The Collaborative International Dictionary of English v.0.48:

Dorsally \Dor"sal*ly\, adv. (Anat.)
   On, or toward, the dorsum, or back; on the dorsal side of;
   dorsad.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dorsally
    adv 1: in a dorsal location or direction