"what is the x-shaped structure that sits just below the brain's ventral surface and represents the merging of the optic nerves from the two eyes

The X-shaped structure is the optic chiasma (part of the optic nerve).

Structure of the optical chiasma: 
The fibers from the ganglion cells of the nasal retina and the temporal retina come together and leave the eyeball to form the two optic nerves (forming an X-shape). Each optic nerve passes through the optic hole of the sphenoid bone and extends in and out of the skull. The two nerves meet at the front and above the pituitary to form the optic chiasma. At this level, fibers from the nasal retina intersect while those from the temporal retina continue in the same trajectory to form the two optical tracts.

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