=Hearing or Audition.= Just as the eye is an organ sensitive to certain frequencies of ether vibration, so the ear is an organ sensitive to certain air vibrations. The reader should familiarize himself with the physiology of the ear by reference to physiologies. The drum-skin, the three little bones of the middle ear, and the cochlea of the inner ear are all merely mechanical means of making possible the stimulation of the specialized endings of the auditory nerve by vibrations of air. As the different colors are due to different vibration frequencies of the ether, so different pitches of sound are due to differences in the rates of the air vibrations. The low bass notes are produced by the low vibration frequencies. The high notes are produced by the high vibration frequencies. The lowest notes that we can hear are produced by about twenty vibrations a second, and the highest by about forty thousand vibrations a second. =Other Sense Organs.= We need not give a detailed statement of the facts concerning the other senses. In each case the sense organ is some special adaptation of the nerve-endings with appropriate apparatus in connection to enable it to be affected by some special thing or force in the environment. In the case of taste, we find in the mouth, chiefly on the back and edges of the tongue, organs sensitive to sweet, sour, salt, and bitter. In the nose we have an organ that is sensitive to the tiny particles of substances that float in the air which we breathe in through the nose. In the skin we find several kinds of sense organs that give us the sensations of cold and warmth, of pressure and pain. These are all special and definite sensations produced by different kinds of organs. The sense of warmth is produced by different organs from those which produce the sense of cold. These organs can be detected and localized on the skin. So, also, pain and touch or pressure have each its particular organ. Within the body itself we have sense organs also, particularly in the joints and tendons and in the muscles. These give us the sensations which are the basis of our perception of motion, and of the position of the body and its members. In the semicircular canals of the inner ear are organs that give us the sense of dizziness, and enable us to maintain our equilibrium and to know up from down. The general nature of the sense organs and of sensation should now be apparent. The nervous system reaches out its myriad fingers to every portion of the surface of the body, and within the body as well. These nerve-endings are specially adapted to receive each its particular form of stimulation. This stimulation of our sense organs is the basis or cause of our sensations. And our sensations are the elementary stuff of all our experience. Whatever thoughts we have, whatever ideas or images we have, they come originally from our sensations. They are built up out of our sensations or from these sensations as they exist in memory.