Axial Skeleton: Structure and Terminology (Lab Bio 203 Study Notes)

Axial Skeleton Overview

The axial skeleton forms the central axis of the human body and provides support and protection for the brain, spinal cord, and thoracic organs. It consists of the skull, vertebral column, and thoracic cage.

Major Components of the Axial Skeleton

• Skull: Protects the brain and forms the structure of the face.

• Vertebral Column: Supports the body and protects the spinal cord.

• Thoracic Cage: Composed of the ribs and sternum, protects the heart and lungs.

Skull Anatomy

The skull is a complex structure composed of cranial and facial bones, joined by immovable joints called sutures. It houses and protects the brain and forms the framework of the face.

Cranial Bones and Key Landmarks

• Frontal Bone: Forms the forehead and the superior part of the eye sockets.

• Parietal Bones: Paired bones forming the superior and lateral aspects of the skull.

• Temporal Bones: Located on the sides of the skull, house the structures of the ears.

• Occipital Bone: Forms the posterior and base of the skull; contains the foramen magnum.

• Sphenoid Bone: Forms part of the base of the skull and the orbits; contains the sella turcica.

• Ethmoid Bone: Forms part of the nasal cavity and the orbits; contains the crista galli and cribriform plate.

Facial Bones and Key Landmarks

• Nasal Bones: Form the bridge of the nose.

• Lacrimal Bones: Small bones forming part of the medial wall of each orbit.

• Zygomatic Bones: Cheekbones; form part of the orbit.

• Maxillae: Upper jawbones; form part of the orbits, nasal cavity, and hard palate.

• Mandible: Lower jawbone; the only movable bone of the skull.

• Vomer: Forms part of the nasal septum.

• Inferior Nasal Conchae: Thin, curved bones in the nasal cavity.

Key Sutures of the Skull

• Coronal Suture: Between frontal and parietal bones.

• Sagittal Suture: Between the two parietal bones.

• Lambdoid Suture: Between parietal and occipital bones.

• Squamous Suture: Between parietal and temporal bones.

Other Important Skull Features

• External Occipital Protuberance: Prominent projection on the occipital bone.

• Foramen Magnum: Large opening in the occipital bone for the spinal cord.

• Styloid Process: Slender projection from the temporal bone.

• Mastoid Process: Rounded projection from the temporal bone, behind the ear.

• Zygomatic Arch: Formed by the zygomatic bone and the zygomatic process of the temporal bone.

• Mandibular Fossa: Depression in the temporal bone that articulates with the mandible.

• Alveolar Process: Ridge containing the tooth sockets in the maxilla and mandible.

Major Foramina and Canals

• Optic Canal: Passage for the optic nerve.

• Jugular Foramen: Passage for the jugular vein and cranial nerves.

• Foramen Ovale: Passage for nerves in the sphenoid bone.

• Carotid Canal: Passage for the internal carotid artery.

• Stylomastoid Foramen: Passage for the facial nerve.

Paranasal Sinuses

• Frontal Sinus: Located in the frontal bone.

• Sphenoidal Sinus: Located in the sphenoid bone.

• Ethmoid Sinus: Located in the ethmoid bone.

• Maxillary Sinus: Largest sinus, located in the maxilla.

Vertebral Column

The vertebral column, or spine, is a flexible structure composed of individual vertebrae separated by intervertebral discs. It supports the head, encloses the spinal cord, and provides attachment points for ribs and muscles.

Regions of the Vertebral Column

• Cervical (7 vertebrae): Neck region; includes the atlas (C1) and axis (C2).

• Thoracic (12 vertebrae): Upper back; each articulates with a pair of ribs.

• Lumbar (5 vertebrae): Lower back; largest vertebrae.

• Sacrum (5 fused vertebrae): Forms the posterior wall of the pelvis.

• Coccyx (4 fused vertebrae): Tailbone.

General Vertebral Anatomy

• Body: Weight-bearing anterior portion.

• Vertebral Arch: Posterior part enclosing the vertebral foramen.

• Vertebral Foramen: Opening for the spinal cord.

• Spinous Process: Posterior projection for muscle attachment.

• Transverse Process: Lateral projections for muscle and ligament attachment.

• Superior and Inferior Articular Processes: Form joints with adjacent vertebrae.

• Intervertebral Foramen: Openings between vertebrae for spinal nerves.

• Intervertebral Disc: Fibrocartilaginous pad between vertebral bodies.

Specialized Cervical Vertebrae

• Atlas (C1): Supports the skull; allows nodding motion.

• Axis (C2): Has the dens (odontoid process) for rotation of the head.

• Transverse Foramen: Unique to cervical vertebrae; passage for vertebral arteries.

Sacrum and Coccyx

• Sacral Promontory: Anterior projecting edge of the sacrum.

• Sacral Foramina: Openings for sacral nerves.

• Median Sacral Crest: Fused spinous processes of sacral vertebrae.

• Lateral Sacral Crest: Fused transverse processes.

• Sacral Canal: Continuation of the vertebral canal.

• Ala: Wing-like projections on each side of the sacrum.

Thoracic Cage

The thoracic cage protects vital organs in the thorax and supports the shoulder girdles and upper limbs. It consists of the sternum, ribs, and thoracic vertebrae.

Sternum Anatomy

• Manubrium: Superior portion; articulates with clavicles and first ribs.

• Body: Middle and largest part; articulates with costal cartilages of ribs 2-7.

• Xiphoid Process: Inferior, small, cartilaginous in youth, ossifies in adults.

• Jugular Notch: Superior indentation of the manubrium.

• Clavicular Notch: Articulation site for the clavicle.

• Sternal Angle: Junction of manubrium and body; landmark for rib 2.

Rib Classification and Anatomy

• Vertebrosternal (True) Ribs (1-7): Attach directly to the sternum via costal cartilage.

• Vertebrochondral (False) Ribs (8-10): Attach to the sternum indirectly via cartilage of rib 7.

• Floating Ribs (11-12): Do not attach to the sternum.

• Head: Articulates with vertebral bodies.

• Neck: Narrow region next to the head.

• Shaft: Main, elongated part of the rib.

• Costal Groove: Groove on the inferior border for nerves and vessels.

Summary Table: Major Axial Skeleton Structures

Region	Main Bones/Structures	Key Features
Skull	Frontal, Parietal, Temporal, Occipital, Sphenoid, Ethmoid, Maxilla, Mandible, Zygomatic, Nasal, Vomer, Lacrimal, Inferior Nasal Concha	Sutures, foramina, sinuses, processes, fossae
Vertebral Column	Cervical, Thoracic, Lumbar vertebrae, Sacrum, Coccyx	Bodies, arches, processes, foramina, discs
Thoracic Cage	Sternum (manubrium, body, xiphoid), Ribs (true, false, floating)	Costal cartilage, notches, grooves, angles

Example: Clinical Application

• Fracture of the Atlas (C1): Can result in instability of the head and neck, potentially damaging the spinal cord.

• Sinus Infections: Inflammation of the paranasal sinuses can cause facial pain and pressure.

Additional info: The axial skeleton is essential for protecting the central nervous system and vital organs, as well as providing attachment points for muscles involved in posture and respiration.