The Skeleton

Overview of the Skeletal System

The skeletal system is a complex structure composed of bones, cartilages, joints, and ligaments. It provides support, protection, and movement for the human body. The skeleton 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: Includes the bones of the limbs and girdles.

The Skull

Structure and Function of the Skull

The skull is formed by two sets of bones: cranial bones and facial bones. These bones are mostly locked together by joints called sutures, except for the mandible. The skull protects the brain, forms the framework of the face, and provides cavities for special sense organs.

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

• Facial bones: Form the face, secure teeth, anchor facial muscles, and provide openings for air and food passage.

Major Functions of the Cranium and Facial Skeleton

• Cranium: Protects the brain and supports head muscles.

• Facial skeleton: Shapes the face, supports teeth, and forms cavities for the senses.

Skull Geography

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

Skull Cavities and Openings

• Major 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

List 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 (squamous region). It also forms the superior wall of the orbits and most of the anterior cranial fossa. Important landmarks include the supraorbital margin, supraorbital foramen, glabella, and frontal sinuses.

Parietal Bones and Sutures

Two parietal bones form most of the superior and lateral aspects of the cranial vault. Four major sutures mark their articulations:

• 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 of the skull and the posterior cranial fossa. It articulates with parietal, temporal, and sphenoid bones. Key features include the foramen magnum, occipital condyles, hypoglossal canal, external occipital protuberance, and nuchal lines.

Temporal Bones

Paired temporal bones make up the inferolateral aspects of the skull and parts of the cranial base. They consist of three regions: squamous, tympanic, and petrous. Important landmarks include the zygomatic process, mandibular fossa, external acoustic meatus, mastoid and styloid processes, and several foramina for nerves and arteries.

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, and hypophyseal fossa. The bone has three pairs of processes (greater wings, lesser wings, pterygoid processes) and several foramina for 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 perpendicular plate forms the superior part of the nasal septum, and the ethmoidal labyrinths contain ethmoidal air cells. Orbital plates contribute to the medial wall of the orbits.

Sutural Bones

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

Facial Bones

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

Maxillary Bones

The maxillary bones (maxillae) are medially fused 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. Important features include the zygomatic process, maxillary sinuses, and several openings for 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 parts 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 in the body that does not articulate directly with another bone. It acts as a movable base for the tongue and provides attachment sites for muscles involved in swallowing and speech.

Special Characteristics of the Orbits and Nasal Cavity

Orbits

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

Nasal Cavity

The nasal cavity is formed by parts of 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 processes of 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.

Paranasal Sinuses

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

Example: The frontal bone forms the forehead and contains the frontal sinuses, which help lighten the skull and enhance voice resonance.

Additional info: Academic context was added to clarify the functions and anatomical relationships of the bones, as well as the significance of the sutures and sinuses.