b. The Midbrain. The midbrain (figure 3-17), or mesencephalon, is the short
section between the forebrain and the hindbrain. It provides the main motor connection
between the forebrain and the midbrain. Dorsally, it presents two pairs of rounded
elevations: the corpora quadrigemina, which act as relay centers in auditory and visual
reflexes, and ventrally the cerebral peduncles, which are two diverging stalks, one
entering each cerebral hemisphere. These peduncles consist of bundles of fibers that
provide pathways for impulses passing to and from the cerebrum.
c. The Hindbrain. During embryonic development, the hindbrain
(rhombencephalon) is divided into two portions: the metencephalon and the
myelencephalon. The metencephalon develops into the cerebellum and pons, and the
myelencephalon into the medulla oblongata (figures 3-17 and 3-19).
(1) The cerebellum (or "little brain") is located in the posterior region of the
skull inferior to the occipital lobes of the cerebrum. It consists of two lateral expansions,
the hemispheres, and a constricted central portion, the vermis, which joins the two
hemispheres. Its internal structure is similar to that of the cerebrum with the gray matter
located in the outer cortical layer. Three pairs of peduncles connect the cerebellum with
the other portions of the brain and spinal cord. The superior peduncles connect the
cerebellum with the midbrain, the middle peduncles connect it with the pons, and the
inferior peduncles connect it with the medulla oblongata. The cerebellum acts as a
reflex center that coordinates muscular movements. It is also the control center for the
(2) The pons (figure 3-14) is anterior to the cerebellum and lies between the
midbrain and the medulla oblongata. As the name implies, it forms a bridge of
connecting fibers, extending between the halves of the cerebellum and joins midbrain
with the medulla. It is located in the midline of the cranial cavity at the base of the skull.
(3) The medulla oblongata is anterior to the cerebellum and extends almost
vertically from the caudal margin of the pons through the foramen magnum to the level
of the cranial border of the atlas where it joins the spinal cord. Internally, its structure
resembles that of the spinal cord with the gray matter generally located centrally. Many
nerve fibers in the medulla cross from one side to the other. As a result, one side of the
brain controls the opposite side of the body. The medulla contains the fourth ventricle,
which is continuous with the central canal of the spinal cord. It serves as a relay station
for the passage of impulses between the brain and the spinal cord, contains centers
controlling cardiac vasomotor and respiratory activities, and regulates many other reflex
activities.
MD0956
3-34