The Temporal Bone
The temporal bone is a large, bilaterally symmetrical bone which forms the base of the cranial vault (along with the occipital bone) and ascends to participate in the lateral walls of the skull. It is divided into four parts: the squamous part, the tympanic part, the styloid process and the petrous part.
The squamous part is a flat plate that builds the lateral wall of the middle cranial fossa. It houses the middle meningeal artery that is marked by a groove in the bone. The zygomatic process arises from the outer surface of the squamous part and articulates with the zygomatic bone. The glenoid fossa sits just below the zygomatic process where is comes into contact with the mandibular condyle. It is part of the temporomandibular joint (TMJ). The squamous part is separated from the tympanic part by the petrous part. The petrotympanic fissure lies behind this division.
The tympanic part is horseshoe-shaped and contributes to the anterior, posterior and inferior walls of the external acoustic meatus and the posterior, non-articulating part of the glenoid fossa. Laterally it is attached to the cartilage of the external acoustic meatus and medially to the tympanic membrane (temporal sulcus).
The styloid process is a spike-like projection from the inferior aspect of the temporal bone. It arises just anteriorly to the stylomastoid foramen, which harbours the facial nerve (CN VII) and the stylomastoid artery.
The petrous part is the pyramid-shaped, medial part of the temporal bone and separates the middle and posterior cranial fossae. It is extremely solid and protects the interior auditory and vestibular apparatus. The internal acoustic meatus is located at the posterior surface leading to the ear canal. The inferior surface contains numerous openings such as the carotid canal (internal carotid artery) and the jugular foramen (internal jugular vein, cranial nerves IX, X and XI). The most posterior part of the petrous portion contains the mastoid process which is filled with mastoid air cells and lined with mucous membrane inside.
The squamosal suture connects the temporal bone and parietal bone and continues as the sphenosquamosal suture anteriorly and the parietomastoid suture posteriorly. The occipitomastoid suture runs between the mastoid part of temporal bone and the occipital bone. The zygomatic process articulates with the temporal bone through the temporozygomatic suture.
Of the four portions of the temporal bone the petrous part and styloid process undergo endochondral ossification whereas the squamous and tympanic part develop through intramembranous ossification. Spread evenly over the entire bone are eight ossification centers that create three of the main fontanelles before birth. The mastoid process is missing in newborns and starts to grow with the increasing traction force of the sternocleidomastoid muscle.
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