The Skeletal System: Structure, Function, and Major Components

Overview of the Skeletal System

Structure and Divisions of the Skeletal System

The human skeletal system is composed of approximately 206 bones and associated cartilages, forming the framework that supports and protects the body. The skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Forms the longitudinal axis of the body and includes the skull, vertebral column, and thoracic cage. It primarily functions to protect vital organs such as the brain, spinal cord, heart, and lungs.

• Appendicular Skeleton: Comprises the bones of the pectoral girdle, upper limbs, pelvic girdle, and lower limbs. It is mainly involved in movement and muscle attachment.

Bone Markings

Types and Functions of Bone Markings

Bones display various surface features known as bone markings, which reflect their functional roles. These markings are classified as depressions, openings, and projections.

• Depressions: Serve as pathways for blood vessels and nerves or as sites for articulations between bones.

• Openings: Allow the passage of delicate structures through bones.

• Projections: Sites where bones articulate or where ligaments and tendons attach.

Bone Marking	Description	Example
Facet	Shallow concave surface where two bones articulate	Rib: Articular facet for articulation with transverse process
Fossa	Indentation in a bone into which another structure fits	Humerus: Distal portion with olecranon fossa
Fovea	Shallow pit	Femur: Fovea capitis

The Skull

Structure and Organization

The skull consists of 22 bones organized into cranial and facial groups. Cranial bones (8) protect the brain, while facial bones (14) form the framework of the face. Most skull bones are fused at immovable joints called sutures, except for the mandible.

• Cranial Bones: Frontal, parietal, occipital, temporal, ethmoid, and sphenoid bones.

• Facial Bones: Maxilla, zygomatic, nasal, lacrimal, palatine, inferior nasal concha, mandible, and vomer.

Cranial Vault and Base

The cranial vault (calvarium) forms the superior portion of the cranial cavity, while the cranial base forms the inferior portion, divided into anterior, middle, and posterior cranial fossae that accommodate the brain.

Cavities of the Skull

The skull contains several cavities, including the cranial cavity (brain), orbits (eyeballs), nasal cavity (smell), oral cavity (taste), and small cavities for hearing and balance.

Disarticulated Skull

Viewing the skull as a disarticulated model helps in understanding the spatial relationships between individual bones, especially complex ones like the ethmoid and sphenoid.

Orbit and Nasal Cavity

The orbit is formed by seven fused bones and houses the eyeball and associated structures. The nasal cavity is lined with mucous membranes and forms the first part of the respiratory tract.

Fetal Skull and Fontanels

In the fetal skull, bones are not yet fused, and membranous areas called fontanels allow for flexibility during birth and brain growth. Major fontanels include anterior, posterior, sphenoid, and mastoid fontanels.

Hyoid Bone

The hyoid bone is a C-shaped bone in the neck that does not articulate with any other bone. It provides attachment points for muscles involved in swallowing and speech.

The Vertebral Column

Structure and Classification

The vertebral column consists of an average of 33 vertebrae, classified by region: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 3–5 fused coccygeal vertebrae. The column supports the head, protects the spinal cord, and allows for flexible movement.

Spinal Curvatures

Normal spinal curvatures include primary (thoracic and sacral) and secondary (cervical and lumbar) curves. Abnormal curvatures include scoliosis (lateral), lordosis (exaggerated lumbar), and kyphosis (exaggerated thoracic).

Structure of a Typical Vertebra

All vertebrae share common features: a body (centrum), vertebral foramen, pedicles, laminae, transverse and spinous processes, and articular processes. These structures provide support, protection, and sites for muscle attachment.

Cervical Vertebrae

Cervical vertebrae are the smallest and have transverse foramina. The atlas (C1) supports the skull, and the axis (C2) allows for rotation of the head.

Thoracic and Lumbar Vertebrae

Thoracic vertebrae articulate with ribs and have heart-shaped bodies. Lumbar vertebrae are the largest, supporting the weight of the torso.

Thoracic Cage

Structure and Function

The thoracic cage consists of the sternum, 12 pairs of ribs, and thoracic vertebrae. It protects the heart and lungs and provides attachment points for muscles involved in respiration.

• True Ribs (1–7): Attach directly to the sternum via costal cartilages.

• False Ribs (8–12): Do not attach directly to the sternum; ribs 11 and 12 are floating ribs.

Pectoral Girdle and Upper Limb

Pectoral Girdle

The pectoral girdle consists of the clavicle and scapula, supporting the upper limb and providing attachment sites for muscles.

Humerus

The humerus is the only bone of the arm, articulating proximally with the scapula and distally with the radius and ulna.

Forearm: Radius and Ulna

The radius and ulna are the bones of the forearm, connected by an interosseous membrane and articulating with the humerus and carpal bones.

Wrist and Hand

The wrist contains eight carpal bones, while the hand consists of five metacarpals and fourteen phalanges.

Pelvic Girdle and Lower Limb

Pelvis and Pelvic Girdle

The pelvis is formed by the sacrum and two coxal bones (ilium, ischium, and pubis). It supports the weight of the upper body and protects pelvic organs.

Femur and Patella

The femur is the largest bone in the body, articulating with the pelvis at the hip joint and with the tibia at the knee. The patella is a sesamoid bone within the tendon of the quadriceps muscle.

Tibia and Fibula

The tibia is the main weight-bearing bone of the leg, while the fibula provides lateral stability to the ankle.

Ankle and Foot

The ankle contains seven tarsal bones, and the foot contains five metatarsals and fourteen phalanges. The foot's arches help support and distribute body weight during movement.

Study Boost: Mnemonics and Visual Analogies

• PEST OF 6: Parietal, Ethmoid, Sphenoid, Temporal, Occipital, Frontal (cranial bones)

• Breakfast at 7, lunch at 12, dinner at 5: 7 cervical, 12 thoracic, 5 lumbar vertebrae

• Stop Letting The People Touch The Cadaver’s Hand: Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate (carpal bones)

Additional info: These mnemonics and analogies are useful for memorizing the names and locations of bones in the skeletal system.