Temporal bone

Temporal bone
Position of temporal bone (shown in green)
Structure of temporal bone (left)
Details
Latin	Os temporale
Articulations	Occipital, parietal, sphenoid, mandible and zygomatic
Identifiers
Gray's	p.138
TA	A02.1.06.001
FMA	52737
Anatomical terms of bone

The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.

The temporal bone supports that part of the face known as the temple and houses the structures of the organ of hearing. The lower seven cranial nerves and the major vessels to and from the brain traverse the temporal bone.

Structure

The temporal bone consists of five parts: The squamous part, which is the biggest part of the temporal bone and lies superior to all parts. Posterior and inferior to the squamous part lies the mastoid portion. Fused with the squamous and mastoid parts and between the sphenoid and occipital bones lies the petrous portion (Petrosal ridge). It is recognizable because of its pyramid shape. The tympanic part is a small portion which lies inferior to the squamous part, anteroir to the mastoid part and superior to the styloid process. Styloid, ( Greek: Pillar Form) a long pointed process is directed downwards, forwards and medially between parotid gland and internal jugular vein. Main article: styloid Process As already mentioned the Styloid process lies inferior to the tympanic part. It is shaped like a thorn.

Borders

Occipitomastoid suture. It separates occipital bone and mastoid portion of temporal bone.  Squamosal suture. It separates parietal bone and squama portion of temporal bone.  Sphenosquamosal suture. It separates sphenoid bone and squama portion of temporal bone.  Zygomaticotemporal suture. It separates zygomatic bone and zygomatic process of temporal bone.

Development

The temporal bone is ossified from eight centers, exclusive of those for the internal ear and the tympanic ossicles: one for the squama including the zygomatic process, one for the tympanic part, four for the petrous and mastoid parts, and two for the styloid process. Just before the close of fetal life [Fig. 6] the temporal bone consists of three principal parts:

1. The squama is ossified in membrane from a single nucleus, which appears near the root of the zygomatic process about the second month.
2. The petromastoid part is developed from four centers, which make their appearance in the cartilaginous ear capsule about the fifth or sixth month. One (proötic) appears in the neighborhood of the eminentia arcuata, spreads in front and above the internal acoustic meatus and extends to the apex of the bone; it forms part of the cochlea, vestibule, superior semicircular canal, and medial wall of the tympanic cavity. A second (opisthotic) appears at the promontory on the medial wall of the tympanic cavity and surrounds the fenestra cochleæ; it forms the floor of the tympanic cavity and vestibule, surrounds the carotid canal, invests the lateral and lower part of the cochlea, and spreads medially below the internal acoustic meatus. A third (pterotic) roofs in the tympanic cavity and antrum; while the fourth (epiotic) appears near the posterior semicircular canal and extends to form the mastoid process (Vrolik).
3. The tympanic ring is an incomplete circle, in the concavity of which is a groove, the tympanic sulcus, for the attachment of the circumference of the tympanic membrane. This ring expands to form the tympanic part, and is ossified in membrane from a single center which appears about the third month. The styloid process is developed from the proximal part of the cartilage of the second branchial or hyoid arch by two centers: one for the proximal part, the tympanohyal, appears before birth; the other, comprising the rest of the process, is named the stylohyal, and does not appear until after birth. The tympanic ring unites with the squama shortly before birth; the petromastoid part and squama join during the first year, and the tympanohyal portion of the styloid process about the same time [Fig. 7, 8]. The stylohyal does not unite with the rest of the bone until after puberty, and in some skulls never at all.

The chief subsequent changes in the temporal bone apart from increase in size are:

1. The tympanic ring extends outward and backward to form the tympanic part. This extension does not, however, take place at an equal rate all around the circumference of the ring, but occurs most rapidly on its anterior and posterior portions, and these outgrowths meet and blend, and thus, for a time, there exists in the floor of the meatus a foramen, the foramen of Huschke; this foramen is usually closed about the fifth year, but may persist throughout life.
2. The mandibular fossa is at first extremely shallow, and looks lateralward as well as downward; it becomes deeper and is ultimately directed downward. Its change in direction is accounted for as follows. The part of the squama which forms the fossa lies at first below the level of the zygomatic process. As, however, the base of the skull increases in width, this lower part of the squama is directed horizontally inward to contribute to the middle fossa of the skull, and its surfaces therefore come to look upward and downward; the attached portion of the zygomatic process also becomes everted, and projects like a shelf at right angles to the squama.
3. The mastoid portion is at first quite flat, and the stylomastoid foramen and rudimentary styloid process lie immediately behind the tympanic ring. With the development of the air cells the outer part of the mastoid portion grows downward and forward to form the mastoid process, and the styloid process and stylomastoid foramen now come to lie on the under surface. The descent of the foramen is necessarily accompanied by a corresponding lengthening of the facial canal.
4. The downward and forward growth of the mastoid process also pushes forward the tympanic part, so that the portion of it which formed the original floor of the meatus and contained the foramen of Huschke is ultimately found in the anterior wall.
5. The fossa subarcuata becomes filled up and almost obliterated.

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  1. Outer surface of petromastoid part. 2. Outer surface of tympanic ring. 3. Inner surface of squama.

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  Figure 7 : Temporal bone at birth. Outer aspect.

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  Figure 8 : Temporal bone at birth. Inner aspect.

Clinical significance

Temporal bone fractures are often divided into three main categories, longitudinal, in which the vertical axis of the fracture parallels the petrous ridge, horizontal, in which the axis of the fracture is perpendicular to the petrous ridge, and oblique, a mixed fracture with both longitudinal and horizontal components. Historically, horizontal fractures were thought to be associated with injuries to the facial nerve, while longitudinal fractures were associated with injuries to the middle ear ossicles. A newer classification system, in which temporal bone fractures are delineated based on disruption of the otic capsule, has been found to be more reliable in predicting complications of temporal bone trauma such as facial nerve injury, sensorineural hearing loss, intracerebral hemorrhage, and cerebrospinal fluid otorrhea.

Other animals

In many animals some of these parts stay separate through life:

• Squamosal: the squama including the zygomatic process
• Tympanic bone: the tympanic part: this is derived from the angular bone of the reptilian lower jaw
• Periotic bone: the petrous and mastoid parts
• Two parts of the hyoid arch: the styloid process. In the dog these small bones are called tympanohyal (upper) and stylohyal (lower).

In evolutionary terms, the temporal bone is derived from the fusion of many bones that are often separate in non-human mammals:

• The squamosal bone, which is homologous with the squama, and forms the side of the cranium in many bony fish and tetrapods. Primitively, it is a flattened plate-like bone, but in many animals it is narrower in form, for example, where it forms the boundary between the two temporal fenestrae of diapsid reptiles.
• The petrous and mastoid parts of the temporal bone, which derive from the periotic bone, formed from the fusion of a number of bones surrounding the ear of reptiles. The delicate structure of the middle ear, unique to mammals, is generally not protected in marsupials, but in placentals, it is usually enclosed within a bony sheath called the auditory bulla. In many mammals this is a separate tympanic bone derived from the angular bone of the reptilian lower jaw, and, in some cases, it has an additional entotympanic bone. The auditory bulla is homologous with the tympanic part of the temporal bone.
• Two parts of the hyoid arch: the styloid process. In the dog the styloid process is represented by a series of 4 articulating bones, from top down tympanohyal, stylohyal, epihyal, ceratohyal; the first two represent the styloid process, and the ceratohyal represents the anterior horns of the hyoid bone and articulates with the basihyal which represents the body of the hyoid bone.

Etymology

From Old French temporal "earthly" and directly from Latin tempus "time, proper time or season," of unknown origin. temporal bones situated on the side of the skull, usually grey hairs appear in this region early on. Or it may relate to the pulsations of the underlying superficial temporal artery, marking the time we have left here. There is also a probable connection with the Greek verb temnion, to wound in battle. The skull is thin in this area and presents a vulnerable area for a blow from a battle ax.

Additional images

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  Position of temporal bone (green). Animation.

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  Shape of temporal bone (left).

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  Cranial bones.

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  Sphenoid and temporal bones

See also

This article uses anatomical terminology; for an overview, see anatomical terminology.

• Temporal fenestrae
• Temporomandibular joint
• Temporal muscle

References

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

External links

• Anatomy diagram: 34256.000-1 at Roche Lexicon - illustrated navigator, Elsevier