In higher animals, a mass of interconnected nerve cells forming the anterior part of the central nervous system, whose activities it coordinates and controls. In vertebrates, the brain is contained by the skull.
The brain is composed of three main regions. At the base of the brainstem, the medulla oblongata contains centres for the control of respiration, heartbeat rate and strength, blood pressure, and thermoregulatory control (the control of body temperature) – all examples of involuntary bodily functions. Overlying this is the cerebellum, which is concerned with coordinating complex muscular processes such as maintaining posture and moving limbs, and the control of balance. The cerebrum (cerebral hemispheres) are paired outgrowths of the front end of the forebrain, in early vertebrates mainly concerned with the senses, but in higher vertebrates greatly developed and involved in the integration of all sensory input and motor output, and in thought, emotions, memory, and behaviour. Sensory information arrives in the cerebrum in the form of nerve impulses that come from receptors – these may be found in sense organs, such as cones (light-sensitive cells) in the retina of the eye, which send impulses to the brain along the optic nerve. The cerebrum processes the information received and can cause impulses to be sent out to the body to produce a response, such as moving towards an object that has been seen. Because a decision is made whether to make this kind of movement, it is an example of a voluntary function.
In vertebrates, many of the nerve fibres from the two sides of the body cross over as they enter the brain, so that the left cerebral hemisphere is associated with the right side of the body and vice versa. In humans, a certain asymmetry develops in the two halves of the cerebrum. In right-handed people, the left hemisphere seems to play a greater role in controlling verbal and some mathematical skills, whereas the right hemisphere is more involved in spatial perception. In general, however, skills and abilities are not closely localized. In the brain, nerve impulses are passed across synapses by neurotransmitters, in the same way as in other parts of the nervous system.
In mammals the cerebrum is the largest part of the brain, carrying the cerebral cortex. This consists of a thick surface layer of cell bodies (grey matter), below which fibre tracts (white matter) connect various parts of the cortex to each other and to other points in the central nervous system. As cerebral complexity grows, the surface of the brain becomes convoluted into deep folds. In higher mammals, there are large unassigned areas of the brain that seem to be connected with intelligence, personality, and higher mental faculties. Language is controlled in two special regions usually in the left side of the brain: Broca's area governs the ability to talk, and Wernicke's area is responsible for the comprehension of spoken and written words.
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