COMPOUND EYE. 331 



lentigen cells of the external layers, are in relation with a re- 

 fracting element, the rhabdomere, which forms their axial border. 

 They are also abundantly invested by black pigment. The 

 several rhabdomeres of a retinula make up the rhabdom (Fig. 

 232, A andD rh). Internally the re tinular cells are continuous 

 through apertures in a basement membrane (bm) with the nerve 

 fibres of the optic ganglion. The crystalline cone (C) like the 

 rhabdom bears evidence of being divided longitudinally. In 

 some cases (Sphaeroma) the rhabdom contains a central space, 

 occupied by one or two hyaline cells (Watase). Moreover 

 the rhabdomere has been found in some cases (Schizopoda, Deca- 

 poda) to present very fine striations perpendicular to its margin 

 (Fig. 232 A rh) which are regarded by some authors as the 

 ultimate ends of nerve fibrillae. The distribution of the pigment 

 in both sets of cells is, as we shall see, dependent in some 

 animals on the degree of illumination to which the eye is sub- 

 jected. 



Beside the cells forming the ommatidia proper other hypoder- 

 mic cells in varying number are present in the interspaces be- 

 tween them (Figs. 232 A p, and 233, 6). These (accessory pig- 

 ment cells) may contain flakes of pigment which is not black but 

 white by reflected and yellow by transmitted light. The 

 function of this pigment appears to be not to absorb, but to 

 reflect the light. It forms the tapetum of crustacean eyes. In 

 the eyes of Insecta, adapted for vision by night (see below), 

 tufts of minute tracheae invest the retinulae, and by their 

 shining surfaces likewise serve as a tapetum (Exner). 



According to Lord Avebury some 4,000 ommatidia compose 

 the eye of the common house-fly, while that of a Dragon-fly 

 (Aeschna) contains 20,000. 



The mode of working of the compound eye was explained in 

 part by the theory of mosaic vision first propounded by Johannes 

 Miiller. The only direction from which the light is able to 

 penetrate to the retinula, of an ommatidium, ensheathed in pig- 

 ment in the manner above described, is that in and near the 

 line of its axis. The light which thus enters, concentrated on 

 the pigmented retinular cells by the refracting media of the 

 ommatidium and reflected from the surrounding tapetum, will 

 set up changes in them, according to its nature, and give rise to a 

 corresponding stimulus in the nerve fibres with which they are 



