The principal parts of the eye are arranged in a globe-like ball called the eyeball. This eyeball is movable in the socket under control of various muscles. The eyeball is almost surrounded by a membrane which is opaque in most parts, but very transparent at the front. This transparent portion of the surrounding membrane is called the cornea, and is quite hard. This is the outside coat of the eye. The second coat of membrane consists of parts of various names and contains the iris. The third coat is the retina, which is the end of the optic nerve entering the eye full from behind and expanded into a membrane which spreads out over the second coat. The retina or optic nerve receives optical impressions focused upon it by the crystalline lens. These impressions are carried along the optic nerve to the brain, and the brain then receives the sensation of seeing the image. The eyeball is hollow, and its three surrounding coats form what is practically the same as the interior of a camera. The crystalline lens of the eye acts the same as the lens in the camera. This crystalline lens is suspended within the eyeball right in front of the transparent opening in the front of the eyeball, and when the rays of light strike this lens it focuses them on the retina, which is the same as the film in your camera. Why Can We Hear? We can hear because nature has provided us with a very wonderful organ called the ear and which catches the sound waves that come through the air into the ear and make a part of the ear vibrate. In man and mammals the ear is generally found on the outside of the body, but the principal part of the ear is located within the skull. What we call ears are only the funnel-shaped extensions on the outside of the head which are not so very important so far as hearing is concerned, because they only help the real ear to hear more easily. The outside of the ear gathers in the sound waves and, because it is much larger than the little hole which takes the sounds in to the real ear, we can detect more sounds by having this funnel-shaped arrangement on the outside. The inside of the ear contains an eardrum or tympanum which is separated from the outside part of the ear by a membrane. Behind this eardrum is the real hearing part of the ear in a labyrinth containing the nerves of hearing. Now, when a sound wave strikes the membrane which hangs over the opening before the eardrum, the membrane vibrates and transmits the sound wave through the eardrum into the inner ear which contains the ends of the nerves by which we hear. These nerves, on receiving the sensation, transmit it to the brain which thus records the impression of sounds. As we descend the scale of animal life from the mammals downward, the ear becomes a more and more simple organ. In the vertebrates which are not mammals, there is no external ear at all, and we find great simplifications of the ear the lower down in the scale we go. What Is a Totem Pole For?