The Collaborative International Dictionary of English v.0.48:

Crus \Crus\ (kr?s), n.; pl. Crura (kr?"r?). [L., the leg.]
   (Anat.)
   (a) That part of the hind limb between the femur, or thigh,
       and the ankle, or tarsus; the shank.
   (b) Often applied, especially in the plural, to parts which
       are supposed to resemble a pair of legs; as, the crura of
       the diaphragm, a pair of muscles attached to it; crura
       cerebri, two bundles of nerve fibers in the base of the
       brain, connecting the medulla and the forebrain.
       [1913 Webster]

WordNet (r) 3.0 (2006):

crus
    n 1: the leg from the knee to foot