Accommodation It is the ability of the crystalline lens to change its shape (the converging power) to focus objects of any distance upon the retina.

Mechanism of accommodation Contraction of the ciliary muscle Relaxation of zonule of Zinn Relaxation of tension on the lens capsule Increased curvature of the anterior lens surface Crystalline lens becomes more convex Near objects are focused on the retina

Unaccommodating Eye

Three aging factors of accommodation in presbyopia:- Decrease in the elastic modules of the lens capsule. Increase in the lens substance (size+ density). Flattening of the lens as whole.

The accommodated simplified eye

Physical and physiological accommodation . Two factors enter into the efficiency of accommodation:- The ability of the lens to change its shape (physical accommodation). The power of the ciliary muscle (physiological accommodation). If the substance of the lens becomes inelastic, as it occurs in progressive age, so that it can no longer change its shape, accommodation can not act even if the ciliary muscle violently contracts. On the other hand a weak or paralysis of the ciliary muscle will not be able to induce change in the lens shape even in children.

Physical accommodation It’s an expression of the actual physical deformation of the lens, and it’s measured in dipoters, thus if the converging power of the eye increases by one diopter we speak of the expenditure of one diopter of accommodation.

Physiological accommodation It’s the action of ciliary muscle and is measured by the unit of diopter which equals to the magnitude of ciliary muscle contraction to raise the refractive power of the lens by 1 diopter. Both physical and physiological accommodation correspond each other during the first 1/2 of life. But during the second half of life (after age of 40) both types of accommodation dissociate where the physical accommodation fails to cover the needs of near work while the ciliary muscle is still intact and powerful but the lens becomes hard this occurs in presbyopia. Conversely a failure of the physiological power of ciliary muscle may appear in states of debility like in famines and in diseases of the parasympathetic nerve supply to that muscle when the muscle fails at any age, while the lens is intact. Since an attempt of ciliary muscle to cover the accommodation deficiency (weakness) by a sustained and exaggerated contraction such as a weakness may be responsible for eye strain and asthenopia.