The Axial Skeleton

The axial skeleton, totalling 80 bones, forms the main vertical axis running through the head, neck and trunk of the body, and its principal function is to protect the vital organs that lie along this axis -- the brain, the spinal cord, and the heart and lungs.

The skull is the most complex part of the axial skeleton and is itself made of several distinct sets of bones. The 8 cranial bones (the frontal, the paired parietal, the paired temporal, the occipital, the sphenoid and the ethmoid) interlock along immovable joints called sutures to form the rounded cranial vault (braincase) that directly encloses and protects the brain. The 14 facial bones (including the paired nasal, maxilla, zygomatic, lacrimal and inferior nasal concha bones, the single mandible and the single vomer) form the face and the bony framework of the eye sockets, nasal cavity and the upper and lower jaws; the mandible (lower jaw) is the only movable bone of the skull, hinged to the temporal bone on each side. Within each middle ear lie three tiny ear ossicles -- the malleus, incus and stapes -- giving 6 ear ossicles in all; these are the smallest bones in the human body and transmit sound vibrations from the eardrum to the inner ear. A single hyoid bone, shaped like a horseshoe and not directly articulating with any other bone, lies in the front of the neck and serves as an anchor point for muscles of the tongue and the larynx.

The vertebral column (backbone), made of 26 bones, extends from the base of the skull to the pelvis and encloses and protects the spinal cord running through it, while also bearing the weight of the head and trunk. Reading from top to bottom it consists of 7 cervical vertebrae in the neck (the first, the atlas, supports the skull, and the second, the axis, allows the head to rotate -- the pivot joint between them is described in section 18.8), 12 thoracic vertebrae in the upper back, each articulating with a pair of ribs, 5 lumbar vertebrae in the lower back, the sacrum (a single bone formed by the fusion of 5 vertebrae during development), and the coccyx (a single small bone formed by the fusion of 4 rudimentary vertebrae, representing the vestige of a tail). …