UNICOBNEAL EYE. 



89 



(figs. 85 & 86 Rf & fi), which are separated from one another by 

 pigment sheaths. In front of these rods are placed the strongly 

 refractile crystalline cones (k), and in front of these again the lens- 

 shaped corneal facets (C & F). 



The eye is enclosed by a firm chitinous layer, which, following 

 the sheath of the entering optic nerve, surrounds 

 its soft parts and reaches as far as the cornea. 

 That part of the eye which is known as optic 

 nerve corresponds in a great measure to the retina 

 itself, and contains a layer of ganglion cells and of 

 nerve fibres. 



A reversed and reduced picture of the object 

 is thrown behind each convex corneal facet (lying 

 far from the sensitive layer of nervous rods), and 

 only the perpendicular rays can be perceived since 

 all the others are absorbed by the pigment. Ac- 

 cordingly the light impressions caused by these 

 axial rays, whose number corresponds with the 

 separate nerve rods, form a mosaic on the retina 

 which repeats the arrangement of the parts of the 

 external object emitting light. The picture which 

 is here formed lacks, however, brilliancy and dis- 

 tinctness. 



2. The second form of eye, which is widely distri- 

 buted in the animal kingdom (the simple eye, 

 Annelids, Insects, Arachnida, Molluscs, Verte- 

 brates) corresponds to a globular camera obscura 

 with collecting lenses (cornea, lens) on its exposed 

 anterior wall on which the light falls and usually 

 with additional dioptric media filling the optic 

 chamber (vitreous humour.) The simple eye of 

 Insects seems to have originated from the simple 

 metamorphosis of part of the integument, beneath 

 which are placed the end organs of the optic nerve 

 (fig. 87). The cuticular covering (CL) projects as a 

 lens- shaped thickening into the subjacent layer of 

 transparent, elongated, hypodermis cells (Gk), 

 within which are placed elongated rod-like nerve- 

 cells with refractile cuticular portions, closely aggregated to form a 

 retina (fig. 87 Rz). The hypodermis cells surrounding the edge of 

 the lens are filled with pigment, and form an iris-like dark ring 



FIG. 86. Three fa- 

 cets Avith retinulae 

 from the com- 

 pound eye of a 

 cockchafer (after 

 Grenacher) . The 

 pigment has been 

 dissolved away 

 from two of them, 

 F, corneal facet. 

 K, crystalline 

 cone. P, pigment 

 sheath. P-, chief 

 pigment ceils. P", 

 pigment cells of 

 the second order. 

 .R, retinulse. 



