. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE FORMATION OF THE HEART 239 the musculi papillares and chordae tendineae so as to form a complete lining of the inner surface of the myocardium. The aortic and pulmonary semilunar valves make their appearance, before the aortic bulb undergoes its longitudinal splitting, as four tubercle-like thickenings of connective tissue situated on the inner wall of the bulb just where it arises from the ventricle. When the division of the bulb occurs, two of the thickenings, situated on opposite sides
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. THE FORMATION OF THE HEART 239 the musculi papillares and chordae tendineae so as to form a complete lining of the inner surface of the myocardium. The aortic and pulmonary semilunar valves make their appearance, before the aortic bulb undergoes its longitudinal splitting, as four tubercle-like thickenings of connective tissue situated on the inner wall of the bulb just where it arises from the ventricle. When the division of the bulb occurs, two of the thickenings, situated on opposite sides, are divided, so that both the pulmonary and systemic aorta? receive three thickenings (Fig. 145). Later the thickenings become hollowed out on the surfaces directed away from the ventricles and are so converted into the pouch-like valves of the adult. Changes in the Heart after Birth.—The T FlG- 145-—Diagrams / . Illustrating the For- heart when first formed lies far forward in the mation of the Semi- neck region of the embryo, between the head £(G^«»- and the anterior surface of the yolk-sac, and from this position it gradually recedes until it reaches its final position in the thorax. And not only does it thus change its rela- tive position, but the direction of its axes also changes. For at an early stage the ventricles lie directly in front of (i. e., ventrad to) the atria and not below them as in the adult heart, and this prim- itive condition is retained until the diaphragm has reached its final position (see p. 322). In addition to these changes in position, which are antenatal, important changes also occur in the atrial septum after birth. Throughout the entire period of fetal life the foramen ovale persists, permitting the blood returning from the placenta and entering the right atrium to pass directly across to the left atrium, thence to the left ventricle, and so out to the body through the systemic aorta (see p. 267). At birth the lungs begin to
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