Skull+and+Cranial+Vault

toc =**Objectives**=

**The Skull**
"Alas, poor Yorick! I knew him, Horatio; a fellow of infinite jest, of most excellent fancy." -- Hamlet, Act V, Scene I


 * 1. Identify the major bony landmarks visible from the anterior, lateral, and superior views of the skull.**

The skull is defined as the bones of the head. It is mostly synonymous with cranium, though cranium specifically refers to two structures: the neurocranium and viscerocranium. The neurocranium houses the brain and is made of large flat protective bones. The viscerocranium makes up the features of the face and is the facial skeleton. The calvaria is the skull cap and is a domed, superior part of the skull. The cranial vault or cranial base is the inside of the cranial cavity that supports the brain and associated structures.

__Frontal View__ The bones visible from the frontal view include the frontal, the maxilla, two zygomatic, two nasal, and the mandible bones.

The frontal bone makes up the forehead and roof of the orbit. Its highest point is called the vertex of the skull. It contains a supraorbital ridge that is a sharp edge between the orbit and the forehead. Just superior is the supraorbital foramen or notch that the supraorbital nerve and vessels pass through. Superior to that is the superciliary ridge just superior to the eyebrow and is more prominent in males.

The maxilla bone makes up the upper jaw and is formed from 2 maxilla bones that have fused during development. Just inferior to the lower border of the orbit, the infraorbital foramen opens to allow the infraorbital nerve and vessels to pass. It contains the superior alveolar processes at the inferior border where the teeth insert. The maxilla articulates with the frontal bone at the lateral sides of the nose and with the two zygomatic bones.

The two zygomatic bones are the cheek bones that contain holes for several nerves and vessels. It articulates with both the frontal and maxilla bones.

The two nasal bones articulates with the nasion, the point of articulation between the frontal and two nasal bones. Together they form the ridge of the nose.

The mandible forms the lower jaw. It contains two mental foramen laterally where the mental nerve and vessels pass through. Its anteriosuperior border contains alveolar processes where teeth insert.

__Lateral__ In the lateral view, the parietal, temporal, sphenoid, and occipital bones are visible.

The two parietal bones makes up much of the superiolateral parts of the skull and articulate with the frontal and occipital bones. The point where the two parietal bones and the frontal bone meet is called the bregma. The point where the two parietal bones and the occipital bone meet is called lambda. The parietal bone has a superior and inferior temporal lines where the temporalis muscles and fascia attach.

The temporal bone is subdivided to the squamous, petrous, mastoid, and tympanic portions. The squamous portion is a flat portion. The petrous portion is a solid portion on the internal aspect of the cranial vault that houses the inner ear and parts of the middle ear. The mastoid portion contains the mastoid process and mastoid air cells. The tympanic portion contains the external auditory meatus (bony part of ear) and middle ear parts.

The sphenoid bone is the lateral part of the greater wing of the sphenoid visible from the external lateral surface of the skull.

The occipital bone is the most posterior bone.


 * 2. Identify certain areas of the skull more vulnerable to injury than others.**

Where the frontal, parietal, sphenoid, and temporal bones meet is a ridge called the pterion that is thin, vulnerable area of the skull because of the middle meningeal artery lying just deep to the pterion.


 * 3. Identify the sutures and their location. Understand the importance and relevance of these sutures in the adult and newborn.**

The junction between the sagittal and coronal sutures is called bregma. The junction between the sagittal and lambdoid suture is called lambda. Bregma and lambda used to be the anterior and posterior fontanelles, respectively. Fontanelles are membranous sites between the corners of the bones of the skull where there is incomplete closure of the sutures after birth. It is important to test this soft spot to detect its existence and any bulges or depressions. This is because the fontanelles are necessary for the growth of the brain and skull and closure of the skull over the next two years after birth. Any bulging or depressions of fontanelle can indicate a change in the intracranial pressure and excessive or insufficient cerebral spinal fluid.


 * 4. Describe the relationship of vessels, nerves, and soft tissues as they related to the bony skull and its foramina and fissures.**

The supraorbital nerve and vessels pass through the supraorbital foramen on the frontal bone. The infraorbital nerve and vessels pass through the infraorbitall foramen on the maxilla bone. The mental nerve and vessels pass through the mental foramena on the mandible. These three pairs of foramena lineup almost in line with each other.

The middle meningeal artery, which is the main blood supply to the cranial dura mater, has branches that pass just deep to the pterion.

**The Cranial Vault**
The interior aspect of the skull.

The cranial vault is divided into 3 fossae that are like steps leading down from anterior to posterior. The anterior cranial fossa is the shallowest, Formed by the frontal bone, cribriform plate of the ethmoid bone, and the lesser wing of the sphenoid. It forms the frontal lobe of the brain and the roof of the orbit.
 * 1. Identify the essential parts of the anterior, middle, and posterior cranial fossae.**

The middle cranial fossae is formed by the greater wing of the sphenoid and the temporal bone. It contains the sella turcica that is a fossa for the pituitary gland and supports the temporal lobe of the brain. It contains the foramen for five or 6 cranial nerves (depending on if you count branches).

The posterior cranial fossa is the deepest and most posterior. It is made up of primarily the occipital bone. It contains the foramen magnum which transmit the spinal cord and the jugular foramen where the internal jugular vein and nerves pass at the lateral border between the posterior and middle cranial fossae. The posterior cranial fossa support the cerebellum, the pons, and the medulla.

**The Calvaria**
Skull Cap


 * 1. Describe the composition of the bones of the calvaria.**

The calvaria or skull cap is composed of two layers of compact bone separated by a layer of spongy bone. The spongy bone is called the diploe and is at the center of the flat bone of the calvaria. It allows the 2 plates of compact bone to act as protection for the brain while reducing the weight of the skull. The spongy diploe has diploic veins running through it.


 * 2. What are the diploic veins? What is their significance?**

The spongy diploe has diploic veins running through it. These veins //do not// have valves. These veins drain into dural venous sinuses which collect all the venous blood from the brain. Because these veins have no valves, they can act as bidirectional channels that can bring disease directly to the dural sinuses.

**Cranial Meninges**

 * 1. Define the cranial meninges. Describe the relationship of the meninges to the internal surface of the skull. Describe the formation of a dural fold.**

The cranial meninges are three layers of connective tissue that serve as protection for the brain. These layers are continuous with those surrounding the spinal cord. Going deep to superficial, the meninges are pia mater, arachnoid mater, and dura mater. The pia mater is intimate with the brain. The arachnoid mater has web-like processes that attach to the dura and makes a subarachnoid space between the arachnoid mater and pia mater and contains cerebral spinal fluid.

Cranial dura mater is composed of an endocranium or periosteal layer and a meningeal layer. The periosteal layer is attached to the inside of the cranial cavity. The meningeal layer is in contact with the arachnoid mater. These two layers are typically not separatable except at four specific places where they form dural folds.

Dural folds are locations where the meningeal layer of the dura mater separates from the periosteal layer and folds in onto itself to form a dual layer reflection. These reflections form compartments in the brain which help support the brain.

The dural folds are the falx cerebi, falx cerebelli, tentorium cerebelli, and diaphragmn sella. The two biggest folds are the falx cerebri and diaphragm sella. The falx cerebri is a sickle-shaped fold that is suspended vertically on the inside of the sagittal suture and separates the right and left cerebral hemispheres. The falx cerebelli is a small, sickle-shaped fold separating the right and left cerebellar hemispheres in the posterior cranial fossa. The tentorium cerebelli is a tent-like projection that is perpendicular to the falx cerebri from the falx cerebelli, separating the cerebral hemispheres from the cerebellar hemispheres. The diaphragm stella is a circular fold covering the pituitary gland to attach it to the pituitary fossa.


 * 2. What is a dural sinus? How are they named? Describe how they usually drain. What is the primary blood supply to the cranial dura?**

The dural sinuses are produced when the periosteal dura separates from the meningeal dura and form a cavity or space. These spaces are lined with endothelium and form the dural venous sinuses. They drain blood from the brain and all eventually empty into the internal jugular vein.

The superior sagittal sinus is on the superior attachment of the falx cerebri. The inferior sagittal sinus is on the inferior free margin of the falx cerebri. The straight sinus is formed at the junction of the falx cerebi and tentorium cerebelli. The transverse sinus runs horizontally along the posterior attachment of the tentorium cerebelli. The sigmoid is the s-shaped continuation of the transverse sinus and ends at the jugular foramen. The cavernous sinus is adjacent to the pituiatarygland in the diaphragmic sella. The superior and inferior petrosal is associated with the tentorum cerebelli along the superior and inferior portions of the petrous portion of the temporal lobe. The confluence of sinuses is the junction point between the straight, superior sagittal, occipital and transverse sinuses.

Blood flows from the diploic veins and drain into the sagittal sinuses (inferior and superior). Blood generally drains posteriorly, with the inferior sagittal sinus draining into the straight sinus and the superior sagittal sinus, straight sinus, and occipital sinus draining towards the confluence of sinuses. At that point, the blood drains into the left and right transverse sinuses and then goes into the sigmoid sinuses and out the jugular foramen into the internal jugular veins. Blood from the cavernous sinuses, drain into the superior and inferior petrosal sinuses. The superior petrosal sinuses drains into the sigmoid sinuses with the transverse sinuses while the inferior petrosal sinuses drains directly into the origin of the internal jugular veins.

The blood supply to the meninges is from the middle meningeal artery which is a branch off the maxillary artery from the external carotid artery. It enters the cranial vault through the foramen spinosum in the middle cranial fossa. It lies outside the periosteal dura, between the periosteal dura and the skull. Because there is no space between the periosteal dura and the skull, it actually makes grooves in the inside surface of the skull. The middle meningeal artier has extensive supply over the skull.

Because of the weak spot at the pterion, a blow to the lateral side of the head can resulting bone fragments that can lacerate the middle menigeal artery. Blood will bleed relatively fast and produce a hemotoma, but because it needs to create a space where there was none, it doesn’t bleed //too// fast. A epidural hematoma can spread asymptomatically until it begins to compress the brain, causing coma and death, so injuries to the lateral side of the skull near the pterion should be thoroughly investigated.