i io INTRODUCTION TO GENERAL PHYSIOLOGY 



Next, supposing that we have immersed in water a spring 

 which is capable of vibration at a certain rate, like the wires of a 

 piano, and that we send, by some means, sound waves of this rate 

 into the water. The spring will be set into sympathetic vibration 

 by resonance in the way previously explained (p. 52 above). But 

 if its rate is not that of the sound waves, it will remain at rest. 

 Have we then anything that might act in this way in the cochlea ? 



One of the component parts of the receptor structure immersed 

 in the liquid of the cochlea is a membrane, the " basilar membrane," 

 to which other parts composing the organ of Corti are attached. 

 This membrane is a strip narrower at one end than at the other, 

 and is stretched transversely by being attached to the bony walls 

 at both sides. In a longitudinal direction it is lax. It also con- 

 tains fibres arranged transversely. Such a membrane can be 

 shown mathematically to have a series of different rates of vibration 

 in order from one end to the other, so that a narrow section 

 would respond by resonance to a higher note than a broader 

 one (E., p. 212). 



All that we need further is a mechanism by which the 

 vibrations of each section can be made to stimulate a particular 

 nerve fibre and we have the means of distinguishing between notes 

 of a different pitch or rate of vibration. The precise means by 

 which this is done is difficult to make out, but it seems to be that 

 represented as a diagram in Fig. 4. There is a series of arches, 

 jointed at the top, arranged along the membrane. One foot of the 

 arch rests on the basilar membrane near one of its attachments, so 

 that it is practically immobile. The other foot rests on a part of 

 the membrane which vibrates up and down as represented. The 

 result of this is a movement of the top of the arch chiefly in the 

 direction from right to left in the plane of the paper, and back 

 again in the opposite direction. Attached to these arches is a 

 membrane ("ret&ufar") with holes in it. Through these holes 

 project stiff hairs attached to cells below it. The points of the 

 hairs appear to be more or less fixed by being stuck against 

 another soft membrane (" tectorial"). When the reticular mem- 

 brane therefore is pulled backwards and forwards by the up and 

 down movements of the basilar membrane, the base of the hair is 

 pulled through, or together with, the cell to which it is attached, 

 and exerts pressure on the termination of the auditory nerve which 

 ramifies in or upon the cell. 



It will be clear that there must be as many nerve fibres and 

 elements of the organ of Corti as it is possible to distinguish in 

 difference of pitch. It is said that about 1 1,000 different notes can 

 be distinguished, and the number of fibres in the cochlear division 

 of the auditory nerve has been found to be 14,000. The number 



