234 ORIGIN OF AUDITORY IMPULSES. [BOOK in. 



consisting as they do of fine filaments tending to cohere together, 

 seem imperfect instruments for vibratory purposes. On the other 

 hand in the case both of the cochlea, the maculae and the cristae, 

 the hairs of the hair-cells have close relations with structures 

 which have the aspect of being ' damping ' organs, namely the 

 tectorial membrane over the organ of Corti, and the otolith 

 membrane over the macula, to which we may add the cupula over 

 the crista. It has indeed been suggested that the otoliths may 

 serve as exciting adjuvants of the vibrations of the fluid endolymph, 

 may act as it were as minute hammers enforcing the blows of the 

 fluid ; but this is negatived by observations on the auditory organs 

 of certain molluscs. In these animals the organ of hearing is 

 a closed spherical vesicle, part of the lining of which is furnished 

 by an auditory epithelium of hair-cells with short rod-like hairs, 

 and the rest by ciliated epithelium, the former being alone in 

 connection with a nerve which seems to function as an auditory 

 nerve. Within the vesicle lies a large otolith, and it has been 

 observed that while under ordinary circumstances when sounds of 

 not too great intensity are directed towards the object the otolith 

 is kept away from the auditory hairs, but that when too intense 

 a sound is brought to bear upon the organ, the otolith is driven 

 by the cilia down upon the auditory hairs, and obviously behaves 

 as a damper. We may probably conclude therefore that the mein- 

 brana tectoria, and the otolith membrane act as dampers ; but if 

 so the view seems natural that the so-called auditory hairs are 

 useful for conveying the damping action from the damper to the 

 hair-cell, and do not serve to communicate the vibrations of sound 

 from the endolymph to the hair-cell. This would lead us to the 

 conception that both in the organ of Corti and in the macula and 

 crista, the vibrations reach the bodies of the hair-cells in some 

 other manner, not by means of the hairs, but by means of the 

 other structures which go to make up the auditory epithelium; 

 and indeed it is much more in respect to the general arrangements 

 of the auditory epithelium than to the special characters of the 

 hair-cells that the organ of Corti, which we must certainly regard 

 as the chief auditory mechanism, is so much more highly differ- 

 entiated than the macula or crista. The hair-cells of the cochlea 

 and of the vestibule are exceedingly alike ; but very different is 

 the organ of Corti, with its basilar membrane, reticulate membrane, 

 rods of Corti, and rod cells specialized into cells of Deiters, from 

 the simple macula or crista, which contain besides the hair-cells 

 nothing except the relatively plain rod cells. We can hardly 

 resist the conclusion that the structures just mentioned play an 

 important part in bringing the vibrations to bear on the hair- 

 cells. But what that part is remains for the present mere subject 

 of speculation. 



850. A complex sound, consisting of vibrations of more than 

 one period, travels as we have said not as a group of discrete waves, 



