Anatomy and Physiology I: Skeletal System

Anatomy and Physiology I: Skeletal System

Divisions of the Skeleton

  • Axial Skeleton: Comprises the 80 bones of the head, neck, and torso. It consists of 74 bones forming the upright axis of the body and 6 tiny middle ear bones.
  • Appendicular Skeleton: Comprises the 126 bones that form the appendages to the axial skeleton, specifically the upper and lower extremities.

Axial Skeleton: Skull

The skull is made up of 28 bones, divided into two major sections: cranial bones and facial bones.

Cranial Bones

There are 8 cranial bones that protect the brain.

  • Frontal Bone:

    • Forms the forehead and the anterior part of the top of the cranium.
    • Contains the frontal sinuses.
    • Forms the upper portion of the orbits.
    • Forms the coronal suture with the two parietal bones.

  • Parietal Bones:

    • Form the bulging top of the cranium.
    • Form several sutures:
      • Lambdoid suture with the occipital bone.
      • Squamous suture with the temporal bone and part of the sphenoid bone.
      • Coronal suture with the frontal bone.

  • Temporal Bones:

    • Form part of the bulging top of the cranium.
    • Form several sutures, similar to parietal bones in related areas:
      • Lambdoid suture with the occipital bone.
      • Squamous suture with the parietal bone and part of the sphenoid bone.
      • Coronal suture with the frontal bone (indirectly, via articulation with the sphenoid which articulates with the frontal).
    • The transcript contains a repetition for temporal bones, listing sutures identical to parietal bones. To clarify, temporal bones primarily form the lambdoid (with occipital), squamous (with parietal and sphenoid), and are part of the base of the skull.

  • Occipital Bone:

    • Forms the lower, posterior part of the skull.
    • Forms immovable joints with three other cranial bones (parietal, temporal, sphenoid) and a movable joint with the first cervical vertebra (atlas).

  • Sphenoid Bone:

    • A bat-shaped bone located in the central portion of the cranial floor.
    • Anchors the frontal, parietal, occipital, and ethmoid bones.
    • Forms part of the lateral wall of the cranium and part of the floor of each orbit.
    • Contains the sphenoid sinuses.
    • Key features visible in diagrams include sella turcica which houses the pituitary gland, optic canal, superior orbital fissure, foramen rotundum, foramen ovale, foramen lacerum, and foramen spinosum.

  • Ethmoid Bone:

    • A complex, irregular bone lying anterior to the sphenoid and posterior to the nasal bones.
    • Forms the anterior cranial floor, medial orbit walls, upper parts of the nasal septum, and sidewalls of the nasal cavity.
    • The cribriform plate, containing olfactory foramina for the sense of smell, is located in the ethmoid bone.
    • Other features include the crista galli (attachment point for dura mater) and the perpendicular plate (forms part of the nasal septum).

Facial Bones

There are 14 distinct facial bones that provide structure to the face and support soft tissues.

  • Maxilla (Upper Jaw):

    • Two maxillae form the keystone of the face, articulating with each other and with the nasal, zygomatic, inferior concha, and palatine bones.
    • Forms parts of the orbital floors, roof of the mouth (hard palate), and floor and sidewalls of the nose.
    • Contains maxillary sinuses.
    • Features include the incisive foramen, palatine process, alveolar process, and infraorbital foramen.

  • Mandible (Lower Jaw):

    • The largest and strongest bone of the face.
    • Forms the only movable joint of the skull with the temporal bone, specifically at the mandibular fossa.
    • Features include the condylar process (forms temporomandibular joint), coronoid process, mental foramen, and mandibular foramen.

  • Zygomatic Bone:

    • Shapes the cheek and forms the outer margin of the orbit.
    • Forms the zygomatic arch with the zygomatic process of the temporal bones.
    • Also known as the malar bone.

  • Nasal Bone:

    • Both nasal bones form the upper part of the bridge of the nose, with cartilage forming the lower part.
    • Articulates with the ethmoid, nasal septum, frontal bone, maxillae, and the other nasal bone.

  • Lacrimal Bone:

    • Paper-thin bone located just posterior and lateral to each nasal bone.
    • Forms part of the nasal cavity and the medial wall of the orbit.
    • Contains a groove for the nasolacrimal (tear) duct.
    • Articulates with the maxilla, frontal, and ethmoid bones.

  • Palatine Bone:

    • Two bones that form the posterior part of the hard palate.
    • The vertical portion forms the lateral wall of the posterior part of each nasal cavity.
    • Articulates with the maxillae and the sphenoid bone.

  • Inferior Nasal Conchae (Turbinates):

    • Form the lower edge projecting into the nasal cavity, creating the nasal meatuses.
    • Articulate with the ethmoid, lacrimal, maxillary, and palatine bones.

  • Vomer Bone:

    • Forms the posterior portion of the nasal septum.
    • Articulates with the sphenoid, ethmoid, palatine, and maxillae.

Eye Orbits

  • The right and left eye orbits are bony cavities that contain the eyes, associated eye muscles, lacrimal apparatus, blood vessels, and nerves.
  • Their walls are thin and fragile, separating orbital structures from the cranial and nasal cavities and paranasal sinuses.
  • Bones contributing to the orbit include the frontal, sphenoid, ethmoid, lacrimal, maxilla, zygomatic, and palatine bones.

Axial Skeleton: Fetal Skull

  • Unique Anatomical Features: The fetal skull possesses distinct features not seen in the adult skull.
  • **Fontanels (