Sacculus

from WordNet (r) 3.0 (2006)

sacculus
    n 1: a small sac or pouch (especially the smaller chamber of the
         membranous labyrinth) [syn: {saccule}, {sacculus}]

from The Collaborative International Dictionary of English v.0.48

Sacculus \Sac"cu*lus\, n.; pl. {Sacculi}. [L., little sack.]
   (Anat.)
   A little sac; esp., a part of the membranous labyrinth of the
   ear. See the Note under {Ear}.
   [1913 Webster]