The Collaborative International Dictionary of English v.0.48:

Mesorchium \Me*sor"chi*um\, n. [NL., fr. Gr. me`sos middle + ? a
   testicle.] (Anat.)
   The fold of peritoneum which attaches the testis to the
   dorsal wall of the body cavity or scrotal sac.
   [1913 Webster]