Circulation in Fetus.—In the fetus there is [direct communication between the two auricles] through the foramen ovale, which normally closes at birth, though occasionally it remains open. There is also communication between the pulmonary artery and the arch of the aorta through the [ductus arteriosus]. The freshly oxidized blood comes to the fetus through the placenta, from which it is brought along the umbilical cord in the umbilical vein to the liver and thence to the inferior vena cava, where it mixes with the blood from the lower extremities. By the inferior vena cava it is carried to the right auricle, where the Eustachian valve—a valve between the inferior vena cava and the auriculo-ventricular opening, larger in the fetus than in later life where it serves no special purpose—guides it across the auricle and through the foramen ovale to the left auricle. From this auricle, together with a small amount of blood from the lungs, it goes to the left ventricle and is distributed by the aorta almost entirely to the head and upper extremities. Hence their large size and perfect development at birth. Returned from the upper extremities by the superior vena cava, the blood enters the right auricle again and, passing over the Eustachian valve this time, descends to the right ventricle, from which the greater part passes by the pulmonary artery and the ductus arteriosus to the descending aorta, though a small amount keeps on through the pulmonary artery to the lungs. In the aorta it mixes with the blood from the left ventricle and part goes to supply the lower extremities, though the greater part is carried back to the placenta through the two umbilical arteries. The fact that the greater part of the blood traverses the liver accounts for its large size at birth, while the lower extremities, which receive for the most part blood that has already circulated through the upper extremities, are of small size and imperfectly developed.
Arteries.—After birth the arterial blood for the general circulation leaves the heart by the [aorta], the main distributing artery of the body. Through this and its branches it is carried throughout the body in what, with the return of the venous blood by the venæ cavæ and other smaller veins, is known as the systemic circulation. The aorta ascends from the left ventricle and arches backward to the left over the root of the left lung to descend along the spinal column at the left to the fourth lumbar vertebra, about opposite the umbilicus, where, considerably diminished in size by the branches it has given off, it divides into the two common iliacs. For convenience its different parts are named, according to their position, the ascending aorta, the arch of the aorta, and the descending aorta, the last being subdivided into the thoracic and the abdominal aorta.
Fig. 44.—The aortæ and their branches.
(Leidy.)
From the ascending aorta come off the coronary arteries which supply the heart muscle itself, as the coronary sinuses carry off the venous blood from the heart. From the arch are given off the left common carotid and left subclavian and the innominate, which divides into the right common carotid and right subclavian.
The common carotids pass up the neck behind the sterno-cleido-mastoid muscles in a line from the sterno-clavicular joint to a point midway between the mastoid process and the angle of the lower jaw and divide opposite the upper border of the thyroid cartilage into the internal and external carotids, of which the former with its branches supplies the anterior part of the brain, the eye and forehead, and the latter the neck and face.
The subclavian is the artery of the upper extremity but its vertebral branch goes to the brain, where with its fellow it forms the basilar artery, whose branches together with the branches of the internal carotid form the circle of Willis at the base of the brain. Other branches of the subclavian are the thyroid axis, with branches to the neck and shoulders; the internal mammary, with branches to the chest walls, mediastinum, and diaphragm, such as the musculo-phrenic and superior epigastric; and the superior intercostal. At the lower border of the first rib, over which it passes, the name axillary is substituted for subclavian, while at the lower border of the axilla, where it starts down the arm, it is called the brachial artery. At the elbow the brachial divides into the radial and ulnar arteries. The axillary artery sends branches to the chest and shoulder and is more frequently injured than any other artery except the popliteal. Aneurism may occur in it and is very likely to occur in the thoracic aorta.
From the thoracic aorta branches go to various of the chest contents, while the abdominal aorta supplies the abdominal viscera. Among the branches of the abdominal aorta are: the celiac axis, which has a gastric, an hepatic, and a splenic branch; the superior and inferior mesenteric to the intestines; the renal; the suprarenal; the spermatic or ovarian; the inferior phrenic; and the lumbar.
The common iliacs divide at the upper edge of the sacrum into the external and internal iliacs, of which the latter with its branches supplies the walls and viscera of the pelvis and the inner part of the thigh. The external iliac and its branches go to the thigh, leg, and foot.
Veins.—Of the veins few need be mentioned by name. The deep veins have the same names as the arteries they accompany, though there are two innominate veins where there is only one innominate artery, the subclavian and internal jugular veins on either side joining to form an innominate vein and the two innominates in turn forming the superior vena cava. Of the superficial veins the external and internal jugular correspond to the common carotid arteries and return the blood from the head and face. The external jugular vein is important because it is the largest superficial vein in the neck and is often cut in suicide. The median vein is found at the bend of the elbow and is used in letting blood and in giving salt solution, while the basilic is on the inner side and the median cephalic on the outer side of the upper arm. Varicosity often occurs in the internal or long saphenous and the external or short saphenous in the leg. The inferior vena cava is formed by the juncture of the two common iliac veins.