2o8 APPLIED ANATOMY. 



THE AORTA. 



The aorta, as it leaves the left ventricle, begins under the left portion of the 

 sternum opposite the lower border of the third left costal cartilage. This is the 

 location of the aortic semilunar valves as already given. It passes upward toward 

 the right for 5 cm. and then forms an arch, extending backward toward the left, to 

 reach the spine on the left side of the body of the fourth thoracic vertebra. The 

 arch is continued down in front of the spine as the thoracic aorta and pierces the 

 diaphragm in the median line, between the two crura of the diaphragm, opposite the 

 twelfth thoracic vertebra. 



The ascending aorta begins behind the left half of the sternum on a level 

 with the lower border of the third costal cartilage. It proceeds upward toward the 

 right until it reaches the level of the lower border of the right second costal cartilage, 

 where the arch begins. 



Immediately above its commencement it has three enlargements, called the 

 sinuses of the aorta (Valsalva), which correspond to the semilunar valves. Of 

 the three semilunar valves two are anterior and one is posterior. From behind the 

 two anterior valves come the right and left coronary arteries. 



Beyond the valves, in the upper right portion of the arch, the aorta is again 

 dilated, forming the great sinus of the arch of the aorta. 



The right limit of the aorta is about even with the right edge of the sternum; 

 sometimes it projects slightly beyond. When it does so it is liable to be wounded by 

 a stab in the second interspace close to the edge of the sternum. On account of the 

 proximity of the aorta to the second interspace, it is here that the stethoscope is placed 

 to hear aortic murmurs. The aorta at this point is covered only by the thin border of 

 the right lung and pleura and the slight remains of the thymus gland. Below, its com- 

 mencement is overlapped on the right by the aitricula dextra (right auricular appendix) 

 of the atrium and on the left by the commencement of the pulmonary artery. 



The ascending aorta is liable to be the seat of aneurism. It may involve either 

 the lower portion in the region of the sinuses or the region of the great sinus at its 

 upper right anterior portion. 



If the aneurism enlarges anteriorly it will show itself first in the second or third 

 interspace. It will bulge the ribs outward in this region. The right lung will be 

 pushed outward and the two layers of the pleura pressed together. It may break 

 externally through the surface or open into the pleural cavity. If it tends to the 

 right it presses on the descending cava and right atrium, thus interfering with the 

 return of the blood from the head and neck and both upper extremities. If it en- 

 larges to the left or backward it may press on the right pulmonary artery and 

 interfere with the free access of blood to the lungs. 



The first portion of the aorta is not united with the pericardium, but simply 

 loosely covered by it, so that this portion of the arch is weaker than the other por- 

 tions, and rupture, with extravasation of blood into the pericardial sac, is not uncom- 

 mon. An aneurism may also rupture into the superior vena cava. 



The arch of the aorta passes anteroposteriorly from the upper border of the 

 second right costal cartilage in front to the left side of the body of the fourth thoracic 

 vertebra behind. It is about 5 cm. (2 in.) long. Its under surface is level with the 

 angle of the sternum (angle of Ludwig), opposite the second costal cartilage. Its 

 upper surface rises as high as the middle of the first piece of the sternum, which is 

 opposite the middle of the first costal cartilage, about 2. 5 cm. ( i in. ) below the top 

 of the sternum. 



Relations. In front of the arch the right lung and pleura cover it slightly, but 

 the left more so; the remains of the thymus gland is between them. The left superior 

 intercostal vein crosses its upper portion to empty into the left innominate vein. The 

 left phrenic and vagus nerves also cross it, the phrenic being the farther forward and 

 the vagus crossing almost in front of the point of origin of the left subclavian artery. 

 Behind lie the trachea, oesophagus, and thoracic duct, also the left recurrent laryn- 

 geal nerve. The bifurcation of the trachea is directly behind and below the lower 

 portion of the arch and the left bronchus passes beneath it. The oesophagus lies 



