Axial Skeleton and Skeletal System Overview

Divisions of the Skeletal System

The skeletal system is the rigid internal framework of the body, consisting of bones, cartilages, and ligaments. It is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Composed of 80 bones, it protects the brain, spinal cord, heart, and lungs. It includes the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Composed of 126 bones, it facilitates movement of the limbs and includes the bones of the upper and lower limbs, pectoral girdle, and pelvic girdle.

Key Functions:

• Supports body weight and allows for movement

• Protects internal organs

• Stores calcium, phosphate, and fat

• Red bone marrow produces blood cells

Skull: Cranium and Facial Bones

Cranial Bones

The cranium consists of several bones fused together to protect the brain and house ear structures. The main cranial bones are:

• Frontal bone: Forehead and upper part of orbits

• Parietal bones: Top of the cranium

• Temporal bones: Sides and floor of the cranium

• Ethmoid bone: Forms part of the cranial floor and nasal cavity

• Sphenoid bone: Central cranial floor, anchors other cranial bones

• Occipital bone: Posterior skull, forms joint with the atlas (C1)

Facial Bones

Facial bones support the face, form the nasal cavity, enclose the eyeballs, and support the teeth. Key facial bones include:

• Lacrimal bones: Medial orbit wall, contains tear duct groove

• Nasal bones: Bridge of the nose

• Maxilla: Upper jaw, orbital floors, roof of mouth

• Mandible: Lower jaw, only movable skull bone

• Zygomatic bones: Cheekbones

• Palatine bones: Hard palate and lateral nasal cavity

• Vomer: Nasal septum

• Inferior nasal conchae: Lateral nasal cavity wall

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within certain skull bones, connected to the nasal cavity and lined with mucosa. They lighten the skull and add resonance to the voice.

• Frontal sinus: Above the eyebrows

• Sphenoid sinus: Within sphenoid bone

• Ethmoid air cells: Lateral sides of ethmoid bone

• Maxillary sinus: Largest, within maxillary bones

Muscles of Facial Expression and Mastication

Facial Expression Muscles

Muscles that create facial expressions originate on the skull and insert into connective tissue and dermis. Major muscles include:

• Occipitofrontalis: Moves scalp and eyebrows

• Orbicularis oculi: Closes the eye

• Orbicularis oris: Moves the lips

• Buccinator: Compresses the cheek

• Corrugator supercilii: Moves the eyebrows

Muscles of Mastication (Chewing)

These muscles move the mandible for biting and chewing:

• Masseter: Elevates mandible

• Temporalis: Elevates and retracts mandible

• Buccinator: Keeps food in place

• Lateral pterygoid: Depresses and protracts mandible

• Medial pterygoid: Elevates and protracts mandible

Hyoid Bone and Associated Muscles

Hyoid Bone

The hyoid bone is a small, U-shaped bone in the upper neck, not directly attached to other bones. It supports muscles involved in swallowing and speech.

• Suprahyoid muscles: Raise hyoid, floor of mouth, and larynx

• Infrahyoid muscles: Depress hyoid and control larynx position

Vertebral Column

Structure and Regions

The vertebral column is a flexible structure composed of vertebrae separated by intervertebral discs. It supports the head, neck, and body, and protects the spinal cord.

• Cervical vertebrae (C1–C7): Neck

• Thoracic vertebrae (T1–T12): Chest, attach to ribs

• Lumbar vertebrae (L1–L5): Lower back

• Sacral vertebrae: Fused to form pelvis

• Coccygeal vertebrae: Fused to form tailbone

Curvatures of the Vertebral Column

The vertebral column has four curvatures that increase strength and flexibility:

• Primary curves: Thoracic and sacrococcygeal (present at birth)

• Secondary curves: Cervical (develops when infant holds head up) and lumbar (develops when child stands/walks)

Structure of a Typical Vertebra

Each vertebra has a body, vertebral arch, vertebral foramen, and processes for muscle and ligament attachment.

• Body: Supports weight

• Vertebral arch: Protects spinal cord

• Vertebral foramen: Passage for spinal cord

• Intervertebral foramen: Passage for spinal nerves

• Processes: Transverse, spinous, superior/inferior articular

Intervertebral Discs and Ligaments

Intervertebral discs are fibrocartilaginous pads between vertebrae, consisting of anulus fibrosus (outer layer) and nucleus pulposus (inner gel-like core). Ligaments stabilize the vertebral column.

• Anterior longitudinal ligament: Prevents excessive backward bending

• Supraspinous ligament: Supports column during forward bending

• Nuchal ligament: Supports skull

• Posterior longitudinal ligament and ligamentum flavum: Support column during forward bending

Cervical Vertebrae: Atlas and Axis

The first two cervical vertebrae (C1 and C2) are specialized for head movement:

• Atlas (C1): Supports the skull, allows "yes" motion

• Axis (C2): Has the dens (odontoid process), allows "no" motion

Thoracic Vertebrae

Thoracic vertebrae have larger bodies, long downward spinous processes, and facets for rib articulation.

• Costal facets: Articulate with rib heads

• Articular facet on transverse process: Articulates with rib tubercle

Thoracic Cage (Rib Cage)

Structure and Function

The thoracic cage consists of 12 pairs of ribs, costal cartilages, and the sternum. It protects the heart and lungs.

• Sternum: Manubrium (superior), body (central), xiphoid process (inferior)

• Ribs: True ribs (1-7, direct attachment), false ribs (8-12, indirect or no attachment), floating ribs (11-12)

Parts of a Typical Rib

• Head: Articulates with vertebral body

• Neck: Narrowed portion

• Tubercle: Articulates with transverse process

• Body (shaft): Main part of rib

Embryonic Development and Fontanels

Development of the Axial Skeleton

The axial skeleton forms during embryonic development via intramembranous and endochondral ossification. Fontanels (soft spots) allow for skull molding during birth and rapid brain growth.

• Fontanels: Anterior, posterior, sphenoidal, mastoid

• Sutures: Interlocking projections between skull bones

Summary Table: Axial Skeleton Components