The Cranial Bones

Sun, 11 Oct 2009 04:41:57 +0100

    The frontal bone forms the roof of the head, nasal cavity, and the superior arch of the bony orbits. It develops in two parts that eventually bond and become one. Commonly, they are bonded by age six. It contains a frontal sinus which is connected to the nasal cavity. Since it is connected to the nasal cavity and other paranal sinuses, it lessens the weight of the skull and acts as a sound chamber for voice resonance.

       The coronal structure seperates the frontal bones from the parietal bones. The sagittal sturcture seperates the right and left parietal bones. The parietal bones form the upper part of the cranium. The inner concave surface of the parietal, along with the inner surface of the other cranium bones, is marked by shallow impressions from the convolutions of the brain and the vessels serving the brain.

    Two Temporal bones form the lower sides of the cranium. Both temporal bones are joined to their adjacent parietal bones by the squamosal suture. Each temporal bone has four parts.
    The first part is the squamous part which is the flattened plate of bone at the sides of the skull. Projecting forward is a zygomatic process which forms the posterior part of the zygomatic arch. On the inferior surface of the squamous part is the mandibular fossa, which recieves the articular condyle of the mandable. This articulation is referred to as the temporomandible joint.
    The tympanic part of the temporal bone contains the external acoustic meatus, or ear canal, located immediately posterior to the mandibular fossa. A thin, pointed styloid process projects downward from the tympanic portion.
    The mastoid process, which is a rounded projection posterior to the external auditory meatus, accounts for the mass of the mastoid part. The mastoid foramen is directly posterior to the mastoid process. The stylomastoid foramen, located between the mastoid and styloid processes, is the passage for part of the facial nerve.
    The structures of the middle and inner ear are located in the dense, bony, petrous part. The carotid canal and the jugular foramen border on the medial side of the petrous part. The carotid canal allows blood into the brain via the internal carotid artery and the jugular foramen lets blood drain from the brain via the internal jugular vein. Three cranial nerves also pass through the jugular foramen.

    The occipital bone forms the back and much of the base of the skull. It is fastened to the parietal bones by the lambdoidal suture. The foramen magnum is the large hole in the occipital bone through which the spinal cord attaches to the brain. On each side of the foramen magnum are the occipital condyles, which articulate with the atlas of the vertebral column. At the anterolateral edge of the occipital condyle is the hypoglossal canal, through which the hypoglossal nerve passes. A condyloid canal lies posterior to the occipital condyle. The external occipital proturberance is a prominent, posterior projection on the occipital bone that can be felt as a definite bump just under the skin. The superior nuchal line is a ridge of the bone extending laterally from the occipital protuberance to the mastoid portion of the temporal bone. Sutural bones are small clusters of irregularly shaped bones that may be found between the joints of certain cranial bones but usually occur the lambdoidal structure.

    The sphenoid bone forms the anterior base of the cranium and can be viewed laterally and inferiorly. This bone resembles a butterfly with outstreched wings. It consists of a body with laterally projecting greater and lesser wings, which form part of the bony orbit. The body is a wedgelike central portion that contains the sphenoidal sinuses and a prominent depression called the sella turcia which supports the pituitary gland. The sella turcia is seen on the floor of the cranium. A pair of pterygoid processes project inferiorly form the sphenoid bone to help form the lateral walls of the nasal cavity. Many foramina are located within the sphenoid bone. They are as follows:
1. The optic canal, which is a large opening through the lesser wing into the back of the orbit for passage of the optic nerve and the ophthalmic artery.
2. The superior orbital fissure is a triangular opening between the wings of the sphenoid for passage of the ophthalmic nerve off of the trigeminal cranial nerve and the oculomotor, trochlear, and the abducens cranial nerves.
3. The foramen ovale is an opening at the base of the ptyergoid process, through which the mandibular branch of the trigeminal nerve passes.
4. The foramen spinosum is a small opening at the posterior angle of the sphenoid for passage of the middle meningeal vessels.
5. The foramen lacerum is an opening between the sphenoid and the petrous part of the temporal bone, through which the internal carotid artery and the meningeal branch of the ascending phryngeal artery pass.
6. The foramen rotundum is an opening located just posterior to the superior orbital fissure at the junction of the anterior and middle portions of the sphenoid bone. The maxillary nerve passes through this foramen.

    The ethmoid bone is located in the anterior portion of the floor of the cranium between the orbits, where it forms the roof of the nasal cavity. An inferior projection of the ethmoid, called the perpendicular plate, contributes in part to the nasal septum that seperates the nasal cavity into two chambers, known as the nasal fossae. A spine of the perpendicular plate, the crista galli, projects superiorly into the cranial cavity and serves as an attachment for the meninges covering the brain. On both lateral walls of the nasal cavity are two scroll-shaped plates of ethmoid, called the superior and middle nasal conchae. At right angles to the perpendicular plate, within the floor of the cranium, is the cribriform plate, which has numerous perforations for the passage of olfactory nerves from the nasal cavity.

the-human-skull--the-cranial-bones

The Cranial Bones