SENSATION AND REACTION 87 



and retinula makes up an element or o?nmatidium of the 

 compound eye and the ommatidia (Fig. 28, o, 29) which 

 correspond in number with the corneal facets, are isolated 

 from each other by pigment cells (Fig. 29, p). As the 

 ommatidia converge inwards from the surface of the eye it 

 is evident that each of them is concerned with the reception 

 of impressions coming along a path represented by a con- 

 tinuation of its axis, since rays of light not closely parallel 

 with this axis, which pass through the lens, will be absorbed 

 by the surrounding dark pigment. The transparent cones 

 allow the light- rays to reach the underlying rods and thus 

 to excite the retinular cells that surround them. From the 

 retinulae nerve impulses pass along the fibres to the optic 

 ganglion, through a series of cell-stations the first of which 

 form a periopticon (Fig. 28, p) lying within the basement 

 membrane of the eye, and the second an epiopticon {e) in 

 the outer region of the optic ganglion. From this latter the 

 impulses pass to the central ganglionic opticon (op), a mass 

 of nerve-cells in connection with the central brain. The 

 nerve fibres passing between the various cell-stations undergo 

 extensive crossing-over (decussation), so that some impulses 

 started in the upper elements of the eye pass to the lower, 

 and those from the lower elements to the upper. Such a 

 scheme for the reception of the impulses in the brain ensures 

 a certain amount of co-ordination of the multitudinous 

 impressions that aflfect the central nerve-masses through the 

 two large compound eyes of a highly organised insect. 



We may now pass to consider the nature of an insect's 

 sight. The eyes are clearly adapted to receive visual im- 

 pressions, but it is by no means certain that an insect sees 

 as a vertebrate animal sees, and such questions as " Can an 

 insect clearly discern the form of surrounding objects ? " 

 or " Can it distinguish between various colours ? " have 

 often been asked, and have been very differently answered 

 by different students. 



In the simple eyes of insects, such as the three ocelli on 

 the crown of a bee's head, or the group of such organs on 

 either side of the head in caterpillars and other larvae, we 



