. Chordate morphology. Morphology (Animals); Chordata. bicuspid (mitral) valve semilunar valves of pulmonary arch mm tricuspid valves interventricular septum bicuspid valve Figure 11-1. Development of the mammalian heart. A, semidiagrammatic frontal section of the heart of a pig; B, anterior holf of A as seen from behind; C, 6-mm pig; D, anterior half of C; E, pig; F, anterior half of E; G, humon; H, anterior half of G. (After Patten, 1 946) between the openings into the fourth and sixth aortic arches. These ridges are extended backward along the truncus wall in a cloc
. Chordate morphology. Morphology (Animals); Chordata. bicuspid (mitral) valve semilunar valves of pulmonary arch mm tricuspid valves interventricular septum bicuspid valve Figure 11-1. Development of the mammalian heart. A, semidiagrammatic frontal section of the heart of a pig; B, anterior holf of A as seen from behind; C, 6-mm pig; D, anterior half of C; E, pig; F, anterior half of E; G, humon; H, anterior half of G. (After Patten, 1 946) between the openings into the fourth and sixth aortic arches. These ridges are extended backward along the truncus wall in a clockwise course of just over one-half turn. The right lateral ridge comes to lie left ventrally in the conus opening, and the left ridge is right dorsal in position. The edges of these extend outward until they meet, thus dividing the truncus. The fused ridges then join the interventricular septum which has also fused with the atrioventricular cushion to complete the separation of the ventricles. Concurrently the sinus venosus is drawn into the wall of the right auricle until the now formed anterior and poste- rior venae cavae open separately, the former in front of the latter. (The development of these vessels will be described later in this chapter.) Then a coronary vessel opens behind the posterior. The sinus tissue is represented by the sinu- atrial node of the adult heart. The second interatrial septum develops until only a small foramen ovale remains. This foramen is not in line with the osteum II of the primary interatrial septum. The cushions margining the openings between the atria and ventricles now begin to develop as flaps supported by chordae tendi- neae and papillary muscles. These braces for the valves are sculpted out of the thick muscular walls of the ventricle. Valves develop from the cushions at the base of the pulmo- nary and systemic trunks, three in each of these. The pulmo- nary drainage has appeared and has entered the left auricle. These pulmonary openings
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