Human Anatomy - The Skeletal System II: Axial Skeleton

Overview of Skeleton

  • Axial Skeleton:
    • Defines the central supporting axis of the body.
    • Composed of:
    • Skull
    • Vertebral column
    • Thoracic cage
  • Appendicular Skeleton:
    • Comprised of:
    • Upper limbs and pectoral girdle
    • Lower limbs and pelvic girdle

Bones of the Skeleton

  • At birth, humans have about 270 bones; however, many of these bones fuse as a person ages, reducing the total to approximately 206 bones in adulthood.
  • Sesamoid bones:
    • Develop within tendons due to stress.
  • Sutural (wormian) bones:
    • Extra bones found in some individuals' skulls, not consistently present.

Surface Features (Markings) of Bones

Table 7.2: Surface Features (Markings) of Bones
  1. Canal:
    • A tubular passage or tunnel in a bone (e.g., auditory canal of the skull).
  2. Condyle:
    • A rounded knob (e.g., occipital condyles of the skull).
  3. Crest:
    • A narrow ridge (e.g., iliac crest of the pelvis).
  4. Epicondyle:
    • A flare superior to a condyle (e.g., medial epicondyle of the femur).
  5. Facet:
    • A smooth joint surface that is flat or slightly concave or convex (e.g., articular facets of the vertebrae).
  6. Fissure:
    • A slit through a bone (e.g., orbital fissures behind the eye).
  7. Foramen:
    • A hole through a bone, usually round (e.g., foramen magnum of the skull).
  8. Fossa:
    • A shallow, broad, or elongated basin (e.g., infraspinous fossa of the scapula).
  9. Line (linea):
    • A raised, elongated ridge (e.g., nuchal lines of the skull).
  10. Meatus:
    • A canal (e.g., external acoustic meatus of the temporal bone).
  11. Process:
    • Any bony prominence (e.g., mastoid process of the skull).
  12. Protuberance:
    • A bony outgrowth or protruding part (e.g., mental protuberance of the chin).
  13. Sinus:
    • A cavity within a bone (e.g., frontal sinus of the skull).
  14. Spine:
    • A sharp, slender, or narrow process (e.g., spine of the scapula).
  15. Sulcus:
    • A groove for a tendon, nerve, or blood vessel (e.g., intertubercular sulcus of the humerus).
  16. Tubercle:
    • A small, rounded process (e.g., greater tubercle of the humerus).
  17. Tuberosity:
    • A rough surface (e.g., tibial tuberosity).

The Skull

  • Composed of 22 bones that are joined by sutures; includes several cavities:
    • Cranial cavity
    • Orbits
    • Nasal cavity
    • Oral cavity
    • Ear cavities
    • Paranasal sinuses
  • Foramina:
    • Refers to holes in the skull through which nerves and blood vessels pass.
Cranial Bones
  1. Frontal Bone (1):
    • Contains the coronal suture and frontal sinus.
  2. Parietal Bones (2):
    • Joined by sagittal suture, lambdoid suture, and squamous suture.
  3. Sphenoid Bone (1):
    • Known for its moth-like shape with greater and lesser wings, contains sphenoidal sinuses and the sella turcica.
  4. Ethmoid Bone (1):
    • Contains the ethmoidal sinus, crista galli, and cribriform plate.
  5. Temporal Bones (2):
    • Comprises four regions: squamous, tympanic, mastoid, and petrous parts.
  6. Occipital Bone (1):
    • Contains the foramen magnum and occipital condyles.
Facial Bones
  • Lie anterior to the cranial cavity, thus forming the face.
List of Facial Bones:
  1. Maxillae (2):
    • Only facial bone possessing a sinus, contains alveolar processes.
  2. Palatine (2):
    • Forms the hard palate.
  3. Zygomatic (2):
    • Create the angles of the cheeks.
  4. Lacrimal (2):
    • The smallest bones of the skull located in the medial wall of each orbit.
  5. Nasal (2):
    • Form the bridge of the nose.
  6. Inferior Nasal Conchae (2):
    • Found in the nasal cavity.
  7. Vomer (1):
    • Functions as the base of the nasal septum.
  8. Mandible (1):
    • The only movable bone of the skull and is the strongest.
The Hyoid Bone
  • Positioned between the chin and larynx, it does not articulate with any other bones.
  • A fracture of the hyoid bone can indicate strangulation because of its location.

Vertebral Column

  • The vertebral column consists of a chain of 33 vertebrae and intervertebral discs categorized as follows:
    • Cervical (7)
    • Thoracic (12)
    • Lumbar (5)
    • Sacral (5)
    • Coccygeal (4)
  • Main functions include supporting the skull and trunk, allowing movement, protecting the spinal cord, absorbing stress from activities, and providing attachment for limbs and muscles.

Normal and Abnormal Curvatures

  • Normal Curves:
    • Initially C-shaped at birth, changes to S-shaped by age 3 with cervical and lumbar curvatures developing later.
  • Abnormal Curvatures:
    • Scoliosis: Abnormal lateral curvature.
    • Hyperkyphosis: Abnormal posterior thoracic curvature.
    • Hyperlordosis: Abnormal anterior lumbar curvature.

Development of the Axial Skeleton

  • Primarily through endochondral ossification.
    • Chondrification: Formation of cartilage from mesenchyme.
    • Ossification: Replacement of cartilage with bone.
Skull Development
  • Neurocranium: Encloses the brain and comprises the base and calvaria.
  • Viscerocranium: Comprises all facial bones developed from pharyngeal arches.
  • Fontanelles: Fibrous membranes present in gaps between cranial bones (e.g., anterior, posterior). These typically close by 24 months.

Pathology of Axial Skeleton

  • Skull Fractures: Can potentially damage the brain, nerves, and meningeal blood vessels.
  • Le Fort Fractures: A type of facial fracture involving the maxilla.
  • Vertebral Fractures and Dislocations: Can lead to damage to the spinal cord.
  • Herniated Discs: Involves the cracking of the anulus fibrosus, which may compress spinal nerves.

Figures and Diagrams

  • Refer to various figures, including various views and anatomical positions of bones and cavities.
  • Diagrams include detailed images for visualizing relationships between bones and surfaces.

Note: All figures are referenced according to the descriptions in the provided material, further enriching the understanding of the axial skeleton anatomy.