A distinction will be drawn in the following remarks, wherever it is possible, between the actual dura mater, and its innermost shining stratum. For, though the latter cannot be demonstrated as a separate serous layer, we are compelled to adopt the distinction by the substantial difference which is exhibited, at least at first, by morbid processes in the two layers. Inflammation, for instance, attacks one of the layers independently of the other, and presents differences accordingly in its course, in its proneness to extend along the surface, and in the products it furnishes, which manifest the analogy between that layer and serous membranes in general. I shall not at present enter into these, or into several similar subjects, as it would be interfering with diseases that are evidently connected with the arachnoid membrane.

1. Inflammation

With the exception of those which are brought on by injury, primary inflammations of the dura mater to any extent, such, for instance, as would lead to the formation of matter, are of rare occurrence.

Inflammations of slight degree, on the other hand, and usually combined with moderate inflammation of the cranial bones, are frequent. These processes are characterized by vascularity and rosy reddening of the dura mater, and by softening of its texture; and they give rise to interstitial infiltration of the membrane, as well as to exudations upon that surface of it which adjoins the bone: such exudations become organized into loose cellular, or thick fibrous tissue, or at length, especially if there be any inflammation of the bone besides, into bone; and they produce an unnatural adhesion to the vitreous table of the skull. They are generally widely spread, especially along the sutures; but sometimes they are confined within a smaller compass, so that they form at one time extensive tracts, at another circumscribed islands. Exudations which, ossify, very commonly present themselves spread out as a layer, that, like the puerperal osteophyte, is at first spongy, but gradually becomes compact: sometimes they form a mass of bone which looks as if it had flowed or dropped upon the membrane, and then coagulated; while, not unfrequently, they are circumscribed osseous plates or nodules, which, though, in course of time, intimately united to the bone, yet originally adhered firmly to the dura mater.

When the inflammation is more intense, and runs a chronic course, the dura mater acquires an increase in thickness, sometimes to the extent of three lines, and even more; it becomes indurated and callous, and usually adheres more closely than natural to the bone. I met with an instance of this effect of inflammation in the dura mater lining the right occipital fossa, in which inflammation came on in the adjoining lateral sinus, and led to its obliteration.

When inflammation is brought on by injury, or passes to the dura mater from neighboring tissues, it frequently terminates in the production of matter, and in suppurative degeneration of the membrane.

These latter cases, therefore, are of great importance, for they are brought on by inflammation and suppuration of the bone, or of neighboring ligamentous structures. They are especially apt to occur in particular localities; thus the dura mater inflames, suppurates, and sloughs from caries of the internal ear, and the labyrinth of the ethmoid bone, from caries of the upper cervical vertebrae, and suppuration of their ligaments. In the dura mater these processes continue circumscribed, but when they reach the inner membranes, they usually spread rapidly into general meningitis.

The characteristics of inflammation of the dura mater are those which are common to all inflamed fibrous tissues: I refer on this point to what has been said at page 94.