*The Hearing of School Children.*—School children not infrequently have defective hearing and for this reason are slow to learn. The hearing is easily tested with a watch, the normal ear being able to hear the watch tick at a distance of at least two feet. Pupils with defective hearing should, of course, have medical attention, and in the classroom should be seated where they can hear to the best advantage. *Summary.*—Sound waves constitute the external stimuli for the sensation of hearing. They consist of progressive vibratory movements of the air that originate in vibrating bodies. Through the larynx and the ear, sound waves are utilized by the body in different ways, but chiefly as a means of communication. The larynx produces sound waves which are reënforced and modified by the air passages. The ear supplies suitable conditions for the action of sound waves upon nerve cells. Both the ear and the larynx are constructed with special reference to the nature and properties of sound waves, and they illustrate the body’s ability to adjust itself to, and to make use of, its physical environment. *Exercises.*—1. For what different purposes are sound waves employed in the body? 2. How do sound waves originate? How are they transmitted? How do they differ from the waves on water? 3. How are sound waves able to act as nerve stimuli? 4. Describe two methods of reënforcing sound waves. Which method is employed in the body? 5. Name all the parts of the body that are directly or indirectly concerned in the production of sound. 6. Describe the larynx. 7. Describe the condition of the vocal cords in speaking and in ordinary breathing. 8. How are sounds differing in pitch and intensity produced by the larynx? 9. How is the sound produced by the vocal cords changed into speech? 10. What parts of the ear are concerned in transmitting sound waves? 11. Give the purposes of the middle ear. 12. Trace a sound wave from a bell to the basilar membrane, and trace the impulse that it causes from there to the brain. 13. Give the purpose of the Eustachian tubes; of the rods of Corti; of the semicircular canals. 14. Give directions for the proper care of the ear. PRACTICAL WORK *To illustrate the Origin of Sound.*—1. Strike a bell an easy blow and hold some light substance, as a pith ball attached to a thread, against the side, noting the result. 2. Sound a tuning fork by striking it against the table. Test it for vibrations as above, or by letting the vibrating prongs touch the surface of water. 3. Pluck a string of a guitar or violin, and find proof that it is vibrating while giving out sound.