L4. Anatomy of the Skull

Question 1

The skull is made up of separate bones divided into 2 groups, what are these?

Answer 1

The cranium (the main part of the skull) and the facial bones

Question 2

The cranium of the skull is further subdivided into 2 parts, what are these? Describe them

Answer 2

The upper part is the cranial vault forming the roof and the lower part is called the cranial floor on which the brain sits upon.

Question 3

The component bones of the skull have a ‘sandwich’ like structure. What is meant by this?

Answer 3

The bones all have an external table and internal table both compact bone.

These are separated by a middle layer of spongey bone (diploe)

Question 4

What are the separate bones making up the cranium

Answer 4

FrontalTemporalParietalOccipitalSphenoid

Question 5

The separate bones are all united what type of joint? What is the name for these?

Answer 5

Immobile joints called sutures

Question 6

The names of these (four) sutures are …

Answer 6

CoronalSagittalLambdoidPterion

Question 7

Describe the Coronal suture

Answer 7

Is in the coronal plane in the anterior portion of the skull joining where the FRONTAL and PARIETAL bones articulate

Question 8

Describe the Sagittal suture

Answer 8

Is the sagittal plane where the 2 parietal bones unite

Question 9

Describe the Lamdoid suture

Answer 9

Is in coronal plane posteriorly on the skull and is the joining of the OCCIPITAL and PARIETAL bones

Question 10

Describe the Pterion suture- what view is it seen best in?

Answer 10

LATERAL VIEW it is seen as an H-shaped sutureIt is more complexWhere the FRONTAL, PARIETAL, TEMPORAL and SPHENOID bones meet

Question 11

What is important to know abut the region of the bones making up Pterion suture in terms of pathology?

Answer 11

Bones are very THIN in this regionThe MIDDLE MENINGEAL ARTERY grooves underneath this bone (dangerous area to have a head knock)

Question 12

What are the components making up the frontal bone? Describe the shape of the bone

Answer 12

1. VERTICAL PART that forms the forehead 2. HORIZONTAL PART that projects into the skull (forms the roof of the orbit)The vertical part curves over and descends down and at the supraorbital margins there is a 90 degree angle/turn into the horizontal part.

Question 13

What is the line of demarcation between the 2 parts of the frontal bone?

Answer 13

The line of demarcation is the 90 degree bend when the horizontal part at the supraorbital margins

Question 14

Describe the two Parietal Bones

Answer 14

They are flat and rectangular bones that are moulded to the shape of the brain (contour). Each parietal bone (right and left) from anterior to posterior meet in the sagittal plane and descends down to the temporal bone.

Question 15

Describe the Occipital Bone

Answer 15

A relatively flat bone that is moulded to the contour of the brain POSTERIORLY.

Question 16

What is the prominent bulge on the posterior aspect of the Occipital bone called?

Answer 16

External Occipital Protuberance (EOP).

Question 17

What is the main part of the temporal bone?Describe it

Answer 17

The SQUAMOUS PART OF THE BONEIt is flat and moulded to the brain (similar to the parietal bone) It is the part of the temporal bone that contributes to the pterion suture. Projecting off the squamous part are three processes or boney projections off the inferior part.

Question 18

What are the processes off the squamous part of the temporal bone?

Answer 18

ZYGOMATIC PROCESS anteriorly MASTOID PROCESS posteriorlySTYLOID PROCESS more deeply (and intermediately)

Question 19

What does the zygomatic process of the temporal bone articulate with?

Answer 19

With the zygoma (cheek bone)

Question 20

Describe the mastoid process of the temporal bone

Answer 20

• It is a chunky process full of air cells inside - It projects down inferiorly - It gives rise to muscles in the neck like sternomastoid muscles- It is relatively superficial compared to other processes

Question 21

Describe the deeply placed styloid process

Answer 21

It comes to a sharp point and is intermediate between the other two processes.It gives rise to midline structure muscles (tongue, pharynx)

Question 22

There is fourth process of the temporal bone that doesn’t project inferiorly (but interiorly)

Answer 22

PETROUS = A wedge/pyramidal shaped piece of bone that projects into the cranial cavity forming part of the floor.

Question 23

Where is the sphenoid bone?

Answer 23

It is very little is represented on the external view of the skull. Look at the cranial base/floor

Question 24

Describe the anatomy of the sphenoid bone (and its main parts)

Answer 24

Looks like a bat with outstretched wings: - BODY (has a depression/pit called the hypophysial fossa) where the pituitary gland sits- WINGS, which have a split/fissure in them (SUPERIOR ORBITAL FISSURE going to the orbit. They then split the wing into a LESSER WING or sphenoid anteriorly and a GREATER WING of sphenoid posteriorly. - PTERYGOID PLATES which give rise to muscles of mastication (oral and laryngeal structures) - two little legs hanging down (not present in the slide)

Question 25

The contents of the cranial vault/sitting on the cranial floor are the brain, cranial nerves and meninges. What is important to note about the inferior aspect of the cranial floor?

Answer 25

IT IS NOT FLAT (has a series of steps) The frontal lobe is higher than the temporal which is higher than the cerebellum + brainstem. Thus the cranial floor must also be stepped.

Question 26

What are the major steps of the inferior cranial aspect?

Answer 26

Anterior Cranial FossaMiddle Cranial FossaPosterior Cranial Fossa

Question 27

There are 12 pairs of cranial nerves each with target structures/areas of distribution outside the skull, what does this mean for the cranial floor?

Answer 27

It must have fissures for the exit of the cranial nerves

Question 28

What is the anterior cranial fossa made up of?

Answer 28

The horizontal plates of the frontal bone and right off the edge of the step is the lesser wing of sphenoid (superior and anterior to the greater wing).

Question 29

What is the ethmoid bone?What are the holes through it?

Answer 29

A thin bone that sits between the eyes and contributes to the nasal structure and the very top of it forms part of the anterior cranial fossa. It has a series of holes (CRIBIFORM) and the olfactory fibres come up from nasal mucosa of the nose through them carrying sense of smell.

Question 30

What is the middle cranial fossa made up of?

Answer 30

The greater wings of sphenoid anteriorly and the Petrous part of the temporal bone posteriorly

Question 31

What is the posterior cranial fossa made up of?

Answer 31

It is all formed by the occipital bone

Question 32

The middle cranial fossa has several foraminae [6]What are these and what goes through each of them?

Answer 32

Hypophysial fossa (pituitary gland)Optic canal (optic nerve, ophthalmic artery)Superior orbital fissure (nerves to extraocular muscles and branches of trigeminal nerve)Foramen rotundum (maxillary nerve)Foramen ovale (mandibular nerve)Foramen spinosum (middle meningeal artery)

Question 33

The Superior orbital fissure of the sphenoid bone is the first hole in an ‘ARC OF FISSURES’ (good way to remember them). How does this arc then descend in the C shape all through the sphenoid bone.

Answer 33

Superior orbital fissure forms the first half of an arc of fissuresRound fissure - foramen rotundumOval fissure - foramen ovaleVery small hole - foramen spinosum (middle meningeal artery enters the cranial cavity and it grooves the bone)

Question 34

What are the foraminae of the posterior cranial fossa and what goes through them?

Answer 34

Internal auditory meatus (hole in the petrous part of the bone) - facial and vestibulocochlear nervesJugular foramen (irregular shaped) in the split between petrous part of temporal bone and occipital bone (is between bones) - IX, X, XI, internal jugular veinHypoglossal canal is in the occipital bone (seen more clearly in the undersurface of the skull) - hypoglossal nerveAnd the Foramen Magnum (brainstem)

Question 35

What are the main meninges of the skull? Describe each one

Answer 35

DURA MATA: outer layer, tough, fibrous and attached to the internal surface of the skull. ARACHNOID: delicate, fibrous layer with a spidery appearance that lines the internal surface of the duraPIA MATA: most delicate of them, it is attached to very intimately to the brain tissue (difficult to disengage it)

Question 36

Associated with these meninges are potential spaces, what are the two potential spaces?Note there is no space (potential or real) between the pia mata and the brain

Answer 36

-Potential extradural space between dura and skull (where blood collects when the meningeal artery tears occur- extradural haemorrhage)- Potential subdural space between dura and arachnoid (blood can also collect here, typically venous blood associated with a subdural venous haemorrhage)

Question 37

A real space exists between meningeal layers, what is this space?

Answer 37

Subarachnoid space between arachnoid and pia mata containing CSF circulation

Question 38

Dural partitions exist that project into the cranial cavity and lodge subdivisions of the brain. What is the the function of these?

Answer 38

Dura that IS NOT lining the internal surface of the skull and delves into the skull between the major subdivisions Believed that the function is to restrict rotatory displacement of the brain (the most damaging)

Question 39

What are the major dural septa? (4)

Answer 39

1. FALX CEREBRI: sickle shaped lying in the sagittal plane (immediately beneath the sagittal suture), between right and left cerebral hemispheres, doesn’t descend the whole way. From the ethmoid bone to the posterior attachment to the EOP (beyond sagittal suture)
2. TENTORIUM CEREBELLI: perpendicular to the falx cerebri and comes in from behind in a horizontal plane (slightly tilted up). Is cresent shaped and doesn’t go all the way in. It grooves in the posterior cranial fossa separating cerebral above and cerebellum below.
3. FALX CEREBELLI: small vertical fold between the hemispheres of the cerebellum4. DIAPHRAGMA SELLAE: grooves the top of the pit of the body of the sphenoid

Question 40

Describe the formation of the Dural Venous Sinuses

Answer 40

At the root of the dural septa there are collections of venous blood in venous sinuses. Venous blood of the brain collects in the dural venous sinuses (endothelial lined surfaces where the outer and inner layers of dura have split to form a partition).

Question 41

Describe drainage of venous blood in the brain

Answer 41

Venous drainage of the brain ends up in these sinuses and also receive from diploic veins, emissary veins from the outside scalp to communicate with the diploic veins and the sinus. (ie from the brain, the skull and the scalp).