age. Before birth, the number of pulsations a minifte averages 140 



During the first year it diminishes to 128 



During the third year it dimishes to 95 



From the eighth to the fourteenth year averages 84 



In adult life the average is 72 



Exercise and digestion increase the frequency of the heart's action. 



Posture influences the number of pulsations a minute; in the male, standing, 

 the average is 81; sitting, 71; lying, 66 independent, for the most part, of 

 muscular effort. 



The Blood Supply to the Heart. The nutrition of the heart, its contrac- 

 tility, the force and frequency of the beat are dependent on and maintained 

 by the introduction of arterialized blood into and the removal of waste 

 products from its tissue. This is accomplished by the coronary arteries and 

 the coronary veins. The arteries and veins on the surface of the heart are 

 known as the extra-mural arteries and veins; those which are found in the 

 substance of the heart are known as intra- mural arteries and veins. During 

 the time of the systole the extra-mural arteries are filled with blood from 

 the aorta; during the time of the diastole, the blood flows into the intra- 

 mural arteries and capillaries, furnishing to the muscle cells an additional 

 supply of nutritive materials and receiving products of waste; at the succeed- 

 ing systole the venous blood is driven from the intra-mural into the extra- 

 mural veins and so into the right auricle. 



The Causation of the Heart Beat. From the fact that the heart will 

 continue to beat for a variable length of time after removal from the body 



