The Collaborative International Dictionary of English v.0.48:

Lacertus \La*cer"tus\ (l[.a]*s[~e]r"t[u^]s), n.; pl. Lacerti
   (-t[imac]). [L., the upper arm.] (Anat.)
   A bundle or fascicle of muscular fibers.
   [1913 Webster]