Cranium, meninges and brain part 2

Question 1

Which structures are found in the cranial base

Answer 1

The cranial base has lots of foramina and can be split into different regions- depending on which part of the brain sits in these regions

Question 2

Summarise the anterior cranial fossa

Answer 2

Parts of the frontal, ethmoid and sphenoid bones form the anterior cranial fossa:

frontal bone in anterior direction
ethmoid bone in the midline
two parts of the sphenoid bone posteriorly, the body (midline) and the lesser wings (laterally)

Question 3

Describe the anterior part of the anterior cranial fossa

Answer 3

Anteriorly, a small wedge-shaped midline crest of bone (the frontal crest) projects from the frontal bone. This is a point of attachment for the falx cerebri. Immediately posterior to the frontal crest is the foramen cecum. This foramen between the frontal and ethmoid bones may transmit emissary veins connecting the nasal cavity with the superior sagittal sinus.

Question 4

Describe the crista galli in anterior cranial fossa

Answer 4

Posterior to the frontal crest is a prominent wedge of bone projecting superiorly from the ethmoid (the crista galli). This is another point of attachment for the falx cerebri, which is the vertical extension of dura mater partially separating the two cerebral hemispheres.

Question 5

Describe the cribiform plate

Answer 5

Lateral to the crista galli
Sieve-like structure, which allows small olfactory nerves to pass through its foramina from the nasal mucosa to the olfactory bulb. The olfactory nerves are commonly referred to collectively as the olfactory nerve

Question 6

What is found on each side of the ethmoid in the anterior cranial fossa

Answer 6

The floor of the anterior cranial fossa is formed by relatively thin plates of frontal bone (the orbital part of the frontal bone) which also forms the roof of the orbit below.
Posterior to both the frontal and ethmoid bones, the rest of the floor of the anterior cranial fossa is formed by the body and lesser wings of the sphenoid. In the midline, the body extends anteriorly between the orbital parts of the frontal bone to reach the ethmoid bone and posteriorly it extends into the middle cranial fossa.

Question 7

What is the boundary between the anterior and middle cranial fossae

Answer 7

Anterior edge of the chiasmatic sulcus, a smooth groove stretching between the optic canals across the body of the sphenoid.

Question 8

Describe the anterior clinoid process

Answer 8

Medially, each lesser wing of the sphenoid curves posteriorly and ends as a rounded anterior clinoid process. These processes serve as the anterior point of attachment for the tentorium cerebelli, which is a sheet of dura that separates the posterior part of the cerebral hemispheres from the cerebellum

Question 9

Summarise the middle cranial fossa

Answer 9

the middle cranial fossa consists of parts of the sphenoid and temporal bones- houses the temporal lobe
the posterior boundaries of the middle cranial fossa are formed by the anterior surface as high as the superior border, of the petrous part of the petromastoid part of the temporal bone

Question 10

Describe the floor in the midline of the cranial fossae

Answer 10

The floor in the midline of the middle cranial fossae is elevated and formed by the body of the sphenoid. Lateral to this are large depressions formed on either side by the greater wing of the sphenoid and the squamous part of the temporal bone. These depressions contain the temporal lobes of the brain.

Question 11

What is the sella turcica

Answer 11

Just posterior to the chiasmatic sulcus is the uniquely modified remainder of the body of the sphenoid- sella turcica- which consists of a deep central area (the hypophyseal fossa) containing the pituitary gland with anterior and posterior walls of bone.

Question 12

Describe the lateral projections and posterior wall of the sella turcica

Answer 12

The lateral projections from the corners of tuberculum sellae (the middle clinoid processes) are sometimes evident.
The posterior wall of the sella turcica is the dorsum sellae, a large ridge of bone projecting upward and forward. At the top of this bony ridge the lateral edges contain rounded projections (the posterior clinoid processes), which are points of attachment for, like the anterior clinoid processes, the tentorium cerebelli.

Question 13

Describe the superior orbital fissure

Answer 13

A diagonal gap, the superior orbital fissure, separates the greater wing of the sphenoid from the lesser wing and is a major passageway between the middle cranial fossa and the orbit. Passing through this fissure are the oculomotor nerve, the trochlear nerve, the opthalamic division of the trigeminal nerve, abducent nerve and opthalamic veins.

Question 14

Describe the foramen rotundum

Answer 14

Posterior to the medial end of the superior orbital fissure on the floor of the middle cranial fossa is a rounded foramen projecting in an anterior direction (foramen rotundum), through which the maxillary division of the trigeminal nerve passes

Question 15

Describe the foramen ovale

Answer 15

Posterolateral to the foramen rotundum is a large oval opening the foramen ovale, which allows structures to pass between the extracranial infratemporal fossa and the middle cranial fossa.
The mandibular division of the trigeminal nerve and lesser petrosal nerve (carrying fibres from the tympanic plexus that originally came from the hypoglossal nerve) and occasionally a small vessel (the accessory middle meningeal artery) pass through this foramen.

Question 16

Describe the foramen spinosum

Answer 16

Posterolateral from the foramen ovale is the foramen spinosum. This opening also connects the infratemporal fossa with the middle cranial fossa. The middle meningeal artery and its associated veins pass through this foramen and, once inside, the groove for the middle meningeal artery across the floor and lateral wall of the middle cranial fossa clearly marks their path.

Question 17

Describe the carotid canal and the foramen lacerum

Answer 17

Posteromedial to the foramen ovale is the rounded intracranial opening of the carotid canal. Directly inferior to this opening is an irregular foramen (the foramen lacerum). Closed in life by a cartilaginous plug, and no structures pass through completely.

Question 18

Describe the trigeminal impression of the temporal bone

Answer 18

Medially, there is a slight depression (trigeminal impression) in the anterior surface of the petrous part of the temporal bone, which marks the location of the sensory ganglion for the trigeminal nerve.

Question 19

What is found lateral to the trigeminal impression

Answer 19

Lateral to the trigeminal impression and on the outer surface of the petrous part of the temporal bone is a small linear groove that passes in a superolateral direction and ends in a foramen (the groove and hiatus for the greater petrosal nerve). The greater petrosal nerve is a branch of the facial nerve.

Question 20

Describe the hiatus for the lesser petrosal nerve

Answer 20

Anterolateral to the groove for the greater petrosal nerve is a second, smaller groove and hiatus for the lesser petrosal nerve, a branch of the tympanic plexus carrying fibres that originally came from the glossopharyngeal nerve.

Question 21

Describe the arcuate eminence

Answer 21

Above and lateral to the small openings of the greater and lesser petrosal nerves, near the superior ridge of the petrous part of the temporal bone, is a rounded protrusion of bone (the arcuate eminence) produced by the underlying anterior semi-circular canal of the inner ear

Question 22

Describe the tegmen tympani

Answer 22

Just anterior and lateral to the arcuate eminence the anterior surface of the petrous part of the temporal bone is slightly depressed. This region is the tegmen tympani, and marks the thin bony roof of the middle ear cavity.

Question 23

Which structures pass through the optic canal

Answer 23

The optic nerve and opthalamic artery

Question 24

What can a fracture in the floor of the middle cranial fossa cause

Answer 24

A fracture through the roof of the temporomandibular joint may lead to the leakage of CSF from the external auditory meatus.
Middle meningeal artery (which passes through the foramen spinosum) high pressure artery- fast filling when bleeds- will raise intracranial pressure. Thus a fracture, of the skull laterally (especially in the region of the pterion) may cause haemorrhage of the middle meningeal artery, resulting in an extradural haematoma ( a collection of blood between the skull and dura) which causes an increase in pressure of the motor area of the cerebral cortex and eventually unconsciousness and death. Urgent surgical drainage is required.

Question 25

What can fractures in the anterior cranial fossa result in

Answer 25

Loss of smell (anosmia)

Question 26

Summarise the posterior cranial fossa

Answer 26

The posterior cranial fossa consists mostly of parts of temporal and occipital bones, with small contributions from the sphenoid and parietal bones. It is the largest and deepest of the 3 cranial fossae and contains the brainstem and the cerebellum.

Question 27

Describe the boundaries of the posterior cranial fossa

Answer 27

The anterior boundaries of the posterior cranial fossa in the midline are the dorsum sellae and the clivus. The clivus is a slope of bone that extends upwards from the foramen magnum. It is formed by contributions from the body of the sphenoid and from the basilar part of the occipital bone.
Laterally, the anterior boundaries of the posterior cranial fossa are the superior border of the petrous part of the petromastoid part of the temporal bone.
Posteriorly, the squamous part of the occipital bone to the level of the transverse groove is the major boundary while laterally the petromastoid part of the temporal bone and small parts of the occipital and parietal bones border the fossa.

Question 28

Describe the foramen magnum

Answer 28

Centrally, in the deepest part of the posterior cranial fossa, is the largest foramen in the skull. It is surrounded by the basilar part of the occipital bone anteriorly, the lateral parts of the occipital bone on either side, and the squamous part of the occipital bone posteriorly.
The spinal cord passes superiorly through the foramen magnum to continue as the brainstem
Also passing through the foramen magnum are the vertebral arteries, the meninges, and the spinal roots of the accessory nerve.

Question 29

Describe the inferior petrosal sinus

Answer 29

The clivus slopes upwards from the foramen magnum. Lateral to the clivus is a groove for the inferior petrosal sinus between the basilar part of the occipital bone and the petrous part of the petromastoid part of the temporal bone

Question 30

Describe the internal acoustic meatus

Answer 30

Laterally, across the upper half of the posterior surface of the petrous part of the temporal bone, is an oval foramen (the internal acoustic meatus). The facial nerve, vestibulotrochlear nerve and labyrinthine artery pass through it

Question 31

Describe the sigmoid sinus

Answer 31

Inferior to the internal acoustic meatus the temporal bone is separated from the occipital bone by the large jugular foramen. Leading to this foramen from the medial side is the groove for the inferior petrosal sinus, and from the lateral side the groove for the sigmoid sinus.

Question 32

What do the sigmoid sinus and the inferior petrosal sinus empty into

Answer 32

The sigmoid sinus passes into the jugular foramen, and is continuous with the internal jugular vein, while the inferior petrosal sinus empties into the jugular vein in the area of the jugular foramen.
Also passing through the jugular foramen are the glossopharyngeal nerve, vagus nerve, accessory nerve, inferior petrosal sinus, sigmoid sinus (forming the jugular vein)

Question 33

Describe the hypoglossal canal

Answer 33

Medial to the jugular foramen is a large rounded mound of the occipital bone (the jugular tubercle). Just inferior to this, and superior to the foramen magnum, is the hypoglossal canal, through which the hypoglossal nerve leaves the posterior cranial fossa, and a meningeal branch of the ascending pharyngeal artery enters the posterior cranial fossa

Question 34

Describe the condylar canal

Answer 34

Just posterolateral to the hypoglossal canal is the small condylar canal that, when present, transmits an emissary vein.

Question 35

What is important to remember about the transverse and sigmoid sinuses

Answer 35

They are intradural venous sinuses.

Question 36

Describe the features of the squamous part of the occipital bone

Answer 36

Running upward in the midline from the foramen magnum is the internal occipital crest
On either side of the internal occipital crest, the floor of the posterior cranial fossa is concave to accommodate the cerebellar hemispheres
the internal occipital crest ends superiorly in a bony prominence (the internal occipital protuberance)
extending laterally from the internal occipital protuberance are grooves produced by the transverse sinuses, which continue laterally, eventually joining a groove for each sigmoid sinus- each of these grooves ten turns inferiorly towards the jugular foramina.

Question 37

Summarise the 3 layers of the meninges

Answer 37

3 layers
Dura mater – thick inelastic – 2 layers (periosteal and meningeal)
Arachnoid mater – elastic – spider-like projections
Pia mater – innermost, thin, delicate layer

Question 38

Outline the meninges

Answer 38

The brain, as well as the spinal cord,is surrounded by three layers of membrane (the meninges)- a tough, outer layer (the dura mater), a delicate, middle layer (the arachnoid mater), and an inner layer firmly attached to the surface of the brain (pia mater)
The cranial meninges are continuous with, and similar to, the spinal meninges through the foramen magnum. The important distinction is that the cranial dura mater consists of two layers- but only one of these (meningeal layer) is continuous for the foramen magnum.

Question 39

Describe the cranial dura mater

Answer 39

The cranial dura mater is a thick, tough, outer covering of the brain. It consists of an outer periosteal layer and an inner meningeal layer.
The outer periosteal layer is firmly attached to the skull, is the periosteum of the cranial cavity, contains the meningeal arteries, and is continuous with the periosteum on the outer surface of the skull at the foramen magnum and other intracranial foramina.

The inner meningeal layer is in close contact with the arachnoid mater and is continuous with the spinal cord dura mater through the foramen magnum

Question 40

Are the two layers of the dura normally stuck together

Answer 40

Yes- except when the inner meningeal layer peels away to help anchor the brain or create sinuses that are filled with blood.
dural partitions- which project inward and completely separate parts of the brain
intracranial venous structures (e.g superior saggital sinus)

Question 41

Summarise the dural partitions

Answer 41

Project into the cranial cavity and partially subdivide the cranial cavity. They include the falx cerebri, tentorium cerebelli, falx cerebelli and diaphragma sellae

Question 42

Describe the falx cerebri

Answer 42

Crescent shaped downward projection of the meningeal dura mater from the dura lining the calva that passes between the two cerebral hemispheres. It is attached anteriorly to the crista galli of the ethmoid bone and frontal crest of the frontal bone. Posteriorly, it is attached to and blends with the tentorium cerebelli.

Question 43

Describe the tentorium cerebellum

Answer 43

Haorizontal projection of the meninges furs that covers and separates the cerebellum in the posterior cranial fossa from the posterior parts of the cerebral hemispheres. It is attached posteriorly to the occipital bone along the grooves for the transverse sinuses.
Laterally, it is attached to the superior border of the petrous part of the temporal bone, ending anteriorly at the anterior and posterior clinoid processes
The anterior and medial borders of the tentorium cerebelli are free, forming an oval opening in the midline(tentorium notch) through which the midbrain passes

Question 44

Describe the flax CEREBELLI

Answer 44

Small midline projection of the meningeal dura mater in the posterior cranial fossa. It is attached posteriorly to the internal occipital crest of the occipital bone and superiorly to the tentorium cerebelli. It’s anterior edge is free and between the two cerebellum hemispheres.

Question 45

Describe the diaphragma Sellae

Answer 45

Final Dural projection, small horizontal shelf of the meningeal dura mater covers the hypophyseal fossa in the sella turcica of the sphenoid bone. There is an opening in the centre of the diaphragma sellae through which passes the infundibulum, connecting the pituitary gland with the base of the skull, and any accompanying vessels.

Question 46

Ultimately, what is the purpose of these dural folds

Answer 46

Stop the movement of the brain

Provide spaces where blood can drain back into the venous system.

Question 47

Describe the arachnoid mater

Answer 47

Thin, avascular membrane that lines, but is not adherent to, the inner surface of the dura mater. From its inner surface thin processes or trabeculae extend downward, cross the subarachnoid space, and become continuous with the pia mater.
Unlike the pia, the arachnoid does not enter the groves or fissures of the brain, except for the longitudinal fissures between the two cerebral hemispheres.

Question 48

Describe the pia mater

Answer 48

Thin, delicate membrane that closely invests the surface of the brain. It follows the contours of the brain, entering the grooves and fissures on its surface, and is closely applied to the roots of the cranial nerves at their origins.

Question 49

Distinguish between the real and potential spaces of the meninges

Answer 49

Real- between arachnoid mater and the pia mater

Potential- dura mater

Question 50

Describe the extradural space

Answer 50

Potential space between the dura mater and bone is the extradurdal space. Normally, the outer or periosteal layer of dura mater is firmly attached to the bones surrounding the cranial cavity
This potential space between dura and bone can become a fluid-filled actual space when a traumatic event results in a vascular haemorrhage. Bleeding into the extradural space due to rupture of the meningeal artery or torn dural venous sinus results in an extradural heamatoma.

Question 51

Describe the sub dural space

Answer 51

Anatomically- a true subdrual space does not exist
Blood collecting in this region (subdural haematoma) due to injury represents a dissection of the dural border cell layer, which is the innermost lining of the meningeal dura. Dural border cells surrounded by extra cellular spaces filled with amorphous material. While very infrequent, an occasional cell junction may be seen between the cells and the underlying arachnoid layer. Bleeding due to the tearing of a cerebral vein as it crosses through the dura to enter a dural venous sinus can result in a subdrual haematoma

Question 52

Summarise the subarachnoid space

Answer 52

Deep to the arachnoid mater is the only normally occurring fluid-filled space associated with the meninges, the subarachnoid space
It occurs because the arachnoid mater clings to the inner surface of the dura mater and does not follow the contour of the brain, while the pia mater, being against the surface of the brain, closely follows the grooves and fissures on the surface of the brain. The narrow subarachnoid space is therefore created between the two membranes.

Question 53

What does the subarachnoid space surround

Answer 53

Surrounds the brain and spinal cord and in certain locations it enlarges into expanded areas (subarachnoid cisterns). it contains CSF and blood vessels.

Question 54

Outline the pathway of the CSF

Answer 54

CSF is produced by the chloroid plexus primarily in the ventricles of the brain. it is a clear, colourless, cell-free fluid that circulates through the subarachnoid space surrounding the brain and spinal cord.
The CSF returns to the venous system though arachnoid villi. These project as clumps (arachnoid granulations) into the superior sagittal sinus, which is a dural venous sinus, and its lateral extensions, the lateral lacunae.