The eyes of insects and several other classes of animals are called compound eyes, and the image they produce must resemble objects depicted in mosaic, or made up of small spots of distinct colours; they consist of hundreds of small tubes, radiating and forming part of a sphere on each side of the head, each of these tubes admits the light from objects at which they point. Some of the animals still lower in the scale have simple spots of transparent membrane with an expansion of optic nerve beneath them; in such creatures the power of vision is probably limited to distinguishing light from darkness. [Illustration: FIG. 86.--SECTION OF THE HUMAN EAR. _a_, external opening; _b_, tympanum; _c_, small bones; _d_, Eustachian tube; _e_, cochlea; _f_, semicircular canals.] _Hearing._--Sound is conveyed to the mind by the vibration of air, and this vibration is communicated to a membrane within the ear, called the "tympanum;" this is in connection (by means of three minute bones) with the expanded auditory nerve, which ramifies within the "vestibule," which is filled with a fluid, so that the vibrations of the air are thus communicated to the nerve of hearing (fig. 86). _Smell._--The interior of the nose is lined by a fine moist membrane, on which the olfactory nerve is distributed. The cause of odour is quite unknown; though, as most volatile substances are odorous, it is supposed to be some volatile matter which comes in contact with the olfactory nerve, or nerve of smelling. The quantity given off in some instances must be exceedingly small, for one grain of musk placed on a piece of glass will give out scent for months, and at the end of that time weigh still one grain. Some animals, as Dogs and Birds of prey, have this sense so acute that it serves them instead of almost every other; the Vulture will scent carrion miles away, and the Dog will track his master by the power of scent. _Taste._--The sense of taste resides chiefly in the tongue, but to some extent in the palate or roof of the mouth; it appears to be a modified sense of smell, combined with common sensation or touch, and is much diminished by loss of smell.