This system has 206
bones and associated cartilage, tendons, and ligaments. Because
bone is rigid, it gives the body a framework, maintains its shape,
and protects vital organs. Bones provide a place for muscles and
supporting structures to attach, and, with the movable joints, form
a system of levers upon which muscles can act to produce body movements.
A joint is a place of union between two or more bones that may be
movable or immovable. Bone also functions as a site for mineral
storage and blood cell formation. Tendons and ligaments are strong
bands of fibrous connective tissue that attach muscles to bones,
and bones to bones, respectively.
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The skeleton has two
parts: the axial skeleton and the appendicular skeleton.
The axial skeleton
includes the skull, the hyoid bone, the vertebral column (spine, sacrum,
and coccyx), the sternum, and the ribs. Its components are aligned along
the long axis of the body.
The appendicular skeleton
includes the bones of the upper extremities (arms, forearms, and hands),
the pectoral (shoulder) girdle, the pelvic (hip) girdle, and the bones
of the lower extremities (thigh, knee, leg, and foot). Its components
are outside the body main axis.
Bone tissue stores
and releases ionic minerals that affect homeostasis (stable internal environment)
of the body. Chief among these minerals is calcium, which is necessary
for proper functioning of the muscles and nervous system. Endocrine system
hormones regulate calcium release and storage.
Cranial bones are
flat, rounded, and fused to protect the brain. The eight cranial
bones are the frontal (1), parietal (2), temporal (2), occipital
(1), sphenoid (1), and ethmoid (1). Two cranial bones meet at a
suture (immovable joint). The four major sutures are the coronal
suture (between the parietal and frontal bones), the lambdoidal
suture (between the occipital and parietal bones), the sagittal
suture (between the two parietal bones), and the squamous suture
(between the parietal and temporal bones).
Facial bones provide
a framework for the facial muscles, form eye sockets, and form jaws for
the teeth. The fourteen facial bones are the vomer (1), maxilla (2), zygomatic
(2), palatine (2), lacrimal (2), nasal (2), inferior nasal conchae (2),
and mandible (1). The hinged mandible (jaw bone) moves freely during mastication
(chewing) and speech.
The vertebral column
has three groups of vertebrae and two sets of fused bones. These vertebrae
include seven cervical (neck) vertebrae, twelve thoracic (upper back)
vertebrae, and five lumbar (lower back) vertebrae. Five fused vertebrae
form the sacrum and from three to five fused small vertebrae form the
coccyx (tail bone). The vertebrae form a column of bone that protects
the spinal cord. The thoracic vertebrae have facets (indentations) upon
their surfaces that articulate (meet) with the ribs.
The twelve pairs of
ribs are long, flattened, and curved bones that form a protective cage
for the heart, lungs, and other internal organs. The vertebrosternal (true)
ribs are the first seven ribs; they are "true" because they attach directly
to the sternum (breast bone). Ribs eight through twelve are the false
ribs because they indirectly attach to the sternum or they lack a sternal
attachment. Ribs eight through ten are the vertebrochondral ribs because
they attach indirectly to the sternum by cartilage. Ribs eleven and twelve
are called floating (vertebral) ribs because they do not attach to the
sternum. Instead, their floating position allows them to bend sideways
while providing protection for the kidneys.
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In the appendicular
skeleton, the pectoral girdle bones include two scapulae (shoulder blades)
and two clavicles (collar bones). The scapula is in the upper back and
articulates with two bones: the humerus and the clavicle. Because the
scapula is part of the shoulder joint, the scapula must be mobile to allow
the upper extremities freedom of movement. The clavicle articulates with
the sternum and the scapula, giving support to the pectoral girdle and
adding stability to the shoulder joint.
The bones of the upper
extremities are the humerus, ulna, radius, carpals (wrist bones), metacarpals
(hand bones), and phalanges (finger bones). The humerus in the arm articulates
with the scapula at the shoulder joint and with the ulna and radius at
the elbow. The radius and ulna in the forearm articulate with the carpals
at the wrist. The ulna articulates with the humerus and forms the elbow.
The carpals are small, flat, irregularly shaped wrist bones. They articulate
with the metacarpals in the hand. The metacarpals articulate with the
In the pelvic girdle
are two hip bones. Each coxa (hip bone) forms from fused bones.
The two coxae, sacrum, and coccyx form the pelvis, a bowl-shaped
cavity that supports and protects many abdominal organs. The three
fused bones of each coxa are the ilium, ischium, and pubis.
The pelvic girdle
articulates with the femur (thigh bone) at the acetabulum (hip joint)
and with the sacrum at the sacroiliac joint. Also, the coxae articulate
with each other at the pubic symphysis, a joint with limited movement.
The femur is the longest and heaviest bone in the body and helps in weight-bearing
At the knee, the femur
articulates with the tibia (shin bone). Suspended within muscle tendons
at the front of the knee joint is the patella (kneecap). The patella is
an example of a sesamoid bone (small bone) that is within a tendon.
Attaching to the lateral
(outer side) of the tibia is the fibula (leg bone). The fibula provides
points of attachment for muscles of the foot and leg and increases the
lateral stability of the ankle. The fibula is not a weight-bearing bone
like the tibia.
In the foot are specialized
bones designed for weight-bearing. Among these are the tarsals (ankle
bones), metatarsals (foot bones), and phalanges (toe bones). These
foot bones form a system of arches that allow the foot to support
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(bone growth) begins during fetal development. Before birth every
skeleton bone appears as a fibrous membrane template (membranous
bone) or a cartilaginous template (cartilaginous bone). These templates
form the basic shapes that mature bone replaces. Membranous bone
develops through intramembranous ossification. Cartilaginous bones
develop through endochondral ossification.
hardness and relative inflexibility, bone is one of the most changing
tissues of the body. At the bone tissue cellular level, reorganization
and replacement of the solid matrix occurs continuously. Two types
of cells are primarily responsible for this reconstructive process:
the osteoblasts and the osteoclasts.
form new bone tissue in response to more demands on bone. When a
person is physically active, the rate of osteoblast activity increases,
which thickens and strengthens the bones. When muscles tense and
flex, their tendons pull on the bones. This action causes the osteoblasts
to secrete proteins that form lamellar (compact) bone and cancellous
(spongy) bone with texture that resembles trabeculae (a web-like
network of tunnels).
bone is a system of canals that contain blood vessels, nerves, and
lymph vessels. These canals form functional units called osteons
(Haversian systems). The osteon is a tube-like structure containing
a central (Haversian) canal through which nerves and blood vessels
pass. Surrounding the central canal is bone matrix, a substance
produced by osteocytes. Within the osteon, osteocytes form lamellae,
concentric layers of hard bone matrix. As osteoblasts create lamellae,
these osteoblasts become trapped within the compact bone and are
Osteocytes lie within
lacunae, small pockets between the lamellae. Canaliculi, tiny canals,
connect the lacunae. Canaliculi also connect with the central canal. These
connections allow nutrients to pass through the canaliculi to the trapped
osteocytes within the dense lamellae.
Osteoclasts work with
osteoblasts. These osteoclasts dissolve bone tissue by secreting enzymes.
Their actions help healthy individuals keep bones light and airy. They
also regulate calcium and phosphate concentrations in body fluids. When
osteoclast activity is not balanced by new bone formation, osteoporosis,
a degenerative bone disease, occurs.
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Within the long bones
are two types of bone marrow: red marrow and yellow marrow. The
yellow marrow has fatty connective tissue and fills the marrow cavity.
During starvation, the body uses the fat in yellow marrow for energy.
The red marrow of
some bones is an important site for blood cell production. Here all erythrocytes
(red blood cells), platelets, and most leukocytes (white blood cells)
form in adults. From the red marrow, erythrocytes, platelets, and leukocytes
migrate to the blood to do their special tasks. Red blood cells carry
oxygen and nutrients to the body tissues. Platelets help in blood clotting.
White blood cells help fight disease and infection.
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Bones of the skeleton
articulate at joints. Joints form three categories of movability:
freely movable, slightly movable, and immovable. Freely movable
joints are also called synovial joints. A typical synovial joint
has a joint capsule, a synovial membrane, synovial fluid, a joint
cavity, and articular cartilage. A joint capsule surrounds the joint,
supporting and stabilizing it. The synovial membrane is within the
joint capsule. This membrane closely surrounds the joint and forms
a joint cavity. The synovial membrane secretes synovial fluid that
lubricates the articular surfaces of the joint. In some joints,
the synovial membrane extends outside the joint capsule to form
a bursa. The bursa cushions the joint. Bursae are in the knee, elbow,
shoulder, and hip. Articular cartilage covers the articular surfaces
of synovial joints to prevent excess wear and tear as they move
against each other.
Six types of synovial
joints are hinge, ball-and-socket, pivot, condyloid (angular or ellipsoidal),
plane (gliding), and saddle. The elbow is an example of a hinge joint.
Here, the convex and concave articulating bones allow movement along one
plane, similar to a door.
The shoulder and hip
are the only ball-and-socket joints in the body. In this type of
joint, one bone has a spherical head that articulates with a corresponding
concavity. This joint frees the joint to move in many directions.
In a pivot joint, one round-shaped articulating bone fits within
a corresponding depression on another bone. This joint allows one
bone to rotate against the other. An example is the radioulnar joint
(joint of the radius and ulna) in the forearm. In a condyloid (angular)
joint, one bone has an oval articulating head that rests within
an oval concavity. This joint permits angular movement of the bones.
The metacarpophalangeal joint (junction between the metacarpals
and phalanges) of the hand are examples of condyloid joints.
Plane joints have
two flat bones joined. The sole movement of the bones is short gliding
motions. An example of this joint is the intertarsal joint (junction between
the tarsal bones) of the feet. Saddle joint bones have convex and concave
surfaces similar to a saddle. This joint allows the joint to move in many
directions. The carpometacarpal joint of the thumb is an example saddle
As their name implies,
amphiarthrosis joints (slightly movable joints) have limited movement.
The two types of amphiarthrosis joints are syndesmosis (fibrous) and symphysis
(cartilaginous). A syndesmosis joint occurs when two bones join by a section
of cartilage. The junction between the tibia and fibula is an example.
A symphysis joint forms when two bones fuse by a fibrocartilage pad. Typical
symphysis joints are between the pubic symphysis (pubic bones in the pelvis),
and in the vertebral column between individual vertebrae. Intervertebral
discs act as weight-bearing shock absorbers for walking, jumping, and
An immovable joint
is called a synarthrosis. The two types of this joint are sutures and
gomphoses. Sutures are joined by short fibers of dense fibrous connective
tissue and are in the skull. The single example of a gomphosis joint is
the teeth sitting within their sockets. An example of a bony fusion joint
is the fusion of the three bones forming a coxa (hip bone): the ilium,
ischium, and pubis.
Ligaments link bones.
These ligaments, sheets of strong, fibrous connective tissue, are identical
with the muscular system tendons. The only difference is their function:
tendons attach muscle to bone and ligaments attach bone to bone.
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