ANATOMY AND PHYSIOLOGY Tere 
and retinal), (5) fenestrate membrane, (6) fibers of the optic 
nerve, (7) trachee. 
The cornea (Fig. 142) is a biconvex transparent portion 
of the external chitinous cuticula. Immediately beneath it are 
the cone cells, which may contain a ' 
. ‘ : Gee Al 
clear fluid or else, as in most insects, 4 
° RURAL 
- 
solid transparent cones. ‘The rhab- w. 
-s. 
~. 
Lr) 
dom is a transparent chitinous rod 
or a group of rods (rhabdomeres ) 
situated in the long axis of the 
ommatidium and surrounded by 
greatly elongated cells, which 
constitute the retinula. Two 
zones of pigment are present: an 
outer zone, of iris pigment, in 
which the pigment in the form of 
fine black granules is contained 
chiefly in short cells that surround 
nk ; ; Portion of compound eye of 
the retinula distally; and an inner fy, Calliphora vomitoria, radial 
section. c, cornea; 7%, iris pig- 
ment; 2, nerve fibers; nc, nerve 
zone of retinal pigment, in which 
Encumpioment..cells. are lone aid) <cls; 7) seUnal plement 2 tea, 
. chea.—After Hr1cxson. 
slender, and enclose the retinula 
proximally. All these parts are hypodermal in origin, as is also 
the fenestrate basement membrane, through which pass trachez 
and nerve fibers. The nerve fibrille, which are ultimate 
branches of the optic nerve, pass into the retinal cells—the end- 
organs of vision. Under the basement membrane is a fibrous 
optic tract of complex structure. 
Physiology.—After much experimentation and discussion 
upon the physiology of the compound eye—the subject of the 
monumental works of Grenacher and Exner—Muller’s ** mo- 
saic ’’ theory is still generally accepted, though it was proposed 
early in the last century. It is thought that an image is 
formed by thousands of separate points of light, each of which 
corresponds to a distinct field of vision in the external world. 
