540 ANIMAL PHYSIOLOGY, 
one side produces blindness of the opposite eye, and in birds, 
etc., the same result follows when their homologues—the optic 
lobes—are similarly treated. There can be no doubt, therefore 
that they are a part of the central nervous machinery of vision, 
and it seems to be probable that the anterior parts of the cor- 
pora quadrigemina alone have this visual function. But, since 
it is the opposite eye that is affected, and in some animals 
(rabbits) that alone, we are led to infer a decussation of the 
optic fibers, or at least of impulses. In dogs, on the other hand, 
the crossing seems to be but partial. From the fact that only 
a part of the visual field is wanting (hemianopsia—i.e., that 
only the half of the usual field of view is visible), and, since 
there may be hemianopsia of both eyes, with unilateral disease 
of the brain, it has been inferred that in man the decussation 
is also incomplete. We may remark incidentally that it has 
lately been maintained that removal of one occipital lobe in the 
monkey leads to heminopsia of the opposite eye. These parts, 
as we have already seen, take some share in the co-ordination of 
muscular movements, and give rise to forced movements after 
unilateral injury. : 
It begins to appear that there are several parts of the brain 
concerned with vision. After removal of almost any part of 
the cerebral. cortex, if of sufficient extent, vision is impaired. 
We may say, then, that, before an object is “seen” in the high- 
est sense, processes beginning in the retina undergo further 
elaboration in the corpora quadrigemina, optic thalami, and, 
finally, in the cerebral cortex. We may safely assume that the 
part played by the latter is of very great importance, making 
the perception assume that highest completeness which is of 
very varying character, no doubt, with different groups of 
animals. In a sense, all mammals may see alike, and, in an- 
other sense, they may see things very differently; for, if we 
may judge. by the differences in this respect between educated 
and uneducated men, the great dissimilarity lies in the inter- 
pretation of what is seen; in a word, the cortex has to do with 
the perfecting of visual impulses. Nevertheless, a break any- 
where in the long and complicated chain of processes must lead 
to some serious impairment of vision. Much of the same sort 
of reasoning applies to the other senses and also to speech. 
To speak, therefore, of a visual center or a speech center in 
any very restricted sense is unjustifiable; at the same time, it 
is becoming clearer that there is in the occipital lobe, rather 
than in other parts of the cortex. an area which takes a pecul- 
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