[28] To show in how many different ways nature may carry out the same purpose, the smelling organs in insects, lobsters, and crabs are on the ends and sides of tiny feelers, which they wave about; and the eyes in lobsters, crawfish, and snails, are on the ends of stalks, which they thrust about in all directions as a burglar handles a bull's-eye lantern. Snakes "hear," or catch the sound-waves, with their flickering, forked tongues; and grasshoppers and locusts have "ear-drums" on the sides of their chests. [29] These are called the _recti_ or "straight" muscles, upper, lower, inner, and outer, according to their position. Then, to roll the eye round and round, there are two little muscles, one above and one below, which run "crosswise" of the orbit, called the upper and lower _oblique_ muscles. [30] The retina is chiefly made up of a great number of fine little nerve cells called, from their shape, the _rods_ and _cones_. These are kept soaked in a colored fluid called the _retinal purple_, which changes under the influence of light, somewhat in the same way that the film on a photographic plate does, thus forming pictures, which are translated by the rods and cones and telegraphed along the fibres of the optic nerve to the brain. Naturally, all parts of the retina are not equally sensitive to light; its centre, which is directly opposite the pupil of the eye, is far the most so, while those around the rim of the cup are dull. This is why, when you are looking, say at some one's face across the room, only the face and a few inches around it are seen perfectly clear and sharp, while the rest of the room is seen only vaguely. [31] As the inside of the eye is dark, or comparatively so, the pupil, or little opening in the centre of the iris, looks black, and was at one time supposed to be a solid body instead of a hole. You can easily watch the pupil changing in size, according to the brightness of the light, from a mere pin-point in very bright sunlight or gaslight, up to the size of the butt-end of a lead pencil in the dark or in a dim light. This change in size is very simply but ingeniously carried out by two sets of tiny muscles. One set of these muscles runs in a ring right around the pupil; and when they shorten, the opening is contracted or narrowed. The other set runs outward through the iris like the spokes of a wheel; and when they shorten, they pull the pupil open. If anyone has had "drops" (_atropin_) put into his eyes in order to have them fitted with glasses, he will know what a disagreeably dazzling thing it is to have the pupil permanently enlarged, so that it cannot _contract_ in a bright light. CHAPTER XXIV THE SPEECH ORGANS