The thoracic aorta(Fig. 530) is contained in the posterior mediastinal cavity. It begins at the lower border of the fourth thoracic vertebra where it is continuous with the aortic arch, and ends in front of the lower border of the twelfth at the aortic hiatus in the diaphragm. At its commencement, it is situated on the left of the vertebral column; it approaches the median line as it descends; and, at its termination, lies directly in front of the column. The vessel describes a curve which is concave forward, and as the branches given off from it are small, its diminution in size is inconsiderable.
Relations.It is in relation, anteriorly, from above downward, with the root of the left lung, the pericardium, the esophagus, and the diaphragm; posteriorly, with the vertebral column and the hemiazygos veins; on the right side, with the azygos vein and thoracic duct; on the left side, with the left pleura and lung. The esophagus, with its accompanying plexus of nerves, lies on the right side of the aorta above; but at the lower part of the thorax it is placed in front of the aorta, and, close to the diaphragm, is situated on its left side.
Peculiarities.The aorta is occasionally found to be obliterated at the junction of the arch with the thoracic aorta, just below the ductus arteriosus. Whether this is the result of disease, or of congenital malformation, is immaterial to our present purpose; it affords an interesting opportunity of observing the resources of the collateral circulation. The course of the anastomosing vessels, by which the blood is brought from the upper to the lower part of the artery, will be found well described in an account of two cases in the Pathological Transactions, vols. viii and x. In the former, Sydney Jones thus sums up the detailed description of the anastomosing vessels: The principal communications by which the circulation was carried on were: (1) The internal mammary, anastomosing with the intercostal arteries, with the inferior phrenic of the abdominal aorta by means of the musculophrenic and pericardiacophrenic, and largely with the inferior epigastric. (2) The costocervical trunk, anastomosing anteriorly by means of a large branch with the first aortic intercostal, and posteriorly with the posterior branch of the same artery. (3) The inferior thyroid, by means of a branch about the size of an ordinary radial, forming a communication with the first aortic intercostal. (4) The transverse cervical, by means of very large communications with the posterior branches of the intercostals. (5) The branches (of the subclavian and axillary) going to the side of the chest were large, and anastomosed freely with the lateral branches of the intercostals. In the second case Wood describes the anastomoses in a somewhat similar manner, adding the remark that the blood which was brought into the aorta through the anastomosis of the intercostal arteries appeared to be expended principally in supplying the abdomen and pelvis; while the supply to the lower extremities had passed through the internal mammary and epigastrics.
In a few cases an apparently double descending thoracic aorta has been found, the two vessels lying side by side, and eventually fusing to form a single tube in the lower part of the thorax or in the abdomen. One of them is the aorta, the other represents a dissecting aortic aneurism which has become canalized; opening above and below into the true aorta, and at first sight presenting the appearances of a proper bloodvessel.
The bronchial arteries (aa. bronchiales) vary in number, size, and origin. There is as a rule only one right bronchial artery, which arises from the first aortic intercostal, or from the upper left bronchial artery. The left bronchial arteries are usually two in number, and arise from the thoracic aorta. The upper left bronchial arises opposite the fifth thoracic vertebra, the lower just below the level of the left bronchus. Each vessel runs on the back part of its bronchus, dividing and subdividing along the bronchial tubes, supplying them, the areolar tissue of the lungs, the bronchial lymph glands, and the esophagus.
The esophageal arteries (aa. æsophageæ) four or five in number, arise from the front of the aorta, and pass obliquely downward to the esophagus, forming a chain of anastomoses along that tube, anastomosing with the esophageal branches of the inferior thyroid arteries above, and with ascending branches from the left inferior phrenic and left gastric arteries below.
Intercostal Arteries (aa. intercostales).There are usually nine pairs of aortic intercostal arteries. They arise from the back of the aorta, and a redistributed to the lower nine intercostal spaces, the first two spaces being supplied by the highest intercostal artery, a branch of the costocervical trunk of the subclavian. The right aortic intercostals are longer than the left, on account of the position of the aorta on the left side of the vertebral column; they pass across the bodies of the vertebræ behind the esophagus, thoracic duct, and vena azygos, and are covered by the right lung and pleura. The left aortic intercostals run backward on the sides of the vertebræ and are covered by the left lung and pleura; the upper two vessels are crossed by the highest left intercostal vein, the lower vessels by the hemiazygos veins. The further course of the intercostal arteries is practically the same on both sides. Opposite the heads of the ribs the sympathetic trunk passes downward in front of them, and the splanchnic nerves also descend in front by the lower arteries. Each artery then divides into an anterior and a posterior ramus.
The Anterior Ramus crosses the corresponding intercostal space obliquely toward the angle of the upper rib, and thence is continued forward in the costal groove. It is placed at first between the pleura and the posterior intercostal membrane, then it pierces this membrane, and lies between it and the Intercostalis externus as far as the angle of the rib; from this onward it runs between the Intercostales externus and internus, and anastomoses in front with the intercostal branch of the internal mammary or musculophrenic. Each artery is accompanied by a vein and a nerve, the former being above and the latter below the artery, except in the upper spaces, where the nerve is at first above the artery. The first aortic intercostal artery anastomoses with the intercostal branch of the costocervical trunk, and may form the chief supply of the second intercostal space. The lower two intercostal arteries are continued anteriorly from the intercostal spaces into the abdominal wall, and anastomose with the subcostal, superior epigastric, and lumbar arteries.
The collateral intercostal branch comes off from the intercostal artery near the angle of the rib, and descends to the upper border of the rib below, along which it courses to anastomose with the intercostal branch of the internal mammary.
The Posterior Ramus runs backward through a space which is bounded above and below by the necks of the ribs, medially by the body of a vertebra, and laterally by an anterior costotransverse ligament. It gives off a spinal branch which enters the vertebral canal through the intervertebral foramen and is distributed to the medulla spinalis and its membranes and the vertebræ. It then courses over the transverse process with the posterior division of the thoracic nerve, supplies branches to the muscles of the back and cutaneous branches which accompany the corresponding cutaneous branches of the posterior division of the nerve.
The subcostal arteries, so named because they lie below the last ribs, constitute the lowest pair of branches derived from the thoracic aorta, and are in series with the intercostal arteries. Each passes along the lower border of the twelfth rib behind the kidney and in front of the Quadratus lumborum muscle, and is accompanied by the twelfth thoracic nerve. It then pierces the posterior aponeurosis of the Transversus abdominis, and, passing forward between this muscle and the Obliquus internus, anastomoses with the superior epigastric, lower intercostal, and lumbar arteries. Each subcostal artery gives off a posterior branch which has a similar distribution to the posterior ramus of an intercostal artery.
The superior phrenic branches are small and arise from the lower part of the thoracic aorta; they are distributed to the posterior part of the upper surface of the diaphragm, and anastomose with the musculophrenic and pericardiacophrenic arteries.
A small aberrant artery is sometimes found arising from the right side of the thoracic aorta near the origin of the right bronchial. It passes upward and to the right behind the trachea and the esophagus, and may anastomose with the highest right intercostal artery. It represents the remains of the right dorsal aorta, and in a small proportion of cases is enlarged to form the first part of the right subclavian artery.