Final answer:
The query refers to parts of the cranial base, specifically the anterior, middle, and posterior cranial fossae. These fossae conform to the brain regions above them and have distinct boundaries and midline structures, increasing in depth from the front of the skull to the back.
Step-by-step explanation:
The student's question pertains to the anatomy of the base of the skull, particularly focusing on the structure and subdivisions of the cranial fossae, and mistakenly mentions the fornix, which is unrelated to the cranial fossae. The base of the skull, known as the cranial base, has three subdivisions: the anterior cranial fossa, the middle cranial fossa, and the posterior cranial fossa. These increase in depth from anterior to posterior and serve as the floor of the cranial cavity, conforming to the shape of the brain regions they contain.
Each fossa has boundaries and is divided at the midline into right and left areas by significant bony structures. The anterior cranial fossa is the most shallow, the middle cranial fossa houses such structures as the sphenoid bone and temporal bone, and the posterior cranial fossa is the deepest, accommodating the brainstem and cerebellum, with openings such as the foramen magnum for the passage of the spinal cord.