Function of Iris Muscles
The iris is the coloured part of the eye, and the muscles of the iris are responsible for controlling the size of the pupil. These muscles are known as the sphincter pupillae and the dilator pupillae. The sphincter pupillae muscles contract to constrict the pupil, while the dilator pupillae muscles contract to dilate the pupil.

Importance of Pupil Size
The size of the pupil is important for regulating the amount of light that enters the eye. When there is less light, the pupil dilates to allow more light in, and when there is more light, the pupil constricts to reduce the amount of light entering the eye. This helps to maintain clear vision and prevent damage to the retina.

Relation with Focal Length and Crystalline Lens
The iris muscles do not directly control the focal length of the eye-lens or the shape of the crystalline lens. The focal length of the eye-lens is determined by the curvature of the lens and the distance between the lens and the retina. The shape of the crystalline lens is controlled by the ciliary muscles, which are located behind the iris.

Relation with Optic Nerve
The optic nerve is responsible for transmitting visual information from the retina to the brain. The iris muscles do not play a direct role in the function of the optic nerve.

Conclusion
In conclusion, the muscles of the iris control the opening and size of the pupil, which is important for regulating the amount of light that enters the eye. They do not play a direct role in controlling the focal length of the eye-lens, the shape of the crystalline lens, or the function of the optic nerve.