236 



THE HUMAN BODY 



of sounds which reach the labyrinth through the general skull- 

 bones instead of through the tympanic chain is imperfect or 

 absent. The recognition of the distance of a sounding body is pos- 

 sible only when the sound is well known, and then not very accu- 

 rately; from its faintness or loudness we may make in some cases a 

 pretty good guess. Judgments as to the direction of a sound are 

 also liable to be grossly wrong, as most persons have experienced. 

 However, when a sound is heard louder by the left than the right 

 ear we can recognize that its source is on the left; when equally 

 with both ears, that it is straight in front or behind; and so on. 



The concha has perhaps something 

 to do with enabling us to detect 

 whether a sound originates before 

 or behind the ear, since it col- 

 lects, and turns with more in- 

 tensity into the external auditory 

 meatus, sound-waves coming from 

 the front. By turning the head 

 and noting the accompanying 

 changes of sensation in each ear 

 we can localize sounds better than 

 if the head be kept motionless. 

 The large movable concha of many 

 animals, as a rabbit or a horse, 

 which can be turned in several 

 directions, is probably an important aid to them in detecting the 

 position of the source of a sound. That the recognition of the 

 direction of sounds is not a true sensation, but a judgment, 

 founded on experience, is illustrated by the fact that we can 

 estimate much more accurately the direction of the human 

 voice, which we hear and heed most, than that of any other 

 sound. 



Nerve-Endings in the Semicircular Canals and the Vestibule. 

 Myelinated fibers (/, Fig. 75) from the vestibular branch of the 

 auditory nerve are distributed along a line across the ampulla of 

 each semicircular canal. They lose their myelin sheath close 

 to the basement membrane, a, which the axons pierce. The 

 axons branch among the epithelium cells, which at this place are 

 several rows thick, but have not yet been traced into direct con- 



FIG. 75. Diagram of epithelium 

 in nervous region of ampulla of a 

 semicircular canal. 



