BRAIN

the concentration of nerve tissue in the anterior end of an animal's body. The brain integrates sensory information and directs motor responses, thus playing a major role in vital instinctive activities; in higher vertebrates it is also the centre of learning. (See also nervous system, human.) In the lower vertebrates the brain is tubular and resembles an early developmental stage of the brain in higher vertebrates. It consists of three distinct regions: the hindbrain, the midbrain, and the forebrain. Although the brain of higher vertebrates undergoes considerable modification during embryonic development, these three regions are still discernible. The hindbrain is composed of the medulla oblongata and the pons. The medulla transmits all signals between the spinal cord and the higher parts of the brain; it also governs such autonomic functions as heartbeat and respiration. The pons is partly made up of tracts connecting the spinal cord with higher brain levels, and it also contains cell groups that transfer information from the cerebrum to the cerebellum. The midbrain, the upper portion of which evolved from the optic lobes, serves as the main centre of sensory integration in fish and amphibians. It also plays a major integration role in reptiles and birds. In mammals the midbrain is greatly reduced, serving primarily as a connecting link between the hindbrain and the forebrain. Connected to the medulla, pons, and midbrain by large bundles of nerves is the cerebellum. Relatively large in humans, this little brain controls balance and coordination by smoothing out the actions of muscle groups. The forebrain includes the cerebral hemispheres and, under these, the diencephalon, which contains the thalamus and hypothalamus. The thalamus is the main relay centre between the medulla and the cerebrum; the hypothalamus is an important control centre for sex drive, pleasure, pain, hunger, thirst, blood pressure, body temperature, and other visceral functions. The hypothalamus produces releasing hormones that govern the secretions of the anterior pituitary, and it also produces oxytocin and the antidiuretic hormone, which are stored in and released by the posterior pituitary. The cerebrum, originally functioning as part of the olfactory lobes, is involved with the more complex functions of the human brain. In humans and other advanced vertebrates, the cerebrum has grown over the rest of the brain, forming a convoluted (wrinkled) layer of gray matter. The degree of convolution is partly dependent on the size of the body. Small mammals (e.g., lesser anteater, marmoset) generally have smooth brains, and large mammals (e.g., whale, elephant, dolphin) have highly convoluted ones. The cerebral hemispheres are separated by a deep groove, the longitudinal cerebral fissure. At the base of this fissure lies a thick bundle of nerve fibres, called the corpus callosum, which provides a communication link between the hemispheres. The left hemisphere controls the right half of the body, and vice versa, because of a crossing of the nerve fibres in the medulla or, less commonly, in the spinal cord. Although in many ways the right and left hemispheres are mirror images of one another, there are important functional distinctions. In most people, for example, the areas that control speech are located in the left hemisphere, while areas that govern spatial perceptions reside in the right hemisphere. Two major furrowsthe central sulcus and the lateral sulcusdivide each cerebral hemisphere into four sections: the frontal, parietal, temporal, and occipital lobes. The central sulcus, also known as the fissure of Rolando, also separates the cortical motor area (which is anterior to the fissure) from the cortical sensory area (which is posterior to the fissure). Starting from the top of the hemisphere, the upper regions of the motor and sensory areas control the lower parts of the body, and the lower regions of the motor and sensory areas control the upper parts of the body. Other functional areas of the cerebral hemispheres have been identified, including the visual cortex in the occipital lobe and the auditory cortex in the temporal lobe. A large amount of the primate cortex, however, is devoted to no specific motor or sensory function; this so-called association cortex is apparently involved in higher mental activities. It makes up the largest share of the human cortex.