The Skeleton: Overview

Divisions of the Human Skeleton

The human skeleton is a structural framework composed of bones, cartilages, joints, and ligaments. It is divided into two major regions: the axial skeleton and the appendicular skeleton. The axial skeleton consists of 80 bones and includes the skull, vertebral column, and thoracic cage. The appendicular skeleton comprises the limbs and girdles that attach them to the axial skeleton.

• Axial Skeleton: Supports and protects the organs of the head, neck, and trunk.

• Appendicular Skeleton: Facilitates movement and interaction with the environment.

The Skull: Structure and Function

Skull Bones and Sutures

The skull is formed by two sets of bones: cranial bones and facial bones. Cranial bones enclose the brain and provide attachment sites for head and neck muscles. Facial bones form the framework of the face, house special sense organs, provide openings for air and food, secure teeth, and anchor muscles for facial expression. Most skull bones are joined by immovable joints called sutures.

• Cranial Bones: 8 bones forming the cranium.

• Facial Bones: 14 bones forming the face.

• Sutures: Coronal, sagittal, lambdoid, and squamous sutures mark the articulations between cranial bones.

Major Functions of the Cranium vs. Facial Skeleton

• Cranium: Protects the brain, provides attachment for muscles.

• Facial Skeleton: Shapes the face, forms cavities for sense organs, provides passageways for air and food, anchors teeth and muscles.

Skull Geography: Vault, Base, and Cavities

The cranium is divided into a vault (calvaria) and a base. The cranial vault forms the superior, lateral, and posterior portions, while the base forms the inferior aspect. Internally, the base is divided into three fossae: anterior, middle, and posterior, which cradle the brain. The skull also contains cavities for the ears, nasal cavity, orbits, and sinuses, and has 85 named openings for nerves and blood vessels.

Cranial Bones: Identification and Features

List of Cranial Bones

• Frontal bone

• Parietal bones (2)

• Occipital bone

• Temporal bones (2)

• Sphenoid bone

• Ethmoid bone

Frontal Bone

The frontal bone forms the anterior portion of the cranium, including the forehead (squamous region). It forms the superior wall of the orbits and most of the anterior cranial fossa. The supraorbital foramen allows passage of the supraorbital artery and nerve. The glabella is the area between the orbits, and the frontal sinuses are located lateral to the glabella.

Parietal Bones and Sutures

Two large parietal bones form most of the superior and lateral aspects of the cranial vault. Four major sutures mark their articulations: coronal, sagittal, lambdoid, and squamous.

• Coronal suture: Parietal and frontal bones

• Sagittal suture: Right and left parietal bones

• Lambdoid suture: Parietal and occipital bones

• Squamous suture: Parietal and temporal bones

Occipital Bone

The occipital bone forms the posterior wall and posterior cranial fossa. It articulates with parietal, temporal, and sphenoid bones. The foramen magnum is a large opening for the spinal cord, flanked by occipital condyles that articulate with the first vertebra. Other features include the hypoglossal canal, external occipital protuberance, external occipital crest, and nuchal lines for muscle attachment.

Temporal Bones

Temporal bones form the inferolateral aspects of the skull and parts of the cranial base. They consist of squamous, tympanic, and petrous regions. Key features include the zygomatic process, mandibular fossa, external acoustic meatus, mastoid and styloid processes, and several foramina for nerves and blood vessels.

Sphenoid Bone

The sphenoid bone is a complex, bat-shaped bone that articulates with all other cranial bones. It contains sphenoidal sinuses, the sella turcica (housing the pituitary gland), and three pairs of processes: greater wings, lesser wings, and pterygoid processes. Several foramina allow passage of nerves and arteries.

Ethmoid Bone

The ethmoid bone is the deepest skull bone. Its superior part is formed by paired cribriform plates, which form the roof of the nasal cavity and floor of the anterior cranial fossa. The crista galli is a point of attachment for the dura mater. The perpendicular plate forms the superior part of the nasal septum, and the ethmoidal labyrinths contain ethmoidal air cells and form the superior and middle nasal conchae.

Facial Bones: Identification and Features

List of Facial Bones

• Mandible

• Maxillary bones (2)

• Zygomatic bones (2)

• Nasal bones (2)

• Lacrimal bones (2)

• Palatine bones (2)

• Vomer

• Inferior nasal conchae (2)

Mandible

The mandible is the largest and strongest bone of the face. It consists of a body (chin) and two upright rami. Key features include the mandibular angle, coronoid process, condylar process, mandibular notch, alveolar process (for teeth), mandibular symphysis, and foramina for nerves and blood vessels.

Maxillary Bones

The maxillary bones are fused medially to form the upper jaw and central facial skeleton. They hold the upper teeth in alveolar processes, form the anterior nasal spine, and contribute to the hard palate. The maxillary sinuses flank the nasal cavity, and several openings allow passage of nerves and blood vessels.

Zygomatic Bones

Zygomatic bones form the cheekbones and inferolateral margins of the orbits. They articulate with the zygomatic processes of the temporal, frontal, and maxillary bones.

Nasal Bones

Nasal bones form the bridge of the nose and articulate with the frontal, maxillary, and ethmoid bones. They attach to cartilage that forms the tip of the nose.

Lacrimal Bones

Lacrimal bones form the medial walls of the orbits and articulate with the frontal, maxillary, and ethmoid bones. The lacrimal fossa houses the lacrimal sac, allowing passage for tears to drain.

Palatine Bones, Vomer, and Inferior Nasal Conchae

• Palatine bones: L-shaped bones forming the posterior hard palate and part of the nasal cavity and orbits.

• Vomer: Plow-shaped bone forming part of the nasal septum.

• Inferior nasal conchae: Paired bones forming part of the lateral walls of the nasal cavity.

The Hyoid Bone

Structure and Function

The hyoid bone is not a bone of the skull. It lies in the anterior neck, inferior to the mandible, and is the only bone that does not articulate directly with another bone. It is anchored by ligaments and acts as a movable base for the tongue and a site of attachment for muscles involved in swallowing and speech.

Special Characteristics: Orbits, Nasal Cavity, and Paranasal Sinuses

Orbits

The orbits are cavities that encase the eyes and lacrimal glands, providing sites for muscle attachment. They are formed by parts of seven bones: frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid.

Nasal Cavity

The nasal cavity is formed by several bones. The roof is formed by the cribriform plates of the ethmoid, the lateral walls by the conchae and perpendicular plates, and the floor by the palatine and maxillary bones. The nasal septum is formed by the vomer and perpendicular plate of the ethmoid, with the anterior part formed by septal cartilage. Spaces between conchae are called meatuses, which increase air turbulence.

Paranasal Sinuses

Paranasal sinuses are mucosa-lined, air-filled spaces formed from five skull bones: frontal, sphenoid, ethmoid, and paired maxillary bones. Their functions include warming and humidifying air, lightening the skull, and enhancing the resonance of the voice.

Table: Bones of the Skull

Classification and Features

The following table summarizes the bones of the skull, their classification, and key features.

Clinical Note: Mastoiditis

Homeostatic Imbalance

The mastoid process contains air cells that can become infected if a middle ear infection spreads. Mastoiditis is a serious condition because the air cells are separated from the brain by a thin bony plate, increasing the risk of infection spreading to the brain.

Summary

The skull is a complex structure composed of cranial and facial bones, each with distinct features and functions. Understanding the anatomy of the skull is essential for anticipating clinical issues and for comprehending the structural basis of the head and face.