Temporal bone anatomy: squamous, petrous, tympanic and mastoid parts

What it is

A paired bone forming part of the side and base of the skull.

Location

Lies below the parietal bone, articulating with the occipital, sphenoid, parietal and zygomatic bones and the mandible.

Function

Houses the middle and inner ear and forms the temporomandibular joint.

Body system

Skeletal · paired (left and right)

FMA ID

FMA52739 (Foundational Model of Anatomy)

Key facts

• It has squamous, tympanic, petrous and mastoid parts and a styloid process.

Clinical relevance

Fractures of the petrous part can damage the facial nerve or hearing structures.

There are two temporal bones, one on each side of the skull. Each forms part of the side wall and base of the cranium, lateral to the temporal lobe of the brain. The temporal bone below and the parietal bone above meet at the squamous suture. (Radiopaedia: Temporal bone.)

In the skull as a whole, the temporal bone forms the lower lateral part of the skull. Its petrous portion houses the structures of the middle and inner ear, and its external acoustic meatus is the bony opening of the ear canal. (OpenStax Anatomy and Physiology 2e, 7.2: The Skull.)

The temporal bone is usually divided into a squamous part, a petromastoid part (made of the petrous and mastoid parts), a tympanic part and the styloid process. Kenhub describes the temporal bones as highly irregular bones with extensive muscle attachments. (Kenhub: Temporal bone.)

Radiopaedia notes that the mastoid part is not a separate piece embryologically: it forms from both the squamous and petrous parts. The mandibular fossa, the socket for the jaw joint, is formed in front by the squamous part and behind by the tympanic part. Small fissures between the parts, such as the petrotympanic (Glaserian) fissure, remain visible in the adult bone. (Radiopaedia: Temporal bone.)

The squamous part is the largest part: a flat, plate-like area whose convex outer surface forms part of the temporal fossa. Its lower part gives origin to temporalis. It meets the sphenoid in front and the parietal bone above. (TeachMeAnatomy: The Temporal Bone.)

The zygomatic process arises from the lower squamous part and projects forwards to join the temporal process of the zygomatic bone, forming the zygomatic arch (cheekbone arch). Masseter attaches to its lateral surface. The root of the zygomatic process forms the articular tubercle, the front boundary of the mandibular fossa. (TeachMeAnatomy: The Temporal Bone.)

The head of the mandible sits in the mandibular fossa to form the temporomandibular joint (TMJ). This is the only synovial joint of the skull; the other skull bones are joined by fibrous sutures or cartilaginous joints. (Kenhub: The skull.)

The petrous part (petrous portion, petrous pyramid, or petrous temporal bone) is a wedge-shaped part of the skull base between the sphenoid and occipital bones. It is pyramid-shaped, with a medial apex, a lateral base, three surfaces and three angles. Its upper (superior) angle carries the superior petrosal sinus and gives attachment to the tentorium cerebelli. (Radiopaedia: Petrous part of the temporal bone.)

The petrous ridge separates the middle and posterior cranial fossae. On its anterior surface, the arcuate eminence is raised by the superior semicircular canal, and the tegmen tympani forms a thin roof over the middle ear. The carotid canal carries the internal carotid artery through the petrous part into the cranial cavity. The apex holds the front opening of this canal and forms the back edge of the foramen lacerum. (Kenhub: Temporal bone.)

On the posterior surface, the internal acoustic meatus opens into the posterior cranial fossa. It carries the facial nerve (CN VII), the vestibulocochlear nerve (CN VIII) with the vestibular ganglion, and the labyrinthine artery to the inner ear. The jugular foramen lies between the petrous part and the occipital bone. (TeachMeAnatomy: Foramina of the Skull.)

The mastoid process is the cone-shaped projection of the mastoid part that you can feel behind your ear. It is absent at birth. It develops from infancy until puberty, its front third coming from the squamous part and its back two-thirds from the petrous part. Mastoid air cells form in about 80% of people by the age of 4, and the amount of air varies a lot between people. (Radiopaedia: Mastoid process.)

Several muscles attach to it. On its outer surface, from top to bottom, are the occipital belly of occipitofrontalis, posterior auricular, sternocleidomastoid, splenius capitis and longissimus capitis. The posterior belly of digastric attaches on its medial side, in the mastoid notch. On the inside of the skull, the sigmoid sulcus behind the mastoid holds the sigmoid sinus. (Radiopaedia: Mastoid process.)

The tympanic part is a curved plate just below the root of the zygomatic process. It forms the front wall, the floor and part of the back wall of the external acoustic meatus. The styloid process is a narrow, pointed projection that runs downwards and forwards from the under surface of the bone, about 2.5 cm long on average. (Kenhub: Temporal bone.)

The styloid process anchors three muscles, styloglossus, stylohyoid and stylopharyngeus, and two ligaments, the stylohyoid and stylomandibular ligaments. These structures all come from the second pharyngeal (branchial) arch. A styloid process longer than 30 mm is classed as elongated, a finding associated with Eagle syndrome. (Radiopaedia: Styloid process.)

The facial nerve enters the temporal bone through the internal acoustic meatus, runs in the facial canal through the petrous part, and leaves the skull through the stylomastoid foramen, which lies between the styloid and mastoid processes. The stylomastoid artery passes through the same foramen. (Kenhub: Temporal bone.)

The temporal bone joins the parietal bone at the squamous and parietomastoid sutures, the sphenoid at the sphenosquamosal and sphenopetrosal joints, the occipital bone at the occipitomastoid suture and petro-occipital fissure, and the zygomatic bone at the zygomaticotemporal suture. It also forms the temporomandibular joint with the mandible. (Radiopaedia: Temporal bone.)

Middle ear infection (otitis media) can spread into the mastoid air cells, causing mastoiditis. From there infection can reach the middle cranial fossa and cause meningitis. Treatment may involve draining the air cells while protecting the facial nerve. (TeachMeAnatomy: The Temporal Bone.)

Temporal bone fractures make up about 20% of skull fractures. They are classed by their direction relative to the long axis of the petrous part (longitudinal, transverse or mixed) and, more usefully, by whether they break through the otic capsule around the inner ear. Fractures that do are 2 to 5 times more likely to paralyse the facial nerve and 7 to 25 times more likely to cause sensorineural hearing loss. Bruising behind the ear (Battle sign) and bleeding from the ear canal suggest a temporal bone fracture. (Radiopaedia: Temporal bone fractures.)

The squamous part also forms one corner of the pterion, where it meets the frontal, parietal and sphenoid bones. This is one of the weakest areas of the skull, and a fracture here can tear the middle meningeal artery, causing an extradural haematoma. (TeachMeAnatomy: The Temporal Bone.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Inferior temporal gyrus · nervous
• Fusiform gyrus · nervous
• Parahippocampal gyrus · nervous
• Hippocampus · nervous
• Anterior choroidal artery · cardiovascular
• Amygdala · nervous
• Superior cerebellar artery · cardiovascular
• Lateral geniculate body · nervous
• Ophthalmic nerve · nervous
• Postcommunicating part of posterior cerebral artery · cardiovascular