(5) Pons. The pons is located anterior and slightly superior to the
cerebellum and between the midbrain and the medulla. The pons acts as a pathway to
higher structures. It contains conduction pathways between the medulla and higher
brain centers. It also serves to connect the two halves of the cerebellum. There is a
respiratory center in the pons which prolongs inspiration (breathing in). The beginnings
of some cranial nerves are in the pons.
(6) Medulla oblongata (brain stem). This part of the brain is an expanded
continuation of the spinal cord. The brain stem is located between the pons and the
spinal cord and is only about one inch long. Contained in the brain stem are the centers
for the regulation of respirations, heartbeat, and basomotor activators. These centers
are often called the vital centers because they are essential to life. Some nerves cross
over at the medulla oblongata which explains why one side of the brain controls
activities on the opposite side of the body.
(7) Thalamus. The thalamus is located in the walls of the third ventricle of
the brain and is the area of arousal and conscious recognition of crude sensations; for
example, temperature and pain. Sensory and afferent impulses go to the thalamus and
are sorted and grouped there. Next, these impulses are sent to the proper area of the
cerebral cortex where the impulses are interpreted. According to the Law of Specific
Nerve Energies, the place at which an impulse ends in the thalamus determines the
sensation to be felt. For example, if the impulse ends in the heat area of the thalamus,
the individual feels heat.
(8) Hypothalamus. The hypothalamus is a small part of the diencephalon.
This portion of the brain has several functions. It monitors the chemical composition of
blood. The hypothalamus acts as a relay station between the cerebrum and the lower
autonomic centers. It controls hormone secretion by the pituitary gland and also
controls the appetite.
b. Spinal Cord. The spinal cord is a cylindrical structure which extends from
the foramen magnum through the spinal foramina of the vertebral column to the upper
portion of the lumbar region. Extension varies from the 12th thoracic vertebra to the
2nd lumbar vertebra. The length of the cord remains fairly constant in adults: 18 inches
in males and 16 inches in females. The conus medullaris is the cone-shaped
termination of the cord. This portion of the spinal cord weighs about one ounce and is
approximately one and one-half inches wide. The spinal cord itself appears wider from
right to left than from anterior to posterior. The size and shape, however, do vary
depending on the vertebral region. For example, the spinal cord presents cervical and
lumbar enlargements, which are areas of nerve origin in the upper and lower lumbar
region. The spinal cord is composed of a series of 31 segments. A pair of spinal
nerves comes out of each segment. The region of the spinal cord from which a pair of
spinal nerves originates is called the spinal segment. Both motor and sensory nerves
are located in the spinal cord.