Encyclopedia of Health and Medicine

The Musculoskeletal System 293
torso
pectoralis major
pectoralis minor
serratus anterior
external oblique
internal oblique
intercostal
diaphragm
transverse abdominus
rectus abdominis
iliopsoas
back
teres major
infraspinatus
rhomboideus major
latissimus dorsi
gluteus maximus
gluteus medius
obturator
leg
pectineus
sartorius
adductor longus
rectus femoris
adductor magnus
gracilis
biceps femoris
semitendinosus
vastus lateralis
vastus medialis
semimembranosus
tibialis anterior
tibialis posterior
gastrocnemius
soleus
peroneus longus
flexor hallucis
extensor
Functions of the Musculoskeletal System
The musculoskeletal system gives the body form
and structure, protects the internal organs, and
provides movement. It determines body height
and mass. It is the foundation for facial features,
hand characteristics, and athletic ability. The
bones of the SKELETON form the core of the structural
body; the muscles build the body’s outward
appearance. In tandem, the bones and the muscles
carry the body through life.
A soft start: the skeleton’s origins The SKELETON
arises from the mesoderm very early in embryonic
development, taking rudimentary form at about
three weeks of gestational age. Hyaline CARTILAGE,
a tough, dense type of connective tissue, forms the
template that will become the ossified (mineralhardened)
skeleton. Though the process of ossification
begins before birth, the greater percentage
of the skeleton is still cartilage at birth to facilitate
passage through the birth canal.
After birth an intricate, HORMONE-regulated
process immediately sets about to convert cartilage
cells (chondrocytes) to BONE cells (osteocytes). This
process of ossification takes the first two decades
of life to reach fruition. Bone tissue continues to
grow and change throughout life even after bone
size reaches stability through another process
called bone remodeling, in which bone-building
cells (osteoblasts) create new bone structure in
synchronization with bone-destroying cells (osteoclasts)
which remove old bone structure.
Framework: the skeleton The 206 bones of the
adult human skeleton give the body shape, protection,
and mobility. There are two divisions of
the skeleton:
• The axial skeleton forms the body’s central
alignment; its bones are primarily those of support
and shelter.
• The appendicular skeleton “hangs from” the
axial skeleton; its bones are primarily those of
movement.
Bones provide the structure that gives the body
resistance against gravity and makes movement
possible. Long bones, such as those in the arms
and legs, function as levers for the skeletal muscles
to generate movement and locomotion. A
honeycombed structure within the long bones
reduces their density and weight while increasing
their STRENGTH. The compact construction of short
bones, such as those in the hands and feet, supports
functions that require greater strength and
less leverage. Flat bones, such as the scapulae
(shoulder blades) and pelvis (hip bones), provide
surface area for firmly anchoring the large skeletal
muscles that make movement possible.
Some bones function as armor, protecting vital
structures and organs. The smooth, thick bones of
the skull completely encase the BRAIN in a chamber
that has few natural points of entry. Vertebrae
separated by cushions of cartilage enclose the
SPINAL CORD, their irregular shapes deflecting access
while at the same time permitting FLEXIBILITY. The
ribs form a cage that contains the HEART and LUNGS,
providing a framework for the bellows-like action
of the lungs with the thick sternum like a shield to
shelter the heart.
Form and function: the muscles The 650 or so
muscles in the body give the body shape and
make movement, including locomotion, possible.
The skeletal muscles cover and protect the bones,
attaching directly to them. Muscles also support
and protect other structures such as BLOOD vessels
and nerves. Most skeletal muscles work in opposing
pairs, with one MUSCLE group contracting and
the other relaxing in synchronization to permit
the balanced, coordinated, and smooth move-