The Skeleton: Structure and Function of the Human Skeletal System

The Skeleton

Overview of the Skeletal System

The human skeleton is a complex framework composed of bones, cartilages, joints, and ligaments. It provides structural support, protects vital organs, and facilitates movement. The skeleton accounts for approximately 20% of body mass and is divided into two major divisions: the axial skeleton and the appendicular skeleton.

• Axial skeleton: Forms the longitudinal axis of the body, supporting the head, neck, and trunk, and protecting the brain, spinal cord, and thoracic organs.

• Appendicular skeleton: Comprises the bones of the limbs and their girdles, enabling locomotion and manipulation of the environment.

Axial Skeleton

Regions and Functions

The axial skeleton consists of 80 bones divided into three major regions:

• Skull

• Vertebral column

• Thoracic cage

Its primary functions are to form the body's longitudinal axis, support the head, neck, and trunk, and protect the brain, spinal cord, and thoracic organs.

The Skull

The skull is formed by two sets of bones: cranial bones (cranium) and facial bones. Most skull bones are flat and firmly locked together by immovable joints called sutures, except for the mandible.

• Cranial bones: Enclose the brain and provide attachment sites for head and neck muscles.

• Facial bones: Form the framework of the face, contain cavities for special sense organs, provide openings for air and food passage, secure teeth, and anchor facial muscles for expression.

Skull Geography

The cranium is divided into the cranial vault (calvaria) and the cranial base, which forms the inferior aspect of the skull. The base is internally divided into three fossae: anterior, middle, and posterior, which support different regions of the brain.

Major Cavities and Openings

• Cranial cavity: Houses the brain.

• Other cavities: Middle and internal ear cavities, nasal cavity, orbits, and sinuses.

• Openings: The skull has 85 named openings (foramina, canals, fissures) for passage of the spinal cord, blood vessels, and cranial nerves.

Cranial Bones

• Frontal bone

• Parietal bones (left and right)

• Occipital bone

• Temporal bones (left and right)

• Sphenoid bone

• Ethmoid bone

Facial Bones

• Mandible

• Maxillary bones (2)

• Zygomatic bones (2)

• Nasal bones (2)

• Lacrimal bones (2)

• Palatine bones (2)

• Vomer

• Inferior nasal conchae (2)

Vertebral Column

General Characteristics

The vertebral column, or spine, extends from the skull to the pelvis and consists of 26 irregular bones called vertebrae, organized into five major regions:

• Cervical: 7 vertebrae

• Thoracic: 12 vertebrae

• Lumbar: 5 vertebrae

• Sacrum: 1 bone (fusion of several vertebrae)

• Coccyx: 1 bone (fusion of several vertebrae)

The vertebral column transmits the weight of the trunk to the lower limbs, surrounds and protects the spinal cord, and provides attachment points for ribs and muscles.

Curvatures

• Cervical and lumbar curvatures: Concave posteriorly

• Thoracic and sacral curvatures: Convex posteriorly

These curvatures increase the resilience and flexibility of the spine.

Abnormal Spinal Curvatures

• Scoliosis: Abnormal lateral rotation of the spine, often in the thoracic region.

• Kyphosis (hunchback): Abnormal dorsal thoracic curvature.

• Lordosis (swayback): Accentuated lumbar curvature.

Ligaments and Intervertebral Discs

• Ligaments: Support the vertebral column and prevent excessive movement.

• Intervertebral discs: Cushionlike pads between vertebrae, acting as shock absorbers. Each disc has a gelatinous nucleus pulposus and a tough anulus fibrosus.

General Structure of Vertebrae

• Body: Weight-bearing region

• Vertebral arch: Formed by pedicles and laminae

• Processes: Spinous, transverse, superior and inferior articular processes

• Vertebral foramen: Encloses the spinal cord

Regional Vertebral Characteristics

Characteristic	Cervical	Thoracic	Lumbar
Body	Small, oval	Heart-shaped, bears costal facets	Massive, kidney-shaped
Spinous process	Short, bifid (except C7)	Long, sharp, projects inferiorly	Short, blunt, rectangular
Vertebral foramen	Triangular, large	Circular	Triangular
Transverse processes	Contain foramina	Bear facets for ribs	Thin and tapered
Movements allowed	Flexion, extension, rotation	Rotation, limited flexion/extension	Flexion, extension, some lateral flexion

Thoracic Cage

Structure and Function

The thoracic cage is composed of the thoracic vertebrae, ribs, sternum, and costal cartilages. It protects vital organs, supports the shoulder girdles and upper limbs, and provides attachment sites for muscles.

• Sternum: Consists of the manubrium, body, and xiphoid process.

• Ribs: 12 pairs; true ribs (1–7) attach directly to the sternum, false ribs (8–10) attach indirectly, and floating ribs (11–12) have no anterior attachment.

Appendicular Skeleton

Pectoral Girdle

The pectoral (shoulder) girdle consists of the clavicles and scapulae, attaching the upper limbs to the axial skeleton and providing mobility and flexibility.

• Clavicle: S-shaped bone articulating with the sternum and scapula.

• Scapula: Triangular bone with borders, angles, and several important processes (spine, acromion, coracoid process).

Upper Limb

• Arm: Humerus

• Forearm: Radius and ulna

• Hand: Carpals (wrist), metacarpals (palm), phalanges (fingers)

Pelvic Girdle

The pelvic (hip) girdle is formed by two hip bones and the sacrum, attaching the lower limbs to the axial skeleton and supporting pelvic organs. The coxal bone is formed by the fusion of the ilium, ischium, and pubis.

Lower Limb

• Thigh: Femur (largest bone in the body)

• Leg: Tibia (weight-bearing) and fibula

• Foot: Tarsals, metatarsals, phalanges

Developmental Aspects of the Skeleton

Infant and Childhood Development

• Infant skull has more bones than adult skull; bones are connected by fontanelles to allow for growth and birth.

• Primary spinal curvatures are present at birth; secondary curvatures develop as the child grows.

Aging and the Skeleton

• Intervertebral discs lose hydration and elasticity with age, increasing the risk of herniation.

• Costal cartilages ossify, and bone mass decreases, increasing fracture risk.