Skull Bones

Introduction

The skull comprises numerous, separate bones fused at the immobile joints, referred to as sutures, except for the mandible, which is fused to the skull by the mobile, synovial Temporomandibular joints (TMJ).

Normally, the human skull has twenty-two bones – fourteen facial skeleton bones and eight cranial bones.

The space containing the brain is the cranial cavity. The calvarium or the skull vault is the upper part of the cranium, forming the roof and the sidewalls of the cranial cavity. The foundation of the skull is the lower part of the cranium, constituting the floor of the cranial cavity.

What are the 22 Bones of the Skull?

The skull bones can be broadly divided into two main sections –

• Facial skeleton bones (14)
• Cranial bones (8)

Skull Bones Anatomy

Facial Bones

Mandible

Mandible forms the strongest and largest facial bones, it comprises the body and the two rami. The U-shaped body has a mental foramen situated on the outer surface near the root apex of the second and first premolars. The foramen’s opening is led backwards, laterally transmitting the mental nerve and the vessels.

On the outer midline surface of the body of the mandible, the symphysis menti constitutes a shallow ridge indicative of the line of fusing the two halves of the mandible at the time of development.

The midline medial surface of the body of the mandible comprises the mental spines, which give rise to the genioglossus muscles (seen above) and geniohyoid muscles (seen below). The mylohyoid lines are oblique ridges running laterally and backwards from the site of mental spines to the region behind and below the third molar tooth, representing the attachment of the mylohyoid muscle.

The submandibular fossa is seen below the posterior section of the mylohyoid line. For the sublingual gland, the sublingual fossa is seen above the anterior section of the mylohyoid line. Mandible’s ramus has an anterior coronoid process and a posterior condyloid process. A small neck is inferior to the head.

Vomer

It is a plow-shaped bone situated in the midline of the nasal fossa. It forms the posterior part of the nasal septum. Not only that, but it also mediates with the maxillary, ethmoid and palatine bones.

Inferior conchae

This structure is a paired one, forming the bony assistance of the inferior turbinate bilaterally. It is of surgical significance in the event when it blocks the inferior meatus and the nasolacrimal duct.

Palatine bones

These irregularly shaped paired bones, each comprising a main horizontal part and a vertical perpendicular plate. The horizontal plate anteriorly mediates with the maxilla and with that of the palatine bone of the opposite side in the midline, forming the posterior component of the hard palate.

The vertical plate superiorly passes behind the maxilla and posteriorly mediates with the lateral pterygoid plate of the sphenoid bone. This vertical plate ends in a small offering to the orbital floor at the posteromedial factor.

It has two foramenae – the lesser and greater palatine that lies in the posterior section of the palate, passing the lesser and greater palatine nerves and the vessels.

Nasal bones

These are rectangular bones forming the bridge of the nose articulating the frontal bone superiorly and with one another at the midline. It is comparatively thick at the superior articulation, inferiorly it is fine. This is a fracture prone site. These bones posteriorly mediate with the frontal process of the maxilla. The anterior nasal aperture is formed by the lower borders with the maxilla.

There are two nasal cavities divided by the nasal septum that is constituted by the vomer, the perpendicular plate of the ethmoid and the septal cartilage. The middle and superior conchae are ledges of bones projecting into the nasal cavity from the ethmoid on both sides.

Lacrimal bones

These are the most fragile and the smallest facial bones and are located at the front part of the medial wall of the orbit. It is a paired bone, anteriorly articulating with the frontal process of the maxilla, and articulates posteriorly with that of the lamina papyracea of the ethmoid. It articulates superiorly with the frontal bone and inferiorly with that of the orbital plate of the maxilla.

There is a groove referred to as the lacrimal groove for the nasolacrimal duct seen in the anterior section of the lateral surface of the lacrimal bone. The lacrimal groove unites anteriorly with the posterior periphery of the frontal process of the maxilla for the formation of the fossa that harbours the lacrimal sac.

Maxillae

Both the maxillae constitute the upper jaw, parts of the lateral walls of the nasal cavities, the anterior sections of the hard palate, and sections of the floors of the orbital cavities. The two bones unite in the midlines at the intermaxillary suture, forming the lower margin of the nasal aperture.

The infraorbital foramen penetrates the maxilla under the orbit. The alveolar process moves downwards, and with the opposite side, constitutes the alveolar arch holding the upper teeth. Each of the hemi maxillas comprises the maxillary sinus, a big pyramid-shaped body, four evident processes – alveolar, frontal, palatine and zygomatic processes.

Maxilla’s body is hollow, comprising the maxillary sinus. The anterior walls of the sinus are the facial surface of the maxilla and typically are thin. The medial wall is the lateral nasal wall. The sinus leads medially and superiorly to the nasal cavity at the middle meatus. The superior roof of the sinus is the orbital floor, the sinus’ floor is the palatine and the alveolar processes of the maxilla. The maxilla posteriorly mediates, with the lacrimal bone forming the anterior part of the medial orbital wall.

Zygomatic bones

It is a strong, thick and diamond-shaped bone forming the anterior and lateral projections to the midface and comprises 4 processes. The zygoma is paired bones constituting the prominence of the cheek. The frontal process constitutes the lateral orbital wall and mediates with the frontal bone at the frontozygomatic suture.

The temporal process constitutes the zygomatic arch, mediating with the temporal bone. The maxillary process mediates with the maxilla forming the infraorbital rim and a section of the floor of the orbit. The fourth process unites with the maxilla on the lateral wall, generating the zygomatic distinction.

The zygoma mediates with the sphenoid bone on the posterior component of the frontal process. The articulation is with the higher wing of the sphenoid bone, constituting the lateral wall of the orbit.

The zygomaticotemporal and zygomaticofacial foramen are the only foramen of the zygomatic bone. Both these branches of the second division of the trigeminal nerve pass from inside the orbit to the surface and provide sensory innervation to the structures associated.

Cranial Bones

Temporal bones

These bones are paired structures at the base and lateral sides of the skull. Each of these bones comprises these processes and parts –

• Tympanic section – comprises the external auditory meatus
• Zygomatic part process – constitutes the zygomatic arch with the temporal process of the zygomatic bone
• Petrous part – found in the foundation of the skull, it harbours the internal acoustic meatus and structures of the inner ear. The mastoid process is deemed as the down growth from the petrous section and both referred to as petromastoid
• Styloid process – provides attachment to the ligaments and muscle
• Squamous part – forms the most superior located and largest section of the temporal bone. It unites the parietal bone to that of the squamous suture constituting part of the pterion

The canals and the foramen are present in this bone area – the external acoustic meatus, carotid canal, foramen lacerum, stylomastoid foramen, internal acoustic meatus and facial nerve canal. Temporal bones mediate with the mandibular bone via the temporomandibular joint (TMJ). The temporal bone’s glenoid fossa mediates with the condylar head of the mandible. The anterior periphery of the glenoid fossa is the articular eminence.

Parietal bones

These are two flat bones constituting most of the vault of the skull. It mediates with one another in the midline at the sagittal suture. It mediates anteriorly with the frontal bone at the coronal suture, the site of the joint between the coronal suture, and the sagittal suture referred to as the Bregma.

These bones posteriorly mediate with the occipital bone at the lambdoid suture, the site of joining the sagittal suture with the lambdoid suture referred to as the lambda. The bones laterally mediate with the squamous temporal bone at the squamous suture. They have paired foramen, which is referred to as the parietal foramen, and have the emissary’s vein.

Occipital bone

These are unpaired trapezoid bones that are the main bones of the occiput (back of the skull). It constitutes a major part of the basilar section of the neurocranium and harbours the cerebellum fully.

The bone superiorly mediates with the parietal bones at the lambdoid suture, forming a section of the vault of the skull. It is the only cranial bone inferiorly to mediate with the cervical spine. It mediates with the sphenoid anteriorly at the base of the skull. This bone comprises these parts – basilar part, squamous part, condylar part.

Out of these, the squamous part is the largest; it is seen posterior to the foramen magnum and is curved from over to downwards. The outer surface has external occipital protuberance and 3 curved lines called nuchal lines. The superior nuchal line and the highest nuchal line run somewhat inferiorly. Further, running inferior is the inferior nuchal line. The internal surface of the squamous section is indicated by the grooves on the internal surface as a result of the venous cranial sinuses – the sigmoid sinus, transverse sinuses and the superior sagittal sinus.

The basilar part is seen anterior to the foramen magnum and next to the petrous section of the temporal bone. It mediates anteriorly with the sphenoid bone. The condylar section is seen lateral to the foramen magnum. It has two kidney-shaped projections which mediate with the first cervical vertebra.

The jugular foramen is seen between the occipital bone and the petrous section of the temporal bone. The hypoglossal nerve leaves the neurocranium via the hypoglossal canal that penetrates through the condylar section of the occipital bone.

Sphenoid bone

These are single bones located at the centre of the skull foundation and are a portion of the middle and anterior cranial fossae. It has several processes which are intricate articulations with the facial and cranial bones found adjacent to it.

It mediates with the ethmoid, temporal, frontal, occipital, parietal, zygoma, vomer, palatine bones and at times the tuberosity of the maxilla. These bones resemble the shape of a butterfly – with a greater, lesser wing, the body and the pterygoid plates. Additionally, it has numerous foraminae and fissures – optic canal, foramen ovale and the superior orbital fissure.

Its body is hollow, forming two cavities distinguished by a fine bony septum. The hollow cavities are the sphenoidal air sinuses, draining into the superior nasal meatus. On the body’s superior surface is a depression for the pituitary gland, referred to as the Sella turcica. The body is seen on the roof of the nasal cavity.

Ethmoid bone

It is a single bone seen in the midsection of the anterior cranial fossa. It establishes a section of the anterior cranial fossa, nasal structure and medial orbital walls. The crista galli in the anterior cranial fossa, a bony upward projection, provides the attachment to the Falx cerebri.

The ethmoid plate that is perpendicular slopes downwards in the midline of the nasal cavity, constituting the anterior and superior parts of the nasal septum. Posteriorly, it mediates with the vomer and with that of the septal cartilages establishing the nasal septum.

The cribriform plates of ethmoids mediate laterally and anteriorly with the frontal bone and posteriorly with the sphenoid bone. The olfactory nerve filaments advance via the cribriform plate.

This bone is a pneumatic bone containing ethmoid air cells that drain into the middle and superior nasal meatus.

Frontal bone

It is a single bone, constituting the anterior part of the calvaria and the higher third of the forehead. The bone laterally mediates with the zygoma at the frontozygomatic suture. In its vault, at its coronal suture, it mediates with the parietal bones.

This bone constitutes a great part of the orbit’s roof. The thickening of this bone in the anterior part constitutes the superciliary arches. The curved altitude provides evidence of the eyebrow part. This rim is crossed over by the supraorbital notch, transiting the frontal vessels and nerves.

The bone comprises the air sinuses – two hollow spaces that are lined with mucous membranes found immediately above the orbital margins. The paranasal sinus is the mucous membrane with the lining of the air-filled bone cavities. These are frontal, maxillary, ethmoid and sphenoid paranasal sinuses. It mediates with the nose, serving the lightning of the facial skeleton acting as voice resonators.

This was a brief on skull bones, different types of facial and cranial bones, and their description.

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