BRAIN, a soft and pulpy organ, which in man occupies the cavity of the cranium, and forms one of the central masses of the nervous system. [NERVOUS SYSTEM] In man and all the higher animals the nervous system consists of four distinct parts—the white threads called Nerves, knots or masses of nervous matter situated along the course of the nerves called Ganglions, a long cord of nervous matter filling the cavity of the vertebral or spinal column called the Spinal Cord, and a large mass of nervous matter now generally considered as a continuation and expansion of the spinal cord called the Brain. The Spinal Cord and Brain constitute the two central masses of the nervous system, that is, the immediate seat of the functions peculiar to this system.
The general MOM of nervous matter designated under the common term Brain, together with its membranes, vessels, and nerves, com pletely fills the cavity of the skull. This mass is divided into three parts, the Cerebrum, or brain proper, which occupies the whole of the superior part of the cavity of the cranium ; the Cerebellum, much smaller than the cerebrum, whence its name, Little Brain, which occupies the lower and back part of the cavity of the cranium ; and the Medulla Oblongata, by much the smallest portion of the mass, situated at the basis of the cavity beneath the cerebrum and cere bellum. The medulla oblongata passes out of the cavity of the cranium into that of the vertebral canal by the foramen magnum of the occipital bone, being continuous with and forming the commence ' went of the spinal cord.
This general nervous mass is closely enveloped in three distinct membranous coverings, two of which have been called metres,' , from the fanciful notion that they give rise to all the othermembranes ' of the body. The external covering termed Dura Mater, from its being of a firmer texture than the other two membranes, incloses the brain with all its appendages, and lines the whole internal surface of the bones of the cranium. It is of a fibrous texture, the 'coin , ponent fibres interlacing each other in every possible direction, and forming by their firmness and density the thickest and strongest membrane of the whole body. By its external surface the dura
meter adheres everywhere to the inner surface of the cranium, just as the periosteum adheres to other bones. When torn from the cranium this surface appears somewhat rough and irregularly spotted with bloody points, which are the lacerated orifices of vessels that pass between the membrane and the surrounding bones. These vessels are much more numerous in the young than in the adult, and aro most abundant at the sutures or junctions of the bones that compose the skull. The inner surface of the dura meter, which is shining and smooth, is lubricated and kept in a state of moisture by a fluid secreted by its own vessels. This membrane performs a twofold office ; it supplies the place of the periosteum to the inner surface of the bones of the cranium, sustaining their nutrient 'vessels ; and it serves as a defence to the brain, and a support to the different masses into which it is divided.
The dura meter gives off several elongations or productions called processes, which descend between certain portions of the brain ; the most remarkable of which is termed the superior longitudinal process, which extends from the fore to the back part of the skull between the lateral halves of the cerebrum. Narrow in front, it becomes gradually broader as it passes backwards, bearing, as has been conceived, some resemblance in shape to a sickle or scythe, whence the common name of it, falx cerebri.
Where the falx cerebri terminates behind, there proceeds a large lateral expansion of the same membrane, extending across the back part of the skull beneath the posterior parts of the cerebrum, and forming a complete floor or vault over the cerebellum. This mem branous expansion is called tentorium, the obvious use of which is to prevent the cerebrum from pressing upon the cerebellum ; while from the middle of the tentorium proceeds another membranous expansion,, which descends between the lobes of the cerebellum, and terminates insensibly at the edge of the foramen magnum, performing for the cerebellum the same office as the fala performs for the cerebrum : hence it is called falx cerebelli.