THORACIC ANEUEYSMS. 269 



ANEURYSM. 



Certain arteries are more prone to be the seat of aneurysm 

 than others, and many of the reasons for this fact are dependent 

 upon anatomical considerations. Aneurysm of the thoracic 

 aorta is more frequent than that of any other vessel. Following 

 upon this artery the popliteal, the superficial femoral, the 

 common carotid, the subclavian, the axillary, the innominate, 

 and the abdominal aorta are not infrequent sites. 



From an anatomical point of view it is convenient to subdivide 

 thoracic aneurysms into three groups, namely, those arising 

 from the ascending aorta, those which spring from the oblique 

 segment of the arch, and those which occur in connexion with 

 the descending portion. These limits are purely artificial, and 

 are often overstepped by the progress of the disease. The only 

 aneurysms which can attain a large size without producing 

 serious internal pressure effects are those which project forwards 

 or upwards from the anterior and upper aspects of the aortic arch. 



The ascending thoracic aorta, or first part of the aortic arch, 

 commences at the lower border of the third left costal cartilage, 

 just beneath the sternal edge, this being the situation of the 

 aortic valves. The vessel inclines upwards, forwards and to the 

 right, and terminates under cover of the right half of the sternum 

 at the level of the " aortic cartilage," which is the second costal 

 cartilage of the right side. This part of the aorta lies in a 

 pericardial sheath which is common to it and the pulmonary 

 artery, so that both vessels are closely invested in the serous 

 layer save where they come into contact with each other. The 

 presence of this pericardial investing sheath accounts for the 

 tendency of aneurysms near the root of the aorta to rupture into 

 the pericardial cavity. The front of the vessel is at first con- 

 cealed by the root of the pulmonary artery and the right auricular 

 appendix, but higher up it is only separated from the sternum 

 by the right pleura and the mediastinal tissues. Saccular 

 aneurysms arising here tend to present in the second and third 

 right intercostal spaces near the sternal edge, but they may 



