544 THE ARTERIES. 



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 of 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 peculiarity not being accompanied by any transposition of the viscera. 



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

 ing and descending trunk, the former of which is directed vertically upward, 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 vessel in the reptilia. 



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

 cially 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 aorta 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 



Vein azygos major. 

 Right 

 pulmonary veil 



Right ' 

 innom. vein. 



Right subclav. / 



Right common carotid. Left subclav. 



Inferior thyroid. "*~ ' ~ \ 



Left innominate win. 1 ** common caroti(L 



FIG. 346. Relation of great vessels at base of heart, seen from above. (From a preparation in the Museum 

 of the Royal College of Surgeons.) 



of cases, affects the right anterior 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 toward 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 into 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 vena 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 

 against this part. The direction of the aneurism is also chiefly toward the right of the median 

 line. If it attains a large size and projects forward, it may absorb the sternum and the cartilages 

 of the ribs, usually on the right side, and appear as a pulsating tumor on the front of the chest, 

 just below the manubrium ; or it may burst into the pericardium, or may compress or open into 

 the right lung, the trachea, bronchi, or oesophagus. 



Regarding the transverse aorta, the student is reminded that the vessel lies on the trachea, 

 the oesophagus, and thoracic duct ; that the recurrent laryngeal nerve winds around it ; and that 

 from its upper part are given off three large trunks, which supply the head, neck, and upper 

 extremities. Now, an aneurismal tumor, taking origin from the posterior part or right aspect of 

 the vessel, its most usual site, may press upon the trachea, impede the breathing, or produce 

 cough, haemoptysis, or stridulous breathing, or it may ultimately burst into that tube, producing 

 fatal haemorrhage. Again, its pressure on the laryngeal nerves may give rise to symptoms which 

 so accurately resemble those of laryngitis that the operation of tracheotomy has in some cases 



