2. In looking from near to distant objects, the lens becomes flatter and thinner (Fig. 161). This change is necessary because the less divergent waves from the distant objects require less converging power on the part of the lens. The method employed in changing the shape of the lens is difficult to determine and different theories have been advanced to account for it. The following, proposed by Helmholtz, is the theory most generally accepted: The lens is held in place back of the pupil by the suspensory ligament. This is attached at its inner margin to the membranous capsule, and at its outer margin to the sides of the eyeball, and entirely surrounds the lens. It is drawn perfectly tight so that the sides of the eyeball exert a continuous tension, or pull, on the membranous capsule, which, in its turn, exerts pressure on the sides of the lens, tending to flatten it. This arrangement brings the elastic force of the eyeball into opposition to the elastic force of the lens. The ciliary muscle plays between these opposing forces in the following manner: _To thicken the lens_, the ciliary muscle contracts, pulling forward the suspensory ligament and releasing its tension on the membranous capsule. This enables the lens to thicken on account of its own elastic force. _To flatten the lens_, the ciliary muscle relaxes, the elastic force of the eyeball resumes its tension on the suspensory ligament, and the membranous capsule resumes its pressure on the sides of the lens. This pressure, overcoming the elastic force of the lens, flattens it. *Movements of the Eyeballs.*—In order that the light may enter the eyeballs to the best advantage, they must be moved in various directions. These movements are brought about through the action of six small muscles attached to each eyeball. Four of these, named, from their positions, the superior, inferior, internal, and external recti muscles, are attached at one end to the sides of the eyeball and at the other end to the back of the orbit (Fig. 162). These, in the order named, turn the eyes upward, downward, inward, and outward. The other two, the superior and inferior oblique muscles, aid in certain movements of the recti muscles and, in addition, serve to rotate the eyes slightly. The movements of the eyeballs are similar to those of ball and socket joints. [Fig. 162] Fig. 162—*Exterior muscles of eyeball.*