Yielding Membranous Expansions in part give way to strong pressure, while their fibres admit of being separated; and in part they become gradually atrophied like cellular tissue, serous and fibrous membranes, muscular coats, etc. Large masses of muscle become pale and thin, and even wholly disappear.

Highly vascular and nervous structures, such as the external investment and the mucous membranes, have their texture so much loosened by inflammation, that they readily tear; or where this is not the case, they become gangrenous.

Parenchymatous structures waste away in consequence of the exudation produced by inflammation, and finally become atrophied.

Vessels are obliterated either in consequence of coalescence, induced simply by perfect compression, or in consequence of inflammation, that is to say, by means of adhesion to a coagulum of blood produced by the inflammatory process.

Nerves undergo atrophy through pressure and tension.

Rigid structures become atrophied in proportion to their deficiency of elasticity. Detritus of the bones is therefore very commonly induced by aneurism, whilst cartilage and fibro-cartilage, as, for instance, the intervertebral cartilages, are longer able to resist this action. This detritus is most frequently observed in the bodies of the vertebrae, in the ribs and the sternum, the clavicle, and also in the scapula in aortic aneurisms, and is often present in so highly developed a degree, that these bones are entirely destroyed, and the osseous wall of the thorax perforated. The vertebral canal has even been seen opened.

The process of resorption induced by the deposition and pressure of an aneurism on the bones, destroys not only the osseous substance itself, but, sooner or later, the aneurismal wall also, which becomes fused as it were with the periosteum and the other fibrous structures that usually invest the bones. The bone is then either very commonly laid bare, or is only covered by a layer of the deposition investing the aneurism, or by the fibrinous coagula in the aneurismal sac. The exposed vertebral column thus very frequently constitutes a portion of the aneurismal wall. In aortic aneurisms which perforate the anterior or lateral wall of the thorax, the roughened and nodular extremities of the ribs, the clavicle, and the sternum, are almost entirely denuded on their inner surface, and project into the sac of the aneurism.

The effects of the aneurism are diffused beyond its own immediate locality to distant organs, and even over the whole organism. These effects are as varied in their nature as the influences from which they arise; but in general they occur more rapidly, are more violent, and are more extensively diffused in proportion to the size of the aneurism, its relation to a main artery, and its vicinity to the heart.

The pressure on the nerves and their tension occasion variously developed symptoms of neuralgia and paralysis.

The pressure of the aneurism gives rise to a varicose condition of the veins below the aneurism, venosity, cyanosis, dropsy, and inflammations, which frequently terminate in gangrene.

Large aneurisms on the trunk of the aorta have a tendency to produce active dilatation of the heart, and this tendency is the more marked in proportion to their vicinity of that organ. They give rise to this disease either in association with insufficiency of the aortic valves, which is, however, generally the case, or independently of this affection. They also induce general venosity, diffused, as it were, from this point, as from a centre.

The pressure on the arteries, and the occlusion resulting from it in the region of the aneurism, may possibly be unattended by injurious results, in consequence of the establishment of a collateral circulation.

The stasis and coagulation of a considerable quantity of blood within a large aneurism, have the effect of withdrawing so large a quantity from the organism, as to occasion symptoms of anaemia, tabes, a watery condition of the blood, general dropsy, and cachexia. The pressure of the aneurism on parenchymatous structures, and the obstruction of their functions, contribute without doubt to the presence of cachexia, and to the development of its special character.

The following must be noticed in reference to the modes of termination of aneurism:

Aneurism very commonly terminates fatally.

This fatal termination is very frequently induced by the results already mentioned, amongst which we may specially place diffused inflammations terminating in gangrene, dropsy of the cavities of the body, hyperemia and acute oedema, more especially of the lungs, cachexia, and general marasmus.

Spontaneous opening or laceration, rupture and extravasation of blood from the rent constitute a very frequent, always extremely unfavorable, and indeed very often rapidly fatal termination of aneurism. We would direct attention to the following particulars in reference to this subject.

The tendency to spontaneous opening does not bear a direct relation to the size of the aneurism, for we find that small aneurisms burst more frequently than larger ones.

The direction in which the aneurism opens, and in which the blood emerges, varies considerably. Aneurisms in the limbs open into the surrounding cellular tissue, in consequence of which a large quantity of blood is extravasated into the intermuscular, subcutaneous cellular tissue, below and between the aponeuroses, the muscular sheaths, etc. Aneurisms of the trunk, and of some of the branches of the aorta, as, for instance, the splenic, open into the large cavities of the body, as the peritoneal sac, one or other of the pleural sacs, or the pericardium, occasioning hemorrhage into the corresponding cavity and the sub-serous cellular substance. Aneurisms of the cerebral arteries open in a similar manner into the sac of the Arachnoid, and into the tissue of the Pia Mater.