The ciliary ganglion is a small cluster of nerve cells located behind the eye, specifically in the orbit. It plays a crucial role in the autonomic nervous system, particularly in controlling functions related to the eye, such as pupil constriction and lens shape adjustment for focusing. This ganglion is part of the parasympathetic nervous system and receives input from the oculomotor nerve, which is responsible for eye movement. The ciliary ganglion sends out fibers to the ciliary muscle and the iris, helping regulate light entry and visual clarity.