. Anatomy, descriptive and applied. Anatomy. 762 THE VASCULAR SYSTEMS the sixth left arch gives off the pulmonary arteries and forms the ductus arteriosus; this duct remains pervious throughout fetal life, but becomes impervious a few days after birth.' The Dorsal Aortae.—Cephalad of the third aortic arches the dorsal aortse persist and form the headward continuation of the internal carotid arteries. Caudad of the third arch the right dorsal aorta disappears as far as the point where the two dorsal aortfe fuse to form the descending aorta. The part of the left dorsal aorta which intervenes bet
. Anatomy, descriptive and applied. Anatomy. 762 THE VASCULAR SYSTEMS the sixth left arch gives off the pulmonary arteries and forms the ductus arteriosus; this duct remains pervious throughout fetal life, but becomes impervious a few days after birth.' The Dorsal Aortae.—Cephalad of the third aortic arches the dorsal aortse persist and form the headward continuation of the internal carotid arteries. Caudad of the third arch the right dorsal aorta disappears as far as the point where the two dorsal aortfe fuse to form the descending aorta. The part of the left dorsal aorta which intervenes between the third and fourth arches disappears, while the remainder persists to form the descending part of the arch of the aorta. A constriction, the aortic isthmus, is sometimes seen in the aorta between the origin of the left subclavian and the attachment of the ligamentum arteriosum. Below this isthmus the aorta expands slightly to form the aortic spindle. Sometimes the right subclavian artery arises from the aortic arch beyond the origin of the left subclavian and passes upward and to the right behind the trachea and oesophagus. This con- dition may be explained by the persistence of the right dorsal aorta and the obliteration of the fourth right arch. In birds the fourth right arch forms the arch of the aorta; in reptiles the foiu-th arch on both sides persists and gives rise to the double aortic arch in these animals. The heart originally lies on the ventral aspect of the pharynx, immediately caudad of the stoma- todeum. With the elongation of the neck and development of the lungs it recedes within the thorax, and, as a consequence, the anterior ventral aortee are drawn out and the original position of the fourth and fifth arches is greatly modified. Thus, on the right side the fourth recedes to -External carotid Ventral aorta Internal carotid Common carotid Aortic arch Right subclavian artery Right piil?nonary artery Trunk of pulmonary, artery. Ductus arteriosus Vertebra
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1913
