THE SKELETON
WHAT ARE THE FUNCTIONS
OF THE SKELETON?
1. Framework or support for the body
2. Bones serve as levers that change magnitude
and direction of the forces created by skeletal
muscles
3. Bones serve as protection for the heart,
lungs, spinal cord, and brain
4. Bone marrow is the site for the formation of
blood cells
5. Bone tissue stores calcium salts essential for
maintaining normal concentrations of calcium
and phosphate ions in body fluids
Bone Types
There are 206 bones that make up the
human skeleton
4 types of bones:
1. LONG- found in arms and legs; longer than
they are wide
2. SHORT- wrist and ankle; equal dimensions
3. FLAT- shoulder blades and cranium; thin
and broad
4. IRREGULAR- vertebrae; complex shapes
BONE STRUCTURE
A long bone has:
DIAPHYSIS- central shaft
EPIPHYSIS (epiphyses)- make up ends of
diaphysis
- the epiphyses of adjacent bones articulate
with each other and are covered by
ARTICULAR CARTILAGE
PERIOSTEUM- thick, double-layered membrane
that covers the bone
- fibrous outer layer and cellular inner layer
ENDOSTEUM- lines marrow cavity and other
inner surfaces
- active during bone growth and repair
BONE TISSUE
Bone tissue is called OSSEOUS TISSUE
- contains the minerals calcium and phosphorus
2 types of bone tissue:
1. Cancellous (spongy) bone- network of bony
rods separated by spaces filled with bone
marrow; fills epiphyses
2. Compact bone- more solid than spongy; forms
the diaphysis
MICROSCOPIC LOOK AT
BONE
RECALL: WHAT ARE BONE CELLS
CALLED?
Osteocytes
- found in small pockets called lacunae
- the lacunae are found between narrow sheets
of calcified matrix known as LAMELLAE
- there are small channels called CANALICULI
that radiate through the matrix, interconnecting
lacunae and linking them to nearby blood vessels
COMPACT AND SPONGY BONE
The basic functional unit of compact bone is the
OSTEON (also called Haversian system)
- in an osteon, osteocytes are arranged around
a central canal that contains one or more
blood vessels
- perforating canals provide passageways for
linking the blood vessels of the central canals
with those of the periosteum or marrow cavity
Spongy bone is arranged differently:
- no osteons
- lamella form rods or plates called
TRABECULAE
- these branch creating an “open” network
- canaliculi radiating from the lacunae of spongy
bone end at the exposed surface of the
trabeculae, and this is where nutrients and
wastes diffuse between the marrow and the
osteocytes
http://www.webbooks.com/eLibrary/Medicine/Physiology/Skeletal/Skeletal.htm
Location of Compact vs. Spongy
bone
COMPACT:
- a layer of compact bone covers bone surfaces
almost everywhere
- usually found where stresses come from a
limited range of directions
Ex: limb bones withstand forces applied at either
end- will not bend
- exception is in disorders that reduce
calcium salts in skeleton  RICKETS
SPONGY:
- found where bones are not heavily stressed or
where stresses arrive from many directions
Ex: at epiphyses of long bones
- much lighter than compact bone  reduces
weight of skeleton
- trabecular network supports and protects
cells of red bone marrow, sites of blood cell
formation
BONE CELLS
- Although osteocytes are most common, there are
other types:
1. OSTEOCYTES- mature bone cells
- maintain normal bone structure by recycling
calcium salts in the bony matrix around
themselves and assisting in repairs
2. OSTEOCLASTS- giant cells
- secrete acids and enzymes that dissolve the
bony matrix and release stored minerals
3. OSTEOBLASTS- cells responsible for
producing new bone
- produce new bone matrix (called
OSTEOGENESIS) and promote the depositing
of calcium salts in the organic matrix
** At any given moment, osteoclasts are
removing matrix and osteoblasts are adding
to it
BONE
DEVELOPMENT
AND GROWTH
Skeletal growth begins about 6 weeks after
fertilization
- continues through adolescence
- some portions do not stop growing until about
age 25
RECALL: OSTEOGENESIS- bone formation
and growth
- during development, cartilage or connective
tissue is replaced by bone
OSSIFICATION- the process of replacing other
tissues with bone
CALCIFICATION- the depositing of calcium
salts
2 types of ossification:
1. INTRAMEMBRANOUS- bone develops
within sheets or membranes of connective tissue
2. ENDOCHONDRIAL- bone replaces
existing cartilage
INTRAMEMBRANOUS
OSSIFICATION
- begins when osteoblasts (bone forming cells)
differentiate within embryonic or fetal
connective tissue
- occurs in the deeper layers of the epidermis
- osteoblasts differentiate from connective tissue
stem cells after organic components of the
matrix secreted by the stem cells become
calcified
- where ossification first begins is called an
OSSIFICATION CENTER
- blood vessels grow into the area of ossification,
because osteoblasts require oxygen and nutrients
- these blood vessels become trapped in the
developing bone
- at first, intramembranous bone resembles spongy
bone, but eventually osteons (of compact bone)
can also be produced
- BONES OF SKULL, LOWER JAW, AND
COLLARBONES are formed this way
ENDOCHONDRIAL
OSSIFICATION
Most bones of the skeleton are formed through this
type of ossification
- bone replaces existing hyaline cartilage
STEPS OF ENDOCHONDRIAL OSSIFICATION:
1. Chondrocytes (cartilage cells) in the cartilage
enlarge, and the surrounding matrix begins to
calcify
- the cells die because as the matrix calcifies, the
nutrient supply slows
2. Bone formation first begins at the
shaft surface
- blood vessels invade
perichondrium, and the cells of
its inner layer differentiate into
osteoblasts and begin to produce
bone matrix
3. Blood vessels invade
inner region of cartilage,
and newly differentiated
osteoblasts form spongy
bone in the center of the
shaft at a PRIMARY
CENTER OF
OSSIFICATION
- bone development fills
the shaft with spongy
bone
4. As the bone enlarges,
osteoclasts break down
some spongy bone, and
create a marrow cavity
- the cartilage does not
completely fill with bone
because the epiphyseal
cartilages on the ends
keep enlarging,
increasing the length of
the bone
5. Centers of the epiphyses
begin to calcify
- blood vessels and
osteoblasts enter these
areas, and
SECONDARY
CENTERS OF
OSSIFICATION form
- epiphyses become filled
with spongy bone
- a small amount of the original cartilage remains
exposed at the joint cavity as ARTICULAR
CARTILAGE
- bone of the shaft and the bone of the
epiphysis are separated by areas of cartilage
called EPIPHYSEAL PLATES
- at puberty, bone growth greatly accelerates,
and osteoblasts produce bone faster than
epiphyseal cartilage expands
- epiphyseal plates become narrower and
narrower until they disappear
- in adults the former location of the plate is
detected in X-rays as the EPIPHYSEAL
LINE (see your diagram!)
- end of epiphyseal growth is called
CLOSURE
As bones get longer, they also increase in diameter
- APPOSITIONAL GROWTH- enlargement of
the diameter of bones at their outer surfaces
- cells of the periosteum develop into osteoblasts
and produce more bony matrix
- new bone is deposited on the outside as the
inside is eroded away by osteoclasts, making the
marrow cavity larger
DIFFERENCES IN BONE
GROWTH
- Timing of epiphyseal closure varies from bone to
bone and from individual to individual
- Epiphyseal plates in arms and legs usually
close by about the age of 18 in women, and
20 in men
- Differences in sex hormones accounts for
variations in size and proportions between men
and women
NORMAL GROWTH
REQUIREMENTS
**Normal bone growth and maintenance requires
a reliable source of minerals such as calcium
salts
- during prenatal development, these minerals are
absorbed from the mother’s bloodstream, so
much in fact that the mother’s skeleton often
loses bone mass during pregnancy
**Vitamin D3 is converted into calcitrol, a
hormone that stimulates the absorption of
calcium and phosphate ions in the digestive tract
- vitamin D3 can be obtained from dietary
supplements, or where else?
SUN EXPOSURE
- vitamin D3 deficiency can cause RICKETS, or
softening of the bones, especially in children
**Vitamin C deficiency can lead to SCURVYweak and brittle bones
REMODELING AND
HOMEOSTASIS
REMODELING
In adults, osteocytes in lacunae maintain surrounding
matrix by continually removing and replacing the
surrounding calcium salts
- osteoclasts and osteoblasts also remain active
even after epiphyseal plates have closed
- the activities of these cells are usually balanced: as
osteoblasts form an osteon, osteoclasts destroy one
- protein and mineral components are removed
and replaced through REMODELING
The purpose of this remodeling is to give bones
the ability to adapt to new stresses
- bones that are heavily stressed become thicker
and stronger
*** Exercise is important in maintaining
normal bone structure***
- degeneration in the skeleton can occur even after
short periods of inactivity
* think of what happens to a person’s leg when
they have to use crutches for a few weeks
MINERAL STORAGE
DID YOU KNOW?
The human body typically contains about 2.2 to
4.4 lbs of calcium, with 99% of it deposited in
the skeleton!
- calcium ion concentrations in the body must
be closely regulated
Ex: if calcium concentrations in body fluids
increase, neurons and muscle cells become
unresponsive; if concentrations decrease, they
become over-excited
- by providing a calcium reserve, the skeleton
helps maintain calcium HOMEOSTASIS in
body fluids
INJURY AND REPAIR
Even though bone is very strong, sometimes
cracks or breaks do occur
- all cracks or breaks in bone are called
FRACTURES
- fractures are classified according to their external
appearance, the site of the fracture, and the
nature of the break
- though bone repair may take anywhere from 4
months to over a year, bones will usually heal
even after severe damage
STEPS IN REPAIR OF A
FRACTURE
1. Blood vessels are broken and bleeding occurs
- blood pools and clots, and a swollen area called a
FRACTURE HEMATOMA forms- this closes off
the injured blood vessels
2. Cells of periosteum and endosteum divide and move
to the fracture zone
- the cells form thickenings called an EXTERNAL
CALLUS and an INTERNAL CALLUS
- at the center of the external callus, cells differentiate
into chondrocytes and build cartilage
3. Osteoblasts replace this cartilage with spongy bone
- when this process is complete, both the internal and
external calluses form a continuous brace of
spongy bone at the fracture site
4. When remodeling of the spongy bone is complete
(4 months to 1 year) fragments of dead bone and
the spongy bone will be gone, and living compact
bone will remain
- bone may just be slightly thicker at fracture site
AGING
Bones become thinner and weaker with age
OSTEOPENIA- inadequate ossification
- all people become slightly osteopenic as they age
- occurs between ages of about 30-40: osteoblast
activity begins to decline as osteoclast activity
continues
- women lose about 8% of their body mass
every 10 years, as compared to the 3% for
men
- epiphyses, vertebrae, and jaws lose more than
other bones  fragile limbs, reduced height,
loss of teeth
OSTEOPOROSIS- reduction in bone mass great
enough to compromise normal function
- 29% of women ages 45 to 79 are considered
osteoporotic (compared to 18% of men)
- this is linked to decreases in the production
of estrogens (sex hormones) after
menopause
EFFECTS:
- bones break easily and do not repair well;
vertebrae may collapse
- estrogen therapies, diets that increase calcium,
and exercise slow development of osteoporosis,
but do not prevent it completely
CHAPTER 6 NOTES
PART 2
SKELETAL ANATOMY
SURFACE FEATURES OF
BONES (BONE MARKINGS)
Some bones have projections, depressions, or even
holes
Projections on the surface of bones are called
processes:
Head- a large rounded articular surface
Neck- the narrow part of a bone between the
head and shaft
Spine- a sharp slender process (back of the
shoulder blade)
CONDYLE- a rounded knuckle-like process
located where the bone articulates with another
bone
CREST- a ridge of bone that is unusually narrow
TROCHANTER- a large projection for the
attachment of muscles
Depressions and openings on the surface of
bones include:
FOSSA- a shallow depression in the surface of a
bone
FORAMEN- rounded passageway for blood
vessels and/or nerves
SINUS- a chamber within a bone, usually filled
with air
DIVISIONS OF THE
SKELETON
RECALL: Skeleton consists of 206 separate bones
and associated cartilage
2 DIVISIONS:
AXIAL SKELETON- forms longitudinal axis of
the body- 80 bones
- skull (+ ossicles and hyoid bone)
- vertebral column
- thoracic cage
APPENDICULAR SKELETON- bones of limbs
and pectoral and pelvic girdles
AXIAL SKELETON
80 BONES
SKULL: CRANIUM AND FACE
SEE FIRST PAGE OF YOUR SKULL
HANDOUT FOR PARTS THAT YOU NEED
TO KNOW!
- REQUIRED PARTS ARE IN BOLD
HYOID BONE
- U-shaped bone that hangs below the skull,
suspended by ligaments from the styloid
processes of the temporal bones
- serves as a base for muscles associated with the
larynx (voice box), tongue, and pharynx
- supports and stabilizes position of the larynx
SKULLS OF INFANTS AND
CHILDREN
At birth the cranial bones
are connected by areas
of fibrous connective
tissue known as
FONTANELS
- “soft spots”
- flexible, permit distortion
of the skull without
damage (such as during
delivery)
VERTEBRAL COLUMN
The vertebral column is made up of a series of 26
bones called vertebrae
- linked together by cartilage to allow flexibility
- protects the spinal cord
- divided into cervical (neck), thoracic (chest),
lumbar (small of back), sacral (between hip
bones), and coccygeal (tail bone) regions
- SEE DIAGRAM
SPINAL CURVATURE
The spinal column is made up of 4 spinal curves:
PRIMARY CURVES- thoracic and sacral curves
- appear late in fetal development
SECONDARY CURVES- cervical and lumbar
curves
- do not appear until months after birth
- cervical develops as infant learns to balance the
head upright; lumbar develops with the ability to
stand
- all 4 curves are developed by the age of
10
ABNORMAL DISTORTIONS:
SCOLIOSIS- abnormal lateral curvature
TYPICAL VERTEBRA
- composed of a BODY with an arch on the
posterior surface
- arch roofs over an opening, the VERTEBRAL
FORAMEN, through which the spinal cord
passes
- there are several processes:
* SPINOUS PROCESS- attachment of ligaments
and muscles of the back
* TRANSVERSE PROCESSES- (2) project on
either side for attachment of muscles
* ARTICULAR PROCESSES- form the joints
connecting the individual vertebrae
SURFACES OF THESE PROCESSES ARE
COVERED WITH CARTILAGE
CERVICAL VERTEBRAE
The first 7 vertebrae are CERVICAL
The top 2 vertebrae are different:
** the first, the ATLAS, supports the skull
- has no body and therefore appears as a ring with
2 transverse processes
** the second, the AXIS, has a protruding upper
surface called the DENS- forms a pivot around
which the axis rotates to allow movement of the
head
THORACIC VERTEBRAE
The next 12 vertebrae are THORACIC
- distinguished by the presence of RIB FACETSprovide attachment for the ribs
- most thoracic vertebrae have 3 facets
LUMBAR VERTEBRAE
The next 5 vertebrae are LUMBAR
- have largest bodies since most of the weight of
the body is supported by this region
SACRUM AND COCCYX
SACRUM- triangular shaped bone containing
foramina that are passageways for nerves and
blood vessels
- consists of 5 fused sacral vertebrae
COCCYX- considered vestigial remains of 4 fused
coccygeal vertebrae
* SEE DIAGRAMS
THORAX: STERNUM AND
RIBS
STERNUM- thin flat bone divided into 3 parts:
* MANUBRIUM- upper part
* BODY- long and narrow
* XIPHOID PROCESS- made of cartilage
- ligaments form the attachment of the clavicles
(collar bones) to each side of the top of the
sternum
- 7 pairs of COSTAL CARTILAGES form the
attachment of the ribs to the body of the
sternum
- there are 12 pairs of ribs
- they are attached posteriorly with the thoracic
vertebrae and anteriorly with the sternum
- first 7 are TRUE RIBS because each has a direct
costal cartilage connection with the sternum
- next 3 pairs are called FALSE RIBS because their
cartilage connections join indirectly with the
sternum
- last 2 pairs are called FLOATING RIBS because
they have no connection with the sternum
* SEE DIAGRAMS
APPENDICULAR
SKELETON
126 BONES
PECTORAL GIRDLE
Consists of 2 pairs of bones:
SCAPULAE (shoulder blades) & CLAVICLES
(collar bones)
SCAPULAE
- flat, triangular-shaped bones
- shelf-like ridge extending across the posterior
surface of the bone called SPINE
- spine ends in a flattened projection called the
ACROMION PROCESS
- clavicle articulates with acromion of the scapula
- other end of the clavicle rests against the top of
the sternum
- beneath acromion is a rounded concave fossa
called the GLENOID CAVITY- head of
humerus fits into this cavity
- underneath the clavicle is the CORACOID
PROCESS- projects forward and serves as the
attachment for several muscles and ligaments
* SEE DIAGRAMS
ARM AND FOREARM
Each arm is made up of 3 main bones:
HUMERUS, ULNA, RADIUS
HUMERUS- long bone of the upper arm
- rounded head of the humerus articulates with
the scapula
- GREATER and LESSER TUBERCLES provide
attachment for muscles
- DELTOID TUBEROSITY runs along the shaft
and provides attachment for the deltoid muscle
- MEDIAL and LATERAL EPICONDYLES project to
either side providing additional muscle attachment
- CONDYLE makes up the inferior surface of the
humerus
* TROCHLEA is the large medial portion of the
condyle
* CORONOID FOSSA is a depression on the lower
anterior surface, while on lower posterior surface is a
notch called the OLECRANON FOSSA
ULNA- longer bone along the back of the
forearm
- at the upper end is the OLECRANON
PROCESS- fits into the olecranon fossa of the
humerus when the arm is extended (point of
elbow)
- on the lateral side of the ulna is the RADIAL
NOTCH for the head of the radius
- the flattened surface at the lower end of the
radius enables it to rotate around the ulna
- STYLOID PROCESSES of radius and ulna
articulate with the carpal (wrist) bones
* SEE DIAGRAMS
WRIST AND HAND
The wrist consists of 8 small bones which are held
tightly together by ligaments
CARPAL BONES- arranged in 2 rows to allow
the wrist to flex
Palm of the hand consists of 5 METACARPAL
BONES
- articulate with bones of the wrist, and bones of
the fingers
The bones of the fingers contain 14
PHALANGES
- each of the 4 fingers has 3 phalanges and the
thumb has 2
- thumb has greatest movement because its
metacarpal is more rounded, and it is attached to
many muscles in the hand
* SEE DIAGRAMS
PELVIC GIRDLE
Consists of 2 large hip bones joined together
- connect with the sacrum to form the pelvis
Each hip bone is composed of 3 fused bones:
ILIUM, ISCHIUM, PUBIS
ILIUM- largest, forms the upper portion of the
hip bone
ISCHIUM- forms the posterior portion
PUBIS- forms the anterior portion
As the hip bone develops, the 3 bones fuse into a
single bone
- eventually the 2 sets of bones meet in front and
form the PUBIC SYMPHYSIS
- SACROILIAC JOINT- forms in the back by the
articulation of the hip bones and the sacrum
DIFFERENCE IN MALE AND
FEMALE PELVIS
MALE- angle of the 2 pubic bones is more
pointed and narrow
- pelvis is deeper and the cavity is smaller
FEMALE- pubic arch is much wider and rounded
- pelvis is shallow and has a larger cavity
- adapted to childbearing
THIGH AND LOWER LEG
FEMUR- upper leg or thigh bone
- longest and strongest bone in the human skeleton
- HEAD is rounded and smooth and articulates with the
ilium at the ACETABULUM
- GREATER and LESSER TROCHANTERS are large
projections that extend laterally from the junction of
the neck and shaft; where tendons attach to the femur
- below head is the NECK- becomes more porous in
older people so common site of fracture
- on posterior surface of femur is an elevation called
LINEA ASPERA which marks attachment of
powerful thigh muscles
- lower end of femur expands into large flattened area
with 2 bony processes on each side: MEDIAL
EPICONDYLE, LATERAL EPICONDYLE
- inferior surfaces of epicondyles form LATERAL and
MEDIAL CONDYLES
- these come together to form the PATELLAR
SURFACE over which the PATELLA (kneecap) glides
TIBIA- larger bone of lower leg joins with processes to
form the knee joint
- patella is joined to tibia with a ligament
FIBULA- on outside of tibia, blunt end articulates with
the tibia below the knee joint
** A fib is a small lie; the fibula is the smaller bone
- lower end of tibia forms projecting ankle bone
on the inside of the leg  MEDIAL
MALLEOLUS
- at the end of the fibula is a lateral projection, the
LATERAL MALLEOLUS, which completes the
ankle joint at the side
ANKLE AND FOOT
Composed of 7 TARSAL BONES
- similar to bones of wrist
- tibia and fibula articulate with a tarsal bone called
the TALUS- allows foot to move up and down
in single plane
- largest bone of tarsals is the heel bone or
CALCANEUS
- 5 METATARSAL BONES- similar to
metacarpal bones
Foot is shaped to form 2 main arches- formed by
the joining of the metatarsals with the tarsals
LONGITUDINAL ARCH
TRANSVERSE ARCH- lies perpendicular to
longitudinal
- strengthen the foot and act as a spring to cushion
movements
- lowered arches or flat feet can occur
PHALANGES- 4 toes have 3 phalanges and the
hallux has only 2 (is like thumb)