Chapter 6: The Skeletal System - Axial Division

Overview of the Axial Skeleton

The human skeletal system consists of $206$ total bones, divided into two primary structural divisions:
- Appendicular Skeleton: Comprises $126$ bones, including the upper and lower limbs, pelvic girdle, and pectoral girdle.
- Axial Skeleton: Comprises $80$ bones arranged along the longitudinal central axis of the body.

Skeletal system with axial components highlighted

Primary functions of the axial skeleton:
- Serves as a rigid framework that protects and supports vital organs located within the dorsal (cranial and vertebral) and ventral (thoracic and abdominopelvic) body cavities.
- Houses and protects the special sense organs responsible for taste, smell, hearing, balance, and vision.
- Provides extensive muscle attachment surfaces for:
- Adjusting the posture and alignment of the head, neck, and trunk.
- Executing respiratory movements by shifting the thoracic cage.
- Anchoring and stabilizing elements of the appendicular skeleton.
Quantitative numerical breakdown of the $80$ axial skeleton bones:
- Skull and Associated Bones ( $29$ total bones):
- Cranium: $8$ bones.
- Face: $14$ bones.
- Auditory Ossicles: $6$ bones (three per middle ear cavity, enclosed within the temporal bones).
- Hyoid Bone: $1$ bone.
- Thoracic Cage ( $25$ total bones):
- Sternum: $1$ bone.
- Ribs: $24$ bones ( $12$ pairs).
- Vertebral Column ( $26$ total bones):
- Vertebrae: $24$ individual bones.
- Sacrum: $1$ bone (formed by the fusion of $5$ sacral vertebrae).
- Coccyx: $1$ bone (formed by the fusion of $3\text{--}5$ coccygeal vertebrae).

Bones of the Cranium

The cranium encloses the cranial cavity, a fluid-filled space containing cerebrospinal fluid that cushions, supports, and protects the brain.
The cranium is formed by $8$ individual cranial bones:
- Occipital bone ( $1$ )
- Parietal bones ( $2$ )
- Frontal bone ( $1$ )
- Temporal bones ( $2$ )
- Sphenoid bone ( $1$ )
- Ethmoid bone ( $1$ )

Posterior view of the adult skull

Occipital Bone:
- Forms the posterior, inferior structure of the cranium and cranial base.
- Contains the foramen magnum, a large aperture connecting the cranial cavity with the vertebral canal for the passage of the spinal cord.
- Features occipital condyles situated on either side of the foramen magnum, articulating with the first cervical vertebra (atlas, $C_1$ ).
- External landmarks for muscle attachment include the external occipital protuberance, external occipital crest, superior nuchal line, and inferior nuchal line.
- Passageways and depressions include the hypoglossal canal (transmitting cranial nerve XII), condyloid fossa, jugular notch, and jugular foramen (formed at the junction with the temporal bone).
- Internal features include the fossa for cerebrum, fossa for cerebellum, internal occipital crest, internal occipital protuberance, and the groove for sigmoid sinus.
Parietal Bones ( $2$ ):
- Form the superior and lateral walls of the cranium.
- Key external surface markings: parietal eminence, superior temporal line, and inferior temporal line (anchor points for the temporalis muscle and fascia).
- Major articulation borders (sutures):
- Sagittal suture: Midline articulation between the left and right parietal bones.
- Coronal suture: Anterior articulation joining the parietal bones with the frontal bone.
- Lambdoid suture: Posterior articulation joining the parietal bones with the occipital bone.
- Squamous suture: Inferolateral articulation joining each parietal bone with the adjacent temporal bone.
Frontal Bone:
- Forms the anterior portion of the cranium, forehead, and superior roof of the orbits.
- Structural features:
- Squamous part (squamous surface): Convex forehead region.
- Frontal (metopic) suture: A midline suture often visible in young skulls; typically fuses completely during development.
- Superciliary arch: Ridge supporting the eyebrow region.
- Supra-orbital margin: Thickened upper rim of the eye orbit.
- Supra-orbital foramen (or supra-orbital notch): Transmits blood vessels and nerves to the forehead.
- Lacrimal fossa: Anterolateral depression on the orbital roof housing the tear gland.
- Internal surfaces feature the frontal crest and frontal air cells (frontal sinus).
Temporal Bones ( $2$ ):
- Form the inferolateral walls of the cranium and part of the cranial floor.
- Subdivisions and landmarks:
- Squamous part (squama): Flat superior region forming the cerebral surface.
- Zygomatic process: Projects anteriorly to articulate with the temporal process of the zygomatic bone, forming the zygomatic arch.
- Mandibular fossa and articular tubercle: Concavity and ridge superior to the temporomandibular joint for articulation with the condylar process of the mandible.
- External acoustic meatus: Entrance to the ear canal.
- Internal acoustic meatus: Opening on the petrous part transmitting nerves (cranial nerves VII and VIII) to the inner ear.
- Mastoid process: Bulging projection posterior to the meatus housing mastoid air cells.
- Styloid process: Needle-like projection serving as an anchor for muscles and ligaments of the tongue and hyoid.
- Petrous part: Dense internal bony ridge housing middle and inner ear structures.
- Associated foramina: Carotid canal, jugular fossa/foramen, stylomastoid foramen, and mastoid foramen.
Sphenoid Bone:
- Central, bat-shaped bone functioning as a structural keystone that articulates with every other cranial bone.
- Structural elements:
- Body: Central mass containing the hollow sphenoidal sinuses.
- Sella turcica: Saddle-shaped depression on the superior surface holding the pituitary gland; consists of the tuberculum sellae, hypophyseal fossa, and dorsum sellae.
- Clinoid processes: Anterior clinoid process, middle clinoid process, and posterior clinoid process surrounding the sella turcica.
- Greater wings: Extend laterally to form parts of the cranial floor and lateral skull wall.
- Lesser wings: Triangular horizontal ridges superior to the greater wings.
- Pterygoid processes: Vertical downward projections consisting of a medial plate and lateral plate for chewing muscle attachment; contain the pterygoid canal.
- Openings and fissures: Optic canal (and optic groove for cranial nerve II), superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, and sphenoidal spine.
Ethmoid Bone:
- Complex, lightweight bone forming part of the anterior cranial floor, medial orbit wall, and superior nasal cavity.
- Elements:
- Cribriform plate: Horizontal plate perforated by cribriform foramina for olfactory nerve fibers (cranial nerve I).
- Crista galli: Vertical midline ridge projecting superiorly from the cribriform plate; serves as an attachment site for the falx cerebri (dural membrane).
- Perpendicular plate: Vertical downward extension forming the superior portion of the bony nasal septum.
- Ethmoidal labyrinth: Lateral masses containing hollow ethmoidal air cells (ethmoid sinus).
- Superior nasal concha and middle nasal concha: Scroll-like delicate projections extending into the nasal cavity to produce air turbulence.

Cranial Fossae

Superior view of horizontal section showing the cranial floor and fossae

The internal floor of the cranium is organized into three distinct depressions or basins called cranial fossae:
- Anterior Cranial Fossa:
- Formed by the frontal bone, ethmoid bone (cribriform plate and crista galli), and lesser wings of the sphenoid bone.
- Supports the frontal lobes of the brain.
- Middle Cranial Fossa:
- Formed by the body and greater wings of the sphenoid bone, as well as the temporal bones (squamous and petrous parts) and parietal bones.
- Houses the temporal lobes of the brain, pituitary gland (in the sella turcica), and major cranial nerves passing through the superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum.
- Posterior Cranial Fossa:
- Deepest fossa, formed predominantly by the occipital bone along with contributions from the temporal and parietal bones.
- Accommodates the cerebellum, pons, and medulla oblongata.
- Contains the foramen magnum, jugular foramen, internal acoustic meatus, and hypoglossal canal.

Bones of the Face

The facial complex comprises $14$ individual bones ( $6$ paired bones and $2$ single bones):
- Paired Facial Bones ( $12$ total):
- Maxillae ( $2$ )
- Palatine bones ( $2$ )
- Nasal bones ( $2$ )
- Zygomatic bones ( $2$ )
- Lacrimal bones ( $2$ )
- Inferior nasal conchae ( $2$ )
- Single Facial Bones ( $2$ total):
- Vomer ( $1$ )
- Mandible ( $1$ )
Maxillae ( $2$ ):
- Form the upper jaw, central facial framework, lower orbital rim, and floor of the orbits.
- Features:
- Body: Central portion housing the large maxillary sinus.
- Orbital surface: Forms the floor of the orbit.
- Frontal process: Projects superiorly to articulate with the frontal bone.
- Zygomatic process: Articulates laterally with the zygomatic bone.
- Alveolar process: Inferior ridge containing tooth sockets (alveoli) for the upper dentition.
- Palatal process: Horizontal plate extending medially; left and right palatal processes unite to form the anterior $\frac{3}{4}$ of the hard palate (bony roof of the mouth).
- Landmarks: Infra-orbital foramen, infra-orbital groove, incisive canal / incisive fossa, and anterior nasal spine.
Palatine Bones ( $2$ ):
- L-shaped bones situated posterior to the maxillae.
- Components:
- Horizontal plate: Forms the posterior $\frac{1}{4}$ of the hard palate.
- Perpendicular plate: Vertical projection forming part of the posterolateral nasal cavity wall.
- Processes and features: Orbital process, conchal crest, ethmoidal crest, nasal crest, greater palatine foramen, and lesser palatine foramen.
Nasal Bones ( $2$ ):
- Small, rectangular bones articulating along the midline to form the bridge of the nose superiorly.
- Articulate superiorly with the frontal bone via the frontonasal suture.
Zygomatic Bones ( $2$ ):
- Form the prominence of the cheeks and the lateral walls and floor of the orbits.
- Feature the temporal process, which extends posteriorly to articulate with the zygomatic process of the temporal bone, completing the zygomatic arch.
- Feature the zygomaticofacial foramen for blood vessel and nerve transmission.
Lacrimal Bones ( $2$ ):
- Smallest facial bones, situated in the anterior medial wall of each orbit.
- Feature a depression called the lacrimal groove, which combines with adjacent maxillary structures to form the nasolacrimal canal (directing tears from the orbital surface into the nasal cavity).
Inferior Nasal Conchae ( $2$ ):
- Independent, paired scroll-like bones extending horizontally into the lower nasal cavity from the lateral walls.
- Increase mucosal surface area and create airflow turbulence to warm, humidify, and filter incoming air.
Vomer ( $1$ ):
- Unpaired, wedge-shaped bone forming the inferior and posterior portion of the bony nasal septum.
- Articulates inferiorly with the maxillae and palatine bones and superiorly with the perpendicular plate of the ethmoid bone.
Mandible ( $1$ ):
- Forms the lower jaw; the only freely movable bone in the adult skull.
- Anatomic regions:
- Body: Curved horizontal portion forming the chin; features the central mental protuberance, mental foramen, and superior alveolar part housing sockets for lower teeth.
- Ramus: Vertical extension on each side ascending toward the temporal bone.
- Angle: Posteroinferior junction where the body meets the ramus.
- Condylar process: Posterior extension of the ramus terminated by a rounded head, which articulates with the mandibular fossa of the temporal bone at the temporomandibular joint (TMJ).
- Coronoid process: Sharp anterior projection of the ramus providing insertion for the temporalis muscle.
- Mandibular notch: Deep U-shaped indentation separating the coronoid and condylar processes.
- Internal medial features: Mandibular foramen (entry point for sensory nerves of lower teeth), mylohyoid line, and submandibular fossa.

The Orbital Complex and Nasal Cavity

Anterior view of the orbital complex

The Orbital Complex:
- Each orbital cavity (eye socket) is a cone-shaped bony structure constructed from $7$ individual skull bones:
1. Frontal bone (roof of orbit)
2. Sphenoid bone (posterior wall and wings)
3. Zygomatic bone (lateral wall and floor)
4. Maxilla (floor)
5. Palatine bone (small portion of floor)
6. Ethmoid bone (medial wall)
7. Lacrimal bone (anterior medial wall)
- Key passages within the orbit:
- Optic canal: Transmits the optic nerve (cranial nerve II) and ophthalmic artery.
- Superior orbital fissure: Passageway for cranial nerves III, IV, VI, and ophthalmic division of V.
- Inferior orbital fissure: Passageway for maxillary nerve and vessels.
- Supra-orbital notch/foramen, infra-orbital groove/foramen, and nasolacrimal canal.
The Nasal Complex & Paranasal Sinuses:
- Bony structures surrounding and forming the nasal cavities.
- Bony Nasal Septum: Midline partition separating left and right nasal cavities; constructed superiorly by the perpendicular plate of the ethmoid bone and inferiorly by the vomer.
- Paranasal Sinuses: Air-filled chambers within specific cranial and facial bones that open into the nasal cavity. These include:
- Frontal sinus (in frontal bone)
- Maxillary sinus (in maxillae; largest sinus)
- Sphenoidal sinus (in sphenoid bone)
- Ethmoidal air cells (in ethmoid bone)
- Functions of the paranasal sinuses:
- Significantly lessen the weight of the skull.
- Produce mucous secretions that clean and moisten nasal passages.
- Serve as resonating chambers during voice production.

The Hyoid Bone

Anterior and superior views of the hyoid bone

A horseshoe-shaped bone situated in the anterior neck region superior to the larynx.
Unique characteristic: Does not articulate directly with any other bone in the skeleton.
Suspended from the styloid processes of the temporal bones via paired stylohyoid ligaments.
Structural components:
- Body: Central region providing muscle attachment surfaces.
- Greater horns (greater cornua): Lateral posterolateral projections supporting larynx muscles.
- Lesser horns (lesser cornua): Superior projections attaching to stylohyoid ligaments.
Functional connections:
- Anchors muscles controlling the tongue, pharynx, and larynx during swallowing and speech.
- Connected inferiorly to the thyroid cartilage of the larynx via the thyrohyoid ligament.

The Infant Skull and Fontanels

Anterior and superior views of an infant skull showing fontanels

Features of fetal and infant skulls:
- Cranial growth is linked to brain expansion; full skull fusion is delayed at birth.
- Skull bones are separated by flexible areas of dense fibrous connective tissue called fontanels ("soft spots").
- Functional significance: Allow deformation and compression of the head during birth through the pelvic canal, and enable rapid brain growth during early infancy.
Major fontanels:
- Anterior fontanel:
- Largest diamond-shaped fontanel situated at the intersection of the coronal, sagittal, and frontal sutures.
- Commonly referred to as the "soft spot"; typically closes between $18\text{--}24$ months of age.
- Posterior fontanel:
- Triangular fontanel located at the junction of the sagittal and lambdoid sutures.
- Closes within $1\text{--}2$ months after birth.
- Sphenoidal fontanels ( $2$ ):
- Anterolateral fontanels located at the junction of the frontal, parietal, temporal, and sphenoid bones.
- Mastoid fontanels ( $2$ ):
- Posterolateral fontanels located at the junction of the parietal, occipital, and temporal bones.
In infants, the frontal bone is divided along the midline by the frontal (metopic) suture.

The Vertebral Column

The adult vertebral column (spine) consists of $26$ total bones:
- $24$ individual vertebrae.
- $1$ sacrum ( $5$ fused sacral vertebrae).
- $1$ coccyx ( $3\text{--}5$ fused coccygeal vertebrae).
Primary functions:
- Encloses and protects the spinal cord within the vertebral canal.
- Supports the head, skull, neck, and trunk.
- Bears and transfers weight down through the pelvic girdle to the lower limbs.
- Maintains erect posture and proper body alignment.
Regional division of the vertebral column:
- Cervical region: $7$ vertebrae ( $C_1\text{--}C_7$ ).
- Thoracic region: $12$ vertebrae ( $T_1\text{--}T_{12}$ ).
- Lumbar region: $5$ vertebrae ( $L_1\text{--}L_5$ ).
- Sacral region: $1$ sacrum ( $5$ fused vertebrae, $S_1\text{--}S_5$ ).
- Coccygeal region: $1$ coccyx ( $3\text{--}5$ fused vertebrae).

Lateral view of the vertebral column and spinal curves

Spinal Curves:
- Anteroposterior curves that distribute body weight during erect standing and movement.
- Primary curves (Accommodation curves):
- Thoracic curve and Sacral curve.
- Sweep posteriorly.
- Present at birth (fetal development) to accommodate thoracic and abdominopelvic viscera.
- Secondary curves (Compensation curves):
- Cervical curve and Lumbar curve.
- Sweep anteriorly.
- Develop postnatally as motor skills progress: cervical curve develops when the infant learns to hold up its head ( $\approx 3\text{--}4$ months); lumbar curve develops as the child learns to stand and walk ( $\approx 12$ months).

Development of spinal curvatures from fetus to adult

Clinical conditions showing abnormal curves of the vertebral column

Abnormal Curvatures of the Spine:
- Kyphosis: Exaggerated posterior curvature of the thoracic spine ("humpback").
- Lordosis: Exaggerated anterior curvature of the lumbar spine ("swayback").
- Scoliosis: Abnormal lateral curvature and rotation of the spine.
General Vertebral Anatomy:
- A typical vertebra consists of three main structural parts:
1. Vertebral body (centrum): Heavy, round, anterior weight-bearing portion; separated by fibrocartilaginous intervertebral discs.
2. Vertebral arch: Formed by paired pedicles (projecting posteriorly from body) and paired laminae (roofing plates extending medially).
3. Vertebral foramen: Central opening bounded by the body and arch; alignment of successive foramina forms the vertebral canal for the spinal cord.
- Projections/Processes ( $7$ total per typical vertebra):
- Spinous process: Posterior midline projection from the junction of the laminae.
- Transverse processes ( $2$ ): Lateral projections extending from pedicle-lamina junctions.
- Superior articular processes ( $2$ ): Project superiorly, carrying smooth superior articular facets.
- Inferior articular processes ( $2$ ): Project inferiorly, carrying smooth inferior articular facets.
- Intervertebral foramina: Lateral gaps formed between pedicles of adjacent articulated vertebrae; allow nerve roots to exit the spinal cord.

Regional Vertebral Anatomy

Cervical Vertebrae ( $C_1\text{--}C_7$ ):
- Smallest vertebrae supporting head weight.
- Feature a relatively wide, triangular vertebral foramen.
- Spinous processes are short and split at the tip, producing a bifid tip ( $C_2\text{--}C_6$ ).
- Feature costal processes extending laterally from the body, uniting with transverse processes to enclose a transverse foramen (transmits vertebral arteries and veins to the brain).
- Atypical Cervical Vertebrae:
- Atlas ( $C_1$ ):
  - Lacks a vertebral body and spinous process.
  - Consists of an anterior arch, posterior arch, anterior tubercle, posterior tubercle, and broad superior articular facets that articulate with occipital condyles.
  - Articulation allows nod-head movements ("yes" movement).
- Axis ( $C_2$ ):
  - Features a vertical tooth-like projection called the dens (odontoid process) on its body, created by fetal fusion of the atlas body with the axis.
  - The dens acts as a pivot axis secured by the transverse ligament inside the atlas ring.
  - Articulation allows head rotation ("no" movement).
  - Lacks an intervertebral disc between $C_1$ and $C_2$ ..
- Vertebra Prominens ( $C_7$ ):
  - Transitions structure toward thoracic type.
  - Features a long, prominent, non-bifid spinous process that forms a visible bump at the base of the neck.
  - Serves as the inferior anchor for the ligamentum nuchae, a fibrous elastic band running to the external occipital crest.

Superior view of articulated atlas and axis

Thoracic Vertebrae ( $T_1\text{--}T_{12}$ ):
- Medium-sized vertebrae articulating with ribs.
- Vertebral body is distinctly heart-shaped.
- Vertebral foramen is smaller and circular.
- Spinous process is long, slender, and points sharply inferoposteriorly (posterocaudal angle).
- Transverse processes point dorsolaterally.
- Costal articulations:
- Bodies feature superior costal facets and inferior costal facets for articulating with rib heads.
- Transverse processes feature transverse costal facets for articulating with rib tubercles ( $T_1\text{--}T_{10}$ ).
Lumbar Vertebrae ( $L_1\text{--}L_5$ ):
- Largest, heaviest, and most robust vertebrae built to bear weight.
- Vertebral body is thick and oval/kidney-shaped.
- Vertebral foramen is small and triangular.
- Spinous process is short, thick, flattened, and hatchet-shaped (resembling a fish tail fin), projecting posteriorly.
- Transverse processes are slender and project laterally.
- Articular processes are oriented vertically to allow flexion/extension while limiting rotation.

The Sacrum and Coccyx

Posterior, lateral, and anterior views of sacrum and coccyx

Sacrum:
- Single triangular bone formed by the fusion of $5$ sacral vertebrae ( $S_1\text{--}S_5$ ); fusion begins around puberty and completes between ages $25\text{--}30$ ..
- Structural orientation:
- Base: Broad superior surface articulating with $L_5$ ..
- Apex: Narrow inferior tip articulating with the coccyx.
- Anatomic features:
- Sacral promontory: Prominent anterior lip along the superior margin of the base.
- Ala: Broad wing-like lateral expansions on either side of the base.
- Sacral canal: Internal hollow tunnel continuing from the vertebral canal.
- Median sacral crest: Midline posterior ridge formed by fused spinous processes.
- Lateral sacral crests: Ridges formed by fused transverse processes.
- Sacral foramina: Four pairs of anterior and posterior openings extending from the sacral canal for nerve transmission.
- Auricular surface: Ear-shaped lateral articular surface forming the sacroiliac joint with the ilium of the pelvic girdle.
- Sacral tuberosity: Rough area posterior to the auricular surface for ligament attachment.
- Sacral hiatus: Inferior opening at the termination of the sacral canal, bounded by paired sacral cornua (horns).
Coccyx:
- Small terminal bone composed of $3\text{--}5$ (typically $4$ ) fused coccygeal vertebrae.
- Provides attachment for pelvic floor muscles and ligaments.
- Features superior projections termed coccygeal cornua that articulate with the sacral cornua.

The Thoracic Cage

Structure formed by the thoracic vertebrae, ribs, and sternum.
Primary functions:
- Forms a protective bony cage enclosing internal thoracic organs (heart, lungs, thymus, major vessels).
- Serves as anchor sites for skeletal muscles engaged in respiration, spinal positioning, and shoulder girdle/upper limb movement.

Anterior view of the thoracic cage structure

Sternum:
- Flat bone situated along the anterior midline of the thorax, comprising three fused parts:
1. Manubrium: Broad superior portion; features a medial jugular notch and lateral clavicular articulations for the pectoral girdle.
2. Body: Elongated central portion; attaches directly to costal cartilages of ribs $2\text{--}7$ ..
3. Xiphoid process: Small, flexible cartilaginous tip at the inferior end that ossifies later in life ( $\approx$ age $40$ ).
Ribs ( $24$ total / $12$ pairs):
- Curved, flat bones classified into three categories based on anterior sternal attachment:
- True Ribs (Ribs $1\text{--}7$ ):
  - Termed vertebrosternal ribs.
  - Connect directly to the sternum via individual strips of hyaline cartilage called costal cartilages.
- False Ribs (Ribs $8\text{--}12$ ):
  - Do not attach directly to the sternum.
  - Vertebrochondral ribs (Ribs $8\text{--}10$ ): Costal cartilages fuse together and join the costal cartilage of rib $7$ superiorly.
  - Floating ribs / Vertebral ribs (Ribs $11\text{--}12$ ): Have no anterior connection to the sternum or costal cartilages; terminate within abdominal wall musculature.
Anatomy of a Typical Rib:
- Head (capitulum): Posterior end carrying two smooth articular facets for articulation with costal facets on thoracic vertebral bodies.
- Neck: Short region connecting the head to the body.
- Tubercle: Elevation containing a facet that articulates with the transverse costal facet of the corresponding thoracic vertebra.
- Angle: Curve where the rib shaft bends anterolaterally.
- Body (shaft): Main structural body; features a inner inferior groove called the costal groove that protects intercostal nerves and blood vessels.
- Sternal end: Anterior terminus attaching to costal cartilage.