THE CIRCULATION. 163 



when the ventricles are passively full, and the blood now 

 begins to back up in the auricles. Dividing the time of a 

 complete heart beat into ten periods, the time occupied in 

 this passive filling of the heart occupies six-tenths of the 

 complete time. 



At this juncture the auricles begin to contract and force 

 their added amount of blood into the ventricles, which, 

 being already practically filled with blood, now become 

 gorged and distended. By this means the auriculo- ventric- 

 ular valves become tightly closed. The time required for 

 the contraction of the auricles is about one-tenth of the 

 time. It is not necessary to state here that the two auricles 

 contract together, and the two ventricles together, as if 

 they were in each case but a single structure. 



The reason why the auriculo-ventricular valves are closed 

 by the forcing of this extra blood into the ventricles, may 

 be explained by comparing the ventricles to a crowded 

 room. It is easily possible to imagine a room so crowded 

 as to make it impossible to open the door, especially if the 

 crowding should be behind the door. As soon as the au- 

 ricles have finished their contraction the ventricles begin to 

 contract with much greater force. Just at the beginning of 

 this contraction, when the muscles of the ventricles are 

 becoming taut and tense, the first sound of the heart is 

 heard. Just how it is produced will be explained further 

 on. By the continued contraction of the ventricles the 

 blood is pressed harder and harder, until finally the pressure 

 is sufficient to open the semilunar valves, and the blood 

 flows into the outgoing vessel. The time required by the 

 ventricles in their contraction is about three-tenths of the 

 total period. As soon as the blood has been pressed into 

 the vessels the muscular fibres of the heart relax. The blood 

 in the pulmonary artery and aorta, being under consider- 

 able pressure, tends to flow back into the relaxed heart, but 

 in so doing it distends the semilunar valves and closes 

 them. This is the moment when the second sound of the 

 heart occurs. This sound is therefore easily explained. It 



