The base of the skull (BOS) is the complex, curved floor of the cranial cavity. It functions as a protective barrier for the brain and serves as the platform upon which the head rests. This intricate bony structure separates the brain from the facial skeleton, the upper neck, and the delicate sinuses below. Its primary purpose is to shield the brain from external trauma while acting as a controlled conduit for vital communication pathways. By providing numerous openings, the base of the skull ensures that the central nervous system can connect with the body’s sensory and motor systems.
Defining the Base of the Skull
The base of the skull is not a flat surface but rather a highly contoured, saddle-shaped region located at the lowest part of the cranium. When viewed from the inside, it is organized into three distinct levels or depressions known as the cranial fossae, which step down from front to back. These fossae cradle the different lobes of the brain.
The anterior cranial fossa is the highest and shallowest, supporting the frontal lobes of the brain and extending over the eye sockets. The middle cranial fossa is deeper and butterfly-shaped, supporting the temporal lobes on either side. The posterior cranial fossa is the deepest and lowest of the three, housing the cerebellum and the brainstem.
From an external perspective, the base of the skull forms the roof of the mouth, the back of the nasal cavity, and the attachment points for the muscles that stabilize the head and neck. It connects directly to the top of the spine at the first cervical vertebra.
Structural Components: The Major Bones
The base of the skull is constructed from a fusion of several bones, with the Occipital, Sphenoid, Temporal, and Ethmoid bones making up the bulk of its structure. The Occipital bone forms the majority of the posterior cranial fossa, creating the strong, curved back floor of the skull. It is characterized by the large central opening, the Foramen Magnum, which provides passage for the spinal cord.
The Sphenoid bone is a single, complex bone situated centrally, often described as a “keystone” because it articulates with almost every other cranial bone. Its wings and body contribute significantly to both the anterior and middle cranial fossae, housing the sella turcica, a saddle-like depression that holds the pituitary gland.
The paired Temporal bones contribute greatly to the middle and posterior fossae, forming the sides and much of the lateral floor of the base. These are robust structures that contain the delicate organs of hearing and balance within their petrous (rocky) portion. The Ethmoid bone, although smaller, forms the central part of the anterior cranial fossa. It is a delicate, sieve-like structure that sits between the orbits, forming part of the roof of the nasal cavity.
Key Passageways for Nerves and Blood Vessels
The base of the skull is perforated by numerous openings, known as foramina and canals, which serve as functional gateways for crucial neurological and vascular structures. The Foramen Magnum is the largest opening, located in the occipital bone, providing the passage through which the brainstem transitions into the spinal cord, accompanied by the vertebral arteries. The vital internal carotid arteries, which supply blood to the brain, ascend through the Carotid Canal, a tortuous bony tunnel within the temporal bone.
Other openings facilitate the exit of the twelve pairs of cranial nerves, which control functions like vision, hearing, and facial movement. The Optic Canal transmits the optic nerve (Cranial Nerve II) and the ophthalmic artery into the orbit. The Jugular Foramen, formed between the temporal and occipital bones, is a large irregular opening that is the primary exit point for the internal jugular vein and three lower cranial nerves: the glossopharyngeal (CN IX), vagus (CN X), and accessory (CN XI) nerves.
Smaller, yet important, openings include the Foramen Rotundum and Foramen Ovale in the sphenoid bone, which transmit branches of the trigeminal nerve (Cranial Nerve V). This network of passages ensures the brain’s continuous communication with the head and neck structures. The tight confines of these bony canals mean that any swelling or injury in the area can easily compress the nerves and vessels traveling through them.
Clinical Significance of Base of Skull Injuries
Injuries to the base of the skull often result from significant blunt force trauma and can lead to severe and distinct clinical signs. A basilar skull fracture involves a break in one or more of the bones forming the cranial floor, most commonly the temporal or occipital bones. Such fractures are concerning because they can disrupt the protective membranes surrounding the brain, specifically the dura mater.
One common sign is the leakage of cerebrospinal fluid (CSF) from the nose (rhinorrhea) or ear (otorrhea), which indicates a tear in the dura mater. The presence of blood mixed with CSF can sometimes be identified by the “halo sign” on absorbent material, where a central spot of blood is encircled by a clear ring of CSF.
Two other highly characteristic signs are Battle’s sign and “raccoon eyes.” Battle’s sign presents as bruising over the mastoid process behind the ear. Raccoon eyes refers to bruising around the eyes. Both ecchymoses are caused by blood tracking from the fracture site into the surrounding soft tissues, though they may take hours or days to appear. Damage to the tight passageways can also injure the cranial nerves, leading to symptoms like hearing loss, facial weakness, or issues with eye movement.