. Chordate morphology. Morphology (Animals); Chordata. ductus caroticus porathyroid III thymus ductus arteriosus thyroid pulmonary artery trachea esophagus. external carotid parathyroid IV pulmonary artery. A3 6 mm B 7 mm Figure 1 1-9. Two stages in the development of the aortic arches of man as seen from the side. (After Prentis and Arey, 1917) third arch becomes the base of the internal carotid. Dorsal connection between the third and fourth arches (carotid duct) is reduced and eventually lost. The left fourth arch, the systemic, continues to develop, while that of the right side disappears.
. Chordate morphology. Morphology (Animals); Chordata. ductus caroticus porathyroid III thymus ductus arteriosus thyroid pulmonary artery trachea esophagus. external carotid parathyroid IV pulmonary artery. A3 6 mm B 7 mm Figure 1 1-9. Two stages in the development of the aortic arches of man as seen from the side. (After Prentis and Arey, 1917) third arch becomes the base of the internal carotid. Dorsal connection between the third and fourth arches (carotid duct) is reduced and eventually lost. The left fourth arch, the systemic, continues to develop, while that of the right side disappears. The fifth arch disappears shortly after its appearance if it appears at all. The ductus arteriosus of the sixth arch atrophies at the time of birth, thus directing all of the blood of this arch through the lung. In terms of the adult, the systemic trunk extends to the left as the base of the systemic arch. The right fourth arch remains as the right subclavian artery. It has lost its connection with the dorsal aorta. The right subclavian and the right common carotid arise from the aortic stem by a common vessel, the brachiocephalic (innominate) artery. The left common carotid and subclavian arise separately from the aortic arch. Reptiles and birds The aortic arches of the reptile pass through the same basic stages as those of the mammal. The primary difference lies in the fact that both systemic arches are retained as con- tinuations of separate divisions of the truncus (Figure 11-11). The subclavian arteries pass to the limbs from the systemic arches and the carotid duct is usually retained (Iguana but not Varanus). The pulmonary arches are like those of the mammal, the ductus arteriosus being closed at the time of hatching with the result that all blood is directed to the lung. The exception to this is Sphenodon which sometimes re- tains this connection. In the bird, the reptile pattern of development is main- tained The right fourth arch becomes the systemic arch, the left d
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