Session 2 - Base Of Skull Flashcards
What forms the base of the skull?
Individual bones joined by fibrous joints known as sutures
Three cranial fossae - small holes inside them for structures to pass through = cranial foramina?
What is in the anterior cranial fossa?
- frontal lobes
- three bones: orbital part, cribriform plate and crista galli of ethmoid bone (olfactory nerves), lesser wings of the sphenoid bone
Only 1 foramen = cribiform plate
What is in the middle cranial fossa?
The temporal lobes rest in the middle cranial fossa, and it is formed of two bones:
• Petrous and squamous parts of the temporal bone – the petrous (meaning ‘rock-like’) is the very hard and bulbous inferior and medial part of the temporal bone. The inner and middle ear cavities are located inside it. The squamous (meaning ‘scale-like’) is the flat, lateral part of the temporal bone.
• Greater wing and body of the sphenoid bone – the body of the sphenoid bone includes a small, rounded cavity about the size of the end of your finger called the pituitary fossa. It is also known as the sella turcica (meaning ‘turkish saddle’) and it is where the pituitary gland is located.
Which foramina are located on either side of the middle cranial fossa?
foramina located on each side of the middle cranial fossa are the:
• Optic canal – transmits the optic nerve into the bony orbit.
• Superior orbital fissure – transmits several nerves that provide motor innervation (oculomotor, trochlear and abducens nerves) and sensation (ophthalmic branch of the trigeminal nerve) to the orbital region.
• Foramen rotundum – transmits the maxillary branch of the trigeminal nerve.
• Foramen ovale – transmits the mandibular branch of the trigeminal nerve.
• Carotid canal – the internal carotid artery passes through this s-shaped canal within the temporal bone to enter the skull. Foramen lacerum is located within this canal, but is covered by cartilage in life.
• Foramen spinosum – transmits the middle meningeal artery
What is inside the posterior cranial fossa?
The occipital lobes, cerebellum and brainstem rest in the posterior cranial fossa and it is formed primarily by the occipital bone, but part of the petrous part of the temporal bone makes up its most anterior border.
The foramina located on each side of the posterior cranial fossa are the:
• Internal auditory meatus – transmits the vestibulocochlear and facial nerves into the inner ear cavity.
• Jugular foramen – transmits the glossopharyngeal, vagus and accessory nerves, and the internal jugular vein.
• Hypoglossal canal – transmits the hypoglossal nerve.
• Foramen magnum – this central singular foramen is very large and allows central nervous system fibres to leave the skull and become the spinal cord.
What can head injuries cause?
A traumatic injury to the head may result in a fracture of the skull. If this occurs, there may be numerous consequences including:
• The brain itself could be directly damaged by the force.
• The fracture could extend through some of the foramina and damage the structures passing through them.
• The dura and arachnoid meninges may be damaged which could cause CSF to leak out. Clinically, this may be suspected if a clear liquid is seen to be leaking from the patient’s nose or ears after a head injury.
• Significant bleeding may occur from the fractured bone or due to damage to intracranial arteries, veins or dural venous sinuses.
What is the Pterion?
The pterion is an area of the skull often referred to as the ‘temple’ and it is located just lateral and posterior to the eyebrow. It is a shallow depression where four bones of the skull converge: the frontal, temporal, sphenoid and parietal bones. Because of this, it is considered to be the weakest part of the skull and prone to fracture if struck. Unfortunately, the middle meningeal artery lies immediately behind the pterion, therefore traumatic injuries to this area may cause an extradural haemorrhage.
What is a craniosynostosis?
The sutures (joints between the bones) of the skull do not completely fuse until a child is around two years old. This allows the brain to increase in size as the child grows quickly in infancy. If certain sutures of the skull fuse together too early, as the brain continues to grow it will cause the skull to become misshapen and this is called craniosynostosis. The skull may be elongated in the longitudinal, transverse or oblique planes, depending on which sutures fuse before they are supposed to.
What are burr holes and craniotomies?
If there is a build-up of pressure within the fixed confines of the skull, this must be relieved, or the brain will eventually be compressed which can lead to death. Common causes of a build of pressure in this way may include intracranial bleeding (such as an extradural haemorrhage) or a brain tumour. To relieve the pressure quickly, a small hole (about 10-15 mm diameter) can be drilled into the skull. This allows the brain to expand enough to relieve the pressure, or it can be used to directly drain the bleeding that’s causing the pressure build-up. To perform surgery on the brain, a larger hole may be needed. This is called a craniotomy, and a circular piece of the skull is removed. This may be replaced later, or a prosthetic implant may be used to close the craniotomy instead.
