SENSATION AND REACTION 83 



There remains for consideration the sense of vision, or 

 of some dimmer perception due to the stimulation of nerve- 

 endings by the impact of light- rays, referred to at the 

 opening of this chapter. An insect's eye is a sense-organ 

 of considerable complexity, but it is, like all the various 

 types of sense-organ, due to a modification of the skin with 

 its overlying cuticle and the connection of certain special 

 sensory organs in the skin with the underlying nervous 

 system. The cuticle covering an eye may be relatively 

 thick, but it must be transparent to allow rays of light to 

 pass through and stimulate the nerve-endings beneath. In 

 many insect larvae groups of small circular or sub-circular 

 simple eyes (ocelli) are readily observed on either side of the 

 head, while many adult insects have two or three ocelli on 

 the vertex (Fig. 10, 6). Their surface presents a glassy aspect, 

 and the deeper structures imperfectly seen through the 

 transparent cuticle (cornea) appear black on account of the 

 dark pigment which is contained in or associated with the 

 visual cells of animals generally. Microscopical examination 

 of the simple eye or ocellus of an insect shows that the trans- 

 parent cornea is convex on the upper surface, and possibly 

 also on the lower, beneath which the living cells of the skin 

 extend in a continuous sheet ; the cornea has of necessity 

 been formed by the activity of these cells. Below them 

 come the receptive cells which form the retina of the eye ; 

 they are elongate and taper towards their inner ends which 

 are produced into the axes of fibres. These retinal cells, 

 w^hich are derived immediately from the skin, may contain 

 a quantity of dark pigment or the pigment may be accumu- 

 lated in special cells intercalated between groups of retinal 

 cells, the retina being thus divided into a number of small 

 cell-groups (retinulae). The structure of such a simple eye 

 suggests that the circular convex cornea acts as a lens, the 

 rays of light converging on the sensitive retina whence nerve 

 impulses pass on through the fibres to the brain. Eyes 

 essentially of the same type as these insect ocelli are found 

 also among worms, molluscs, and other animals. Further 

 discussion of their function may be postponed until we 



