Accommodation for Near and Distant Objects

The Muscles and Ligaments

The shape of the lens is controlled by a ring of smooth muscle called the ciliary muscles. These are connected to the lens by inelastic fibrous strands called suspensory ligaments.

• There is an inverse physical relationship between the two structures.
• The lens is naturally elastic. It naturally wants to "spring" into a fatter shape, but it is pulled flat when tension is applied by the ligaments.

Focusing on a Near Object

Focusing on something close requires active muscle contraction, which is why staring at a screen for too long can lead to eye strain. The causal sequence of events is:

1. The ciliary muscles contract, which decreases the diameter of the muscle ring (moving the muscle mass inward).
2. This causes the suspensory ligaments to loosen (become slack).
3. Because the tension is released, the elastic lens becomes thicker (more convex).
4. The thicker lens refracts light rays more strongly to compensate for the diverging rays from the near object.
5. The light rays converge and focus exactly onto the retina.

Focusing on a Distant Object

Looking at distant objects is the natural "resting state" of the eye and causes less fatigue because the muscles are relaxed. The sequence is:

1. The ciliary muscles relax, increasing the diameter of the muscle ring (moving the muscle mass outward).
2. This pulls the suspensory ligaments tight (taut).
3. The tension pulls the lens thin (flatter or less convex).
4. The thinner lens refracts light rays only slightly, as the light rays from distant objects are already nearly parallel.
5. The light rays converge and focus exactly onto the retina.