618 



THE NERVOUS SYSTEM. 



will be but two-thirds that of the tip of the manubrium, but will 

 have one and one-half times its force. 



The action of the tensor tympani and that of the stapedius 

 have been already described (p. 604). 



It is interesting to note that sound may be conducted to the in- 

 ternal ear through the bones of the skull so as to cause the sensa- 

 tion of hearing. Thus if a vibrating body, as a tuning-fork, is held 

 between the teeth, it can be heard though the ears are closed ; 

 indeed, it sounds more loudly when the ears are closed than when 

 they are open. The sound is conducted by the bones to the 

 internal ear, and also, doubtless, some of the sound is due to 

 vibrations of the membraca tympani. 



This fact is made use of in the audiphone, a fan-like device 

 held in the teeth by the deaf. If the essential portions of the 

 auditory apparatus are so diseased as to cause deafness, no such 

 device as the audiphone will be of any use. 



Theories of Hearing. Two theories have been advanced to 



FIG. 396. The chain of 

 auditory ossicles, anterior 

 view: 1, head of malleus; 

 2, long process of incus; 3, 

 stapes (after Testut). 



FIG. 397. Ligaments of the ossicles and 

 their axis of rotation. The figure represents 

 a nearly horizontal section of the tympanum, 

 carried through the heads of the malleus and 

 incus : M, malleus ; I, incus ; t, articular tooth 

 of incus ; Ig.a. and Ig.e, external ligament of 

 malleus; Ig.inc, ligament of the incus; the 

 line a-x represents the axis of rotation of the 

 two ossicles (from Foster, after Testut). 



explain the physiology of hearing : (1) The piano theory and (2) 

 the telephone theory. 



The Piano Theory. This is by far the older, and may be 

 regarded as the theory usually held to explain what takes place in 

 the cochlea. The cochlear division of the auditory nerve sends 

 into the modiolus of the cochlea branches that pass in between the 

 plates of the lamina spiralis, where they form a plexus in which 

 are ganglion-cells, from which the nerve-filaments pass to the 

 organ of Corti, terminating, it is believed, in the hair-cells. 



The waves already referred to as being set in motion in the 

 endolymph pass over and under these cells, with which the nerve- 

 filaments are connected, and cause the basilar membrane on which 

 they rest to vibrate. This motion is communicated to the outer 

 rods of Corti, which in turn pass it to the hairs of the special audi- 

 tory cells through the medium of the perforated membrane, and from 



