Chapter 6- Skeleton
System
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I. Structure of bones
A. Functions
1. The skeleton has many functions, but
the most obvious is supporting the weight
of the body.
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2. The skeletal system includes the
bones of the skeleton and the cartilages,
ligaments, and other connective tissues
that stabilize or connect them. This
system has five primary functions:
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a.
Support- Provides structural support for the
entire body. Individual bones or groups of
bones provide a framework for attachment of
soft tissue organs.
b.
Storage-The calcium salts of bones are very
important to bones. Also, bones store lipids in
areas of yellow marrow as energy reserve.
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c. Blood cell production- RBC, WBC and
other blood elements are produced within
the RED marrow.
d. Protection- Delicate tissues and organs
are surrounded by skeletal elements for
protection.
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e. Leverage- Bones of the skeleton
function as levers that change the
magnitude and direction of the forces
generated by skeletal muscles.
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B. Organization of the Skeleton System
1.
Osteology is the study of bones.
a.
Each bone is an organ that plays a part in
the total functioning of the skeleton system.
b.
An adult human has approximately 206
bones.
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c. Number of bones differs from person to
person depending on age and genetic
variations.
d. At birth, a baby has about 270 bones. As
the bones develop (ossification – hardening of
the bones) the number decreases.
e. During adolescence, the number
decreases, due to gradual fusion of
separate bones.
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f. Some adults have extra bones within
the sutures (joints) of the skull called
suture.
g. Additional bones may develop in
tendons in response to stress as the
tendons repeatedly move across a joint.
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C. Macroscopic features of bone
1.
The bones of the human skeleton have four general
shapes, long, short, flat and irregular. (See page 128)
2.
Long bones are longer than they are wide whereas
short bones are of roughly equal dimensions.
3. Long bones examples: bones in limbs.
(Femur or humerus)
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4. Short bones include the bones of the wrist
(carpal) and ankles (tarsal)
5. Flat bones are thin and relatively broad,
such as the parietal bones in the skull, the ribs
and the scapulae.
6. Irregular bones have complex shapes that
do not fit easily into any other category.
Example- vertebrae of the spinal column.
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7. A long bone has a central shaft, or diaphysis
and an expanded portion at each end or
epiphysis.
8. The diaphysis surrounds a central marrow
cavity.
9. The ends or epiphyses are covered by
cartilage.
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10. The two types of bone tissue are
compact/dense bone or spongy
bone/cancellous.
11. The outer surface of a bone is covered by a
periosteum, which consists of a fibrous outer
layer and a cellular inner layer.
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12. Inside the bone, a cellular
endosteum lines the marrow cavity and
other surfaces.
13. See figure 6.2 on page 129.
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D. The Axial skeleton
consists of the bones
that form the axis of
the body and that
support and protect
the organs of the
head, neck and torso.
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1. Skull- has two main
sets of the bones: the
cranial bones that form
the cranium and the
facial bones that support
the eyes, nose, and jaws.
There are 22 bones total
in the skull.
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2. Auditory ossicles
are present in the
middle ear chamber
of each ear and serve
to transmit sound.
There are 6 bones
total.
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3. The hyoid bone is
located above the
larynx and below the
lower jaw, supports
the tongue and
assists in swallowing.
One bone total.
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4. Vertebral column
(backbone) consists
of 26 individual
vertebrae separated
by cartilaginous
intervertebral discs.
In the pelvic region
several vertebrae are
fused to form the
sacrum.
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5. The rib cage or
thoracic cage
includes the 12
pairs of ribs, the
flattened sternum
and the costal
cartilage that
connects the ribs
to the sternum.
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E. The appendicular
skeleton is composed
of bones of the upper
and lower extremities
and the bony girdles,
which anchor the
appendages to the
axial skeleton.
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1. Pectoral girdle is
made up of the paired
scapulae and the
clavicles. The primary
function of the pectoral
girdle is to provide
attachment for the
muscles that move the
brachium and forearm.
Pectoral girdle has 4
bones.
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2. Upper extremityEach upper extremity
consists of a proximal
humerus, an ulna and
radius, carpal bones of
the wrist and the
metacarpal and
phalangeal bones of the
hand. Upper extremity
has 60 bones.
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3. Pelvic girdles are
formed by two ossa
coxae (hipbones)
untied by the
symphysis pubis.
The pelvic girdle has
2 bones.
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4. The lower
extremity consists of
a femur, tibia, fibula,
tarsals bones of the
ankle, and the
metatarsal and
phalangeal bones of
the foot. The patella
is also included. The
lower extremity has
60 bones.
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F. Cells in bone
1.
Osteocytes, mature bone cells. Osteocytes
maintain normal bone structure by recycling the
calcium salts in bony matrix around themselves.
2.
Osteoclasts, acids and enzymes secreted by
osteoclasts dissolve the bony matrix and release
the stored minerals through osteolysis or
resorption.
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3. Osteoblasts- the cell responsible
for the production of new bone, a
process called ostogenesis
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II. Bone development
A. Skeletal growth begins about 6 weeks
after fertilization, when the embryo is
about 0.5 in long.
1. At this time, all skeletal elements
are made of cartilage
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2. Bone growth continues through
adolescence, and portions of the skeleton
usually do not stop growing until around age
25.
3. Osteogenesis is bone formation and growth.
4. During development, cartilage or connective
tissue is replaced by bone.
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5. Replacing tissue with bone is called
ossification.
6. There are two major forms of ossification.
a.
Intramembraneous ossification bone
develops within sheets.
b.
Endochondral ossification bone is replaced
with cartilage.
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B. Intramembranous ossification
1. Intramembranous ossification begins
when osteoblasts start to differentiate
into stem cells where they become
calcified.
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2. The place where ossification first
occurs is called an ossification center.
3. An ossification proceeds and new
bone branches.
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C. Endochondral ossification
1.
Most of the bones of the skeleton are formed
through the endochondral ossification of
existing hyaline cartilage.
2.
The cartilages develop first; they are like
miniature cartilage models of the future bone.
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3. By the time an embryo is 6 weeks old, the
cartilage models of the future limb bones begin
to be replaced by true bone.
a. Step 1- Endochondral ossification starts as
chondrocytes within the cartilage model enlarge
and the surrounding matrix begins to calcify.
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b. Step 2- Bone formation first occurs at the
shaft surface. Blood vessels invade the area
and osteoblasts begin producing bone matrix.
c. Step 3- Blood vessels invade the inner
region of the cartilage and newly
osteoblasts from spongy bone within the
center of the shaft at the primary center
of ossification
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d. Step 4- As the bone enlarges,
osteoclasts break down some of the
spongy bone and create a marrow cavity.
e. Step 5- The centers of the epiphyses
begin to calcify.
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4. When sex hormone production
increases at puberty, bone growth
accelerates dramatically, and osteoblasts
begin to produce bone faster than
epiphyseal cartilage expands. = growing
pains.
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D. Bone growth and body portions
1.
The timing of epiphyseal closure varies from
bone to bone and individual to individual.
2.
Toes may complete ossification by age 11.
3.
Some of the pelvis or the wrist may continue
to enlarge up to 25 years old!
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4. The epiphyseal plates in the arms and
legs usually close by age 18 in women
and 20 in men.
5. Differences in sex hormones account
for variations in body size and proportions
between men and women.
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E. Requirements for normal bone growth
1.
Normal bone growth and maintenance cannot
occur without a reliable source of mineral,
especially calcium.
2.
During prenatal development these minerals
are absorbed from the mother’s bloodstream.
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3. The demands are so great that the maternal
skeleton often loses bone mass during
pregnancy.
4. From infancy to adulthood, the diet must
provide adequate amounts of calcium and
phosphate to be able to absorb and transport
these minerals to sites of bone formation.
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5. Vitamin D plays an important role in
normal calcium metabolism.
6. The vitamin is converted in the liver
into calcitriol, a hormone that stimulates
the absorption of calcium and phosphate
ions in the digestive track.
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7. Rickets is a
condition marked by
a softening and
bending of bones
that occurs in
growing children as a
result of vitamin D
deficiency.
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8. Vitamin A and Vitamin
C are also important.
9. A deficiency of
vitamin C will cause
scurvy. This condition
causes a reduction in
osteoblast activity that
leads to weak and brittle
bones.
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10. Hormones can also play an important
role in normal skeletal growth and
development – such as growth hormones,
thyroid hormones, and sex hormones.
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III. Remodeling and
Homeostatic Mechanisms
A. Support and storage depends on the
bones.
1. The turn over rate for bone is quite
high.
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2. In adults, roughly 18% of the portion and
mineral components are removed and replaced
each year through the process of remodeling.
3. Each part of every bone may not be
affected, but new bone is always being formed.
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B. Remodeling and support
1.
Regular mineral turnover gives each bone the ability
to adapt to new stress.
2. Heavily stressed bones become thicker,
stronger, and develop more pronounced surface
ridges; bones not subject to ordinary stresses
become thin and brittle
3. Regular exercise is very important in
maintaining bone strength.
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4. Degenerative changes in the skeleton occur
after even brief periods of inactivity. Such as
using a crutch.
5. After a few weeks, the unstressed leg will
lose up to about a third of its bone mass.
6. The bones rebuild just as quickly when
they again carry their normal weight.
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C. Homeostasis and mineral storage
1. Bones are important mineral reservoirs,
and calcium is the most abundant mineral
in the human body.
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2. By providing a calcium reserve, the
skeleton helps maintain calcium
homeostasis in body fluids.
3. This function can directly affect the
shape and strength of the bones in the
skeleton.
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D. Injury and repair
1.
Despite its mineral strength, bone cracks or
even breaks if subjected to extreme loads,
sudden impact, or stresses from unusual
directions.
2.
All such cracks and breaks in bones constitute
a fracture.
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3. Fractures are classified according to
their external appearance, the site of the
fracture and the nature of the break.
4. See page 135.
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Comminuted fracture
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Transverse fractures
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Spiral Fracture
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5. Bones will usually heal even after they have
been severely damaged, as long as the
circulatory supply and the cellular components
of the endosteum and periosteum survive.
6. To repair a break can take from 4
months to a year following a fracture.
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7. Steps to heal a break:
a. Step 1- Many blood vessels are
broken and extensive bleeding occurs. A
hematoma forms to clot the blood.
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b. Step 2- An internal callus forms as a
network of spongy bone unities the inner
surfaces, and an external callus of cartilage and
bone stabilizes the outer edges.
c. Step 3- The cartilage of the external
callus has been replaced by bone.
Fragments of dead bone and areas of
bone closest to the break have been
removed and replaced.
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d. Step 4- A swelling initially marks the
location of the fracture. Over time this
region will be remodeled with little
evidence of a fracture.
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E. Aging and the Skeletal System
1.
2.
The bones of the skeleton become thinner
and relatively weaker as normal part of the
aging process.
Inadequate ossification is called osteopenia.
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3. The reduction in bone mass occurs between
the ages of 30-40.
4. Osteoblast activity decline, while osteoclast
activity continues at normal levels.
5. Once the reduction begins, women lose
about 8% of their skeletal mass every decade.
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6. Men’s skeletons deteriorate about 3% per
decade.
7. Osteoporosis is a condition that produces a
reduction in bone mass great enough to
compromise normal function.
8. About 29% of women between 45-79
can be considered to have this disease.
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9. The increase in incidence after menopause
has been linked to decreases in the production
of estrogens.
10. The incidence in men for the same age is
18%.
11. Because bones are more fragile, vertebrae
may collapse.
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IV. An overview of the
skeleton system
A. Skeletal terminology
1. Each bone in the human skeleton not
only has a distinctive shape, but also has
characteristics external features.
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2. Depressions and openings indicate
sites where blood vessels and nerves lie
alongside or penetrate the bone.
3. These landmarks are called bone
markings, or surface features.
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B. Skeletal divisions
1. The skeletal system consists of 206
bones that are divided into axial and the
appendicular.
2.
See page 139!!!!
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C. The axial division
1.
The axial division creates a framework that
supports and protects organ systems in the
dorsal and ventral body cavities.
2. The axial has three functions: adjust
the positions of the head, neck and trunk,
performs respiratory movements,
stabilize or position elements of the
appendicular skeleton.
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3. Remember that the axis skeleton
consists of the head neck and torso.
4. Look on pages 140-141 for the
bones in the skull
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V. Bones of the face
The maxillary bones
1.
The maxillary bones articulate with all other
facial bones except the mandible.
2.
The maxillary bones contain large maxillary
sinuses, which lighted the portion of the
maxillary bones above teeth.
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3. Know the
sinuses on page
144.
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IV. Skulls of infants and
children
A. Many centers of ossification are
involved in the formation of the skull.
1. As the fetus develops, the individual
centers begin to fuse.
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2. At birth, the fusion has yet to be completed,
and there are two frontal bones, four occipital
bones, and several sphenoid and temporal
elements.
3. The skull organizes around the developing
brain, and as the time of birth approaches, the
brain enlarges rapidly.
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4. The bones in the skull are also growing, but they
fail to keep pace and at birth the cranial bones are
connected to fontanels.
5. The fontanels, or “soft spots,: are quite flexible and
permit distortion of the skull without damage.
6. Such distortions normally occur during
delivery to allow the baby out of the birth canal.
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C. Spinal curvature
1.
The vertebral column is not straight and rigid.
2.
The spinal column has four spinal curves.
3.
The thoracic and sacral curves are called
primary curves because they appear late in fetal
development, as the thoracic and abdominal
organs enlarge.
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4. The cervical and lumbar curves known as
secondary curves do not appear until months
after birth.
5. The lumbar curve forms as an in infant
starts to crawl. Actually crawling for a long
time promotes good curves in the back.
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6. The cervical curve forms as infants
learn to balance their head and lift their
head up.
7. Several abnormal distortions of spinal
curvature may appear during childhood
and adolescence.
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C. The cervical vertebrae
1.
The seven cervical vertebrae extend from the
head to the thorax.
2.
Distinctive features of a typical cervical
vertebra include; 1- an oval, concave body, 2- a
relatively large vertebral foremen, 3- a stumpy
spinous process, usually with a notched tip and
4- round transverse foramina within the
transverse processes.
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3. The first two vertebrae have unique
characteristics that allow for specialized
movements.
4. The atlas holds up the head,
articulating with the occipital. It is
named after Atlas according to Greek
myth that held the world on his
shoulders.
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D. The thoracic vertebrae
1.
There are 12 thoracic vertebrae.
2. Features of the thoracic vertebra include:
a.
b.
c.
See
Characteristic heart shaped body
Large, slender
One or more pairs of ribs
pages 148-149
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E. The lumbar vertebrae
1.
The distinctive features of lumbar vertebrae include:
a. Vertebral body that is thicker and more oval
than that of a thoracic vertebra.
b. A relatively massive, stumpy spinous process.
c. Bladelike transverse process.
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VII. The appendicular
division
A. The pectoral girdle
1. Movement of the clavicle and scapula
position the shoulder joint and provide a
base for arm movement.
2.
See pages 150-156.
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VIII. Articulations
A. Joints, or articulations, exist wherever two
bones meet.
1.
The characteristic structure of a joint
determines the type of movement that may
occur.
2.
Each joint reflects a workable compromise
between the need for strength and the need for
movement.
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B. Classification of a joint
1. Joints are classified according to their
structure or function.
2. The structure classification is based on
the anatomy of the joint.
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3. Joints are classified as fibrous, cartilaginous
or synovial.
4. The first two reflect the type of connective
tissue binding them together.
5.
Synovial joints prevent bone-to-bone contact.
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6. An immovable joint is a synarthrosis.
7. A slightly movable joint is
amphiarthrosis.
8. A freely moveable joint is diarthrosis.
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C. Immovable joints/synarthroses
1.
A synarthrosis can be fibrous or cartilaginous.
2.
Two examples can be found in the skull.
3.
In a suture the bones of the skull are interlocked and
bound together by dense connective tissue.
4.
In a gomphosis a ligament binds each tooth in the
mouth within a bony socket.
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D. Slightly movable
joints
(amphiarthroses)
1.
Permits very limited
movement- example
in the leg the fibula
and the tibula.
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E. Diarthrosis (free
movement)
1.
Most of the joints
give free range of
movement such as
the shoulder or wrist.
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F. Types of joints
1.
There are five types of joints 1- gliding (wrist
and ankle), hinge (knee and elbow) 3- ball and
socket (shoulder and hip) 4- immovable –
(skull) 5- pivot (neck)
G. Types of movement
1.
See pages 160-161
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Abduction
Abduction is movement away from the midline, or to abduct.
Adduction
Adduction is movement toward the midline, or to add.
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Flexion
Flexion is to bend at a joint, or to reduce the angle.
Extension
Extension is to straighten at a joint, or to increase the angle, for
example, from 90 degrees to 180 degrees.
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Medial Rotation
Medial rotation is to turn inward.
Lateral Rotation
Lateral rotation is to turn outward.
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Supination
Supination is to rotate the forearm so that the palm faces forward.
Pronation
Pronation is to rotate the forearm so that the palm faces backward.
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IX. Clinical consideration
A. Development disorders
1.
Minor defects of the extremities are relatively
common malformation’s
2.
Extra digits – polydactyly is the most common
limb deformity.
3.
Usually an extra digit is incompletely formed
and does not function.
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4. Syndactyly, or
webbed digits is
another common limb
deformity.
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5. Talipes- clubfootthe sole of the foot is
twisted medially. It
isn’t sure if it is
caused by restricted
movement in utero
causing this
condition.
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6. Cleft palate is a treatable birth
defect in which the roof of the
mouth (palate) does not develop
normally during pregnancy, leaving
an opening (cleft) that may go
through to the nasal cavity
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B. Trauma and injury
1.
The most common type of bone injury is a
fracture- a cracking or breaking of a bone.
2.
Spontaneous, or pathologic fractures result
from diseases that weaken the bones.
3.
Most fractures are called traumatic fracturesbecause they are caused by injury.
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THE END
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