The middle sacral artery, as it runs down the front of the sacrum, gives branches to the back of the pelvic wall.
Injections made by Sir W. Turner have shown that, both in the thoracic and abdominal cavities, slender anastomosing communications exist between the visceral and parietal branches.
The arteries to the pelvis and hind limbs begin at the bifurcation of the aorta into the two common iliacs.
The common iliac artery, after a short course, divides into the internal and external iliac arteries. The internal iliac enters the pelvis and divides into branches for the supply of the pelvic walls and viscera, including the organs of generation, and for the Iliac system. great muscles of the buttock. The external iliac descends behind Poupart’s ligament into the thigh, where it takes the name of femoral artery. The femoral descends along the front and inner surface of the thigh, gives off a profunda or deep branch, which, by its circumflex and perforating branches, supplies the numerous muscles of the thigh; most of these extend to the back of the limb to carry blood to the muscles situated there. The femoral artery then runs to the back of the limb in the ham, where it is called popliteal artery. The popliteal divides into two branches, of which one, called anterior tibial, passes between the bones to the front of the leg, and then downward to the upper surface of the foot; the other, posterior tibial, continues down the back of the leg to the sole of the foot, and divides into the internal and external plantar arteries; branches proceed from the external plantar artery to the sides of the toes, and constitute the digital arteries. From the large arterial trunks in the leg many branches proceed, to carry blood to the different structures in the limb.
The wall of an artery consists of several coats (see fig. 2). The outermost is the tunica adventitia, composed of connective tissue; immediately internal to this is the yellow elastic coat; within this again the muscular coat, formed of involuntary. Structure of arteries. muscular tissue, the contractile fibre-cells of which are for the most part arranged transversely to the long axis of the artery; in the larger arteries the elastic coat is much thicker than the muscular, but in the smaller the muscular coat is relatively strong; the vaso-motor nerves terminate in the muscular coat. In the first part of the aorta, pulmonary artery and arteries of the retina there is no muscular coat. Internal to the muscular coat is the elastic fenestrated coat, formed of a smooth elastic membrane perforated by small apertures. Most internal of all is a layer of endothelial cells, which form the free surface over which the blood flows. The arteries are not nourished by the blood which flows through them, but by minute vessels, vasa vasorum, distributed in their external, elastic and muscular coats.
| Fig. 2.—Diagram of the structure of an artery. A, tunica adventitia; E, elastic coat; M, muscular coat; F, fenestrated coat; En, endothelium continuous with the endothelial wall of C, the capillaries. |
Embryology
| Fig. 3.—Diagram of the Embryonic Arterial Arches. 1, 2, 3, 4, 5, 6, point to the six arches. (The black parts are obliterated in the adult human subject.) |
| V.Ao. Ventral Aorta. A.Ao. Arch of Aorta. D.Ar. Ductus Arteriosus. In. Innominate Artery. R.I.C.-L.I.C. Right and Left Internal Carotid Arteries. D.B. Duct of Botalli. R.S.-L.S. Right and Left Subclavian Arteries. R.V.-L.V. Right and Left Vertebral Arteries. P.A. Posterior Auricular Artery. Oph. Ophthalmic Artery. D.Ao. Dorsal Aorta. P.T. Pulmonary trunk. R.P.A.-L.P.A. Right and Left Pulmonary Arteries. R.C.C.-L.C.C. Right and Left Common Carotid Arteries. E.C. External Carotid Artery. Oc. Occipital Artery. I.M. Internal Maxillary Artery. |
| Fig. 4.—Diagram of the Human Aorta and its branches. S.T., Superficial Temporal Artery. |
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 explanation 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 backward toward the tail end and form the dorsal aortae; here, according to A.H. Young (Studies in Anatomy, Owens College, 1891 and 1900) they again turn toward the ventral side and become, after a transitional stage, the hypogastric, placental, allantoic or umbilical arteries. This authority does not believe that the middle sacral artery of the adult is the real continuation of the single median dorsal aorta into which the two parallel dorsal vessels just mentioned soon coalesce, though until recently it has always been so regarded. 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, 1). 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 disappears, but is probably partly represented in the adult by the internal maxillary artery, one branch of which, the infraorbital, 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 subclavian, 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, 6) 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, 5). 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, In.) 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 vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere (Jena, 1905).