Objectives:
1. To identify different bones forming the skull.
2. To know the different cavities contained in the skull.
3. To identify the different main sutures of the skull.
4. To be able to differentiate between the different foramina of the skull.
5. To know the relationship between bones of the skull and direction of articulation.
The bones of the skull are divided into two groups: 1. The cranial bones, which are eight in number. They are called so because they surround the brain tissue and protect the brain from shocks.
2. The facial bones, which are fourteen in number. They form the texture of the face and facial muscles attachment site. Also they surround the facial cavities such as orbital, nasal and oral.
The skull will be discussed here from different views as it is handled in the lab.
The superior view of the skull
1. The cranial roof
The frontal bone is located anteriorly ending at the coronal suture.
The two parietal bones are located laterally separated by the sagittal suture medially making the lateral walls of the cranial roof.
The occipital bone is the most posterior bone of the skull, separated from the two parietal bones by the lambdoid suture.
2. The cranial floor
The internal surface of the cranial floor is not a straight one; it contains three main depressed surfaces each is termed as fossa. The anterior fossa starts from the most anterior point of the cranial floor to the lesser wing of the sphenoid bone. The middle fossa is extended from the lesser wing to the petrous portion of the temporal bone. The posterior fossa extends from the petrous portion to the most posterior point of the cranial floor.
The frontal bone lies in the anterior fossa forming the anterolateral part of the cranial floor.
The ethmoid bone is lying at the anteromedial part of the cranial floor. The most superior structure of ethmoid bone is the cresta galli, which is surrounded laterally by the cribriform plate containing large number of olfactory foramena.
The sphenoid bone which occupies the middle fossa also called the key stone bone because it articulates with all other cranial bones. The sphenoid bone looks like the butterfly having two lesser wings, two greater wings, body which contains the sphenoid sinus, back holding the pituitary gland in the sella turcica. Each greater wing contains three different foramina at its medial side: foramen rotendum, foramen ovale and foramen spinosum. The superior orbital fissure is located between the lesser wing and greater wing while the two optic foramina are situated between the body and the lesser wings.
The two lateral sides of the cranial floor are occupied by the two temporal bones containing the internal auditory meatus, carotid canal and jugular foramen in the petrous portion of the bone.
The most posterior half of the cranial floor is covered by the occipital bone containing the foramen magnum for the spinal cord passage and the hypoglossal canal medially.
The Lateral View
1. The cranial bones
The frontal bone lies at the most anterior part of the skull.
The inferior part of the parietal bone at the middle of the skull can be seen laterally separated from the temporal bone inferiorly by the squamosal suture.
The most lateral side of the sphenoid bone can be distinguished just inferior to the frontal bone.
The temporal bone is occupying the middle region of the lateral side of the skull. The squama of the temporal bone is the thin flat part of the bone. The mastoid portion of the temporal bone lies at the posterior part of the temporal bone, contains many processes. Anteriorly the zygomatic process of the temporal bone is extending to meet the temporal process of the zygomatic bone forming the zygomatic arch. The external auditory meatus is posterior to the zygomatic process. The mastoid process is nipple like structure located posterior and inferior to the external auditory meatus. The styloid process which is pointed needle like process is located at the inferior surface of the skull. The mandibular fossa lies between the zygomatic process and the external auditory meatus inferiorly.
The occipital bone occupies the most posterior part of the skull.
2.The facial bones
The zygomatic bone is located at the angle of the skull just anterior to the temporal bone. The two bones articulate together forming the zygomatic arch. The zygomatic bone articulates superiorly with frontal bone and anteromedially with the maxilla.
The maxilla is located anterior to the zygomatic bone contains the infraorbital foramen. This bone is considered as the key stone bone of the facial bones because it articulates with all other facial bones except the mandible bone. The maxilla contains alveolar process housing the upper teeth.
The lacrimal bone can be seen at the medial wall of the orbit, containing the lacrimal fossa which houses the lacrimal sac.
The nasal bone forms the superior part of the nose. The two nasal bones articulate medially with each other.
The Inferior View
1. The cranial bones
The sphenoid bone lies at the middle third of the skull. Only from this view the pterygoid processes can be seen forming the lateral posterior region of the nasal cavity.
The temporal bone contains the styloid process, a very pointed one is medial to the mastoid process, a rounded nipple like process. The mandibular fossa is a depressed surface for articulation with the mandible bone making the only movable joint of the skull which is the temporomandibular joint. Anterolaterally the zygomatic process is seen as part of the zygomatic arch.
The occipital bone is occupying the posterior half of the skull inferiorly, attaches the sphenoid bone anteromedially. The large hole which is called the foramen magnum is found in the middle of the bone. At the two lateral sides of the foramen magnum two rounded processes are found and called the occipital condyles for the articulation with the vertebral column. The external occipital protuberance is found at the most posterior point of the skull.
2. The facial bones
The anterior third of the skull from this view is occupied by the facial bones, forming the hard palate. The anterior two third of the hard palate is formed by the palatine process of the maxilla.
The horizontal plate of the palatine bone forms the posterior third of the hard palate.
The two zygomatic arches are located at the lateral sides of the skull. Each zygomatic arch is formed by the articulation between the temporal process of the zygomatic bone and the zygomatic process of the temporal bone.
The Anterior View
1. The cranial bones
Limited number of cranial bones and structures are seen from this view. The superior third of skull is occupied by the frontal bone which has the thin flat structure called frontal squama forming the forehead. At the mid point between the two orbits and superiorly, the frontal bone contains frontal sinus. The superior edge of the orbit is formed mainly from the supraorbital margin of the frontal bone. The supraorbital foramen is located at the medial third of the supraorbital margin.
The ethmoid bone participates in forming a small porion of the medial orbital walls. The perpendicular plate which is the superior portion of the nasal septum is also part of the ethmoid bone. The wall between the nasal cavity and the orbit are formed by the lateral masses of the ethmoid bone and contain the ethmoid sinuses.
The sphenoid bone can be seen at the roof and lateral wall of the orbit. The optic foramen can be seen at the most posterior point of the orbit.
2. The facial bones
The nasal bones are the most superior facial bones. They articulate with each other medially forming the superior part of the bridge of the nose.
The lacrimal bones are the smallest bones participate to form the medial walls of the orbit. Each bone contains one lacrimal fossa for the lacrimal sac.
The two maxillae are considered as the key stone bones. The infraorbital foramen lies inferior to the orbit. The alveolar process holds the upper teeth. At the two medial sides of the maxillae, the maxillary sinuses are found.
The inferior nasal conchae are found as pair at the lateral walls of the nasal cavity as transverse bony extensions, serve in filtration, moisturizing and worming up of the air in the nasal cavity. The superior nasal concahe and the middle nasal conchae are parts of the ethmoid bone.
The zygomatic bones exist at the two lateral sides of the skull.
The vomer bone is found at the floor of the nasal cavity. It forms the inferior part of the nasal septum.
The mandible bone is a single bone formed from a body at the anterior side and two rami extending posteriorly forming the mandibular angle. Each ramus extends superiorly ending with two processes, the condylar process and the coronoid process. At the internal surface of the mandible, the mandibula foramena are found, while the mental foramena are found at the body of the mandible anteriorly. The alveolar process is found at the superior surface of the mandible for the lower teeth.
The following chick list that helps you to know what to see in each view of the skull:
1. Superior view:
Cranial roof: Frontal bone
Parietal bone
Occipital bone; foramen magnum
Coronal suture
Sagittal suture
Lambdoid suture
Cranial floor: Anterior cranial fossa
Middle cranial fossa
Posterior cranial fossa
Sphenoid bone; sella turcica, body, lesser wing, greater wing, superior orbital fissure, foramen rotendum, foramen ovale, foramen spinosum and optic foramen.
Temporal bone; petrous portion, internal auditory meatus, carotid canal and jugular foramen
Ethmoid bone; cresta galli and cribriform plate
2. Lateral view: Frontal bone
Parietal bone
Squamosal suture
Temporal bone; squamal portion, zygomatic process, external auditory meatus and mastoid process
3. Anterior view: Frontal bone; frontal squama, frontal sinus, supraorbital margin and supraorbital foramen
Coronal suture
Nasal bone
Lacrimal bone; lacrimal fossa
Zygomatic bone; temporal process
Maxilla; infraorbital foramen, maxillary sinus and alveolar processes
Ethmoid bone; ethmoidal sinus and perpendicular plate
Mandible bone; body, mental formena and mandibular angle
4. Inferior view: Maxilla; palatine plate and alveolar processes
Palatine bone; horizontal plate
Sphenoid bone; pterygoid processes
Vomer bone
Temporal bone; mastoid portion, mastoid process, styloid process and mandibular fossa
Occipital bone; foramen magnum external occipital protuberance and occipital condyles
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