Final answer:
The sphenoid bone is termed the keystone of the cranial floor because it articulates with nearly all other cranial bones and forms a central part of the skull's base, contributing to structures such as the sella turcica and the greater and lesser wings.
Step-by-step explanation:
The sphenoid bone can be called the keystone of the cranial floor because it connects with almost every other bone in the skull. This single, complex bone is situated in the central skull and forms a significant portion of the base of the cranial cavity. The sphenoid bone houses important features such as the lesser wings, which form the boundary between the anterior and middle cranial fossae, and the sella turcica, a saddle-like structure at the midline containing the pituitary fossa for the pituitary gland. The greater wings of the sphenoid extend laterally to form the anterior floor of the middle cranial fossa and contribute to the structure of the lateral skull, also providing openings for the passage of nerves and blood vessels.