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 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 limbs and girdles that attach them to the axial skeleton.

The Skull: Structure and Function

Skull Bones and Their Functions

The skull is formed by two sets of bones: cranial bones and facial bones. - Cranial bones (8): Enclose the brain and provide attachment sites for head and neck muscles. - Facial bones (14): 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. - Sutures: Immovable joints that connect most skull bones, except the mandible.

Major Functions of the Cranium vs. Facial Skeleton

- Cranium: Protects the brain, forms the cranial cavity, and provides muscle attachment sites. - Facial skeleton: Shapes the face, supports teeth, forms the orbits and nasal cavity, and provides passageways for air and food.

Skull Geography: Vault, Base, and Cavities

The cranium is divided into a vault (calvaria) and a base. - Cranial vault: Forms the superior, lateral, and posterior portions of the skull. - Cranial base: Forms the inferior aspect and is divided into three fossae: anterior, middle, and posterior. The brain sits within these fossae. - Cranial cavity: The space enclosed by the cranium.

Other Cavities and Openings

- Ear cavities: Middle and internal ear. - Nasal cavity: Passage for air. - Orbits: House the eyeballs. - Sinuses: Air-filled spaces that lighten the skull and enhance voice resonance. - Openings: Foramina, canals, and fissures provide passageways for nerves and blood vessels.

Cranial Bones

List and Description of Cranial Bones

There are eight cranial bones: - Frontal bone: Forms the forehead and superior wall of the orbits. - Parietal bones (2): Form most of the superior and lateral aspects of the cranial vault. - Occipital bone: Forms the posterior wall and base of the skull. - Temporal bones (2): Form the inferolateral aspects and part of the cranial base. - Sphenoid bone: Keystone bone that articulates with all other cranial bones. - Ethmoid bone: Deepest skull bone, forms part of the nasal cavity and orbits.

Frontal Bone

The frontal bone forms the anterior portion of the cranium and the superior wall of the orbits. - Squamous region: Forehead. - Supraorbital margin: Area under the eyebrows. - Supraorbital foramen: Passage for artery and nerve. - Glabella: Area between the orbits. - Frontal sinuses: Located lateral to the glabella.

Parietal Bones and Sutures

The two parietal bones articulate with other cranial bones via four major sutures: - 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 base of the skull. - Foramen magnum: Passage for the spinal cord. - Occipital condyles: Articulate with the first vertebra. - Hypoglossal canal: Passage for cranial nerve XII. - External occipital protuberance and crest: Attachment sites for ligaments and muscles. - Nuchal lines: Attachment sites for neck and back muscles.

Temporal Bones

The temporal bones are paired and form the inferolateral aspects of the skull. - Squamous part: Zygomatic process forms the zygomatic arch. - Tympanic part: Surrounds the external acoustic meatus. - Petrous part: Houses middle and internal ear cavities. - Foramina: Jugular foramen, carotid canal, foramen lacerum, internal acoustic meatus, stylomastoid foramen. - Mastoid and styloid processes: Muscle attachment sites.

Sphenoid Bone

The sphenoid bone is a complex, bat-shaped bone that articulates with all other cranial bones. - Sphenoidal sinuses: Located within the body. - Sella turcica: Encloses the pituitary gland. - Processes: Greater wings, lesser wings, pterygoid processes. - Foramina: Optic canals, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum.

Ethmoid Bone

The ethmoid bone is the deepest skull bone. - Cribriform plates: Form the roof of the nasal cavity. - Crista galli: Attachment for dura mater. - Perpendicular plate: Forms the superior part of the nasal septum. - Ethmoidal labyrinths: Contain ethmoidal air cells and form superior and middle nasal conchae. - Orbital plates: Contribute to the medial wall of the orbits.

Facial Bones

List and Description of Facial Bones

There are 14 facial bones: - Mandible: Lower jawbone. - Maxillary bones (2): Upper jaw and central facial skeleton. - Zygomatic bones (2): Cheekbones. - Nasal bones (2): Bridge of the nose. - Lacrimal bones (2): Medial walls of orbits. - Palatine bones (2): Posterior hard palate and part of nasal cavity. - Vomer: Part of nasal septum. - Inferior nasal conchae (2): Lateral walls of nasal cavity.

Mandible

The mandible is the largest and strongest facial bone. - Body: Contains alveolar process for teeth. - Rami: Upright branches. - Mandibular angle: Junction of rami and body. - Coronoid process: Muscle attachment. - Condylar process: Part of temporomandibular joint. - Mandibular and mental foramina: Passageways for nerves and blood vessels.

Maxillary Bones

Maxillary bones are fused to form the upper jaw. - Alveolar processes: Hold upper teeth. - Anterior nasal spine: Below the nose. - Palatine process: Forms two-thirds of the hard palate. - Frontal process: Forms lateral bridge of nose. - Zygomatic processes: Articulate with zygomatic bones. - Maxillary sinuses: Flank the nasal cavity. - Openings: Inferior orbital fissure, infraorbital foramen, incisive fossa and canal.

Zygomatic Bones

Zygomatic bones form the cheekbones and part of the orbits. - Articulate with: Temporal, frontal, and maxillary bones.

Nasal Bones

Nasal bones form the bridge of the nose. - Articulate with: Frontal, maxillary, and ethmoid bones. - Attach to: Cartilage forming the tip of the nose.

Lacrimal Bones

Lacrimal bones form the medial walls of the orbits. - Lacrimal fossa: Houses the lacrimal sac for tear drainage.

Palatine Bones, Vomer, and Inferior Nasal Conchae

- Palatine bones: L-shaped, form posterior hard palate and part of nasal cavity and orbits. - Vomer: Plow-shaped, forms part of nasal septum. - Inferior nasal conchae: Form part of lateral walls of nasal cavity.

The Hyoid Bone

Characteristics and Function

The hyoid bone is unique as it does not articulate directly with any other bone. - Location: Anterior neck, inferior to the mandible. - Function: Movable base for the tongue and attachment site for muscles involved in swallowing and speech.

Special Characteristics of the Orbits and Nasal Cavity

Orbits

Orbits are cavities that encase the eyes and lacrimal glands. - Formed by: Frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, and ethmoid bones. - Function: Protect the eyes and provide attachment sites for eye muscles.

Nasal Cavity

The nasal cavity is formed by several bones and contains spaces called meatuses. - Roof: Cribriform plates of ethmoid. - Lateral walls: Superior and middle conchae of ethmoid, perpendicular plates of palatine, and inferior nasal conchae. - Floor: Palatine and maxillary bones. - Nasal septum: Vomer and perpendicular plate of ethmoid (posterior), septal cartilage (anterior).

Paranasal Sinuses

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

Table: Bones of the Skull

Classification and Properties

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

Additional info:

- The skeleton's anatomy is foundational for understanding clinical issues such as pelvic dimensions affecting labor and delivery. - Sutural bones are tiny, irregular bones found within sutures; their significance is unknown and not everyone has them. - Mastoiditis is a clinical condition where infection spreads to the mastoid process, potentially reaching the brain due to thin bone separation. ----------------------------------------