Skull Anatomy — Vocabulary Flashcards (Bones, Sutures, Fontanelles, and Ossicles)

The Skull: Overview

The skull is the body’s most complex bony structure, organized into two sets of bones: the Cerebral Cranium (Calvaria) and the Facial Cranium. The Cerebral Cranium consists of the bones that form and protect the brain and provide attachment sites for some head and neck muscles, while the Facial Cranium provides the framework of the face, supports sense organs and teeth, and creates openings for air and food. The bones of the skull are joined by sutures, with only the mandible attached by a freely movable joint. The Skull consists of a total of $22$ bones: $8$ in the Cranial (Cerebral) Cranium and $14$ in the Facial Cranium. The middle ear contains the six auditory ossicles: two malleus, two incus, and two stapes.

Bones of the Cerebral Cranium (Calvaria)

The cerebral cranium comprises $8$ bones in total. There are four single, unpaired bones: frontal, ethmoid, sphenoid, and occipital. There are four paired bones: temporal and parietal bones (two temporal and two parietal bones). These bones encase the brain and form the cranial vault. The calvaria protects the brain, provides attachment for scalp and facial muscles, and contributes to the shape of the head.

Bones of the Facial Cranium

The facial cranium comprises $14$ bones. There are two single bones: the mandible and the vomer. The remaining twelve are paired: maxilla, nasal bone, lacrimal bone, palatine bone, zygomatic bone, and inferior nasal concha. These bones form the structure of the face, support the sense organs and teeth, and contribute to the nasal and oral cavities.

Mnemonic for Facial Bones

A handy memory aid for the facial bones is: Virgil Can Not Make My Pet Zebra Laugh! which stands for the bones: Vomer, Conchae (inferior nasal concha), Nasal, Maxilla, Mandible, Palatine, Zygomatic, Lacrimal. This mnemonic helps recall the order and presence of the facial bones.

Sutures

Sutures are the immovable joints that connect skull bones. The major sutures are:

Coronal suture: between the frontal and paired/parietal bones.
Sagittal suture: between the two parietal bones.
Lambdoidal suture: between the parietal bones and the occipital bone.
Squamous suture: between the parietal bone and the temporal bone.
These sutures mark the boundaries between cranial bones and allow for growth during infancy and childhood.

Fontanelles

Fontanelles are the soft spots in an infant’s skull where sutures have not yet fused. They allow for growth of the brain and skull during infancy and facilitate passage through the birth canal. The two primary fontanelles are:

Anterior fontanelle: the junction where the two frontal and two parietal bones meet; remains soft until about age 2 years.
Posterior fontanelle: the junction of the two parietal bones and the occipital bone; typically closes earlier than the anterior fontanelle, usually within the first several months of life.

Key Points about the Skull Landmarks

The meeting points of sutures are named, for example: Bregma is the point where the coronal and sagittal sutures meet, and Lambda is where the sagittal and lambdoid sutures meet.
In newborn imaging, a normal skull shows fontanelles and sutures in characteristic locations and stages of closure.

The Base of the Skull: Internal View

The base of the skull forms three fossae in its interior: the anterior cranial fossa, the middle cranial fossa, and the posterior cranial fossa. These basilar divisions house parts of the brain and provide openings (foramina) for neurovascular structures. The internal view reveals important features such as the olfactory foramina in the cribriform plate, the sphenoid body and wings, the optic canal, the superior and inferior orbital fissures, the foramen ovale, foramen spinosum, foramen lacerum, the jugular foramen, and the hypoglossal canal, among others. The internal view also highlights the petrous portion of the temporal bone that houses the inner ear structures.

The Base of the Skull: Openings and Passages

Anterior cranial fossa contains structures related to the frontal lobes and the cribriform plate with olfactory foramina.
Middle cranial fossa includes foramina and openings such as the optic canal, foramen rotundum, foramen ovale, and foramen spinosum, along with the sella turcica (the saddle-shaped housing of the pituitary gland).
Posterior cranial fossa contains the foramen magnum and other openings such as the jugular foramen and hypoglossal canal.

The Anterior View of the Skull

From the anterior view, major bones visible include the frontal bone (unpaired), the paired parietal bones, the sphenoid bones (unpaired), the ethmoid bones (unpaired), the temporal bones (paired), and the occipital bone (unpaired). Surface landmarks visible include:

Supraorbital foramen/notch and supraorbital margin on the frontal bone.
Glabella, frontonasal suture, and nasal bones along the midline.
Infraorbital foramen below the orbit, and the mental foramen on the mandible in the lower face region.

The Lateral View of the Skull

In the lateral view you will see sutures such as the coronal suture anteriorly and the lambdoid suture posteriorly, as well as the squamous suture between the parietal and temporal bones. Key features include the external acoustic meatus (ear canal) on the temporal bone, the mastoid process, the styloid process, and the zygomatic process of the temporal bone. The pterion is a notable junction where several sutures meet (frontal, parietal, sphenoid, and temporal).

The Posterior View of the Skull

From the posterior view, the lambdoid suture and the sagittal suture are often visible along with the occipital bone. The external occipital protuberance and the superior nuchal lines are notable surface landmarks.

Frontal Bone

Forms the forehead and the roofs of the orbits. It also forms the superior part of the orbital rims and contains frontal sinuses. It contributes to the anterior cranial fossa. It articulates posteriorly with the parietal bones via the coronal suture.
Major markings include the supraorbital margin (the arch above the orbital opening), the superciliary arches (ridge above the orbital margin), and the glabella (midline point between the paired superciliary arches). The supraorbital notch (or foramen) is a notable feature for transmission of nerves and vessels.

Occipital Bone

Forms the posterior portion of the cranium and part of the posterior cranial fossa. It articulates with the temporal and parietal bones. The foramen magnum is located at its base and communicates with the vertebral canal.
Major markings include the external occipital protuberance and the superior and inferior nuchal lines, the occipital condyles, and the hypoglossal canal (for the XII nerve). The condylar canal (emissary vein) is located near the condyle.

Parietal Bones

Cover much of the top and sides of the brain. They form the superior and lateral aspects of the skull. Major markings include the superior temporal line (attachment for the temporal fascia) and the inferior temporal line (attachment for the temporal muscle).
The parietal bones articulate with several neighboring bones via the coronal, sagittal, squamous, and lambdoid sutures.

Temporal Bones

Lie inferior to the parietal bones and form the inferolateral portion of the skull. Regions include the squamous region, tympanic region (around the external auditory meatus), mastoid region, and petrous portion (which houses the inner ear). The temporal bone contributes to the zygomatic process and the mandibular fossa. The bone also contains mastoid air cells. The term “temporal” derives from Latin for time.
Major markings include: the squamous part, the zygomatic process, the tympanic portion surrounding the external auditory canal, the mastoid process, the styloid process, and the petrous portion with the otic labyrinth and the internal acoustic meatus.

The Sphenoid Bone

A butterfly-shaped bone that forms part of the floor of the anterior, middle, and posterior cranial fossae. It is often called the “keystone” of the cranial floor because it articulates with all the other cranial bones. It consists of a central body, greater wings, lesser wings, and pterygoid processes.
Key features include the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, and the sella turcica (the saddle-like depression housing the pituitary gland). The greater wings contribute to the middle cranial fossa and the orbits, while the lesser wings and the sella turcica contribute to the skull base.

The Ethmoid Bone

Lies between the nasal and sphenoid bones, forming most of the medial bony region between the nasal cavity and the orbits; includes the ethmoid sinuses. It contributes to the medial wall of the orbits and the walls of the nasal cavity.
Major markings include the cribriform plate with olfactory foramina, the crista galli, and the perpendicular plate. It also contributes to the superior and middle nasal conchae (ethmoidal air cells).

Other Facial Bones

Maxillary bones (maxillae): The second largest facial bones, forming the midface and articulating with all other facial bones except the mandible. They contain the maxillary sinuses, form part of the inferior orbital fissure, and contribute to the hard palate and nasal cavity. Major markings include the orbital surface, zygomatic process, frontal process, alveolar margin, infraorbital foramen, and anterior nasal spine.
Nasal bones: Thin bones forming the bridge of the nose; join at the midline via the frontonasal suture.
Lacrimal bones: Small bones forming part of the medial wall of each orbit; contain the lacrimal fossa that houses the lacrimal sac.
Zygomatic bones: Form the lateral wall and part of the orbit; major markings include the frontal process, temporal process, and zygomaticotemporal and zygomaticofacial sutures.
Vomer: A roughly triangular bone forming the inferior part of the nasal septum; articulates with the sphenoid and the perpendicular plate of the ethmoid.
Palatine bones: L-shaped bones with horizontal and vertical portions; contribute to the posterior part of the hard palate, the lateral wall of the nasal fossa, and the posterior part of the orbital floor. The horizontal plate forms part of the hard palate; the vertical plate forms part of the nasal cavity floor.
Inferior nasal conchae: Thin, curved bones projecting medially to form the lateral walls of the nasal cavity.

Paranasal Sinuses

Paranasal sinuses are air-filled cavities lined with mucous membrane, located within the frontal, ethmoid, sphenoid, and maxillary bones. They have several proposed functions: to lighten the skull, to resonate and enrich the voice, and to produce mucus that drains into the nasal cavity. The major sinuses are:

Frontal sinus (within the frontal bone).
Ethmoidal air cells (ethmoid bone) with anterior and middle groups draining into the middle nasal meatus and posterior group draining into the superior meatus.
Sphenoid sinus (within the sphenoid bone) draining into the sphenoethmoidal recess.
Maxillary sinus (largest, within the maxilla) draining into the middle nasal meatus.

Notes on Sinus Drainage and Anatomy

Frontal sinus drains to the anterior part of the middle meatus.
The maxillary sinus is the largest sinus and drains into the middle nasal meatus.
The ethmoidal sinuses have anterior and middle groups draining into the middle meatus and posterior groups draining into the superior meatus.

The Hyoid Bone

A small, U-shaped bone located in the neck between the mandible and larynx. It is unique in that it does not articulate directly with another bone. It is suspended by ligaments and muscles and serves as an anchoring structure for the tongue and for muscles associated with swallowing and speech. It consists of five parts: a central body, two greater horns (posterior extension from the body), and two lesser horns (smaller superior extensions from the body).

The Auditory Ossicles (Middle Ear Bones)

The middle ear contains three tiny bones in each ear, collectively known as the auditory ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). Each ossicle has specific regions:

Malleus: has a manubrium that attaches to the tympanic membrane and a head that articulates with the incus. It also has anterior and lateral processes.
Incus: consists of a body and two processes (short and long). The head articulates with the malleus, and the lenticular process of the long arm articulates with the head of the stapes. The short process attaches to the cavity wall.
Stapes: consists of a head, two crura (anterior and posterior), and a footplate that fits into the oval window. The head articulates with the incus.

Notable Integrations and Significance

The sphenoid bone is the “keystone” of the cranial floor due to its extensive articulation with all other cranial bones, stabilizing the skull’s architecture.
The sella turcica houses the pituitary gland, a critical endocrinological structure, defensively protected within this bony saddle.
The orbital and nasal cavities, along with the paranasal sinuses, are intimately associated with facial expression, respiration, and voice resonance.
The hyoid bone, while not part of the skull proper, is essential for the mechanics of swallowing and speech due to its role as an anchoring structure for tongue and laryngeal muscles.
Fontanelles and sutures permit rapid skull growth in infants and allow for deformation during birth and brain growth. Their progression to fusion marks developmental milestones.

Quick Reference: Key Landmarks by Bone (highlights)

Frontal: supraorbital margin/notch, glabella, frontal sinuses, coronal suture with parietals.
Occipital: foramen magnum, external occipital protuberance, superior and inferior nuchal lines, occipital condyles, hypoglossal canal.
Parietal: sagittal, coronal, lambdoid, squamous sutures; superior and inferior temporal lines.
Temporal: external acoustic meatus, mastoid process, styloid process, zygomatic process, mandibular fossa, petrous portion.
Sphenoid: body, greater and lesser wings, pterygoid processes, sella turcica, foramina (rotundum, ovale, spinosum).
Ethmoid: cribriform plate with olfactory foramina, crista galli, perpendicular plate, ethmoidal air cells, superior and middle nasal conchae.
Maxilla: orbital surface, infraorbital foramen, alveolar margin, palatine process, frontal process, zygomatic process.
Mandible: body, ramus, angle, condylar process, coronoid process, mental foramen, alveolar process, mandibular notch.
Zygomatic: frontal and temporal processes forming the cheek prominence and part of the orbit.
Lacrimal: medial orbital wall, lacrimal fossa.
Nasal: nasal bridge, frontonasal and inter-nasal sutures.
Vomer: inferior nasal septum, perpendicular plate, articulates with sphenoid and ethmoid.
Palatine: horizontal and vertical plates forming posterior hard palate and part of the nasal cavity and orbital floor.
Inferior nasal concha: lateral wall of nasal cavity, aids in air conditioning.
Hyoid: body, greater horns, lesser horns.
Auditory ossicles: malleus, incus, stapes with their respective parts and connections to the tympanic membrane and oval window.

Note: Several figures in the source labeling include a watermark or signature (“ADAM”) and additional figure-specific annotations. The textual content above consolidates the anatomical information and common descriptive terms relevant for exam study and practical understanding of skull anatomy.