SENSE OF EQUILIBRIUM 239 



exterior, nor are they hallucinations. They may be due to 

 disturbances of the auditory apparatus or to abnormal condi- 

 tions of surrounding organs. Thus, buzzing or ringing in the 

 ears may result from the hyperemia of the parts and the 

 increased rush of blood, or from disease in the auditory nerve or 

 some other portion of the apparatus. Hallucinations are purely 

 creations of a disordered brain. 



Musical sounds are distinguished by the mind by three factors 

 loudness, pitch, and quality. Every musical tone is produced 

 by a succession of regular alternate rarefactions and condensa- 

 tions of the air. It is their periodicity which makes them musical, 

 otherwise they are known as noises. The range of musical 

 notes that can be appreciated by the human ear is about seven 

 octaves. There are about 3000 hair cells in the organ of Corti, 

 and it is easily seen that this would allow an enormous capa- 

 bility to differentiate sounds and musical tones. This corre- 

 sponds to a range of from 40 to about 4000 vibrations per second. 

 The range of audibility, on the other hand, is about eleven 

 octaves, or from 16 to 38,000 vibrations per second. With less 

 than 16 vibrations per second the ear perceives only separate 

 shocks, while with more than the larger number the sensation 

 of sound is not produced. 



The distance and direction of sounds are not perceived directly, 

 but are estimated by their loudness and quality combined 

 with reasoning from past experience. When one ear is totally 

 deaf, all sounds seem to originate from the side of the healthy 

 ear. When the eyes are closed, a sound directly overhead is 

 imperfectly localized, but seems to come from a point ahead 

 and above the person. The quality as well as the loudness of 

 the sound varies with the distance from its source, because the 

 lower tones die away first, making the overtones more promi- 

 nent. This is taken advantage of by ventriloquists, who, by 

 modifying the intensity and quality of the voice, produce an 

 imitation of the effect of distance. The ear is capable of appre- 

 ciating very small intervals of time; 132 auditory impulses per 

 second are heard separately, while in the eye all above 24 per 

 second are fused together. 



Sense of Equilibrium. By sense of equilibrium or equi- 

 poise is meant a state of the body in which all the muscles are 

 under control so as to resist the effect of gravity whenever 



