BRAIN, the encephalon, or center of the nervous system and the seat of con sciousness and volition in man and the higher animals. It is a soft white and gray matter contained in the skull of vertebrated animals. The invertebrate have, instead of a true brain, nervous ganglia, situated near the end of the body. The external gray matter is softer than the white matter and consists of variously shaped nerve cells communicat ing with nerve fibers, and thereby re ceiving and discharging impressions. The internal white matter is composed almost entirely of medullated nerve fibers which transmit nerve impulses from one point to another. The brain is enveloped by three membranes called the Dura mater, which is dense and elastic; the Arachnoid, thin and double; and the Pia mater, which covers the whole surface of the brain and supplies the nervous tissues. These support the blood vessels which nourish the cranium and the brain, and also contain a clear fluid, the cerebrospinal fluid, which re moves the product of the brain waste and serves like a water cushion to di minish the effect of external shocks. The brain consists of two principal parts, the cerebrum and cerebellum, connected by bands of fibers. The cerebrum— great brain or cerebral hemisphere—oc cupies in man the upper part of the head and is seven or eight times larger than the cerebellum, which lies behind it and below it in a peculiar cavity of the skull. The cerebrum is divided into two por tions, the right and left hemispheres, by the longitudinal fissure, the hemispheres being at the same time transversely con nected by a band or bridge of nervous matter called the corpus callosum. The cerebellum, like the cerebrum, is also di vided into right and left hemispheres, connected by a bridge of nervous matter called the pans Varolii, under which is the medulla oblongata, or continuation of the spinal marrow. At the base of the brain are several masses of nervous mat ter or ganglia known as the corpora striata (two), and optic thalami (two), and corpora quadrigemina (four) ; and there are in it certain cavities or ven tricles.
Cerebrum.—The cerebrum underlies the whole vault of the cranium and cov ers all the rest of the brain. The hemi spheres, which are divided by a deep median fissure, are covered by a thin layer of matter, or nerve cells, and thrown into ridges or furrows called con volutions or gyri and fissures. These ridges are the result of the folding of the cerebral surface during the growth of the brain. On the outer surface of these hemispheres is the fissure of Syl vius, which is a deep incision at the base of the brain and runs in several directions. The fissure of Rolando runs almost vertically from the fissure of Syl vius nearly to the border of the hem ispheres. On the inner surface, half way between the central fissure and the rear end of the brain, is the parieto occipital incision. These fissures form the boundaries of the various lobes of the cerebrum. (1) The frontal lobe is that part of the outer and corresponding median surfaces which lies anterior to the fissure of Rolando, and is probably associated with the exercise of the higher mental faculties. (2) The temporo
sphenoidal lobe lies below the fissure of Sylvius. (3) The occipital lobe lies be hind the parieto-occipital fissure and in cludes the corresponding parts on the outer surface. (4) The parietal lobe is bounded by the fissures of Rolando and Sylvius, and by the occipital lobe. (5) The central lobe or island of Reil lies at the bottom of the fissure of Sylvius and is hidden in the adult by overreaching adjacent lobes. Secondary fissures on these lobes divide them into convolutions. On the under surface of the cerebrum are two olfactory nerves and two optic nerves. The olfactory nerves cross like the letter X, wind around the two cer ebral peduncles and terminate in the ol factory thalami and the olfactory lobe. These peduncles pass from under the surface of the hemispheres and approach each other as they enter the Varolii. If we press apart the two cerebral hemi spheres, we come upon the corpus cal losum, which is a band of white fibers connecting the convolutions of both hemi spheres. On dividing these and remov ing some white fibers and a layer of connected tissue called velum interposi tum, with its vascular margin, we ex pose the two lateral and the third ven tricles of the cerebrum, the former occu pying the hemispheres, the latter lying between them and continued backward through a narrow channel (the aque duct of Sylvius) into the fourth ven tricle, which lies behind the pons Varolii and the medulla. Projecting into the third and lateral ventricles are rounded masses of gray matter, the corpus stri atum and optic thalamus, called the basal ganglia. The whole cerebrum is sur rounded by a thin, convoluted envelope of gray matter about a quarter of an inch thick. Within this lies the centrurn ovale, composed of white nerve fibers passing in all directions and difficult to unravel. Connecting the two hemi spheres is the corpus callosum. Under neath are the septa lucida and the f or nix. Into the ventricles project the ovoid optic thalamus and the caudate nucleus of the corpus striatum. A wedge-shaped mass of gray matter, the lenticular nu cleus of the corpus striatum, is separated from the first two nuclei by a hand of white fibers. The internal capsule is composed of two parts, an interior and posterior limb, which meet each other at an obtuse angle (the knee). Directly outside the lenticular nucleus is the white external capsule, separated by the claustrum, a thin band of gray matter, from the island of Reil. Some of the fibers of the claustrum ovale connect the frontal and occipital lobes, others con nect the basal ganglia with the cortical gray matter. An important group, the corona cortex, passes from the internal capsule to the whole of the cortex. That part of the corona radiata entering the occipital lobes is called the optic radia tion of Gratiolet, who considered it to be the central expansion of the optic nerve.