The brain stem , which is only two and a half inches long, is seen here cut from top to bottom and laid open, in a similar way to which a book is opened. The central dark area is the reticular formation. The left side shows the motor nuclei of the cranial nerves and the right side illustrates their sensory nuclei. The anatomy of the brain includes: the motor nuclei of cranial nerves, this one represents the oculomotor nucleus; the choroid plexus, which secretes a clear, watery fluid--the cerebrospinal fluid; a sensory nucleus of the fifth cranial nerve; the thalamus, which receives both motor and sensory nerve fibers as they pass into and out of the brain; the trigeminal nerve, which is comprised of single fibers which unite to form a nerve bundle; the pituitary stalk, which is part of the forebrain and lies above the pons; the optic nerve, which splits to enter the chiasma and lies in front of the brain stem; the pons, which is made up of transverse nerve fibers interwoven with longitudinal nerve fibers; the medulla oblongata, which is the lower part of the brain stem; the spinal cord, a cord of central gray matter surrounded by white matter; motor nerve bundles, which emerge from the right side of the brain and cross to supply the body's left side; the sensory nerve bundles, which arise from the left side of the body and pass to the right side of the brain after crossing in the medulla; and the motor and sensory nerve fibers, which pass through the brain stem on their way into and out of the spinal cord, and cross in the medulla to serve and supply areas on the opposite sides of the body.
