Chapter 7A

Chapter 7: The Skeletal System - The Axial Skeleton

Overview

  • The human skeleton consists of 206 named bones grouped into two principal divisions:

    • Axial Skeleton: 80 bones

    • Appendicular Skeleton: 126 bones

Divisions of the Skeletal System

  • Axial Skeleton Includes:

    • Skull Bones

    • Auditory Ossicles (ear bones)

    • Hyoid Bone

    • Ribs

    • Sternum (breastbone)

    • Vertebral Column

  • Appendicular Skeleton Includes:

    • Bones of the upper limbs (arms)

    • Bones of the lower limbs (legs)

    • Bones forming girdles that connect limbs to the axial skeleton

Types of Bones

  • Bones are classified into five main types based on shape:

    • Long Bones: Greater length than width (e.g., femur)

    • Short Bones: Cube-shaped (e.g., wrist bones)

    • Flat Bones: Thin layers of parallel plates (e.g., skull bones)

    • Irregular Bones: Complex shapes (e.g., vertebrae)

    • Sesamoid Bones: Shaped like sesame seeds (e.g., patella)

Sutural Bones

  • Sutural Bones: Small, extra bone plates located within the sutures of cranial bones.

    • Sutures are areas where flat bones meet.

Bone Surface Markings

  • Surface markings on bones are structural features adapted for specific functions, including:

    • Depressions and Openings:

      • Allow passage of soft tissues

      • Form joints

    • Processes:

      • Projections or outgrowths forming joints

      • Serve as attachment points for ligaments and tendons

Types of Bone Surface Markings

  • Fissure: Narrow slit between bones for passage of blood vessels or nerves.

  • Foramen: Hole for the passage of blood vessels, nerves, or ligaments.

  • Fossa: Shallow depression (e.g., mandibular fossa).

  • Sulcus: Furrow on a bone for passage of blood vessel, nerve, or tendon.

  • Meatus: Tubelike opening.

  • Condyle: Rounded projection with a smooth articular surface.

  • Facet: Smooth, flat, slightly concave articular surface.

  • Head: Rounded articular process supported on a neck.

  • Crest: Prominent ridge or elongated process.

  • Epicondyle: Roughened projection on a condyle.

  • Line: Long, narrow ridge or border, less prominent than a crest.

  • Spinous Process: Sharp, slender projection.

  • Trochanter: Very large projection (only on femur).

  • Tubercle: Variably sized rounded projection.

  • Tuberosity: Variably sized projection with a rough, bumpy surface.

The Skull

  • The Skull contains 22 bones, excluding the 3 middle ear bones on both sides.

    • Associated features include processes, ridges, lines, depressions, and foramen.

The Mandible

  • The Mandible (lower jaw) is the largest and strongest facial bone and is the only moveable skull bone, aside from the auditory ossicles.

Paranasal Sinuses

  • Paranasal Sinuses: Mucous membrane-lined cavities in the frontal, maxillary, sphenoid, and ethmoid bones.

    • Functions include enhancing voice resonance and increasing surface area of nasal mucosa for moisture.

Fontanels

  • Fontanels are mesenchyme-filled spaces between cranial bones present at birth, which close between 6 months to 2 years of age.

Hyoid Bone

  • The Hyoid Bone supports the tongue by attaching to muscles and is located at the top of the larynx.

The Vertebral Column

  • Known as the spinal column, it's made up of 26 vertebrae divided into five regions: cervical, thoracic, lumbar, sacrum, and coccyx.

    • Sacrum (5 fused vertebrae) and coccyx (4 fused vertebrae) are not counted as individual bones.

    • The column protects the spinal cord.

Intervertebral Discs

  • Intervertebral Discs: Separate vertebrae and are located from the second cervical to the sacrum.

    • Composed of an outer fibrocartilage ring (annulus fibrosus) and a soft inner nucleus (nucleus pulposus).

    • Functions include shock absorption and creating space between vertebrae.

Composition of a Typical Vertebra

  • Composed of various parts including:

    • Vertebral Arch: Includes the lamina and pedicle.

    • Processes: Spinous and transverse processes.

    • Vertebral Body: The main weight-bearing portion.

Thoracic Vertebrae

  • Support the ribs and include special structures for rib attachments.

Lumbar Vertebrae

  • The largest and strongest vertebrae, supporting the body's weight without special structures for rib attachment.

Sacrum and Coccyx

  • The triangular sacrum is formed from 5 fused vertebrae, fusing between 16 and 30 years of age.

  • The coccyx consists of 4 fused vertebrae, fusing between 20 to 30 years of age.

Thoracic Bones

  • The Sternum is a flat bone at the center of the anterior thoracic wall divided into three segments:

    • Manubrium (upper)

    • Body (middle)

    • Xiphoid Process (lower)

  • Ribs provide thoracic cavity support with:

    • 12 pairs of ribs extending from thoracic vertebrae to sternum.

    • True Ribs: First 7 pairs (directly attached to sternum).

    • False Ribs: Next 5 pairs (indirectly or not connected).

    • Bony portions end near the sternum and connect via costal cartilage.

Disorders of the Skeletal System

  • Common disorders affecting the skeleton include:

    • Herniated Disc: Nucleus pulposus leaks out due to disc tear.

    • Scoliosis: Increased lateral curvature of the spine.

    • Kyphosis: Increased thoracic curve, bent forward.

    • Lordosis: Increased lumbar curve, bent backwards.

    • Spina Bifida: Congenital defect in the vertebral column, varying degrees of severity.

Conclusion

  • Understanding the anatomy and function of the axial skeleton is essential for recognizing and addressing various disorders and clinical conditions related to the spinal structure and its associated components.