CH7.Study Notes on the Skeletal System: The Axial Skeleton

Chapter 7: The Skeletal System: The Axial Skeleton

Divisions of the Skeletal System

• The human skeleton consists of 206 named bones.

• Bones are categorized into two principal divisions:

  • Axial Skeleton

  • Consists of bones that lie around the longitudinal axis of the human body.

  • Comprises:

    • Skull bones

    • Auditory ossicles (six small bones in each ear)

    • Hyoid bone (the only bone in the body that does not articulate with any other bone)

    • Ribs

    • Sternum (breastbone)

    • Bones of the vertebral column

  • Appendicular Skeleton

  • Includes the bones of the upper and lower limbs (extremities).

  • Comprises girdles that connect the limbs to the axial skeleton.

Table 7.1: The Bones of the Adult Skeletal System

Divisions of the Axial Skeletal System

• Skull

• Sternum

• Ribs

• Vertebrae

  • Cervical

  • Thoracic

  • Lumbar

  • Sacrum

  • Coccyx

Types of Bones

• Bones can be classified into five types based on shape:

  • Long bones: Greater length than width, slightly curved for strength.

  • Examples: Femur, tibia, fibula.

  • Short bones: Cube-shaped, nearly equal in length and width.

  • Examples: Carpal, tarsal.

  • Flat bones: Thin, composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue.

  • Examples: Cranial bones, sternum.

  • Irregular bones: Complex shapes that cannot be grouped into other categories.

  • Examples: Vertebrae, hip bones.

  • Sesamoid bones: Protect tendons from excessive wear and tear.

  • Examples: Patella (kneecap), bones in hands and feet.

Bone Surface Markings

• Bones exhibit characteristic surface markings that are structural features adapted for specific functions.

• There are two major types of surface markings:

  1. Depressions and Openings: Allow the passage of blood vessels and nerves or form joints.

  2. Processes: Projections or outgrowths that form joints or serve as attachment points for ligaments and tendons.

Skull

• The skull (cranium) consists of 22 bones grouped into two categories:

  • Cranial bones: Eight cranial bones form the cranial cavity.

  • Examples: Frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, ethmoid bone.

  • Facial bones: Fourteen facial bones form the face.

  • Examples: Two nasal bones, two maxillae, two zygomatic bones, mandible, two lacrimal bones, two palatine bones, two inferior nasal conchae, and one vomer.

Skull Features

• The cranial and facial bones protect and support special sense organs and the brain.

• The skull forms several smaller cavities:

  • Nasal cavity

  • Orbits (eye sockets)

  • Paranasal sinuses (lined with mucous membrane)

• Most skull bones are fused together by immovable joints called sutures.

• The facial bones form the framework of the face providing support for the entrances to the digestive and respiratory systems.

Cranial Bones

• Frontal Bone

• Parietal Bones

• Temporal Bones

• Occipital Bone

• Sphenoid Bone

• Ethmoid Bone

Facial Bones

• Nasal Bones: Form the bridge of the nose.

• Maxillae: Form the upper jawbone.

• Zygomatic Bones: Commonly referred to as cheekbones, they form the prominences of the cheeks.

• Lacrimal Bones: Form a part of the medial wall of each orbit; the smallest bones in the face.

• Palatine Bones: Form the posterior portion of the hard palate.

• Inferior Nasal Conchae: Form a part of the inferior lateral wall of the nasal cavity.

• Vomer: Forms the inferior portion of the nasal septum.

• Mandible: Lower jawbone, the largest and strongest facial bone; it is the only movable skull bone.

Skull Special Features

• Nasal Septum: Divides the interior of the nasal cavity into two sides, composed of the vomer bone and the perpendicular plate of the ethmoid bone.

• Orbits: Eye socket formed by the union of seven bones:

  • Frontal bone

  • Lacrimal bone

  • Ethmoid bone

  • Zygomatic bone

  • Maxillary bone

  • Palatine bone

  • Sphenoid bone

• Foramina & Fossa:

  • Foramen: Opening for blood vessels, nerves, or ligaments of the skull.

  • Fossa: A shallow depression on the bone surface.

Unique Features of the Skull

• Sutures: Immovable joints that hold most skull bones together, including sagittal, coronal, squamous, and lambdoidal sutures.

• Paranasal Sinuses: Cavities within cranial and facial bones near the nasal cavity, lined with mucous membranes that produce secretions draining into the nasal cavity, serving as resonating chambers that intensify and prolong sounds.

• Fontanels: Areas of unossified tissue at birth, commonly known as "soft spots" that allow brain growth and skull shape changes during childbirth. They are gradually replaced by bone to become sutures.

Paranasal Sinuses

• Include the Frontal, Ethmoid, Sphenoid, and Maxillary sinuses.

Fontanels

• Located in the forehead (frontal bone), links during development are visible as the anterior fontanel and posterior fontanel, with lateral views showing the mastoid fontanel.

Vertebral Column

• Also referred to as the spine, backbone, or spinal column.

• Functions:

  • Protect the spinal cord.

  • Support the head.

  • Serve as an attachment point for ribs, pelvic girdle, and muscles.

• The vertebral column is curved, enhancing strength, maintaining balance in an upright position, absorbing shocks during walking, and offering protection against fractures.

Composition of the Vertebral Column

• Composed of 26 vertebrae:

  • 7 cervical vertebrae (C1-C7) in the neck region.

  • 12 thoracic vertebrae (T1-T12) posterior to the thoracic cavity.

  • 5 lumbar vertebrae (L1-L5) supporting the lower back.

  • 1 sacrum, formed by five fused sacral vertebrae.

  • 1 coccyx, formed by four fused coccygeal vertebrae.

Intervertebral Discs

• Located between the bodies of adjacent vertebrae, they:

  • Form strong joints.

  • Permit movements of the vertebral column.

  • Absorb vertical shock.

  • Typically consist of:

  • A body (weight-bearing).

  • A vertebral arch (surrounds the spinal cord).

  • Several processes (attachment points for muscles).

Regions of the Vertebral Column

• Cervical Region:

  • Cervical vertebrae (C1–C7)

  • The atlas (C1): First cervical vertebra that supports the skull.

  • The axis (C2): Second cervical vertebra allows side-to-side head rotation; features a structure called the Dens that replaces the body of the Atlas.

• Thoracic Region: Thoracic vertebrae (T1–T12).

• Lumbar Region: Lumbar vertebrae (L1–L5), the largest and strongest vertebrae.

• Sacrum: Triangular bone formed from the union of five sacral vertebrae (S1–S5).

• Coccyx: Formed by the fusion of usually four coccygeal vertebrae, similar in shape to the sacrum.

Sacrum and Coccyx

• Features and structures include the anterior view showing the body of the first sacral vertebra, transverse ridges (sites of vertebral fusion), and the coccyx.

Thorax

• The thoracic cage comprises:

  • Sternum

  • Ribs

  • Costal cartilages

  • Thoracic vertebrae

• Functions:

  • Encloses and protects organs in the thoracic and abdominal cavities.

  • Provides support for the bones of the upper limbs.

  • Plays a role in breathing.

Sternum

• Also known as the "breastbone", located in the center of the thoracic wall, consisting of:

  • Manubrium

  • Body

  • Xiphoid process

Ribs

• Twelve pairs of ribs provide structural support to the thoracic cavity:

  • The first seven are classified as true ribs.

  • The remaining ribs are false ribs (last two are termed floating ribs).

Costal Cartilages

• These contribute to the elasticity of the thoracic cage, allowing for expansion during breathing.