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 distinctly laminated than in the rest of the eyestalk, and it contains no calcareous matter. But it is directly continuous 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.
FIG. 28.—Astacus fluviatilis.—A, a vertical section of the eye-stalk (× 6); B, a small portion of the same, showing the visual apparatus more highly magnified; a, cornea; b, outer dark zone; c, outer white zone; d, middle dark zone; e, inner white zone; f, inner dark zone; cr, crystalline cones; g, optic ganglion; op, optic nerve; sp, striated spindles.
The cornea is divided into a great number of minute, usually square facets, by faint lines, which cross it from side {119} to side nearly at right angles with one another. A longitudinal section shows that both the horizontal and the vertical contours of the cornea are very nearly semicircular, and that the lines which mark off the facets merely arise from a slight modification of its substance between the facets. The outer contour of each facet forms part of the general curvature of the outer face of the cornea; the inner contour sometimes exhibits a slight deviation {120} 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, which 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 {121} one of the facets of the cornea, while its summit lies in the middle dark zone. Each of these visual pyramids consists of an axial structure, the visual rod, invested by a sheath. The latter extends inwards from the margin of each facet of the cornea, and contains pigment in two regions of its length, the intermediate space being devoid of pigment. As the position of the pigmented regions in relation to the length of the pyramid is always the same, the pigmented regions necessarily take the form of two consecutive zones when the pyramids are in their natural position.
The visual rod consists of two parts, an external crystalline cone (fig. [28], B, cr), and an internal striated spindle (sp). The crystalline cone consists of a transparent glassy-looking substance, which may be made to split up longitudinally into four segments. Its inner end narrows into a filament which traverses the outer white zone, and, in the middle dark zone, thickens into a four-sided spindle-shaped transparent body, which appears transversely striated. The inner end of this striated spindle narrows again, and becomes continuous with nerve fibres which proceed from the surface of the optic bulb.
The exact mode of connection of the nerve-fibres with the visual rods is not certainly made out, but it is probable that there is direct continuity of substance, and that each rod is really the termination of a nerve fibre.