THE ACTION OF THE HEART 



309 



above the pressure in the auricles; during this time, again, the 

 valves of the heart are all shut. 



Diagram of the Events of a Cardiac Cycle. In the following 

 table the phenomena of the heart's beat are represented with 

 reference to the changes of form which are seen on an exposed 

 working heart. Events in the same vertical column occur simul- 

 taneously; on the same horizontal line, from left to right, succes- 

 sively. 



Function of the Auricles. The ventricles have to do the work of 

 pumping the blood through the blood-vessels. Accordingly their 

 walls are far thicker and more muscular than those of the auricles; 

 and the left ventricle, which has to force the blood over the Body 

 generally, is stouter than the right, which has only to send blood 

 around the comparatively short pulmonary circuit. Th'e circu- 

 lation of the blood is in fact maintained by the ventricles, and we 

 have to inquire what is the use of the auricles. Not unfrequently 

 the heart's action is described as if the auricles first filled with 

 blood and then contracted and filled the ventricles; and then the 

 latter contracted and drove the blood into the arteries. From the 

 account given above, however, it will be seen that the events are 

 not accurately so represented, but that during all the pause blood 

 flows on through the' auricles into the ventricles, which latter are 

 already nearly full when the auricles contract; this contraction 

 merely completing their filling. The real use of the auricles'is to 

 afford a reservoir into which the veins may empty while the com- 

 paratively long-lasting ventricular contraction is taking place. 



If the heart consisted of the ventricles only, with valves at the 

 points of entry and exit of the blood, the circulation could be 



