pages 539

lowed, after a slight pause, by (2) a simultaneous, but more prolonged, contraction of both ventricles, named the ventricular systole, and (3) a period of rest, during which the whole heart is relaxed. The atrial contraction commences around the venous openings, and sweeping over the atria forces their contents through the atrioventricular openings into the ventricles, regurgitation into the veins being prevented by the contraction of their muscular coats. When the ventricles contract, the tricuspid and bicuspid valves are closed, and prevent the passage of the blood back into the atria; the musculi papillares at the same time are shortened, and, pulling on the chordæ tendineæ, prevent the inversion of the valves into the atria. As soon as the pressure in the ventricles exceeds that in the pulmonary artery and aorta, the valves guarding the orifices of these vessels are opened and the blood is driven from the right ventricle into the pulmonary artery and from the left into the aorta. The moment the systole of the ventricles ceases, the pressure of the blood in the pulmonary artery and aorta closes the pulmonary and aortic semilunar valves to prevent regurgitation of blood into the ventricles, the valves remaining shut until reopened by the next ventricular systole. During the period of rest the tension of the tricuspid and bicuspid valves is relaxed, and blood is flowing from the veins into the atria, being aspirated by negative intrathoracic pressure, and slightly also from the atria into the ventricles. The average duration of a cardiac cycle is about 8/10 of a second, made up as follows:

Atrial systole, 1/10.		Atrial diastole, 7/10.
Ventricular systole, 3/10.		Ventricular diastole, 5/10.
Total systole, 4/10.		Complete diastole, 4/10.

The rhythmical action of the heart is muscular in origin—that is to say, the heart muscle itself possesses the inherent property of contraction apart from any nervous stimulation. The more embryonic the muscle the better is it able to initiate and propagate the contraction wave; this explains why the normal systole of the heart starts at the entrance of the veins, for there the muscle is most embryonic in nature. At the atrioventricular junction there is a slight pause in the wave of muscular contraction. To obviate this so far as possible a peculiar band of marked embryonic type passes across the junction and so carries on the contraction wave to the ventricles. This band, composed of special fibers, is the atrioventricular bundle of His (p. 537). The nerves, although not concerned in originating the contractions of the heart muscle, play an important role in regulating their force and frequency in order to subserve the physiological needs of the organism.

4c. Peculiarities in the Vascular System in the Fetus

The chief peculiarities of the fetal heart are the direct communication between the atria through the foramen ovale, and the large size of the valve of the inferior vena cava. Among other peculiarities the following may be noted. (1) In early fetal life the heart lies immediately below the mandibular arch and is relatively large in size. As development proceeds it is gradually drawn within the thorax, but at first it lies in the middle line; toward the end of pregnancy it gradually becomes oblique in direction. (2) For a time the atrial portion exceeds the ventricular in size, and the walls of the ventricles are of equal thickness: toward the end of fetal life the ventricular portion becomes the larger and the wall of the left ventricle exceeds that of the right in thickness. (3) Its size is large as compared with that of the rest of the body, the proportion at the second month being 1 to 50, and at birth, 1 to 120, while in the adult the average is about 1 to 160.

The foramen ovale, situated at the lower part of the atrial septum, forms a free communication between the atria until the end of fetal life. A septum (septum secundum) grows down from the upper wall of the atrium to the right of the primary

Contents -SUBJECT INDEX -BIBLIOGRAPHIC RECORD -ILLUSTRATION INDEX