440 ARTERIES. 



DESCENDING PART OF THE ARCH. 



The Descending Portion of the Arch has a straight direction*, inclining down- 

 wards on the left side of the body of the fourth dorsal vertebra, at the lower 

 border of which it takes the name of 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 fourth dorsal vertebra. 

 On its right side are 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. 

 Boot of left lung. 



Right side. /Arch of AortaA Leftside. 



(Esophagus. ( Descending ) Pleura. 



Thoracic duct. 



Behind. 

 Left side of body of fourth dorsal vertebra. 



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

 tion 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. Occasion- 

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



In 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. 



In Conformation. The aorta occasionally divides, as in some quadrupeds, into an ascending 

 and a descending trunk, the former of which is directed vertically upwards, and subdivides 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 vessels 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, being crossed at its commencement by the pulmonary artery and right 

 auricular appendix, and 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 fact that the regurgitation 

 of blood upon the sinuses takes 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 sac 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 may press upon the superior cava. 



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

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



