118 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
The sonorous vibrations transmitted through the 
water in which the crayfish lives to the fluid and solid 
contents of the auditory sac are taken up by the delicate 
hairs of the ridge, and give rise to molecular changes 
which traverse the auditory nerves and reach the cerebral 
ganglia. 
The vibrations of the luminiferous ether are brought 
to bear upon the free ends of two large bundles of nerve 
fibres, termed the optic nerves (fig. 25, on), which proceed 
directly from the brain, by means of a highly complex eye. 
This is an apparatus, which, in part, sorts out the rays of 
light into as many very small pencils as there are separate 
endings of the fibres of the optic nerve, and, in part, 
serves as the medium by which the luminous vibrations 
are converted into molecular nerve changes. 
The free extremity of the eyestalk presents a convex, 
soft, and transparent surface, limited by an oval contour. 
The cuticle in this region, which is termed the cornea, 
(fig. 28, a), is, in fact, somewhat thinner and less dis- 
tinctly laminated than in the rest of the eyestalk, and it 
contains no calcareous matter. But it is directly con- 
tinuous with the rest of the exoskeleton of the eyestalk, 
to which it stands in somewhat the same relation as the 
soft integument of an articulation does to the adjacent 
hard parts. 
The cornea is divided into a great number of minute, 
usually square facets, by faint lines, which cross it from side 
