"The ejection period may therefore be divided into two phases, that of maximum ejection and that of reduced ejection. The ventricular muscle suddenly relaxes at the point marked IV., causing a sudden fall of pressure in the ventricle and a slight fall in the aorta. The latter is, however, arrested almost at once by the closure of the aortic valves, marked by the sharp depres sion, the dicrotic notch, in the aortic tracing. The pressure in the ventricle continues to fall until at the point VI. it drops below that in the auricle and the auriculo-ventricular valves open, allow ing the inflow of blood from the pulmonary veins and auricles. Between V. and VI. the relaxation is isometric, since all the valves guarding the orifices of the ventricles are closed.
"The pressure in the ventricles then continues to fall more slowly until it reaches the line of zero pressure, and remains at or near this line during the greater part of diastole. With a big inflow there may be a slight rise towards the end of diastole, which may be accentuated by the auricular contraction. If the chest is opened the pressure in the ventricle never sinks below zero during any part of diastole. In the closed chest the pressure in the heart cavities during diastole will be negative, on account of the negative pressure within the thorax." The duration of the separate phases of the heart beat depends naturally on the rate of the beat. In all cases of change of heart rate, the period of diastole is affected relatively much more than the period of systole. Owing to the absence of valves between the auricles and the large veins, changes of pressure within the auricle will be transmitted along the veins.
In every case the auricular pressure tracing exhibits the follow ing features (fig. 6) : (I) The first positive wave, which occurs during auricular systole. (2) The second positive wave, which is due to the sharp closure of the auriculo-ventricular valves. (3) The third positive wave, which is due to the filling of the auricles while the auriculo-ventricular valves are closed. (4) A negative wave, which is due to the rapid emptying of the auricle after the opening of the auriculo-ventricular valves. The chief function of the auricle is not to propel blood into the ventricle by its contraction, but to serve together with the big veins as a reservoir for the blood which flows in from the body but which cannot enter the ventricle while the latter is in a state of contraction.
The Apex Beat.—The pulsation which is felt over the region of the heart is known as the "apex beat," and was formerly thought to be due to the twisting forward of the apex at each systole. Its origin is, however, different. During diastole, the ven tricles form a flabby flattened cone, lying against the chest wall and slightly depressed by the latter. In systole, the ventricles become hard and rigid, and assume the form of a rounded cone. This sudden change in shape and hardening of the ventricular wall pushes out that part of the chest wall which is in immediate proximity to the ventricle, giving rise to the "apex beat." The Sounds of the Heart.—When the ear is applied to the chest above the cardiac region, two sounds may be heard; the first, which is heard most intensely over the apex, is a duller and longer sound than the second, which is heard best over the base of the heart. The first and second sounds resemble the syllables lubb dup–lubb dup. The first sound is of twofold origin. It arises from the sudden closure of the auriculo-ventricular valves, and from the contraction of the thick muscular wall of the ventricles. The second sound is due to the sharp closure of the auricular and pulmonary valves.
When fluid escapes through a narrow orifice, vibrations are set up in the fluid giving rise to various sounds. Under normal condi tions, when the valves of the heart are closed completely, sounds produced in this way are either absent or negligible ; in abnormal conditions, e.g., after disease affecting the orifices of the heart or the valves, these vibrations may become loud enough to be easily heard. These murmurs or bruits as they are called are of great importance, for they enable the physician to judge the condition of the valves, and to determine which valve is affected.
The Electrocardiogram.—The contractions of the different cavities of the heart are accompanied by electrical changes which can be recorded if any two parts of the heart are connected to a sensitive galvanometer. The apparatus which is generally used for this purpose is the string galvanometer, in which a very delicate thread of silvered quartz or of platinum is stretched between the poles of a strong magnet. If we lead off, not from the heart itself, but from tissues which are in contact with the heart, we shall still obtain the electrical changes at each heart beat. An electrocardi ogram so obtained is reproduced in fig. 7.