THE AUDITORY APPARATUS. 885 



THE AUDITORY APPARATUS. 



PLAN OF THE STRUCTURE OF THE EAR. 



The auditory nerve is excited normally by waves of sound, 

 which are supposed to set in vibration the end-organs of the auditory 

 nerve. These lie in the endolymph of the labyrinth of the inner ear, on 

 membranous expansions of the cochlea, the saccule and utricle, and the 

 semicircular canals. The waves of sound are first communicated to the 

 labyrinthine fluid, producing wave-motions that set up similar vibrations 

 in the nerve-endings. The stimulation of the auditory nerve is brought 

 about, therefore, by the mechanical irritation produced by the un- 

 dulations of the labyrinthine fluid. 



The labyrinthine fluid is enclosed in the extraordinarily dense and 

 hard mass constituting the petrous portion of the temporal bone (Fig. 

 314). At one situation in the shape of a small, rounded triangle (fenes- 

 tra rotunda), the boundary is formed of a delicate, yielding membrane, 

 the opposite side of which is in contact with the air in the tympanum 

 (P). Not far from the fenestra rotunda is the fenestra ovalis (o), into 

 which the basal plate of the stapes (s) is fixed by means of a yielding 

 membranous ring. The outer surface of this also is in contact with the 

 air in the tympanum. As the labyrinthine fluid is enclosed at these two 

 places by flexible boundaries, it is evident that it is capable of an undu- 

 latory movement, as yielding limiting membranes are able to follow these 

 undulations. 



If it be asked further, in what ways the waves of sound can set the 

 labyrinthine fluid in movement, three different methods suggest them- 

 selves: 



i. Conduction through the bones of the skull. This takes place 

 especially when solid, sounding bodies are placed directly on the head 

 (for example, a tuning-fork; the sound is then propagated most strongly 

 in the direction of the prolonged handle of the tuning-fork), also when 

 the sound is transmitted to the head through fluids (for example, water 

 under which the head is submerged). If the external auditory canal 

 is stopped up, the vibrations of the tuning-fork are more strongly heard. 

 From this it has been concluded that the vibrations in the bone set the 

 air in the middle ear and the auditory canal in vibration, and that this 

 is communicated to the tympanic membrane, so that the stimulation 

 arises from this, as under normal circumstances craniotym panic stimu- 

 lation. Waves of sound in the air are practically not transmitted to the 

 bones of the skull, as is shown by the inability to hear when the ears 

 are closed. 



Of the soft parts belonging to the head, only those that are directly in contact 

 with the bones conduct sound well; of the detached portions, the cartilaginous 

 part of the external ear is the best conductor. Even under the most favorable 

 circumstances, conduction through the bones of the skull affords less favorable 

 conditions for excitation of the auditory nerves than conduction of the sound 

 through the auditory canal. For example, if a vibrating tuning-fork be held 

 between the teeth until its sound is no longer heard, its tone may be still distinctly 

 heard if it is brought quickly in front of the ear. Sounds are also better con- 

 ducted through the bones of the skull if the oscillations arc- not freely transmitted 

 by the bones to the tympanic membrane, and by this to the air of the auditory 

 canal. Therefore, sounds are heard better if the ears are closed at the same time, 



