The term “cranial” is an adjective meaning “of or relating to the cranium,” which is the anatomical name for the skull. This bony structure forms the framework of the head and acts as the primary enclosure for the brain.
Anatomy of the Cranium
The cranium, often thought of as a single structure, is actually a complex assembly of 22 bones that are tightly interlocked in adults. These bones are generally classified into two major groups based on their function and location. The neurocranium, or braincase, is the posterior and superior portion that directly surrounds the brain.
The neurocranium is formed by eight bones that meet at immovable joints called sutures. These include the single frontal, occipital, sphenoid, and ethmoid bones, along with the paired parietal and temporal bones. This part of the cranium is primarily responsible for forming the protective cavity that houses the brain.
The second major division is the viscerocranium, commonly known as the facial skeleton. This consists of 14 bones that create the structure of the face, including the orbits for the eyes and the nasal cavity. The only movable joint in the entire cranium is the one connecting the mandible, or lower jaw, to the temporal bones.
Primary Function: Protecting the Central Nervous System
The defining role of the cranium is to provide mechanical protection for the delicate brain tissue within a rigid bony helmet. This protective role is multi-layered, extending beyond the skull itself to include specialized membranes and fluid. The skull’s rigid, fixed volume is crucial for maintaining a stable pressure environment for the brain.
Beneath the bone lie three protective membranes known collectively as the meninges: the dura mater, arachnoid mater, and pia mater. The dura mater is the thick, tough outermost layer that is closely associated with the inner surface of the skull. The arachnoid mater is the middle layer, and the pia mater is the innermost, thin layer that directly adheres to the surface of the brain.
The subarachnoid space, located between the arachnoid and pia mater, is filled with cerebrospinal fluid (CSF). This clear fluid acts as a hydraulic cushion, absorbing sudden impacts and protecting the brain from abrupt movements. The CSF also provides buoyancy, minimizing pressure on the floor of the cranium.
Understanding Cranial Nerves
Another aspect of the cranial region involves the cranial nerves, which are a set of twelve pairs of nerves that emerge directly from the brain or the brainstem. These nerves are distinct from the spinal nerves that branch out from the spinal cord, and they exit the cranium through various small openings, or foramina. Cranial nerves are responsible for supplying sensory and motor functions primarily to the head and neck.
These nerves are categorized as sensory, motor, or mixed, indicating whether they carry information to the brain, away from the brain, or both. For instance, the optic nerve, the second cranial nerve, is purely sensory, transmitting visual information from the retina to the brain. In contrast, the trigeminal nerve is a mixed nerve, providing sensation to the face and controlling the muscles used for chewing.
One of the most extensive is the vagus nerve, or the tenth cranial nerve, which is unique because it travels far beyond the head and neck. It extends down into the chest and abdomen, where it plays a significant role in regulating automatic bodily functions. The vagus nerve controls heart rate, breathing, and the smooth muscle contractions of the digestive tract.