L4. Anatomy of the Skull Flashcards
The skull is made up of separate bones divided into 2 groups, what are these?
The cranium (the main part of the skull) and the facial bones
The cranium of the skull is further subdivided into 2 parts, what are these? Describe them
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.
The component bones of the skull have a ‘sandwich’ like structure. What is meant by this?
The bones all have an external table and internal table both compact bone.
These are separated by a middle layer of spongey bone (diploe)
What are the separate bones making up the cranium
FrontalTemporalParietalOccipitalSphenoid
The separate bones are all united what type of joint? What is the name for these?
Immobile joints called sutures
The names of these (four) sutures are …
CoronalSagittalLambdoidPterion
Describe the Coronal suture
Is in the coronal plane in the anterior portion of the skull joining where the FRONTAL and PARIETAL bones articulate
Describe the Sagittal suture
Is the sagittal plane where the 2 parietal bones unite
Describe the Lamdoid suture
Is in coronal plane posteriorly on the skull and is the joining of the OCCIPITAL and PARIETAL bones
Describe the Pterion suture- what view is it seen best in?
LATERAL VIEW it is seen as an H-shaped sutureIt is more complexWhere the FRONTAL, PARIETAL, TEMPORAL and SPHENOID bones meet
What is important to know abut the region of the bones making up Pterion suture in terms of pathology?
Bones are very THIN in this regionThe MIDDLE MENINGEAL ARTERY grooves underneath this bone (dangerous area to have a head knock)
What are the components making up the frontal bone? Describe the shape of the bone
- 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.
What is the line of demarcation between the 2 parts of the frontal bone?
The line of demarcation is the 90 degree bend when the horizontal part at the supraorbital margins
Describe the two Parietal Bones
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.
Describe the Occipital Bone
A relatively flat bone that is moulded to the contour of the brain POSTERIORLY.
What is the prominent bulge on the posterior aspect of the Occipital bone called?
External Occipital Protuberance (EOP).
What is the main part of the temporal bone?Describe it
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.
What are the processes off the squamous part of the temporal bone?
ZYGOMATIC PROCESS anteriorly MASTOID PROCESS posteriorlySTYLOID PROCESS more deeply (and intermediately)
What does the zygomatic process of the temporal bone articulate with?
With the zygoma (cheek bone)
Describe the mastoid process of the temporal bone
- 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
Describe the deeply placed styloid process
It comes to a sharp point and is intermediate between the other two processes.It gives rise to midline structure muscles (tongue, pharynx)
There is fourth process of the temporal bone that doesn’t project inferiorly (but interiorly)
PETROUS = A wedge/pyramidal shaped piece of bone that projects into the cranial cavity forming part of the floor.
Where is the sphenoid bone?
It is very little is represented on the external view of the skull. Look at the cranial base/floor
Describe the anatomy of the sphenoid bone (and its main parts)
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)
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?
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.
What are the major steps of the inferior cranial aspect?
Anterior Cranial FossaMiddle Cranial FossaPosterior Cranial Fossa
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?
It must have fissures for the exit of the cranial nerves
What is the anterior cranial fossa made up of?
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).
What is the ethmoid bone?What are the holes through it?
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.
What is the middle cranial fossa made up of?
The greater wings of sphenoid anteriorly and the Petrous part of the temporal bone posteriorly
What is the posterior cranial fossa made up of?
It is all formed by the occipital bone
The middle cranial fossa has several foraminae [6]What are these and what goes through each of them?
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)
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.
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)
What are the foraminae of the posterior cranial fossa and what goes through them?
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)
What are the main meninges of the skull? Describe each one
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)
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
-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)
A real space exists between meningeal layers, what is this space?
Subarachnoid space between arachnoid and pia mata containing CSF circulation
Dural partitions exist that project into the cranial cavity and lodge subdivisions of the brain. What is the the function of these?
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)
What are the major dural septa? (4)
- 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)
- 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.
- 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
Describe the formation of the Dural Venous Sinuses
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).
Describe drainage of venous blood in the brain
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).
