. An American text-book of obstetrics. For practitioners and students. sponding mandibular arch, the last PHYSIOLOGY OF PREGNANCY. 109 within the tissues of the imperfectly defined fifth visceral bow. The first pairearliest appears and soonest disappears, all five at no time being found simul-taneously fully developed, since by the twentieth day, when all are present, theanterior arches have already partly atrophied. These aortic arches in man andin mammals transiently represent the branchial circulation of gill-bearingtypes; their identity in the higher animals is lost in the metamorphosis wr
. An American text-book of obstetrics. For practitioners and students. sponding mandibular arch, the last PHYSIOLOGY OF PREGNANCY. 109 within the tissues of the imperfectly defined fifth visceral bow. The first pairearliest appears and soonest disappears, all five at no time being found simul-taneously fully developed, since by the twentieth day, when all are present, theanterior arches have already partly atrophied. These aortic arches in man andin mammals transiently represent the branchial circulation of gill-bearingtypes; their identity in the higher animals is lost in the metamorphosis wrhichthey undergo in the development of permanent trunks. The fate of the several aortic arches and their relations to persistent struc-tures is briefly as follows (Fig. 98): (1) The first or mandibular aortic arch early in the fourth week loses itsmiddle segment, the anterior limb taking part in the formation of the external Iagus carotid. Internal carotid. Common laryngeal nerve. Right subclavian^ Innominate artery. Ascending Vertebral artery. Arch of subclavian. Ductus arteriosus. Pulmonary trunk. Descending aorta. Fig. 98.—Diagram illustrating the fate of the aortic arches in mammals and man (modified from Rathke). carotid artery and its branches; the posterior or aortic limb aids in formingthe internal carotid artery. (2) The second arch has a fate identical with that of the first, its straighterventral and dorsal limbs taking part in producing the carotids. (3) The third arch, which remains almost complete, gives rise to the connec-tion between the external and internal carotid arteries, to the latter of whichthe arch particularly contributes. (4) The fourth arch undergoes important changes resulting in its retentionon the two sides, since from it are largely derived the innominate, togetherwith the subclavian and vertebral arteries on the right side, and the importantarch of the aorta on the left. 110 AMERICAN TEXT-BOO
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