The petrous part of the

temporal bone The temporal bone is a paired bone situated at the sides and base of the skull, lateral to the temporal lobe of the cerebral cortex. The temporal bones are overlaid by the sides of the head known as the temples where four of the cranial bone ...

is pyramid-shaped and is wedged in at the base of the skull between the sphenoid and

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lob ...

s. Directed medially, forward, and a little upward, it presents a base, an apex, three surfaces, and three angles, and houses in its interior the components of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

. The petrous portion is among the most basal elements of the skull and forms part of the endocranium. Petrous comes from the Latin word ''petrosus'', meaning "stone-like, hard". It is one of the densest bones in the body. In other mammals, it is a separate bone, the petrosal bone. The petrous bone is important for studies of ancient DNA from skeletal remains, as it tends to contain extremely well-preserved DNA.

Base

The base is fused with the internal surfaces of the

squamous Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

, tympanic, and mastoid parts.

Apex

The apex, which is rough and uneven, is received into the angular interval between the posterior border of the great wing of the

sphenoid bone The sphenoid bone is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the basilar part of occipital bone, basilar part of the occipital bone. The sphenoid bone is one of the seven bon ...

and the basilar part of the

; it presents the anterior or internal opening of the carotid canal, and forms the postero-lateral boundary of the foramen lacerum.

Surfaces

Anterior surface

The anterior surface forms the posterior part of the middle cranial fossa of the base of the skull, and is continuous with the inner surface of the squamous portion, to which it is united by the petrosquamous suture, remains of which are distinct even at a late period of life. It is marked by depressions for the convolutions of the brain, and presents six notable points: # near the center, the arcuate eminence (eminentia arcuata), which indicates the location of the superior semicircular canal. # in front of and a little lateral to this eminence, a depression indicating the position of the

tympanic cavity The tympanic cavity is a small cavity surrounding the bones of the middle ear. Within it sit the ossicles, three small bones that transmit vibrations used in the detection of sound. Structure On its lateral surface, it abuts the external audit ...

: Here, the layer of bone that separates the tympanic from the cranial cavity is extremely thin, and is known as the ''tegmen tympani'' # a shallow groove, sometimes double, leading lateralward and backward to an oblique opening, the hiatus for greater petrosal nerve, for the passage of the greater petrosal nerve and for the petrosal branch of the middle meningeal artery # lateral to the hiatus, a smaller opening, occasionally seen, for the passage of the lesser superficial petrosal nerve # near the apex of the bone, the termination of the carotid canal, the wall of which in this situation is deficient in front # above this canal the shallow trigeminal impression for the reception of the trigeminal ganglion.

Posterior surface

The posterior surface forms the anterior part of the posterior cranial fossa of the base of the skull, and is continuous with the inner surface of the mastoid portion. Near the center is a large orifice, the internal acoustic opening, the size of which varies considerably; its margins are smooth and rounded, and it leads into the internal auditory meatus a short canal, about 1 cm. in length, which runs lateralward. It transmits the facial and acoustic nerves and the internal auditory branch of the basilar artery. The lateral end of the canal is closed by a vertical plate, which is divided by a horizontal crest, the falciform crest, into two unequal portions. Each portion is further subdivided by a vertical ridge into an anterior and a posterior part. * In the ''portion beneath the falciform crest'' are three sets of foramina; these openings together with this central canal transmit the nerves to the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...

. ** one group, just below the posterior part of the crest, situated in the area cribrosa media, consists of several small openings for the nerves to the saccule; ** below and behind this area is the foramen singulare, or opening for the nerve to the posterior semicircular duct; ** in front of and below the first is the tractus spiralis foraminosus, consisting of a number of small spirally arranged openings, which encircle the canalis centralis cochleæ. * The ''portion above the crista falciformis'' presents behind, the area cribrosa superior, pierced by a series of small openings, for the passage of the nerves to the utricle and the superior and lateral semicircular ducts, and, in front, the area facialis, with one large opening, the commencement of the canal for the facial nerve (aquæductus Fallopii). Behind the internal acoustic meatus is a small slit almost hidden by a thin plate of bone, leading to a canal, the aquæductus vestibuli, which transmits the ductus endolymphaticus together with a small artery and vein. Above and between these two openings is an irregular depression that lodges a process of the dura mater and transmits a small vein; in the infant, this depression is represented by a large fossa, the subarcuate fossa, which extends backward as a blind tunnel under the superior semicircular canal. # Falciform crest # Area facialis, with (2’) internal opening of the facial canal # Ridge separating the area facialis from the area cribrosa superior # Area cribrosa superior, with (4’) openings for nerve filaments # Anterior inferior cribriform area, with (5’) the tractus spiralis foraminosus, and (5’’) the canalis centralis of the cochlea. # Ridge separating the tractus spiralis foraminosus from the area cribrosa media # Area cribrosa media, with (7’) orifices for nerves to saccule # Foramen singulare.

Inferior surface

The inferior surface is rough and irregular, and forms part of the exterior of the base of the skull. It presents eleven points for examination: # near the apex is a rough surface, quadrilateral in form, which serves partly for the attachment of the Levator veli palatini and the cartilaginous portion of the

auditory tube The Eustachian tube (), also called the auditory tube or pharyngotympanic tube, is a tube that links the nasopharynx to the middle ear, of which it is also a part. In adult humans, the Eustachian tube is approximately long and in diameter. It ...

, and partly for connection with the basilar part of the

through the intervention of some dense fibrous tissue # behind this is the large circular aperture of the carotid canal, which ascends at first vertically, and then, making a bend, runs horizontally forward and medially; it transmits into the cranium the internal carotid artery, and the carotid plexus of nerves # medial to the opening for the carotid canal and close to its posterior border, in front of the jugular fossa, is a triangular depression; at the apex of this is a small opening, the aquæductus cochleæ, which lodges a tubular prolongation of the dura mater establishing a communication between the perilymphatic space and the subarachnoid space, and transmits a vein from the cochlea to join the internal jugular # behind these openings is a deep depression, the jugular fossa, of variable depth and size in different skulls; it lodges the bulb of the internal jugular vein # in the bony ridge dividing the carotid canal from the jugular fossa is the small inferior tympanic canaliculus for the passage of the tympanic branch of the

glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper Medulla oblongata, medulla, just anterior (closer to the nose) to t ...

# in the lateral part of the jugular fossa is the mastoid canaliculus for the entrance of the auricular branch of the vagus nerve # behind the jugular fossa is a quadrilateral area, the jugular surface, covered with cartilage in the fresh state, and articulating with the jugular process of the

# extending backward from the carotid canal is the vaginal process, a sheath-like plate of bone, which divides behind into two laminæ; the lateral lamina is continuous with the tympanic part of the bone, the medial with the lateral margin of the jugular surface # between these laminæ is the styloid process, a sharp spine, about 2.5 cm. in length # between the styloid and mastoid processes is the stylomastoid foramen; it is the termination of the facial canal, and transmits the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of ta ...

and stylomastoid artery # situated between the tympanic portion and the mastoid process is the tympanomastoid fissure, for the exit of the auricular branch of the

vagus nerve The vagus nerve, also known as the tenth cranial nerve (CN X), plays a crucial role in the autonomic nervous system, which is responsible for regulating involuntary functions within the human body. This nerve carries both sensory and motor fibe ...

Angles

The superior angle, the longest, is grooved for the superior petrosal sinus, and gives attachment to the tentorium cerebelli; at its medial extremity is a notch, in which the

trigeminal nerve In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...

lies. The posterior angle is intermediate in length between the superior and the anterior. Its medial half is marked by a sulcus, which forms, with a corresponding sulcus on the occipital bone, the channel for the inferior petrosal sinus. Its lateral half presents an excavation — the jugular fossa — which, with the jugular notch on the occipital, forms the jugular foramen; an eminence occasionally projects from the center of the fossa, and divides the foramen into two. The anterior angle is divided into two parts—a lateral joined to the squamous part by a suture (petrosquamous), the remains of which are more or less distinct; a medial, free, which articulates with the spinous process of the sphenoid. At the angle of junction of the petrous and the squamous parts are two canals, one above the other, and separated by a thin plate of bone, the septum canalis musculotubarii; both canals lead into the tympanic cavity. * The upper one (semicanalis m. tensoris tympani) transmits the tensor tympani. * the lower one (semicanalis tubae auditivae) forms the bony part of the auditory tube.

Petrous bone in ancient DNA studies

In ancient DNA studies, scientists extract and sequence DNA from ancient skeletal remains of humans and other species. In many cases the DNA is highly degraded, and contaminated by DNA from soil microbes. In 2015 it was found that the petrous bone has remarkably well-preserved DNA. A 2017 study comparing DNA from different skeletal sites concluded that "The inner part of petrous bones and the cementum layer in teeth roots are currently recognized as the best substrates for (ancient DNA) research ... Both substrates display significantly higher endogenous DNA content (average of 16.4% and 40.0% for teeth and petrous bones, respectively) than parietal skull bone (average of 2.2%)." Consequently, petrous bones are now the most widely-used skeletal site for the study of ancient DNA.

Additional images

File:Gray187.png, Base of skull. Inferior surface. File:Gray193.png, Base of the skull. Upper surface. File:Gray1028.png, Dissection of the muscles of the palate from behind. File:Temporal bone1.jpg, Temporal bone File:Slide25hhhh.JPG, Bones of the front of the skull, viewed from behind (interior surface). The petrous part of the temporal is labeled in the lower left. File:Slide4pppp.JPG, Another view of the bones of the front of the skull, viewed from behind, with the petrous part of the temporal labeled bilaterally.

References

{{Authority control Bones of the head and neck Otorhinolaryngology