In the vast majority of eases of this kind the products of inflammatory states of the pericar dium or endocardium, or its appendages, are present ; in short, a diseased state of the valves constantly co-exists with hypertrophy and dila tation. These conditions of the cavities are very frequently traceable to some obstacle to the cir culation through the heart, and sometimes it would seem that the valvular disease preceded and gave rise to the hypertrophous and dilated cavity; but it is not impossible nor unlikely that the valvular disease may follow the hyper trophy, and may result from the violence of contraction of the enlarged ventricle. Dilata tion of the aorta at its commencement and its arch is frequently the consequence of this dis ease in the left ventricle, and dilatation of the pulmonary artery ensues upon it in the right ventricle.
Dilatation of the cavities of the heart.— gg When the heart is incapable of sufficiently expelling its contents, whether in consequence of obstruction in the vessels from it, of regurgi tation into it through imperfect valves, of want of power, of irritability, or of both, it becomes distended, and in time permanently dilated." We have already described that kind of dilata tion which is the most common, namely, that accompanied by hypertrophy ; dilatation also occurs in connexion with an opposite condition of the parietes, namely, attenuation of them. The muscular tissue has lost its tone, and yields, as it were, without resistance to the dis tending force. It is laid down by authors that a third variety of dilatation may exist, what they call simple dilatation, or that in which, while the cavity is dilated, the parietes are of their na tural size. It seems to me impossible that any cavity of the heart can, in a dilated state, conti nue of the natural thickness without hypertrophy, in the absence of which dilatation implies neces sarily a diminution in thickness; during dia stole the parietes of the heart's cavities are thin ner than during systole ; what a contracted muscle gains in one dimension it loses in ano ther; and the same may be said of a relaxed or distended muscle. Again, if we contrast a con tracted with a dilated bladder, it seems evident that we cannot inflate the former, however in completely, without producing a manifest dimi nution in the thickness of its walls. Hence I infer, that if the parietes of any cavity be per fectly natural, they must become thinned under the influence of the force which produces the dilatation ; and, on the other hand, if we find that the parietes of a dilated cavity possess the normal thickness, we may be assured that it is slightly hypertrophous. It appears then to be
most correct to limit the varieties of dilatation to two, that with hypertrophy and that with attenuation,or the passive aneurism of Corvisart. In this latter form of dilatation, then, we see a manifest alteration of the muscular tissue ; it is paler, softer, less resisting, less elastic than natural. When the heart is emptied of its con tents, the walls do not at all return upon them selves, but remain flaccid ; nor when cut do they show any disposition to retract; and it is this state of the muscular substance which will serve best to enable the anatomist to distinguish morbid dilatations from those which result from mechanical distension of the cavity by a eoagu lum formed at the time of death. An obvi ously diseased state of both the internal and external membranous coverings of the heart is constantly present along with this form of dila tation. These membranes lose their transpa rency in several parts, apparently from some abnormal deposit subjacent to them : the white spot so often seen upon the external surface of the right ventricle is an almost invariable at tendant upon the dilated heart. Dilatation may affect any or all of the heart's cavities ; but it is met with by far the most frequently in the right ventricle, and very commonly both ven tricles are dilated, in which case the right cavity is generally more capacious than the left.
An extreme case of dilatation is afforded in an example quoted by Bouillaud " the right cavities were so dilated and their walls so at tenuated, that the auricle was converted into a kind of transparent membrane, and the ventricle was reduced only to the ordinary thickness of the auricle." In determining as to the degree of attenua tion of the walls which may accompany any particular case of dilatation of the auricles, the anatomist must bear in mind that even in the natural state the interval between the musculi pectinati of the right auricle is only composed of the endocardium and pericardium, separated by a very fine and transparent cellular tissue, and by a few muscular fibres crossing obliquely from one pectinate muscle to the next one. I have twice seen a perfectly natural right auricle carefully put up as a museum specimen of morbid attenuation of the parietes, owing to ignorance or forgetfulness of this fact.