THE MEMBRANES OF THE BRA [N.
The Central Nervous System, or the Cerebro-Spinal axis, is contained in the cavity of the cranium and spinal canal. This cavity is lined by a tough fibrous membrane, the dura mater, which is closely adherent to the inner surfaces of the bones of the skull, especially at the sutures, base, and the margin of the foramen magnum, but is loosely at tached by fibrous tissue to the bones forming the spinal canal. Its internal surface is smooth, covered by a serous surface which is the outer wall of the arachnoid cavity, and it is perforated by numerous openings corresponding to the foramina of its bony enclosure. These openings are for the transmission of nerves and blood vessels, and the membrane is prolonged outwards, to be continuous with their sheaths and with the periosteum of the skull. As an instance, the dura mater forms a sheath for the optic nerve, expands to form sclerotic coat of the eyeball and lines the walls of the orbit. The dura mater contains between its layers the meningeal arteries and channels for the conveyance of venous blood, called sinuses. The falx cerebri and the tentorum cerebelli are projections of the dura mater extending into the fissures of the brain, respectively between the hemispheres of the cerebrum and between the occipital lobes and the cerebellum. The use of these processes are to sustain the parts of the brain in their proper position.
The arachnoid is a thin fibrous membrane which surrounds the brain, spinal cord and the roots oche nerves. It covers the convolutions of the brain, but does not dip into the sulci between them. It is somewhat loosely attached to the sur face of the brain, but closely adherent to the pons, medulla and spinal cord, as well as to the roots of the nerves which it encloses and supports between their origin and the fora mina of the data mater before mentioned. The intervals between the arachnoid membrane and the pia mater beneath are called the sub-arachnoid spaces. These communicate with each other and with the ventricles of the brain and in certain pathological conditions are the seat of effusion. The arachnoid is continuous with the inner serous surface of the dura mater enclosing the arachnoid cavity. Upon each side of the spinal cord, between the roots of the nerves and throughout its whole length, this membrane sends out deli cate projections which are attached to the inner surface of the dura mater; the function of which is to preserve the position of the spinal cord in the spinal canal. Together they are
called the ligament= dentatum.
The pia mater immediately invests the nervous sub stance of the brain and spinal cord, is the vascular membrane, and is made up of an intricate network of blood vessels of various sizes,which break up into numerous minute vessels that penetrate the nervous mass and are distributed to it. In a sec tion of the brain these vessels are divided and are seen as points or specks on the surface of the section, and are called puncta vasculosa. Those blood vessels which are distributed to the grey masses, or nerve centres within the brain, are large and numerous, while those in the white matter are smaller and fewer in number. Projections of the pia mater extend through fissures into the ventricles, or cavities within the brain, with which they will be described. Attached to these projections of the pia in the ventricles are several close capillary networks, one for each ventricle, which are called the choroid plexuses and which will also be described with the ventricles of the brain. The network of blood vessels constituting the pia mater is held together a.nd supported by fine filaments of fibrous tissue attached on the outer side to the inner surface of the arachnoid membrane, and on the inner extending into the substance of the brain with the framework. of which it is con tinuous. The arachnoid membrane before described is a condensation of the fibrous tissue of the pia rnater over the surface of the brain covered externally by a serous sur face, in the same manner as the internal surface of the dura mater is covered int,ernally by a membrane of the sante serous character. These serous surfaces are continuous, therefore it may be said, that in reality there are but two imembranes of the brain, viz., the dura nutter and the pia mater, each having a serous surface which together enclose a serous cavity.
To sum up: The brain is supplied with a, dura nutter for protection, the arachnoid to accommodate its movements and the pia meter to provide for its nourishment