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brain

A. pituitary gland B. cerebrum C. skull D. corpus callosum E. thalamus F. hypothalamus G. pons H. cerebellum I. medulla J. spinal cord (Carlyn Iverson)

n.

1. 1. 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.
   2. A functionally similar portion of the invertebrate nervous system.
2. 1. Intellectual ability; mind: a dull brain; a quick brain.
   2. Intellectual power; intelligence. Often used in the plural: has brains and good looks. See synonyms at mind.
3. A highly intelligent person.
4. The primary director or planner, as of an organization or movement. Often used in the plural.
5. The control center, as of a ship, aircraft, or spacecraft.

tr.v. Slang., brained, brain·ing, brains.

1. To smash in the skull of.
2. To hit on the head.

idioms:

beat (one's) brains (out)

1. Informal. To exert or expend great mental effort: She beat her brains out during the examination.

on the brain

1. Obsessively in mind: The coach has winning on the brain.

pick (someone's) brain (or brains)

1. To explore another's ideas through questioning.

rack (one's) brain Informal.

1. To think long and hard: I racked my brain for hours trying to recall her name.

[Middle English, from Old English brægen.]

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A collection of specialized cells (neurons) in the head that regulates behavior as well as sensory and motor functions. The three main parts of the brain in vertebrates are the cerebrum, the cerebellum, and the brainstem that connects them with each other and with the spinal cord (see illustration). The two cerebral hemispheres are separated by a midline fissure that is bridged by a massive bundle of axons running in both directions, the corpus callosum. Each hemisphere has a core of groups of neurons (the basal ganglia); an outer shell of neurons in layers (the cerebral cortex); and massive bundles of axons for communication within the cerebrum and with the rest of the brain. These bundles are called white matter because of the waxy myelin sheaths surrounding the axons. See also Neuron.

Midsagittal (midline, medial) section through the human brain. (After C. R. Noback, The Human Nervous System, 4th ed., McGraw-Hill, 1991)

The basal ganglia comprises three main groups. (1) The thalamus receives axons from all sensory systems and transmits information to the cortex. It also receives feedback from cortical neurons during sensory processing. (2) The striatum, comprising bundles of axons cutting through the groups of neurons, also has two-way communication with the cortex and assists in the organization of body movement. (3) The hypothalamus receives orders from the cortex and organizes the chemical systems that support body movement. One output channel is hormonal, and controls the pituitary gland (hypophysis) which in turn controls the endocrine system. The other channel is neural, comprising axons coursing through the brainstem and spinal cord to the motor neurons of the autonomic nervous system, which regulates the heart, blood vessels, lungs, gastrointestinal tract, sex organs, and skin. The autonomic and endocrine systems are largely self regulating, but they are subject to control by the cortex through the hypothalamus. See also Autonomic nervous system; Endocrine system (vertebrate); Neurobiology.

The cortex is also called gray matter because it contains the axons, cell bodies, and dendrites of neurons but there is very little myelin. An index of the capacity of a brain is cortical surface area. In higher mammals, the cortical surface increases more rapidly than the volume during fetal development; as a result the surface folds, taking the form of convexities (gyri) and fissures (sulci) that vary in their details from one brain to another. However, they are sufficiently reliable to serve as landmarks on the cerebral hemisphere that it can be subdivided into lobes. Four lobes make up the shell of each hemisphere, namely the frontal, parietal, temporal, and occipital lobes. Each lobe contains a motor or sensory map (an orderly arrangement of cortical neurons associated with muscles and sensory receptors on the body surface). The central sulcus delimits the frontal and parietal lobes. The precentral gyrus contains the motor cortex whose neurons transmit signals to motor neurons in the brainstem and spinal cord which control the muscles in the feet, legs, trunk, arms, face, and tongue of the opposite side of the body. The number of neurons for each section is determined by the fineness of control, not the size of the muscle; for example, the lips and tongue have larger areas than the trunk. Within the postcentral gyrus is the primary somatosensory cortex. Sensory receptors in the skin, muscles, and joints send messages to the somatosensory cortical cells through relays in the spinal cord and the thalamus to a map of the opposite side of the body in parallel to the map in the motor cortex. The lateral fissure separates the temporal lobe from the parietal and frontal lobes. The cortex on the inferior border of the fissure receives input relayed through the thalamus from the ears to the primary auditory cortex. The occipital lobe receives thalamic input from the eyes and functions as the primary visual cortex.

In humans, the association cortex surrounds the primary sensory and motor areas that make up a small fraction of each lobe. The occipital lobe has many specialized areas for recognizing visual patterns of color, motion, and texture. The parietal cortex has areas that support perception of the body and its surrounding personal space. Its operation is manifested by the phenomenon of phantom limb, in which the perception of a missing limb persists for an amputee. Conversely, individuals with damage to these areas suffer from sensory neglect. The temporal cortex contains areas that provide recognition of faces and of rhythmic patterns, including those of speech, dance, and music. The frontal cortex provides the neural capabilities for constructing patterns of motor behavior and social behavior. It was the rapid enlargement of the frontal and temporal lobes in human evolution over the past half million years that supported the transcendence of humans over other species. This is where the capacity to create works of art, and also to anticipate pain and death, is located. Insight and foresight are both lost with bilateral frontal lobe damage, leading to reduced experience of anxiety, asocial behavior, and a disregard of consequences of actions.

A small part of frontal lobe output goes directly to motor neurons in the brainstem and spinal cord for fine control of motor activities, such as search movements by the eyes, head, and fingers, but most goes either to the striatum from which it is relayed to the thalamus and then back to the cortex, or to the brainstem from which it is sent to the cerebellum and then through the thalamus back to the cortex. In the cerebellum, the cortical messages are integrated with sensory input predominantly from the muscles, tendons, and joints, but also from the eyes and inner ears (for balance) to provide split-second timing for rapid and complex movements. The cerebellum also has a cortex and a core of nuclei to relay input and output. Their connections, along with those in the cerebral cortex, are subject to modification with learning in the formation of a working memory (the basis for learned skills). See also Memory; Motor systems.

The cerebellum and striatum do not set goals, initiate movements, store temporal sequences of sensory input, or provide orientation to the spatial environment. These functions are performed by parts of the cortex and striatum deep in the brain that constitute another loop, the limbic system. Its main site of entry is the entorhinal cortex, which receives input from all of the sensory cortices, including the olfactory system. The input from all the sensory cortices is combined and sent to the hippocampus, where it is integrated over time. Hippocampal output returns to the entorhinal cortex, which distributes the integrated sensory information to all of the sensory cortices, updates them, and prepares them to receive new sensory input. This new information also reaches the hypothalamus and part of the striatum (the amygdaloid nucleus) for regulating emotional behavior. Bilateral damage to the temporal lobe including the hippocampus results in loss of short-term memory. Damage to the amygdaloid nucleus can cause serious emotional impairment. The Papez circuit is formed by transmission from the hippocampus to the hypothalamus by the fornix, then to the thalamus, parietal lobe, and entorhinal cortex. The limbic system generates and issues goal-directed motor commands, with corollary discharge to the sensory systems that prepares them for the changes in sensory input caused by motor activity (for example, when one speaks and hears oneself, as distinct from another).

Each hemisphere has its own limbic, Papez, cortico-thalamic, cortico-striatal, and cortico-cerebellar loops, together with sensory and motor connections. When isolated by surgically severing the callosum, each hemisphere functions independently, as though two conscious persons occupied the same skull, but with differing levels of skills in abstract reasoning and language. The right brain (spatial)-left brain (linguistic) cognitive differences are largely due to preeminent development of the speech areas in the left hemisphere in most right- and left-handed persons. Injury to Broca's area (located in the frontal lobe) and Wernicke's area (located in the temporal lobe) leads to loss of the ability, respectively, to speak (motor aphasia) or to understand speech (sensory aphasia). Studies of blood flow show that brain activity during intellectual pursuits is scattered broadly over the four lobes in both hemispheres. See also Aphasia; Central nervous system; Hemispheric laterality.

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A computer's "brains" are its central processing unit. See CPU.

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The brain is a pinkish-grey, wrinkled organ that fills the skull — looking, for all the world, like a huge walnut. It is hard to believe, from its appearance, that this ugly lump of jelly contains the mechanisms of thought, perception, will, and consciousness, that it is the seat of our personality. Its 100 000 million nerve cells, each with an average of 100 00 connections from other neurons, makes the human brain the most complicated and least understood object in the known universe.

The brain and the spinal cord constitute the central nervous system. In the human embryo the brain grows from three swellings in the head end of a tube of developing nervous tissue. The frontmost swelling differentiates into the cerebral hemispheres, consisting mainly of the thalamus, the hypothalamus, the corpus striatum (involved in the control of movement) and the cerebral cortex. The two rear swellings form the brain stem (midbrain, pons, and medulla) and the cerebellum. When we look at the outside of the human brain we see little more than the cerebral cortex, which is very enlarged in humans compared with other mammals.

The brain is surrounded by protective membranes, the meninges, continuous with those covering the spinal cord. The outermost layer, the dura mater, is tough and protects the brain physically. Beneath the dura is the arachnoid mater, through which cerebral arteries and veins penetrate to reach the brain. The surface of the brain is intimately covered by the innermost layer, the pia mater, from which tiny blood vessels plunge into the cortex. A clear fluid, cerebrospinal fluid (CSF), which is secreted inside cavities called cerebral ventricles, within the brain, circulates in the subarachnoid space between the arachnoid and the pia. CSF protects the brain, both physically and chemically. The brain, hungry for oxygen and glucose, receives its blood through a rich system of arteries derived from two major sources, the internal carotid arteries and the vertebral arteries. Sudden blockage or haemorrhage in an artery (a stroke) can have catastrophic consequences, including almost immediate loss of consciousness or function, and even death.

The adult human brain weights about 1400 g, but there is much individual variation. The side view of the brain is dominated by the highly convoluted cerebral hemispheres, with the brain stem protruding from below, bearing the cerebellum on its back. The axis of each cerebral hemisphere, from the frontal pole, back to the occipital pole, and then down and around to the temporal pole, forms a C-shape — a reminder of the folding process that occurs during embryological development. Each hemisphere is divided into four lobes. On the surface of the lobes are variously named convolutions or gyri, with fissures, or sulci, some of them very deep, separating the gyri (see Fig. 1). The exact pattern of fissures varies enormously from brain to brain, and even between the two hemispheres, but some are very distinctive. The lateral sulcus, one of the first to appear in the embryo, divides the frontal from the temporal lobe. Likewise, the central sulcus divides the frontal from the parietal lobe. The rearmost of the four lobes is the occipital lobe, but there is no sulcus to define its limit on the lateral surface. The two hemispheres, roughly mirror-images of each other, are separated by the huge Sylvian fissure described in 1660 by Franciscus Sylvius, a physician and anatomist in Leyden.

Fig. 1 (a) the whole brain from the left side; (b) a mid-line section
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If a cut is made into the depth of the Sylvian fissure, dividing the brain in two, a complex series of structures is revealed on the inner surface of the hemisphere (Fig. 1b). Most apparent is the corpus callosum (Latin: ‘beam-like body’), a massive tract of nerve fibres (axons) connecting the 2 hemispheres. Like the side view of the entire hemisphere, the cut corpus callosum appears as an upside-down C-shape. So too does a smaller longitudinal fibre tract below it called the fornix (Latin for ‘arch’ — Roman prostitutes fornicated beneath the arches!). Just below this is a hole, leading into the cerebral ventricles. This interventricular foramen communicates between the third ventricle (a midline cavity with the thalamus in its wall) and the lateral ventricle, deep within the hemisphere. The third ventricle dips down between the hypothalamus of each side, below which we can see the pituitary gland. The thalamus joins to the brain stem below. The intricate folded pattern of the cerebellum fills most of the space between the bottom of the occipital lobe, above, and the upper surface of the brain stem, below. Beneath the cerebellum the tent-shaped fourth ventricle is visible, communicating at this level with the subarachnoid space around the brain. The fourth ventricle communicates with the third ventricle via a narrow tube, the cerebral aqueduct, which runs up through the midbrain.

The cerebral hemispheres consist of a thin outer rind of grey matter, containing mainly the bodies of nerve cells (neurons), surrounding a core of white matter (named after the whitish colour of the axons of the neurons). Deep within the hemispheres are a number of important cell groups (nuclei), as well as the ventricular system. Axons arising in the cerebral cortex and those running to it traverse the internal capsule, a thick band of white matter in each hemisphere. The largest of the deep nuclei is the corpus striatum (named because of its striped appearance when cut), which is of vital importance in integration of muscular action. Another mass of grey matter behind the corpus striatum is the thalamus, which lies in the walls of the third ventricle. It is a relay station for sensory and motor pathways on their way to the cerebral cortex. Just below the thalamus is the hypothalamus. Although small, it is one of the most important parts of the brain, for it participates in a number of vital activities. It regulates a variety of hormonal functions by direct action on the pituitary gland, and exerts control over the autonomic nervous system, the ‘vegetative’ part of the nervous system, which controls the involuntary activity of, for example, our gastrointestinal tract, heart, and blood vessels.

The hypothalamus is also an integral part of the limbic system (‘limbus’ is Latin for a border, and the limbic system forms an almost circular boundary to the inner surface of the cerebral hemisphere). The limbic system is involved in vital cyclical activity — including appetites and sexual cycles, and emotions such as fear, anger, and aggression — and in all-important short-term memory. It involves not only the hypothalamus but also the thalamus, part of the cerebral cortex called the hippocampus (Latin for ‘sea-horse’, because of its shape), and their interconnections. The hippocampus sends its axons backwards in the fornix, which then curves forward, like an arch, to meet the fornix of the other side, ending in the mamillary bodies of the hypothalamus. A tract then conveys axons up to the thalamus, which then sends fibres indirectly to the hippocampus again. So the circuit is completed.

The corpus striatum receives information from the cerebral cortex, the thalamus, and a nucleus called the substantia nigra (‘black substance’), in the midbrain. In Parkinson's disease, the cells in the substantial nigra that project to the corpus striatum degenerate and this leads to problems with motor control and co-ordination (muscle rigidity and tremor).

The cerebral cortex is one of the major features of the mammalian brain, and especially in humans it reaches a very high level of development. It is responsible for the initiation of movements, and for interpreting input from all our sensory systems, as well as for integrating motor and sensory activity necessary for speech and other cognitive functions. It is the seat of our very thoughts, personality, and character.

The cerebellum has on its surface a series of tight folds, called folia, similar to, but narrower than, the gyri of the cerebral cortex. The cerebellum consists mainly of two hemispheres that receive their major input from the spinal cord and the cerebral cortex. However, a small, but important, part receives information from the vestibular system, the apparatus in the inner ear that signals information about our position in space and, therefore, helps us balance ourselves. The cerebellum is responsible for unconscious control of motor activity. Although voluntary movement is thought to be initiated in the cerebral cortex, the cerebellum guides such movements. Further, it is involved in learning new skills of movement, often a painfully frustrating business. For instance, when we learn to drive a car, our initial attempts are clumsy and full of errors. We have to learn to co-ordinate movements of hand, eye, and foot in order to turn the key and to control gears, brake lever and accelerator, and clutch and brake peddles so that the vehicle is set in motion and safely stopped again. At first, the whole process demands huge mental effort, as if we were using our cerebral cortex consciously to call up the various movements and muscle groups we need. However, after many attempts, our efforts become smoother and less laborious, and we find that we are achieving the desired results with much less stalling of the motor or threat to the bodywork. Later still, we discover that we can drive around without really thinking about it much, and we are sometimes surprised, if distracted by other preoccupations, to realize that we are on the road and driving safely without clear memories of starting the vehicle and getting under way. We have successfully completed a motor ‘apprenticeship’, with the cerebellum taking over the routine management of the task from the cerebral cortex. It is as if the cerebellum were a programmable computer controlling the output of the motor system, and its programs have been slowly improved to take more and more change of the operation. Think of learning to play a sport or a musical instrument; but think also of walking, talking, and writing. In all these, and many more activities, we can look upon the first, hesitant steps as being essentially cortical, while the final, polished result is more cerebellar.

The brain stem extends between the thalamus and the spinal cord, gradually decreasing in size and in the complexity of its internal structure. It is divided, from top to bottom, into the midbrain, the pons (bridge), and the medulla oblongata (usually simply referred to as the medulla). The entire brain stem is largely hidden from view by the highly developed masses of the cerebral and cerebellar hemispheres. The midbrain is attached to the base of the cerebral hemispheres by the cerebral peduncles, two massive, flattened bundles of nerve fibres. The longitudinal orientation of the cerebral peduncles is abruptly interrupted by the pons, which gives the impression of a giant ring, slipped on to the brain stem between the peduncles and the medulla. The medulla merges gradually with the spinal cord.

The brain stem contains much white matter, with ascending and descending tracts that can be traced in continuity with those of the spinal cord, including various sensory pathways from the skin and organs, and the corticospinal or pyramidal tract, conveying motor information from the cerebral cortex down to the spinal cord. There are also various groups of neurons (nuclei) within the brain stem. Several of these give rise to the cranial nerves, through which the brain sends and receives information to and from the head and the organs of the trunk. Other groups of brain stem neurons are vital to the life of the body and to the conscious function of the brain: they generate the rhythmic nerve impulses that maintain breathing, regulate the heart and circulation, and activate the cerebral cortex itself.

Investigation of brain function

Compared with the pulsating heart or blood-filled liver, the brain looks rather unimpressive. No wonder, then, that many ancient cultures chose those other organs as their assumed seat of the mind or soul. Now that we generally accept that the brain is responsible for action, perception, and understanding, one of the greatest scientific challenges is to explain how it works.

The clues to the functions of the brain were once provided only by ‘nature's experiments’: the consequences of damage caused by disease or injury. The advent of anaesthesia allowed investigation of the effects in animals of more precisely localized damage and of the responses to electrical stimulation at particular sites. The development of microelectrode techniques made it possible to record the electrical activity of individual neurons in the brains of anaesthetized animals, or in isolated slices of brain tissue. The human brain has been stimulated during neurosurgery under local anaesthetic, and the resulting movements of the body and sensations described by the patient have identified particular regions concerned with motor and sensory function. Electrical activity can also be recorded from the human brain through electrodes on the scalp (electroencephography). Finally, new technologies developed in the twentieth century provided ways of ‘mapping’, non-invasively, the function of the living human brain. These imaging techniques can show, for example, the regional distribution of blood flow or metabolic activity, reflecting neuronal activity in the various parts of the brain during different actions or sensations. Thus they are assisting in the understanding of healthy function, as well as in the diagnostic localization of abnormalities.

— Laurence Garey

See nervous system. See also brain stem; central nervous system; cerebral cortex; cerebral ventricles; cerebrospinal fluid; hypothalamus; imaging techniques; magnetic brain stimulation; thalamus.

Traditionally the brains of sheep and calves are stewed and eaten; probably not advisable because of the risk of transmitting the agents responsible for various degenerative brain diseases, including scrapie and bovine spongiform encephalopathy (BSE).

noun

1. The seat of the faculty of intelligence and reason: head, mind. Informal gray matter. See thoughts.
2. The faculty of thinking, reasoning, and acquiring and applying knowledge. brainpower, intellect, intelligence, mentality, mind, sense, understanding, wit. Slang smart (used in plural). See ability/inability, thoughts.
3. A person of great mental ability: intellect, intellectual, mind, thinker. See ability/inability.

Idioms beginning with brain:
brain someone

See also beat one's brains out; blow one's brains out; on one's mind (the brain); pick someone's brains; rack one's brains.

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n

Definition: mind, intelligence
Antonyms: body, physicality

n

Definition: very smart person
Antonyms: dumbo, dumdum, simpleton

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For more information on brain, visit Britannica.com.

The brain contains something over 10¹¹ neurons, each connected to something over three thousand others; this makes something over 10¹⁴ connections. If each connection is capable of ten different ‘weights’ or levels of activation, then the number of distinct brain states possible is 10 to the power of 10¹⁴. By comparison, the number of elementary particles in the universe is estimated at a miserly 10⁸⁷. The progress of neuroscience in understanding brain function increases the urgency of reconciling the scientific view of a person as a conglomerate of connected cells, with the personal view of a unified, conscious, single self subject to experiences and capable of rational and voluntary action. If this reconciliation cannot be managed, then either the scientific view drives out the personal view (see eliminativism), or we end with some kind of dualism whereby the mental is different from and additional to the physical. Reconciling approaches include functionalism and physicalism, both hoping to show how mental explanation of events is a consistent supplement to their physical explanation, not a rival to it.

That part of the central nervous system contained within the skull. See also cortex, cerebellum, and medulla oblongata.

Brain
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The central organ in the nervous system, protected by the skull. The brain consists of the medulla, which sends signals from the spinal cord to the rest of the brain and also controls the autonomic nervous system; the pons, a mass of nerve fibers connected to the medulla; the cerebellum, which controls balance and coordination; and the cerebrum, the outer layer of which, the cerebral cortex, is the location of memory, sight, speech, and other higher functions.

The cerebrum contains two hemispheres (the left hemisphere and the right hemisphere), each of which controls different functions. In general, the right hemisphere controls the left side of the body and such functions as spatial perception, whereas the left hemisphere controls the right side of the body and functions such as speech.

Under the cerebral cortex are the thalamus, the main relay center between the medulla and the cerebrum; and the hypothalamus, which controls blood pressure, body temperature, hunger, thirst, sex drive, and other visceral functions.

Encephalon; that part of the central nervous system contained within the cranium, comprising the forebrain, midbrain and hindbrain, and developed from the embryonic neural tube. It is connected at its base with the spinal cord. The brain is a mass of soft, pinkish gray nerve tissue. For specific brain diseases see under headings relating to etiology and lesion.

• b. abscess — common signs caused by an abscess in the brain are circling, rotation of the head, abnormal reflexes in one eye. The CSF may show evidence of infection.
• b. aneurysm — see berry aneurysm.
• b. anoxia — acute or chronic insufficiency of the blood supply to the brain causes anoxia which causes clinical signs that vary with the severity of the deprivation. Acute anoxia causes muscle tremor, recumbency, convulsions and death or recovery if the anoxia is relieved soon enough. Chronic anoxia causes lethargy, weakness, blindness and sometimes convulsions. In either case there may be permanent damage.
• b. case — the cranium.
• b. cestodal cyst — see coenurosis.
• b. coup lesion — a derivation from contrecoup.
• b. dead — irreversible coma with apnea, loss of all brainstem reflexes and absence of activity on an electroencephalogram.
• b. decompression — relieving the pressure within the cranial vault. This may be done surgically by opening the cranium, or medically by administering hypertonic solutions of slowly metabolized materials, such as mannitol, intravenously.
• b. edema — an important part of a number of acute diseases, e.g. lead poisoning, encephalitis, salt poisoning in swine, polioencephalomalacia of ruminants and hypoxia due to any cause. Clinically manifested by blindness, opisthotonos, nystagmus, recumbency and tonic convulsions. Inherited in polled and horned Herefords; calves are recumbent at birth and are never able to stand but consciousness is normal. See also neuraxial edema.
• b. ependymal lining — see ependyma.
• b. hematoma — may occur with trauma, in extradural, subdural or intraparenchymal locations. They can cause progressive increase in intracranial pressure and eventually death.
• b. hemorrhage — intracranial hemorrhage affecting the brain usually follows traumatic injury but spontaneous hemorrhage may result from an intrinsic vascular lesion. Loss of consciousness is a common sign followed by residual signs depending on the locality and size of the hemorrhage. Ataxia and convulsions are common sequelae.
• b. herniation — displacement of brain from the cranial vault through the foramina (tentorial notch or foramen magnum) or ventral to dural septae. The usual causes are brain edema or hemorrhage with resulting increase in intracranial pressure.
• b. hypoxia — see brain anoxia (above).
• b. infarction — see feline ischemic encephalopathy.
• b. inflammation — see encephalitis, encephalomyelitis, meningoencephalitis.
• b. ischemia — see brain anoxia (above).
• b. laceration — occurs in cranial trauma that fractures the skull, causes severe acceleration or deceleration, or penetrates the skull and brain tissue.
• b. necrosis — see encephalomalacia.
• b. pigmentation — occurs in phalaris spp. poisoning; a characteristic greenish brown color grossly of the gray matter in brainstem nuclei and spinal cord, caused by a suspected lysosomal storage of granules of pigment material; usually associated with some degree of Wallerian degeneration within spinal cord tracts.
• b. sand — see acervuli.
• b. scanning — a radiographic, magnetic or nuclear medical procedure for the detection of brain tumors, abscesses, hematomas and other intracranial lesions. Not widely used in veterinary medicine because of the expensive equipment required.
• b. spongy degeneration — see bovine spongiform encephalopathy.
• b. staggers — see dummy.
• b. trauma — injury to the brain, including that caused by migrating worm larvae, will have diffuse effects including the development of edema, and local effects due to pressure by displaced bone or to hemorrhage. Initial shock, manifested as unconsciousness, is likely to be followed by residual localizing signs, e.g. facial paralysis, head rotation.
• b. tumors — cause signs suggestive of local space-occupying lesion in the cranial cavity, including the increased intracranial pressure syndrome, blindness with disturbance of ocular reflexes, head rotation, circling and jacksonian epileptic episodes.
• b. ventricles — see third, fourth, fifth ventricle.

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A cynical view of the world by Ambrose Bierce

n.

An apparatus with which we think what we think. That which distinguishes the man who is content to be something from the man who wishes to do something. A man of great wealth, or one who has been pitchforked into high station, has commonly such a headful of brain that his neighbors cannot keep their hats on. In our civilization, and under our republican form of government, brain is so highly honored that it is rewarded by exemption from the cares of office.

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IN BRIEF: The "command center" of the nervous system, located in the head.

The human brain is very large compared to most animals.