The Human Skeleton

Overview of the Skeletal System

The human skeleton is a complex framework of bones, cartilages, joints, and ligaments that provides structural support, protection, and movement for the body. It is divided into two major divisions: the axial skeleton and the appendicular skeleton.

• Axial skeleton: Consists of 80 bones, including the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Composed of the bones of the limbs and girdles that attach them to the axial skeleton.

The Skull

General Structure and Function

The skull is a bony structure that forms the head and houses the brain and primary sensory organs. It is composed of two sets of bones: cranial bones and facial bones. Most skull bones are joined by immovable joints called sutures, except for the mandible.

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

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

Major Regions and Cavities of the Skull

The cranium is divided into the cranial vault (calvaria) and the cranial base. The cranial vault forms the superior, lateral, and posterior aspects of the skull, while the base forms the inferior aspect. The cranial base is further divided into three fossae: anterior, middle, and posterior, which cradle the brain.

• Cranial cavity: Encloses the brain.

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

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

Cranial Bones

List and Description of Cranial Bones

The cranium consists of eight 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 and the superior walls of the orbits. Key features include the squamous region (forehead), supraorbital margins (under the eyebrows), supraorbital foramen (passage for artery and nerve), glabella (between orbits), and frontal sinuses.

Parietal Bones and Sutures

The two parietal bones form most of the superior and lateral aspects of the cranial vault. They articulate with other cranial bones at four major sutures:

• Coronal suture: Between parietal and frontal bones

• Sagittal suture: Between right and left parietal bones

• Lambdoid suture: Between parietal and occipital bones

• Squamous suture: Between parietal and temporal bones

Occipital Bone

The occipital bone forms the posterior wall and base of the skull. It contains the foramen magnum (passage for the spinal cord), occipital condyles (articulate with the first vertebra), external occipital protuberance, and nuchal lines (muscle attachment sites).

Temporal Bones

The paired temporal bones form the inferolateral aspects of the skull and part of the cranial base. They have several regions:

• Squamous region: Forms part of the zygomatic arch and mandibular fossa.

• Tympanic region: Surrounds the external acoustic meatus.

• Petrous region: Houses the middle and internal ear cavities.

Key features include the mastoid process, styloid process, jugular foramen, carotid canal, and internal acoustic meatus.

Sphenoid Bone

The sphenoid bone is a complex, bat-shaped bone that articulates with all other cranial bones. It contains the sella turcica (houses the pituitary gland), sphenoidal sinuses, 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, forming part of the nasal cavity and the medial wall of the orbits. Key features include the cribriform plates (roof of nasal cavity), crista galli (attachment for dura mater), perpendicular plate (part of nasal septum), and ethmoidal air cells (sinuses).

Sutural Bones

Sutural bones are tiny, irregular bones that may appear within sutures. Their significance is unknown and not everyone has them.

Facial Bones

List and Description of Facial Bones

The facial skeleton consists of 14 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 facial bone, forming the lower jaw. It consists of a body (chin) and two upright rami. Key features include the mandibular angle, coronoid process (muscle attachment), condylar process (forms temporomandibular joint), alveolar process (tooth sockets), mandibular symphysis, and foramina for nerves and blood vessels.

Maxillary Bones (Maxillae)

The maxillae are fused medially to form the upper jaw and central facial skeleton. They hold the upper teeth, form the anterior nasal spine, and contribute to the hard palate, lateral bridge of the nose, and orbits. Key features include the palatine process, frontal process, zygomatic process, maxillary sinuses, and several foramina for nerves and vessels.

Zygomatic Bones

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

Nasal Bones

The 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

The lacrimal bones form part of the medial walls of the orbits and articulate with the frontal, maxillary, and ethmoid bones. The lacrimal fossa houses the lacrimal sac, allowing tears to drain into the nasal cavity.

Palatine Bones, Vomer, and Inferior Nasal Conchae

• Palatine bones: L-shaped bones forming the posterior part of the 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; largest of the three pairs of conchae.

The Hyoid Bone

Structure and Function

The hyoid bone is unique in that it does not articulate directly with any other bone. Located in the anterior neck inferior to the mandible, it is anchored by ligaments and serves as a movable base for the tongue and an attachment site for muscles involved in swallowing and speech.

Special Characteristics of the Orbits and Nasal Cavity

Orbits

The orbits are bony cavities that house the eyes and lacrimal glands and provide attachment sites for eye muscles. Each orbit is 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 made 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 posteriorly by the vomer and perpendicular plate of the ethmoid, and anteriorly by septal cartilage. Spaces between the conchae are called meatuses, which increase air turbulence.

Paranasal Sinuses

Paranasal sinuses are mucosa-lined, air-filled spaces in five skull bones: frontal, sphenoid, ethmoid, and paired maxillary bones. They function to warm and humidify air, lighten the skull, and enhance the resonance of the voice.

Table: Bones of the Skull

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