The Cranial Fossae

1 Anterior Cranial Fossa: Houses frontal lobes.

o Includes all of Frontal and Ethmoid bones, and the Lesser Wing of the Sphenoid.

2 Middle Cranial Fossa: Houses temporal lobes.

o Includes the Greater Wing of the Sphenoid, Sella Turcica, and the majority of Temporal Bones.

1. Posterior Cranial Fossa: House occipital lobes, cerebellum, and medulla

o Includes the Occipital bone.

DURAL FOLDS: Extensions of Dura Mater that subdivide the cranial cavity and give support to the brain, such as when bungee jumping.

· FALX CEREBRI: Dural fold going sagittally between the cerebral hemispheres.

§ It connects anteriorly to the Crista Galli of the Ethmoid Bone and the Internal Occipital Protuberance posteriorly.

§ The Superior Sagittal Sinus is the superior limit of the Falx Cerebri, within the dura.

§ The Inferior Sagittal Sinus is the inferior limit of the Falx Cerebri, within the dura.

· TENTORIUM CEREBELLI: Lateral dural fold separating the cerebrum from the cerebellum.

§ The Falx Cerebri holds it down posteriorly, where they intersect.

§ The Transverse Sinus runs through it posteriorly.

§ The Superior Petrosal Sinus runs through it anteriorly.

§ INCISURA TENTORIA: The hole in the Tentorium Cerebelli that allows the cerebrum to communicate with the cerebellum.

· DIAPHRAGMA SELLAE: Small donut-shaped dural fold covering the Pituitary Gland, at the Sella Turcica of the Sphenoid Bone.

DURAL SINUSES: Venous sinuses which drain all blood from the brain. They are formed within the dura mater layer of the meninges. The Dura Mater splits into two layer in the cranium: A Periosteal (outer) Layer and a Meningeal (inner) Layer.The dural sinuses form between them.

1. SUPERIOR SAGITTAL SINUS: Runs within superior aspect of the Falx Cerebri.

· ARACHNOID VILLI: Granulations from the Arachnoid Mater project into this sinus, providing a way to empty CerebroSpinal Fluid into the Superior Sagittal Sinus. This is how we recycle cerebrospinal fluid (CSF).

· It drains into the CONFLUENCE OF SINUSES ------> TRANSVERSE SINUS

2. INFERIOR SAGITTAL SINUS: Runs within the inferior aspect of the Falx Cerebri.

· It joins the GREAT VEIN OF GALEN to form the Straight Sinus.

· From there Blood Drainage is as follows: STRAIGHT SINUS ------> TRANSVERSE SINUS

· The Sigmoid Sinus also communicates with the Internal Vertebral Venous Plexus.

3. OCCIPITAL SINUS: Lies in the Falx Cerebellia. It drains blood as follows: OCCIPITAL SINUS ------>CONFLUENCE OF SINUSES ------> TRANSVERSE SINUS
4. CONFLUENCE OF SINUSES: The junction between the Superior Sagittal, Straight, and Transverse Sinuses. It leads into the Transverse Sinus. The occipital sinus is caudal to it.
5. TRANSVERSE SINUS: It receives blood from all three sinuses above: Superior and Inferior Sagittal, and Occipital. It drains blood as follows:

· TRANSVERSE SINUS ------> SIGMOID SINUS ------> INTERNAL JUGULAR VEIN

6. SUPERIOR PETROSAL SINUS: Drains the Cavernous Sinus ------> OCCIPITAL SINUS
7. INFERIOR PETROSAL SINUS: Drains the Cavernous Sinus ------> INTERNAL JUGULAR VEIN directly.
8. CAVERNOUS SINUS: VERY IMPORTANT because it contains lots of nerves and vessels running through it.
• It is located at the Sella Turcica of the Sphenoid, surrounding the Pituitary Gland, within the Diaphragma Sella dural fold.
• It receives blood from:

i. Superior and Inferior Ophthalmic Veins

ii. Superficial Middle Cerebral Vein

iii. Sphenoparietal Sinus

• It drains blood into:

i. Inferior and Superior Petrosal Sinuses.

ii. Pterygoid Plexus.

· It has the following running through it:

§ All of Cranial Nerves III - VI, except V3

· All of the above run, in numerical order, along the lateral wall of the cavernous sinus from superior to inferior, EXCEPT for the Abducens Nerve (VI) which runs directly through the cavernous sinus.

§ The Internal Carotid Artery. This is also completely surrounded within the Cavernous Sinus.

· The Sphenoidal Paranasal Sinus, in the sphenoid bone, is inferior and medial to the cavernous dural sinus.

· SPHENOID FRACTURE: A fracture at the base of the sphenoid could produce a bone fragment that could sever the internal carotid in cavernous sinus ------> dural hematoma.

MENINGEAL ARTERIES: Arteries that supply the meninges and bone -- not the brain per se.

1. Middle Meningeal Artery is a branch off the first part of the Maxillary. It is the principle supplier to the dura.
2. Anterior Meningeal Artery
3. Posterior Meningeal Artery

HEMATOMA: Bleeding within and around the meningeal layers

1. EPIDURAL HEMATOMA: Middle Meningeal Artery is easily cut with a skull fracture. This will put pressure in the epidural space and smush the brain.

· Flow of blood will go through dura superficially. The extent of the hematoma is limited by suture lines.

2. SUBDURAL HEMATOMA: Blood flow will not stop at the skull sutures in this case. However, hematoma usually will not cross the midline.

3. SUBARACHNOID HEMATOMA: Blood in this space intermixes with CSF and can lead to headache, stiff neck, and loss of consciousness.

· Unlike the ones above, these are very painful.

4. INTRACEREBRAL HEMORRHAGE: Results in death or paralysis by stroke.

INTERNAL CAROTID ARTERY: Major supplier of blood to the brain.

Course: Common Carotid ------> Internal Carotid ------> Into the Foramen Lacerum ------> medially to Carotid canal ------> then Anteriorly through Cavernous Sinus ------> Then superiorly again.

Circle Of Willis: The anastomotic arterial connections supplying the cranial cavity. The two main supplies to the brain are the Internal Carotid and Vertebral Arteries, and they communicate through the Circle of Willis

· Branches of the VERTEBRAL ARTERY

o Both Left and Right Vertebrals give off Posterior Inferior Cerebellar arteries

o Then the two Vertebral Arteries converge to form the singular Basilar Artery

o Basilar Artery gives off the Anterior Inferior Cerebellar and Superior Cerebellar arteries.

o The Basilar Artery then becomes the Posterior Cerebral Artery.

· POSTERIOR COMMUNICATING ARTERY connects the Posterior Cerebral (from Basilar) to the Internal Carotid Artery. This is the major anastomosis between the Carotid and Vertebral arterial channels.

· Branches of the INTERNAL CAROTID

· ANTERIOR COMMUNICATING ARTERY: Connects the Anterior Cerebral Arteries to each other. This is the major anastomosis of the Right and Left Internal Carotids with each other.

VENTRICLES OF THE BRAIN: Cerebrospinal fluid is produced in the ventricular system in the brain.

· Median and Lateral Apertures: Holes that drain the cerebrospinal fluid down into the spinal column.

· Interventricular Foramen: A hole that connects the Lateral Ventricle to the Third Ventricle.

· Cerebral Aqueduct: A hole that connects the Third Ventricle to the Fourth Ventricle.

· Cerebrospinal fluid is continuous with the optic tract, until it reaches the back of the eye.

· Again, cerebrospinal fluid from the ventricles communicates with the Superior Sagittal Sinus via Arachnoid Villi penetrating the sinus.

INTERNAL HYDROCEPHALY: The internal buildup of cerebrospinal fluid. There is no feedback inhibitory mechanism for stopping production of cerebrospinal fluid.

· This could be caused by an obstruction at the cerebral aqueduct.

EXTERNAL HYDROCEPHALY: Cerebrospinal fluid buildup outside the CNS. Caused by excess production of CSF, leading to increased intracranial pressure.

Category: Anatomy Notes