Bone Function
As mentioned previously, bones are generally classified into four types according to shape: long, short, flat, or irregular. They range in size from the all-powerful leg bone the femur- about 20 inches long, more than an inch across at midshaft to the pisiform, the smallest of the wrist bones, shaped like a split pea; this bone lies at the base of the little finger, familiarly known as the pinkie. But whatever their size or shape, almost every bone in the body is designed to fit a particular need. The most notable exception is the coccyx, our tailbone.
| Figure 8. The anti-griavity bones. |
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The arrangement of the individual bones is as precise, orderly and purposeful as the full skeletal system itself, and their distribution from top to bottom is extremely balanced. Most of the bones in our body are structured in a symmetrical fashion. That is, many of our bones are matched on each side of the body. This matched design allows us to balance and stabilize ourselves in the face of the various forces that act on our bodies. Although we will be discussing only the skeletal system, keep in mind that the sensory and balance organs of the nervous system, the muscles, and the bones work together to help achieve this stability. Let's start at the top and work our way down the major parts of the skeleton!
The skull, the "top" of the skeletal system, has 29 bones
that are fused together to form the cranium, or brain
case, the face, and the ear bones. The only part of the skull that can
move freely is the jawbone. The spine, to which are
attached the pectoral (shoulder) girdle, rib cage and the
pelvic (hip) girdle, has 26 vertebrae.
The ribs number 24, 12 on each side. The two girdles, so named because of
their shape, mark the upper and lower limits of the body's trunk, or
central area. From them, respectively, stem the bones of the upper and
lower limbs, the arms and the legs, respectively. Each
limb has 30 bones apiece. Of the 60 bones in the two upper limbs,
all but 6 are concentrated in the hands and wrists; of the 60 bones in the
2 lower limbs, all but 8 are concentrated in the ankles and feet. Thus,
appropriately, more than half of all the bones in the body support those
parts of our bodies that maintain the busiest daily work schedule - our
extremities.
The femur is not, by any means, the only anti-gravity bone in our bodies
but it is the largest. The spine, pelvic girdle, tibia (lower
leg), and the bones of the foot (particularly,
the talus and calcaneus bones) are all
important in our day-to-day "struggle" to stand and move against gravity.
Another superb example of how each bone was designed with a purpose is the
vertebrae of the spinal column. To help bear the weight
of the body, it is formed like a solid cylinder, but it actually consists
of alternating layers of bone and cartilage. These compressible cartilage
disks between the vertebrae absorb shock and keep the vertebrae from
grinding together when the spine bends. At the back of the bony cylinder,
a ring permits passage of the spinal nerve cord, and also
serves to protect it. At the back of the ring are three sharp projections,
or spurs, which join with the ribs and anchor the muscles
of the back. The flexibility of the spine and its ability to stretch and
compress contribute to the actual height changes that occur to all of us
during the day. In fact, when you have reached the exciting time in your
life when you have begun to drive a car, notice how you must adjust your
rear-view mirror upwards in the morning and then read just it downwards in
the evening. This happens because, after a full day of fighting off
gravity, you actually shrink in size! Let's discuss how the absence of
gravity might affect your height as well.
After about age 25, a person's height can go only one way - and that is
down. A man or woman might lose an eighth of an inch
between ages 25 and 40 as the spongy disks between the vertebrae in the
spine shrink, causing the bones to move closer together. The back begins
to bend forward after age 40. From age 20 to age 70, a woman may shrink
about 2 inches, while a man might lose about an inch. In space, however,
there is a height increase as the human vertebral column
lengthens and straightens, probably because gravity does not compress the
body. In fact, on past U.S. space flights, more than two-thirds of
astronauts reported back pain. This back pain may be associated with the
stretching of the spine. On previous space missions, spinal measurements
were performed and the astronauts were found to increase their
height from 6 to 8 cm above their greatest early morning height here on
Earth (Figure 9), and there was a flattening of the normal spinal
curve.
Space flight also causes bone loss presumably because astronauts are not
required to stand and support themselves to create "loading forces" on the
bones. This bone loss could be a limiting factor for long-duration
missions, such as a Mars expedition or extended stays on a space station.
Before effective countermeasures can be devised, a
thorough knowledge of how much bone loss occurs,
which bones are affected, and how fast
the bone loss appears during weightlessness is needed from actual space
flight data and from Earth-based models that simulate the
disuse (lack of use) of bones. We are going to examine a
space flight investigation designed by Dr. Emily Morey-Holton and her team
from across the United States that uses laboratory rats to study the
effects of space flight on bone formation and calcium metabolism. This
results of this and other related investigations can offer some important
clues and strategies about how to cope with some of the debilitating bone
problems that exist here on Earth, such as osteoporosis. Let's get
started!
Figure 9.(a) As the intervertebral distance increases in microgravity,
the muscles, ligaments, and/or nerves are stretched and can cause pain.
(b) Space flight data has confirmed the lengthening and straightening of
the spine during exposure to microgravity.
