This is the name given to the membrane which immediately surrounds the nervous centre. It is a vascular net-work of extreme delicacy and fineness of texture, and may be considered as the nutritive membrane of the cerebro-spinal organs. In its tissue, the arteries running to the brain are ramified with infinite minuteness, and inosculate with the radicles of the veins which spring from it. It follows closely all the cerebral convolutions; penetrates the furrows, and into the ventricles, and covers even the thin layers of the cerebellum. It becomes denser and more fibrous around the spinal cord, of which it forms the neurilemma or envelope, as well as at the origin of the nerves.
To recapitulate, the greater and lesser brain, and the spinal cord, contained in the skull and spinal canal inclosed in three superposed membranes, are united by a common centre, the isthmus of the encephalon. Nerves spring from the brain and spinal cord.