The Skeletal System

Introduction to the Skeleton

The skeletal system is composed of bones, cartilages, joints, and ligaments. It provides the framework for the body, protects internal organs, and enables movement. There are approximately 206 named bones in the adult human body, though this number can vary slightly among individuals.

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage (80 bones). It forms the central axis of the body and protects vital organs such as the brain, heart, and lungs.

• Appendicular skeleton: Comprises the limbs and the pectoral (shoulder) and pelvic girdles (126 bones). It is primarily responsible for movement and manipulation of the environment.

Key Point: The axial skeleton provides support and protection, while the appendicular skeleton facilitates movement.

The Skull

Overview and Functions

The skull is a complex structure consisting of cranial bones, facial bones, and associated bones. It houses and protects the brain, forms the structure of the face, and contains cavities for sensory organs and air passage.

• Cranial bones: Form the protective case around the brain.

• Facial bones: Shape the face and provide attachment points for muscles.

• Associated bones: Involved in hearing and swallowing (e.g., auditory ossicles, hyoid bone).

• Cavities and sinuses: Air-filled spaces that lighten the skull and enhance vocal resonance.

Functions of the skull:

• Protects the brain

• Forms facial structures

• Provides openings for air and food

• Anchors teeth

• Houses sensory organs (sight, hearing, smell, taste)

Cranial Bones

The cranium is made of eight bones connected by immovable joints called sutures. These bones include:

• Frontal bone (1): Forehead region

• Parietal bones (2): Upper sides of the head

• Occipital bone (1): Back of the head; contains the foramen magnum for the spinal cord

• Temporal bones (2): Sides of the head near the ears; contain the auditory meatus and processes (styloid, mastoid, zygomatic)

• Sphenoid bone (1): Butterfly-shaped bone at the base of the skull; contains the sella turcica, which houses the pituitary gland

• Ethmoid bone (1): Sieve-like bone between the orbits and nasal cavity; contains the crista galli and cribriform plate for olfactory nerves

Facial Bones

The facial skeleton consists of 14 bones that form the structure of the face, anchor teeth, and contribute to the nasal and orbital cavities.

• Nasal bones (2): Bridge of the nose

• Maxillae (2): Upper jaw and part of the hard palate

• Zygomatic bones (2): Cheekbones

• Mandible (1): Lower jaw; only freely movable bone of the skull

• Lacrimal bones (2): Medial walls of the orbits

• Palatine bones (2): Posterior part of the hard palate

• Vomer (1): Forms part of the nasal septum

• Inferior nasal conchae (2): Lateral walls of the nasal cavity

Cavities and Sinuses of the Skull

The skull contains several cavities and sinuses that serve important functions:

• Orbital cavity: Houses the eyes; formed by seven bones (frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, ethmoid)

• Nasal cavity: Formed by the maxilla, nasal, sphenoid, vomer, inferior nasal conchae, palatine, and ethmoid bones

• Paranasal sinuses: Air-filled spaces in the frontal, ethmoid, sphenoid, and maxilla bones; lighten the skull, warm and humidify air, and enhance vocal resonance

The Vertebral Column (Spine)

Structure and Regions

The vertebral column consists of 24 vertebrae, the sacrum, and the coccyx. It supports the body, protects the spinal cord, and allows flexible movement.

• Cervical vertebrae (7): Neck region (C1–C7); C1 (atlas) supports the head, C2 (axis) allows rotation

• Thoracic vertebrae (12): Upper back (T1–T12); articulate with the ribs

• Lumbar vertebrae (5): Lower back (L1–L5); largest and strongest

• Sacrum: Five fused vertebrae; part of the pelvis

• Coccyx: Three to five fused vertebrae; tailbone

Curvatures: The spine has four curvatures (cervical, thoracic, lumbar, sacral) that act like a spring to absorb shock.

Intervertebral discs: Pads of connective tissue between vertebrae that cushion and allow movement; absent between C1 and C2.

The Thoracic Cage

Structure and Function

The thoracic cage protects the thoracic cavity and provides structure for the lungs. It consists of the thoracic vertebrae, ribs, and sternum.

• Ribs (12 pairs): Flat bones that wrap around the chest

• True ribs (1–7): Attach directly to the sternum via costal cartilage

• False ribs (8–12): Attach indirectly or not at all to the sternum; ribs 11 and 12 are floating ribs with no sternal attachment

• Sternum: Flat bone consisting of the manubrium, body, and xiphoid process

• Intercostal cartilage: Connects ribs 1–10 to the sternum, allowing flexibility

The Pectoral Girdle

Structure and Function

The pectoral (shoulder) girdle attaches the arms to the axial skeleton and consists of two bones on each side:

• Clavicle (collarbone): Articulates with the sternum and scapula

• Scapula (shoulder blade): Articulates with the clavicle and humerus; contains the acromion process and glenoid cavity (shoulder socket)

Bones of the Upper Limb

Arm and Forearm

• Humerus: The only bone of the upper arm; articulates with the scapula at the shoulder and with the radius and ulna at the elbow

• Radius: Lateral bone of the forearm (thumb side); allows rotation of the hand

• Ulna: Medial bone of the forearm (pinky side); forms a hinge joint with the humerus

Wrist and Hand

• Carpals (8): Short bones of the wrist

• Metacarpals (5): Long bones of the palm; numbered I (thumb) to V (pinky)

• Phalanges (14): Bones of the fingers; each finger has three (proximal, middle, distal) except the thumb, which has two (proximal and distal)

The Pelvic Girdle

Structure and Function

The pelvic girdle is formed by the right and left coxal (hip) bones, which together with the sacrum and coccyx form the pelvis. Each coxal bone is made of three fused bones:

• Ilium: Superior, flared region; forms the iliac crest

• Ischium: Posterior, lower region; forms the obturator foramen and the bones you sit on

• Pubis: Anterior, lower region; forms the pubic symphysis

• Acetabulum: Deep socket formed by all three bones; articulates with the femur

Sexual Dimorphism of the Pelvis

The shape of the pelvis differs between males and females, reflecting adaptations for childbirth in females.

Bones of the Lower Limb

Thigh and Leg

• Femur: Longest and strongest bone; head fits into the acetabulum of the pelvis

• Patella: Sesamoid bone (kneecap) embedded in the tendon of the quadriceps muscle

• Tibia: Larger, medial bone of the lower leg; bears most of the weight

• Fibula: Smaller, lateral bone of the lower leg; stabilizes the ankle

Ankle and Foot

• Tarsals (7): Short bones of the ankle; includes the talus (articulates with tibia) and calcaneus (heel bone)

• Metatarsals (5): Long bones forming the arches of the foot; numbered I (medial) to V (lateral)

• Phalanges (14): Bones of the toes; each toe has three (proximal, middle, distal) except the big toe (hallux), which has two

Key Point: The arches of the foot, formed by the metatarsals and tarsals, help distribute body weight and provide springiness to the step.

Summary Table: Major Bones of the Skeleton

Additional info: The number of bones in the skeleton can vary due to anatomical variations such as extra ribs or fused vertebrae. The structure of the skeleton is closely related to its function, with differences in bone shape, size, and articulation reflecting the demands placed on different regions of the body.