





PECULIARITIES IN THE VASCULAR SYSTEM OF THE FETUS 5c 



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 atrio- 

 ventricular openings into the ventricles, regurgitation into the veins being pre- 

 vented 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 chordae tendineae, 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 T 8 T of a second, made up as follows: 



Atrial systole, T V Atrial diastole, T V 



Ventricular systole, 3%. Ventricular diastole, ^$-. 



Total systole, T \ . Complete diastole, T V 



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 w r ave; 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. 



PECULIARITIES IN THE VASCULAR SYSTEM OF THE FETUS. 



The chief peculiarities of the fetal heart are the direct communication between 

 he 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 





