292 GENERAL ANATOMY. 



Simple dilatation is especially met with in the large trunks; 

 it affects equally the whole circumference, and the tumour 

 resulting from it has an ovoid form. It has often been ob- 

 served in the aorta, particularly at its curvature, and some- 

 times in the pulmonary artery. 



The dilatation, with an alteration of the parietes, affects the 

 aorta and the different parts of the aortic system even to the 

 ramifications. The arteries of the superior members are more 

 seldom affected than those of the inferior. The alteration and 

 dilatation which results from it, are most commonly lateral. 

 This is the affection that authors have described, since Fernel, 

 under the name of true aneurism. The altered parietes are 

 rather thickened than thinned in it. The blood contained in 

 these two kinds of dilatations is fluid. 



435. Aneurism is caused by the injury or rupture, in a 

 word, by the solution of continuity of the arterial parietes, 

 commonly preceded by the dilatation of these parietes, and 

 always preceded by their alteration. It consists in a cavity 

 formed by the outer membrane, dilated and strengthened on 

 all sides by the cellular tissue and by the other surrounding 

 parts ; lined internally by a thin and in some places polished 

 membrane, resembling very much the inner membrane of 

 arteries. This cavity communicates with that of the vessel, 

 through a passage sometimes regular, at others irregular, made 

 in the inner and middle membranes; it is filled with coagu- 

 lated blood, and with layers of fibrine more or less firm, dif- 

 ferently altered, and perhaps mixed with organizable matter 

 produced by the parietes of the cavity. The blood, in its cir- 

 culatory course, penetrates continually into the accidental 

 cavity. 



Sometimes aneurisms enlarge indefinitely, and cause death 

 by the compression they produce on the neighbouring organs 

 and by the disturbance of their functions. At others it is 

 ruptured either externally or internally, and causes death by 

 hemorrhage or by effusion. At other times it inflames, sup- 

 purates and opens like a large abscess, and then sometimes 

 hemorrhage occurs, or, on the contrary, the artery being ob- 

 literated by inflammation, a radical cure may follow. Some- 



