144 THE SENSE ORGANS [CH. 



a level below the inner ends of the pseudocones, we find a second 

 set of these pigment cells embracing the distal ends of the retinulae. 

 Four of these are of considerable size, and very densely pigmented, 

 especially at their distal ends, which are somewhat enlarged. 

 They lie rather close in between the retinula-cells. Smaller 

 pigment cells lie outside these. Their nuclei can be fairly easily 

 made out. 



The whole layer of pigment described above is known as the 

 iris, or iris tapetum. 



Below each pseudocone there lies the excessively elongated 

 portion of the ommatidium known as the retinula. This is formed 

 from four visual or retinula-cells (rt), placed close together, with 

 their long axes parallel. These cells are considered to be highly- 

 specialized unipolar nerve-cells, placed with the pole or nervous 

 end in contact with the fenestrate membrane. The opposite or 

 distal end is drawn out into an extremely long border, carrying 

 a row of excessively fine fibrils, arranged like the teeth of a fine 

 comb. The retinula-cells of each ommatidium are so arranged 

 that their fibrils pass inwards, and unite to form the peculiar 

 structure known as the great rod or rhabdome (rh). The formation 

 could be very roughly imitated by taking four ordinary long 

 hair-combs and standing them up on end with their teeth all 

 meeting inwards. The rhabdome itself appears to have a pale 

 central axis surrounded by a dark outer portion. The fibrils 

 which form it pass backwards right through the cell to which 

 they belong, finally emerging at the pole in the form of the nerve- 

 fibre process. The rhabdome is supposed to receive a stimulus 

 from the rays of light focussed on it by the dioptric apparatus of 

 the eye (lens and pseudocone), and to transmute this into the 

 form of a nervous stimulus which is transmitted to the brain. 



The retinula-cells themselves are somewhat flattened, sub- 

 triangular or trapezoidal in cross-section, and stand out from the 

 rhabdome in the manner seen in fig. 62 D-G. They contain 

 pigment-granules throughout their entire length, but more 

 especially thickly for about one-fifth of their length, at the level 

 shewn in fig. 62, rp. This is called the retinal pigment. 



The bases of the retinulae rest upon the fenestrate or basement 

 membrane (fm), which forms the inner boundary of the eye proper. 



