(3) Pons. The pons lies in front of the cerebellum and is continuous with the
medulla oblongata, although separated from it in front by a shallow furrow. "Pons"
means bridge, and it is actually a bridge of tissue. Nerve tracts go through the pons,
conveying sensory impulses to the cerebral cortex, and motor impulses away from it.
(4) Medulla oblongata. The medulla oblongata is the lowest part of the
brain, extending from the pons to the spinal cord. It is pyramidal in shape and contains
nerve tracts between the spinal cord and the cerebellum. There are several vital
centers in the medulla: the vasomotor center, for control of the blood pressure; the
cardioinhibitory center, for control of the heart rate through the vagus nerve; the
respiratory center, for control of rate and depth of respiration, and control of respiratory
movements through the vagus, the phrenic, and the intercostal nerves; and the
temperature control center for the control of body heat. Several of the nerves that leave
the brain through the skullwithout going down the spinal cordcome from the
medulla and the pons. These are the cranial nerves.
c. Ventricles (figure 217). Within the brain is a system of intercommunicating
cavities called the ventricles which are filled with cerebrospinal fluid. The lateral
ventricles are in the cerebral hemispheres on either side. They are connected with
each other by an opening which also empties into the centrally located third ventricle.
The third ventricle, in the midbrain, is triangular in shape and empties below and behind,
into the fourth ventricle. The fourth ventricle is in front of the cerebellum, and behind the
pons and the upper half of the medulla oblongata. On the roof of this ventricle are three
openings through which the cerebrospinal fluid flows into the subarachnoid space of the
brain and spinal cord.
d. Spinal Cord. The spinal cord is that part of the central nervous system within
the vertebral column; it extends from the medulla oblongata to the level of the second
lumbar vertebra. If a transverse section of the cord is examined, the white and gray
matter composing it can be seen. The gray matter is in the interior of the cord, in the
shape of an H (figures 218 and 219). Within the gray matter lie the cell bodies. The
posterior horns contain the cells of sensory nerves, and the anterior horns contain cells
of motor nerves. The white matter surrounding the gray matter is composed of bundles
of nerve fibers, motor, and sensory, which are the nerve tracts of the cord. The
ascending nerve tracts contain sensory nerves, and carry various kinds of sensations to
the brain, such as pain, temperature, and posture. The descending nerve tracts contain
motor nerves, and control either voluntary or involuntary activities of the muscles, the
glands, and the organs. Most of these tracts (both ascending and descending) cross
over to the opposite side before reaching the brain.