Diencephalon

The diencephalon develops from the back of the forebrain and is located between the cerebral hemispheres and above the midbrain. It generally surrounds the third ventricle and is composed largely of gray matter organized into nuclei. Among these, a dense nucleus, called the "thalamus," bulges into the third ventricle from each side. Another region that includes many nuclei is the "hypothalamus," which lies below the thalamic nuclei and forms the lower walls and the floor of the third ventricle. Other parts of the diencephalon include (1) the "optic tracts" and "optic chiasma," which is formed by the optic nerve fibers crossing over; (2) the "infundibulum," a conical process behind the optic chiasma to which the pituitary gland is attached; (3) the "posterior pituitary gland," which hangs from the floor of the hypothalamus; (4) the "mammillary bodies," which appear as two rounded structures behind the infundibulum; and (5) the "pineal gland," which forms in a cone-shape from the roof of the diencephalon. Above the hypothalamic sulci (fissures) the side of the walls of the third ventricle are interconnected across the cavity of the ventricule by extensions of gray matter that form the "interthalamic adhesion." The "thalamus" serves as a central relay station for sensory impulses ascending from other parts of the nervous system to the cerebral cortex. It receives all sensory impulses (except those associated with the sense of smell) and channels them to appropriate regions of the cortex for interpretation. In addition, all regions of the cerebral cortex communicate with the thalamus by means of descending fibers. The thalamus seems to produce a general awareness of certain sensations such as pain, touch, and temperature. The "hypothalamus" serves as a link between the nervous and endocrine systems, and is connected to the cerebral cortex, thalamus, and other parts of the brain stem so that it can receive impulses from them and send impulses to them.