*To locate Warm and Cold Sensation Spots.*—Slowly and evenly draw a blunt-pointed piece of metal over the back of the neck. If it be of the same temperature as the skin, only touch sensations will be experienced. If it be a little colder (the temperature of the room) sensations of cold will be felt at certain spots. If slightly warmer than the body, heat sensation spots will be found on other parts of the skin. If the heat and cold sensation spots be marked and tested from day to day they will be found to remain constant as to position. Inference. CHAPTER XXI - THE LARYNX AND THE EAR Man is a social being. His inclinations are not to live alone, but to be a part of that great human organization known as society. For men to work together, to be mutually helpful one to another, requires the ability to exchange ideas and this in turn requires some means of communication.(115) One means of communication is found in certain movements of the atmosphere, known as _sound waves_. In the exchange of ideas by this means there are employed two of the most interesting divisions of the body—the larynx and the ear. The first is an instrument for the production of sound waves; the second is the sense organ which enables the sound waves to act as stimuli to the nervous system. *Nature of Sound Waves.*—If some sonorous body, as a bell, be struck, it is given a quivering, or vibratory, motion. This is not confined to the bell, but is imparted to the air and other substances with which the bell comes in contact. These take up the movements and pass them to objects more remote, and they in turn give them to others, until a very considerable distance is reached. Such progressive vibrations are known as waves, and, since they act as stimuli to the organs of hearing, they are called _sound waves_. Sound waves _always originate in vibrating bodies_.(116) They are transmitted chiefly _by the air_, which, because of its lightness, elasticity, and abundance, readily takes up the vibrations and spreads them in all directions (Fig. 148). While these vibratory movements of the atmosphere are correctly classified as waves, they bear little resemblance to the waves on water. Instead of being made of crests and troughs, as are the water waves, the sound waves consist of alternating successions of slightly condensed and rarefied layers of air. Then, while the general movement of the water waves is that of ever widening circles _over a surface_, the sound waves spread as enlarging spherical shells _through_ the air. In sound waves, as in all other waves, however, it is only the form of the wave that moves forward. The individual particles of air that make up the wave simply vibrate back and forth. [Fig. 148] Fig. 148—Diagram illustrating the spreading of sound waves through air.