604 ANIMAL PHYSIOLOGY. 
blind in the sense that it is probable that other animals (ants, 
etc.) can perceive colors not included in our spectrum, and also 
in the sense that all parts of the retina are not equally sensitive 
to rays of a certain wave-length; but some persons are unable 
to perceive certain colors at all. 
The macula lutea, and especially the fovea centralis, are the 
points of greatest retinal sensitiveness. When the images of 
objects are thrown on these parts, they are seen with complete 
distinctness; and it is to effect this result that the movements 
of the two eyes in concert take place. An object is seen as one 
when the position of the eyes (visual axes) is such that the im- 
ages formed fall on corresponding parts of the retina. Binocu- 
lar vision is important to supply the sensory data for the idea of 
solidity. It is important to remember that, before an object is 
“seen” at all, the sensory impressions furnished by the retina 
and conveyed inward by the optic nerve are elaborated in the 
brain and brought under the cognizance of the perceiving ego. 
We recognize many visual illusions and imperfections of vari- 
ous kinds, the course of which it is difficult to locate in any 
one part of the visual tract, such as are referred to “irradia- 
tion,” “contrast,” etc. There may also be visual phenomena 
that are purely subjective, and others that result from sugges- 
tion rather than any definite sensory basis of retinal images. 
Hence what one sees depends on his state of mind at the time. 
This applies to appreciation of size and distance also, though 
in such cases we have the visual angle, certain muscular move- 
ments (muscular sense), the strain of accommodation, etc., as 
guides. 
HEARING. 
As the end-organ of vision is protected both without and 
within, so is the still more complicated end-organ of the sense 
of hearing more perfectly guarded against injury, being in- 
closed within a membranous as well as bony covering and sur- 
rounded by fluid, which must shield it from stimulation, except 
through this medium. 
Hearing proper, as distinguished from the mere recognition 
of jars to the tissues, can, in fact, only be attained through the 
impulses conveyed to the auditory brain-centers, as originated 
in the end-organ by the vibrations of the fluid with. which it is 
bathed. 
