This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

The Cerebral Cortex

The cerebrum's outermost layer is called the cerebral cortex. This layer averages 3.2 mm thick in human beings, less than a quarter inch. The layer of cortex covers the surface of the cerebrum and goes down into the fissures.

What is the cerebral cortex? What does "cortex" mean? What makes gray matter gray and white matter white?

The word cortex means rind or shell in Latin and refers to a dense outer layer of neurons on any brain structure, not just the cerebrum. (Smaller brain structures like the thalamus and cerebellum also have cortical layers.)

When a brain is preserved, cortical areas (areas of cortex) appear gray because they contain dense concentrations of cell bodies. This is called gray matter ("grey" matter in Britain and most of the rest of the world; "gray" is the American spelling). Technically, that phrase refers only to the cortical areas. Other brain tissue is called white matter : It consists of communication lines: neural fibers called axons which run from the cells in the cortical layers to other parts of the brain. The axons appear white because in this area of the brain they are coated with a fatty substance, myelin.

How large is the cortical area, if unfolded?


Gray and White Matter

A slice of brain tissue is shown above. You can see how the gray matter extends down into the fissures. This is significant because it creates a large surface area on the brain for nerve cell bodies. The cerebral cortex, if unfolded, would have about the same surface area as a small umbrella.

What does a PET scan usually show, during complex activity?

The cerebral cortex has many specialized areas that work together in ordinary thought processes. Brain scans typically show that several widely separated areas of the cortex are active simultaneously when a mental task is performed. Often there is a characteristic pattern. For example, the same five or six different areas may light up each time a person perceives pain.

What are examples of the brain's "incredible" adaptability?

The cerebral cortex is important to humans, but people can live without major portions of it, especially if the damage is done early in life. Babies are sometimes born with half the cerebral cortex missing, yet they grow up to have normal mental abilities. Hydrocephalics, who suffer compression of brain tissue due to fluid accumulation in the brain, can have normal intellects even in severe cases where fluid occupies 95% of the skull. "One of these patients gained a first-class honors degree (Sheffield University) in mathematics and is socially completely normal. And yet...has virtually no brain" (Lewin, 1980, p.1232, quoted in Maratsos & Matheny, 1994).


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