mi AUDITORY SENSATIONS 383 



there can be no doubt of the fact that the elaborate 

 apparatus of the cochlea is able to translate, so to speak, 

 the sonorous vibrations which reach it into stimulations 

 of nerve fibres, the molecular changes of which are 

 transmitted along the auditory nerve as nervous im- 

 pulses. Passing along the auditory nerve, these mole- 

 cular changes, these nervous impulses, reach certain parts 

 of the brain, situated in the cortex of the temporo- 

 sphenoidal lobe, below the fissure of Sylvius (see Lesson 

 XI.), and there in turn set up those molecular disturbances 

 of nervous matter which form the immediate cause of the 

 states of feeling called " sounds." Thus the auditory 

 nerve may be said, and a similar statement may be made 

 in the case of the other nerves of special sensations, to be 

 provided with two "end-organs."' There is the periphe- 

 ral end-organ (the apparatus of the cochlea and laby- 

 rinth . by which the physical agent is enabled to excite 

 the sensory nerve-fibres ; and there is the central end- 

 organ, in the brain, in which the nervous impulses of 

 the sensory nerve excite the special state of feeling 

 which we call the special sensation. The central end- 

 organ of hearing is often spoken of as the auditory 

 sensorium. 



Between the emission of sound from a body and its 

 appreciation by the hearer there is a series of events of 

 diti'erent kinds. There are the vibrations started Vjy the 

 sounding body, and passing through the air, the tym- 

 panum, the perilymph, and the endolymph : these are all 

 of one order. Then there are the changes in the peri- 

 pheral end-organ, in the apparatus of the cochlea and 

 labyrinth ; these are of another order. Then follow the 

 molecular disturbances travelling along the auditory 

 nerve ; these are of still another order. Lastly, there 

 are the changes in the central end-organ, in the brain ; 

 these, though resembling the preceding in so far as they 

 are clianges of nervous matter, are yet of still another 

 order, and probably comprise in themselves a whole .series 



