Axial Skeleton

Axial Skeleton (80 bones)

Skull (28)

• Cranium (8)
  • Frontal bone (1) – forms forehead and superior surface of orbits
  • Parietal bones (2) – found on both sides of the skull, posterior to the frontal bone
  • Occipital bone (1) – forms the posterior and inferior portions of the cranium
  • Temporal bones (2) – found below the parietal bones, contributing to the sides and base of the cranium
  • Sphenoid bone (1) – forms part of the floor of the cranium
  • Ethmoid bone (1) – found anterior to the sphenoid bone, consisting of two honeycombed masses of bone

• Facial (14)
  • Maxillary bones (2) – forms the floor and medial portion of the orbit rim, walls of the nasal cavity, and the anterior roof of the mouth (hard palate)
  • Zygomatic bones (2) – found on each side of the skull, articulating with the frontal bone and the maxilla to complete the lateral wall of the orbit. Along the lateral margin, each gives rise to a slender bony extension that curves laterally and posteriorly to meet a process from the temporal bone, together forming the zygomatic arch.
  • Palatine bones (2) – form the posterior surface of the hard palate. The superior surfaces of each horizontal portion contribute to the floor of the nasal cavity. The superior tip of the vertical portion of each forms a small portion of the inferior wall of the orbit.
  • Mandible (1) – forms the lower jaw.
  • Lacrimal bones (2) – located within the orbit on its medial surface and articulate with the frontal, ethmoid, and maxillary bones.
  • Nasal bones (2) – form the bridge of the nose and articulate with the superior frontal bone and the maxillary bones.
  • Inferior nasal conchae (2) – project from the lateral walls of the nasal cavity.
  • Vomer (1) – The inferior margin articulates with the paired palatine bones and, with the ethmoid bone, supports a prominent partition that forms part of the nasal septum. (See more under Parts of Skull Bones.)

• Middle Ear (Auditory Ossicles) (6)
  • Malleus (2) – attaches at three points to the interior surface of the tympanum (tympanic membrane)
  • Incus (2) – attaches the malleus to the inner bone (stapes)
  • Stapes (2) – seated within the “oval window.”

Hyoid Bone (1) – U-shaped and hangs below the skull, suspended by ligaments from the styloid processes of the temporal bones, and serves as a base for muscles associated with the tongue and larynx.

Vertebral Column (26)

• Cervical vertebrae (7) – extend from the head to the thorax (C1-C7)
• Thoracic vertebrae (12) – extend from the cervical portion to the lumbar section (T1-T12)
• Lumbar vertebrae (5) – continues from the thoracic vertebrae to the sacrum (L1-L5)
• Sacrum (1) – forms the posterior wall of the pelvis
• Coccyx (1) – is one mass of four to five small coccygeal vertebrae that have fused into one, commonly called the tailbone

Thoracic Cage (25)

• True ribs (14) – consist of 7 pairs of bone that reaches the anterior body wall. They are connected to the sternum by separate cartilaginous extensions (costal cartilages).
• False ribs (10) – consist of ribs 8-12 that do not attach directly to the sternum. The last two pairs are floating ribs because they have no connection with the sternum.
• Sternum (1) – has three parts in the adult. The manubrium articulates with the clavicles of the appendicular skeleton and with the cartilages of the first pair of ribs. The body, or gladiolus, ends at the xiphoid process.