364 ARTERIES. 



DESCENDING PART OF THE ARCH. 



The descending portion of the arch has a straight direction, inclining downwards 

 on the left side of the body of the third dorsal vertebra, at the lower border of 

 which it becomes the thoracic aorta. 



Relations. Its anterior surface is covered by the pleura and root of the left 

 lung ; behind, it lies on the left side of the body of the third dorsal vertebra. On 

 its right side lie the oesophagus and thoracic duct ; on its left side it is covered 

 by the pleura. 



PLAN OF THE RELATIONS OF THE DESCENDING PART OF THE ARCH. 



In front. 

 Pleura. 

 Hoot of left lung. 



Right side. \^ Left side. 



(Esophagus. / A r h tft of \ Pleura. 



Thoracic duct. 



Behind. 

 Left side of body of third dorsal vertebra. 



The ascending, transverse, and descending portions of the arch vary in position 

 according to the movements of respiration, being lowered, together with the 

 trachea, bronchi, and pulmonary vessels, during inspiration by the descent of the 

 Diaphragm, and elevated during expiration, when the Diaphragm ascends. These 

 movements are greater in the ascending than the transverse, and in the latter 

 than the descending part. 



Peculiarities. The height to which the aorta rises in the chest is usually about an inch below 

 the upper border of the sternum ; but it may ascend nearly to the top of that bone. Occasionally 

 it is found an inch and a half, more rarely three inches, below this point. 



Direction. Sometimes the aorta arches over the root of the right instead of the left lung, as 

 in birds, and passes down on the right side of the spine. In such cases, all the viscera of the 

 thoracic and abdominal cavities are transposed. Less frequently, the aorta, after arching over 

 the root of the right lung, is directed to its usual position on the left side of the spine, this pecu- 

 liarity not being accompanied by any transposition of the viscera. 



Conformation. The aorta occasionally divides into an ascending and a descending trunk, as 

 in some quadrupeds, the former, directed vertically upwards, subdividing into three branches, 

 to supply the head and upper extremities. Sometimes the aorta subdivides soon after its origin 

 into two branches, which soon reunite. In one of these cases, the oesophagus and trachea were 

 found to pass through the interval left by their division ; this is the normal condition of the vessel 

 in the reptilia. 



Surgical Anatomy. Of all the vessels of the arterial system, the aorta, and more especially 

 its arch, is most frequently the seat of disease ; hence it is important to consider some of the 

 consequences that may ensue from aneurism of this part. 



It will be remembered that the ascending part of the arch is contained in the pericardium, just 

 behind the sternum, its commencement being crossed by the pulmonary artery and right auricular 

 appendage, having the root of the right lung behind, the vena cava on the right side, and the 

 pulmonary artery and left auricle on the left side. 



Aneurism of the ascending aorta, in the situation of the aortic sinuses, in the great majority 

 of cases affects the right coronary sinus ; this is mainly owing to the regurgitation of blood upon 

 the sinuses, taking place chiefly on the right anterior aspect of the vessel. As the aneurismal 

 sac enlarges, it may compress any or all of the structures in immediate proximity with it, but 

 chiefly projects towards the right anterior side ; and, consequently, interferes mainly with those 

 structures that have a corresponding relation with the vessel. In the majority of cases, it bursts 

 in the cavity of the pericardium, the patient suddenly drops down dead, and, upon a post-mortem 

 examination, the pericardial bag is found full of blood ; or it may compress the right auricle, or 

 the pulmonary artery, and adjoining part of the right ventricle, and open into one or the other 

 of these parts, or it may compress the superior cava. 



Aneurism of the ascending aorta, originating above the sinuses, most frennently implicates 

 the right anterior wall of the vessel ; this is probably mainly owing to the blood being impelled 



