A STUDY OF THE CIRCULATION OF BLOOD 135 



The Action of the Valves of the Heart. Let us now review 

 in succession the events that occur within the chambers of 

 the heart from the beginning of one heart beat to the begin- 

 ning of the next. The walls of the auricles begin their con- 

 traction in the region where the veins are bringing in the 

 blood. In this way the blood is prevented from being pushed 

 backward, and all of it is forced into the ventricles. As long 

 as the auricles are contracting, the mitral and tricuspid valves 

 are at least partly open; but when this contraction ceases, 

 these valves are 

 forced by the ji 



blood in the ven- 

 tricles into a 

 horizontal posi- 

 tion, but are 

 prevented from 

 going farther by 

 the chordae ten- 

 dineae and the 

 papillary mus- 

 cles. Mean- 

 while the semi- 

 lunar valves are 

 kept closed by 

 the pressure of the blood in the aorta and pulmonary artery, 

 and for an instant the blood is held fast in the grip of the 

 ventricles. 



The contraction of the ventricles now begins, and the 

 pressure on the two sets of valves increases. The mitral 

 and tricuspid valves, that close the passage back to the auri- 

 cles, cannot open, and so the semilunar valves are forced 

 back, and the ventricles drive on the blood they have held 

 into the two large arteries (pulmonary and aorta) already 

 mentioned. 



During the contraction of the ventricles the auricles have 

 been relaxing and filling with blood. When the ventricles 



A B 



FIG. 50. Diagrams to show the Action of the Valves 

 of the Heart. 



A = position of valves during pause. 

 B = position of valves during the contraction of the 

 ventricle. 



