1 1 64 THE EAR. 



It is possible, as suggested by Gray, 1 that when a sound-wave falls on 

 the membrana while the intrinsic muscles are in a state of tension, a slight 

 change may be produced in this tension. This change will be often 

 different in the two ears, and thus we may judge of the direction of a 

 sound by the muscular sense. If this be so, then our powers for 

 estimating the direction of sound are ultimately referable to the muscular 

 sense. 



THE INTERNAL EAR. 

 By JOHN G. M'KENDRICK. 



The internal ear consists of a complicated series of sacs and tubes 

 filled with fluid. In certain situations the walls of the sacs contain 

 highly differentiated epithelial structures, which are intimately related 

 to the terminal filaments of the auditory nerve. The problem is to 

 explain how the pressures transmitted by the foot of the stapes affect 

 these terminal structures in such a way as to excite sensations corre- 

 sponding to the pitch, intensity and quality of tones. It is evident 

 that the subject may be approached from the physical or from the 

 physiological side. The physical phenomena of sound fall within the 

 scope of the principles of mechanics; but when we attempt to apply 

 those principles to an explanation of the functions of the internal ear, 

 we find the task surrounded with difficulties. 



We shall in this article discuss the question purely as regards its 

 physiological aspects, presuming an acquaintance with the anatomical 

 structure of the internal ear, 2 and also with the general principles of 

 acoustics. 3 



General mechanism of internal ear. As the base of the stapes 

 is opposite the vestibule in which lie the utricle and saccule, it is 

 evident that these latter structures are the first to receive the 

 mechanical impulses communicated by the chain of bones. The utricle 

 (communicating with the semicircular canals) and saccule (communicat- 

 ing by the canalis reuniens with the ductus cochlearis) are surrounded by 

 the perilymph. It must also be noted that the cavity of the vestibule 

 containing perilymph opens on the one hand into the scala vestibuli, 

 while on the other it is in free communication with the perilymphatic 

 space around the ampullae and the tubes of the semicircular canals. 

 These perilymphatic spaces in the vestibule, around the canals, and in 

 the scalse of the cochlea, are contained in corresponding cavities in the 

 bone, the walls of which are, except at the fenestra rotunda, perfectly 

 rigid. The fenestra rotunda, however, is covered by a membrane. Suppose 

 no fenestra rotunda had existed, any pressure of the base of the stapes 

 inwards would have produced no effect on the membranous structures, 

 as the fluid would be practically incompressible ; in other words, the base 

 of the stapes, in such conditions, would have been immovable. As a 

 portion of the wall of the bony cavity is yielding, namely, the fenestra 

 rotunda, it is clear that an inward movement of the base of the stapes 

 becomes possible, and this inward movement will always be accom- 

 panied by an outward movement of the membrane covering the round 

 window. In this manner, pressures or impulses can be communicated 



1 Proc. Roy. Soc. Edin., 1897, vol. xxi. p. 443. 



2 Quain's "Anatomy," 9th ed., vol. iii. part 3, p. 71. 



3 Rayleigh, "The Theory of Sound," 2 vols. London, 1896. 



