DILATATION OF VEINS, 263 



aneurism an opportunity of bursting. The pleural surface h the 

 least dangerous in this respect ; for the pleural sac is merely an 

 interstice in the connective tissue, whose opposed surfaces readily 

 cohere. The pressure of the aneurism is thus transferred to the 

 lung, and the fatal issue is for a time delayed. It cannot take 

 place till both of the pleural laminae have been perforated and 

 the spongy parenchyma with the smaller bronchi laid open in 

 consequence. Those aneurisms which force their way towards 

 the larger bronchi or the trachea itself burst at a much earlier 

 })eriod. Aneurisms of the ascending aorta ultimately burst 

 through the skin ; rupture may also occur into the oesophagus, 

 pericardium or pulmonary artery ; more rarely, into the perito- 

 neum or the retro-peritoneal connective tissue. 



§ 227. Clots often form in the cavity of the an em-ism. The 

 first impulse to their formation is usually given by irregularities 

 of the internal surface. Retardation of the blood-current is also 

 an important factor; hence the peculiar frequency with which 

 the blood coagulates in saccular aneurisms with narrow necks. 

 These aneurismal thrombi are always exquisitely laminated ; the 

 outer layers, those first deposited, are usually quite decolorised, 

 tough and fibrous, like all clots which have been exposed to con- 

 tinued pressure ; no trace of commencing organisation can ever 

 be detected. Hence it is that the formation of thrombi in an 

 aneurism, though directly lessening the size of its cavity, seldom 

 leads to a spontaneous cure. This has only been known to occur 

 in aneurisms of small arteries which have been quite shut oft^" 

 from the parent- vessel. For the most part, the filling-up of an 

 aneurismal sac with coagula proves inadequate to resist the im- 

 pulse of the blood. Should the sac burst, the thrombus is torn 

 up and the blood forces its way out between its fragments. 

 Aneurismal coagula may also undergo softening, an event which 

 facilitates the detachment of fragments and increases the risk of 

 embolism. 



^. Dilatation of Veins. — Phlehedasij. 



§ 228. Dilatations of veins, notwithstanding their greater 

 frequency, notwithstanding the many anatomical peculiarities 

 which they present, are but a feeble copy of arterial dilatations. 

 It has been already stated that an '^ endophlebitis chronica," 

 analogous to atheromatous disease of the arteries, can hardly be 



