Final answer:
The sphenoid bone is called the keystone of the cranium because it joins with almost every other bone of the skull, forming a central part of the cranial base and contributing to the structure and protection of the brain.
Step-by-step explanation:
The sphenoid bone can be referred to as the keystone of the cranium because of its central and critical role in the structure of the skull. Just like a keystone in architecture which is the central, wedge-shaped stone in an arch that holds all other stones in place, the sphenoid bone connects with almost every other bone of the skull, thus maintaining the integrity of the cranial structure. Its importance is further highlighted by its contribution to the base of the central skull as well as the sides of the skull. Moreover, it contains multiple openings that allow for the passage of nerves and blood vessels, showing how integral it is to both the form and function of the skull.
The sphenoid forms part of the eye sockets along with the frontal bone and contributes to the structure that houses critical parts of the brain, such as the pituitary gland within the sella turcica. The greater wings of the sphenoid extend laterally forming part of the central base of the skull, underscoring its keystone status and significance. The sphenoid is also involved in forming the boundaries between different cranial fossae, and in shaping the overall anatomy of the skull which protects the brain.