Axial Skeleton Study Notes

Skeletal System Overview

• Total Bones: Adult skeleton consists of $206$ bones.
• Axial Skeleton: $80$ bones positioned along the longitudinal axis.
• Appendicular Skeleton: $126$ bones, including upper and lower limbs, shoulder girdle, and pelvic girdle.

Cranial Bones

• Cranial Cavity (8 Bones): Encloses the brain within the calvaria and cranial floor.
  • Frontal Bone (1): Singular, pneumatized bone forming the forehead, orbit, and floor of cranial cavity; features the nasion and supraorbital foramen.
  • Parietal Bones (2): Paired bones forming the sides and roof of cranial cavity; joined by coronal/frontal and sagittal sutures.
  • Temporal Bones (2): Paired pneumatized bones forming inferior lateral cranium and cranial floor.
  • Surface Markings: Squamous suture, external auditory meatus, mastoid process, styloid process, zygomatic process, mandibular fossa.
  • Internal Markings: Middle ear ossicles (malleus, incus, stapes), internal acoustic meatus, jugular foramen, carotid canal.
  • Occipital Bone (1): Formed at posterior and inferior cranium; features lambdoid suture, external occipital protuberance, superior and inferior nuchal lines, foramen magnum, and occipital condyles.
  • Sphenoid Bone (1): Bat-shaped bone articulating with all other cranial bones; pneumatized; features greater wing, lesser wing, sella turcica, superior orbital fissure, and pterygoid processes.
  • Ethmoid Bone (1): Major contributor to nasal cavity, orbits, and facial structure; features crista galli, cribriform plate, olfactory foramen, superior and middle nasal conchae, and perpendicular plate.

Facial Bones, Orbits, and Sinuses

• Facial Bones (14 Bones):
  • Nasal (2): Form the bridge of the nose.
  • Lacrimal (2): Anterior portion of medial wall of orbit; contains lacrimal fossa.
  • Palatine (2): Form posterior edge of hard palate, parts of orbit, and nasal cavity.
  • Inferior Nasal Conchae (2): Curved bones on lateral nasal wall that swirl and filter air.
  • Vomer (1): Inferior bone of the nasal septum.
  • Maxillae (2): Upper jaw, roof of mouth, floor of orbit, nasal floor; features incisive foramen and palatine process.
  • Zygomatic (2): Form cheekbones and lateral orbit floor; features zygomatic arch and temporal process.
  • Mandible (1): Only movable facial bone forming temporomandibular joint (TMJ); features mental foramen, angle, condylar process, mandibular foramen, and coronoid process.
• Nasal Septum Structure: Composed of perpendicular plate of ethmoid, vomer, and septal cartilage.
• Orbits: Formed by $3$ cranial bones and $4$ facial bones; feature $5$ openings for nerves/vessels plus the lacrimal fossa.

• Paranasal Sinuses: Four air-filled spaces lined with ciliated pseudostratified columnar epithelium (frontal, ethmoidal, sphenoidal, maxillary).
  • Functions: Humidify/warm air, act as buffer zones, decrease bone mass, provide voice resonance.

Hyoid Bone

• Location & Features: U-shaped bone located in anterior neck between mandible and larynx; does not articulate directly with any other bone.
• Anatomical Parts: Body, greater horns, and lesser horns.

Vertebral Column

• Regions & Vertebrae Count:
  • Cervical Vertebrae ($7$): C1 (Atlas) articulates with occipital condyles; C2 (Axis) features dens (odontoid process); C7 is vertebra prominens. Feature transverse foramina and bifid spinous processes.
  • Thoracic Vertebrae ($12$): Articulate with ribs via costal facets and demi-facets on bodies (T1–T8) and transverse process facets.
  • Lumbar Vertebrae ($5$): Large bodies designed for weight bearing.
  • Sacrum ($1$): Formed by $5$ fused sacral vertebrae (S1–S5); features sacral canal, sacral promontory, anterior/posterior sacral foramina, median sacral crest, and sacral hiatus.
  • Coccyx ($1$): Formed by $4$ fused coccygeal vertebrae (Co1–Co4).
• Curvatures: Four normal curves (cervical, thoracic, lumbar, sacral).
• Intervertebral Discs: Fibrocartilage structures with outer annulus fibrosus and inner nucleus pulposus situated between vertebral bodies.

Thorax

• Thoracic Cage Components: Sternum, ribs, and costal cartilages.
• Sternum: Flat bone divided into manubrium (with suprasternal notch and clavicular notch), body, and xiphoid process.
• Rib Classifications ($12$ Pairs):
  • True Ribs (1–7): Vertebrosternal ribs; attach directly to sternum via costal cartilage.
  • False Ribs (8–10): Vertebrochondral ribs; attach indirectly to sternum via shared costal cartilage.
  • Floating Ribs (11–12): Vertebral ribs; no anterior connection to sternum.
• Rib Structures: Head (with superior and inferior facets), neck, tubercle (with costal facet), costal angle, and body.