224 THE HUMAN BODY. 



Function of the Auricles. The ventricles have to da 

 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 com- 

 paratively short pulmonary circuit. The circulation of the 

 blood is in fact maintained by the ventricles, and we 

 have to inquire what is the use of the auricles. Not un- 

 frequently 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 con- 

 traction merely completing their filling and finishing the- 

 closure of the auriculo-ventricular valves. The real use of 

 the auricles is to afford a reservoir into which the veins may 

 empty while the comparatively long-lasting ventricular 

 contraction is taking place: they also largely control the 

 amount of work done by the heart. 



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

 at the points of entry and exit of the blood, the circulation 

 could be maintained. During diastole the ventricle would 

 fill from the veins, and during systole empty into the ar- 

 teries. But in order to accomplish this, during the systole 

 the valves at the point of entry must be closed, or the ven- 

 tricle would empty itself into the veins as well as into the 

 arteries; and this closure would necessitate a great loss of 

 time which might be utilized for feeding the pump. 

 This is avoided by the auricles, which are really reser 

 voirs at the end of the venous system collecting blood 

 when the ventricular pump is at work. When the ven- 

 tricles relax, the blood entering the auricles flows on 

 into them: but previously, during the -fW of the cardiac 

 cycle occupied by the ventricular systole, the auricles 

 have accumulated blood, and when they at last con- 



