Bone and Axial Skeleton: Structure, Identification, and Key Features

Bone and Axial Skeleton

Objectives and Overview

The axial skeleton forms the central axis of the human body and includes the skull, vertebral column, and thoracic cage. Understanding the structure and function of bones within this system is fundamental to anatomy and physiology. This guide covers identification, structure, and key features of bones and the axial skeleton, as well as related terminology and models.

• Identify major structures of long bones, compact and spongy bone, and the axial skeleton.

• Describe bone markings and their significance.

• Distinguish between different vertebrae and thoracic cage components.

Actual Bones: Structure of a Typical Long Bone

Key Parts of a Long Bone

• Diaphysis: The shaft or central part of a long bone.

• Epiphysis: The expanded ends of a long bone, which articulate with other bones.

• Epiphyseal Line: The remnant of the epiphyseal plate (growth plate), marking the site where bone growth has ceased.

Example: The femur has a diaphysis (shaft) and two epiphyses (proximal and distal ends).

Bone Tissue Types

Compact Bone vs. Spongy Bone

• Compact Bone: Dense, strong bone tissue forming the outer layer of bones; provides strength and support.

• Spongy Bone: Porous, lighter bone tissue found at the ends of long bones and inside flat bones; contains red bone marrow.

Bone Histology Model

Microscopic Structure of Bone

• Osteon (Haversian System): The basic functional unit of compact bone, consisting of concentric layers (lamellae) around a central canal.

• Central Canal (Haversian Canal): Contains blood vessels and nerves; runs longitudinally through the osteon.

• Concentric Lamellae: Rings of calcified matrix surrounding the central canal.

• Osteocytes in Lacunae: Mature bone cells housed in small spaces (lacunae) between lamellae.

• Perforating (Volkmann's) Canals: Canals that run perpendicular to the central canals, connecting blood vessels and nerves between osteons.

Spongy Bone Features

• Osteon: Less organized in spongy bone; not as prominent as in compact bone.

• Trabecula (plural: trabeculae): Lattice-like network of bone tissue in spongy bone, providing structural support.

Skull and Associated Bone Models

Major Bones and Markings of the Skull

• Frontal Bone: Forms the forehead.

• Parietal Bones (2): Form the sides and roof of the cranium.

• Occipital Bone: Forms the back and base of the skull.

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

• Occipital Condyles: Articulate with the first cervical vertebra (atlas).

• Temporal Bones (2): Form the lower sides of the skull.

• Mastoid Process: Rounded projection for muscle attachment.

• Sphenoid Bone: Butterfly-shaped bone at the base of the skull; contains the sella turcica (depression for the pituitary gland).

• Optic Foramen (Canal): Passage for the optic nerve (CN II).

• Ethmoid Bone: Located between the eyes; forms part of the nasal septum.

• Cribriform Plate: Contains olfactory foramina for the olfactory nerve.

• Perpendicular Plate: Forms the superior part of the nasal septum.

• Maxilla: Upper jaw bone.

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

• Palatine Bone: Forms part of the hard palate.

• Zygomatic Bone: Cheekbone.

• Lacrimal Bone: Forms part of the medial wall of the orbit.

• Nasal Bone: Forms the bridge of the nose.

• Hyoid Bone: U-shaped bone in the neck; does not articulate with other bones.

Sutures of the Skull

Major Cranial Sutures

• Sagittal Suture: Between the left and right parietal bones.

• Coronal Suture: Between the frontal and parietal bones.

• Squamous Suture: Between the temporal and parietal/frontal bones.

• Lambdoidal Suture: Between the parietal and occipital bones.

Vertebral Column

Regions and Types of Vertebrae

• Cervical Vertebrae (7): Smallest vertebrae; found in the neck.

• Thoracic Vertebrae (12): Articulate with the ribs; found in the upper/mid-back.

• Lumbar Vertebrae (5): Largest vertebrae; found in the lower back.

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

• Coccyx (4 fused): Tailbone.

General Vertebral Features

• Body: Thick, weight-bearing anterior portion.

• Vertebral Foramen: Opening for the spinal cord.

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

• Spinous Process: Posterior projection; palpable along the back.

• Intervertebral Foramen: Openings between vertebrae for spinal nerves.

Special Cervical Vertebrae

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

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

Thoracic Cage

Sternum and Ribs

• Sternum: Flat bone at the center of the chest; consists of:

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

  • Body: Main, elongated portion.

  • Xiphoid Process: Inferior tip; cartilaginous in youth, ossifies with age.

• Ribs (12 pairs):

  • 7 pairs of true ribs: Attach directly to the sternum via costal cartilage.

  • 3 pairs of vertebrochondral ribs: Indirectly attach to the sternum.

  • 2 pairs of vertebral (floating) ribs: Do not attach to the sternum.

Key Table: Comparison of Bone Types and Features

Feature	Compact Bone	Spongy Bone
Location	Outer layer of bones	Ends of long bones, inside flat bones
Structure	Osteons (Haversian systems)	Trabeculae
Function	Strength, support	Lightweight, houses marrow

Additional info:

• Bone markings (such as foramina, processes, and condyles) serve as attachment points for muscles, passageways for nerves and blood vessels, and articulation points for joints.

• Understanding the differences among cervical, thoracic, and lumbar vertebrae is essential for identifying vertebral regions in anatomical models and clinical practice.