576 ANIMAL PHYSIOLOGY. 
eye (Fig. 418), being too short, parallel rays are focused be- 
hind the retina; hence no distinct image of distant objects can 
Fia. 418.—Anomalies of refraction in the hypermetropic eye (after Landois). 
be formed, and they can only be seen clearly by the use of con- 
vex glasses, except by the strongest efforts at accommodation. 
When the eye is passive, no objects are seen distinctly beyond 
a certain distance—i. e., the near point is abnormally distant 
(eight to eighty inches). The defect is to be remedied by the 
use of convex glasses. 
4, Presbyopia, resulting from the presbyopic eye of the old, 
is owing to defective focusing power, partly from diminished 
elasticity (and hence flattening) of the lens, but chiefly, proba- 
bly, to weakness of the ciliary muscle, so that the changes 
required in the shape of the lens, that near objects may be dis- 
tinctly seen, can not ‘be made. The obvious remedy is to aid 
the weakened refractive power by convex glasses. It is prac- 
tically important to bear in mind that, as soon as any of these 
defects in refractive power (though the same remark applies 
to all ocular abnormalities) are recognized, the remedy should 
be at once applied, otherwise complications that may be to a 
large extent irremediable may ensue. 
VISUAL SENSATIONS. 
We have thus far considered merely what takes place in the 
eye itself or the physical causes of vision, without reference to 
those nervous changes which are essential to the perception of 
an object. It is true that an image of the object is formed on 
the retina, but it would be a very crude conception of nervous 
processes, indeed, to assume that anything resembling that 
image were formed on the cells of the brain, not to speak of 
the superposition of images inconsistent with that clear mem- 
ory of objects we retain. Before an object is “seen,” not only 
peers 
