Til K CI II C UL A TWN. 



Their contents can not go back into the veins, for they 

 are full; but the ventricles have just emptied them- 

 selves, and are opening for a new supply. The blood is 

 therefore forced suddenly on through the passages, with 

 their open valves, into the ventricles. Directly the ven- 

 tricles, now full, begin to contract. The blood sets back 

 against the valves it has passed, and shuts them tight, 

 just as a crowd trying to get through a door which 

 opens towards them will often close it: but the passages 

 into the pulmonary artery and the aorta are clear, 

 their valves (the semilunar} open out; into them the 

 blood pours, the ventricles still contracting until all is 

 squeezed out. Then the blood in the arteries sets back, 

 and shuts the semilunar valves; and the ventricles re- 

 lax, and open for another supply. 



SOUNDS OP THE HEART. 



SECTION IV. 1, If the ear be placed upon the chest 

 over the heart, two sounds can be distinctly heard, re- 

 peated with each beat. ' One is quickly followed by the 

 other, and then there is an interval. The second is 

 shorter, and higher pitched than the first. They may be 



indicated by the signs 4 +~ The first sound is 



lub - dub. 



caused chiefly by the closing of the tricuspid and mitral 

 valves, as the ventricles contract. The second sound is 

 caused by the closing of the semilunar valves after the 

 blood has passed into the pulmonary artery and aorta. 

 When the heart is diseased, these sounds are changed ; 

 and the changes in sound indicate to the ear of the 

 physician the particular changes in the heart. 



