Which axis travels through the line of sight in Listing's Plane?

• A. X axis
• B. Z axis
• C. Y axis
• D. Line-of-sight axis

Answer: (C)

In Listing’s Plane, any eye rotation from the primary position can be described as a single rotation about an axis that lies in that plane. The plane itself is defined relative to the eye’s orientation so that the line of sight in the primary position is perpendicular to it, and the possible rotation axes lie within the plane spanned by the horizontal (X) and vertical (Y) directions.

Among those axes, the vertical axis is the one that travels through the line of sight in this framework. That vertical (Y) axis serves as the rotation axis for movements that sweep the gaze left or right while staying consistent with Listing’s Law, and it lies in Listing’s Plane, aligning with how the eye’s instantaneous rotation is constrained.

The horizontal (X) axis also lies in Listing’s Plane, but it does not correspond to the axis that passes through the line of sight in the same way, and the Z axis is perpendicular to Listing’s Plane. So the axis that travels through the line of sight in Listing’s Plane is the vertical (Y) axis.