Making a Hard Head Harder: The Bones of the Skull - dummies

Making a Hard Head Harder: The Bones of the Skull

By Janet Rae-Dupree, Pat DuPree

Of the 80 named bones in the axial skeleton, 29 are in (or very near) the skull. In addition to the hyoid bone, 8 bones form the cranium to house and protect the brain, 14 form the face, and 6 bones make it possible for you to hear.

Fortunately for the cramming student, most of the bones in the skull come in pairs. In the cranium there’s just one of each of the following:

  • Frontal bone (forehead)

  • Occipital bone (back and base of the skull) containing occipital condyles, which articulate with the atlas of the vertebral column

  • Ethmoid bone (made of several plates, or sections, between the eye orbits in the nasal cavity)

  • Sphenoid bone (a butterfly-shaped structure that forms the floor of the cranial cavity)

But there are two temporal bones each housing: (1) the hearing organs in the auditory meatus; (2) a styloid process, a long pointed process for anchoring muscles; (3) a mandibular fossa articulating with the condyle of mandible; and (4) a zygomatic process that projects anteriorly joining the zygomatic bone forming the cheek prominence.

There are two parietal bones (roof and sides of the skull). These bones are attached along sutures called the following:

  • Coronal (located at the top of the skull between the two parietal bones)

  • Squamosal (located on the sides of the head surrounding the temporal bone)

  • Sagittal (along the midline atop the skull located between the two parietal bones)

  • Lambdoidal (forming an upside-down V — the shape of the Greek letter lambda — on the back of the skull)

In the face, there is only one mandible (jawbone) and one vomer dividing the nostrils, but there are two each of maxillary (upper jaw), zygomatic (cheekbone), nasal, lacrimal (a small bone in the eye socket), palatine (which makes up part of the eye socket, nasal cavity, and roof of the mouth), and inferior nasal concha, or turbinated, bones.

The mandible has two lateral condyles that articulate in the mandibular fossa of the temporal bone. (The maxillary bones are also called, simply, the maxilla.) Inside the ear, there are two each of three ossicles, or bonelets, which also happen to be the smallest bones in the human body: the malleus, incus, and stapes.

The floor of the cranial cavity contains several openings, or foramina (the singular is foramen), that allow various nerves and vessels to connect to the brain.

  • The holes in the ethmoid bone’s cribriform plate are olfactory foramina that allow olfactory — or sense of smell — receptors to pass through to the brain. A process called the crista galli extends into the brain cavity for the attachment of the meninges of the brain.

  • A large hole in the occipital bone called the foramen magnum allows the spinal cord to connect with the brain.

  • The sphenoid bone is riddled with foramina.

    • The optic foramen allows passage of the optic nerves.

    • The jugular foramen allows passage of the jugular vein and several cranial nerves.

    • The foramen rotundum allows passage of the trigeminal nerve, which is the chief sensory nerve to the face and controls the motor functions of chewing.

    • The foramen ovale allows passage of the nerves controlling the tongue, among other things.

    • The foramen spinosum allows passage of the middle meningeal artery, which supplies blood to various parts of the brain.

    • The sphenoid bone also features the sella turcica, or Turk’s saddle, that cradles the pituitary gland and forms part of the foramen lacerum, through which pass several key components of the autonomic nervous system.

Encased within the frontal, sphenoid, ethmoid, and maxillary bones of the skull are several air-filled, mucous-lined cavities called paranasal sinuses.

Although you may think their primary function is to drive you crazy with pressure and infections, the sinuses actually lighten the skull’s weight. They make it easier to hold your head up high; they warm and humidify inhaled air; and they act as resonance chambers to prolong and intensify the reverberations of your voice.

Mastoid sinuses drain into the middle ear (hence the earache referred to as mastoiditis). Maxillary sinuses are flanked by the bones of the maxilla, the upper jaw. The paranasal sinuses are the ones that drain into the nose and cause so much trouble when you cry or have a cold.