120 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
from the general curvature of the inner face, but usually 
nearly coincides with it. 
When a longitudinal or a transverse section is taken 
through the whole eyestalk, the optic nerve (fig. 28, 
A, op) is seen to traverse its centre. At first narrow 
and cylindrical, it expands towards its extremity into 
a sort of bulb (B, g), the outer surface of which is curved 
in correspondence with the inner surface of the cornea. 
The terminal half of the bulb contains a great quantity 
of dark colouring matter or pigment, and, in section, 
appears as what may be termed the inner dark zone (f). 
Outside this, and in connection with it, follows a white 
line, the inner white zone (e), then comes a middle dark 
zone(d); outside this an outer pale band, whick may 
be called the outer white zone (c), and between this and 
the cornea (a) is another broad band of dark pigment, the 
outer dark zone (b). 
When viewed under a low power, by reflected light, this 
outer dark zone is seen to be traversed by nearly parallel 
straight lines, each of which starts from the boundary 
between two facets, and can be followed. inwards through 
the outer white zone to the middle dark zone. Thus the 
whole substance of the eye between the outer surface of 
the bulb of the optic nerve and the inner surface of the 
cornea is marked out into as many segments.as the 
cornea has facets; and each segment has the form 
of a wedge or slender pyramid, the base of which is 
four-sided, and is applied against the inner surface: of 
