Posterior Cranial Fossa

The temporal bones are a pair of irregularly-shaped bones which constitute the lateral wall and base of the skull. The temporal consists of essentially four portions; the squamous, mastoid, petrous and tympanic.

Borders

• Occipitomastoid suture – separates occipital bone and mastoid portion.
• Squamosal suture – separates parietal bone and squamous portion.
• Sphenosquamosal suture – separates sphenoid bone and squamous portion of temporal bone. 
• Zygomaticotemporal suture – separates zygomatic bone and zygomatic process of temporal bone.

Muscular Attachments

• Temporalis
• Masseter
• Sternocleidomastoid
• Posterior belly of digastric
• Splenius capitis

Squamous

Description

• Flat and plate-like. Largest and most-superiorly part of the temporal bone, forming lateral wall of middle cranial fossa.

External surface 

• Smooth and convex.
• Above the external acoustic meatus, there is a groove for the middle temporal artery.
• Attachments: temporal muscle.
• Forms part of the temporal fossa.

Internal surface

• Has impressions following the contour of the temporal lobe and a groove for the middle meningeal vessels.
• Forms lateral part of the middle cranial fossa.

Borders

• Superior – squamosal suture with the parietal bone.
• Anteroinferior – articulation with the greater wing of the sphenoid bone.

Attachments

• Lower part of the squamous bone: site of origin of the temporalis muscle.

Projections

• Zygomatic process – long, arch process projecting from the lower region of the squamous portion articulating with the zygomatic bone to form zygomatic arch

Mastoid

Size, shape, location

• Downward projecting, conical process, most posterior part of the temporal bone. 
• External surface: Roughened surface for muscular attachments

Landmarks

• Mastoid notch: depression on the medial surface.
• Sigmoid sulcus: deep groove on the medial surface in which the sigmoid sinus is located.

Attachments

• Sternocleidomastoid
• Splenius capitis
• Longissimus capitis
• Posterior belly of diagastric (insert onto mastoid notch)

Petrous

Size, shape, location

• Pyramid-shaped. Fused with the squamous and mastoid parts and between the sphenoid and occipital bones lies the petrous part, which is shaped like a pyramid.

Contents

• Acoustic labryinth

Base: fused with the internal surfaces of the squamous and mastoid parts
Apex: presents the anterior opening of the carotid canal and forms the postero-lateral border of the foramen lacerum
Anterior surface: forms the posterior part of the middle cranial fossa; contains arcuate eminence, tegmen tympani, groove and hiatus for greater petrosal nerve, hiatus for the lesser superficial petrosal nerve, termination of the carotid canal, trigeminal impression
Posterior surface: froms the anterior part of the posterior cranial fossa; contains internal acoustic opening
Inferior surface: quadrangular area, opening of the carotid canal, jugular fossa

Tympanic

Posterior surface: forms the anterior wall, the floor and part of the posterior wall of the external acoustic meatus; contains the tympanic sulcus (for the attachment of the tympanic membrane)
Anterior surface: forms the posterior part of the mandibular fossa and a part of the external acoustic meatus
Borders: lateral, superior, inferior (its lateral part – vaginal process – splits to enclose the root of the styloid process)
Processes: styloid process

Main bone of the occiput.

Shape, size, location

Trapezoidal-shaped flat bone.

Structure

• Basilar part: Thick, quadrilateral shaped
• Sqaumous part: Curved, expanded plate behind the foramen magnum and is the largest part of the occipital bone

Outer surface

• Marked by the external occipital protuberance, located near the middle of the outer surface of the squamous part

Inner surface

• Forms the base of the posterior cranial fossa.
• Foramen magnum: large oval opening with a large anteroposterior diameter through which the medulla oblongata traverses.

Borders

• The superior borders extend from the superior to the lateral angles: they are deeply serrated for articulation with the occipital borders of the parietals, and form by this union the lambdoidal suture.
• The inferior borders extend from the lateral angles to the inferior angle; the upper half of each articulates with the mastoid portion of the corresponding temporal, the lower half with the petrous part of the same bone.
• These two portions of the inferior border are separated from one another by the jugular process, the notch on the anterior surface of which forms the posterior part of the jugular foramen.