Figure 4.

Schematic trends of the orientation of the normal (arrow) to the slices as one goes into the canal (increasing Z). (a) On the left, $\theta$ decreases monotonically, (b) while on the right, it reaches a minimum and increasing further on. Note that due to the arbitrariness of the orientation of the XYZ axes outside the ear, the minimum of the graphs need not be at $\theta =0$. In fact, it generally is not.