Complex sentence: The muscle weakness of botulism characteristically starts in the muscles supplied by the cranial nerves--a group of twelve nerves that control eye movements, the facial muscles and the muscles controlling chewing and swallowing.
Split into simpler sentences: The muscle weakness of botulism characteristically starts in the muscles supplied by the cranial nerves. A group of twelve nerves controls eye movements, the facial muscles and the muscles controlling chewing and swallowing.