STEP 1: Navigate to the Segment Editor module and create a segmentation of the skull.
STEP 2: Rotate your segmentation so that the LEFT orbit is oriented towards you.
STEP 3: Navigate to the Markups Module.
STEP 4: Create a Closed Curve.
STEP 5: Place points around the orbital aperture of the LEFT orbit following the natural contour of the margin.
STEP 6: Now rotate your segmentation and adjust the points as necessary to best fit the anatomy of the orbital margin. Make sure to look at it from all angles.
STEP 7: Now that you've finished collecting your closed curve, click on the Measurements dropdown menu on the left hand side of the screen. Select the measurements you'd like to collect. "Length" of this closed curve will give you Circumference, and "Area" will give you the area of the Orbital Aperture.
You're done! Your measurements are now visible in the Measurements section of the left hand menu.