The Skeletal System

The Skeletal System

Parts of the skeletal system
Bones (skeleton)
Two subdivisions of the skeleton
Axial skeleton
Appendicular skeleton

Functions of Bones
Support the body
Protect soft organs
Skull and vertebrae for brain and spinal cord
Rib cage for thoracic cavity organs
Allow movement due to attached skeletal muscles
Store minerals and fats
Calcium and phosphorus
Fat in the internal marrow cavity
Blood cell formation (hematopoiesis)

Bones of the Human Body
The adult skeleton has 206 bones
Two basic types of bone tissue
Compact bone
Spongy bone
Small needle-like pieces of bone
Many open spaces

Classification of Bones on the Basis of Shape

Long bones
Typically longer than they are wide
Shaft with heads situated at both ends
Contain mostly compact bone
All of the bones of the limbs (except wrist, ankle, and kneecap bones)
Short bones
Generally cube-shaped
Contain mostly spongy bone
Includes bones of the wrist and ankle
Sesamoid bones are a type of short bone which form within tendons (patella)
Flat bones
Thin, flattened, and usually curved
Two thin layers of compact bone surround a layer of spongy bone
Irregular bones
Irregular shape
Do not fit into other bone classification categories
Hip bones

Anatomy of a Long Bone
Composed of compact bone
Ends of the bone
Composed mostly of spongy bone
Outside covering of the diaphysis
Fibrous connective tissue membrane
Perforating (Sharpey’s) fibers
Secure periosteum to underlying bone
Supply bone cells with nutrients
Articular cartilage
Covers the external surface of the epiphyses
Made of hyaline cartilage
Decreases friction at joint surfaces
Epiphyseal plate
Flat plate of hyaline cartilage seen in young, growing bone
Epiphyseal line
Remnant of the epiphyseal plate
Seen in adult bones
Marrow (medullary) cavity 
Cavity inside of the shaft
Contains yellow marrow (mostly fat) in adults
Contains red marrow for blood cell formation in infants
In adults, red marrow is situated in cavities of spongy bone and epiphyses of some long bones

Bone Markings
Surface features of bones
Sites of attachments for muscles, tendons, and ligaments
Passages for nerves and blood vessels
Categories of bone markings
Projections or processes—grow out from the bone surface
Terms often begin with “T”
Depressions or cavities—indentations
Terms often begin with “F”

Microscopic Anatomy of Compact Bone
Osteon (Haversian system)
A unit of bone containing central canal and matrix rings
Central (Haversian) canal
Opening in the center of an osteon
Carries blood vessels and nerves
Perforating (Volkmann’s) canal
Canal perpendicular to the central canal
Carries blood vessels and nerves

Microscopic Anatomy of Bone
Cavities containing bone cells (osteocytes)
Arranged in concentric rings called lamellae
Rings around the central canal
Sites of lacunae
Tiny canals
Radiate from the central canal to lacunae
Form a transport system connecting all bone cells to a nutrient supply

Formation of the Human Skeleton
In embryos, the skeleton is primarily hyaline cartilage
During development, much of this cartilage is replaced by bone
Cartilage remains in isolated areas
Bridge of the nose
Parts of ribs

Bone Growth (Ossification)

Epiphyseal plates allow for lengthwise growth of long bones during childhood
New cartilage is continuously formed
Older cartilage becomes ossified
Cartilage is broken down
Enclosed cartilage is digested away, opening up a medullary cavity
Bone replaces cartilage through the action of osteoblasts
Bones are remodeled and lengthened until growth stops
Bones are remodeled in response to two factors
Blood calcium levels
Pull of gravity and muscles on the skeleton
Bones grow in width (called appositional growth)

Types of Bone Cells
Osteocytes—mature bone cells
Osteoblasts—bone-forming cells
Osteoclasts—giant bone-destroying cells
Break down bone matrix for remodeling and release of calcium in response to parathyroid hormone
Bone remodeling is performed by both osteoblasts and osteoclasts

Bone Fractures
Fracture—break in a bone
Types of bone fractures
Closed (simple) fracture—break that does not penetrate the skin
Open (compound) fracture—broken bone penetrates through the skin
Bone fractures are treated by reduction and immobilization

Common Types of Fractures
Repair of Bone Fractures
Hematoma (blood-filled swelling) is formed
Break is splinted by fibrocartilage to form a callus
Fibrocartilage callus is replaced by a bony callus
Bony callus is remodeled to form a permanent patch
The Axial Skeleton
Forms the longitudinal axis of the body
Divided into three parts
Vertebral column
Bony thorax
The Skull
Two sets of bones
Facial bones
Bones are joined by sutures
Only the mandible is attached by a freely movable joint
Paranasal Sinuses
Hollow portions of bones surrounding the nasal cavity
Functions of paranasal sinuses
Lighten the skull
Give resonance and amplification to voice
The Hyoid Bone
The only bone that does not articulate with another bone
Serves as a moveable base for the tongue
Aids in swallowing and speech
The Fetal Skull
The fetal skull is large compared to the infant’s total body length
Fetal skull is 1/4 body length compared to adult skull which is 1/8 body length
Fontanels—fibrous membranes connecting the cranial bones
Allow skull compression during birth
Allow the brain to grow during later pregnancy and infancy
Convert to bone within 24 months after birth
The Vertebral Column
Each vertebrae is given a name according to its location
There are 24 single vertebral bones separated by intervertebral discs
Seven cervical vertebrae are in the neck 
Twelve thoracic vertebrae are in the chest region
Five lumbar vertebrae are associated with the lower back
Nine vertebrae fuse to form two composite bones
Primary curvatures are the spinal curvatures of the thoracic and sacral regions
Present from birth
Form a C-shaped curvature as in newborns
Secondary curvatures are the spinal curvatures of the cervical and lumbar regions
Develop after birth
Form an S-shaped curvature as in adults
Sacrum and Coccyx
Formed by the fusion of five vertebrae
Formed from the fusion of three to five vertebrae
“Tailbone,” or remnant of a tail that other vertebrates have
The Bony Thorax
Forms a cage to protect major organs
Consists of three parts
True ribs (pairs 1–7)
False ribs (pairs 8–12)
Floating ribs (pairs 11–12)
Thoracic vertebrae

The Appendicular Skeleton
Composed of 126 bones
Limbs (appendages)
Pectoral girdle
Pelvic girdle
The Pectoral (Shoulder) Girdle
Composed of two bones
Articulates with the sternum medially and with the scapula laterally
Scapula—shoulder blade
Articulates with the clavicle at the acromioclavicular joint
Articulates with the arm bone at the glenoid cavity
These bones allow the upper limb to have exceptionally free movement
Bones of the Upper Limbs
Forms the arm
Single bone
Proximal end articulation
Head articulates with the glenoid cavity of the scapula
Distal end articulation
Trochlea and capitulum articulate with the bones of the forearm
The forearm has two bones
Ulna—medial bone in anatomical position
Proximal end articulation
Coronoid process and olecranon articulate with the humerus
Radius—lateral bone in anatomical position
Proximal end articulation
Head articulates with the capitulum of the humerus
Eight bones arranged in two rows of four bones in each hand
Five per hand
Phalanges—fingers and thumb
Fourteen phalanges in each hand
In each finger, there are three bones
In the thumb, there are only two bones
Bones of the Pelvic Girdle
Formed by two coxal (ossa coxae) bones
Composed of three pairs of fused bones
Pelvic girdle = 2 coxal bones, sacrum
Bony pelvis = 2 coxal bones, sacrum, coccyx
The total weight of the upper body rests on the pelvis
It protects several organs
Reproductive organs
Urinary bladder
Part of the large intestine
Gender Differences of the Pelvis
The female inlet is larger and more circular
The female pelvis as a whole is shallower, and the bones are lighter and thinner
The female ilia flare more laterally
The female sacrum is shorter and less curved
The female ischial spines are shorter and farther apart; thus the outlet is larger
The female pubic arch is more rounded because the angle of the pubic arch is greater
Bones of the Lower Limbs
Femur—thigh bone
The heaviest, strongest bone in the body
Proximal end articulation
Head articulates with the acetabulum of the coxal (hip) bone
Distal end articulation
Lateral and medial condyles articulate with the tibia in the lower leg 
The lower leg has two bones
Tibia—Shinbone; larger and medially oriented
Proximal end articulation
Medial and lateral condyles articulate with the femur to form the knee joint
Fibula—Thin and sticklike; lateral to the tibia
Has no role in forming the knee joint
The foot
Tarsals—seven bones
Two largest tarsals
Calcaneus (heel bone)
Metatarsals—five bones form the sole of 
the foot
Phalanges—fourteen bones form the toes

Arches of the Foot
Bones of the foot are arranged to form three strong arches
Two longitudinal
One transverse

Articulations of bones
Functions of joints
Hold bones together
Allow for mobility
Two ways joints are classified

Functional Classification of Joints
Immovable joints
Slightly moveable joints
Freely moveable joints

Structural Classification of Joints
Fibrous joints
Generally immovable
Cartilaginous joints
Immovable or slightly moveable
Synovial joints
Freely moveable

Fibrous Joints

Bones united by collagenic fibers
Allows more movement than sutures but still immobile
Example: Distal end of tibia and fibula

Cartilaginous Joints

Bones connected by cartilage
Slightly movable
Example: Pubic symphysis, intervertebral joints

Synovial Joints

Articulating bones are separated by a joint cavity
Synovial fluid is found in the joint cavity
Features of Synovial Joints
Articular cartilage (hyaline cartilage) covers the ends of bones
Articular capsule encloses joint surfaces and lined with synovial membrane
Joint cavity is filled with synovial fluid
Reinforcing ligaments 
Structures Associated with the Synovial Joint
Bursae—flattened fibrous sacs
Lined with synovial membranes
Filled with synovial fluid
Not actually part of the joint
Tendon sheath
Elongated bursa that wraps around a tendon

Inflammatory Conditions Associated 
with Joints

Bursitis—inflammation of a bursa usually caused by a blow or friction
Tendonitis—inflammation of tendon sheaths
Arthritis—inflammatory or degenerative diseases of joints
Over 100 different types
The most widespread crippling disease in the United States
Initial symptoms:  pain, stiffness, swelling of the joint
Clinical Forms of Arthritis
Most common chronic arthritis
Probably related to normal aging processes
Rheumatoid arthritis
An autoimmune disease—the immune system attacks the joints
Symptoms begin with bilateral inflammation of certain joints
Often leads to deformities
Gouty arthritis
Inflammation of joints is caused by a deposition of uric acid crystals from the blood
Can usually be controlled with diet
More common in men

Developmental Aspects of the Skeletal System

At birth, the skull bones are incomplete
Bones are joined by fibrous membranes called fontanels
Fontanels are completely replaced with bone within two years after birth

Skeletal Changes Throughout Life
Long bones are formed of hyaline cartilage
Flat bones begin as fibrous membranes
Flat and long bone models are converted to bone
Fontanels remain until around age 2
Epiphyseal plates become ossified and long bone growth ends
Size of cranium in relationship to body
2 years old—skull is larger in proportion to the body compared to that of an adult
8 or 9 years old—skull is near adult size and proportion
Between ages 6 and 11, the face grows out from the skull
Curvatures of the spine
Primary curvatures are present at birth and are convex posteriorly 
Secondary curvatures are associated with a child’s later development and are convex anteriorly
Abnormal spinal curvatures (scoliosis and lordosis) are often congenital
Bone-thinning disease afflicting 
50 percent of women over age 65 
20 percent  of men over age 70
Disease makes bones fragile and bones can easily fracture
Vertebral collapse results in kyphosis (also known as dowager’s hump)
Estrogen aids in health and normal density of a female skeleton