Axial Skeleton Study Notes

The Skeleton: Axial

Chapter 7 Overview

This chapter focuses on the axial skeleton, which includes the skull, vertebral column, and thoracic cage.

Major Divisions of the Skeleton

  • Axial Skeleton: Includes the bones of the skull, vertebral column, and thoracic cage.
  • Appendicular Skeleton: Comprises limbs and their attachments.
  • Appendages: Refers to limbs and structures attached to the axial skeleton.

The Skull

General Characteristics
  • Most skull bones are flat and interlocked through joints called sutures, except for the mandible which is a movable bone.
  • Sutures have a serrated, saw-tooth appearance, including:
    • Coronal Suture
    • Sagittal Suture
    • Squamous Suture
    • Lambdoid Suture
Cranial Anatomy
  • Cranium is divided into:
    • Cranial Vault (Calvaria): Forms the superior, lateral, and posterior portions of the skull as well as the forehead.
    • Cranial Base: Forms the inferior aspect of the skull, consisting of three internal fossae: anterior, middle, and posterior cranial fossae.
Cranial Cavities
  • The skull contains several cavities:
    • Middle and internal ear cavities
    • Nasal cavity
    • Orbits that house the eyeballs
  • Additional structures include sinuses (e.g., frontal sinus, maxillary sinus) and air cells.
Skull Openings
  • The skull boasts 85 named openings including foramina, canals, and fissures that allow passage for the spinal cord, blood vessels, and 12 pairs of cranial nerves.
Cranial Bones
  • Comprised of eight cranial bones:
    • Frontal Bone
    • Parietal Bones (two: left and right)
    • Occipital Bone
    • Temporal Bones (two: left and right)
    • Sphenoid Bone
    • Ethmoid Bone
Occipital Bone Features
  • Forms most of the skull's posterior wall and posterior cranial fossa.
  • Articulates with the parietal, temporal, and sphenoid bones.
  • Contains the Foramen Magnum, which allows the brain to connect with the spinal cord.
  • Features occipital condyles that articulate with the first cervical vertebra (the atlas).
Temporal Bones
  • Paired bones comprising the inferior lateral aspects of the skull and parts of the cranial base. Each has three major regions:
    • Squamous Part: Contains the zygomatic processes that articulate with the zygomatic bone to form the zygomatic arch.
    • Petrous Part: Houses the inner ear.
    • Mastoid Part: Contains the mastoid process.
Sphenoid Bone Characteristics
  • Complex, bat-shaped bone with three pairs of processes:
    • Greater Wings
    • Lesser Wings
    • Pterygoid Processes
  • Houses the Pituitary Gland, often referred to as the "master gland."
Ethmoid Bone Features
  • Contains structures like the cribriform plate with cribriform foramina, crista galli, and orbital plates.
  • Forms part of the nasal septum and contributes to the walls of the nasal cavity.

Facial Bones

Overview
  • The facial skeleton comprises bones that form the structure and shape of the face.
  • Includes:
    • Zygomatic Bones: Provide the structure for the cheekbones and inferolateral margins of the orbits, articulating with the temporal, frontal, and maxillary bones.
    • Palatine Bones: Comprised of horizontal and perpendicular plates responsible for forming parts of the hard palate and lateral walls of the nasal cavity.
    • Vomer: This bone forms part of the nasal septum.
Lacrimal Bones
  • Small, thin bones forming part of the medial wall of the orbits which house the lacrimal glands responsible for tear production.

The Vertebral Column

General Characteristics
  • Extends from the skull to the pelvis, also known as the spine or spinal column.
  • Functions to support weight, protect the spinal cord, and provide attachment for ribs and muscles.
  • Comprises 26 irregular bones called vertebrae organized into five regions.
Vertebral Regions
  1. Cervical Region (C1-C7): 7 vertebrae, concave posteriorly.
  2. Thoracic Region (T1-T12): 12 vertebrae, convex posteriorly.
  3. Lumbar Region (L1-L5): 5 vertebrae, concave posteriorly.
  4. Sacral Region: 5 fused vertebrae forming the sacrum.
  5. Coccygeal Region: 4 fused vertebrae forming the coccyx.
Vertebral Curvatures
  • Four main curvatures:
    • Cervical and Lumbar Curvatures: Concave posteriorly.
    • Thoracic and Sacral Curvatures: Convex posteriorly.
Structure of Vertebrae
  • Each vertebra has seven processes:
    • Spinous Process: Projects posteriorly.
    • Transverse Processes (2): Project laterally.
    • Superior Articular Processes (2): Protrude superiorly.
    • Inferior Articular Processes (2): Protrude inferiorly.
Special Features of Cervical Vertebrae
  • Small, oval bodies; bifid spinous processes (except C2).
  • Vertebral foramen is triangular and large, facilitating a broad neural passageway.
  • Contains foramina.
  • Superior articular facets direct superoposteriorly and inferior facets direct inferoanteriorly.

Thoracic Cage

General Characteristics
  • Composed of thoracic vertebrae, sternum, and ribs.
  • Functions include:
    • Protecting vital organs in the thoracic cavity.
    • Supporting shoulder girdles and upper limbs.
    • Providing attachment sites for muscles of the neck, back, chest, and shoulders.
Ribs
  • Comprising 12 pairs of ribs:
    • True Ribs (1-7): Attach directly to the sternum.
    • False Ribs (8-10): Do not attach directly to sternum (vertebrochondral).
    • Floating Ribs (11-12): Not attached to any cartilage.
Rib Anatomy
  • Main Parts of Rib:
    • Shaft: Flat bone making up most of the rib.
    • Costal Groove: Houses nerves and vessels.
    • Head: Articulates with adjacent vertebrae.
    • Neck: Constricted portion beyond the head.
    • Tubercle: Knob-like structure lateral to neck.

Infant Skull Characteristics

  • The infant skull comprises more bones than the adult skull, as bones such as the mandible and frontal are unfused.
  • Connected by fontanelles, allowing for birth and growth:
    • Four Fontanelles: Anterior, posterior, mastoid, and sphenoidal.