ANEURISM OF THE AORTA. 419 



and its entire calibre. If the dilatation terminate abruptly, it is called 

 a cylindrical aneurism. If it decrease gradually, it is a fusiform 

 aneurism. Diffuse aneurism is always a true one, in Scarptfs sense, 

 and is most commonly met with in the ascending and transverse por- 

 tion of the arch of the aorta. It is very often combined with the form 

 next to be described, that is to say, circumscribed pouches often form 

 upon the dilated portion of the artery. 



Circumscribed aneurism consists in the dilatation of a shorter por- 

 tion of the artery. Here, too, the artery is sometimes widened in all 

 directions, the tumor involving its entire diameter. Far more fre- 

 quently, however, one side alone is dilated, and the aneurism, whose 

 walls form an angle with those of the normal part of the vessel, as- 

 sumes the appearance of a tumor situated on its side. Secondary 

 pouches, in the form of elevations of varying size, are often observed 

 upon these sac-like dilatations, just as in the other kind. At the out- 

 set, the disease almost always bears the character of a true aneurism 

 of Scarpa, consisting of all three of the aortic coats ; but, when the 

 sac has attained some magnitude, the inner tunic only extends for a 

 short distance into it. When at its period of fullest development, the 

 middle tunic, too, dwindles, and finally disappears totally, while there 

 still remain traces here and there of the tunica intima, in a state of 

 degeneration. 



Aneurisms attached by a neck must be regarded as a peculiar 

 species of the sac-like form. In these cases, a very small spot on the 

 arterial wall gives way. If the dilatation be large, the wall of the 

 aneurism wraps itself around that of the artery. Thus a duplicature 

 is formed, which, looked at from within, presents a prominent ridge, 

 while, from without, the tumor seems to have been constricted at its 

 base. In these saccular aneurisms, the tunica media can only be 

 traced for a short distance, and soon disappears on the far side of the 

 neck, the wall then consisting of the intima and adventitia (aneurisma 

 mixtum internum seu herniosum). When very large, all the tissues 

 gradually disappear under the pressure ; and the adjacent structures, 

 to which the aneurism becomes adherent, finally furnish its wall. If 

 the enlargement be slow, the new wall may become very firm by pro- 

 liferation of the connective tissue ; but, if rapid, the wall remains thin, 

 and soon bursts. If the tumor encounter any resisting body, such as 

 bone, the latter undergoes absorption (uSur), just like the sac-wall, and, 

 after destruction of the periosteum, the bone is laid bare, and projects 

 aaked into the pouch. 



In the cavities of aneurisms of large size, especially in the sac- 

 shaped ones, we almost always find deposits of fibrin arranged in sep- 

 arate layers. Those attached to the walls are yellow, dry, and firm ; 



