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.*

*Binocular Vision.*--In addition to directing the eyeballs so that light
may enter them to the best advantage from different objects, the muscles
also enable two eyes to be used as one. Whenever the eyes are directed
toward the same object, an image of this object is formed on the retina of
each. Double vision is prevented only by having the images fall on
corresponding places in the two eyes. This is accomplished by the muscles.
In each act of seeing, it becomes the task of the superior and inferior
recti muscles to keep the eyes in the same plane, and of the external and
internal recti muscles to give just the right amount of convergence. If
slight pressure is exerted against one of the eyes, the action of the
muscles is interfered with and, as a consequence, one sees double. The
advantages of two eyes over one in seeing lie in the greater distinctness
and broader range of vision and in the greater correctness of judgments of
distance.

*Visual Sensations.*--The visual sensations include those of _color_ and
those of a _general sensibility to light_. Proof of the existence of these
types of sensation is found in color blindness, a defect which renders the
individual unable to distinguish certain colors when he is still able to
see objects. Color sensations are the results of light waves of different
lengths acting on the retina. While the method