Next Lesson - The Temporomandibular Joint (TMJ)
Core
The skull, or cranium, is the bony skeleton of the head. It is made up of 22 individual bones which are joined by fibrous joints called sutures. Most of these bones form by intramembranous ossification (bone forming directly within a sheet of connective tissue). Parts of the cranial base (the floor of the skull) develop instead by endochondral ossification (bone replacing a cartilage model).
The skull can be broadly divided into two parts:
- Neurocranium (8 bones) - this is the part which encases the brain. It has a roof (calvaria), a floor and a cranial cavity (which holds the brain). Its 8 bones are the frontal, occipital, sphenoid and ethmoid bones, plus two parietal and two temporal bones. The sphenoid is a butterfly-shaped bone in the middle of the skull base, and the ethmoid is a delicate bone between the eyes.
- Viscerocranium (14 bones) - this is the skeleton of the face. It supports the soft tissue of the face and houses the eyes, oral cavity and upper respiratory tract.
The facial skeleton is made up of 14 bones:
- Zygomatic Bones (2) - these are the cheekbones. They articulate (form joints) with the frontal, sphenoid and temporal bones and the maxillae.
- Lacrimal Bones (2) - these are the smallest bones of the facial skeleton, roughly the size of a fingernail. They each have a groove for the nasolacrimal duct (the tear duct, which drains tears into the nose).
- Nasal Bones (2) - these form the bony bridge of the nose.
- Palatine Bones (2) - these form part of the hard palate, the bony roof of the oral cavity (mouth).
- Inferior Nasal Conchae (2) - these protrude into the nasal cavity and enhance the warming and humidification (moistening) of inspired (breathed-in) air by increasing surface area.
- Vomer Bone - this forms part of the nasal septum posteriorly.
- Maxillae (2) - these bones make up the majority of the upper facial skeleton. Together they form the upper jaw, which is fixed to the cranial base. They also form part of the hard palate.
- Mandible - this U-shaped bone forms the lower jaw. It is moveable as it articulates with the cranial base at the temporomandibular joint (TMJ).
Diagram - Osteology of the viscerocranium
Public Domain Source by العربيَّة [Public domain]
The calvaria is the roof of the neurocranium. It comprises 4 bones: frontal, occipital and two parietal bones.
The bones of the calvaria are joined by sutures. Important suture lines to note are:
- Coronal Suture - joins the frontal bone with the two parietal bones.
- Sagittal Suture - joins the two parietal bones in the midline.
- Lambdoid Suture - joins the occipital bone to the two parietal bones.
Diagram - Sutures of the skull: 1) Coronal; 2) Sagittal; 3) Lambdoid
Public Domain Source by Julius Kollmann [Public domain]
In infants, the skull is immature, leaving membranous gaps (areas of soft fibrous tissue) between plates of ossified (hardened) bone. This permits enlargement of the skull in line with the growth of the brain. These unossified areas are termed fontanelles: there is an anterior fontanelle and posterior fontanelle. These fontanelles typically close at about 12-20 months and 1-3 months, respectively, although the exact timing varies between healthy infants.
The areas of the adult skull which correspond to the previous locations of the anterior and posterior fontanelles are called bregma and lambda, respectively.
Diagram - The anterior and posterior fontanelles
Creative commons source by BruceBlaus, edited by Dr. Joshua Bray [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The bone of the calvaria is arranged in 3 layers:
- Inner Table - the innermost layer of cortical (dense, compact) bone.
- Diploe - the middle layer composed of spongy bone.
- Outer Table - the outermost layer of cortical bone.
This trilaminar (three-layered) arrangement ensures adequate protective strength without excessive weight.
The outer table of bone is covered with periosteum (a layer of connective tissue). The periosteum is bound to the suture line. The inner table is lined by the dura mater (the tough outer membrane around the brain), specifically its endosteal layer, which is the layer next to the bone. The dura also folds inwards to form partitions that run across the cranial cavity.
The cranial floor (also called the cranial base) is the inferior portion of the neurocranium which the brain rests upon. It is formed by parts of the frontal, ethmoid, sphenoid, temporal and occipital bones.
The cranial floor is anatomically divided into three depressions known as the cranial fossae:
- Anterior Cranial Fossa - bounded posteriorly by the lesser wings (thin, wing-shaped projections) of the sphenoid. Its floor consists of the frontal, ethmoid and sphenoid bones. It holds the frontal lobe of the brain. The ethmoid bone has a projection called the crista galli. This serves as an attachment for the falx cerebri (the sickle-shaped fold of dura mater between the two halves of the brain).
- Middle Cranial Fossa - bounded anteriorly by the lesser wings of the sphenoid and posteriorly by the petrous (dense, rock-like) part of the temporal bones. Its floor consists of the sphenoid and temporal bones. It holds the temporal lobes of the brain. The sphenoid bone has a saddle-shaped depression called the sella turcica, which holds the pituitary gland.
- Posterior Cranial Fossa - bounded anteriorly by the petrous part of the temporal bones. Its floor is made up of the temporal and occipital bones. It accommodates the brainstem and cerebellum.
Diagram - The cranial fossae
Creative commons source by Anatomist90, edited by Dr. Joshua Bray [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The cranial floor contains a series of canals, foramina (holes) and fissures (narrow slits) which transmit (let pass through) important structures such as arteries, veins and nerves. These are summarised in the table below. Please read the table with these corrections: the abducens nerve (the nerve that moves the eye outwards) is cranial nerve VI (not XI), the facial nerve is cranial nerve VII (not XII) and the vestibulocochlear nerve (the nerve for hearing and balance) is cranial nerve VIII (not XIII). The table also leaves out the vertebral arteries, which pass through the foramen magnum (the largest opening in the skull base, in the occipital bone) together with the structures it lists.
Table - Summary of the cranial foramina and their contents
SimpleMed original by Dr. Joshua Bray
Diagram - Openings of the cranial floor
Creative commons source by OpenStax College, edited by Dr. Joshua Bray [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
For more information about the cranial nerves, check out our articles: Cranial Nerves I-VI and Cranial Nerves VII-XII.
The skull is a very strong structure and so skull fractures require significant blunt trauma or a penetrating injury.
Fractures of the calvaria (also called the cranial vault) can be either linear or depressed:
- Linear - a full thickness crack in the bone. There are often multiple fracture lines radiating from the site of injury.
- Depressed - fracture of the skull where the bony fragments displace inwards, resulting in an indentation. These fractures are more likely to result in brain injury.
A relatively thin area of bone is located on the lateral aspect of the skull and is known as the pterion. Anatomically it is defined as the junction between the frontal, parietal, temporal, and sphenoid bones. Closely underlying the pterion is the middle meningeal artery (the artery that supplies the meninges, the membranes around the brain). Therefore, blunt trauma to the lateral head can fracture the pterion and tear the middle meningeal artery, resulting in an extradural haemorrhage (bleeding between the skull and the dura mater).
Diagram - Skull with the pterion circled
Public Domain Source by LadyofHats Mariana Ruiz Villarreal, edited by Dr. Joshua Bray [Public domain]
Fractures of the cranial floor (basal skull fractures) are linked to more severe head injuries and must not be missed. They may present with specific signs. Any one of them is a reason for an urgent CT head scan (NICE advises a scan within 1 hour). Some signs take time to appear: bruising around the eyes typically develops 1 to 3 days after the injury, so a normal examination soon after the injury does not rule out a fracture. The signs are:
- CSF Rhinorrhoea (fluid running from the nose) - fracture of the cribriform plate (the thin, sieve-like roof of the nose) of the ethmoid bone can cause cerebrospinal fluid (CSF) to leak into the nasal cavity.
- Periorbital Ecchymoses (bruising around the eyes, called ‘Raccoon Eyes’ or, in NICE guidance, ‘panda eyes’) - fracture of the anterior cranial fossa can cause blood to enter the orbit (eye socket) and collect in the periorbital tissue. This bruising points to a fracture when there is no direct injury around the eyes.
- CSF Otorrhoea (fluid running from the ear) - fracture through the petrous part of the temporal bone can cause CSF to leak into the middle ear and into the ear canal.
- Haemotympanum - fracture through the petrous bone can cause blood to collect in the middle ear.
- Battle’s Sign - fracture through the petrous bone can result in bruising around the mastoid process (the bony lump behind the ear).
Fractures of the facial skeleton are common. The nasal bones are most susceptible due to their prominent nature on the face. The zygomatic arches and mandible are also commonly affected. The mandible is commonly fractured at more than one site due to its ring-like structure, often involving two fracture points.
Image - Orthopantomogram (a panoramic X-ray of the jaws) showing a fracture through the body of the mandible (indicated by the white arrow)
Creative commons source by Coronation Dental Specialty Group [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
- 7576
.svg){kind=link}
