The human brain has two types of crevasse, mostly seen on the surface.
The first type of crevasse is the longitudinal fissure. It is the deepest and
oldest, as vertebrate brains all divided into two largely independent,
cooperating hemispheres. It reaches down into the centre of the brain, where
the corpus callosum connects the two hemispheres.
The second type of crevasse is the sulcus, which only descends into the
brain a short ways, as shown in this picture from Gyrus.
The sulcus and gyrus result from the folding of the 465 square inch 2D
surface of the cortex into a space small enough to fit in the skull. The cortex
surface consists of a regular array of neurons arranged in stacks of 6 deep. In
primitive vertebrates the 2D surface (with different neuron depths) was
deposited directly on the inner surface of the skull during development. Since
that limits the size of the brain to the size of the skull’s surface area, some
vertebrate experienced a mutation that allowed a slight wrinkling that evolved
into the gyri and sulci that humans (and other animals) have.
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