The meninges of the brain consist of three separate coverings: the outer being the dura mater, the middle the arachnoid, and the inner the pia mater.

The dura mater or fibrous covering of the brain is tough and strong and intended to protect it. Injuries of the skull without a laceration of this membrane are much less serious than when it is involved. When it is torn, not only is the brain beneath likely to be injured, but an opportunity is given for infection to enter and affect the brain itself and even produce a hernia cerebri or hernial protrusion of brain matter through the rent.

The dura mater is composed of two layers, the outer one acting as a periosteum to the bones. The two layers are in most places closely united, but at others they separate and form sinuses or canals, connected with the veins and carrying venous blood. The falx cerebri (Fig. 27) which is the fibrous partition separating the hemispheres of the brain from one another, as well as the tentorium, which separates the cerebrum from the cerebellum, is formed by the inner layer of the dura mater projecting inward and forming a partition. On the floor of the skull, the dura mater accompanies the nerves and gives them a sheath. The Gasserian ganglion of the fifth nerve is held in a pocket formed by the separation of the two layers of the dura mater.

The cerebral blood sinuses have already been considered. The dura is nourished by the meningeal arteries; bleeding from these has already been alluded to (page 17). Thin fibres of the dura pass to the bone, also branches of the meningeal arteries and veins pass to the inner table and diploŽ: these all serve to fasten the dura to the skull. This attachment is firmest on the base of the skull. On the vault, after an opening has been made through the skull by a trephine, the dura can be readily separated from the bone by means of a thin, flat, steel spatula. On account of the small size of the vessels passing from the dura to the bone, this procedure is not usually accompanied by much hemorrhage. In separating the dura from the base of the skull, as is done in operations on the Gasserian ganglion, the bleeding from this source is often quite free. The dura is liable to be torn in lifting it from the bone if the greatest care is not exercised.

The middle meningeal artery, at a distance of 4 cm. (1 1/2 in.) posterior to the angular process of the frontal bone and about the same above the zygoma, usually passes within the bone for a distance of 1 01 2 cm. Therefore, in operating in the temporal region, if the dura is detached the vessel will be torn and free bleeding will follow. The vessel is liable to be torn in endeavoring to remove bony fragments in fractures of this region. The dura is also more firmly attached in the median line; and on each side of the median line are the depressions in the parietal bone which lodge the Pacchionian bodies. The largest are usually located at a distance of from 2 to 5 cm. posterior to a line drawn across the skull from one external auditory meatus to the other. They are prolongations from the arachnoid and are surrounded by blood from the longitudinal sinus.

Fig. 27.   Vault of the skull opened and brain removed, showing the falx cerebri and tentorium.

Fig. 27. - Vault of the skull opened and brain removed, showing the falx cerebri and tentorium.