Axial Skeleton: Bones and Features for Identification

Axial Skeleton Overview

The axial skeleton forms the central axis of the human body and includes the skull, vertebral column, and thoracic cage. It provides support, protection for vital organs, and attachment points for muscles.

Skull

Cranial Bones

The cranial bones protect the brain and form the structure of the head. Key bones and features include:

• Frontal (1): Contains the frontal sinuses.

• Parietal (2): Marked by temporal lines.

• Occipital (1): Features the occipital condyles (articulate with the atlas) and the foramen magnum (passage for the spinal cord).

• Temporal (2): Includes petrous and squamous parts, external acoustic meatus, mastoid process, zygomatic process, and mandibular fossa.

• Sphenoid (1): Contains greater and lesser wings, sella turcica (houses pituitary gland), optic canals, and sphenoid sinus.

• Ethmoid: Features cribriform plate, perpendicular plate (forms part of nasal septum), and superior and middle nasal conchae.

Facial Bones

• Zygomatic (2): Forms the cheekbones; temporal process is part of the zygomatic arch.

• Lacrimal (2): Contains the lacrimal fossa for tear ducts.

• Nasal (2): Forms the bridge of the nose.

• Inferior nasal conchae (2): Increases surface area in the nasal cavity.

• Maxillary (2): Contains maxillary sinuses and alveolar process (holds teeth).

• Vomer (1): Forms the lower part of the nasal septum.

• Palatine (2, fused): Forms the posterior part of the hard palate.

• Mandible (1): Lower jaw; features alveolar processes, body, angle, rami, coronoid processes, and condylar processes (articulate with temporal bone at TMJ).

Sutures and Fontanelles

• Sutures: Immovable joints between skull bones. Major sutures include coronal, sagittal, squamous, and lambdoid.

• Fontanelles: Soft spots in the fetal skull that allow for growth and flexibility during birth.

Hyoid Bone

The hyoid bone is unique as it does not articulate with any other bone. Located in the throat, it supports the larynx and is anchored by neck muscles.

Vertebral Column

The vertebral column supports the body, protects the spinal cord, and provides attachment points for ribs and muscles. It consists of five regions:

• Cervical (7): Neck region

• Thoracic (12): Upper back, articulates with ribs

• Lumbar (5): Lower back, supports most body weight

• Sacral (5, fused): Forms the sacrum

• Coccygeal (3-5, fused): Forms the coccyx (tailbone)

Intervertebral discs separate vertebrae and act as shock absorbers.

Typical Vertebra Structure

• Body: Weight-bearing portion

• Vertebral arch: Formed by pedicles and laminae

• Vertebral foramen: Passage for the spinal cord

• Processes: Transverse, spinous, superior & inferior articular processes

Cervical Vertebrae (C1–C7)

• Typical cervical vertebrae: Small, with transverse foramina for vertebral arteries, bifid spinous processes, and short transverse processes.

• Atlas (C1): Lacks a body and spinous process; has anterior and posterior arches; articulates with occipital condyles for nodding motion.

• Axis (C2): Has a dens (odontoid process) that allows rotation of the head.

Thoracic Vertebrae (T1–T12)

• Medium-sized, heart-shaped body with superior and inferior costal facets for rib articulation.

• Long, downward-pointing spinous processes.

• Transverse costal facets on transverse processes for rib tubercles.

Lumbar Vertebrae (L1–L5)

• Large, robust bodies for weight bearing.

• Short, thick spinous processes and long, slender transverse processes.

Sacrum and Coccyx

• Sacrum: Five fused vertebrae; features median sacral crest and sacral hiatus.

• Coccyx: Three to five fused segments; serves as an attachment for ligaments and muscles.

Thoracic Skeleton

Ribs

There are 12 pairs of ribs, each articulating with thoracic vertebrae. Key features include:

• Head: Has articular facets for vertebral bodies.

• Neck: Connects head to shaft.

• Tubercle: Contains tubercular facet for articulation with transverse costal facet of vertebra.

• Shaft: Main body of the rib.

• Costal cartilages: Connect ribs to sternum (ribs I–VII) or to cartilage above (ribs VIII–X); ribs XI and XII are floating ribs with no costal cartilage.

Sternum

• Manubrium: Superior portion

• Body: Central portion

• Xiphoid process: Inferior tip

Summary Table: Vertebral Regions and Key Features

Region	Number of Vertebrae	Key Features
Cervical	7	Transverse foramina, bifid spinous process, atlas & axis
Thoracic	12	Costal facets for ribs, long spinous process
Lumbar	5	Large body, short spinous process
Sacral	5 (fused)	Median sacral crest, sacral hiatus
Coccygeal	3–5 (fused)	Small, fused segments

Additional info:

• Fontanelles allow for brain growth and skull flexibility in infants.

• The temporomandibular joint (TMJ) is formed by the articulation of the mandibular condyle with the mandibular fossa of the temporal bone.

• Intervertebral discs are composed of an outer annulus fibrosus and an inner nucleus pulposus, providing cushioning between vertebrae.