Part of the central nervous system located in the skull. Controls mental and physical actions of the organism.

The brain, with the spinal cord and network of nerves, controls information flow throughout the body, voluntary actions, such as walking, reading, and talking, and involuntary reactions, such as breathing and heartbeat. The human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, the brain of an average adult weight about 3 lb (1.4 kg). At birth, the average human infant's brain weighs 13.7 oz (390 g); by age 15, the brain has nearly reached full adult size. The brain is protected by the skull and by a threelayer membrane called the meninges. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics, do not. The four...