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Skeletal System
Introduction
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The framework of bones and cartilage that
protects our organs and allows us to move is
called the skeletal system.
The branch of medicine that deals with the
preservation and restoration of the skeletal
system, articulations (joints), and associated
structures is called orthopaedics.
The skeletal system performs the
following functions:
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Support
Protection (for internal organs)
Movement
Mineral storage
Storage of blood cell-producing cells
Storage of energy
Types of Bones
There are five principal types of bones based on shape
1. Long bones (e.g. thighs, legs, toes, arms, forearms,
and fingers)
 greater length than width
 consist of a shaft and extremities (ends)
 slightly curved for strength
 consist mostly of compact bone (dense bone with
few spaces) but also contain considerable amounts of
spongy bone (bone with large spaces)
2. Short bones (e.g. wrist, ankle
bones)
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Somewhat cube-shaped and nearly equal in
length and width
Spongy except at the surface where there is a
thin layer of compact bone
3. Flat bones (e.g. cranial bones,
sternum, ribs, scapulas)
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Generally thin and composed of two more or
less parallel plates of compact bone enclosing
a layer of spongy bone
Flat bones afford considerable protection and
provide extensive areas for muscle attachment
4. Irregular bones (e.g. vertebrae,
and certain facial bones)
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Have complex shapes and cannot be grouped
into any of the other three categories
They vary in the amount of spongy and
compact bone
5. Sesamoid bones
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Are small bones in tendons where
considerable pressure develops, for instance,
the wrist
Their number varies greatly from person to
person
All people have at least two sesamoid bones:
the patella (kneecap)
Divisions of the Skeletal System
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The adult human skeleton consists of 206 bones grouped as
the axial skeleton and the appendicular skeleton.
The axial division consists of the bones of the skull, auditory
ossicles, hyoid bone, ribs, breastbone, and the backbone.
The appendicular division consists of the bones of the upper
and lower extremities (limbs), plus the bones called girdles,
which connect the extremities to the axial skeleton.
There are 80 bones in the axial division and 126 in the
appendicular. Listed below are the divisions of the skeletal
system.
Anatomical Position (AP)
AP is the reference position used to describe the
location of anatomical parts and to describe
and explain human movement.
The subject is:
 standing upright with feet flat on the floor
 arms at the side of the body
 facing the observer
 palms are facing forward (supinated)
Anatomical Position
Directional Terms
Note: the midline is an imaginary vertical line
that divides the body into equal left and right
sides
Superior: nearer the head
 e.g. the heart is superior to the liver
Inferior: farther away from the head
 e.g. the stomach is inferior to the lungs
Directional Terms
Anterior (ventral): nearer to or in front of the
body
 e.g. the sternum is anterior to the heart
Posterior (dorsal):
nearer to or at the back of
the body
 e.g. the esophagus is posterior to the trachea
Directional Terms
Medial: closer to the midline of the body
 e.g. the ulna is on the medial side of the
forearm
Lateral: farther away from the midline of the
body
 e.g. the ear is lateral to the nose in all
individuals
Directional Terms
Proximal: nearer to the attachment of an extremity
(limb) to
 the trunk; nearer to the point of origin
 e.g. the humerus is proximal to the radius
Distal: farther from the attachment of an extremity
(limb) to the
 trunk; farther away from the point of origin
 e.g. the phalanges are distal to the carpals (wrist
bones)
Directional Terms
Superficial: on or near the surface of the body
 e.g. the skin is the most superficial organ of
the body
Deep: farther away from the surface of the body
 e.g. the ribs are deep to the skin of the chest
Planes of Motion and Axes
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Biomechanics is the study of human
movements through the use of physics. As
such, biomechanics has its own language and
terminology. The language of biomechanics
establishes a common reference system of
standard terms. Planes and axes of motion are
an important part of this language.
Planes of Motion
A plane of motion can be defined as the two dimensional space cut by a
moving body or the plane along which movements occur. There are
generally three planes used to describe segmental and body movements in
physical activity. These are:
Sagittal plane
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a vertical plane that cuts the body into right and left sides
Frontal plane
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vertical plane that cuts the body into anterior (front) and posterior (back)
parts
Transverse plane
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horizontal plane that cuts the body into superior and inferior parts
Anatomical Planes
Anatomical Axes
An axis is the point about which rotation of a body or of a body segment occurs. There
are three axes of rotation. Each axis is associated with a plane of motion and the
axis is perpendicular to that plane.
Horizontal axis (…think East and West)
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passes through the body from side to side
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perpendicular to the sagittal plane
Antereoposterior axis (axis)
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passes through the body from front to back
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perpendicular to the frontal plane
Longitudinal axis (…think North and South pole)
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passes through the body from top to bottom
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perpendicular to the transverse plane
Planes & Movements
Movements in the sagittal plane around a horizontal axis
 (e.g. front roll, back roll, cycling, running)
Flexion
 flexion at a joint results in a decrease of the angle
between the two segments that meet at that joint
Extension
 extension at a joint results in an increase of the angle
between the two segments that meet at that joint
 if the movement occurs beyond the extended
position, the action is called hyperextension
Examples of flexion and extension
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shoulder flexion and extension
elbow flexion and extension
wrist flexion and extension
fingers flexion and extension
hip flexion and extension
knee flexion and extension
ankle dorsi flexion and plantar flexion
tilt of pelvis under
Dorsi flexion:
bringing the toes toward the shin
Plantar flexion: pointing the toes away from the shin
(toward the floor)
Planes & Movements
Movements in the frontal plane around a antereoposterior axis
 (e.g. cartwheel, jumping jacks, galloping)
Abduction
 occurs when a body part is moved away from the midline of
the body
 e.g. shoulder, hip, fingers
Adduction
 occurs when a body part is moved toward the midline of the
body
 e.g. shoulder, hip, fingers
 remember “add to your midline”
 e.g. shoulder, hip, fingers
Movements & Planes
Inversion
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Turning the sole of the foot inward at the ankle (so the sole of the foot
faces toward the midline)
Eversion
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turning the sole of the foot outward at the ankle (so the sole of the foot
faces away from the midline)
Elevation
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raising a part to a superior position
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e.g. raising your shoulders toward your ears; closing your jaw
Depression
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lowering a part to an inferior position
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e.g. lowering your shoulders to normal or lower than normal position;
lowering your jaw to an open position
More…
Protraction
 Sticking jaw out (pouting)
Retraction
 Bringing jaw back to anatomical position
 Lateral bending
 bending of the spinal column in the frontal plane to
the left or right
 e.g. bending side to side at the waist
And More…
Movements in the transverse plane around a longitudinal axis
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(e.g. twist, pirouette)
Rotation
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the movement of a bone around its own axis; this is also known as a pivot
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e.g. the head, neck, and trunk can pivot around the longitudinal axis
Internal (medial) rotation
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Rotation towards the midline
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E.g. turning forearn in towards body
External (lateral) rotation
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Rotation away from midline
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E.g. turning forearm away from body
More…
Pronation
 rotation of the forearm and hand to the palms down position
Supination
 rotation of the forearm and hand to the palms up position
(remember holding a cup of “soup”)
Protraction
 Shoulder rounding (hunching shoulders)
Retraction
 Bringing shoulders back to anatomical position, or squeezing
shoulder blades together at back
Special movements
Circumduction
 a combination of abduction, adduction, flexion and extension
 this action describes a circle
 e.g. moving the shoulder in a circle (swimming, windmill
throw in baseball); can also be done at the hip joint
Opposition
 Bringing thumb towards fingers
Reposition
 Returning thumb back to anatomical position
Skeletal Surface Markings
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The surfaces of bones have various structural
features adapted to specific functions. These
features are called surface markings. Long
bones that bear a great deal of weight have
large, rounded ends that can form sturdy
joints, for example. Other bones have
depressions that receive the rounded ends.
Depressions and Openings
Foramen
an opening through which blood
vessels, nerves, or ligaments
pass
Example:
Meatus
a tubelike passageway running
within a bone
Example:
Paranasal
sinus
an air-filled cavity within a bone
connected to the nasal cavity
Fossa
a depression in or on a bone
Example:
Processes that form Joints
Condyle
a large, rounded articular
prominence
Example:
Head
a rounded articular projection
supported on the constricted
portion (neck) of a bone
Example:
Facet
a smooth, flat surface
Example:
Processes to which tendons, ligaments and other connective tissues
attach
Tuberosity
a large, rounded, usually
roughened process
Example:
Spinous
a sharp, slender projection
process
Example:
Trochanter a large, blunt projection
found only on the femur
Example:
Crest
Example:
a prominent border or ridge