c. The specially constructed sacrum, at the lower end of the vertebral column,
receives the body weight from above and transfers it to the pelvic bones of the lower
4-33. PROTECTION OF THE SPINAL CORD AND ITS MEMBRANES
Whereas the cranium protects the brain, the neural arches protect the spinal cord
and its membranes (meninges). The neural arches of the individual vertebrae arch over
the spinal cord and its membranes. The continuous series of neural arches forms a
continuous spinal canal.
4-34. MOTION OF THE VERTEBRAL COLUMN
Together, the vertebrae, the intervertebral discs, and the associated ligaments
form a semiflexible rod. This allows a certain amount of motion to the vertebral column
in addition to its supporting role.
a. Role of Processes. The spinous and transverse processes of the neural
arches serve as attachments for skeletal muscles. By acting as levers, these processes
enable the skeletal muscles to move the vertebrae.
b. Role of Intervertebral Discs. The intervertebral discs between adjacent
vertebrae serve several functions.
(1) First, they allow motion to occur between adjacent vertebrae. The
relative thickness of the individual intervertebral disc determines the amount of motion
possible between the adjacent vertebrae. The total movement of the vertebral column
(spine) is the sum of the motions of the individual intervertebral discs.
(2) Secondly, the intervertebral disc acts as a shock absorber. As such, it
minimizes the shocks that are transmitted to the vertebral column by the contact of the
heels with the floor during walking, jumping, etc.
(3) During the course of a day standing and sitting, the individual becomes
about an inch shorter than he was at the beginning of the day. This is less true of older
individuals. After a good night's rest in a horizontal position, these discs regain their
original thickness. As an astronaut works at zero gravity, he retains his full height.
(4) With age, individuals tend to lose height. This is because the
intervertebral discs shrink somewhat over the years. Since these discs also become
less flexible, there is less compression from morning until night. Thus, the height in the
evening is closer to the morning height than with a younger person.