The two hemispheres are connected mainly by a thick bundle of nerve fibers called the corpus callosum, or “hard body,” because of its tough consistency. A smaller bundle, the anterior commissure, connects just the two temporal lobes. Although the corpus callosum is a good landmark for students of brain anatomy, its contribution to behavior has been difficult to pin down. Patients in whom the corpus callosum has been severed (a way of ameliorating epilepsy by restricting seizures to one side of the brain) go about their everyday business without impairment. Careful testing does turn up a gap between sensations processed by the right brain and the language centers of the left brain—for instance, a person with a severed corpus callosum is unable to name an object placed unseen in the left hand (because stimuli perceived by the left half of the body are processed in the right hemisphere). On the whole, though, it appears that the massive crossing-over of nerve fibers that takes place in the brainstem is quite adequate for most purposes, at least those related to survival.
Although the cerebral cortex is quite thin, ranging from 1.5 to 4 millimeters deep (less than 3/8 inch), it contains no fewer than six layers. From the outer surface inward, these are the molecular layer, made up for the most part of junctures between neurons for the exchange of signals; the external granular layer, mainly interneurons, which serve as communicating nerve bodies within a region; an external pyramidal layer, with large-bodied “principal” cells whose axons extend into other regions; an internal granular layer, the main termination point for fibers from the thalamus; a second, internal pyramidal layer, whose cells project their axons mostly to structures below the cortex; and a multiform layer, again containing principal cells, which in this case project to the thalamus. The layers vary in thickness at different sites on the cortex; for example, the granular layers (layers 2 and 4) are more prominent in the primary sensory area and less so in the primary motor area.
Extensive and intricate as the human brain is, and with the almost limitless variation of which it is capable, it is built from relatively few basic units. The fundamental building block of