212 THE EAR 



This theory receives confirmation from the following facts : 



(1) Birds and other animals whose calls have a short range of 

 pitch have short basilar membranes. 



(2) Prolonged subjection to a definite note produces degenera- 

 tion in a definite part of the membrane e.g. in boiler makers' 

 disease there is inability to hear high notes with degeneration of 

 the short fibres ; animals give evidence of deafness to low notes 

 when the long fibres of the membrane have been destroyed. 



Against this theory are arrayed most psychologists and not a 

 few physiologists and anatomists. They find in it no adequate 

 explanation of certain phenomena. 



(1) The parrot has only half a whorl of cochlea, but is able to 

 imitate speech and to whistle musical notes over a fair range, while 

 the guinea pig, with one and a half whorls more than man, produces 

 only squeaks and grunts. 



(2) The bird has a short basilar membrane, about ^ that of 

 the mammal, but has numerous hair cells. These cells are set in 

 the bird in rows of 30 or 40, and in the mammal in rows of 4. 

 If equal lengths of membrane vibrate sympathetically to the 

 same note, then as the bird has ten times the number of hair 

 cells stimulated as the mammal, it ought to hear the sound 

 correspondingly louder. This does not appear to be so. 



(3) It is rather difficult to see how, even taking differences 

 in tension into account, sufficient resonators could be obtained 

 to receive the wide range of notes that it is known we do receive. 

 Displacement Theory. 



The latest form of this theory is put forward by Sir Thomas 

 Wrightson, an engineer. He is supported by Professor Arthur 

 Keith, the eminent anatomist. He considers that the ear is not 

 a physiological piano played upon by the sound waves, but a 

 delicate spring weighing every phase of a sound wave, simple 

 or compound, and transmitting to the brain a record of every 

 fluctuation of pressure in the endolymph of the scala media. 

 Every variation of pressure transmitted by the stapes to the 

 perilymph is in turn transmitted to the membrane closing the 

 fenestra rotunda. The cochlear system is a closed one, in shape 

 rather like a long drawn out, doubled over hour glass, with the 

 stapes operating at one end. The only relief for the motion of 

 displacement is at the fenestra rotunda, at the opposite end, 

 whose membrane moves to and fro simultaneously with the 

 stapes. These movements are transmitted to the endolymph 

 enclosed in the tube, the scala media, which ends blindly at the 



