FoundationalConceptsNeuroscience

gyri (singular: gyrus) and sulci (singular: sulcus), respectively, and area
consequence of the cerebrum in many mammals being a highly folded
structure. While mice, rats, and squirrels do not exhibit this cerebral
folding, capybaras (the world’s largest rodents), as well as dogs, cats,
and primates, all have folding of the outer layer of the cerebrum. This
outer layer is called the cerebral cortex (cortex in Latin means bark, or
outer layer).
In the human brain, the cerebral cortex is actually a sheet of neural
tissue, around 3 millimeters thick (about 1/8 inch), with its thickness
varying from region to region. The sheet is highly folded so that its
large size can fit inside the skull. If the cerebral cortex could be care-
fully removed from the human brain, unfolded and laid flat, it would
be about the size of a small newspaper opened up, around 2.5 square
feet.
Figure 2.7 shows two views of the human brain: dorsal (top) and
lateral (side). Many gyri and sulci are visible. Some prominent features
are indicated. Among these are the four lobes of the cerebral cortex:
frontal, parietal, occipital, and temporal. There are actually eight
lobes, four in the right hemisphere and four in the left hemisphere.
There are several prominent landmark grooves: the central sulcus
separates the frontal lobe from the parietal lobe, the lateral fissure
separates the temporal lobe from the frontal and parietal lobes, and
the very prominent longitudinal fissure divides the right and left
cerebral hemispheres. The boundary between the occipital lobe and
the temporal and parietal lobes is not so clear at the gross anatomical
level. Figure 2.8 gives two more views of the human brain: ventral
(bottom or underside) and medial (inside, revealed by cutting the
brain in half down the middle, along the longitudinal fissure). Cranial
nerves are described in Chapter 7. The corpus callosum is a bundle of
approximately 200 million nerve fibers connecting the right and left