. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. DEVELOPMENT OF THE ARTERIAL SYSTEM 245 cervical nerve arises just above the point of union of the two aortic arches (Fig. 150, s), and extends out into the limb bud, forming the subclavian artery.* Further down twelve pairs of lateral branches, arising from the thoracic portion of the aorta, rep- resent the intercostal arteries, and still lower four pairs of lum- bar arteries are formed, the fifth lumbars being represented by two large branches, the common iliacs, which seem from their size t
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. DEVELOPMENT OF THE ARTERIAL SYSTEM 245 cervical nerve arises just above the point of union of the two aortic arches (Fig. 150, s), and extends out into the limb bud, forming the subclavian artery.* Further down twelve pairs of lateral branches, arising from the thoracic portion of the aorta, rep- resent the intercostal arteries, and still lower four pairs of lum- bar arteries are formed, the fifth lumbars being represented by two large branches, the common iliacs, which seem from their size to be the continuations of the aorta rather than branches of it. The true continuation of the aorta is, however, the middle sa- cral artery, which represents in a degenerated form the caudal prolongation of the aorta of other mammals, and, like this, gives off lateral branches corre- sponding to the sacral segments. In addition to the segmental FlG. I50.—diagram showing the Re- lateral branches arising from nations op the Lateral Branches to 0 the Aortic Arches. the aorta, Visceral branches, EC> External carotid; h, lateral branch Which have their origin rather cacompanying the hypoglossal nerve; IC, ° internal carotid; ICo, intercostal; IM, m- from the Ventral surface, also ternal mammary; s, subclavian; v, verte- ^„„,,~ TV, ~™u„mr, ~t - mm bral; I to VIII, lateral cervical branches; OCCUr. In embryos of 5 mm. I; 2) lateral thoracic branches. these branches are arranged in a segmental manner in threes, a median unpaired vessel passing to the digestive tract and a pair of more lateral branches passing to the mesonephros (see p. 339) corresponding to each of the paired branches passing to the body wall (Fig. 151). As * It must be remembered that the right subclavian of the adult is more than equiva- lent to the left, since it represents the fourth branchial vessel + a portion of the dorsal longitudinal trunk + the lateral segmental branch (see Fig. 142).. Please not
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