The alteration in the shape of the lens is accomplished by the action of the muscular layer, already named, which radiates from the edge of the cornea to the ciliary region of the choroid coat, where it is attached. When the ciliary muscle contracts, it draws the choroid coat and the connections of the suspensory ligament of the lens slightly forward, the junction of the cornea and sclerotic being its fixed point. Under ordinary circumstances, the eye being at rest, the suspensory ligament is tense and exerts a radial traction on the anterior part of the capsule of the lens, tending to stretch it flat; this affects the shape of the soft lens and reduces its convexity. When the ciliary muscle shortens, it draws forward the attachment of the suspensory ligament, relaxes it, and removes the tension of the capsule, so that the unconstrained elastic lens bulges into its natural form. The posterior surface cannot extend backward, because there it is in contact with the vitreous humor, which is held more firmly against it by the increased tension of the hyaloid membrane during the contraction of the ciliary muscle.
Some circular muscular fibres help to relax the ligament and relieve it from the increased pressure which the contraction of the radiating fibres must indirectly cause on the vitreous humor.
The act of accommodation is a voluntary one, the nerve bearing the impulse to the ciliary and iris muscles, coming from the 3d nerve by the ciliary branches of the lenticular ganglion. The local application of the alkaloid of the belladonna plant (atropin) causes paralysis of the ciliary muscle and wide dilatation of the pupil; and the alkaloid of the Calabar bean (physostigmin) produces contraction of the muscle of accommodation and extreme contraction of the pupil.