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noun The portion of the vertebrate central nervous system that is enclosed within the cranium, continuous with the spinal cord, and composed of gray matter and white matter. It is the primary center for the regulation and control of bodily activities, receiving and interpreting sensory impulses, and transmitting information to the muscles and body organs. It is also the seat of consciousness, thought, memory, and emotion.
noun A functionally similar portion of the invertebrate nervous system.
noun Intellectual ability; mind: a dull brain; a quick brain.

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In anatomy, the soft grayish and whitish mass filling the cranial cavity of a vertebrate, consisting of ganglionic nerve-cells and nerve-fibers, with the requisite sustentacular and vascular tissue; the encephalon (which see); the part of the cerebrospinal axis which is contained in the cranium. It is divided by anatomists into — (1) the prosencephalon, comprising the cerebral hemispheres (or lateral halves of the cerebrum) with the olfactory lobes; (2) the thalamencephalon comprising the thalami optici, and other parts about the third ventricle; (3) the mesencephalon, comprising the parts about the Sylvian aqueduct, corpora quadrigemina above and crura cerebri below; (4) the epencephalon, comprising the cerebellum and pons Varolii; and (5) the metencephalon or medulla oblongata, extending from the pons to the foramen magnum. The prosencephalon is now usually divided into the rhinencephalon, or olfactory lobes, and prosencephalon proper. The thalamencephalon is also called diencephalon. By Huxley and others the epencephalon of the above nomenclature is called metencephalon, and the next segment (the fifth) is then named myelencephalon. Common English equivalents of the above five segments are forebrain, ' tweenbrain, midbrain, hindbrain, and afterbrain; these are terms translated directly from the nomenclature of the German anatomists, who call them respectively vorderhirn, zwischenhirn, mittelhirn, hinterhirn and nachhirn. Haeckel calls them protopsyche, deutopsyche, mesopsyche, metapsyche, and epipsyche. These five segments are fundamentally distinct, and correspond embryologically to as many cerebral vesicles or brain-bladders which arise from three primitive vesicles by subdivision. The simplest and a common division of the brain is into the cerebrum or brain proper, the cerebellum or little brain, the pons Varolii, and the medulla oblongata. (See cuts under serebral and corpus.) The human brain is distinguished for the relatively enormous size and surface-complexity of the cerebrum or prosencephalon, which completely covers the cerebellum and olfactory lobes, and is marked by many deep flssures or sulci separating gyri or convolutions. The cerebrum is divided into right and left halves, or cerebral hemispheres, connected by the great transverse commissure or corpus callosum. Each hemisphere is divided into three primary lobes, frontal, pariental, and occipital, and many more detailed subdivisions of its surface are recognized. The interior of the brain (which is primitively hollow) is traversed in the adult by a set or system of connected cavities known as ventricles or cœliæ. The first and second of these are the right and left ventricles of the hemispheres, or procœliæ the third is the diacœlia; the fourth is the epicœlia; passages connecting these are the foramina of Monro and the aqueduct of Sylvius. The brian and adjoining protions of the spinal cord give rise to 12 pairs of nerves, called cranial nerves because they emerge from foramina in the base of the skull. (See cranial.) Brain-substance is of two kinds, gray ganglionic or cellular nerve-tissue, and white commissural or fibrous nerve-tissue. The gray matter which invests the cerebrum and cerebellum is also called the cortical substance, in distinction from the white or medullary substance of the interior. A brain is in fact a collection of gray ganglia united by white commissures. Besides the cortex, there are several ganglia or collections of gray matter in the interior, as the corpora striata, the optic thalamil the optic lobes or corpora quadrigemina, the corpora dentata of the cerebellum, and the corpora olivaria of the medulla oblongata. Connected with the brain are two non-nervous structures, the conarium or epiphysis cerebri and the pitnitary body or hypophysis cerebri. The brain is covered by three membranes or meninges, of which the external is the dura mater, the middle the arachnoid, and the inner the pia mater. Most mammals have a brain like that of man, but in descending the mammalian scale the cerebrum becomes relatively smaller and has fewer if any convolutions, the corpus callosum becomes rudimentary, and the olfactory lobes enlarge. (See cuts under gyrus and sulcus.) In the brain of birds the hemispheres are smooth, there is no corpus callosum or pons Varolii, and the optic lobes are of immense size. There is no brain in the lowest vertebrate, Amphioxus. The average weight of the brain in adult males of the European type is about 1,400 grams (49.5 ounces); in women about 1,250 grams (44 ounces). The brain is in its highest activity the organ of consciousness or mind, and its general function is that of furnishing the most complex and extensive outgoing stimulation of muscles and other active tissues as a response, more or less immediate, to the most complex and extensive incoming sensory stimulation. With functions of this high degree of complexity are associated in some parts much simpler functions resembling those of the spinal cord. The cortex of the cerebral hemispheres is the portion of the brain in which the most complex coördinations seem to be effected, and which is most directly involved in mental acts. Certain parts of the cortex are, however, peculiarly related to certain special incoming or outgoing stimulations, and are called sensory or motor centers. (See cerebral, and cerebral localization, under localization.) The corpus striatum is usually regarded as especially concerned with stimulations passing downward, and the optic thalamus with those passing upward; among the latter, those of sight are connected with the hinder part of the thalamus. The nates are involved in the sight-function, and the testes seem to have close relations with the stimuli entering by the auditory nerve. The cerebellum is concerned with the coördination of muscular contractions is the carrying out of voluntary actions, while the medulla oblongata contains a large number of centers for comparatively simple functions, as vasomotor action, cardiac action, respiration, deglutition, etc.(See also cut under encephalon.) From its complexity, the brain is usually spoken of in the plural in certain relations: as, to beat out or to rack one's brains.
In entomology, the principal ganglion of the nervous system, situated in the head, over the esophagus, and formed by the coalescence of serveral supra-esophageal ganglia. The nerves of the eyes and antennæ are directly connected with it, and it gives off two inferior branches which surround the esophagus and unite beneath in the subesophageal ganglion. Sometimes this ganglion is regarded as a part of the brain, being distinguished as the cerebellum, while the principal or upper ganglion is called the cerebrum.
The same or a corresponding portion of the nervous system in many other invertebrates.

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You will excuse the dull vanity of this epistle, as my brain is a chaos of absurd images, and full of business, preparations, and projects. — Life of Lord Byron
But once you see it for the twentieth time, your brain has been around every corner of that scene, and you remember it, sure, but the shock power's gone. — The 6th Target
Those that conduct impulses downward from the brain are the descending tracts and those that conduct them upward to the brain are the ascending tracts. — The Human Brain
Either my explanation or your brain is at fault, and I will not insist upon which. — When The World Screamed
NOONAN: I specifically mean that it was not his brain, and though his brain was a very good, first-rate one, it wasn't his brain, it wasn't his talents, it wasn't his looks. — CNN Transcript Feb 3, 2002

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