1 Medullary body: 2  the 'white matter' of the cerebrum; consists of myelinated axons. 3  types of axons include: 4  commissural fibers-conduct impulses between cerebral hemispheres (and form the corpus callosum). 5  projection fibers-conduct impulses in & out of the cerebral hemispheres.ike the brain, the spinal cord consists of gray matter and white matter. The gray matter (cell bodies & synapses) of the cord is located centrally & is surrounded by white matter (myelinated axons).