![]() Its distal part on the right loses its connection with the dorsal aorta and disappears. The proximal part of the sixth arch on each side becomes the stem of the right or left pulmonary artery. On the left the distal part of the definitive aortic arch is formed by the fourth aortic arch and the segment of the paired dorsal aorta between the carotid duct and the seventh dorsal intersegmental artery on the right side these segments form the proximal part (base) of the right subclavian artery. ![]() The external carotid artery, a new vessel, joins the internal carotid artery at its point of origin. The proximal segment of the internal carotid artery is formed by the third aortic arch, its distal segment by the cranial extension of the paired dorsal aorta rostral to the carotid duct. Aortic arches—On both sides, the common carotid artery is derived by an elongation of that segment of the horn of the aortic sac between the origins of the third and fourth aortic arches. The corresponding segment of the right horn forms the brachiocephalic artery. Aortic sac—The segment of the left horn of the aortic sac between the origins of the fourth and sixth aortic arches becomes the proximal part of the definitive aortic arch. In most instances the changes differ on each side. Many dramatic changes occur in the aortic sac, aortic arches and the paired dorsal aortas. The arteries between the heart and the brain necessarily increase in length and narrow in relation to surrounding structures. ARTERIES DERIVATIVES OF THE AORTIC SAC, AORTIC ARCHES AND PAIRED DORSAL AORTAS After the heart moves caudally through the cervical segments, the neck region becomes apparent. Venous channels from the right and left lung sacs join near the heart to form the common pulmonary vein, which empties into the left atrium. The transverse part of the sinus venosus and the left sinus horn become incorporated into the coronary sinus in the dorsal part of the atrioventricular sulcus. The left ventricle becomes conical shaped with its blunted end forming the apex of the heart. ![]() CHANGES IN THE EXTERNAL CONFIGURATION The heart moves into the upper thoracic region. The right venous valve (valve of sinus venosus) grows caudally to join the fused endocardial cushion near the orifices of the inferior vena cava and coronary sinus. This causes the septum secundum to lengthen along the right side of the septum primum partially covering the ostium secundum. Most of the septum spurium and the left venous valve become incorporated into the septum secundum obliterating the septovalvular space. Simultaneously the cranial part of the septum primum breaks down forming the ostium secundum. SEPTUM FORMATION IN THE RIGHT ATRIUM The ostium primum disappears as the caudal edge of the septum primum fuses with the endocardial cushions. The attachments to the ventricular wall are replaced by dense connective tissue cords called chordae tendineae, which are connected to fingerlike processes of ventricular muscle known as papillary muscles. ![]() Two flaps develop around the left canal forming the left atrioventricular (mitral) valve. Three flaps develop around the right canal forming the right atrioventricular (tricuspid) valve. The bulges become flaps of tissue that are attached in some areas to the ventricular wall. FORMATION OF THE ATRIOVENTRICULAR (A-V) VALVES The tissue around each atrioventricular canal bulges into the canal and the ventricle below it. The partition later becomes thin and fibrous to form the membranous part of the interventricular septum. As a result a thick partition is formed that closes the foramen. Simultaneously, it also fuses with the lower edge of the conus septum. CLOSURE OF THE INTERVENTRICULAR FORAMEN The dorsal (inferior) endocardial cushion expands into the interventricular foramen and fuses with the upper edge of the muscular portion of the interventricular septum. As they grow into the lumen, they become cuplike and together form a semilunar valve in each vessel. HEART FORMATION OF THE SEMILUNAR VALVES Three tubercles form on the inside of both the ascending aorta and pulmonary artery at the junction of the truncus arteriosus and conus cordis (truncoconal junction). Search results: subcardinal vein Search Atlas 5-6 Atlas 6-7 Atlas 7-8
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