. Cunningham's Text-book of anatomy. Anatomy. THE PEIMITIVE VASCULAR SYSTEM. 67 2nd arches atrophied 3rd arches 4th arches ," 5th arches ' ,' 6th arches Dorsal aorta. X 1st arches atroph: Pulmonary arteries Fig. External carotids .' Ventral root of 3rd arch / Ventral root of 4th and 5th arches / Truncus aorticus 85.—Schema of Aortic Arches of ax Embryo, 9 mm. long. (After Tandeln, modified.) The second and third arches have atrophied and the transitory fifth has appeared. advanced considerably in development. Two aortic arches, on each side, now connect the cephalic end of the heart with
. Cunningham's Text-book of anatomy. Anatomy. THE PEIMITIVE VASCULAR SYSTEM. 67 2nd arches atrophied 3rd arches 4th arches ," 5th arches ' ,' 6th arches Dorsal aorta. X 1st arches atroph: Pulmonary arteries Fig. External carotids .' Ventral root of 3rd arch / Ventral root of 4th and 5th arches / Truncus aorticus 85.—Schema of Aortic Arches of ax Embryo, 9 mm. long. (After Tandeln, modified.) The second and third arches have atrophied and the transitory fifth has appeared. advanced considerably in development. Two aortic arches, on each side, now connect the cephalic end of the heart with the primitive dorsal aorta. The umbilical artery and vitelline arteries are quite separate, and each umbilical artery springs, by a number of roots which anastomose together, from the caudal part of the corre- sponding dorsal aorta. The vitelline arteries are still numerous, but that which rises opposite the twelfth mesodermal somite is becoming the main artery of the yolk-sac; eventually its proximal part is transformed into the superior mesenteric artery of the foetus. When the embryo has attained a length of 5 mm., and is about five weeks old, it possesses about thirty-eight mesodermal somites,and five aortic arches are present on each side. Commencing from the cranial end, they are the first, second, third, fourth, and sixth; the fifth arch appears sub- sequently between the fourth and the sixth. All five arches pass to the corresponding dorsal aorta, but the three most caudal, on each side, spring from the cranial end of the heart, which is now called the aortic trunk, whilst the two most cranial rise from a common stem which constitutes their ventral roots, and which springs, also, from the aortic trunk (Fig. 84). A little later the aortic trunk gives off only two branches on each side, (1) a stem common to the first five arches, for the fifth has now^ appeared, and (2) the sixth arch (Fig. 85). The fifth arch is very transitory. Whilst it is present it runs from the co
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