562 AUDITORY SENSATIONS. [BOOK HI. 



insufficient for the work assigned to them. Moreover, they ap- 

 pear not to be elastic. Lastly, they are wholly absent in birds, 

 who very clearly can appreciate musical sounds. This last fact 

 proves indubitably that the rods in question are not absolutely 

 essential for the recognition of tones. In the face of these diffi- 

 culties it has been suggested that the basilar membrane, which is 

 present in birds as well as in mammals, and which, being tense 

 radially but loose longitudinally, i. e. along the spiral of the cochlea, 

 may be considered as consisting of a number of parallel radial 

 strings, each capable of independent vibrations, is the sought-for 

 organ of analysis; for it may be shewn mathematically that a 

 membrane so stretched in one direction only is capable of vibrating 

 in such a manner. And the radial dimensions of the basilar 

 membrane give a much greater range of difference than do the 

 rods of Corti, diminishing in man downwards from '495 mm. at 

 the hamulus to "04125 mm. near the bottom of the spiral, whereas 

 the difference in length of the latter is simply that between "048 

 and '085 mm. for the inner, and between *019 and '085 for the outer 

 fibres. According to this view, a particular simple vibration 

 reaching the scala tympani of the cochlea throws into sympathetic 

 vibrations a small portion of the basilar membrane, the vibrations 

 of which in turn so affect the structures overlying it, that sensory 

 impulses are generated. These sensory impulses reaching the brain 

 give rise to a corresponding sensation of a particular tone. 



The remarkable reticular membrane which has such peculiar 

 relations with the hair-cells, and through them with the basilar 

 membrane, must, one might imagine, have some special function ; 

 but it is impossible at present to assign to it any satisfactory duty. 

 The structural arrangements seem, if anything, to indicate that when 

 a segment of the basilar membrane is thrown into vibrations, the 

 overlying hair-cells, reticular membrane, and rods of Corti vibrate 

 en masse together with it. But this renders the whole matter still 

 more difficult. Indeed the whole subject is in the highest degree 

 obscure, and the most we can say is that the organ of Corti as 

 a whole seems to be in some way connected with the appreciation 

 of tones, but that at present it is very hazardous to attempt to 

 explain how it acts, or to assign particular functions to particular 

 parts. The distinction between the inner and outer hair-cells 

 seems to be very parallel to that between the rods and the cones of 

 the retina ; but even this analogy may be a fallacious one. 



It has been observed that among the auditory hairs of the 

 Crustacea, some will vibrate to particular notes ; but the auditory 

 hairs of the mammal are far too much of the same length to permit 

 the supposition that they can act as organs of analysis. 



If the organ of Corti is the means by which we appreciate 

 tones, it is evident that by it also we must be able to estimate 

 loudness, for the quality of a musical sound is dependent on the 

 relative intensity, as well as on the nature, of the overtones. And 



