90 PHYSIOLOGY FOR BEGINNERS CHAP. 



and the semilunar valves of the aorta are closed by the great 

 pressure in the aorta keeping the pockets pressed together 

 so that no blood returns to the ventricle. 



The aorta with its branches is distended with blood, and 

 as more and more blood is forced into it by the left ventricle, 

 the distension is kept up and some of the blood already in it 

 is forced along the branches, and so through the capillaries in 

 all parts of the body (except the lungs) into the veins, and 

 finally along the inferior or superior vena cava to the right 

 auricle. The left ventricle forces the blood all the way from 

 the heart through all parts of the body (except the lungs) back 

 to the heart again. Much greater force is required to do this 

 than to send the blood only through the lungs, because it has 

 to drive blood through a much larger number of capillaries. 

 The walls of the left ventricle are therefore much thicker than 

 those of the right. 



The Cardiac Impulse. The apex of the heart lies close 

 to the chest wall, and with each beat of the heart is suddenly 

 pressed against it. This striking of the chest wall by the 

 apex of the heart is called the cardiac impulse. It is easily 

 felt if the finger is placed on the chest between the fifth and 

 sixth ribs on the left side, about one inch below and half an 

 inch to the inner side of the left nipple. 



Sounds of the Heart. Listen to the beat of the heart 

 of a person by putting the ear against the chest just where the 

 cardiac impulse is felt. Two sounds are heard the first dull 

 and relatively long, the second short and sharp ; the two may 

 be likened to the syllables " lub," " dup." The two sounds 

 succeeding each other very rapidly are followed by a relatively 

 longer but stili brief pause, and then repeated again. The 

 first sound, the " lub," occurs when the ventricles are contract- 

 ing, and is caused partly by the " rumbling " of the muscular 

 tissue and partly by the vibrations of the mitral and tricuspid 

 valves set up by the pressure on them. The second, sharp 

 sound, the " dup," is caused by the semilunar valves of the 

 aorta and pulmonary artery being thrown into vibrations at 

 their sudden closure. This second sound is heard louder if the 

 ear is placed on the chest over the aorta*at the level of the 

 second costal cartilage. 



Bate of Beat. The heart beats about 72 times a minute, 



