The Posterior Cranial Fossa
The floor of the cranial cavity is divided into three distinct depressions. They are known as the anterior cranial fossa, middle cranial fossa and posterior cranial fossa. Each fossa accommodates a different part of the brain.
The posterior cranial fossa is the most posterior and deep of the three cranial fossae. It accommodates the brainstem and cerebellum.
In this article, we shall look at the borders, contents and clinical correlations of the posterior cranial fossa.
The posterior cranial fossa is comprised of three bones: the occipital bone and the two temporal bones.
It is bounded as follows:
- Anteriorly and medially it is bounded by the dorsum sellae of the sphenoid bone. This is a large superior projection of bone that arises from the body of the sphenoid.
- Anteriorly and laterally it is bounded by the superior border of the petrous part of the temporal bone.
- Posteriorly it is bounded by the internal surface of the squamous part of the occipital bone.
- The floor consists of the mastoid part of the temporal bone and the squamous, condylar and basilar parts of the occipital bone.
The posterior cranial fossa houses the brainstem and cerebellum.
The brainstem is comprised of the medulla oblogata, pons and midbrain and continues down through the foramen magnum to become the spinal cord. The cerebellum has an important role in co-ordination and fine motor control – more information here.
Alongside the gross anatomical structures of the brainstem and cerebellum, the posterior cranial fossa also accommodates associated arteries and nerves. Some key structures will be discussed with regards to their foramina below.
There are several bony landmarks and foramina present in the posterior cranial fossa (a foramen is simply a hole that allows the passage of a structure – usually a blood vessel or nerve).
The internal acoustic meatus is an oval opening in the posterior aspect of the petrous part of the temporal bone. It transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII) and labyrinthine artery.
A large opening, the foramen magnum, lies centrally in the floor of the posterior cranial fossa. It is the largest foramen in the skull. It transmits the medulla of the brain, meninges, vertebral arteries, spinal accessory nerve (ascending), dural veins and anterior and posterior spinal arteries. Anteriorly an incline, known as the clivus, connects the foramen magnum with the dorsum sellae.
The jugular foramina are situated either side of the foramen magnum. Each transmits the glossopharyngeal nerve, vagus nerve, spinal accessory nerve (descending), internal jugular vein, inferior petrosal sinus, sigmoid sinus and meningeal branches of the ascending pharyngeal and occipital arteries.
Immediately superior to the anterolateral margin of the foramen magnum is the hypoglossal canal. It transmits the hypoglossal nerve through the occipital bone.
Posterolaterally to the foramen magnum lies the cerebellar fossae. These are bilateral depressions that house the cerebellum. They are divided medially by a ridge of bone, the internal occipital crest.
Clinical Relevance: Cerebellar Tonsillar Herniation
Cerebellar tonsillar herniation is the downward displacement of the cerebellar tonsils through the foramen magnum. It is also known as ‘coning’.
It is produced by a raised intracranial pressure, which has a varied aetiology. Causes include hydrocephalus, space occupying lesions, and a malformed posterior cranial fossa.
Cerebellar tonsillar herniation results in the compression of the pons and medulla, which contain the cardiac and respiratory centres. Thus, a herniation of this type ultimately results in death from cardiorespiratory arrest.