The aorta is usually divided by anatomists into three portions ; the curved portion from the heart to the third thoracic vertebra is called the Arch of the aorta ; the remaining portion of the vessel, to which the name of descending aorta has been sometimes given, is called Thoracic aorta above the diaphragm, and .Ab dominal aorta below that muscle.
The Arch of the aorta is divided into three portions, for the purpose of describing its nu merous important relations to surrounding parts with greater accuracy ; these are, first, the _ascending or anterior limb ; second, the trans verse portion ; and, thirdly, the descending or .posterior limb. The commencement of the is covered anteriorly and to the left by pulmonary artery, on the right by the right auricular appendage, the tip of which overlaps it in front, and behind it rests on the sinus of the left auricle. The ascending limb of the arch lies first in front of the right pulmonary artery, as that vessel crosses behind it in its course to the right lung, and then it gets in front of the right bronchus, and the cluster of bron chial glands which fill up the angle formed by the bifurcation of the trachea ; it is bounded on the right side by the superior vena cava, and on the left by the pulmonary artery ; an teriorly it is separated from the sternum by the anterior margins of both lungs, which here approximate, and by the narrowest part of the anterior mediastinum, where the attached sur faces of the opposite pleura touch. This portion of the aorta is contained within the bag of the pericardium, the serous layer of which invests it in every part except where it lies in contact with the pulmonary artery.
The transverse portion of the arch is shorter than the ascending limb. The three great arte ries of the head and upper extremities arise from its superior sides; inferiorly it rests on the left bronchial tube ; in front it has the cellular membrane of the anterior mediastinum, the thymus gland, and the inferior part of the vena innominata ; behind it rests on the trachea a little above its bifurcation, and on the left re current nerve. The posterior limb is the shortest portion of the arch; it lies immediately behind the division of the pulmonary artery, which is connected to it by a ligament, the remains of the ductus arteriosus ; and it is crossed by the left par vagum ; on the right side it is in con tact with the oesophagus, thoracic duct, and left side of the body of the third thoracic ver tebra ; the rest of the circumference of the thoracic aorta is covered by the left pleura, and is in contact with the internal surface . of the
left lung. In the generality of adults having the chest well formed, and the heart and the arch of the aorta free from disease, the origin of the aorta is opposite the sternal articulation of the cartilage of the fourth rib of the left side in the male, and the intercostal space above it in the female ; the ascending limb of the arch, which is behind the middle bone of the sternum in the greater part of its length, may be felt pulsating on the right side of the sternum in the second intercostal space ; the highest part of the transverse portion of the arch is on a plane with the centre of the sternal extremities of the first pair of ribs, and about an inch below the upper margin of the ster num : the arch of the aorta terminates oppo site the lower edge of the cartilage of the second rib of the left side.
The thoracic aorta descends in the posterior mediastinum, and advances from the left side to the front of the thoracic portion of the spine, crossing in its course the left intercostal veins, and the left vena azygos when that vein exists; in front it is covered by the left bronchus, the pos terior surface of the pericardium, the lower ex tremity of the cesophagus, and the left stomachic cord of the par vagum ; on the right side it is bounded by the cesophagus, thoracic duct, and vena azygos ; on the left side it is covered by the pleura, and in contact with the internal surface of the left lung, and at its lower extremity the left splanchnic nerve comes into contact with it, and most frequently accompanies it through the diaphragm.
The abdominal aorta, which enters the•abdo men between the crura of the diaphragm, des cends along the front of the abdominal ver tebrae and the left lumbar veins; it is covered in front by the solar plexus of nerves, the stomach, pancreas, transverse portion of the duodenum, the splenic and left renal veins, the small intestine, and the root of the mesentery ; on the right side it is bounded by the abdomi nal vena cava, and the commencement of the thoracic duct, and on the left it is covered by the peritoneum going to form the left layer of the mesentery. The termination of the aorta in the common iliacs and the middle sacral arteries is a little below the level of the um bilicus.