Although inflammation of the Muscular Substance of the Heart is less frequent than endocarditis, it is much more frequent than is usually supposed. Its anatomical characters and its terminations are the same as those exhibited in inflammation of the muscular substance generally, but there are, nevertheless, many points connected with this subject which demand special notice, both on account of their importance and peculiarity.

It occurs independently in the middle layers of the muscular substance most remote from the pericardium on the one hand and from the endocardium on the other, and in original or consecutive combination with pericarditis and endocarditis. The pericardium and the endocardium are always implicated in inflammation of the adjacent layer of muscle and conversely intense pericarditis, and more especially intense, endocarditis influence the adjacent structure to various depths. It moreover most frequently affects the true fleshy walls of the heart, but sometimes its trabecule, and in some cases both simultaneously.

It also commonly occurs in the form of larger or smaller centres which are in some cases spread over a large portion of one cavity of the heart (as, for instance, the left ventricle), in which case, the wall of the heart is found to be affected throughout more or less of its thickness, when the disease is associated either with pericarditis or endocarditis singly or with both conjointly. In some rare cases one portion of the heart is found to be so thoroughly affected, that there are only a few layers of the muscular wall which are not implicated.

The seat of the affection is almost exclusively the left ventricle, which it attacks at every point, although less frequently at the septum; the apex is commonly attacked when the disease is very extensive. The right ventricle is very rarely affected, although we have observed the disease in an intense degree of development in the anterior wall of the conus arteriosus. It is of very rare occurrence, as far as we know, in the auricles. (See our remarks, in a future page, on Aneurism of the Heart).

Inflammation of the substance of the heart always gives rise to dilatation of the cavity implicated, and this dilatation is proportional to the extent of the inflammation and to the number of its centres. When combined in an early stage with endocarditis it occasionally results in the formation of an acute aneurism of the heart (of which we shall subsequently speak), in consequence of a laceration of the tissue which has been loosened by the process of inflammation. Finally, as we have already remarked, centres of inflammation are not unfrequently the cause of spontaneous ruptures of the heart.

This affection commonly results in induration and in suppuration, although it much more frequently assumes the former than the latter mode of termination.

In the former we find, in place of the muscular substance, a white fibroid (cellulo-fibrous) tissue, either in the form of small stripes, or spread over a more extended surface, according to the size of the centres of inflammation and the mass of the inflammatory product; or we may observe, where the indurated product of inflammation is accumulated in larger quantities at definite points and forms a tissue of this nature-nodular, roundish or irregularly shaped, ramified tumors, having the toughness of callus, which protrude either externally, or internally into the cavity of the heart. This form of striped indurations is frequently found to be deposited in the same subject in great quantity on the most different strata of the muscular substance of the heart, especially where an accurate investigation shows us the residua of pre-existing endocarditis, combined with consecutive dilatations and hypertrophy. Professor Bochdalek has drawn attention to this fact and to the frequency of carditis, which has hitherto been overlooked and generally denied.

The more widely extended inflammations of the muscular substance of the heart exhibiting this termination are of especial importance. They affect either the inner layers of the walls of the heart, together with the trabeculaeand the base of the papillary muscles, including the endocardium; or the external layers, together with the pericardium; or, lastly, the wall of the heart throughout its whole thickness, including both the pericardium and the endocardium. Occasionally we find that contiguous portions of the innermost, the middle, and the external layers of the muscular substance of the heart, are in turn attacked. The muscular substance is here found to be replaced by a fibroid tissue, while the walls of the heart, the trabecule, and papillary muscles, appear to be converted into a white callous tissue; - a process in which the endocardium so far participates, that it not only enters to a corresponding extent into the same metamorphosis, and becomes identified with this tissue; but it even generally exhibits a gradually decreasing fibroid thickening beyond the limits of the metamorphosis in the muscular substance. We also observe at the pericardium exudations, which are either well defined, or spread over the whole heart, and have been converted into cellular or fibrous tissues; and these give rise to adhesions.

These generally-diffused metamorphoses, which affect the wall of the heart throughout its whole thickness, not only exert an influence, in a general sense, on the increase of the dilatation of the respective cavities of the heart by means of the inflammatory process, but also specially on the origin of defined saccular dilatations - true chronic aneurism of the heart - which we shall subsequently consider more at large.

The fibroid tissue in the wall of the heart, in the trabeculae and in the papillary muscles, becomes, not unfrequently, in the course of time, the seat of calcareous deposit, constituting what is termed ossification of the walls of the heart, which invariably depends on the pre-existing alterations of texture of the muscular substance of the heart, which we just described.