A panoramic radiograph shows the anterior loop and incisive canal (right side) and the anterior loop (left side). 1. The lower mandibular cortex as the plane of reference. 2. The line perpendicular to the line passing through the mesial border of the mental foramen. 3. The line perpendicular to the line passing through the most mesial point of the anterior loop of the mental nerve with the mandibular incisive canal. 4. The distance between lines 2 and 3, corresponding to the mesial length of the extent of the anterior loop or incisive canal.