The three membranes, placed one above the other, which line the cranium and the vertebral canal, which envelop the encephalon and the spinal cord, and extend to all their inequalities, are designated by this term. They are divided into the dura mater, the arachnoid, and the pia mater. The name mater, which is given to the external and internal meninges, appears to have been derived from the Arabs, who thus designated the covering of any body whatever.
The dura mater is a resistant, fibrous membrane, which lines the cavity of the skull and of the spinal canal. It adheres very slighdy to the walls of this canal, but more strongly to the arch of the skull, and closely to its base. The arachnoid and the pia mater are interposed between the dura mater and the nervous centre. It is separated from the spinal cord by a space filled with the fluid peculiar to the spinal column; but it is, on the contrary, directly in contact with the encephalon, some portions of which it holds in place.
On the median line of the cranial arch is a triangular canal, circumscribed by the dura mater, called the superior longitudinal sinus, which performs the functions of a large vein. Below this sinus it forms a fold or vertical partition, which is called the falx cerebri (falx, a sickle), which descending into the great fissure, separates the cerebral hemispheres. In its lower border is the inferior longitudinal sinus. Between the posterior lobes of the cerebrum and the cerebellum the dura mater covers the latter with the tentorium or tent of the cerebellum, which isolates it from the cerebral lobes. It also forms the falx cerebelli, which springs from the base of the skull between the two hemispheres of the lesser brain; and, lastly, it extends beyond the cranium, and furnishes the covering of the optic nerve, and the periosteum of the cavity of the orbit.
These folds and partitions formed by the dura mater around and between the organs of the encephalon, serve to maintain the different parts in place, to prevent their collision in shocks received on the body, and to prevent the parts from failing upon each other in certain positions; as, for instance, in lying on one side the falx cerebri prevents the weight of one hemisphere from resting on or compressing the other. The disposition of the sinuses of the dura mater is not less remarkable; they are, as already stated, venous canals with inextensible walls, through which the circulation is easy, and in no danger from obstruction or suspension; and when there is an afflux of blood it cannot compress the brain, as would be the case if the sinuses were replaced by veins with extensible walls.