SENSE ORGANS AND SENSATIONS 255 



Many other examples might be given showing that our 

 visual idea of the world around us is not a simple sensa- 

 tion or impression, but an unconscious inference, judgment, 

 or conclusion built up from a number of simple sensations 

 taken separately or blended together and compounded with 

 results of lifelong experience. In looking at a piece of fine 

 silk or cloth we seldom stop to think that its tissue may be 

 resolved into many simple component threads; and in quite 

 the same way we fail to realize that even our quickly formed 

 judgments of the size, distance, form, or color of objects are 

 likewise tissues woven out of many threads, most of which we 

 have been slowly and laboriously spinning since childhood's 

 days in the hidden factory of individual experience. 



17. Sound and hearing. When the string of a violin, 

 piano, or harp " sounds," one can observe that it is in rapid 

 vibration ; and the same thing is true of all sounding bodies. 

 These vibrations are imparted to the air, water, or other 

 surrounding medium, and through this medium they are 

 transmitted as waves of sound. It is these waves, or vibra- 

 tions, which, on entering the ear, excite the sensation of sound. 

 The more rapid the vibrations, the higher is the pitch of the 

 note ; and the greater their amplitude, the louder the sound. . 



The ear is an organ specially adapted to receive these vibra- 

 tions of air and to transform them into nervous impulses. 

 It is subdivided by anatomists into the outer ear, the middle 

 ear, and the inner ear. 



18. The outer ear. The outer ear consists of the expanded 

 pinna (or that part which we commonly call " the ear ") and 

 a tube along which the vibrations of sound pass inward to 

 the tympanic membrane, or drum. Glands along this canal 

 secrete wax which guards the approach to the drum. It is a 

 bad habit to pick at this wax, and especially to dig into the 

 ear with any pointed instrument, for there is always danger 

 of perforating the drum. If trouble is suspected, a physician 

 should be consulted. 



