THE CIRCULATORY SYSTEM. 457 



that the aneurism looks like a swelling attached to one side of the artery: 

 The aneurism may commence as a dilatation of all the coats of the vessel ; 

 but the middle coat soon atrophies, so that the wall is composed of the 

 inner and outer coats ; or the inner coat is destroyed by eudarteritis, so 

 that the outer coat alone forms the wall of the aneurism ; or the dilata- 

 tion may commence at the seat of rupture of one or more coats of the 

 vessels. As the aneurism increases in size it may press upon and lead to 

 partial destruction of neighboring tissues and viscera, so that portions of 

 these tissues and viscera may take the place of the wall of the aneurism. 

 The cavity of the aneurism is filled with fluid or clotted blood, or with 



FIG. 256. ANEURISM OF THE POPLITEAL ARTERY. 

 Showing lamellated clot. 



layers of fibrin (Fig. 256) which adhere closely to its wall. The com- 

 munication between the aneurism and the artery may be small or large. 

 If arterial branches are given off from the aneurism they may remain 

 open or become plugged with fibrin ; or their walls are thickened and 

 their cavities narrowed by endarteritis. Death may occur by the press- 

 ure and interference of the aneurism with the adjoining viscera, or by 

 rupture. 



Dissecting Aneurisms are those in which, owing to a solution of con- 

 tinuity of the inner layers of the artery, the blood gets between the media 

 and adventitia, and forces its way for a greater or less distance between 

 them. Or it may separate the media into two layers. 



Spurious or False Aneurisms are most frequently connected with ves- 

 sels of the extremities. When an artery is wounded the blood escapes 

 into the surrounding soft parts, and a cavity is formed filled with blood 

 and broken-down tissue. 



