602 ANIMAL PHYSIOLOGY. 
of vision speedily corrected, he will realize the truth of the 
above remark. Precisely the same data furnished by the eye 
are in one mind worked up in virtue of past experience (educa- 
tion) into an elaborate conception, while to another they an- 
swer only to certain vague forms and colors. And herein lies 
the great superiority of man’s vision over that of all other 
animals, 
Within the limits of their mental vision do all creatures see. 
Man has not the keen ocular discriminating power of the hawk; 
he can neither see so far nor so clearly; nor has he the wide 
field of vision of the gazelle; but he has the mental resource 
which enables him to make more out of the materials with 
which his eyes furnish him. It is by virtue of his higher cere- 
bral development that he has added to his natural eyes others 
in the microscope and telescope, which none of Nature’s forms 
can approach. : 
Pathological._There may be ulceration of the cornea, inflam- 
mation of this part, or various other disorders which lead to 
opacity. The low vitality of this region, probably owing to 
absence of blood-vessels, is evidenced by the slowness with 
which small ulcers heal. Opacity of the lens (cataract) when 
complete causes blindness, which can be only partially reme- 
died by removal of the former. Inflammations of any part of 
the eye are serious, from possible adhesions, opacities, etc., fol- 
lowing. Should such be accompanied by great excess of intra- 
ocular tension, serious damage to the retina may result. Of 
course, atrophy of the optic nerve (due to lesions in the brain, 
etc.) is irremediable, and involves blindness. Inspection of the 
internal parts of the eye (fundus oculi) often reveals the first 
evidence of disease in remote parts, as the kidneys. 
From what has been said of the movements of the two eyes 
in harmony, etc., the student might be led to infer that disease 
of one organ, in consequence of an evident close connection of 
the nervous mechanism of the eyes, would be likely to set up 
a corresponding condition in the other unless speedily checked. 
Such is the case, and is at once instructive and of great prac- 
tical moment. 
Paralysis of the various ocular muscles leads to squinting, 
as already noticed. 
Brief Synopsis of the Physiology of Vision.— All the other parts 
of the eye may be said to exist for the retina, since all are re- 
lated to the formation of a distinct image on this nervous ex- 
pansion. The principal refractive body is the crystalline lens. 
