This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The remaining portion of the aorta, from the lower border of the fourth thoracic vertebra down, is called the descending aorta. It is divided into the thoracic and abdominal portions. The thoracic aorta begins at the lower border of the fourth and ends at the lower border of the twelfth thoracic vertebra. At its beginning it lies on the left side of the vertebral column, but as it descends it comes somewhat forward but does not reach the middle line. It lies in the posterior mediastinum more toward the left side than toward the right.
In front above are the pericardium, the pulmonary artery, left bronchus, left pulmonary veins, and oesophagus. Behind is the vertebral column. To the right are the oesophagus above, the vena azygos major, and the thoracic duct. To the left are the left lung and the pleura, which it grooves, and a quite small portion of oesophagus below.
Aneurism, when involving the thoracic aorta, tends to cause absorption of the vertebras and ribs, and to present posteriorly; as the space is not so restricted as is the case higher up the tumor has a freer opportunity to expand and the suffering is not so great nor are the symptoms so marked. It may rupture into the left pleura or oesophagus and may erode through the bodies of the vertebras into the spinal canal.
These aneurisms may exist many years and attain a large size.