From the point at which its arch is considered to terminate — the lower margin of the third dorsal vertebra — the aorta descends along the fore part of the spine to the fourth lumbar vertebra, where it divides into the common iliac arteries. The direction of this part of the vessel is not vertical, for as its course is influenced by the spine, upon which it rests, it is necessarily concave forwards in the dorsal region, and convex forwards in the lumbar. Again, its commencement is at the left side of the bodies of the vertebras; its termination also inclines a little to the left, whilst about the last dorsal vertebra it is nearly upon the median line. From this arises another slight curve, the convexity of which is to the right side. Within the thorax, where the offsets are small, the aorta diminishes but little in size; in the abdomen the diminution is considerable, in consequence of large branches being furnished to the viscera in that cavity.

That part of the aorta (below the arch) which is situated in the thorax is called the thoracic aorta [aorta thoracica descendens]; it extends from the lower border of the third dorsal vertebra on its left side, to the opening in the diaphragm in front of the last dorsal vertebra. It lies in the back part of the interpleural space (the posterior mediastinum), being before the spine and behind the root of the left lung and the pericardium ; its left side is in contact with the corresponding pleura and lung, and close on the right side are the azygos vein, the thoracic duct and oesophagus. The latter tube, however, towards the lower part of the thorax, inclines in front of the artery, and near the diaphragm gets somewhat to its left side. The small azygos vein crosses behind the thoracic aorta.

The branches derived from the thoracic aorta are numerous but small. They are distributed to the walls of the thorax, and to the viscera contained within it — the latter being much the smaller and least numerous branches.

The branches to the viscera are very irregular in their number and place of origin. They are as follows:

The pericardiac branches [a. pericardiacae] are some very small and irregular vessels which pass forwards and ramify on the pericardium.

The bronchial arteries (bronchiales) are the proper nutritious arteries of the substance of the lung : they accompany the bronchial tubes in their ramifications through that organ, and they also supply the bronchial glands, and in part the oesophagus. These vessels vary frequently in number, and in their mode of origin. The bronchial artery of the right side arises from the first aortic intercostal artery, or by a common trunk with the left bronchial artery from the thoracic aorta ; on the left side there are generally two bronchial arteries, both of which arise from the thoracic aorta, one near the commencement of that trunk, and the other, named inferior bronchial, lower down. Each artery is usually directed to the back part of the corresponding bronchus, along which it runs, dividing and subdividing with the successive bronchial ramifications in the substance of the lung.

Peculiarities of the bronchial arteries. — The place of origin is liable to much variation.

The artery of the right side has been found to arise singly from the aorta, from the internal mammary, or from the inferior thyroid. The bronchial arteries of the two sides have been seen to arise by a common trunk from the subclavian (Haller). Two such common trunks, each furnishing a branch to the right and left lungs, have been observed, in a single case, to descend into the thorax after arising, one from the internal mammary, and the other from the superior intercostal artery. Some other peculiarities in these arteries of less note have from time to time been recorded - it is necessary only to refer to one, viz., the occurrence of two distinct bronchial arteries for each lung.

The OEsophageal arteries [a. oesophageae,] are variable in size and number; there are usually four or five, which arise from the fore part or right side of the aorta, and run obliquely downwards upon the oesophagus, supplying its coats. The lower branches of these vessels anastomose with the ascending offsets of the coronary artery of the stomach, whilst the upper branches communicate in a similar way with those of the inferior thyroid artery.

The glands and loose tissue in the posterior mediastinum also receive small branches (posterior mediastinal) , [a. mediastinicse].

The branches furnished by the aorta to the walls of the thorax are named the intercostal from their course between the ribs.

The intercostal arteries, (inferior or aortic intercostals) [intercostales], arise from the posterior part of the aorta, and run outwards upon the bodies of the vertebrae, after which they lie along the intercostal spaces. They are usually ten in number, the upper intercostal space and occasionally a second space also, being supplied by the superior intercostal (a branch of the subclavian artery).

Owing to the position of the aorta to the left side of the spine, the right aortic intercostals cross over the front of the vertebra, furnishing many small branches to those bones ; and they are therefore a little longer than the arteries of the left side. As these vessels pass outwards, they are covered by the pleura, and crossed by the sympathetic nerve ; those of the right side also pass behind the oesophagus, the thoracic duct, and the azygos vein.

Having reached the middle of its corresponding intercostal space, each aortic intercostal artery divides into two branches, of which one (a dorsal branch), passes backwards, and will be presently described; whilst the other or anterior division continues outwards between the ribs.

This anterior branch, or proper intercostal artery, [a. intercostalis,] passes outwards, at first between the pleura with a thin fascia (in front), and the external intercostal muscle (behind) ; and afterwards between the two layers of intercostal muscles. Having gained the lower border of the rib above, near the angle of the bone, by passing obliquely upwards from the middle of the intercostal space, the artery furnishes several small branches, and one long and slender branch which inclines downward and approaches the border of the rib below, supplying the bone and the intercostal muscles. After giving off these branches, the artery continues along the lower border of the rib above, and after supplying the intercostal muscles, anastomoses with the anterior intercostal branches derived from the internal mammary artery, and with the thoracic branches of the axillary artery.

The first of the aortic intercostal arteries has an anastomosis with the superior intercostal, which is derived from the subclavian artery, and the last three are prolonged amongst the abdominal muscles, where they communicate with the epigastric artery in front, with the phrenic arteries at the side, and with the lumbar branches of the abdominal aorta lower down.

Each intercostal artery is accompanied, as it runs outwards between the ribs, by a corresponding vein, and by one of the dorsal nerves.

The posterior or dorsal branch [ramus dorsalis], of each intercostal artery passes backwards to the inner side of the anterior costo-transverse ligament, with the posterior branch of the corresponding spinal nerve ; and having furnished an offset to the spinal canal, reaches the muscles of the back, and divides into an internal and an external branch. The internal branch is directed towards the spinous processes, on or through the semispinalis dorsi, and ramifies in the muscles and the skin. The external branch turns outwards under the longissimus dorsi, and is distributed between that muscle and the sacro-lumbalis ; some reach the superficial muscles.

The spinal branches of the aortic intercostal arteries are distributed partly to the cord and its membranes, and partly to the bones, in the same manner as the spinal branches of the lumbar arteries, to the description of which reference is to be made.

From Quain's Anatomy.