The two parietal bones articulate in the midline, forming the rounded dome of the skull. The parietal bones are fashioned like rectangles, with four edges and angles on each. The frontal border is the articulation of the parietal bone with the frontal bone. The sagittal border of the parietal bone is the portion of the bone that articulates with the adjacent parietal bone. The occipital margin refers to the portion of the parietal bone that articulates with the occipital bone, while the temporal margin refers to the portion of the parietal bone that articulates with the temporal bone.

The two parietals articulate at the sagittal suture. The parietal bones articulate with the frontal bone at the coronal suture. Along the lambdoidal suture, the parietal bones also articulate with the occipital bone. The sagittal suture runs anterior to the coronal suture and posterior to the lambdoidal suture.

While the exterior surface of each parietal bone seems relatively smooth, the inside surface is characterized with grooves that resemble blood arteries. Indeed, these grooves are referred to as the middle meningeal vessel grooves because they are where blood vessels that travel through the brain's meningeal layers rest.

The occipital bone comprises the posterior section of the skull (see Figure 10-13) and is divided into two segments: the larger squamous segment and the smaller basilar segment. Except for the lambdoidal (or parietal) border, where the occipital bone articulates with the parietal bones, and the mastoid margin, where the occipital bone articulates with the temporal bones, the squamous region is generally nondescript. The inion or external occipital protuberance is the point at which the squamous portion of the occipital bone spreads most posteriorly. Two ridges termed the superior and inferior nuchal lines run transversely along the occipital bone from where the squamous section finishes and the basilar portion starts.

In terms of landmarks, the basilar part of the occipital bone is more significant. The most prominent feature is the foramen magnum, a big hole in the basilar portion. On its route down the spinal column, the spinal cord passes through the foramen magnum. Two prominent occipital condyles are located anterior to the foramen magnum and lateral to it. The occipital condyles connect the base of the skull with the first cervical vertebra (C1, or the atlas). Each condyle has a hypoglossal canal anterior to it. The pharyngeal tubercle is located anterior and medial to the condyles. It is where the pharynx's superiormost border joins to the base of the skull. Finally, a depression known as the condylar fossa exists posterior to the condyles. A tiny hole known as the condylar canal exists inside this fossa.

2 MAJOR PARTS OF TEMPORAL BONES

1. Squamous portion:

- Zygomatic process

2. Petrous portion:

- Tympanic section

- Cochlea and semicircular canals

- Mastoid section

- Mastoid process

- Mastoid air cells

- (l) part = air cells diminish

- (A and S) part = air cells are progressively larger until they give way to the tympanic antrum

- Upper limit of tympanic antrum is tegmen tympanum

- Roof of tympanic cavity

- Styloid process

- In petrous portion, (P and I) to zygomatic process

-Point of attachment of three muscles (styloglossus, stylohyoid, stylopharyngeus) and two ligaments (stylomandibular, stylohyoid)

- Mandibular fossa

- Depression on (I) surface, between styloid and zygomatic processes

- Condylar process of mandible rests within this fossa forming the temporomandibular joint

The Ethmoid Bone is classified as bone of cranium, but also contributes to facial part of skull. It has 5 parts:

- Cribriform plate

- Crista galli

- perpendicular plate

- 2 ethmoidal labyrinths

CRIBRIFORM PLATE

- Partition between nasal (below) and cranial (above) cavities

- Perforated by small openings

- Olfactory nerve (CN I)

CRISTA GALLI (COCK’S COMB)

- Housed within cranial cavity

- Anterior point of attachment for falx cerebri (fold of dura mater that separates two cerebral hemispheres of brain)

PERPENDICULAR PLATE

- Articulates with frontal and nasal bones

(A) and vomer and rostrum of sphenoid (P)

- Cartilaginous nasal septum is attached to anterior perpendicular plate

ETHMOIDAL LABYRINTHS

- Contain highly irregular air cells (ethmoid paranasal sinuses)

- Present at birth

- Separated from medial walls of orbits of eyes by lamina papyracea

- Superior and middle nasal conchae

One of the bones of the skull, the sphenoid bone, is thought to be the most complicated. When it is removed from the skull, it looks a little like a butterfly. Within the skull, it can be found next to the ethmoid and next to the foramen magnum and the basilar part of the occipital bone. It has seven parts: a single corpus, two pairs of wings, two pairs of wings, and two pairs of pterygoid processes. The greater and smaller wings spread out from the body, while the pterygoid processes spread out below. The front surface of the corpus forms the back wall of the nasal cavity. Toward the bottom, there is a small bump that connects with the vomer.

The two smaller wings spread out laterally from the upper–lower part of the body. Greater orbital fissures start at the lower parts of the lesser wings and go all the way to the upper parts of the body. The frontal bone has plates called orbital plates that connect to the front of the bone where the wings meet. Jugum is the point where all of the smaller wings come together to make one big one. After the optic canal, which is where the two optic nerves (cranial nerve II) come out of the eyeball and cross over, they go through the optic sulcus, which is where the optic chiasma is found. This is where the two optic nerves meet. The sella turcica, which is right next to the chiasmatic sulcus, is where the pituitary gland lives.

SUMMARY:

- (P) to ethmoid and immediately (A) to foramen magnum and basilar portion of occipital bone

- 7 major parts:

- Corpus

- Greater wings

- Lesser wings

- Pterygoid processes

- Articulates with all other cranial bones + maxillae, vomer, palatines, zygomatics

- Contributes to orbits of eye + nasal and pharyngeal cavities

PARTS OF SPHENOID BONE

- CORPUS

- (A) surface = (P) wall of nasal cavity

- (I) surface = has small ridge called rostrum (articulates with vomer)

- Hollow

- 2 chambers separated by midline septum, creating pair of sphenoid paranasal sinuses

- Not present at birth, starts at year 3 (sphenoid paranasal sinuses)

- Ethmoid paranasal sinuses = only sinuses that are present at birth

Note: ALL paranasal sinuses:

- Lined with mucoperiosteum

- Drain into nasal cavity proper

- Sphenoid PS = spaces above

(S) nasal conchae

- Frontal and maxillary PS = medial nasal meatus

- Ethmoid PS = (S and M) nasal meatuses

LESSER WINGS

- (I) surface = upper boundaries of superior orbital fissures

- (A) margins articulate with orbital plates of frontal bone

- Come together at the jugum

- (P) to jugum is the chiasmatic sulcus (houses optic chiasma where two optic nerves/ CN II cross over)

- SELLA TURCICA = houses pituitary gland, immediately

(P) to chiasmatic sulcus

GREATER WINGS

- Part of (S, L, I) walls of orbits

- (L) margins articulate with zygomatic bones

PTERYGOID PROCESSES

- Composed of 2 laminae (medial and lateral pterygoid lamina)

- Separated by pterygoid notch

- Articulate with

palatine bones anteriorly

- LATERAL PTERYGOID LAMINA = point of articulation for medial and lateral pterygoid muscles

- MEDIAL PTERYGOID LAMINA = terminates in hamulus