7.2: The Skull

Learning Outcomes

  1. Describe the location, structural features, and functions of each of the cranial bones, and the main sutures that unite them

  2. Describe the location, structural features, and functions of each of the facial bones and the hyoid bone

  3. Explain the structural features and functions of the orbit, nasal cavity, and paranasal sinuses

  4. Compare and contrast the skull of a fetus or infant to that of an adult

Overview of Skull structure

  • 2 Groups of bones: Facial and cranial

  • 8 Cranial bones:

    • Four Single bones

      • frontal

      • occipital

      • ethmoid

      • sphenoid

    • two paired bones:

      • temporal

      • parietal

  • 14 facial bones

    • Paired

      • maxillary

      • zygomatic

      • nasal

      • lacrimal

      • paltine

      • inferior nasal conchal bones

    • Unpaired

      • Mandible

      • vomer

  • Besides the mandible, all skull bones are united through sutures

  • 8 cranial bones form the cranial cavity, which is divided into:

    • cranial vault or calvaria (superior)

    • cranial base (inferior)

      • divided into anterior, middle and posterior cranial fossae

        Illustration of cranial cavity, with labels

Smaller cavities

  • The orbits contain the eyeballs

  • Nasal cavity holds the sensory receptors for smell

  • oral cavity surrounds the mouth

  • Other cavities house sensory organs for hearing and balance

  • Many bones contain air filled sacs called sinuses

  • four bones around the nasal cavity have large sinuses called paranasal sinuses

Cranial Bones

  • Pneumonic:

  • PEST OF 6

Parietal Bones

  • The two bones meet at the sagittal suture to form the lateral wall

  • Meet the frontal bone at the coronal suture

  • Each bone meets a temporal bone at the squamous suture

  • Both meet the occipital bone at the lambdoid suture

skull sutures parietal bone frontal occipital temporal

Ethmoid Bone

  • Is anterior, between the eyes, and posterior the nasal bones

  • the cribriform plate forms the roof of the nasal cavity, which have holes in it called cribriform (olfactory) foramina that have nerves that detect smell

  • The Crista galli is superior to the plate, where membranes from the brain attach

  • Inferior to the crista galli is the perpendicular plate, forming part of the nasal septum

Sphenoid bone

  • articulates with every cranial bone

  • anterior view looks like a bat, superior view looks like a stingray

  • 4 main components:

    • Central Body:

      • Hollow containing sphenoidal sinuses

      • anterior of central body are 2 optic foramina, that lead to 2 optic canals

      • On superior surface, has saddle-shaped depression called sella turcica, which houses the pituitary gland

    • Greater wings:

      • extend laterally from the body

      • wings of the bat

      • with temporal bone, form part of the middle cranial fossae

      • with frontal bone, form part of the posterior walls orbit

      • 3 foramina for blood vessels and nerves:

        • anterior foramen rotundum

        • middle foramen ovale

        • posterior foramen spinosum

    • Lesser wings:

      • Wings of stingray

      • project from the superior surface of the body

      • in the anterior can be found superior orbital fissures, a slit between the lesser and greater wings, which contain nerves for eye movement

This image shows the location and structure of the sphenoid bone. A small image of the skull on the top left shows the sphenoid bone highlighted in ochre yellow. The top panel shows the superior view of the sphenoid bone and the bottom panel shows the posterior view of the sphenoid bone.

Temporal Bones

  • forms lateral region

  • 4 regions:

    • Squamous:

      • broad flat surface

      • has the zygomatic process, which forms part of the zygomatic arch and mandibular fossa (where the mandible attaches)

    • Tympanic:

      • has external acoustic meatus

      • inferior to the external acoustic meatus is the styloid process

    • Mastoid:

      • Is thick and located posterior and lateral to the styloid process

      • filled with tiny sinuses called mastoid are cells

    • Petrous:

      • internal surface of the temporal bone, where it forms part of the middle cranial fossa

      • contains 4 important features:

        • internal acoustic meatus

          • a canal leading to the middle ear

        • jugular foramen

          • posterior opening where temporal bone meets the occipital bone, through which the internal jugular vein and nerves pass through

        • carotid canal

          • internal carotid artery passes and enters the cranial cavity

        • foramen lacerum

          • Allows passage of small blood vessels and nerves

  • Test yourself - Temoporal bone lateral view labled

Occipital Bone

  • On anterior and inferior surface is the foramen magnum, where the spinal cord passes through

  • on the sides of the foramen magnum are the occipital condyles, which articulate with the first cervical vertebra

  • The posterior surface has two parallel ridges the superior and inferior nuchal lines

  • The external occipital protuberance runs through the superior nuchal line and continues as external occipital crest

Frontal bone

  • forms the frontal squama

  • inside is the frontal sinuses, which are part of the paranasal sinuses

  • Has 3 features:

    • supraorbital margins

      • sharp ridges forming the superior and superomedial boundaries of the orbit

    • supraorbital foramen

      • middle of the supraorbital margin

      • allows blood vessels and nerves to forehead

    • glabella

      • smooth region between the 2 supraorbital margins

Anterior view illustration of the frontal bone.

Facial bones

  • Pneumonic:

  • Virgil Is Now Making My Pet Zebra Laugh

Vomer bone

  • Deepest facial bone

  • forms the inferior and posterior of the nasal septum

inferior nasal conchae

  • Located in the lateral walls of the nasal cavity, inferior to the middle nasal conchae of the ethmoid bone

Nasal bones

  • form the bridge of the nose

  • articulate with the hyaline cartilage that forms the nose

Mandible

  • only moveable bone in the skull

  • consists of the mandibular body, and the mandibular rami (ramus = branch)

  • mandibular body forms the chin and meets the rami at the mandibular angles

  • on the inside of the body, on each side, is a small mental foramen

  • the bottom teeth are housed in deep sockets, dental alveoli, within a ridge called the alveolar process

  • each ramus is topped with a u-shaped mandibular notch, inferior to which is the mandibular foramen

  • Anterior side of the mandibular notch is the coronoid process, which serves as an attachment point for muscles of mastication

  • Posterior side has the condylar process, at the top of which is the mandibular condyle, which articulates the mandibular fossa, to form the temporomandibular joint

  • Mandible bone - Lateral view - Test yourself - Answers

Maxilla

  • the maxillae fusion creates a bony midline protrusion called the anterior nasal spine

  • medial the alveolar processes, they form 2 horizontal palatine processes, meeting at the intermaxillary suture

  • the 2 processes form the anterior of the palate

  • The anterior and superior surfaces of the maxillary bone form part of the inferomedial walls of the orbit

  • immediately below the inferior margin, arteries and nerves pass through the inferior orbital fissure and the infraorbital foramen, around which you find large maxillary sinuses

  • laterally articulates with with zygomatic bones via zygomatic process

Palatine bones

  • 2 L-shaped bones

  • posterior nasal cavity, between the maxillae and pterygoid process of the sphenoid

  • each bone is made of the perpendicular plate and the horizontal plate

  • Horizontal forms the posterior part of the hard palate

  • perpendicular are vertical projections that form part of the lateral walls of the nasal cavity

zygomatic bones

  • Along the zygomatic process of the temporal and maxillae

  • Form majority of cheekbone or zygomatic arch

  • form lateral wall and part of inferior border of the orbit

lacrimal bones

  • Smallest and most delicate facial bones

  • deep in the medial wall of the orbit

  • contain a depression called the lacrimal fossa, where tears drain

Cavities of the Skull

Orbit

  • Houses eyeball, and lacrimal gland (produces tears)

  • formed by 7 bones

    • Frontal forms the superior and posterosuperior walls

    • maxilla forms the posteroinferior wall (partly along with the palatine bone)

    • zygomatic bone forms the anterolateral wall

    • sphenoid forms posterior wall

    • ethmoid, lacrimal, and palatine bones form the medial wall

Nasal Cavity

  • Posterior wall:

    • formed by the sphenoid body and pterygoid process

  • lateral walls:

    • made of ethmoid, perpendicular plate of palatine bone, inferior nasal conchae, and maxilla

  • roof and floor:

    • cribriform plate of ethmoid bone forms roof

    • floor is hard palate, made of palatine bones and palatine process of maxillae

  • anterior wall:

    • nasal bones and maxillae form it

    • cartilage and connective tissue of nose attach to the margins of the bones

  • nasal septum:

    • nasal cavity is divided into 2, each being a nasal fossa, divided by nasal septum

    • anteriorly made of hyaline cartilage

    • posteriorly made of perpendicular plate of ethmoid bone and vomer

Paranasal Sinuses

  • Include sinuses within frontal, sphenoid, ethmoid, and maxillary bones

  • lined with mucous membranes

  • lighten the skull and aid in voice resonance

  • Sinuses can be blocked from excessive mucous, causing sinus headaches, which the pain relates to the areas of the sinuses

    • above the eyes: frontal sinuses

    • between the eyes: ethmoidal sinuses

    • behind the eyes: sphenoidal sinuses

    • below the eyes: maxillary sinuses

Oral Cavity

  • roof is formed by hard palate, the same of the floor of the nasal cavity

  • anterior and lateral walls are maxillae and mandible

  • floor and posterior wall are soft tissue

Fetal Skull

  • Soft spots are fontanels, resulting from ossification not being completed, and the skull not fully solidifying

  • Fontanel spots

    • anterior fontanel

      • where the coronal and sagittal sutures meet

      • between the frontal and parietal bones

    • posterior fontanel

      • between the parietal and occipital bones at the apex of the lambdoid suture

    • sphenoid fonanel

      • temples, where the sphenoid bones meet other cranial bones

    • mastoid fontanels

      • junction of the lambdoid and squamous sutures

      • where the parietal, temporal and occipital bones meet

  • Sutures between bones are not fully fused

Hyoid Bone

  • small C-shaped bone (not an actual bone) in the superior neck

  • suspended by muscles and ligaments that attach to the styloid process and the larynx

  • important as it supports many bones for swallowing and speech