Around the pharynx are developed five pairs of aortic arches. These commence anteriorly from the two primitive aortae, and, passing along the side of the pharynx, end in the aortae as they descend to become united in the dorsal region of the embryo. The points of origin of the arches are termed their anterior roots, and the points of termination their posterior roots.

Though all these arches do not exist at the same time, still, in describing the vessels which arise from them, they may be conveniently considered together.

On the right side the fifth arch disappears completely. On the left side the anterior root and neighboring part of the fifth arch are transformed into the pulmonary artery; the remaining part of this arch continues as the ductus arteriosus, which connects the pulmonary artery with the permanent aorta.

The fourth left arch, in mammalia, becomes the permanent aorta. At the junction of the fourth and fifth left posterior roots the left subclavian artery is given off. In birds the right fourth arch is transformed into the permanent aorta; and in examining the development of the aortic arch of the chick, it must be borne in mind that it is on the opposite side to that it occupies in man.

On the right side the anterior root of the fourth arch, and the part of the aortic trunk leading to it, persist as the innominate artery, the fourth arch being represented by the right subclavian artery.

The part of the primitive aortic trunk joining the fourth and third right anterior roots becomes the common carotid artery of the same side, while arising from this is the internal carotid, which, taking the position of the third arch, passes to the posterior roots, and occupies the trunk of the primitive aorta from the third to the first arches.

Diagram of the aortic arches; the permanent vessels arising from them are shaded darkly.

Fig. 307. Diagram of the aortic arches; the permanent vessels arising from them are shaded darkly. (Allen Thomson, after Rathke).

1,2,3,4,5- Primitive aortic arches of right side.

I, II, III, IV. Pharyngeal clefts of the left side, showing the relationship of the clefts to the aortic arches.

A. Aorta. P. Pulmonary artery, d. Ductus arteriosus, a'. Left aortic root. a. Right aortic root. A'. Descending aorta, pn.pn'. Right and left vagi. s. s'. Right and left subclavian arteries. v. v'. Right and left vertebral arteries, c. Common carotid arteries, ce. External carotid, ci ci'. Right and left internal carotid.

The external carotid, arising from the common carotid at the third anterior root, occupies the position of the vessel joining this root to those of the second and first arch.

On the left side the common carotid and its branches are developed similarly to those on the right, the only difference being that the common carotid arises from the aorta and not from the innominate.

The iliac arteries are developed from the hypogastric. At first they appear as branches, but with-the growth of the limbs they become so much larger that after birth they appear to be the main branches from the point of division of the aorta, the hypogastric arteries now being merely small branches of the iliac vessels.

The Arterial System 327Plan of principal veins of the foetus of about four weeks old.

Fig. 308. 4. Plan of principal veins of the foetus of about four weeks old. B. Veins of the liver at an earlier period. C. Veins after the establishment of the placental circulation. D. Veins of the liver at the same period.

Primitive jugular veins, dc. Ducts of Cuvier. ca. Cardinal vems, ci. Inferior vena cava. /. Ductus venosus. u. Umbilical vein. p. Portal vein. o. Vitelline vein. cr. External iliac veins, o'. Right vitelline vein, u'. Right umbilical vein. l'. Hepatic veins (venae revehentes). p'p'. Venae advehentts. m. Mesenteric veins, az. Azygos vein, ca'. Remains of left cardinal vein. //. Cross branch from left jugular, which becomes the left brachio-cephalic vein, ri. Right innominate vein. s.s. Subclavian veins, h. Hypogastric veins, il. Division of inferior vena cava into the common iliac veins.

With the development of the organs and limbs, vessels in connection with those above described arise in the mesoblast. It is, however, beyond the scope of this work to describe in detail the origin of the lesser vessels.