The hippocampus is another major structure of the limbic system. Named for its fanciful resemblance to the shape of a sea horse, the hippocampus is located at the base of the temporal lobe near several sets of association fibers. These are bundles of nerve fibers that connect one region of the cerebral cortex with another, so that the hippocampus, as well as other parts of the limbic system, exchanges signals with the entire cerebral cortex. The hippocampus has been shown to be important for the consolidation of recently acquired information. (In contrast, long-term memory is thought to be stored throughout the cerebral cortex. The means by which short-term memory is converted into long-term memory has posed a particularly challenging riddle that only now is beginning to yield to investigation; see Chapter 8 .)
Recent work with a variety of animals has found dense clusters of receptor sites for tetrahydrocannabinol, the active ingredient of marijuana and related drugs, in the hippocampus and other nearby structures of the limbic system. This localization helps explain the effects of marijuana, which range from mild euphoria to wavering attention to temporarily weakened short-term memory. A loss of short-term memory is also seen in certain syndromes of alcoholism and in Alzheimer 's disease, which involves some degeneration of the hippocampus and other limbic structures.
The cerebral cortex occupies by far the greatest surface area of the human brain and presents its most striking aspect. Also known as the neocortex, this is the most recently evolved area of the brain. In fact, the enormous expansion in the area of the cerebral cortex is hypothesized to have begun only about 2 million years ago, in the earliest members of the genus Homo; the result today is a brain weighing approximately three times more than would be expected for a mammal our size. The cortex is named for its resemblance to the bark of a tree, because it covers the surface of the cerebral hemispheres in a similar way. Its wrinkled convoluted appearance is due to a