EMBRYOLOGY The earliest appearance of the blood vessels is dealt with under VASCULAR SYSTEM. Here will be briefly described the fate of the main vessel which carries the blood away from the truncus arteriosus of the developing heart (q.v.). This ventral aorta, if traced forward, soon divides into two lateral parts, the expla nation being that there were originally two vessels, side by side, which fused to form the heart, but continued separate anteriorly. The two parts run for a little distance toward the head of the embryo, ventral to the alimentary canal, and then turn toward the dorsum, passing one on either side of that tube to form the first aortic arch. Having reached the dorsum they turn back ward toward the tail end and form the dorsal aortae. The anterior loop between the ventral and dorsal aortae already described as the first aortic arch is included in the maxillary or first visceral arch of the soft parts (see fig. 3, I). Later, four other well marked aortic arches grow behind this in the more caudal visceral arches, so that there are altogether five arterial arches on each side of the pharynx, through which the blood can pass from the ventral to the dorsal aorta. Of these arches the first soon dis appears, but is probably partly represented in the adult by the internal maxillary artery, one branch of which, the infra-orbital, is enclosed in the upper jaw, while another, the inferior dental, is surrounded by the lower jaw. Possibly the ophthalmic artery also belongs to this arch. The second arch also disappears, but the posterior auricular and occipital arteries probably spring from it, and at an early period it passed through the stapes as the transitory stapedial artery. The third arch forms the beginning of the internal carotid. The fourth arch becomes the arch of the adult aorta, between the origins of the left carotid and left sub clavian on the left side, and the first part of the right subclavian artery on the right. The apparent fifth arch on the left side (fig. 3) remains all through foetal life as the ductus arteriosus, and, as the lungs develop, the pulmonary arteries are derived from it. J. E. V. Boas and W. Zimmermann have shown that this arch is in reality the sixth and that there is a very transitory true fifth arch in front of it (fig. 3) . The part of the ventral aorta from which this last arch rises is a single median vessel due to the same fusion of the two primitive ventral aortae which precedes the formation of the heart, but a spiral septum has appeared in it which divides it in such a way that while the anterior or cephalic arches communicate with the left ventricle of the heart, the last one communicates with the right (see HEART). The fate of the ventral and dorsal longitudinal vessels must now be followed. The fused part of the two ventral aortae, just in front of the heart, forms the ascending part of the adult aortic arch, and where this trunk divides between the fifth and fourth arches (strictly speaking, the sixth and fifth), the right one forms the innominate (fig. 3) and the left one a very short part of the transverse arch of the aorta until the fourth arch comes off (see fig. 4). From this point to the origin of the third arch is common carotid, and after that, to the head, external carotid on each side. The dorsal longitudinal arteries on the head side of the junction with the third arch form the internal carotids. Between the third and fourth arches they are obliterated, while on the caudal side of this, until the point of fusion is reached on the dorsal side of the heart, the left artery forms the upper part of the dorsal aorta, while the right entirely disappears. Below this point the thoracic and abdominal aortae are formed by the two primitive dorsal aortae which have fused to form a single median vessel. As the limbs are developed, vessels bud out in them. The subclavian for the arm comes from the fourth aortic arch on each side, while in the leg the main artery is a branch of the caudal arch which is curving ventralward to form the umbilical artery. From the convexity of this arch the internal iliac and sciatic at first carry the blood to the limb, as they do permanently in reptiles, but later the external iliac and femoral become developed, and, as they are on the concave side of the bend of the hip, while the sciatic is on the convex, they have a mechanical advantage and become the permanent main channel.
For further details see O. Hertwig, Handbuch der vergleichen den and experimentellen Entwickelungslehre der Wirbeltiere (Jena, 1905).
For the heart and vascular systems in other animals see HEART,