The Collaborative International Dictionary of English v.0.48:

Decussate \De*cus"sate\, v. t. [imp. & p. p. Decussated; p.
   pr. & vb. n. Decussating.] [L. decussatus, p. p. of
   decussare to cross like an X, fr. decussis (orig. equiv. to
   decem asses) the number ten, which the Romans represented by
   X.]
   To cross at an acute angle; to cut or divide in the form of
   X; to intersect; -- said of lines in geometrical figures,
   rays of light, nerves, etc. Decussate