Functions of the Cranium

The skeleton of the head and neck includes the skull, middle ear ossicles, hyoid bone, and cervical vertebrae. The skull comprises several separate bones united at immobile joints called sutures. The mandible is an exception to this rule, as it is united to the skull by the mobile, synovial Temporomandibular Joints (TMJ). The skull has 22 bones, organized into a cranial skeleton (8 bones) that surrounds the brain and a facial skeleton (14 bones). The cranial cavity is the space containing the brain. The skull vault (calvarium) is the upper part of the cranium and forms the roof and side walls of the cranial cavity. The Base of the skull is the lowest part of the cranium and forms the floor of the cranial cavity.

1.0Cranium

2.0Cranium Bones

• Frontal bone: one bone 
• Ethmoid bone: one bone 
• Sphenoid bone: one bone 
• Occipital bone: one bone 
• Parietal bones: paired (2 bones) 
• Temporal bones: paired (2 bones)

3.0Facial Bones

• Zygomatic bones: paired (2 bones)  Maxillae: paired (2 bones)  
• Nasal bones: paired (2 bones)  
• Lacrimal bones: paired (2 bones)  
• Palatine bones: paired (2 bones)  
• Inferior conchae: paired (2 bones) 
• Mandible: one bone  
• Vomer: one bone

4.0The Cranial Bones

Frontal Bone 

• The frontal bone is a single cranial bone that forms the anterior portion of the calvaria and the upper third of the face (forehead). 
• The frontal bone articulates laterally with the zygoma at the frontozygomatic suture and medially with the (maxilla at the front maxillary suture and nasal bones at the frontonasal sutures). 
• In the Base of the skull, inferiorly and deeply in the anterior cranial fossa, it articulates with the ethmoid and the wings of the sphenoid bone. In the vault, it articulates with the parietal bones at the coronal suture. 
• The frontal bone forms a large portion of the roof of the orbit. The thickening of the frontal bone in the anterior region forms the superciliary arches (supraorbital ridges). These curved elevations give the prominence of the eyebrow region. 
• The supraorbital notch or foramen crosses this rim and transmits the frontal vessels and nerves (supraorbital and supra trochlear nerves). 
• The frontal bone contains the frontal air sinuses (paranasal sinus), two hollow spaces lined with mucous membranes just above the orbital margins. 
• The paranasal sinuses are mucous membrane-filled bone cavities; they are four in number: 
• The maxillary (the largest), Frontal, sphenoid, and ethmoid paranasal sinuses communicate with the nose, serve to lighten the facial skeleton and act as voice resonators. 

Ethmoid Bone

• The ethmoid bone is a single bone that lies in the mid-area of the anterior cranial fossa. It constitutes part of the nasal structure, medial orbital walls and anterior cranial fossa. 
• In the anterior cranial fossa, the (crista galli)- bony projection upward- gives attachment to Falx cerebri (meningeal layer of dura mater). 
• The perpendicular plate of ethmoids descends downward in the nasal cavity's midline to form the nasal septum's superior and anterior portions. It articulates posteroinferiorly with the vomer and septal cartilage to form the nasal septum. 
• The cribriform plate (perforated bony plates) of ethmoid articulates anteriorly and laterally with the frontal bone and posteriorly with the sphenoid bone, and the filaments of the olfactory nerve (cranial nerve I) pass through the cribriform plate. 
• Hanging bilaterally from the cribriform plate are the superior and middle nasal conchae, which descend laterally into the nasal cavity. Lamina papyracea -paper-like- is an extremely thin plate of the ethmoid bone that constitutes most of the medial orbital wall. 
• The ethmoid bone is a pneumatic bone containing the ethmoid air cells (ethmoid paranasal sinus), which drain into the superior and middle nasal meatus. 

Sphenoid Bone

• The sphenoid bone is a single bone in the middle of the skull base, a part of the anterior and middle cranial fossae. 
• This complex bone has many processes with delicate articulations with the adjacent cranial and facial bones. 
• The sphenoid bone articulates with the frontal, ethmoid, parietal, temporal, occipital, vomer, zygoma, palatine bones, and sometimes the tuberosity of the maxilla.

• The sphenoid bone looks like a butterfly in shape; it has a smaller wing, a greater wing, and a body and pterygoid plates (2 medial and two lateral). 
• It also contains many foramina and fissures, such as the optic canal (Optic N. CNII), foramen rotundum (Maxillary N. CNV2), foramen ovale (Mandibular N. CNV3), foramen spinosum (middle meningeal artery), and superior orbital fissure (Oculomotor N.CNIII, Trochlear N. CNIV, Ophthalmic N. CNV1, Abducens N. CNVI, and Ophthalmic Vein). 
• The body of the sphenoid bone is hollow and forms two cavities separated by a thin bony septum. 
• The hollow cavities are the sphenoidal air sinuses; these drain into the superior nasal meatus. On the superior surface of the body, there is depression for the pituitary gland (hypophysis), Known as the hypophysial fossa or (Sella turcica). 
• The sphenoid's body lies on the roof of the nasal cavity, which is why pituitary gland surgery is performed by passing an endoscope through the nose (transnasal). 
• In children, the sphenoid occipital joint is cartilaginous (sphenoid occipital synchondrosis) and is considered a growth centre for the skull base. At the completion of growth, it is transformed into a suture.

Temporal Bone

• The temporal bone is a paired bone at the lateral side and Base of the skull. Each temporal bone consists of the following parts and processes: The petrous part (also called the pyramid) 
• It is located at the Base of the skull and houses the internal acoustic meatus and structures of the inner ear (internal auditory canal). 
• The mastoid process (which is (contains mastoid air cells that act as reservoirs of air to equalize the pressure in the middle ear) is a down growth from the petro, known as petromastoid. 
• Tympanic part (contains the external auditory meatus), 

• Styloid process (gives attachment to muscle and ligaments) 
• Zygomatic part process (form the zygomatic arch with the temporal process of the zygomatic bone).
• The foramenae and canals in the temporal bone are the foramen lacerum, carotid canal, internal acoustic meatus, external acoustic meatus, stylomastoid foramen and facial nerve canal (the inner opening is the internal acoustic, and the outer opening is the stylomastoid foramen- between styloid and mastoid).

• The temporal bone articulates with the mandibular bone through the TemproMandibular Joint (TMJ), and the glenoid fossa of the temporal bone (at the root of the zygomatic arch) articulates with the condylar head of the mandible. 

Occipital Bone

• The occipital bone is a single, trapezoidal-shaped bone that forms the main structure at the back of the skull (occiput). 
• It constitutes a significant portion of the basilar region of the neurocranium and completely encloses the cerebellum.  Superiorly, it connects with the parietal bones at the lambdoid suture, contributing to the cranial vault.  
• Inferiorly, it is the only cranial bone that articulates with the cervical spine.   Anteriorly, it joins the sphenoid bone at the base of the skull.   
• The occipital bone consists of three main sections:   Basilar part (labeled as no. 1 in the figure),   Condylar part (labeled as no. 2), and the Squamous part (labeled as no. 3), located on either side of the foramen magnum. 
• The squamous part is the largest of all four; it lies posterior to the foramen magnum and is curved from above downward (convex externally and concave internally).  
• The external surface of the occipital bone features several distinct landmarks:  

• The external occipital protuberance is a noticeable midline prominence that serves as the attachment point for the trapezius muscle.  
• Three curved lines, known as nuchal lines, provide attachment points for various muscles and ligaments.  

• The bone is further divided into specific regions:  
• The basilar part is located anterior to the foramen magnum and adjacent to the petrous portion of the temporal bone. It articulates with the sphenoid bone at the skull base.  
• The condylar parts, positioned laterally to the foramen magnum, include two kidney-shaped prominences called the occipital condyles. These structures articulate with the first cervical vertebra, forming the atlanto-occipital joint.  

Parietal Bones

• Parietal bones are two flat bones that form the majority of the vault of the skull (calvaria). They articulate with each other in the midline at the sagittal suture. 
• Anteriorly, it articulates with the frontal bone at the coronal suture, the joint area between the coronal suture and the sagittal suture known as Bregma. 
• Posteriorly, the paired bones articulate with the occipital bone at the lambdoid suture; the area joining the sagittal suture with the lambdoid suture is known as lambda. 
• Laterally, the bones articulate with squamous temporal bone at the squamous suture. 
• The bones have paired foramenae (one in each bone), known as parietal foramen and contain an emissary vein. 

5.0The Facial Bones

The Nasal Bones 

• The nasal bones are rectangular. They form the bridge of the nose, and they articulate with the frontal bone superiorly and with each other at the midline. 
• They are relatively thick at the superior articulation, but they are much thinner inferiorly. It is in this area that most fractures occur. 
• The nasal bones articulate posteriorly with the frontal process of the maxilla. The lower borders with the maxilla form the anterior nasal aperture. 

• The nasal cavity is divided into two cavities by the nasal septum, formed by the vomer, the perpendicular plate of the ethmoid and septal cartilage (the above figure on the right). 
• The superior and middle conchae are shelves of bone that project into the nasal cavity from the ethmoid on each side. The inferior conchae are separate bones.

The Maxillary Bones

• It is  from the upper jaw, the front portion of the hard palate, parts of the lateral walls of the nasal cavities, and portions of the floors of the orbital cavities. 
• They meet at the midline along the intermaxillary suture, shaping the lower margin of the nasal aperture. 
• Below the orbit, the maxilla is perforated by the infraorbital foramen. 
• The alveolar process projects downward, and together with its counterpart from the opposite side, forms the alveolar arch that houses the upper teeth. 
• Each hemimaxilla contains a pyramid-shaped body known as the maxillary sinus (antrum of Highmore) and features four prominent processes: the frontal, alveolar, zygomatic, and palatine processes.
• The body of the maxilla is hollow and contains the maxillary sinus (pyramid-shaped). The sinus's anterior wall is the maxilla's facial surface and is usually thin. 
• The medial wall is the lateral nasal wall. 
• The sinus opens superiorly and medially into the nasal cavity at the middle meatus. 
• The superior wall or roof of the sinus is the orbital floor, and the sinus floor is the maxilla's palatine and alveolar processes. Posteriorly, the maxilla articulates with the lacrimal bone to form the anterior portion of the medial orbital wall.

The Zygoma

• The zygoma (zygomatic bone, malar bone) is a paired bone that makes up the essence of the cheek prominence. 
• This thick, strong, diamond-shaped bone forms the lateral and anterior projections to the midface and comprises four processes. 
• The frontal process forms the lateral orbital wall and articulates with the frontal bone at the frontozygomatic suture. 
• The temporal process forms the zygomatic arch and articulates with the temporal bone. 
• The maxillary process articulates with the maxilla to form the infraorbital rim and part of the orbit's floor. 
• Finally, the fourth process joins the maxilla on the lateral wall, producing the zygomatic eminence. 
• This area of thickened bone is usually available for fixation in treating zygomaticomaxillary complex (ZMC) fractures. 
• The zygoma articulates with the sphenoid bone on the posterior aspect of the frontal process. This articulation is with the greater wing of the sphenoid bone and forms the lateral wall of the orbit. The only foramina of the zygomatic bone are the zygomaticofacial foramen and the zygomaticotemporal. 
• The masseter muscle is inserted on the inferior aspect. Its force direction is down and backwards, and its contraction contributes to the displacement of the complex fracture of the zygoma.

Vomer 

• The vomer is a plough-shaped bone located in the nasal fossa's midline and forms the posterior portion of the nasal septum. 
• It articulates with the palatine, maxillary, and ethmoid bones. 

Palatine Bones

• These are irregularly shaped paired bones; each comprises a major horizontal portion and vertical perpendicular plates. 
• The horizontal plate articulates anteriorly with the maxilla and with the palatine bone of the opposite side in the midline to form the posterior aspect of the hard palate. 
• The vertical plate passes superiorly behind the maxilla and articulates posteriorly with the lateral pterygoid plate of the sphenoid bone. 
• The vertical plate terminates in a small contribution to the orbital floor at the posteromedial aspect. 
• The palatine bone has two foramenae; the greater and lesser palatine lies in the palate's posterior part and transmits the greater and lesser palatine nerves (branches of the maxillary nerve) and vessels. 

Inferior Nasal Concha

• It is a paired bone that forms the bony support of the inferior turbinate bilaterally. 
• It is of surgical importance only when it obstructs the inferior meatus and the nasolacrimal duct.

Lacrimal Bones

• It is a paired bone. The smallest and most fragile bones of the face are situated at the front part of the medial wall of the orbit. 
• Each lacrimal bone articulates anteriorly with the frontal process of the maxilla. Posteriorly, it articulates with the lamina papyracea of the ethmoid; superiorly, it articulates with the frontal bone; and inferiorly, it articulates with the orbital plate of the maxilla. 
• Lacrima is Latin for "tear", so the name of the bone corresponds with its relation to the nearby lacrimal structures. 
• The lacrimal groove is a groove for the nasolacrimal duct, situated in the anterior part of the lateral surface of the lacrimal bone. 
• The lacrimal groove fuses anteriorly with the posterior border of the frontal process of the maxilla to form the fossa that houses the lacrimal sac. 

The Mandible

• The mandible, the largest and strongest facial bone, consists of a U-shaped body and two rami connected at an angle. 
• The body features a mental foramen on its external surface near the root apices of the first and second premolars. This foramen, which opens backward and laterally, transmits the mental nerve (a terminal branch of the inferior alveolar nerve) and vessels. 
• The external midline of the body shows the symphysis menti, a shallow ridge marking the fusion of the two mandibular halves during development. 
• On the internal midline surface, the mental spines (genial tubercles) serve as attachment points for the genioglossus muscles above and the geniohyoid muscles below.  

• The mylohyoid line, an oblique ridge extending backward and laterally from the mental spines to a point below and behind the third molar tooth, provides attachment for the mylohyoid muscle, which forms the floor of the mouth. 
• Below the posterior part of this line lies the submandibular fossa, housing the superficial part of the submandibular salivary gland, while the sublingual fossa above the anterior portion accommodates the sublingual gland.