THE ARCH OF THE AORTA 



583 



Between the origin of the left subclavian artery and the attachment of the ductus 

 arteriosus the lumen of the fetal aorta is considerably narrowed, forming what 

 is termed the aortic isthmus (isthmus aortae), while immediately beyond the ductus 

 arteriosus the vessel presents a fusiform dilatation which His has named the aortic 

 spindle (aortenspindel) the point of junction of the two parts being marked in 

 the concavity of the arch by an indentation or angle. These conditions persist, 

 to some extent, in the adult, where His found that the average diameter of the 

 spindle exceeded that of the isthmus by 3 mm. (about one-eighth of an inch). 



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

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

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



In Direction. Sometimes in man, as is normal in birds, the aorta arches over the root of 

 the right instead of the left lung, and passes down on the right side of the vertebral column; 

 such an arrangement is usually found to be associated with transposition of other viscera. 

 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 vertebral column, this peculiarity not being accompanied by any 

 transposition of the viscera. 



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

 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 the division of the aorta; 

 this is the normal condition of the vessel in the reptilia. 



Applied Anatomy. Of all the vessels of the arterial 

 system, the aorta, and more especially its arch, is most fre- 

 quently the seat of disease; hence it is important to con- 

 sider some of the consequences that may ensue from 

 aneurism of this part. Aortic aneurisms usually occur 

 along a spiral line, the so-called "surf-line of the aorta," 

 which begins at the anterior sinus of Valsalva and is lost in 

 the dorsomesal wall of the descending aorta, as shown in 

 Fig. 436. 



It will be remembered that the ascending aorta 5s con- 

 tained in the pericardium, just behind the sternum, being 

 crossed at its commencement by the pulmonary artery and 

 right auricular appendix, and having the right pulmonary 

 artery behind, the superior vena cava on the right side, and 

 the pulmonary artery and left auricle on the left side. 



A neurism of the ascending aorta, in the situation of the 

 sinuses of Valsalva, in the great majority of cases, affects the 

 anterior sinus; this is mainly owing to the fact that the 

 regurgitation of blood upon the sinuses seems to take place 

 chiefly on the anterior aspect of the vessel. As the aneuris- 

 mal sac enlarges it may compress any or all of the structures 

 in immediate proximity to it, but chiefly project toward the 



right anterior side, and, consequently, interferes mainly with those structures which have a 

 corresponding relation with the vessel. In the majority of cases it bursts into the cavity of the 

 pericardium, the patient suddenly drops dead, and, upon a postmortem 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 or even rupture into the superior vena cava. 



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

 the right anterior wall of the vessel, where, as has been explained, there exists a normal dilata- 

 tion, the great sinus of the aorta; 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. It attains a large size and projects forward, it may cause absorption of the sternum and 

 the cartilages of the ribs, usually on the right side, and appears as a pulsating tumor on the front 

 of the thorax, 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 arch of the aorta, the student is reminded that the vessel lies on the trachea, 

 the oesophagus, and thoracic duct; that the left 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 



FIG. 436. The "surf-line of the 

 aorta, " showing the commoner points 

 of origin of aneurisms. 



