A computer... actually... The brain, along 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 digestion. The human brain is a soft, shiny, grayish-white, mushroom-shaped structure. The brain of an average adult weighs about 3 lb (1.4 kg). At birth, the average infant s brain weighs 13.7 oz (390 g); by age 15, it has nearly reached full adult size. The brain is protected by the skull and a three-layered membrane called the meninges. The brain s surface is covered with many bright red arteries and bluish veins that penetrate inward. Glucose, oxygen, and certain ions pass easily across the blood-brain barrier into the brain, although other substances, such as antibiotics, do not. The four principal sections of the human brain are: the brain stem, the diencephalon, the cerebrum (divided into two large paired cerebral hemispheres), and the cerebellum. http://www.minddisorders.com/A-Br/Brain.html http://www.yourdictionary.com/brain http://medical-dictionary.thefreedictionary.com/Human+brain+cell