Axial Skeleton

The axial skeleton forms the central axis of the human body and includes the skull, vertebral column, sternum, and ribs. Understanding the terminology and anatomical features of these bones is essential for students of Anatomy & Physiology.

Skull

The skull protects the brain and supports the structures of the face. It is composed of several bones, each with distinct landmarks.

• Frontal bone: Forms the forehead and the superior part of the eye sockets (orbits).

• Supraorbital foramen (notch): Opening above each orbit allowing passage of blood vessels and nerves.

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

• Occipital bone: Forms the posterior and base of the skull.

• Foramen magnum: Large opening in the occipital bone for the spinal cord to connect with the brain.

• Jugular foramen: Passage for the internal jugular vein and cranial nerves IX, X, and XI.

• Hypoglossal foramen: Passage for the hypoglossal nerve (cranial nerve XII).

• Occipital condyle: Articulates with the first cervical vertebra (atlas) to allow nodding motion.

• Temporal bone: Forms the sides and base of the skull; houses structures of the ear.

• External acoustic (auditory) canal (meatus): Canal leading to the eardrum and middle ear.

• Internal acoustic (auditory) canal (meatus): Passage for nerves of hearing and balance.

• Styloid process of temporal bone: Slender pointed projection for muscle and ligament attachment.

• Mastoid process: Rounded projection behind the ear for muscle attachment.

• Stylomastoid foramen: Opening for the facial nerve (cranial nerve VII).

• Sphenoid bone: Butterfly-shaped bone at the base of the skull.

• Sella turcica: Saddle-shaped depression housing the pituitary gland.

• Optic canal (foramen): Passage for the optic nerve (cranial nerve II).

• Superior orbital fissure: Slit for passage of nerves and vessels to the orbit.

• Lesser wing & Greater wing: Parts of the sphenoid bone forming portions of the cranial floor.

• Foramen rotundum & Foramen ovale: Openings for cranial nerves and blood vessels.

• Ethmoid bone: Forms part of the nasal cavity and the medial wall of the orbit.

• Crista galli: Vertical projection for attachment of brain membranes.

• Cribriform plate: Contains tiny foramina for olfactory nerve fibers.

• Nasal bone: Forms the bridge of the nose.

• Inferior nasal conchae: Curved bones in the nasal cavity increasing surface area.

• Lacrimal bone: Small bone forming part of the medial orbit wall.

• Zygomatic bone: Cheekbone; forms part of the orbit.

• Zygomatic Arch: Bony arch on the side of the skull formed by the zygomatic and temporal bones.

• Maxilla: Upper jawbone; forms part of the orbit, nasal cavity, and hard palate.

• Infraorbital foramen: Opening below the orbit for nerves and vessels.

• Palatine bone: Forms part of the hard palate and nasal cavity.

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

• Body of mandible: Horizontal portion holding the lower teeth.

• Mental foramen: Openings on the anterior mandible for nerves and vessels.

• Angle: Junction of the body and ramus of the mandible.

• Ramus: Vertical extension of the mandible.

• Coronoid process of mandible: Insertion point for the temporalis muscle.

• Condyle of mandible (condylar process): Articulates with the temporal bone to form the temporomandibular joint (TMJ).

Vertebrae Structures

The vertebral column consists of individual vertebrae, each with characteristic features.

• Body of vertebra: Thick, disc-shaped anterior portion that bears weight.

• Pedicle: Short, thick process connecting the body to the arch.

• Lamina: Flattened part of the arch forming the posterior wall of the vertebral foramen.

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

• Spinous process: Posterior projection for muscle and ligament attachment.

• Vertebral foramen: Opening for the spinal cord.

Vertebrae Types

• Cervical vertebra: Seven vertebrae (C1–C7) in the neck region.

• Transverse foramen: Openings in cervical vertebrae for vertebral arteries.

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

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

• Dens (odontoid process): Peg-like projection from the axis (C2) around which the atlas rotates.

• Thoracic vertebra: Twelve vertebrae (T1–T12) articulating with ribs.

• Costal Facet: Articulation site for ribs on thoracic vertebrae.

• Lumbar vertebra: Five vertebrae (L1–L5) in the lower back; largest and strongest.

• Sacral vertebrae (sacrum): Five fused vertebrae forming the posterior pelvis.

• Coccygeal vertebrae (coccyx): Three to five fused vertebrae forming the tailbone.

Sternum

The sternum is a flat bone forming the anterior midline of the thorax.

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

• Jugular (sternal) notch: Central indentation at the superior border of the manubrium.

• Sternal angle: Junction of manubrium and body; landmark for rib counting.

• Body of sternum: Main, elongated portion.

• Xiphoid process: Inferior, cartilaginous tip; ossifies with age.

Ribs

The ribs protect thoracic organs and aid in respiration. There are 12 pairs, classified as follows:

• True ribs (1–7): Attach directly to the sternum via costal cartilages.

• False ribs (8–12): Attach indirectly or not at all to the sternum.

• Floating ribs (11–12): Subset of false ribs; do not attach to the sternum.

Hyoid Bone

The hyoid bone is a U-shaped bone in the neck that does not articulate with any other bone. It serves as an attachment for tongue and neck muscles, aiding in swallowing and speech.

Summary Table: Major Axial Skeleton Structures

Example: The foramen magnum of the occipital bone is a critical structure, as it allows the passage of the spinal cord from the brain to the vertebral column.

Additional info: The axial skeleton also provides attachment points for muscles involved in posture, respiration, and movement of the head and neck.