Chapter 7: The Skeleton – Structure and Function of the Human Skeletal System

The Skeletal System: Overview

Composition and Divisions

The skeletal system is a structural framework composed of bones, cartilages, joints, and ligaments. It accounts for approximately 20% of body mass and is divided into two major regions: the axial skeleton and the appendicular skeleton.

• Bones: Provide support, protection, and leverage for movement.

• Cartilage: Occurs in isolated areas, providing flexibility and cushioning.

• Ligaments: Connect bones and reinforce joints.

The two major divisions are:

• Axial skeleton: Forms the longitudinal axis of the body.

• Appendicular skeleton: Includes the limbs and their girdles.

Axial Skeleton

Regions and Functions

The axial skeleton consists of 80 bones divided into three major regions: the skull, vertebral column, and thoracic cage. Its primary functions are to form the longitudinal axis of the body, support the head, neck, and trunk, and protect the brain, spinal cord, and thoracic organs.

• Skull: 28 bones (including middle ear bones)

• Vertebral column: 26 bones

• Thoracic cage: 26 bones (24 ribs)

The Skull

The skull is the most complex bony structure in the body, formed by two sets of bones: cranial bones and facial bones. Most skull bones are flat and firmly locked together by 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.

Cranial Cavities and Openings

• Cranial cavity: Contains the brain.

• Middle and internal ear cavities, nasal cavity, sinuses, and orbits: House special sense organs.

• Foramina, canals, fissures: 85 named openings for passage of 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: Largest, strongest facial bone; forms the lower jaw.

• Maxillary bones (maxillae): Form the upper jaw and central facial skeleton.

• Zygomatic bones: Form cheekbones and inferolateral margins of orbits.

• Nasal bones: Form the bridge of the nose.

• Lacrimal bones: Form medial walls of orbits; house lacrimal sac.

• Palatine bones: L-shaped bones forming part of the hard palate.

• Vomer: Forms part of the nasal septum.

• Inferior nasal conchae: Form part of the lateral walls of the nasal cavity.

The Vertebral Column

General Characteristics

The vertebral column, or spine, extends from the skull to the pelvis. It transmits the weight of the trunk to the lower limbs, surrounds and protects the spinal cord, and provides attachment points for ribs and muscles. The column contains 26 irregular bones called vertebrae, organized into five major regions.

• Cervical: 7 vertebrae

• Thoracic: 12 vertebrae

• Lumbar: 5 vertebrae

• Sacrum: 1 bone (formed from fusion of several bones)

• Coccyx: Fused bones forming the terminus of the column

Curvatures

• Cervical and lumbar curvatures: Concave posteriorly

• Thoracic and sacral curvatures: Convex posteriorly

Ligaments and Intervertebral Discs

• Anterior and posterior longitudinal ligaments: Continuous bands running down the front and back of the spine, supporting and preventing excessive bending.

• Ligamentum flavum: Connects adjacent vertebrae, providing strength.

• Intervertebral discs: Cushionlike pads between vertebrae acting as shock absorbers. Composed of:

  • Nucleus pulposus: Inner gelatinous nucleus providing elasticity and compressibility.

  • Anulus fibrosus: Outer collar of collagen and fibrocartilage limiting expansion of the nucleus pulposus.

General Structure of Vertebrae

• Body (centrum): Anterior weight-bearing region.

• Vertebral arch: Forms the vertebral foramen.

• Vertebral foramen: Enclosure for the spinal cord.

• Vertebral canal: Series of vertebral foramina.

• Intervertebral foramina: Lateral openings for passage of spinal nerves.

Processes of Vertebrae

• Spinous process: Projects posteriorly.

• Transverse processes (2): Project laterally.

• Superior articular processes (2): Protrude superiorly.

• Inferior articular processes (2): Protrude inferiorly.

Regional Vertebral Characteristics

• Cervical vertebrae (C1–C7): Smallest, lightest; C7 is vertebra prominens, a palpable landmark.

• Thoracic vertebrae (T1–T12): Increase in size, articulate with ribs, have heart-shaped bodies and circular vertebral foramina.

• Lumbar vertebrae (L1–L5): Massive bodies, short and thick pedicles, flat spinous processes, triangular vertebral foramina.

Characteristic	Cervical (C3–C7)	Thoracic	Lumbar
Body	Small, oval, wide side to side	Larger than cervical; heart shaped; bears two costal facets	Massive; kidney shaped
Spinous process	Short; bifid (except C7); projects directly posterior	Long; sharp; projects inferiorly	Short; blunt; rectangular; projects directly posteriorly
Vertebral foramen	Triangular; large	Circular	Triangular
Transverse processes	Contain foramina	Bear facets for ribs (except T11, T12)	Thin and tapered
Superior and inferior articular processes	Superior facets directed superoposteriorly; inferior facets directed inferoanteriorly	Superior facets directed posteriorly; inferior facets directed anteriorly	Superior facets directed posteromedially (or medially); inferior facets directed anterolaterally (or laterally)
Movements allowed	Flexion and extension; lateral flexion; rotation; region with greatest range of movement	Rotation; lateral flexion possible; but restricted by rib attachment; flexion and extension limited	Flexion and extension; lateral flexion; rotation prevented

Sacrum and Coccyx

• Sacrum: Triangular bone made from five fused vertebrae; forms the posterior wall of the pelvis.

• Coccyx: Tailbone formed from three to five fused vertebrae; minimal function.

Thoracic Cage

Structure and Function

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

Sternum

• Manubrium: Superior portion

• Body: Midportion

• Xiphoid process: Inferior end; not ossified until ~age 40

• Jugular notch: Central indentation in superior border of manubrium

• Sternal angle: Horizontal ridge across front of sternum

• Xiphisternal joint: Point where sternal body and xiphoid process fuse

Appendicular Skeleton

Structure

The appendicular skeleton consists of the bones of the limbs and their girdles. The pectoral girdle attaches the upper limbs to the body trunk, while the pelvic girdle attaches the lower limbs.

Developmental Aspects of the Skeleton

Infant and Aging Skeleton

• Infant skull: Has more bones than adult skull; bones are unfused and connected by fontanelles (fibrous membranes).

• Fontanelles: Allow for ease of birth and brain growth; eventually ossify.

• Primary curvatures: Thorax and sacrum are convex at birth, resulting in a C-shaped spine.

• Secondary curvatures: Cervical and lumbar regions develop convexity anteriorly as the child grows.

• Aging: Intervertebral discs become thinner, less hydrated, and less elastic; risk of disc herniation increases; bones lose mass, increasing fracture risk.

Clinical Terms

• Orthopedist/Orthopedic surgeon: Specialist in skeletal system disorders.

• Pelvimetry: Measurement of the pelvis.

• Spina bifida: Congenital defect of the spine.

• Spinal fusion: Surgical procedure to join two or more vertebrae.

End of Chapter Review Questions

• Review questions #1–7 (not included in detail here).