The heart's action is accompanied by two distinct sounds, which can be heard by bringing the ear into firm, direct contact with the praecordial region, or indirectly by the use of the stethoscope.*
One sound follows the other quickly, and then comes a short pause; consequently, they are spoken of as the first and second sounds.
The first sound is heard at the beginning of the ventricular systole. It is a low, soft, prolonged tone, and is most distinctly heard over the fifth intercostal space.
The second sound is heard at the moment when the two sets of semilunar valves are closed and made tense, that is when the blood ceases to escape from the ventricles. It is a sharp, short sound, and is best heard at the second costal cartilage on the right side.
The cause of the first sound is not so evident. Possibly several factors aid in its production. The principal events occurring at the same time as the first sound may be enumerated thus: -
Fig. 121. Writing Lever and Tambour, (a) Joint of the lever; (d) Air chamber; (f) Rubber tubing connecting it with cardiac tambour.
* A flexible stethoscope to listen to one's own heart sounds can easily be made by fitting the mouthpiece to one end of a piece of rubber tubing about 18 inches long, and to the other end the bowl of a wooden pipe. The bowl is applied over the different regions of the heart, and the mouthpiece firmly fitted in the ear.
1. The heart's impulse.
2. The rush of blood into the arteries.
3. The contraction of the heart muscle.
4. The sudden tension of the ventricular chambers and the auriculo-ventricular valves.
It has already been seen that the heart's impulse is caused by a sudden change in shape and density of the muscle, and not by a knock against the chest. The first sound is heard more clearly when the chest wall is removed, so that the apex beating against the thorax cannot help to cause the sound.
The character of the sound is quite unlike that which could be produced by the passage of the blood through the arterial orifices.
The sound is not unlike the muscular tone which accompanies the continuous (tetanic) contraction of the skeletal muscles. It corresponds in time with the contraction of the cardiac muscle. In disease where the heart muscle is weak, the sound becomes faint or inaudible, although the valves are made tense by an intra-ventricular force sufficient to overcome the pressure in the arteries. Otherwise the circulation would cease. An abnormal presystolic sound, like in character to the systolic sound, is now supposed by some physicians to be produced by the auricular systole; but this cannot depend on the vibrations of valves.
All this evidence tends to show that the sound is produced by the contraction of the muscle tissue of the heart, or, in short, that it depends upon some sudden physical change occurring during the cardiac muscle contraction.
Against the view that the muscular tone is the cause of the first sound is urged the supposition that only tetanus causes a muscle sound, and a single contraction is not accompanied by any tone. Though in many ways it differs from the single contraction of other muscles, yet the heart beat is no doubt a single contraction. But the tone which may be heard during the normal contraction of skeletal muscle has not been proved to depend on regularly recurrent contractions such as occur in the tetanus produced by an interrupted current; and a kind of thud, very like the first sound of the heart, may be elicited by the single stimulation of a skeletal muscle.
On the other hand, the auriculo-ventricular valves are made tense at the beginning of the sound, and injury or disease of these valves is said to be associated with a weak or altered first sound: this is often observed in disease of the mitral valve. The blood is said by some to be necessary for the production of the sound, because the gentle closure and immediate subsequent tension of these valves have a share in causing it.
As before remarked, the valvular tension would not account for the presystolic sound occasionally heard, and there is no doubt that the first sound can be heard in an empty heart, removed from the animal, in which the valves cannot become tense, or even in the ventricles after they are separated from the valves.
The sound has been analyzed with suitable resonators, and two distinct tones made out - one high and short, corresponding to the tension of the valves; the other long and low, corresponding in duration with the muscle contraction.
The reasons given for thinking that the heart muscle cannot produce a tone suggest that the sudden state of tension of the ventricular wall when tightened over the blood may give rise to vibrations, and be an important item in causing the first sound. This would explain the faintness of the sound, both when the valves were injured and the muscle weak, and when the blood was prevented from entering. It would also explain the presystolic sound, which requires a certain auricular tension for its production.
From the foregoing statements it would appear probable that both the tension of the valves and the muscle are concerned in the production of the first sound.
The production of the second sound is more easily explained. Occurring just after the ventricle is emptied, it is synchronous with the closure and sudden tension of the semilunar valves at the aorta and pulmonary orifices. The blood in the aorta forcibly closes the valves as soon as the ventricular pressure begins to wane. This sudden motion causes a vibration of the valves, which is rapidly checked by the continuous pressure of the column of blood.