1. Deficient And Excessive Development

1. This membrane is sometimes entirely wanting in consequence of the absence of the brain; and portions of it are deficient, when the development of the brain is in any way arrested; thus the tentorium or the falx is sometimes wholly or partially wanting.

2. When the brain is double, there are two more or less complete sacs of dura mater.

2. Anomalies In Size, Form, And Position

These include the unnaturally small or large capacity of the sac of the dura mater, which results from corresponding anomalies in the formation of the brain, whether want of symmetry between its two halves, or displacement of them from their natural situation; or which results from an unnatural direction of the falx, from partial dilatation and (hernial) protrusion through the cranium, etc. And at any period of life, the dura mater may be distended and more or less attenuated, and its internal processes displaced in various directions by hypertrophy of the brain, hydrocephalus, morbid growths in the brain, etc.

The thickness of the dura mater is often manifestly increased, not only in consequence of inflammation, but also in persons who are advanced in life. Sometimes also it becomes thin, either in its whole extent, as when the brain is hypertrophied; or at certain parts where it is subjected to pressure; thus it is found thin and cribriform where it has been exposed to the pressure of the Pacchionian bodies.

3. Solutions Of Continuity

Under this head we class together anomalies in the adhesion of the dura mater to the cranium, and solutions of the continuity of the membrane itself.

Besides the thickening which the dura mater undergoes in advanced life, it acquires also a closer adhesion to the cranium; for as new bony matter is at different periods deposited on the inner surface, especially of the vault of the cranium, and the number of vessels with their investments passing between the membrane and bone becomes increased, the union between them is rendered more intimate. The adhesion generally commences, and continues strongest, along the sutures. In very old age the adhesion is often remarkably less firm than it is in middle life; the cranial bones are then atrophied.

The adhesion between the dura mater and cranium is frequently rendered much looser, or even entirely broken for a more or less considerable space, by concussion of the skull; the Separation may take place at the part where the blow was struck, or on the opposite side of the head. Its extent is often increased by subsequent extravasation between the membrane and the bone. Purulent and sanious effusions gradually force the dura mater and the bone apart.

The dura mater and cranium are frequently found separated from one another in cases of cephalhematoma, by an extravasation of blood, which takes place spontaneously from the bone.

Various kinds of solution of continuity befall the dura mater from incised, punctured, and gunshot wounds, and from injuries which break and shatter the skull. The membrane may split also, from being much distended by pressure from within in cases of hydrocephalus; and these ruptures of the dura mater are not limited to cases of hemicephalus; in some extremely rare instances, rupture occurs in hydrocephalus during extra-uterine life. Such a case is preserved in the Vienna Museum; it is a rupture of the dura mater, near the right parietal protuberance, more than an inch in length; it occurred in a hydrocephalic boy, and was followed by the extravasation of blood and of serum from the ventricle, between the dura mater and the cranium, and thence under the pericranium.

The Pacchionian bodies very often exert such pressure on the dura mater, as to give rise very gradually to solution of its continuity and concomitant atrophy. They force the fibres of that membrane asunder, and having pressed through it, lie in immediate contact with the skull.

Somewhat rarely it happens, that the dura mater is forcibly separated into layers by an extravasation of blood within it after concussion of the skull: such a separation is still more rare as a consequence of suppuration. Those extravasations which have been supposed to be collections of blood between the serous and fibrous strata of the membrane, with the exception of a few cases in which a small effusion has raised its innermost layer, must have been extravasations into the sac of the arachnoid, which, after acquiring an adhesion to the dura mater, have become encysted: they will be considered among the diseases of the arachnoid.