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Other Terms: Os temporale, Os temporal
The temporal bone has an interesting developmental history. This bone is actually four separate bones that have, over evolutionary time, become a single bone in the adult mammal. The four parts of the human temporal bone are the squamous, tympanic, petromastoid, and styloid. The squamous part of the bone is the thin lateral plate contributing to the lateral wall of the cranium. It projects anteriorly as the zygomatic process. Antero-inferiorly it forms the mandibular fossa for the temporomandibular joint. The squamous part is similar to other flat membrane bones of the skull. It has outer compact lamellae with an inner core of cancellous bone. The styloid part is represented by the styloid process. This projection of bone derives from the upper elements of the hyoid branchial arch. The petrous and mastoid regions form the thick pyramidal base that projects anteromedially. It begins posterior to the external acoustic meatus and ends where it forms a junction with the basi-occipital and greater wing of the sphenoid. The name petrous describes its rock like appearance. This is the thickest part of the temporal bone. It arises from the otic capsules that stabilize the delicate internal ear structures. The mastoid is the posterolateral protuberance of the petrous portion that is easily palpable just posterior to the ear. The styloid and petrous portions are endochondral in formation containing more compact bone. The compact bone of the petrous has internal cavities and labyrinths to house the middle and inner ear anatomy. The mastoid, a secondary outgrowth of the petrous portion has many pneumatic spaces that connect to the middle ear region. A tympanic part of the temporal bone is the ring-like plate that forms the walls of the external acoustic meatus. The tympanic part forms intramembranously as a new addition of bone or possibly as a homologue of the post-dentary angular bone of earlier vertebrates.
The term temporal arises from the Latin tempus meaning time. The word time was used for this region because it is typically on the sides of the skull where hair first becomes gray, showing the ravages of time.
The temporal bone articulates with five bones: the parietal, occipital, sphenoid, mandible, and zygomatic. The temporoparietal suture is a squamous suture formed between the superior margin of the squamous temporal bone and the inferior border of the parietal bone. The temporal bone is beveled internally and overlaps the parietal bone forming a lap joint. The posterior borders of the squamous and mastoid parts of the temporal bone form a serrate suture with the squama of the occipital bone. Medially the mastoid margin articulates with the jugular process of the occipital bone. The anterior margin of the squamous temporal bone articulates with the posterosuperior margin of the greater wing of the sphenoid. The petrous temporal bone articulates with the postero-inferior part of the greater wing of the sphenoid near the sphenoid spine. A mandibular fossa, formed inferiorly on the squamous temporal bone forms a concave surface for the condyle of the mandible. This forms the synovial temporomandibular joint. The zygomatic process of the squamous temporal bone forms a serrate suture with the corresponding process of the zygomatic bone. This union forms the zygomatic arch that bridges the tendon of the temporalis muscle.
The ontogenetic ossification patterns of the temporal bone reflect its phylogenetic origins. The four parts of the bone each ossify as separate centers. The petromastoid part forms from many centers that correspond with the cartilaginous otic capsules. These appear during the fifth fetal month and are fully ossified by the seventh month. The squamous part of the temporal bone ossifies from a single center that arises during the end of the second embryonic month. This center appears at the root of the zygomatic process and spreads radially from this point. The petromastoid and squamous parts join during the first postnatal year. The styloid ossifies in cartilage at the superior end of the hyoid branchial arch. It forms from two centers corresponding to the tympanohyal and stylohyal. The tympanohyal is the more proximal center. It appears in late fetal life and is completely ossified soon after birth. It joins the petromastoid and squamous portions during the first postnatal year. The stylohyal is the more distal center, forming the tip of the process. It does not begin to ossify until after birth. It does not fuse with the tympanohyal until puberty. Sometimes it never fuses with the base of the styloid, in this case it connects with the base by cartilage. This is the reason many prepared skulls lack complete styloid processes. The tympanic part ossifies from a single center intramembranously. It appears during the third intrauterine month. At birth it forms an incomplete ring that has just united with the squamous part of the bone. After birth it grows rapidly to form the greater part of the elongated external acoustic canal.