The Skeletal System

The Skeletal System: Comprehensive Notes

Divisions of the Skeletal System

• The human skeleton is composed of $206$ named bones.

• These bones are categorized into two primary divisions:

  • Axial Skeleton: Consists of $80$ bones.

  • Appendicular Skeleton: Consists of $126$ bones.

Axial Skeleton

• Components: Includes the skull, vertebral column, ribs, and hyoid bone.

• Detailed Bone Count:

  • Skull ($29$ bones):

    • Cranium: $8$

    • Face: $14$

    • Hyoid bone: $1$

    • Auditory ossicles: $6$ (see Figure 7.5)

  • Vertebral column: $26$ bones

  • Thorax ($25$ bones):

    • Sternum: $1$

    • Ribs: $24$

  • Total Axial Bones: $80$

Appendicular Skeleton

• Components: Consists of the bones of the upper and lower limbs (appendages), the pectoral girdle, and the pelvic girdle.

• Girdles' Function: The pectoral girdle holds the upper limbs in place, and the pelvic girdle holds the lower limbs in place.

• Detailed Bone Count:

  • Pectoral (shoulder) girdles ($4$ bones):

    • Clavicle: $2$

    • Scapula: $2$

  • Upper limbs ($60$ bones):

    • Humerus: $2$

    • Ulna: $2$

    • Radius: $2$

    • Carpals: $16$

    • Metacarpals: $10$

    • Phalanges: $28$

  • Pelvic (hip) girdle ($2$ bones):

    • Hip, pelvic, or coxal bone: $2$

  • Lower limbs ($60$ bones):

    • Femur: $2$

    • Patella: $2$

    • Fibula: $2$

    • Tibia: $2$

    • Tarsals: $14$

    • Metatarsals: $10$

    • Phalanges: $28$

  • Total Appendicular Bones: $126$

• Total Bones in an Adult Skeleton: $206$ ($80 + 126$)

Bone Surface Markings

• Bones display characteristic surface markings, which are structural features adapted for specific functions.

• There are two primary types of surface markings:

  • Depressions and Openings: These allow the passage of soft tissues (blood vessels, nerves, ligaments) or form joints.

    • Fissure: A narrow slit between bones for the passage of blood vessels or nerves.

      • Example: Superior orbital fissure.

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

      • Example: Foramen magnum.

    • Fossa: A shallow depression.

      • Example: Mandibular fossa.

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

    • Meatus: A tubelike opening.

      • Example: External auditory meatus.

  • Processes: These are projections or outgrowths that either form joints or serve as attachment points for ligaments and tendons.

    • Condyle: A rounded projection with a smooth articular surface.

      • Example: Condylar process.

    • Facet: A smooth, flat, slightly concave articular surface.

      • Example: Facet of superior articular process.

    • Head: A usually rounded articular process supported on a neck.

    • Crest: A prominent ridge or elongated process.

      • Example: Median sacral crest.

    • Epicondyle: A usually roughened projection located superior to a condyle.

    • Line or Linea: A long, narrow ridge or border (less prominent than a crest).

    • Spinous process: A sharp, slender projection, often found on vertebra bones.

    • Trochanter: A very large projection found ONLY on the femur.

    • Tubercle: A variably sized rounded projection.

      • Example: Articular tubercle.

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

      • Example: Sacral tuberosity.

Cavities of the Skull

• The skull contains several important cavities:

  • Cranial cavity (endocranium): The largest cavity, which encloses, protects, and supports the brain.

  • Orbits: The eye sockets.

  • Oral cavity.

  • Nasal cavity.

  • Paranasal sinuses: Including the frontal, ethmoidal, and maxillary sinuses.

Cranial Bones ($8$ Bones)

• Frontal Bone ($1$):

  • Forms the forehead.

  • Contains the supraorbital foramen.

• Parietal Bones ($2$).

• Temporal Bones ($2$):

  • Seen bilaterally.

  • External acoustic/auditory meatus: A small bony ring surrounding the external ear opening.

  • Temporomandibular joint (TMJ): Where the mandible articulates with the mandibular fossa of the temporal bone.

  • Mastoid process: A bump located behind the external ear opening.

  • Styloid process: A