The Animal Self: Neurobehavioral Correlates 143“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitystem.” An animal with the forebrain separated from the brain stem loses
consciousness but can survive for some time.
Structures anterior to the midbrain are collectively called the
forebrain, commonly known as the cerebrum or the “brain” proper in
higher animals. It is made up of a mantle of gray matter (cerebral cor-
tex) and a deeper part of the brain called white matter. The gray mat-
ter comprises neuronal cell bodies and their unmyelinated processes,
whereas the white matter consists mainly of nerve tracts of myelinated
axons. Embedded in the white matter are pockets of gray matter (called
“nuclei”), making up of neuronal cell bodies, that include such struc-
tures as the amygdala, basal ganglia, and thalamus. The cerebral cortex
is a latecomer in evolution and correlates with the appearance of intel-
ligence. It is architectonically arranged in layers. In lower vertebrates
such as fish, the cortex has only three layers of neurons, and is referred
to as the allocortex or archicortex, whereas in higher animals includ-
ing man it has six layers and is called the neocortex.^5 Starting from the
amphibians, an olfactory cortex for the sense of smell came into being,
which became relatively insignificant in primates as the sense of smell
lost its importance. During evolution, a rudimentary neocortex first
appeared in reptiles and became increasingly important thereafter. In
the human brain the neocortex occupies over 90% of the cerebral cortex.
It grows so much in bulk that it folds inward upon itself (to increase the
surface area), forming gyri (ridges) and sulci (furrows), giving the brain
surface a peculiar wrinkled and bumpy look. The human cerebral cortex
is topographically differentiated, with areas dedicated to such functions
as vision, hearing, touch, movement, and a large area for associative
and integrative purposes. The prefrontal area (the region in front of the
motor and pre-motor areas) is particularly important in humans, as it is
believed to be the seat of rational thinking and executive function, the
derangement of which has been implicated in schizophrenia.
Attention should also be paid to the area called the limbic system,
best seen from the medial side of the brain, consisting of the older cortex
and its adjacent structures surrounding the brain stem (see Fig. 7.11).