A word remains to be said about the beat of the auricle. During most of the period when the ventricle is relaxed the auricle is also quiet and blood is pouring directly through it from the veins into the ventricle; but just an instant before the ventricular beat begins, one-tenth of a second to be exact, the auricle contracts, emptying what blood it contains into the nearly filled ventricle; thus, when the ventricle beats, which it does immediately, closing the intake valve at the same time, the auricle is empty and so is able to accommodate the inflow of blood from the vein during the three-tenths of a second that the intake valve is shut. Both sides of the heart work exactly together, the two auricles beating simultaneously, and the two ventricles. Of course it is necessary that this be so, for if they did not keep pace exactly, one with the other, there would be a piling up of the blood either in the lungs or in the veins leading from the body to the heart, and the efficiency of the circulation would be seriously impaired. We can get a good deal of information about the way our hearts are behaving simply by holding the hand against the chest directly over the heart or by pressing the ear against the chest of some one else and listening to the heart’s action. The physician makes use of a stethoscope, which is merely an apparatus for conducting clearly the sounds which the heart makes, so that it is not necessary to apply the ear to the chest. When one listens thus to the heart he finds that with every beat there are two distinct sounds: the first is a rather dull sound which comes just at the beginning of the beat of the ventricle, the second is a sharp sound occurring just at the end of the ventricular beat. As we saw in Chapter IX, sound is always the result of vibrations, and a great deal of study has been devoted to an attempt to find out where the vibrations come from that cause the heart sounds. It is now generally believed that the first sound is partly the result of vibrations set up in the contracting heart muscle and partly due to vibrations from the sharp closing of the intake valves. The second sound is known to be wholly due to the sudden closing of the outflow valves. The sounds are chiefly of importance in that they enable the physician to determine whether the valves are holding tight or whether there is a leakage of blood through them. In case the intake valve leaks, there will be a backward jet of blood from the ventricle into the auricle with every beat of the heart. This will cause a sort of hissing or murmur which can be heard with the stethoscope in connection with the first sound. If the outflow valve is the one that leaks, blood will squirt back into the ventricle from the aorta, while the ventricle is relaxing. The murmur in this case will come just after the second sound. The skillful physician by comparing the loudness of the murmur when the stethoscope is pressed at different points on the chest and back can