The human brain is a highly sophisticated
organ, and is responsible for control of the rest of the body.
It receives 20% of the blood flow of the body. The brain
consists of two hemispheres, a right and a left, and the two
halves are connected by a band of tissue known as the corpus
callosum. The "top" and the "bottom"
of the brain are separated by a fibrous membrane known as
the tentorium. Above the tentorium (supratentorial)
, the brain is divided into several lobes, and below the tentorium
(infratentorial) lie the cerebellum and the brainstem.
Above the tentorium, the lobes are named frontal, parietal,
occipital and temporal. Each lobe is responsible
for a different function, and information is sent to the spinal
cord by way of "relay stations" known as the basal
ganglia, located deep within the brain. There are other
deep structures and paths within the brain which are responsible
for the sending information from one portion of the brain
to the other, and modifying and enhancing thought and movement
Bathing the brain is the cererbrospinal
fluid, which is a clear, colorless fluid which protects and
cushions the brain. Most of the spinal fluid is located
within the deep portions of the brain known as the ventricles.
Some of it percolates around the surface of the brain in what
is known as the the subarachnoid spaces.
The brain derives its blood supply
from four arteries, two internal carotid arteries and two
vertebral arteries. There is some degree of redundancy
of blood flow supplied by the circle of Willis, which is a
ring of blood vessels at the base of the brain. This
helps to prevent stroke should one of the feeding arteries
become occluded. The brain also has a very elaborate
venous drainage system consisting of veins which empty into
venous sinuses, which are large venous channels created by
folds of dura (the tough covering of the brain).
There are 12 cranial nerves which
enter the brain. These supply ones ability for smell,
vision, eye movement, facial sensation, hearing, facial movement,
taste, tongue movement, shoulder shrug and head turning.
The spinal cord enters the skull through
a hole in the skull bas known as the foramen magnum.
It merges with the lowest portion of the brainstem known as
the medulla oblongata.