of manatee brain and its postulated sensory and motor
Vis = visual cortex; Aud = auditory cortex; SI = main
somatosensory cortex; SII = second somatosensory cortex;
MI = main motor cortex.
drawing of the manatee brain shown above indicates the sensory
and motor areas of cerebral cortex as suggested by the external
morphology as well as by the microscopic appearance of the manatee
cortex. This picture indicates the well accepted notion that
cerebral cortex is the target region where the sensory environment
is mapped in an orderly topographic fashion.
The cerebral cortex consists of collections of cellular regions
arranged as a sheet which covers the entire forebrain in mammals.
It is generally subdivided into two general zones: the neocortex
and the paleocortex, which merge into transitional cortical
zones together where they adjoin.
The neocortex is a seven-layered sheet of neurons which
may be quite elaborately convoluted into an extensive array
of gyri which may contain a large number of independently developed
cortical areas. These cortical areas are richly interconnected
with each other and with the thalamus, basal forebrain and even
more distant brainstem targets. These connections are usually
very specific and often reciprocal. Neocortex lies on the dorsal,
dorsomedial, dorsolateral, frontal, caudal and ventrolateral
portions of each hemisphere. The cortical regions in the two
hemispheres are connected to each other via a large fiber bundle
known as the corpus callosum. The major sensory inputs from
the environment or the body itself project to cerebral neocortex
in ways that represent the real features of the external world.
The paleocortex consists of cortical regions on the midline
around the corpus callosum, ventrally beneath the basal ganglia
and amygdala, rostrally just behind the anterior olfactory nuclei
and caudally adjacent to the hippocampus. The general subdivisions
of the paleocortex are the entorhinal, pyriform and limbic regions.
These regions of cerebral cortex can be seen from the coronal
series of sections displayed in the brain
atlas and in the array of sections cut in coronal, horizontal
and sagittal planes which are displayed below. These sections
were all stained to show fiber stains, and cerebral cortex is
revealed as the paler grey sheet overlying the forebrain.
The cell bodies themselves are revealed below as purple dots
in a coronal section through the center of the manatee's brain
stained with thionin.
Cerebral cortex viewed at greater magnifications are displayed
below. The strips of cortex shown here are taken from several
different cortical areas from front to back of the brain: