The brain is the most mysterious structure in the human body. Its anatomy does
not yield obvious clues about its function and, in fact, for many centuries people did not
know exactly what it did. Biochemical research and use of imaging techniques in recent
decades have revealed a great deal about the intricate dance between molecules and cells
that underlies the characteristics that make us most human—creativity, intelligence, and
The brain communicates with the rest of the body through the spinal cord and
twelve pairs of cranial nerves. This chapter considers both the brain and cranial nerves.
Overview of the Brain
The brain has changed dramatically in the course of vertebrate evolution, and is
especially developed in humans. The average brain weighs about 1600 g (3.5 pounds) in
males and about 1450 g (3.2 pounds) in females, a sex difference proportional to body
size. It is divided into three main portions, the cerebrum, the cerebellum, and the
The cerebrum is the largest part of the brain, and the two cerebral hemispheres
fold over the other structures. The cerebellum, located posteriorly and inferiorly to the
cerebrum, is the second largest part. The brainstem is defined in this chapter as
everything but the cerebrum and cerebellum, consisting of the diencephalon, midbrain,
pons, and medulla oblongata. The brainstem ends at the foramen magnum, where the
CNS continues as the spinal cord.
Gray and White Matter
Gray matter (neuron cell bodies, dendrites, and synapses) forms the surface layer,
or cortex, over the cerebrum and cerebellum. Deep masses called nuclei are also
composed of gray matter. White matter lies deep to the gray matter of the cortex, and is
composed of tracts that connect one part of the brain to another, or connect the brain and
spinal cord. The white matter gets it color from myelin.
Connective tissue covers the brain in a similar fashion to the spinal cord. The
layers consist of the dura mater, arachnoid, and pia mater. In the cranial cavity, the dura
mater consists of two layers, an outer periosteal layer that connects to bone at several
points, and an inner meningeal layer. Dural sinuses separate the two layers in some
places. The sinuses collect blood that has circulated in the brain and ultimately empty it
into the internal jugular veins of the neck.