ANATOMY AND PHYSIOLOGY 



8 9 



that surround the retinula distally; and an 

 inner zone of retinal pigment, in which the 

 pigment cells are long and slender, and en- 

 close the retinula proximally. All these parts 

 are hypodermal in origin, as is also the fenes- 

 trate basement membrane, through which pass 

 tracheae and nerve fibers. The nerve fibrillae, 

 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 experimenta- 

 tion and discussion upon the physiology of the 

 compound eye the subject of the monumental 

 works of Grenacher and Exner Miiller's 

 "mosaic" theory is still generally accepted, 

 though it was proposed early in the last cen- 

 tury. 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. Each ommatidium is 

 adapted to transmit light along its axis only 

 (Fig. 143), as oblique rays are lofet by absorp- 

 tion in the black pigment which surrounds the 

 crystalline cone and the axial rhabdom. Along 

 the rhabdom, then, light can reach and affect 

 the terminations of the optic nerve. Each 

 ommatidium does not itself form a picture; it 

 simply preserves the intensity and color of the 

 light from one particular portion of the field of 

 vision ; and when this is done by hundreds or 

 thousands of contiguous ommatidia, an image 

 results. All that the painter does, who copies 

 an object, is to put together patches of light 

 in the same relations of quality and position 

 that he finds in the object itself and this is 

 essentially what the compound eye does, so far 

 as can be inferred from its structure. 



- c 



pc 



tv 



FIG. 142. Structure of an 

 ommatidium of Calliphora 

 vomitoria. A, radial section 

 (chiefly); B, transverse sec- 

 tion through middle region; 

 C, transverse section through 

 basal region; bm, basement 

 membrane; c, cornea; n, 

 nucleus; nv, nerve fibrillae; 

 pc, pseudocone; pg 1 , pg 2 , cells 

 containing iris pigment; pg 3 , 

 cell containing retinal pigment; 

 r, one of the six retinal cells 

 which compose the retinula; rh, 

 rhabdom, composed of six rhab- 

 domeres; /, trachea; tv, tracheal 

 vesicle. After HICKSON. 



Exner, removing the cones with the corneal 

 cuticula (in Lampyris), looked through them from behind with the aid of 



