456 



THE CIRCULATORY SYSTEM. 



there may be gummata formed in the walls or a diffuse formation of new 

 cells and new fibrous tissue, especially in the adveutitia of the vessels. 

 This may extend to the middle and internal layer and lead to narrowing 

 or closure of the lumen. Such lesions may be circumscribed or extensive, 

 and may result in aneurism or rupture. l 



ANEURISM. 



An aneurism is a dilatation of an artery, either cylindrical, fusiform, 

 or sacculated, the walls consisting of all or part of the coats of the artery 

 variously altered. 



The conditions which commonly lead to aneurisms are arterio-sclero- 

 sis, either with or without associated degeneration, or the local weakening 



of the walls of the vessels 

 which often leads to com- 

 pensatory arterio - sclerosis, 

 or the blocking of a vessel 

 by emboltis. Aneurism may 

 develop at the point of par 

 ti.il rupture of the walls of 

 an artery. Aneurisms may 

 follow the local necrotic and 

 ulcerative process in the 

 walls of the vessels or on the 

 heart valves incited by in- 

 fectious agents, which as 

 emboli or otherwise have 

 reached these parts. 



In cylindrical or fusiform 

 aneurism there is apt to be 

 at first a distention of all 

 the walls of the vessel ; but 

 this is often irregular, so 

 that the sac may bulge more 

 in one place than in another 

 (Fig. 255). The walls may 

 become thin or through 

 new-formed tissue they may 

 become thickened ; they may 

 undergo degeneration or cal- 

 cification. Such aneurisms are most frequent in the aorta, but may oc- 

 cur in other vessels. 



In the sacculated aneurism there is either a dilatation of the entire 

 circumference of an artery over a short portion of its length ; or there 

 may be a dilatation of only a small portion of one side of the wall, so 



1 For a careful study of alterations of the blood-vessels in syphilis see Abramow, 

 Ziegler's Beitr. z. path. Anat., Bd. xxvi., p. 202, 1899, bibliography. 



FIG. 255. FCSIFORM ANEURISM OF THE ARCH OF THE AORTA. 



