The Thoracic Aorta consists of three parts-ascending, arch, and descending. The ascending portion, about 2 inches long, runs upwards, forwards, and to the right to the level of the upper border of the second right costal cartilage. It lies within the pericardium, and is invested, along with the pulmonary artery behind which it lies at its origin, by the serous layer. At its commencement it presents three hemispherical projections, the aortic sinuses of Valsalva, and also a prominence of the right lateral wall, called the great aortic sinus. In front are the pulmonary artery, right auricular appendix, pericardium, right pleura and lung, mediastinal tissue and sternum ; behind, the right branch of the pulmonary artery and right bronchus ; on the right, the superior vena cava and part of the right auricle ; on the left, the pulmonary artery.

The arch of the aorta commences opposite the upper border of the second right costal cartilage, and ends opposite the lower border of the fourth dorsal vertebra. The highest point is opposite the centre of the manubrium and third dorsal spine. The concavity of the arch is directed downwards, and also backwards and to the right. A constriction between the origin of the left subclavian and the obliterated ductus arteriosus is called the isthmus, and the expanding portion beyond the spindle. In front and to the left are the left mediastinal pleura, phrenic nerve, inferior cardiac branch of vagus, superior cardiac branch of sympathetic, vagus trunk, and superior intercostal vein, remains of thymus, lung, and pleura, mediastinal tissue, and manubrium. Behind and to the right are trachea, oesophagus, thoracic duct, and left recurrent laryngeal nerve ; above are the innominate and left common carotid and subclavian arteries, and left innominate vein ; below are the bifurcation of the pulmonary artery, obliterated ductus arteriosus, left recurrent laryngeal nerve, and root of the left lung.

The descending portion, commencing at the lower border of the fourth dorsal vertebra, lies in close relationship to the left side of the column, but inclines to the middle line as it approaches the aortic opening in the diaphragm. In front are the root of the left lung, (esophagus, base of heart, and pericardium ; on the right, oesophagus, vena azygos major, thoracic duct and pleura ; on the left, pleura and lung ; and behind, vertebral column.

The thoracic aorta is a frequent seat of aneurism. When the ascending portion is affected, the tumour ultimately projects on the right margin of the sternum, at the level of the second and third costal cartilages, all of which structures may become eroded, while the clavicle may be displaced forwards. Owing to pressure upon the superior vena cava, there is engorgement of the veins of the head, neck, and upper limbs. This form may rupture into the pericardium, and cause death by pressure on the heart.

Where the arch is affected, the tumour presents at the root of the neck and suprasternal notch. Pressure on the veins produces engorgement ; on the left recurrent laryngeal nerve, spasm of the cords with high-pitched voice, and dyspnoea, and, later, aphonia from crushing of the nerve. Pressure on the trachea produces harsh breathing, dyspnoea, and cough ; on the left bronchus, dyspnoea ; on the oesophagus, dysphagia ; and on the phrenic nerve, hiccough, and, later, paralysis of the left side of the diaphragm. Tracheal tugging, detected on slightly raising the cricoid cartilage with the finger and thumb, is due to the expansile pulsation affecting the trachea. Owing to irritation of the cervical sympathetic, which conveys fibres to the eye from the lower cervical and upper dorsal regions of the cord (through the rami communicantes, to the carotid plexus, and so to the ciliary ganglion within the orbit), there is dilatation of the pupil on the affected side, while, should the sympathetic fibres be destroyed by increased pressure or stretching, the pupfl contracts from unopposed action of the third nerve. Where the descending aorta is affected, the bodies of the vertebrae are frequently eroded, while the intervertebral discs are comparatively resistant. There is intense gnawring pain in the back, referred to the areas supplied by the intercostal nerves involved, and herpes zoster may develop.

In all cases where there is external swelling, pulsation of an expansile character is a feature which serves to distinguish aneurism from most tumours or abscesses. In addition to presenting the symptoms mentioned, the deep forms may be diagnosed by X rays. Treatment by needling, with the object of slightly injuring the sac wall and producing the formation of a white thrombus with subsequent formation of firm fibrous tissue, is hopeful if the case be got early (Macewen).