The Temporal Bone
The temporal bones are a pair of bilateral, symmetrical bones that constitute a large portion of the lateral wall and base of the skull . They are highly irregular bones with extensive muscular attachments and articulations with surrounding bones.
There are a number of openings and canals in the temporal bone through which structures enter and exit the cranial cavity. The temporal bone also houses the structures forming the middle and inner ear.
When the skull is viewed from a posterior aspect, the temporal bones can be seen on the lateral extremities of the skull, with a rounded prominence, the mastoid process, being the most prominent feature visible. The temporal bone is composed of several parts, these being:
- the squamous part
- the zygomatic process
- the petromastoid part
- the tympanic part
- the styloid process
Divisions of the Temporal Bone
The squamous part is the anterior superior portion of the temporal bone that forms the lateral part of the middle cranial fossa. It has the appearance of a large flattened plate. Its external surface is smooth and slightly convex. Above the external acoustic meatus, there is a groove on the external surface of the bone for the middle temporal artery. The internal surface of the squamous part is concave shaped. Its surface has impressions that follow the groove and contour of the temporal lobe of the cerebrum that rests against it. There is also a groove on this surface for the middle meningeal vessels.
There is an anterior projection from the squamous part of the temporal bone, the zygomatic process. This is located on the lower portion of the squamous part. It initially projects laterally, then turns to pass anteriorly to articulate with the temporal process of the zygomatic bone to form the zygomatic arch. There is a small tubercle inferior to the zygomatic process at its root: the articular process.
The mandibular notch (fossa), the socket for articulation of the temporal bone with the head of the mandible , is also present on the squamous part of the temporal bone.
This part of the temporal bone is usually split into two: the petrous part and the mastoid part.
The mastoid part is the most posterior part of the temporal bone. Its outer surface is roughened by muscular attachments. There is a downward conical projection called the mastoid process from the mastoid part.
A number of muscles are attached to the mastoid process, these being the sternocleidomastoid , splenius capitis and longissimus capitis muscles. There is a depression on the medial surface of the mastoid process, the mastoid notch, onto which the posterior belly of the digastric is inserted. The medial surface of the mastoid process has a deep groove called the sigmoid sulcus, in which the sigmoid sinus is located.
The petrous part is a wedge shaped mass of bone located between the sphenoid and occipital bones within the cranial cavity. It is the most medial part of the temporal bone, and it is the landmark dividing the middle and posterior cranial fossae from each other. It has a base, an apex and three surfaces: anterior, posterior and inferior. It also has three borders: superior, anterior and posterior. The acoustic labyrinth is located within the petrous part.
The anterior region of the petrous part forms the posterior limit of the middle cranial fossa. Its internal surface is grooved by the inferior temporal gyrus, as well as the trigeminal ganglion. The trigeminal impression is separated from another hollow posteriorly by a bony ridge. This hollow partially encloses the internal acoustic meatus and the cochlea. The ridge is limited posteriorly by the arcuate eminence, which is raised superiorly by the superior semicircular canal. The lateral aspect of the petrous part articulates with the squamous part of the temporal bone. Between this articulation laterally and the arcuate eminence medially, is a thin plate of bone called the tegmen tympani. This bone forms the roof of the middle ear. The lateral and posterior semicircular canals lie deep to the posterior slope of the arcuate eminence.
The posterior area of the petrous part of the temporal bone contributes to the anterior limit of the posterior cranial fossa. It is continuous with the internal portion of the mastoid part. There is a depression below the arcuate eminence, the subarcuate fossa, which lies above the opening of the internal acoustic meatus.
The inferior surface is highly irregular and contributes to the external surface of the cranial base. Near the apex, there is a quadrangular area associated with the attachment of the levator veli palatini muscle and the pharyngotympanic tube. Posterior to this region is an opening for the carotid canal, and more posteriorly again is the jugular fossa.
The superior border is the longest border of the petrous part of the temporal bone. It is inside the cranial cavity, and is grooved by the superior petrosal sinus. The posterior border forms part of the fossa for the inferior petrosal sinus. The anterior border is attached laterally to the squamous part of the temporal bone; medially it articulates with the greater wing of the sphenoid bone. Two canals exit the skull at the junction between the petrous and squamous parts: the upper one containing the tensor tympani muscle, the lower one containing the pharyngotympanic tube.
The tympanic part of the temporal bone is a curved plate immediately below the origin of the zygomatic process. Its concave posterior surface forms the anterior wall, floor and part of the posterior wall of the external acoustic meatus. The external acoustic opening is clearly visible on this part.
The tympanic part of the temporal bone fuses with the petrous part internally, and the squamous and mastoid parts posteriorly. Its posterior surface forms the anterior wall, floor and part of the posterior wall of the external acoustic meatus. The anterior surface forms the posterior part of the mandibular fossa, and its lateral part forms part of the external acoustic meatus. The inferior border is sharp, and forms the vaginal process of the styloid process. The central region is thin and often perforated. The stylomastoid foramen lies between the styloid process and the mastoid process. It is the terminal end of the facial canal, transmitting the facial nerve and the stylomastoid artery.
The styloid process is a narrow, pointed projection that extends downwards and anteriorly from the inferior surface of the temporal bone.
Its length is variable, but is usually on average 2.5cm in length. It is usually straight, but can sometimes have a curvature, usually on the anterior surface. Its proximal part is enclosed in the tympanic plate; the distal end is the site of a number of muscles and ligaments. The parotid gland lies lateral to the styloid process; the facial nerve crosses its base; the external carotid artery passes through the parotid gland crossing the tip of the styloid process; and medially is the attachment of the stylopharyngeus muscle, separating the styloid process from the internal jugular vein.
The temporal bone articulates with a number of other flat bones of the skull at joints called sutures:
- the occipitomastoid suture separates the mastoid part of the temporal bone from the occipital bone posteriorly
- the squamosal suture separates the squamous part of the temporal bone from the parietal bone posteriorly and superiorly
- the sphenosquamosal suture separates the squamous part from the greater wing of the sphenoid bone anteriorly
- the zygomaticotemporal suture separates the zygomatic process of the temporal bone from the temporal process of the zygomatic bone, forming the zygomatic arch.
The temporal bone also articulates with the mandible at the temporomandibular joint . At this hinge joint, the rounded head of the mandible articulates with a socket formed by the mandibular fossa and the articular process of the temporal bone.
A number of muscles are attached to different features of the temporal bone. The temporalismuscle originates from the temporal fossa, which is formed partially by the lateral aspect of the temporal bone. The sternocleidomastoid, splenius capitis, longissimus capitis and digastric are all attached to the mastoid process of the temporal bone. Attaching to the styloid process are the:
- stylohyoid muscles
There are a number of openings in the temporal bone through which structures entering and exiting the cranial cavity pass.
- Anteromedially, the temporal bone forms the posterior boundary of the foramen lacerum, through which the greater petrosal nerve passes.
- The carotid canal is a passageway through which the internal carotid artery passes through the petrous part of the temporal bone to emerge in the middle cranial fossa.
- The internal acoustic meatus is another canal passing through the petrous part of the temporal bone between the posterior cranial fossa and the external acoustic meatus. Passing through this opening are the facial nerve (CN VII), the vestibulocochlear nerve (CN VIII) and the labyrinthine artery. The vestibulocochlear nerve terminates in the temporal bone. The facial nerve continuous outwards, exiting the temporal bone through the stylomastoid foramen.
- Posteriorly, the petrous part of the temporal bone forms the anterior extremity of the jugular foramen. Formed at the jugular foramen where the sigmoid sinus exits the skull is the internal jugular vein. There are also a number of other structures passing through this opening. These include three of the cranial nerves: the glossopharyngeal nerve (CN IX), the vagus nerve (CN X) and the accessory nerve (CN XI). The posterior meningeal artery also passes through this opening.
- There are also some smaller openings in the temporal bone. There are two hiatuses through which the greater and lesser petrosal nerves exit the geniculate ganglion in the facial canal. There is also a mastoid foramen, through which emissary veins and sometimes a branch of the occipital artery pass.
There is a region on the lateral aspect of the skull where the temporal, parietal, sphenoid and frontal bones converge. This is called the pterion, it is one of the weakest areas of the skull, and is thus susceptible to fracture. Passing deep to the pterion is a large branch of the maxillary artery supplying the dura : the middle meningeal artery. Fracture at the pterion can produce a complete laceration of this artery and its accompanying vein resulting in bleeding into the epidural space between the dura mater and the skull. This results in an epidural hemorrhage.
The temporal bone itself is less easily fractured. Such fractures usually occur in blunt force trauma.
Mastoiditis is inflammation of the air cells located in the mastoid process, and is often caused by severe middle ear infection (otitis media). Mastoiditis can spread from the temporal bone into the cranial cavity, causing meningitis, or inflammation of the meninges.