The Skeletal System
Functions of the Skeletal System
• Support- The skeleton forms a rigid framework to which
are attached the soft tissues and organs of the body
• Protection- The skull, vertebral column, rib cage, and
pelvic girdle enclose and protect vital organs and sites for
blood cell production is found in the protected hollows
of certain bones.
• Movement- Bones act as levers when muscles contract,
causing movement at the joints.
• Mineral Storage- The matrix of bone is composed
primarily of calcium and phosphorous, with small
amounts of magnesium and sodium thrown in.
– About 99% of the calcium and 90% of the phosphorous found in
the body is found in bones and teeth.
– These minerals give bones rigidity and account for ≈ 2/3 the
weight of the bone.
• Hemopoiesis – Red bone marrow produces red and
white blood cells and platelets in adults.
Types of Bones
Types of Bones
• LONG BONES- have a
tubular shaft and
articular surface at each
end.
• The major bones of the
arms (humerus, radius,
and ulna) and the legs
(the femur, tibia, and
fibula) are all long
bones. Phalanges are
also long bones
• SHORT BONES- Short
bones in the human
body are cubelike -- the
length, width, and
height measurements
are all about the same.
• Short bones include the
carpal bones (hands,
wrist) and tarsal bones
(feet, ankles).
Types of Bones
• IRREGULAR BONES- are
irregular in size and
shape and are usually
quite compact. They
include the bones in the
vertebral column, the
patella (kneecap),
mandible (jaw), maxilla
(upper jaw).
• FLAT BONES- thin and
have broad surfaces.
The flat bones include
the scapula (wingbone),
the ribs, and the
sternum (breastbone)
and some skull bones.
• Bone is a variety of connective tissues built mainly by the
deposition of calcium phosphate. The matrix also
contains collagenous fibers, bone cells and a large
amount of water.
• osteoblasts- cells that help form bone
• osteoclasts- cells that help eat away old bone.
• osteocytes- mature osteoblasts that have ended their
bone-forming careers and are trapped in the bone
matrix. These cells engage in metabolic exchange with
the blood that flows through the bones.
Diaphysis- The shaft of a long bone.
Compact bone tissue. Contains
the Medullary cavity, which
contains fatty yellow bone
marrow (where fat is deposited)
and is lined with endosteum
(squamous epithelium).
On either end of the diaphysis are
the Epiphysis- the ends of the
bone which consist of spongy
bone surrounded by compact
bone. Red bone marrow (where
blood is made) is found within
the pores of the spongy bone.
A Typical Long Bone
Separating the epiphysis from
the diaphysis is the
epiphyseal plate, an area
of mitotic activity where
linear bone growth occurs
(elongation). An
epiphyseal line replaces
the plate when bone
growth is complete.
The entire bone is covered by
periosteum, a connective
tissue that is the site of
tendon-muscle attachment
and diametric bone growth
(widening)
A Typical Long Bone
The periosteum contains
collagenous fibers and many
osteoblasts, which are cells that
can develop into osteocytes.
The ends of the epiphysis are
covered with articular cartilage, a
thin, smooth cartilage that
facilitates the movement of the
joints.
A Typical Long Bone
Microscopic structure
• Under the microscope
dense, compact bone
shows a definite and
characteristic pattern
of arrangement.
• Haversian canals- Small
canals that run parallel to
the long axis (shaft)
of the bone. These canals
are interconnected with
one another and contain
a blood vessel, a nerve
and a lymph vessel.
• Each Haversian canal is surrounded by concentric layers of bone matrix
(called lamallae) and concentric rings of bone forming cells
(osteoblasts).
Osteoblasts and Osteocytes
Bone building cells
Osteoblasts synthesize and
secrete collagen fibers
and other organic
components needed to
build the matrix of the
tissue
Osteoblasts surround
themselves with matrix,
become trapped in their
secretions and become
osteocytes (mature bone
cells)
• Do not undergo Mitosis
• Osteocytes - maintain
metabolism
• Bone cells remain alive
and once they have been
completely surrounded by
the hard bone matrix
they are called osteocytes.
• The osteocytes are
embedded in fluid-filled
cavities within the
concentric lamellae.
• These cavities are known
as lacunae and occur at
regular intervals in these
concentric layers of bone
tissue.
• The lacunae are connected to one another and to the Haversian canals
by a system of interconnecting canals known as canaliculi.
• Each Haversian canal, its concentric lamellae, lacunae with osteocytes
and canaliculi forms a long cylinder and is called a Haversian system.
Separate Haversian systems are joined to each other by means of
interstitial lamellae.
Microscopic Structure of Bone and Cartilage
• Compact bone is
composed of osteocytes
within lacunae arranged
in concentric circles called
lamellae
• This surrounds a central
canal. The entire complex
is called a Haversian
system
• Canaliculi connect
osteocytes to central
canal and to each other
Spongy Bone
The arrow labeled o points to an osteocyte in
its lacuna. Canaliculi (not visible here) connect
the lacunae of osteocytes to each other and to
the marrow spaces between the trabeculae.
Spongy (cancellous) bone is
lighter and less dense than
compact bone.
Spongy bone is a network of
irregularly-shaped sheets and
spikes of bone (trabeculae).
The spaces between the
trabeculae contain red or
yellow marrow, depending
on a person's age and on
which bone it is.
The trabeculae are organized to
provide maximum strength
similar to braces that are
used to support a building.
Spongy bone is located in the
hipbone and sternum of
adults.
A single Haversian system, or
osteon, in compact bone.
The Haversian canal in the
center contains the blood
vessel, with associated
nerve.
b
Concentric layers of extracellular matrix (a, for example),
and the osteocytes are fully encapsulated by the matrix.
At the edges of the image the cement line are where one
osteon meets another (b).
Bone Formation and Growth
• ossification- the formation of bone by the activity of
osteoblasts and osteoclasts and the addition of minerals and
salts. There are two types of ossification: intramembranous
and endochondral.

In the fetus, most of the skeleton is made up of cartilage, a tough,
flexible connective tissue that has no minerals or salts. By the end
of the eighth week after conception, the skeletal pattern is
formed in cartilage and connective tissue membranes, and
ossification begins.
• The replacement of hyaline cartilage by bone is called
endochondral ossification. Most of the bones of the skeleton
are formed in this manner.
• Intramembranous ossification involves the replacement of
sheet-like connective tissue membranes with bony tissue.
Certain flat bones of the skull and some of the irregular bones
are formed in this manner.
Endochondral Ossification
• During the third month after conception, the perichondrium (the
membrane of fibrous connective tissue) that surrounds the hyaline
cartilage models becomes infiltrated with blood vessels and osteoblasts
and changes into a periosteum.
– The osteoblasts form a collar of compact bone around the diaphysis.
– At the same time, the cartilage in the center of the diaphysis begins to
disintegrate.
– Osteoblasts penetrate the disintegrating cartilage and replace it with spongy
bone.
• Ossification continues from this center toward the ends of the bones.
After spongy bone is formed in the diaphysis, osteoclasts break down
the newly formed bone to open up the medullary cavity.
• When ossification is complete, the hyaline cartilage is totally replaced
by bone except in two areas. A region of hyaline cartilage remains over
the surface of the epiphysis as the articular cartilage and another area
of cartilage remains between the epiphysis and diaphysis. This is the
epiphyseal plate or growth region.
• Bone resorption is the process by which
osteoclasts break down bone and release the
minerals.
• http://www.argosymedical.com/flash/aging_bone
/landing.html
• Bones grow in length at the epiphyseal plate.
• The cartilage in the region of the epiphyseal plate next to the
epiphysis continues to grow by mitosis. The chondrocytes (cartilage
cells) in the region next to the diaphysis, age and degenerate.
Osteoblasts move in and ossify the matrix to form bone.
• This process continues throughout childhood and the adolescent
years until the cartilage growth slows and finally stops. When
cartilage growth ceases, usually in
the early twenties, the
epiphyseal plate
completely ossifies
so that only a thin
epiphyseal line remains
and the bones can no
longer grow in length.
Skeletal Formation
Intramembranous ossification- the development of bone
from tissue or membrane
• During fetal development
and infancy, the
membranous bones at the
top and sides of the
cranium are separated by
sutures –immovable joints.
• There are six large areas,
called fontanels, that
permit the skull to undergo
changes in shape during
childbirth and allow rapid
growth of the brain during
infancy.
• The fontanels close by 2
years of age.
• babys-soft-spots
Divisions of the Skeleton
• Axial- blue
• Appendicular- tan
SKULL
• Cranium -8 bones
• face – 14 bones
• middle ear- 6 bones
see table on pg 131
• Sinus- a cavity inside the
bones
• PARANASAL sinuses –
– 4 pair
– sinusitis- an inflammation
of the sinus membrane.
1. Cranium – 8 large, flat bones
a. Frontal 1
b. Parietal – 2
c. Temporal – 2
d. Occipital – 1
e. Sphenoid – 1
f. Ethmoid – 1
These bones
articulate with
one another to
enclose and
protect the
brain.
The bones of the
skull are held
together by
immovable
joints called
sutures
Frontal
Parietal
Sphenoid
Temporal
Occipital
Ethmoid
Maxilla
Zygomatic
2. Facial – 14 bones
a. Maxillae – 2
b. Palatine – 2
c. Zygomatic – 2
Ethmoid
d. Lacrimal – 2
e. Nasal – 2
Zygomatic
f. Vomer – 1
g. Inferior Nasal Conchae – 2
h. Mandible – 1
Vomer
Nasal
Lacrimal
Palatine
Inferior Nasal
Conchae
Maxillae
Mandible
Sinus
• a cavity inside the
bones
• PARANASAL sinuses –
– 4 pair
– sinusitis- an
inflammation of the
sinus membrane.
Spine- vertebral column
• vertebrae form a
flexible curved rod.
• Divided into 5 sections–
–
–
–
cervical- 7 vertebrae
thoracic – 12 vertebrae
lumbar- 5 vertebrae
sacrum- single fused
area
– coccyx- single fused area
cervical
thoracic
lumbar
sacrum
coccyx
The vertebral
column encases
and protects the
spinal cord,
which runs from
the base of the
cranium down
the dorsal side
reaching the
pelvis.
Thorax
• Ribs- 12 pair
• Sternum
• Thoracic vertebrae
• All ribs attach to the
vertebrae.
• 10 pair attach to the
sternum…7 pair with
calcified bone tissue
and 3 pair with
cartilage.
• 2 pair are “floating”
APPENDICULAR SKELETON
Appendicular Skeleton
1. Shoulder Girdle
a. Clavicle- the main
connection between the
upper arm and the rest
of the axial skeleton.
Important site for
muscle attachments.
b. Scapula- an attachment
site for numerous
muscles which support
movement and
stabilization of the
shoulder.
c. Humerous
Clavicle
Scapula
Humerous
2. Upper Limb
1. Arm
a. Humerus
2. Forearm
b. Radius
c. Ulna
3. Hand
a. Carpals wrist
b. Metacarpals palm
c. Phalanges fingers
Pelvic Girdle
Ilium
Sacroiliac joint
• The Pelvis
– Hip bones
• Ilium
• Pubic bone
• Ischium
Acetabulum
– Acetabulum- Each hip bone
consists of an ilium, an
ischium, and a pubis, all three
of which in the adult are
fused at the acetabulum to
form a single bone.
• Sacroiliac joint
Pubic bone
Ischium
Lower Limb
1. Thigh
a. Femur
b. Acetabulum
2. Leg
a. Tibia
b. Fibula
c. Patella
3. Foot
a. Tarsals
b. Metatarsals
Joints of the Body
An articulation, or joint, is where two bones come together. In
terms of the amount of movement they allow, there are
three types of joints: immovable, slightly movable and
freely movable.
Three types:
• Synarthroses- immovable
• Amphiarthroses - slightly movable
• Diarthroses- freely movable
Allow for the movement of body parts
Looser fit of joint equals more movement and the converse is
true.
Classification of joints
Synarthroses -In these joints,
the bones come in very
close contact and are
separated only by a thin
layer of fibrous connective
tissue. The sutures in the
skull are examples of
immovable joints.
Structural classification
• Fibrous Joints
• Two examples
Fibrous connective tissue
Fibula
– Suture line
– Fibrous connective tissue
Suture line
• Amphiarthroses – Slightly
movable.
• Structural classificationhyaline cartilage or
fibrocartilage.
• The ribs connected to the
sternum by costal cartilages
are slightly movable joints
connected by hyaline
cartilage.
• The symphysis pubis is a
slightly movable joint in which
there is a fibrocartilage pad
between the two bones.
• The joints between the
vertebrae and the
intervertebral disks are also of
this type.
Fibrocartilage disc
Vertebra
Cartilage
Diarthroses
Most joints in the adult body are
diarthroses, or freely movable joints.
The ends of the opposing bones are
covered with hyaline (articular)
cartilage.
Separated by a space called the joint
cavity.
The joints are enclosed in a dense fibrous
joint capsule.
The outer layer of the capsule consists of
the ligaments that hold the bones
together.
The inner layer is the synovial membrane
that secretes synovial fluid into the
joint cavity for lubrication.
Because all of these joints have a
synovial membrane, they are
sometimes called synovial joints.
Movement of diarthrosis (synovial)
joints
Factors that limit movement
• Bone structure
– Hinge joint (elbow)
– Ball and socket (hip)
• Joint ligaments
– Degree of tension on ligaments
• Hormones
– Pubic symphysis; effects of relaxin
Synovial Joint Movement -
saddle joint
ball and socket
condyloid joint
Pivot joint
hinge joint
Gliding joint (plane joint)
Characterized by flat
articulating surfaces
• Articulation of
vertebrae with other
vertebrae and bones of
the ankle and wrist.
• Range of Motion: least moveable
Holding your forearm steady while your hand points
upward and then waving side-to-side with your
hand is an example of this joint's functioning.
Hinge joint
Permit motion only in one
plane—backward and forward
--- where convex surface of
one bone fits into concave
surface of second bone.
Movements permitted:
– Flexion
– Extension
• Ex: Movement between the
humerus and ulna, phalanges.
The knee and ankle joints are
also hinge joints, but allow
very slight side to side
movement.
• The articular surfaces are
connected together by
ligaments.
Pivot Joint
• Rotation of one bone
around another --found
in the neck, forearms,
knees.
• A hole in one bone fits
over a pointed part of
the other bone, so that
one bone can turn on
top of the other.
Ball and Socket
• The most maneuverable type
of joint.
• The shoulder and hip are
both ball-and-socket joints.
• The distal bone is capable of
motion around an indefinite
number of axes, which have
one common center. It
enables the bone to move in
a 360° angle.
• These joints allow for
forward motion, backward
motion and circular rotation.
The head of the femur joins the pelvis at the
acetabulum (socket).
Condyloid Joint
• Two bones fit together
with an odd shape (e.g.
an ellipse), and one
bone is concave, the
other convex.
• Between the
metacarpals of the
hand and the first
phalanx of the fingers
Saddle Joint
• Has two saddle-shaped
surfaces at right angles
to each other.
• Found only in the
thumb.
Joint Type Movement at joint
Examples
Hinge
Flexion/Extension
Elbow/Knee
Pivot
Rotation of one bone around
another
Top of the neck
Ball and
Socket
Flexion/Extension/Adduction
/Abduction/Internal &
External Rotation
Shoulder/Hip
Saddle
Flexion/Extension/Adduction
/Abduction/Circumduction
joint of the thumb
Condyloid
Flexion/Extension/Adduction
/Abduction/Circumduction
Wrist
Gliding
Gliding movements
Intercarpal joints
Structure
Anatomical terms of motion
Movement- all descriptions of position and movement
are based on the assumption that the body is in
anatomical position
• Muscles may move parts of the skeleton relatively
to each other, or may move parts of internal organs
relatively to each other.
• Most terms of a motion have clear opposites, and
are treated in pairs.
• Flexion- Bending movement that decreases the angle
between two parts. Bending the elbow, or clenching a
hand into a fist, are examples of flexion. When sitting
down, the knees are flexed. Flexion of the hip or
shoulder moves the limb forward (towards the anterior
side of the body). Dorsiflexion- Flexion of the entire
foot superiorly, as if taking one's foot off an
automobile pedal.
• Extension- The opposite of flexion; a straightening
movement that increases the angle between body
parts. In a conventional handshake, the fingers are fully
extended. When standing up, the knees are extended.
Extension of the hip or shoulder moves the limb
backward (towards the posterior side of the body).
Plantarflexion- Flexion of the entire foot inferiorly, as if
pressing an automobile pedal. Occurs at ankle.
• Abduction pulls a structure or part away from the
midline of the body (or, in the case of fingers and
toes, spreading the digits apart, away from the
centerline of the hand or foot). Raising the arms
laterally, to the sides, is an example of abduction.
• Adduction pulls a structure or part towards the
midline of the body, or towards the midline of a
limb. Dropping the arms to the sides, or bringing
the knees together, are examples of adduction. In
the case of the fingers or toes, adduction is closing
the digits together.
Additional motions without clear
opposites are as follows:
• Rotation - A motion that occurs when a part turns
on its axis. The head rotates on the neck, as in
shaking the head 'no'.
• Circumduction - The circular movement of a body
part, such as a ball and socket joint or the eye. It
consists of a combination of flexion, extension,
adduction, and abduction. "Windmilling" the
arms is an example of circumductive movement.
• range of motion/animations.cfm
Structural differences between the
male and female skeletons.
• In general, male skeletons are larger and
heavier than female skeletons.
• The small bulge at the back of the head
known as the external occipital
protuberance is usually more pronounced
in men.
• The male jawbone or mandible is typically
angular and square-shaped at the chin
area, while the female jawbone tends to
be more rounded and pointed. The brow
ridges of men are often more prominent
than those of women.
• The female pelvis is smaller, shallower and
wider, and the cavity is more circular in
shape. The coccyx or tailbone is more
movable in female skeletons. The sacrum
is wider and flatter in females.