Chapter 7 - The Skeletal System
• Individual bones are the organs of the
skeletal system. A bone contains very
active tissues.
Bone Structure
Bone classification
• Classified according to their shapes:
• Long - longitudinal axis and expanded
ends
Ex: forearm and thigh bones
• Short - cube like with lengths and widths
about equal Ex: wrist and ankle
• Flat - platelike with broad surfaces
Ex: ribs, scapula, some skull bones
• Irregular - variety of shapes, usually
connected to several other bones.
Ex: vertebrae and facial bones
• Sesamoid - round, usually small and
nodular embedded within tendons next to
joints where tendons are compressed Ex:
kneecap
Parts of a long bone
• Epiphysis at each end that articulates
(forms a joint) with another bone
• Outer surface of epiphysis covered with a
layer of hyaline cartilage called articular
cartilage
• The shaft of a bone is called the diaphysis
• Except for the articular cartilage, a bone is
covered by fibrous tissue called the
periosteum.
• Wall of diaphysis made of compact bone
that has a continuous matrix with no gaps.
• Epiphysis made of spongy bone that has
irregular interconnecting spaces between
bony plates (trabeculae).
• Both compact and spongy bone are strong
and resist bending.
• The diaphysis contains a medullary cavity
filled with marrow.
Microscopic structure
• Osteocytes (bone cells) located in lacunae
which form concentric circles around
osteonic canals
Compact bone
• Osteocytes and layers of intercellular
material concentrically clustered around
an osteonic canal to form osteons
cemented together.
• Osteonic canals contain blood vessels that
nourish the cells of osteons.
• Perforating canals connect osteonic
canals transversely and communicate with
the bone's surface and the medullary
cavity.
Spongy bone
• Composed of osteocytes and intercellular
material but the cells don not aggregate
around osteonic canals
• Cells lie within trabeculae
• Diffusion from the surface of thin bony
plates nourishes cells of spongy bones.
Bone Development and Growth
• Bones form by replacing existing
connective tissue in an embryo and
continue to grow and develop into
adulthood.
Intramembraneous Ossification
• Flat, broad bones in skull
• Membranelike layers of unspecialized
connective tissue appear at future bone
sites
• Unspecialized connective tissue cells
arrange blood vessels in layers
• Connective tissue cells differentiate into
osteoblasts, which deposit spongy bone
• Osteoblasts become osteocytes when a
bony matrix completely surrounds them.
• Connective tissue on the surface of each
developing structure forms a periosteum.
• Osteoblasts on the inside of periosteum
deposit compact bone over the spongy
bone.
• Process of bone formation is called
ossification. Ossification timeline - see
table 7.2 page 209.
Endochondral Ossification
• Masses of hyaline cartilage form models of
future bone
• Cartilage tissue breaks down, periosteum
develops
• Blood vessels and osteoblasts invade
tissue
• Osteoblasts form spongy bone
• Osteoblasts deposit a thin layer of
compact bone
• Osteoblasts become osteocytes when
bony matrix surrounds them.
• Osteocytes are mature bone cells
Growth of an endochondral bone
• Primary ossification center appears in the
diaphysis and bone develops towards the
ends
• Secondary ossification centers appear in
the epiphysis and spongy bone forms
around them
• An epiphyseal disk (band of cartilage)
remains between the primary and
secondary ossification centers.
An epiphyseal disk consists of 4
layers of cells
• resting cells - anchor disk to bony tissue of
epiphysis
• young reproducing cells - undergo mitosis
• older enlarging cells - enlarge and thicken
the disk causing bone to lengthen
• dying cells - thin layer
• Long bones continue to lengthen until the
epiphyseal disks are ossified.
• Growth in thickness occurs as compact bone is
deposited beneath the periosteum.
• Osteoclasts erode bone tissue inside which later
fills with marrow.
• Throughout life osteoclasts resorb bone tissue
(resorption) and osteoblasts replace bone
(deposition).
• 3% -5% of calcium exchanged each year.
• The total mass of bone remains nearly constant.
Factors affecting bone development,
growth, and repair.
Nutrition
• Vitamin D necessary for proper calcium
absorption
• Bones deformed without - rickets in
children and osteomalacia in adults
• Vitamin A needed for bone resorption
Lack of retards bone development
• Vitamin C needed for collagen synthesis
Lack of results in bones abnormally
slender and fragile
Sunlight
• Converts dehydrocholesterol produced by
cells in the digestive tract into vitamin D.
• Vitamin D allows calcium to be absorbed
Hormones
• Insufficient secretion of pituitary growth
hormone may result in dwarfism.
• excessive secretion of GH may result in
giantism (>8ft.) or acromegaly( hands,
feet, and jaw enlarge).
• Deficiency of thyroid hormone delays bone
growth.
• Male and female sex hormones promote
bone formation and stimulate ossification
of the epiphyseal disks.
Physical stress
• Skeletal muscle contractions pulls on bone
attachments which stimulates bone tissue
to thicken and strengthen.
Bone Function
• Support and Protection
• Body movevement
• Blood Cell Formation (hematopoiesis)
Begins in the yolk sac, outside the
embryo
Occurs later in the liver, spleen and
bone marrow
• Red marrow houses developing red blood
cells, white blood cells, and blood
platelets.
• Red in color due to hemoglobin.
• In adults found in spongy bone of the skull,
ribs, sternum, clavicles, vertebrae, and
pelvis.
• Yellow marrow stores fat and replaces red
marrow as we age.
Inorganic salt storage
• Bone matrix is made of collagen and
inorganic mineral salts.
• Salts are about 70% of matrix by weight.
• Most salt is calcium phosphate in the form
of hydroxyapatite.
• Calcium is needed for blood clot formation,
nerve impulse conduction, and muscle cell
contraction
• When blood is low in calcium osteoclasts
resorb bone, releasing calcium salts.
• When blood is high in calcium osteoblasts
are stimulated to form bone tissue and
store calcium salts.
• Bone also stores small amounts of
sodium, magnesium, potassium, and
carbonate ions.
Skeletal Organization
Number of bones
• Usually a human has 206 bones, but the
number may vary.
• Extra bones in sutures (flat bones in skull
grow together) are called sutural bones.
Divisions of the skeleton
• Axial- skull, hyoid bone (supports tongue),
vertebral column, and thorax
• Appendicular - pectoral girdle, upper
limbs, pelvic girdle, and lower limbs.
Skull
• Consists of twenty-two bones, which
include eight cranial bones, thirteen facial
bones, and one mandible.
Cranium
• Encloses and protects the brain, and
provides attachments for muscles.
• Some bones contain air-filled sinuses that
help reduce the weight of the skull.
• Cranial bones include the frontal bone,
parietal bones, occipital bone, temporal
bones, sphenoid bone, and ethmoid bone.
Facial skeleton
• Form the basic shape of the face and
provide attachments for muscles.
• Consist of 13 immovable bones and a
movable lower jawbone (mandible).
• Facial bones include the maxillary
bones(2), palatine bones(2), zygomatic
bones(2), lacrimal bones(2), nasal
bones(2), vomer bone, inferior nasal
conchae(2) and mandible.
Infantile skull
• Incompletely developed bones, connected
by fontanels (soft spots) enable the
infantile skull to change shape slightly
during childbirth.
• Small face with a prominent forehead and
large orbits.
• Bones are thin and somewhat flexible and
less easily fractured than adults.
Figure 07.31a
Vertebral Column
• Extends from the skull to the pelvis and
protects the spinal cord.
• Composed of vertebrae separated by
intervertebral disks.
• Vertebrae connected to one another by
ligaments.
• An infant has thirty-three vertebral bones - 5
eventually fuse to become the sacrum and 4
fuse to become the coccyx.
• An adult has twenty-six vertebrae.
• The vertebral column has four curvatures cervical, thoracic, lumbar, and pelvic.
Cervical Vertebrae (7)
• The bones of the neck.
• Smallest and most dense vertebrae
• Transverse processes have transverse
foramina (passageways for arteries
leading to the brain).
• 2 - 5 are forked to provided attachments
for muscles
• 7th vertebrae is longer and called vertebra
prominens
• The atlas (first vertebra) supports the
head.
• The axis (second vertebra) provides a
pivot when the head is turned from side to
side.
Thoracic vertebrae(12)
• Larger than cervical vertebrae.
• Their long spinous processes slope
downward, and facets on the sides of
bodies articulate with the ribs.
• Increase in size starting with 3 rd one.
Lumbar vertebrae(5)
• Large and strong to support more weight.
• Their transverse processes project
posteriorly at sharp angles, and their
spinous processes are directed
horizontally.
Sacrum
• Formed of 5 vertebrae that gradually fuse
between ages 18-30.
• Triangular structure that has rows of
dorsal sacral foramina ( nerves and blood
vessels pass through).
• United with the coxal bones at the
sacroiliac joints.
• The sacral promontory (1 st sacral
vertebrae) provides a guide for
determining the size of the pelvis.
Coccyx
• Composed of four vertebrae that fuse by
age 25.
• Acts as a shock absorber when a person
sits.
Thoracic Cage
• includes the ribs, thoracic vertebrae,
sternum, and costal cartilages .
• Supports the pectoral girdle and upper
limbs, protects viscera, and functions in
breathing.
Ribs
• Twelve pairs are attached to the twelve
thoracic vertebrae.
• Costal cartilages of the true ribs (first 7)
join the sternum directly.
• False ribs (last 5) join indirectly or not at all
to sternum. - last 2 pairs called floating
ribs
• A typical rib has a shaft, head, and
tubercles that articulate with the vertebrae.
Sternum
• The sternum consists of a manubrium
(upper), middle body, and xiphoid process
(projects downward).
• Articulates with costal cartilages and
clavicles.
Pectoral Girdle
• Composed of two clavicles and two
scapulae.
• Forms an incomplete ring that supports
the upper limbs and provides attachments
for muscles that move the upper limbs.
Clavicles
• Rodlike bones that run horizontally between the
sternum and shoulders.
• Hold the shoulders in place and provide
attachments for muscles.
Scapulae
• Broad, triangular bones located on either side of
upper back.
• Articulate with the clavicle and humerus and
provide attachments for muscles.
Upper Limb
• Provide the frameworks and attachments
of muscles, and function in levers that
move the limb and its parts.
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Humerus
Extends from the scapula to the elbow.
Smooth rounded head with narrow
depression called the anatomical neck
Radius
Located on the thumb side of the forearm
between the elbow and wrist.
Disklike head, to allow for rotation.
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Ulna
Longer than the radius and overlaps the
humerus posteriorly.
Articulates with the radius laterally.
Hand
Composed of a wrist, palm, and five
fingers.
It includes 8 carpal bones that form a
carpus, five metacarpals and fourteen
phalanges.
Pelvic Girdle
• Consists of two coxal bones that articulate
with each other anteriorly and with the
sacrum posteriorly.
• The sacrum, coccyx, and pelvic girdle form
the pelvis.
• The girdle provides support for body
weight and attachments for muscles, and
protects visceral organs.
Coxal bones
• Each coxal bone consists of an ilium,
ischium and pubis which are fused in the
region of the acetabulum
• • Ilium isthe largest portion of the coxal
bone, joins the sacrum at the sacroiliac
joint.
• The ischium is the lowest portion of the
coxal bone.
• The pubis is the anterior part of coxal bone
and is fused anteriorly at symphysis pubis.
Greater and lesser pelves
• The lesser pelvis is below the pelvic brim;
the greater pelvis is above it.
• The lesser pelvis functions as a birth
canal; the greater pelvis helps support
abdominal organs.
Differences between male and
female pelves
• Differences related to the function of the
female pelvis as a birth canal.
• Usually the female iliac bones are more
flared (broader hips), pubic arch angle is
greater, more distance between the ischial
spines and the ischial tuberosities, and the
sacral curvature is shorter.
• Bones of female pelvis usually lighter and
show less evidence of muscle
attachments.
Lower Limb
• Bones of the lower limb provide the
frameworks of the thigh, leg, and foot.
Femur
• Extends from the hip to the knee.
Patella
• Flat sesamoid bone in the tendon that
passes anteriorly over the knee.
• Controls the angle of this tendon and
functions in lever actions associated with
lower limb movements.
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Tibia
Located on the medial side of the leg.
Larger of the 2 lower leg bones
Articulates with the talus of the ankle.
Fibula
Located on the lateral side of the tibia.
articulates with the ankle but does not
bear body weight.
Foot
• Consists of an ankle, an instep, and five
toes.
• It includes seven tarsals that form the
tarsus, five metatarsals and fourteen
phalanges.
Types of Fractures
Healing of Fractures
• Compound (open) fracture is where the
bone sticks through the skin
• Simple (closed) fracture is when the bone
does not go through the skin
• Complete fracture is when the break is all
the way through the bone
• The neck of the femur is the site of most
hip fractures
Osteoporosis
• Bones become thinner and brittle
Risk factors
• Postmenopausal women
• Lack of exercise
• Increased alcohol consumption
• Light skin
• Low calcium intake