VISION. 603 
The iris serves to regulate the quantity of light admitted to 
the eye, and to cut off too divergent rays. In order that objects 
at different distances may be seen distinctly, the lens alters in 
shape in response to the actions of the ciliary muscle on the 
suspensory ligament, the anterior surface becoming more con- 
vex. Accommodation is associated with convergence of the 
visual axes and contraction of the pupil. The latter has circu- 
lar and radiating plain muscular fibers (striped in birds, that 
seem to be able to alter the size of the pupil at will), governed 
by the third, fifth, and sympathetic nerves. Contraction of 
the pupil is a reflex act, the nervous center lying in the front 
part of the floor of the aqueduct of Sylvius, while the action 
of the other center (near this one) through the sympathetic 
nerve is tonic. 
Accommodation through the ciliary muscle is governed by 
a center situated in the hind part of the floor of the third ven- 
tricle near the anterior bundles of the third nerve, which latter 
is the medium of the change. There are certain imperfections 
common to all human eyes, such as spherical and chromatic 
aberration, a limited degree of astigmatism, etc. When rays 
of light are focused anterior to the retina, the eye is myopic; 
when posterior to it, hypermetropic. 
The presbyopic eye is one in which the mechanism of accom- 
modation is at fault, chiefly the ciliary muscle. The point of 
entrance of the optic nerve (blind-spot) is insensible to light; 
and visual impulses can be shown to originate in the layers of 
rods arid cones, probably through stimulation from chemical 
changes effected by light acting on the retina. The sensation 
outlasts the stimulus; hence positive after-images occur. Nega- 
tive after-images occur in consequence of excessive stimulation 
and exhaustion of the retina, or disorder of the chemical pro- 
cesses that excite visual impulses. When stimuli succeed one 
another with a certain degree of rapidity, sensation is continu- 
ous. The eye can distinguish degrees of light within certain 
limits, varying by about 745 of the total. 
Objects become fused or are seen as one when the rays from 
them falling on the retina approximate too closely on that sur- 
face. The brain, as well as the eye itself, is concerned in such 
discriminations, the former probably more than the latter. The 
various color sensations we have are the result either of definite 
single sensations or the fusion physiologically of two or more 
of these, and have no reference to the fusion of pigments ex- 
ternal to the eye. All human eyes are to some extent color- 
