The Skeleton: Overview

General Organization of the Human Skeleton

The human skeleton is a structural framework composed of 206 bones, accounting for approximately 20% of total body weight. It is divided into two main groups: the axial skeleton and the appendicular skeleton.

• Axial skeleton: Consists of the bones of the skull, vertebral column, and rib cage. It provides support and protection for the brain, spinal cord, and thoracic organs.

• Appendicular skeleton: Includes the bones of the upper and lower limbs, as well as the pectoral and pelvic girdles, which attach the limbs to the axial skeleton.

Axial Skeleton

Skull: Structure and Function

The skull is the most complex bony structure in the body, composed of 22 bones grouped into cranial and facial bones. Most skull bones are flat and are joined by immovable joints called sutures (except for the mandible).

• Cranial bones: Enclose and protect the brain and house organs of hearing and balance.

• Facial bones: Form the framework of the face, house sensory organ cavities, provide openings for air and food, secure teeth, and anchor facial muscles.

• The skull contains orbits for the eyes, paranasal sinuses, and 85 openings for nerves, blood vessels, and the spinal cord.

Cranium: Vault and Base

The cranium is divided into two main parts: the cranial vault and the cranial base.

• Cranial vault: Forms the superior, lateral, and posterior aspects of the skull, including the forehead.

• Cranial base: Forms the skull's inferior aspect and is divided into three fossae: anterior, middle, and posterior cranial fossae, which support the brain's major regions.

Main Cranial Bones

The cranium consists of eight bones: paired parietal and temporal bones, and unpaired frontal, occipital, sphenoid, and ethmoid bones. Their curvature provides strength while maintaining lightness.

• Frontal bone: Dome-shaped, forms the forehead and roof of the orbits, contains the frontal sinuses, and articulates with the parietal bones.

• Parietal bones: Paired bones forming the superior and lateral aspects of the skull, making up most of the cranial vault.

• Occipital bone: Forms the posterior base of the skull, contains the foramen magnum (passage for the spinal cord), and articulates with the first cervical vertebra via the occipital condyles.

• Temporal bones: Paired bones forming the inferior and lateral skull, with regions including the squamous, tympanic, and petrous parts. Key features include the zygomatic process, mandibular fossa, external acoustic meatus, styloid and mastoid processes.

• Sphenoid bone: Complex, butterfly-shaped bone that articulates with all other cranial bones, forms part of the cranial base, and contains the sphenoid sinuses.

• Ethmoid bone: Deeply situated, forms part of the nasal cavity and orbits, contains the cribriform plate (for olfactory nerves), perpendicular plate (nasal septum), and ethmoid sinuses.

Major Cranial Sutures

Sutures are immovable joints that firmly unite the bones of the adult skull. The four main sutures are:

• Coronal suture: Between the frontal and parietal bones

• Squamous suture: Between the parietal and temporal bones

• Lambdoid suture: Between the occipital and parietal bones

• Sagittal suture: Between the two parietal bones

Facial Bones

Overview and Classification

The facial skeleton consists of 14 bones, with the mandible and vomer being unpaired. The paired bones are the maxillae, zygomatic, nasal, lacrimal, palatine, and inferior nasal conchae.

• Mandible: The largest and strongest facial bone, forms the lower jaw, and contains the alveolar margin for teeth, mandibular condyle, and mental foramina.

• Maxillae: Fused medially, form the upper jaw and anterior hard palate, contain maxillary sinuses, and alveolar margins for upper teeth.

• Zygomatic bones: Cheekbones, articulate with the maxilla, frontal, and temporal bones, and contribute to the orbit.

• Nasal bones: Form the bridge of the nose.

• Lacrimal bones: Small bones in the medial orbit, each with a lacrimal fossa for the lacrimal sac.

• Palatine bones: L-shaped, form part of the hard palate and nasal cavity.

• Vomer: Forms part of the nasal septum.

• Inferior nasal conchae: Thin, curved bones projecting into the nasal cavity.

Orbits and Nasal Cavity

The orbits are bony cavities that house the eyes and are formed by seven bones: frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid. The nasal cavity is formed by contributions from the ethmoid, vomer, maxilla, palatine, and inferior nasal conchae.

Paranasal Sinuses

Paranasal sinuses are mucosa-lined, air-filled spaces in the frontal, maxillary, sphenoid, and ethmoid bones. They lighten the skull, enhance voice resonance, and help warm and humidify inhaled air.

Hyoid Bone

The hyoid bone is unique as it does not articulate with any other bone. It supports the tongue and provides attachment for muscles involved in swallowing and speech. It is horseshoe-shaped with a body and two pairs of horns (greater and lesser).

Summary Table: Cranial and Facial Bones