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 most of the body, is stouter than the right, which has only to send blood around the comparatively 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 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 ventricles 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 the blood flows on through the auricles into the ventricles, which latter are already nearly full when the auricles contract; this contraction merely completes the filling of the ventricles, and finishes the closure of the auriculo-ventricular valves. The main 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.

What do we hear on listening over the heart region of a living person's chest? What are the sounds called? How does the first differ from the second? What words give some idea of their character? What is the origin of the second sound? Of the first? What occurs as regards the heart sounds in many forms of heart disease?

What work have the ventricles to do? How do their walls differ from those of the auricles? Which ventricle has the thicker wall? Why? What part of the heart maintains the blood flow?