The living human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, it is a 3 lb (1.4 kg) mass of nerve tissue that keeps us alive and
A comparison of the brains of an earthworm, an insect, a bird, and a human. Illustration by Hans & Cassidy. Courtesy of Gale Group.
functioning. On average, the brain weighs 13.7 oz (390 g) at birth, and by age 15 grows to approximately 46 oz (1,315 g). The human brain is composed of up to one trillion nerve cells—100 billion of them are neurons, and the remainder are supporting (glial) cells. Neurons receive, process, and transmit impulses, while glial cells (neuroglia) protect, support, and assist neurons. The brain is protected by the skull and by three membranes called the meninges—the outermost the dura mater, the middle the arachnoid, and the innermost the pia mater. Also protecting the brain is cerebrospinal fluid, a liquid that circulates between the arachnoid and pia mater in the subarachnoid space. 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 capillary walls are believed to create a blood-brain barrier that protects the brain from a number of biochemicals circulating in the blood.
The parts of the brain can be studied in terms of structure and function. Four principal sections of the human brain are the brain stem (the hindbrain and midbrain), the diencephalon, the cerebrum, and the cerebellum.