. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 88 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. action of light is of the highest interest, but it cannot be taken as proving a direct participation of the visual purple in the visual process, inasmuch as the visual purple is not present in those parts of the eye in which alone a distinct image is formed, viz., the macula lutea and, generally, the outer segments of the cones. The pigment of the eye seems to be of importance for absorbing the superfluous rays of light which would be i
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 88 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEBAL. action of light is of the highest interest, but it cannot be taken as proving a direct participation of the visual purple in the visual process, inasmuch as the visual purple is not present in those parts of the eye in which alone a distinct image is formed, viz., the macula lutea and, generally, the outer segments of the cones. The pigment of the eye seems to be of importance for absorbing the superfluous rays of light which would be injurious to the per- ception of an image. It is distributed partly immediately outside the retina, forming the choroid coat of the eye, which extends also inwards between the individual retinal elements; and partly in front of the lens, giving rise to a transversely placed curtain, the iris which is pierced by an opening, thepivpil, capable of contrac- ting and dilating. In the higher grades of development the whole eye is, as a rule, enclosed in a hard, connective tis- sue coat, the sclerotic, and thus marked oft' as an eye bulb. The arrangements by which the shining points of an object act in regular ar-. Fio 85.—Diagrammatic representation of the compound eye of aLibellula. C, cornea; K, crystalline cone ; P, pigment; S, nerve rods of retina; Fb, layer of fibres : Gz, layer of ganglion cells ; Rf, retinal fibres; Fk, crossing of fibres. regular rangement on corre- sponding points of the optic nerve and so render possible the perception of an image vary, and are closely dependent upon the whole structure of the eye. Leaving out of consideration the simplest eyes, such as we find in Worms and the lower Crustacea, two types of eye are to be distin- guished. * 1. The first form occurs in the so-called facetted eyes* (figs. 85 & 86) of Arthropods (Crustacea and Insects). The retina of such eyes has a hemispherical form, the convex surface being directed out- wards, and cons
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