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.