Bones of the Skull

The most complex bony structure of the body is Skull and is formed by the cranium and facial bones.

Cranium –It protects the brain and is the site of attachment for head and neck muscles.
Facial bones−It supply the framework of the face, the sense organs, and the teeth

Cranium

          Formed from eight large bones
               Unpaired bones include
               • Occipital bone
               • Frontal bone
               • Sphenoid bone
               • Ethmoid bone

               Paired bones include
               • Temporal bones
               • Parietal bones

• Occipital bone

          It is the most posterior of the cranial bones forming the posterior wall and base of the skull. It is consist of two parts; squamous part and basilar part. In between these part is the foramen magnum of the occipital bone through which passes the spinal cord. The squamous part lies posterior to the foramen magnum and the basilar part lies anterior to the foramen magnum. On the inferior surface of the basilar part just anterior to the foramen magnum lie two projections called as occipital condyles which project inferiorly and posteriorly. The occipital condyle make joint with the superior articular facets of the 1st cervical vertebrae called as atlanto−occipital joint. This joint transmits the weight of skull bones to the vertebral column and helps in making movements like when we say Yes (flexion anteriorly and posteriorly). On the posterior external part of the squamous part is the external occipital protuberance and beneath it lies two curve lines called as superior nuchal line and inferior nuchal line. Just anterior and lateral to the foramen magnum on both sides are the hypoglossal canal for the passage of hypoglossal nerve. On both sides of the hypoglossal canal lies a depression which is the passage way for sigmoid sinus. The basilar part of the occipital bone attaches to the sphenoid bone and petrous part of the temporal bone. Laterally between the petrous part of the temporal bone and occipital bone lie the jugular foramen through which passes the 9th, 10th and 11th cranial nerves plus internal jugular vein.

• Temporal bones

          They form the lateral walls of the skull. Anteriorly articulate with the zygomatic bones, medially with the sphenoid bone and posteriorly with the occipital bone. Like the occipital bone it has two parts; squamous part which is superior part and petrous part which lies inferiorly. From the inferior surface of the petrous part projects two protuberances the mastoid process which lies posterior to the external auditory meatus and the styloid process anterior and medial to the external auditory meatus. Just posterior to the external auditory meatus is an opening called stylomastoid foramen through which passes the 7th cranial nerve (facial). Petrous part has a cavity which lodges middle and internal parts of ear. On the front, temporal bone form zygomatic process which articulates with the temporal process of zygomatic bone forming zygomatic arch.

• Frontal bone

          It forms the anterior portion of the cranium (floor of anterior cranial fossa) and makes the roof and lateral wall of the orbit. It also gives shape to the forehead. Anteriorly it articulates with the nasal bones while posterior with the sphenoid bone (posteroinferiorly) and parietal bone (posterosuperiorly). Above the nasal bones is the body of frontal bone having cavities called as frontal sinuses. Laterally frontal bone articulates with the zygomatic bone and helps in the formation of zygomatic arch.

• Sphenoid bone

          It is only bone which articulates with all the other bones of the cranium. It forms the floor of the middle cranial fossa. Anteriorly it articulates with the frontal and ethmoidal bones, laterally with the temporal bones and posteriorly with the occipital bone. Sphenoid bone has two pair of wings: greater wings and lesser wings. These wings give sphenoid bone bat shaped appearance. The greater wings form the floor and wall of middle cranial fossa. The lesser wings form the anterior wall of the middle cranial fossa. The most anterior part of the lesser wings is called sphenoid ridges (concha) which separates the anterior cranial fossa from the middle cranial fossa. Between the two greater wings lies the body of sphenoid bone. The body has two anterior projections called as anterior clinoid processes and two posterior projections called as posterior clinoid processes. In-between these two projections lie a pituitary fossa which accommodates the pituitary gland. Both anterior and posterior clinoid process along with pituitary fossa form a saddle like structure called as sella turcica. From the inferior surface of the body of sphenoid bone arise two projections called as lateral and medial pterygoid processes. In between these processes lies the pterygoid fossa.

           Through the lesser wing of the sphenoid bone passes a canal called as optic canal through which passes the optic nerve. Between the lesser and greater wings anteriorly lies a fissure called superior orbital fissure through which passes the ophthalmic division of trigeminal nerve. Most anterior of the greater wing has a foramen rotundum through which passes the maxillary division of trigeminal nerve. Posterior to the foramen rotundum is foramen ovale through which passes the mandibular division of trigeminal nerve. Posterior and lateral to the foramen ovale is the foramen spinosum through which passes the middle meningeal artery.

• Ethmoid bone

          It is one of the smallest among the cranial bones. It helps in the formation of some part of anterior cranial fossa. Anteriorly it articulates with the nasal bones, posteriorly with the sphenoid bone and laterally with the frontal bone. Ethmoid bone has four parts.

          • Cribriform plate is the most superior part of the ethmoid bone. It has central upward projection called as crista galli (site of attachment of falx cerebri or durra matter). The two horizontal plate of cribriform plate contain foramina for the passage of olfactory nerve.

          • Perpendicular plate projects downward and helps in the formation of the nasal septum. It has two borders. Anterior border articulates in front with the frontal bone while the upper part of the posterior border articulates with the sphenoid bone and lower part of the posterior border with the vomer bone.

          • 2 Labyrinths (Medial and Lateral labyrinths): The lateral labyrinth forms the medial wall of the orbit which is called lamina papyraceae. The medial labyrinth forms two projections, which protrudes inside the nasal cavity forming two projections called as the superior nasal conchae and middle nasal conchae. These nasal conchae contain air cells which make the ethmoid sinuses. Superiorly these sinuses communicate with the frontal sinus while laterally with the maxillary sinuses.

• Parietal bones

          They form the roof and lateral walls of the cranium. Parietal bones articulate with the frontal bone anteriorly by coronal suture, posteriorly with occipital bone by lambdoid suture and laterally with the temporal and sphenoid bones through the squamous sutures. Parietal bones contain no foramina but contain numerous grooves on its inferior surface for the passage of the blood vessels.

Facial Bones
          Facial skeleton is composed of fourteen bones of which only the mandible and vomer are unpaired. The paired bones are the maxillae, zygomatics, nasals, lacrimals, palatines, and inferior conchae.

• Mandible bone

          It is unpaired, strongest and the only moveable bone of the skull. Mandible has two parts i.e. two rami and one body. The rami join the body at the angle called as angle of the mandible. The superior part of the mandible has two projections called as coronoid process (anteriorly) and condyloid process (posteriorly). The condyloid process makes joint with the mandibular fossa of the temporal bone making the only moveable joint of the skull called as temporal mandibular joint. There are two foramen in the mandible; mental foramen which lies just below the 2nd premolar teeth and mandibular foramen which lies on the medial aspect of the ramus of the mandible. The upper margin of the body of mandible has inferior alveolar processes which fit the lower teeth.

• Vomer bone

          It is the unpaired bone which forms the central part of the floor of the nasal cavity. Superiorly it articulates with the ethmoid bone, inferiorly with the palatine bone and palatine processes of the maxillae.

• Nasal bones

          They form the roof and anterior wall of the nose. Superiorly they articulate with the frontal bone, posteriorly with the lacrimal bones and inferiorly with the maxillae.

• Maxillary bones

          They articulate with all the bones of the facial skeleton except the mandible. Superiorly they articulate with the nasal and lacrimal bones, laterally with the zygomatic bones, medially with the vomer and inferior nasal conhae and posteriorly with the palatine bones. Both maxillary bones contain air filled cavities called as maxillary sinuses which open into the nasal cavity.

• Zygomatic bones

          They form the lateral wall of the facial skeleton. They are also called cheek bones as they form the prominence of the cheeks. Anteriorly they articulate with the maxillary bone, posteriorly with the temporal bone and superiorly with the ethmoidal bone.

• Lacrimal bones

          They lie posterior to the nasal bones. Through the lacrimal bones passes the lacrimal canal which is a passage for tears into the nasal cavity.

• Palatine bones

          They lie posterior to the maxillary bones (palatine processes). They form the posterior one third of the hard palate separating the nasal cavity from the oral cavity.

• Inferior nasal conchae

          These are the two small bones which form the inferior lateral wall of the nasal cavity. Superiorly articulate with the middle nasal conchae of the ethmoid bone while laterally with the maxillary bones.