7.2 The Skull

The skull is a complex structure that protects our brain and houses sensory organs. It's made up of numerous bones that fit together like a puzzle, forming the brain case and facial skeleton. These bones are connected by sutures, which allow for slight movement and growth.

Inside the skull, we find various compartments and spaces that accommodate different structures. The cranial fossae house parts of the brain, while the orbits protect our eyes. The nasal cavity and paranasal sinuses play crucial roles in breathing and voice production.

Bones and Sutures of the Skull

Bones of skull and face

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• Brain case (neurocranium) encloses and protects the brain
  • Frontal bone forms the forehead and superior portion of the eye sockets (orbits)
  • Parietal bones (2) form the superior and lateral walls of the cranial cavity
  • Temporal bones (2) form the inferior lateral walls of the cranial cavity and contain the structures of the middle and inner ear
  • Occipital bone forms the posterior and inferior portions of the cranial cavity and has a large opening (foramen magnum) for the spinal cord
  • Sphenoid bone forms part of the base of the cranial cavity and has greater and lesser wings that contribute to the orbit walls
  • Ethmoid bone forms part of the anterior cranial fossa, the medial walls of the orbits, and the superior portion of the nasal septum
• Facial bones (viscerocranium) form the structure of the face
  • Maxillae (2) form the upper jaw and the anterior portion of the hard palate
  • Palatine bones (2) form the posterior portion of the hard palate
  • Zygomatic bones (2) form the cheekbones and contribute to the lateral walls of the orbits
  • Lacrimal bones (2) form part of the medial walls of the orbits and contain the nasolacrimal ducts
  • Nasal bones (2) form the bridge of the nose
  • Vomer forms the inferior portion of the nasal septum
  • Inferior nasal conchae (2) are thin, curved bones that project into the nasal cavity
  • Mandible is the lower jaw bone and the only movable bone of the skull

Suture lines and adjacent bones

• Coronal suture is a transverse suture that runs between the frontal and parietal bones
• Sagittal suture is a midline suture that runs between the two parietal bones
• Lambdoid suture is a posteriorly positioned suture that runs between the parietal and occipital bones
• Squamous sutures are located on the lateral sides of the skull between the parietal and temporal bones
• Occipitomastoid sutures are located posteriorly between the occipital bone and the mastoid portions of the temporal bones
• Cranial sutures are fibrous joints that connect the bones of the skull
  • Fontanelles are soft spots where cranial sutures intersect in infants, allowing for brain growth

Skull Structure and Development

• Calvaria, or skullcap, is formed by the frontal, parietal, and occipital bones
• Diploe is the spongy bone layer between the inner and outer tables of compact bone in flat skull bones
• Craniometry is the measurement of skull dimensions
  • Cephalic index is a ratio of skull width to length, used to classify skull shapes
  • Facial angle measures the slope of the face in relation to the cranium

Cranial Fossae, Temporal Regions, and Paranasal Sinuses

Cranial fossae and temporal regions

• Anterior cranial fossa is the most superior fossa and is bounded by the frontal bone anteriorly, the lesser wings of the sphenoid laterally, and the cribriform plate of the ethmoid posteriorly. It contains the frontal lobes of the brain
• Middle cranial fossa is situated posterior to the anterior fossa and is bounded by the greater wings of the sphenoid laterally, the temporal bones inferolaterally, and the anterior portion of the occipital bone posteriorly. It contains the temporal lobes of the brain and the pituitary gland
• Posterior cranial fossa is the most inferior fossa and is bounded by the occipital bone posteriorly, the temporal bones laterally, and the posterior portion of the sphenoid bone anteriorly. It contains the cerebellum, pons, and medulla oblongata
• Infratemporal fossa is located below the level of the zygomatic arch and contains the muscles of mastication (temporalis, masseter, medial and lateral pterygoids), the maxillary artery, and the mandibular nerve (V3)
• Pterygopalatine fossa is a small space located between the maxilla and the pterygoid process of the sphenoid bone. It contains the pterygopalatine ganglion, the maxillary nerve (V2), and the terminal branches of the maxillary artery

Paranasal sinuses: location and function

• Frontal sinuses are located within the frontal bone, superior to the orbits. They drain into the middle meatus of the nasal cavity
• Maxillary sinuses are the largest paranasal sinuses and are located within the maxillae, inferior to the orbits. They drain into the middle meatus of the nasal cavity
• Ethmoid air cells are numerous small sinuses located within the ethmoid bone, between the orbits. They drain into the superior and middle meati of the nasal cavity
• Sphenoid sinuses are located within the body of the sphenoid bone, posterior to the ethmoid air cells. They drain into the sphenoethmoidal recess of the nasal cavity
• Functions of paranasal sinuses:
  1. Lighten the weight of the skull by reducing the amount of bone mass
  2. Humidify and warm inspired air as it passes through the nasal cavity
  3. Serve as resonating chambers for voice production, contributing to the quality and timbre of the voice
  4. Provide mucus to keep the nasal cavity moist and trap inhaled particles and pathogens

Orbits, Nasal Cavity, and Foramina

Orbit structure and contents

• Bony structure of the orbit:
  • Frontal bone forms the roof of the orbit
  • Maxilla and zygomatic bone form the floor of the orbit
  • Ethmoid and lacrimal bones form the medial wall of the orbit
  • Greater and lesser wings of the sphenoid and the zygomatic bone form the lateral wall of the orbit
• Openings of the orbit:
  1. Optic canal transmits the optic nerve (CN II) and the ophthalmic artery
  2. Superior orbital fissure transmits the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves; the ophthalmic division of the trigeminal nerve (V1); and the ophthalmic vein
  3. Inferior orbital fissure transmits the maxillary nerve (V2) and the infraorbital vessels
• Contents of the orbit include the eyeball, extraocular muscles (superior, inferior, medial, and lateral rectus; superior and inferior oblique), and various nerves and vessels that supply the structures of the orbit

Anatomy of nasal cavity

• Nasal septum is a midline partition that divides the nasal cavity into right and left halves. It is formed by the perpendicular plate of the ethmoid bone superiorly and the vomer inferiorly
• Nasal conchae are scroll-like projections from the lateral walls of the nasal cavity that increase the surface area for humidification and warming of inspired air
  • Superior nasal concha is the smallest and most superiorly located
  • Middle nasal concha is larger and is located inferior to the superior concha
  • Inferior nasal concha is the largest and most inferiorly located

Foramina and openings in skull

• Foramen magnum is the largest opening in the skull, located in the occipital bone. It transmits the medulla oblongata, vertebral arteries, and spinal accessory nerves (CN XI)
• Jugular foramen is located between the occipital bone and the petrous part of the temporal bone. It transmits the glossopharyngeal (CN IX), vagus (CN X), and spinal accessory (CN XI) nerves, as well as the internal jugular vein
• Stylomastoid foramen is located in the temporal bone, posterior to the styloid process. It transmits the facial nerve (CN VII)
• Foramen ovale is located in the greater wing of the sphenoid bone. It transmits the mandibular division of the trigeminal nerve (V3)
• Foramen rotundum is located in the greater wing of the sphenoid bone, anterior to the foramen ovale. It transmits the maxillary division of the trigeminal nerve (V2)
• Foramen spinosum is located in the greater wing of the sphenoid bone, posterior to the foramen ovale. It transmits the middle meningeal artery
• Carotid canal is located in the petrous part of the temporal bone. It transmits the internal carotid artery into the cranial cavity