The term ‘Skull’ includes the mandible, likewise the term ‘Cranium’ is the ‘Skull’ without the mandible (Figs 1.1 and 1.2). The cranial cavity has a roof (cranial vault) and floor (base of the skull).
The frontal bone occupies the upper third of the anterior view of the skull; the rest is formed by the maxillae and mandible. The frontal bone extends downwards to form the upper margins of the orbits. Medially the frontal bone articulates with the frontal process of each maxilla. Laterally the frontal bone projects as the zygomatic process to make the frontozygomatic suture with the zygomatic bone at the lateral margin of orbit (Figs 1.3 to 1.6). The frontal bone articulates with the parietal bones at the coronal sutures (which run transversely).
The temporal bone consists of five parts-Squamous, mastoid, petrous, tympanic and styloid process. The squamous portion forms part of wall of temporal fossa and gives rise to zygomatic process. The mastoid portion contains the mastoid antrum, in adults it elongates into mastoid process. The mastoid antrum communicates with the remainder of mastoid air cells and with the epitympanum via the aditus ad antrum. The petrous portion is wedge-shaped and lies between the sphenoid bone anteriorly and occipital bone posteriorly. The tympanic portion lies below the squamous part and in front of the mastoid process. The styloid portion forms the styloid process.
The temporal fossa is the area bounded by the superior temporal line, zygomatic arch and the frontal process of the zygomatic bone. The zygomatic arch is formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone. The zygomatic process of the maxilla articulates with the zygomatic bone. The zygomatic bone forms the bony prominence of the cheek (Figs 1.7 to 1.10).
The styloid process is a part of the temporal bone, from its tip the stylohyoid ligament passes to the lesser horn of hyoid bone. At the base of the skull medial to the styloid process the petrous bone is deeply hollowed out to form the jugular fossa with an opening called as jugular foramen through which the internal jugular vein passes. Anterior to the jugular foramen the petrous part of the temporal bone is perforated by the carotid canal, allows the internal carotid artery to pass through it (Fig. 1.11). Between the basiocciput and the body of sphenoid bone lies the foramen lacerum, it allows the small emissary vein and meningeal branch of ascending pharyngeal artery to pass through it. The roof of the infratemporal fossa is pierced medially by the foramen ovale, through which passes the mandibular nerve, lesser petrosal nerve, accessory meningeal artery and emissary veins. The base of the spine of sphenoid is perforated by the foramen spinosum which allows the middle meningeal vessels to pass through it.
Figs 1.1A to D: CT scan multiplanar reconstruction images of skull: (A) Frontal view; (B) View from back; (C) Lateral view; (D) View from below
The stylomastoid foramen lies behind the base of styloid process. Medial to the third molar tooth on either side is the greater palatine, foramen between the horizontal plate of palatine bone and the palatine process of the maxilla, the greater palatine vessels and nerves pass through it. Behind the greater palatine, there are numerous small openings called the lesser palatine foramina in the pyramidal process of palatine bone through which the lesser palatine vessels and nerves pass.
There are two parietal bones on either side of skull. They are seen better on lateral views of skull and they articulate with the frontal bone anteriorly at the coronal sutures. Posteriorly, the parietal bones articulate with occipital bone and temporal bone mastoid process at lambdoid suture. The bregma is the area in midline where the coronal sutures and the two parietal bones meet. Behind the bregma, the parietal bones articulate in the midline sagittal suture. This midline sagittal suture ends at the lambda in posteriorly. The lambda is the area posterior where the sagittal suture ends in midline and the apex of occipital bone reaches out to join it in midline.
The mastoid region of the temporal bone articulates with the parietal and occipital bones posteriorly, the mastoid process projects down at the sides. Inferiorly the parietal bones articulate with the squamous portion of temporal bone on either side.
The occipital bone on its lower surface has a ridge which is pointing towards the base of the mastoid process; this is called the external occipital protuberance. The basiocciput extends forward from the foramen magnum and fuses with the basisphenoid. The foramen magnum is located in the basilar part of the occipital bone (basiocciput). The pharyngeal tubercle is a slight bony prominence in front of the foramen magnum. One-third of the foramen magnum lies in front and two-thirds behind an imaginary line joining the tips of the mastoid processes. This is contrary to the occipital condyles, where two-thirds of the condyles lie in front of this imaginary line.
The internal surface of the base of skull is divided into the anterior, middle and posterior cranial fossa. The orbital part of the frontal bone forms a large part of anterior cranial fossa. The anterior cranial fossa extends up to the posterior edge of the lesser wing of sphenoid. The anterior cranial fossa articulates with the cribriform plate medially. The crista galli is a sharp projection of the cribriform plate.
The sphenoid bone contributes to the middle cranial fossa. The small midline body of sphenoid bone contains the sella turcica (means ‘Turkish saddle’), a small elevation in front of sella turcica is called tuberculum sellae (Fig. 1.5). The tuberculum sellae has three small spikes, the middle spike is called the middle clinoid process, the two lateral spikes are called anterior clinoid process. At the posterior edge of the sella turcica is an elevation called the dorsum sellae, which has two lateral spikes called the posterior clinoid process. A fibrous portion of the dura forms the roof of the sella turcica extending from the tuberculum sellae to the dorsum sellae and is called the diaphragm sellae. The diaphragm sellae has a central opening to allow the pituitary stalk and vessels to pass through it.
The posterior cranial fossa extends from the petrous temporal bone anteriorly to the internal occipital protuberance in the midline. The floor is formed by the foramen magnum, basiocciput and posterior part of sphenoid bone. The dorsum sellae slopes downwards in front of foramen magnum, this slope is called the clivus.
The mandible or the jaw bone is a U-shaped, a horizontal central part with two lateral ramus on each side. The posterior border of each ramus has a condyle with a neck which articulates with the temporal bone forming the temporomandibular joint, while the anterior border of each ramus is sharp and is called the coronoid process (Figs 1.1 to 1.4).
The temporormandibular joint is a synovial joint between the head (condyle) of the mandible and mandibular fossa on the undersurface of the squamous part of the temporal bone. The joint is separated into the upper and lower cavities by a fibrocartilaginous disc within it. There is no hyaline cartilage within the joint which makes it an atypical synovial joint. The synovial membrane lines the inside of the capsule and the intracapsular posterior aspect of the neck of the mandible. The articular disc is attached around its periphery to the inside of the capsule and to the medial and lateral poles of the head of the mandible. The joint is more stable with the teeth in occlusion than when the jaw is open. The movements at the temporomandibular joint are depression and elevation (opening and closing of the jaws), side to side grinding movements, retraction and protaction movements (retrusion and protrusion).
THE NASAL CAVITY AND NASAL SEPTUM
The nasal cavity is pear-shaped, broader below and narrower at the top. From its lateral walls the conchae project into the nasal cavity. There are three conchae—Superior, middle and inferior conchae. The superior concha is small and is found high in nasal cavity, its lower edge overlies the superior meatus. The sphenoethmoidal recess lies above and behind the superior concha and receives the ostia of sphenoidal sinus. The middle concha lies between the superior and inferior concha. The area in front of the middle meatus is the atrium of nose. Posteriorly, the middle meatus is related to the splenopalatine foramen. The inferior concha lies below the middle concha articulates anteriorly with the maxilla and posteriorly with the palatine bone. The nasal septum (Fig. 1.12) is normally in the midline, it consists of bone (vomer) and cartilage. It has a lower free margin, superiorly it articulates with the medial ends of frontal bone and also the frontal process of maxilla. The two maxillae on either side meet in the midline and project forwards as the anterior nasal spine at the lower margin of the nasal aperture. The vomer articulates with the sphenoid body and forms the posterior border of the septum. The septal cartilage forms the anterosuperior part of the septum. The floor of the nose is formed by the upper surface of the hard palate. The central part of the roof of nose is the cribriform plate of the ethmoid.
THE PARANASAL SINUSES
The paranasal sinuses all arise as evaginations from the nasal fossa. It comprises of frontal sinuses, maxillary sinuses, sphenoid sinuses and ethmoidal sinuses. The nasal cavity contains the superior meatus, middle meatus and the inferior meatus. The superior meatus drains the posterior ethmoidal air cells and sphenoidal sinuses. The middle meatus drains the frontal sinuses, maxillary sinuses and anterior ethmoidal air cells. The osteomeatal complex comprises of the uncinate process, ethmoid infundibulum, maxillary sinus ostium, middle turbinate, frontal recess and ethmoid bulla. The inferior meatus has opening for the nasolacrimal duct (Figs 1.8 to 1.12).
The maxillary sinus lies in the body of maxilla, the sinus is triangular in shape, the apex in the zygomatic process of maxilla and the base towards the lateral wall of the nose. The roof of the sinus is the floor of the orbit. The floor of the sinus is formed by the alveolar part of maxilla. The infratemporal fossa and pterygopalatine fossa lies behind the posterior wall of maxillary sinus. The ostium of maxillary sinus is on the superomedial aspect of the sinus and opens into the middle meatus on the same side into the nasal cavity (Figs 1.2B and 1.3B).
The ethmoidal sinus lies between the nasal cavity and orbit. The sinus is divided by multiple thin bony septa into the anterior and posterior group of ethmoidal air cells. The lateral wall of the ethmoidal sinus forms a part of the medial wall of orbit; it is paper thin and is called the lamina papyracea. The ostia of anterior ethmoidal air cells drain into the middle meatus. The ostia of posterior ethmoidal air cells drain into the superior meatus.
The sphenoidal sinus occupies the body of sphenoid bone. A vertical septum divides the cavity into two unequal halves. The roof of sphenoid sinus is formed by pituitary fossa and middle cranial fossa. Laterally the sphenoid sinus is related to the cavernous sinus and internal carotid artery. Posteriorly, the sphenoid sinus is related to the posterior cranial fossa and pons. The ostium of sphenoidal sinus is in the anterior wall of the sinus and opens into the superior meatus or into the sphenoethmoidal recess.
The frontal sinuses are formed within the frontal bone on either side near midline. Its floor forms the roof of orbit medially. Posteriorly the frontal sinus is related to anterior cranial fossa. The ostium of frontal sinus is at its lower medial edge and drains into the middle meatus in nasal cavity or in some cases into the anterior ethmoidal air cells.
The bony orbit is a cavity, shaped like a pyramid with its apex posteriorly and the base forming the orbital margins anteriorly.
The orbital roof is formed by the frontal bone, which separates the orbit from the anterior cranial fossa. The orbital floor is formed by the orbital plate of the maxilla, portions of the palatine bone and the zygoma (Figs 1.10, 1.13 and 1.14). The maxillary portion of orbital floor is usually involved in blow out fractures. The medial orbital wall is the thinnest of all the orbital walls and comprises of frontal process of the maxilla, lacrimal bone, lamina papyracea and bony sphenoid.
The lateral wall of orbit is formed by the zygoma and greater wing of sphenoid. The superior orbital fissure is a space between the greater and lesser wings of sphenoid. The inferior orbital fissure is formed by the maxilla, the palatine bone and the greater wing of sphenoid. The optic canal lies within the lesser wing of sphenoid, the optic nerve and ophthalmic artery encased in the dural sheath pass through it.
Structures passing through the superior orbital fissure: Superior ophthalmic vein, the rectus muscles (superior, inferior, medial and lateral), lacrimal nerve, frontal nerve, trochlear nerve, oculomotor nerve, abducent nerve, nasociliary nerve.
Structures passing through the inferior orbital fissure: Infraorbital artery, inferior ophthalmic vein, zygomatic nerve, infraorbital nerve.
Structures passing through the optic canal: Optic nerve, ophthalmic artery.