THE HEAD

Introduction to the skull

Superficial face; scalp

Parotid region and deep face

Intracranial cavity; meninges

Eye and orbit

Cranial nerves

Oral cavity and contents

Ear and temporal bone

Introduction to the skull

Development of the skull

Base of skull: endochondral ossification

• Brain and cranial nerves develop before the skull, so when the chondrocranium develops, its components form around the nerves and form foramina.
• The chondrocranium ossifies from a number of centers.
• The last piece of cartilage to ossify is between the body of the sphenoid bone and the occipital bone, anterior to the foramen magnum: this is the spheno-occipital synchondrosis (the epiphyseal plate for growth in length of the base of the skull and it ossifies at age 25).

Bones of calvarium: intramembranous ossification

• The bones of the calvarium also ossify from separate centers and they meet to form sutures. The process is completed at about 3 years.

Bones of the face are partly basal and partly calvarial bones so they ossify both by intramembranous and endochondral ossification.

CALVARIAL BONES

The bones of the cranium and the underlying lobes of the brain have the same name:

• The frontal bone (which houses the frontal lobe of the brain) ossifies from 2 centers and there are right and left frontal bones at birth. The metopic suture separating the 2 normally ossifies leaving one large frontal bone.
• The parietal bones are separated by the sagittal suture and from the frontal bone by the coronal suture.

At birth, the anterior fontanelle (bregma) is a diamond-shaped area between the 2 frontal bones and the 2 parietal bones. It pulsates and bulges when the baby cries. It closes by 18 months to 2 years.

• The parietal bones and the occipital bone meet at the posterior fontanelle (lambda).
• The side wall of the skull is completed by the squamous part of the temporal bone and the greater wing of the sphenoid bone, at the pterion. It is located 4 cm above the midpoint of the zygomatic arch and is the site of surgical exploration for the middle meningeal artery.
• The flat bones of the skull (frontal, parietal, temporal and occipital) are formed by a sandwich of diploë (cancellous bone containing red bone marrow) between 2 layers of compact bone.
• The bones are drained by diploic veins (usually 4 on each side) which open into the nearest convenient venous sinus.
• The flat bones of the skull are also pierced by emissary foramina transmitting emissary veins connecting veins of the scalp with the dural venous sinuses inside the skull (possible route for infection). They may be seen in the parietal bone or in the temporal bone posterior to the external auditory meatus.

In an X-ray of the skull, there is the need to differentiate the following normal structures from fractures :

• the sutures (possible metopic suture),
• the diploic veins,
• and the middle meningeal artery.

The pineal gland near the center of the brain may contain corpora aranaceae (calcareous granules) and it can be used to detect displacement of the brain.

• The occipital bone: the external occipital protuberance is located inferior to lambda. The superior nuchal lines run lateral from the external occipital protuberance and the inferior nuchal lines are situated inferiorly.

THE CRANIAL FOSSAE: 3 fossae forming 3 steps down to foramen magnum from front to back.

1) Anterior cranial fossa:

• Orbital plate of the frontal bone
• Lesser wing of the sphenoid bone
• The cribriform plate of the ethmoid with the crista galli transmits olfactory nerves from the upper part of the nasal cavity; this is a possible route for infection or escape route for CSF in skull fractures.
• Optic canal for transmission of optic nerve.
• Anterior clinoid processes projecting posteriorly to the posterior clinoid processes of the sphenoid bone.

2) Middle cranial fossa

• is formed by the greater wing of the sphenoid bone and the temporal bone. It is occupied by the temporal lobe of the brain.
• The greater wings and the body of the sphenoid bone form a butterfly shape.
• The most posterior tip of the greater wing contains the foramen spinosum.
• Anteriorly to the foramen spinosum is the foramen ovale and then the foramen rotundum. The foramen rotundum leads to the pterygopalatine fossa.
• The superior orbital fissure is a gap between the lesser wing and the greater wing of the sphenoid leading to the orbit, just lateral to and below the optic nerve.
• The body of the sphenoid bone contains the hypophyseal fossa for the pituitary gland. This fossa is also called the sella turcica and it is shaped like a 4-poster bed (clinoid processes; clinical = patient is in bed). Its maximum length is 14mm and its depth is 8mm. Measurements are important because pituitary tumors cause ballooning of the sella.
• The foramen lacerum is located lateroposterior to the sella.

The middle meningeal artery enters the skull through the foramen spinosum and its groove can be traced laterally and anteriorly on the squamous portion of the temporal bone before dividing into anterior and posterior (frontal and parietal) branches.

• The anterior branch crosses the greater wing at the pterion and then breaks up into branches that run superiorly and posteriorly.
• The posterior branch runs posteriorly supplying bone and meninges.

3) Posterior cranial fossa

• is formed by the temporal bone and the occipital bones.
• houses the cerebellum.

The posterior cranial fossa contains:

• the foramen magnum for the lower end of the medulla.
• The clivus, anterior to the foramen magnum, which leads up to the body of the sphenoid bone.
• The transverse sinus
• The sigmoid sinus which ends at the jugular foramen.
• The hypoglossal canal (anterior condylar) canal and the posterior condylar canal.
• The internal auditory meatus on the posterior aspect of petrous temporal bone.
• The arcuate eminence on the petrous portion of the temporal bone, marking the position of the superior semicircular canal.

The superior sagittal sinus is continuous with the right transverse sinus whereas the smaller straight sinus is continuous with the left transverse sinus. The right jugular foramen is thus usually larger than the left.

THE EXTERIOR OF THE BASE OF THE SKULL

• Recognize the above- mentioned foramina from the exterior.
• The spine of the sphenoid bone near the foramen spinosum.
• The jugular foramen is occupied by the jugular bulb (for expansion of the internal jugular vein) in life.
• The styloid process, the mastoid process and the stylomastoid foramen between the 2 processes.
• Anterior to the jugular foramen and in the petrous portion of the temporal bone lies the carotid canal.
• The spine of the sphenoid bone lies medial to the mandibular fossa. The mandibular fossa articulates with the head of the mandible to form the temporomandibular (TMJ) joint.
• The foramen ovale lies at the base of the lateral pterygoid plate.
• The medial pterygoid plate (with the pterygoid hamulus at its base) and the lateral pterygoid plate are parts of the sphenoid bone.
• The inferior orbital fissure leads anteriorly from the pterygoid region to the orbit.
• The hard palate is formed by the palatine process of the maxilla and by the horizontal plate of the palatine bone. There are 3 foramina in the horizontal plate, the larger being the greater palatine foramen.
• The vomer bone in the posterior opening (the choanae) of the nasal cavity is attached by a fibrous joint to the undersurface of the body of the sphenoid.

The internal carotid artery:

• enters the carotid canal,
• runs anteromedially to the foramen lacerum
• and then runs superoanteriorly to enter the cranium through the internal orifice of the foramen lacerum.
• It then turns anteriorly and lies on the side of the sella turcica. It now lies in the cavernous sinus.
• It then turns superoposteriorly, medial to the anterior clinoid process and breaks up into 3 branches.

THE SKELETON OF THE FACE

In the orbit are found:

• the superior orbital fissure
• the optic canal
• The greater and lesser wings of the sphenoid.
• The zygomatic bone and the (hollow) maxilla are in the lateral wall of the orbit.
• The ethmoid bone and the small fragile lacrimal bone are in the medial wall .

The supraorbital, infraorbital and mental foramina, lie on a vertical line and transmit the ophthalmic, maxillary and mandibular divisions of the trigeminal (Vth cranial) nerve.