Anterior cranial fossa >Middle cranial fossa > Posterior cranial fossa
Definition:
The inside view of the cranium is known as a cranial cavity.
Contents of cranial cavity:
The cranial cavity contains the brain, pineal and hypophysis cerebri, parts of the cranial and spinal nerves, blood vessels, meninges and cerebrospinal fluid.
Bones that make up the cranial cavity:
The cranial cavity is contained by the frontal, parietal, sphenoid, temporal and occipital bones, and in part the ethmoid, all lines by fibrous endocranium, external zone of dura mater and pericranium.
Importance of cranial cavity:
A cranial cavity forms the floor of the brain, while the roof of the brain is formed by the skull cap or calvaria.
Divisions of cranial cavity:
The cranial cavity is divided into three cranial fossa.
- Anterior cranial fossa which accommodates the anterior lobe of brain.
- The middle cranial fossa, much wider than the anterior cranial fossa contains the 2 temporal lobes of brain.
- The posterior cranial fossa is much shallower and wider than the middle cranial fossa, and it accommodates the occipital lobes of the brain.
Anterior cranial fossa:
Anteriorly it contains sphenoid bone.
Sphenoid bone consists of three parts
a) -Body
b) -Lesser wings, 2 in number
c) -Greater wing, also 2 in number
The body of sphenoid is again subdivided into anterior, middle and posterior parts.
Anterior part lies in anterior cranial fossa, middle in middle cranial fossa and posterior in posterior cranial fossa. So sphenoid bone is common in all three fossae.
Boundaries and foramens of anterior cranial fossa:
Anteriorly and laterally is bounded by the frontal bone
Floor:
The floor of anterior cranial fossa id formed by the orbital plate of the frontal bone, ethmoid cribriform plate , anterior border of sphenoid’s lesser wings and anterior part of the body.
Posteriorly:
Bounded by posterior border of lesser wing of sphenoid, anterior clinoid process and sulcus chiasmaticus.
Ethmoid:
Ethmoid is present in the centre of cranial fossa and it forms part of its floor. It forms 4 sutures, 3 with frontal and 1 with sphenoid.
Crista galli
It is a sharp upward projection of ethmoid bone in the midline, for the attachment of falx cerebri.
Foramen cecum
Between the crista galli and the crest of the frontal bone is a small aperture, the foramen cecum. In 85 % of the subjects, it is closed. While in 15% subjects, it is patent or open for the passage of vein of nose which drains into superior sagittal sinus.
The importance of foramen cecum is that the infection can travel easily from the nose to the venous sinus of the brain.
Cribriform plate of the ethmoid:
This is the perforated part of the ethmoid. The upper surface of the cribriform plate supports the olfactory bulbs, and the small perforations in the cribriform plate are for the olfactory nerves.
Clinical importance: If the cribriform plate is injured, then there will be a loss of sensation of smell.
Anterior and posterior Ethmoidal canals:
There are 2 anterior Ethmoidal canals on the sides of the upper surface of ethmoid for the passage of anterior Ethmoidal vessels and nerves. While posterior Ethmoidal canals give passage to post Ethmoidal vessels only.
Clinical importance of orbital plate of frontal bone:
The orbital part of the frontal born not only forms the floor of the anterior cranial fossa but also the roof of the orbit. If an injury occurs to this plate, then bleeding from eyes could occur.