I Googled commissure cerebral hemispheres and this was the first article. I believe it has your answer, plus a link for further information about this commissure.
http://en.wikipedia.org/wiki/Cerebral_hemispheres
The commissure that is cut in "split brain" experiments is called the corpus callosum. It is the main bundle of nerve fibers that connects the left and right cerebral hemispheres of the brain. The cutting of this commissure is typically done surgically in order to treat severe epilepsy by reducing the spread of seizures between the hemispheres.
