The lower part of the skull is called the skull base. The brain rests on its inner surface. Through openings in the skull base, a total of twelve cranial nerves and blood vessels enter the neck as well as the facial skull.
What is the base of the skull?
The skull base represents a cranial fossa on which the brain rests. It is also called the base cranii. In English, it is called the scull base. Through the base of the skull, the brain is connected to the neck and to the facial skull by several nerves and blood vessels. The inner base of the skull (Basis cranii interna) is divided into the anterior, middle and posterior cranial fossa. It is the side facing the brain. The outer skull base (Basis cranii externa) is again the side facing away from the brain. Strictly speaking, it represents the facial skull. By strict definition, only the base cranii interna is called the skull base. In total, it is composed of five bones, namely the frontal bone (Os frontale), the ethmoid bone (Os ethmoidale), the sphenoid bone (Os sphenoidale), the occipital bone (Os occipitale) and the temporal bone (Os temporale).
Anatomy and structure
The internal skull base consists of anterior cranial fossa (fossa cranii anterior), middle cranial fossa (fossa cranii media), and posterior cranial fossa (fossa cranii posterior). The anterior cranial fossa is composed of the ethmoid bone and the temporal and frontal bones, which are attached laterally. The anterior cranial fossa is demarcated from the middle cranial fossa (fossa cranii media) by the lesser sphenoid wing. The fossa cranii media houses part of the brain stem, the middle part of the brain, and the temporal lobe of the cerebrum. The middle cranial fossa is divided into two halves by the so-called Türkensattel. In the middle of the turciform saddle (sella turcica) is a depression for the pituitary gland (fossa hypophysialis). The posterior fossa (fossa cranii posterior) is composed of the three cranial bones occipital bone (os occipitale), sphenoid bone (os sphenoidale) and temporal bone (os temporale). In the center of the posterior cranial fossa lies the occipital hole (foramen magnum). Through the foramen magnum, the medulla oblongata exits the interior of the skull and passes into the spinal cord. In the posterior fossa, there are other passage points for cranial nerves and arteries.
Function and Tasks
The skull base has the task of protecting the brain from external influences. At the same time, it has many passage points for cranial nerves and blood vessels that keep the brain connected to the rest of the body. Without the skull base, however, the brain would very easily suffer shocks and lose its ability to function. A total of twelve passages through the skull base are necessary for nerves and blood vessels to make contact with the rest of the body’s organs. The canalis opticus is a passage in the anterior sphenoid bone of the middle cranial fossa. Both the optic nerve and the ophthalmic artery pass through this opening. Both are responsible for supplying the eyes. The hypoglossal nerve, which is responsible for the motor function of the tongue, passes through the hypoglossal nerve canal. The internal jugular vein (internal jugular vein) enters the neck through the foramen jugulare (the jugular hole). The internal carotid artery (internal carotid artery) passes through the canalis caroticus. A bony canal in the petrous bone, the canalis musculotubarius, serves as an opening for the eustachian tube. The purely sensory maxillary nerve exits the cranial cavity through the foramen rotundum. Other passage points in the skull base include the foramen ovale, the foramen spinosum, and the foramen lacerum for important nerve cords, as well as the porus acusticus internus for the auditory canal and the foramen alare caudale for the maxillary artery.
Diseases
Skull base fracture represents a serious disease of the skull base. A skull base fracture always occurs after a strong force in the head area mostly by accidents but also by blows or kicks. In this case, bone fractures occur in the anterior, middle or posterior cranial fossa. The frontobasal (nose and base of the skull) and the laterobasal fracture (ear and base of the skull) occur most frequently. Cerebrospinal fluid and blood typically leak from the nose and ear. Due to the actual trauma or bleeding into the brain, occasionally clouding of consciousness and neurological deficits take place. Because many nerves pass through small openings in the base of the skull, their entrapment may occur.As a result, paralysis and sensory loss may occur. A skull base fracture is a highly life-threatening condition whose outcome cannot be predicted. However, there are also diseases of the skull base that are characterized by space-occupying growth processes in this area. In most cases, these are benign skull base tumors. Despite their benign nature, however, these tumors can cause considerable discomfort. For example, they are capable of destroying the bony structures of the skull base and growing around cranial nerves or blood vessels. When cranial nerves are affected, such symptoms as visual disturbances, olfactory and gustatory disturbances, paralysis of facial muscles, facial pain or numbness in parts of the face, and hearing loss may occur. Furthermore, tinnitus, dizziness, dysphagia or weakness of the facial, head and shoulder muscles may also occur. Tumors at the base of the skull are not always responsible for these symptoms. Inflammations and injuries in this area are also possible causes. The examination is performed by imaging techniques such as MRI or CT. In the event of injury, immediate action must be taken. The benign tumors should only be treated surgically if the quality of life is severely limited. Sometimes growth-inhibiting drugs can stop the tumor’s growth process.
