Page:EB1911 - Volume 27.djvu/958

 valves are attached by chordae tendinae to two papillary muscles; these are pillars of muscle which rise up from the inner surface of the ventricles.

The edges of these valves which come into opposition are exceedingly thin and delicate, while the outer parts, which bear the full systolic pressure of the blood, are tough. The cardiac muscle, by its contraction, limits the size of the auriculo-ventricular orifices and so maintains the competency of the valves. It is the papillary muscles and chordae tendineae which pull down the diaphragm formed by the closed valves (the floor of the auricles), thus expanding the auricles and enabling the valvular as well as the muscular parts of the wall of the ventricles to approach together and wring out the blood. The thin, moist, film-like edges of the valves of the heart come into perfect apposition and prevent all leakage, while the fibrous parts give strength and support. The ventricles are never completely emptied, for some blood remains in contact with the auriculo-ventricular valves up, to the end of systole and ensures their closure. Incompetency of the valves may arise when the right heart is greatly dilated. The aortic and pulmonary valves consist of three semilunar, pocket-shaped cusps. A fibrous nodule is placed centrally in the free edge of each cusp, whence numerous tendinous fibres radiate to the attached borders of the cusp. The rest of the free edges which come into apposition are thin and delicate. Opposite the cusps are bulgings of the aortic walls—the sinuses of Valsalva. From the anterior one arises the right coronary artery and from the left posterior, the left coronary artery, these vessels supply the substance of the heart with blood. Eddies formed in the sinuses during the period of systolic output bring the semilunar valves into a position, so that they close without noise or jar at the moment when the intraventricular becomes less than the aortic pressure. The auriculo-ventricular valves are likewise floated up by eddies, and brought into apposition at the moment the intraventricular pressure surmounts that in the auricles.

The heart in size is about equal to the closed fist of a man. The average weight of the heart in the new-born baby is about 24 grms., in the adult 300 grms. The percentage which the heart weight bears to the whole body weight is 0·76 in the new-born and 0·46 in the adult. While the whole body increases in weight 21-fold, the heart increases only 12·74-fold (Vierordt, Karl, 1818–1884). The average weight of the male and female heart is almost the same. The average volume of the whole heart is about 270 c.c. The capacity, estimated by filling the heart with wax, is for each auricle about 100–150 c.c., and 150–230 c.c. for each ventricle. There are considerable sources of error in such measurements. The muscle of the left ventricle is about 1·6 cm. in thickness, and of the right ventricle 0·5 cm. The left ventricle has twice the muscular mass of the right. The circumference of the left auriculo-ventricular orifice is about 14·0 cm.; of the right, about 12·5 cm.; of the aortic orifice, 8·0 cm.; of the pulmonary orifice, 9·0 cm. The average diameter of the vena cava superior is about 23 mm.; of the vena cava inferior,

34 mm.; of each of the four pulmonary veins about 13-14 mm.; of the pulmonary artery, 28 mm.; of the aorta, 32 mm.

The physiologist or physician has many means at his disposal of examining the heart’s action. By palpation with the hand over the region of the heart, its stroke, the cardiac impulse, can be felt. By auscultation with the ear directly, or with use of the stethoscope the sounds of the heart can be heard. By percussion the anatomical limits of the organ can be defined. The cardiac impulse can be recorded by tambour methods of registration, the heart sounds by means of the microphone and capillary electrometer, while the volume and movements of the heart can be studied with the help of the Röntgen rays.

The impulse is caused by the sudden hardening of the muscular mass of the ventricles against the wall of the thorax. It is synchronous with the beginning of systole. The position at which the impulse is felt varies with changing posture of the body, as different parts of the thorax come in turn in contact with the ventricle. In the supine position it is usually to be felt in the fifth intercostal space 3 inches from the midsternal line. The chest wall is driven out by the systole only where the heart muscle touches it; at other places it is slightly drawn in. This in drawing is attributed to the expulsion of the blood out of the thorax by the left ventricle. The thorax is a closed cavity and the vacuum therein produced by systolic output into the arteries of the head, limbs and abdomen is filled by (1) the drawing of air into the lungs, (2) the drawing of venous blood into the great veins and right auricle, (3) the slight in drawing of the chest wall. The impulse is recorded by placing a small cup, or receiving tambour, over the spot where it is most evident, and connecting the inside of the cup by a tube to a recording tambour. The cup can be closed by a rubber dam, or an air-tight junction can be effected by pressing it upon the skin. The stroke of the heart is transmitted as a wave of compression to the air within the system of tambours. The recording tambour is brought to write on a drum, moved by clockwork, and covered with a paper smoked with lamp-black. From the record so obtained we can obtain information as to the time relations of the heart-beat, but no accurate information as to its energy or amount of contraction. The movements of the heart consist of a series of contractions which succeed each other with a certain rhythm. The period of contraction is called the systole and that of relaxation the diastole. The two auricles contract and relax synchronously, and these movements are followed by the synchronous contraction and relaxation of the ventricles. |Finally, there is a short period when the whole heart is in diastole. The whole series of movements is known as the cardiac