Figure 12-4. Human brain; bottom view.
c. The Cerebrum. Attached to the forebrainstem are the two cerebral
hemispheres (Figure 12-5). Together, these two hemispheres make up the cerebrum.
Among related species, the cerebrum is the newest development of the brain.
(1) Cerebral hemispheres. The cerebrum consists of two cerebral
hemispheres, right and left. They are joined together by a very large fiber tract known
as the corpus callosum (the great commissure).
(2) Lobes. Each hemisphere can be divided into four lobes. Each lobe is
named after the cranial bone it lies beneath--parietal, frontal, occipital, and temporal.
(Actually, there are five lobes. The fifth is hidden at the bottom of the lateral fissure. It
is known as the insula or insular lobe. It is devoted mainly to visceral activities.)
(3) Gyri and sulci. The cerebral cortex, the thin layer at the surface of each
hemisphere, is folded. This helps to increase the amount of area available to neurons.
Each fold is called a gyrus. Each groove between two gyri is called a sulcus.
(a) The lateral sulcus is a cleft separating the frontal and parietal lobes
from the temporal and occipital lobes. Therefore, the lateral sulcus runs along the
lateral surface of each hemisphere.