THE COMPOUND OR FACETTED EYE 



258 



The retinula and rod. The retinula is morphologically a nerve- 

 end cell, situated at the end of a nerve-fibril arising from the optic 

 nerve. The elements of the retinula of Musca are six in number 

 and surround the rhabdom (Fig. 264), which consists of a bundle of 

 six long, delicate chitinous rods, more or less firmly united together 

 (Fig. 264, K). 



The six elements of the retinula of Musca are in their outer or 

 distal portion free from one another, but towards their base are 

 fused into a sheath (Fig. 264, r). They are true nerve-end cells, as 

 shown by Muller and by Max Schultze, their views having been con- 

 firmed by Grenacher and by Hickson. The relations of the nerves 

 to the rods after passing through the basal membrane is seen in 

 Fig. 266. 



The pigment. The cones or pseudocones are mostly buried in 

 pigment, as well as the rods ; and the pigment forms two layers. 

 The outer of the two layers is called the iris pigment (Fig. 265, e, 

 iris tapetum), and the inner (/) the retinal pigment. 



Between the ommatidia internally there occur, according to Hick- 

 son, pigment cells (Fig. 264, p.g^), each of which stands on the basilar 

 membrane and sends a fine process outwards towards the internal 

 process of the external pigment-cell (p.# 2 )- A long, slender tracheal 

 vesicle also passes in between the retinulse. 



The basilar membrane. This is a thin fenestrate 

 membrane (Fig. 261) separating the cones and 

 rods from the optic tract (Fig. 264, fr.ra). It is 

 perforated for the passage of tracheal diverticula 

 and of the optic nerve fibrils. It separates the 

 dioptric or instrumental portion of the eye from 

 the percipient portion, i.e. the optic tract. 



The optic tract. This is the optic ganglion of 

 earlier writers, and appears to be the percipient 

 portion of the eye, as opposed to the dioptric 

 portion. If the reader will examine Figs. 249 

 and 261, he will see that it consists of three 

 distinct ganglionic swellings, i.e. the opticon, epi- 

 opticon, and periopticon, whose structure is very 

 complicated. In Musca (Fig. 261) the first gan- 



FIG. 265. Two om- 



glionic swelling (opticon) is separated from the matiaia from the eye 



, . , , . , , .... -i i T> -i of Colymbetes fuscus, 



brain by a slight constriction, which JBerger regards x ieo -. a, cornea ; b, 



, , , ,, , , . ,, , , , , cone ; c, rhabdom ; d, 



as the homologue of the optic nerve of the other basal membrane, with 



,1 j . , r- i nerve filaments below 



arthropods. It consists ot a very fine granular n ; 6 , iris pigment; /, 

 matrix traversed throughout by a fine mesh work E^*, from sharp". M 



