3-33. THE BRAIN
The brain, or encephalon (figures 3-18 and 3-19), is the center of the nervous
system. It fills the cranial cavity of the skull and weighs about 1,300 grams. It is the
most complex mass of nervous tissues and serves as the coordinating center for all
somatic and visceral functions of the body. As noted above, the adult brain develops
from three embryonic regions: the forebrain, the midbrain, and the hindbrain.
a. The Forebrain. The forebrain, or prosencephalon, of the embryo develops
into the cerebrum and the diencephalon, which consists of the thalamus, and the
hypothalamus.
(1) The cerebrum, also known as the telencephalon, is the main portion of
the brain, constituting about seven-eighths of its total weight. The deep longitudinal
fissure divides it into right and left cerebral hemispheres. These hemispheres consist
essentially of an outer region of gray matter, the cerebral cortex, which spreads over an
inner mass of white matter. Several bands of nerve tissue, called commissures,
connect the two hemispheres. The corpus callosum, also known as the great
commissure of the brain, is the most prominent. It consists of bands of nerve fibers that
form an arch below the longitudinal fissure and connect the two hemispheres. The
hemispheres are divided into lobes, named after the bones of the cranium in which they
lie. Although the functioning of the cerebrum is not completely understood and most of
its coordinating activities involve more than one lobe, it has been found that certain
functions are localized in the four lobes as follows: frontal lobe (higher thought
processes and voluntary actions); parietal lobe (sensory and motor functions); temporal
lobe (hearing); and occipital lobe (sight).
(2) The diencephalon, which consists of the thalamus and hypothalamus, is
located between the cerebrum and midbrain. The thalamus is a large ovoid mass of
gray matter located at the base of the cerebrum. It consists of two large nuclear
masses separated by the third ventricle (figure 3-16) and connected by a bridge of
gray matter, the connexus interthalamicus, also called the intermediate mass. It serves
as a relay center for sensory impulses en route to the cerebral cortex. The
hypothalamus is inferior to the thalamus and forms the floor and part of the lateral wall
of the third ventricle. It is involved in the regulation of many metabolic processes and
the functioning of certain viscera.
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