The Collaborative International Dictionary of English v.0.48:

Myelencephalon \My`e*len*ceph"a*lon\, n. [NL., from Gr. myelo`s
   marrow + E. encephalon.] (Anat.)
   (a) The brain and spinal cord; the cerebro-spinal axis.
       Sometimes abbreviated to myelencephal.
   (b) The metencephalon. --Huxley.
       [1913 Webster]