VISION. 601 
It is, of course, impossible to trace each step by which the 
vertebrate eye has been developed from more rudimentary 
forms, though the data for such an attempt have greatly 
accumulated within the last few years; and it is not to be for- 
gotten that even the vertebrate eye has many imperfections, 
so that no doctrine of complete adaptation, according to the 
argument from design as usually understood, can apply. 
Certain acquired imperfections of the eye seem to be multi- 
plying at the present day, such as myopia, weakness of the 
accommodative mechanism, etc. The excessive use of the eyes, 
necessitating undue exercise of this apparatus or strain of the 
accommodation, is the fruitful source of evil. A good light— 
that is, one both sufficient in quantity and falling in the right 
direction upon the eyes and the objects to be viewed, together 
with adequate ventilation of the rooms occupied—is of great 
importance, though, as in the case of other organs, it is impos- 
sible to avoid wholly the penalties of over-use of the visual 
apparatus. 
It is of great importance to recognize that what we really 
see depends more upon the brain and the mind than the eye. 
If any one will observe how frequent are his incipient errors 
------—*=;- Cortical centre 
Brain above ' 
medulla \  . (3A... -Centre in optic thalamus 
Centre in region of.-\---—~--. Optic nerve 
corp. quadrigemina 
+ 
Fie. 444.—Diagram intended to illustrate the elaboration of visual impulses, bepinning in 
retina and culminating in the cerebral cortex. Course of impulses is indicated by arrows. 
Knowledge of auditory centers is not yet exact enough to permit of the construction of a 
diagram, though doubtless eventually the central processes will be localized as with vision. 
The ater remark applies to the other senses to nearly the same extent, possibly quite as 
much. 
