EVENTS DURING A CARDIAC PERIOD. 217 



but during the systole it, especially in its ventricular por- 

 tion, becomes hard and rigid, and diminished in size so as 

 to force blood out of it. 



The Cardiac Impulse. The human heart lies with its 

 apex touching the chest-wall between the fifth and sixth 

 ribs on the left side of the breast-bone. At every beat a 

 sort of tap known as the "cardiac impulse," or "apex 

 beat," may be felt by placing the finger at that point. 



Events occurring within the Heart during a Cardiac 

 Period. Let us commence just after the end of the ventric- 

 ular systole. At this moment the semilunar valves at the 

 orifices of the aorta and the pulmonary artery are closed so 

 that no blood can flow back from those vessels. The whole 

 heart, however, is soft and distensible, and yields readily to 

 blood flowing into its auricles from the pulmonary veins 

 and the hollow veins; this blood passes on through the open 

 mitral and tricuspid valves, and fills up the dilating ventri- 

 cles as well as the auricles. As the ventricles fill, back cur- 

 rents are set up along their walls, and carry up the flaps 

 of the auriculo-ventricular valves, so that by the end of the 

 pause they are nearly closed. At this moment the auricles 

 contract; this contraction commences at and narrows the 

 mouths of the veins so that blood cannot easily flow back 

 from the auricles into them; the flabby and dilating ventri- 

 cles oppose much less resistance, and so the general result is 



How during the systole? How is its bulk changed in systole? 



Where does the apex of the heart touch the chest-wall ? What is 

 the cardiac impulse ? 



What is the position of the semilunar valves just after the end of 

 a ventricular systole? What results from their closure? In what 

 condition is the heart in general ? What parts of it does blood enter? 

 From what vessels? What cavities does this blood fill ? What hap- 

 pens as the ventricles fill? What is the position of the valves at the 

 end of the pause ? Where does the auricular contraction commence ?. 

 What is the main result of the auricular contraction ? 



