d. Ventricles. Within the brain, there are interconnected hollow spaces filled
with cerebrospinal fluid (CSF). These hollow spaces are known as ventricles. The right
and left lateral ventricles are found in the cerebral hemispheres. The lateral ventricles
are connected to the third ventricle via the interventricular foramen (of Monroe). The
third ventricle is located in the forebrainstem. The fourth ventricle is in the
hindbrainstem. The cerebral aqueduct (of Sylvius) is a short tube through the
midbrainstem which connects the third and fourth ventricles. The fourth ventricle is
continuous with the narrow central canal of the spinal cord.
11-10. THE HUMAN SPINAL CORD
a. Location and Extent. Referring to figure 4-4, you can see that the typical
vertebra has a large opening called the vertebral (or spinal) foramen. Together, these
foramina form the vertebral (spinal) canal for the entire vertebral column. The spinal
cord, located within the spinal canal, is continuous with the brainstem. The spinal cord
travels the length from the foramen magnum at the base of the skull to the junction of
the first and second lumbar vertebrae.
(1) Enlargements. The spinal cord has two enlargements. One is the
cervical enlargement, associated with nerves for the upper members. The other is the
lumbosacral enlargement, associated with nerves for the lower members.
(2) Spinal nerves. A nerve is a bundle of neuron processes which carry
impulses to and from the CNS. Those nerves arising from the spinal cord are spinal
nerves. There are 31 pairs of spinal nerves.
b. A Cross Section of the Spinal Cord (figure 11-6). The spinal cord is a
continuous structure which runs through the vertebral canal down to the lumbar region
of the column. It is composed of a mass of central gray matter (cell bodies of neurons)
surrounded by peripheral white matter (myelinated processes of neurons). The gray
and white matter are thus considered columns of material. However, in a cross section,
this effect of columns is lost.
(1) Central canal. A very narrow canal, called the central canal, is located in
the center of the spinal cord. The central canal is continuous with the fourth ventricle of
(2) The gray matter. In the cross section of the spinal cord, one can see a
central H-shaped region of gray matter. Each arm of the H is called a horn, resulting in
two posterior horns and two anterior horns. The connecting link is called the gray
commissure. Since the gray matter extends the full length of the spinal cord, these
horns are actually sections of the gray columns.
(3) The white matter. The peripheral portion of the spinal cord cross section
consists of white matter. Since a column of white matter is a large bundle of processes,
it is called a funiculus. In figure 11-6, note the anterior, lateral, and posterior funiculi.