medial-external axis medial-lateral axis http://upload.wikimedia.org/wikipedia/commons/3/34/Anatomical_Directions_and_Axes.JPG spatial M-L axis ML axis Properly, the ML axis is a half axis; practically, its usage is less clumsy and less linguistically biased than "left-right". The terms may still be used relatively to describe locations along the LR axis. The gills are medial to the operculum, but lateral to the heart. BSPO:0001001 medio-lateral axis mediolateral axis proximal-distal axis An axis that extends from the center of organism to one or other side.