Norma Basalis Interna

THE ANTERIOR CRANIAL FOSSA

The Boundaries of theAnterior Cranial Fossa

Boundary Bone

Anterior Squamous frontal bone

Laterally Squamous frontal bone

Midline Ethmoid bone

The Boundaries of theAnterior Cranial Fossa

Boundary Bone

Posterior 1.Posterior margin of the lesser wing of sphenoid2.Anterior margin of the chiasmatic sulcus

Boundary BoneFloor Orbital plate of frontal

bone,cribriform plate of ethmoid bone, the crista galli, and the lesser wing of sphenoid

Special Features of the Anterior Cranial Fossa

1. Frontal crest2. Foramen cecum3. Ethmoidal notch4. Crista galli5. Cribriform plate of ethmoid bone6. Jugum of sphenoid bone7. Anterior and posterior ethmoidal foramina8. Grooves for anterior meningeal vessels

Frontal crest

- A ridge arising at the termination of the sagittal sulcus on the cerebral surface of the frontal bone and ending at the foramen cecum.

- for anterior attachment of the falx cerebri

Foramen cecum

- foramen varies in size in different subjects, and is frequently impervious; when open, it transmits a vein from the nose to the superior sagittal sinus.

Ethmoidal notch

- separates the two orbital plates; it is quadrilateral, and filled, in the articulated skull, by the cribriform plate of the ethmoid.

Crista galli

- vertical plate of bone projecting upward along the midline from the cribriform plate. It is continuous below with the bony nasal septum and anchors the anterior end of the falx cerebri.

• Cribriform plate of ethmoid bone

- is that horizontal part of the ethmoid bone between the midline and the frontoethmoidal suture. It is thin, perforated with many small holes and easily fractured. It forms the roof of the nasal cavity on each side.

Jugum of sphenoid bone

• a plane surface on the sphenoid bone, in front of the sella turcica, connecting the two lesser wings, and forming part of the anterior cranial fossa.

Foramina in the Anterior Cranial Fossa and their Contents

Name Contents

Cribriform Olfactory nerve fibers

Anterior ethmoid Ant. Ethmoid vessels and nerves

Posterior ethmoid Post. Ethmoid vessels and nerves

Foramen cecum Origin of superior venous sinus

Clinical Significance

Fractures in the floor of the anterior cranial fossa may involve the cribriform plate of the ethmoid, resulting in leakage of CSF through the nose (CSF rhinorrhea). CSF rhinorrhea may be a primary indication of a cranial base fracture which increases the risk of meningitis, because an infection could spread to the meninges from the ear or nose.

THE MIDDLE CRANIAL FOSSA

The Boundaries of theMiddle Cranial Fossa

Boundary Bone

Anterior 1. Lesser wings of sphenoid2. Anterior clinoid process3. Anterior margin of the

chiasmatic groove

Boundary Bones

Posterior 1. Superior angles of the petrous temporal

2. Dorsum sellae

Boundary Bone

Lateral 1. Squamous temporal2. Petrous temporal3. Greater wings of sphenoid4. Parietal bones

Boundary Bones

Floor 1. Squamous temporal2. Petrous temporal3. Greater wing of sphenoid4. Sella tursica

Special Features of the Middle Cranial Fossa

1. Sella tursica2. Posterior clinoid processes3. Carotid sulcus (groove)4. Carotid canal5. Superior orbital 6. fissure7. Optic foramina8. Foramen rotundum9. Foramen ovale10. Foramen spinosum

Special Features of the Middle Cranial Fossa

11. Foramen lacerum12. Arcuate emminence13. Depression for trigeminal ganglion14. Grooves for the greater and lesser petrosal nerves15. Groove for the superior petrosal sinus16. Grooves for the branches of middle meningeal arteries

Sella tursica

• A saddlelike prominence on the upper surface of the sphenoid bone of the skull, situated in the middle cranial fossa and dividing it into two halves.

• a depression on the upper surface of the sphenoid bone, lodging the pituitary gland.

Posterior clinoid processes

Posterior angle of the dorsum sellae which deepen the sella turcica, and give attachment to the tentorium cerebelli.

Carotid sulcus (groove)

it lodges the internal carotid artery and the cavernous sinus.

Carotid canal

is the large circular aperture On the interior surface of the temporal bone, behind the rough surface of the apex

Superior orbital fissure

lies between the lesser and greater wings of the sphenoid bone. Through it pass the branches of the ophthalmic division of the trigeminal (V1), the oculomotor (III), trochlear (IV), and abducens (VI) nerves, and the superior ophthalmic vein.

Optic foramina

the optic nerve and ophthalmic artery (with accompanying sympathetis nerve fibres) into the orbital cavity.

Foramen rotundum

lies in the floor of the middle cranial fossa at the junction of the body and greater wing of the sphenoid. It transmits the maxillary division of the trigeminal nerve (V2) from the cranial cavity into the pterygopalatine fossa.

Foramen ovale

is a large oval opening in the posterior part of the greater wing of the sphenoid, half-way across the floor of the middle cranial fossa. It transmits the mandibular division of the trigeminal nerve (V3) from the cranial cavity into the infratemporal fossa, along with the lesser petrosal branch of the glossopharyngeal nerve (IX).

Foramen spinosum• small round hole in the greater wing of the

sphenoid just posterolateral to the foramen ovale. The middle meningeal branch of the maxillary artery passes upward through the foramen into the middle cranial fossa, accompanied by its postganglionic sympathetic nerve plexus and the nervus spinosus, a recurrent meningeal branch of the mandibular division of the trigeminal nerve.

Foramina in the Middle Cranial Fossa and their Contents

Name Contents

Optic Optic n. and opthalmic a.

Supraorbital Fissure III, IV, VI, opthalmic V nn; sympathetic nn; opthalmic vv.

Rotundum Maxillary N.

Ovale Mandibular n., meningeal a.

Spinosum Middle meningeal a.

Grooves for greater petrosal n.

Greater petrosal n., petrosal br. Middle meningeal a.

Carotid canal Internal carotid a. and sympathetic nn.

THE POSTERIOR CRANIAL FOSSA

The Boundaries of thePosterior Cranial Fossa

Boundary Bone

Anterior 1. Dorsum sellae2. Basal occipital3. Crest of petrous temporal

Lateral Parietal bone

Boundary Bone

Posterior Squamous occipital boneFloor Occipital and temporal

bones

Special Features of the Posterior Cranial Fossa

1. Foramen magnum2. Hypoglossal canal3. Jugular foramen4. Internal acoustic

meatus5. Internal occipital crest6. Grooves for tansverse

and sigmoid sinuses7. Mastoid foramen

Foramen magnum

the large slightly oval opening in the center of the floor of the posterior cranial fossa. The brain stem becomes continuous with the spinal cord through the foramen, while the spinal accessory nerves (XI) pass upward from cervical cord segments into the cranial cavity.

Hypoglossal canal

• pass anterolaterally through the occipital bone immediately above the occipital condyles. They transmit the hypoglossal (XII) nerves.

Jugular foramen

• a large, irregular widening of the petrooccipital suture below the internal auditory meatus. The sigmoid and inferior petrosal dural venous sinuses pass through the foramen to join the internal jugular vein in the jugular fossa. The glossopharyngeal (IX), vagus (X), and spinal accessory (XI) nerves also exit the posterior cranial fossa via the jugular foramen

Internal acoustic meatus

(also internal acoustic meatus, internal auditory canal, and internal acoustic canal) is a canal in the petrous part of the temporal bone of the skull that carries nerves from inside the cranium towards the middle and inner ear compartments namely cranial nerve VII and cranial nerve VIII.

Internal occipital crest

gives attachment to the falx cerebelli

Mastoid foramen

is a large hole in the posterior border of the temporal bone. It transmits a vein to the transverse sinus and a small branch of the occipital artery to the dura mater.

Foramina in the Posterior Cranial Fossa and their Contents

Name Contents

Magnum Spinal cord, accessory n.,vertebral aa. Anterior and posterior spinal aa.

Jugular Inf. petrosal and trans. sinuses, meningeal brs. of occipital and ascending pharyngeal aa., IX,X, XI nn.

Hypoglossal XII n., meningeal br. of ascending pharyngeal a.

Internal auditory meatus

VII, VIII nn.; labyrinthine (int. auditory) a.

Other Bones Involved in Norma Basalis Interna

1. Palatine Bone

2. Ethmoid Bone

3. Sphenoid Bone

THE PALATINE BONE

• A pair of L-shaped bones, presenting two main parts that are at right angles with each other

Characteristic Features:

1. Horizontal plate- forms the opposite side of the posterior and lesser portion of the hard palate.

2. Perpendicular part- The longer process

found immediately in front of the medial pterygoid plate of sphenoid; it forms the lateral wall of the nasal cavity.

Characteristic Features of the Palatine Bone

3. Pyramidal process- is directed backwards and laterally from the junction of the horizontal and perpendicular plates.

4. Orbital and sphenoidal recesses- two irregular projections that stand up from the upper end of the perpendicular plate.

THE ETHMOID BONE

• Is irregular in shape, inserted into a notch between the orbital plates of the frontal bone, in front of the sphenoid bone. It forms part of the nasal, cranial, and orbital cavities.

Characteristic Features

1. Cribriform plate- a thin, horizontal plate, separating the nasal cavity from the cranial cavity. It is perforated by small holes for the passage of the olfactory nerves.

2. Perpendicular plate- is an irregular pentagonal plate of bone forming the upper anterior part of the bony nasal septum. - extends upward as the crista galli which serves for the attachment of the falx cerebri.

3. Ethmoidal labyrinth- are found on each side of the perpendicular plate and contain air sinuses – the anterior, middle, and posterior ethmoidal paranasal sinuses.

The Inferior View

The Lateral View

The Superior View

THE SPHENOID BONE

Is found anterior to the occipital bone, between the two temporal bones at the base of the skull.

CHARACTERISTIC FEATURES OF THE SPHENOID BONE

1. The Body- is the central mass in front of the basilar part of the occipital bone. - saddle-shaped depression superiorly called the sella tursica or the hypophyseal fossa (location of the pituitary gland or hypophysis cerebri). - hollow due to the presence of the sphenoidal paranasal sinuses.

2. Greater wings- form the floor of the middle cranial fossa and the side of the skull. It is the location of the “ r o s” foramens.

3. Lesser wings- are flattened, triangular plates whose roots surround the optic foramen.

4. Pterygoid processes- is made up of the lateral and medial laminae enclosing the pterygoid fossa. Superior to the medial pterygoid is the scaphoid fossa while inferior to it is the pterygoid hamulus.