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384 CAUSES OF ANEURISM. an aneurism. ‘This explains the frequency of intracranial aneurism at special points, such as the middle cerebral arteries, or at bifurea- tions of tho other arteries. Of course there are many such aneurisms without any history of embolism, but still there will often be a history of valvular ene accompanied by vegetations, fragments of which becoming detached might produce embolisms.* Spontaneous thrombosis may, of course, have similar conse- quences.

Form und Contents of Aneurism.—The form of sacculated anen- risms is generally globular, but they may, partly owing to

accidental conditions in the coats of the Fra, 91, aifccted vessels, partly from the pressure exerted by surrounding tissues, agsume an oyal or more or less irregular outline. Upon the original ancurism, an cyolution of secondary dilatations is sometimes met with, and these may even give rise to a further or tevtiary multiplication of the disease, so as to induce a sort of mulbe: appearance in the tumonr ; it is the variety to which Cruvcilhicr has applied the term ‘“anéyrisme sous Vaspect @ampoules a bosselures.”

The contents of aneurismal sacs aro fibrinous coagula, which form in success- ive layers, and accordingly present a con- centric arrangement, like the annular rings in perennial plants. ‘The resemblance may be traced still further in the gradual eon- densation of the outer or external layers, owing to «absorption and compression.

Aneurism of thepoxterior Lhese also lose their colour, and become tibial artery, with the nerve fawn-coloured or white; while towards aprend ver the nck part the interior we continue to recognize the Citinely Ubiterated be cone dark colour of the blood. The accumula- centri¢ layers of fibrne, tion may proceed to such an extent as to Browing paler towards the q),Jiterate the cavity, and thus establish a

et spontaneous cure. A considerable amount

(St. George’s Musewm, : eee : rE

B41.) of organization is observed to take place in

some deposits, manifested by the formation

of fibres. The existence of a distinct membrane is assumed by most writers to cnvelope the coagulum, It is a point to which Lizot particularly drew attention, and upcn which Hasse dwells forcibly. The latter states that he has never met with an ancurism in which this adventitions membrane was not present. Mr, Bowioan has observed, that the membrane enveloping the coagula in an ancu-

% J.W. Ogle: “Trans. Path. Soe.” 1957, vol. viii. p, 170; ‘St. George's Hosp. Reports,” 1867, p. 2&5; ‘‘ Metical Times and Gazette," 1866, vol. i. p. 196; see also Church: “Trans. Path. Soc.,” vol. sx. p. 109; Semple: Ibid., p. 112.

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