The Collaborative International Dictionary of English v.0.48:

Postcommissure \Post*com"mis*sure\, n. [Pref. post- +
   commisure.] (Anat.)
   A transverse commisure in the posterior part of the roof of
   the third ventricle of the brain; the posterior cerebral
   commisure. --B. G. Wilder.
   [1913 Webster]