es traversing the pons pass on to the cerebellum (or “little brain”), which is concerned primarily with the coordination of complex muscular movement. In addition, nerve fibers running through the pons relay sensations of touch from the spinal cord to the upper brain centers.

Many nerves for the face and head have their origin in the pons, and these nerves regulate some movements of the eye-ball, facial expression, salivation, and taste. Together with nerves of the medulla, nerves from the pons also control breathing and the body's sense of equilibrium.

What had been the middle bulge in the neural tube develops into the midbrain, which functions mainly as a relay center for sensory and motor nerve impulses between the pons and spinal cord and the thalamus and cerebral cortex. Nerves in the midbrain also control some movements of the eyeball, pupil, and lens and reflexes of the eyes, head, and trunk.

THALAMUS AND HYPOTHALAMUS

Deep in the core area of the brain, just above the top of the brainstem, are structures that have a great deal to do with perception, movement, and the body's vital functions. The thalamus consists of two oval masses, each embedded in a cerebral hemisphere, that are joined by a bridge. The masses contain nerve cell bodies that sort information from four of the senses—sight, hearing, taste, and touch—and relay it to the cerebral cortex. (Only the sense of smell sends signals directly to the cortex, bypassing the thalamus.) Sensations of pain, temperature, and pressure are also relayed through the thalamus, as are the nerve impulses from the cerebral hemispheres that initiate voluntary movement.

The hypothalamus, despite its relatively small size (roughly that of a thumbnail), controls a number of drives essential for the functioning of a wide-ranging omnivorous social mammal. At the autonomic level, the hypothalamus stimulates smooth muscle (which lines the blood vessels, stomach, and intestines) and receives sensory impulses from these areas. Thus it controls the rate of the heart, the passage of food through the alimentary canal, and contraction of the bladder.

The hypothalamus is the main point of interaction for the



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