218 



PHYSIOLOGY 



CHAP. 



We have already insisted that the transformation of the 

 physical process of sound-vibration into the physiological process 

 of the neural excitation which arouses auditory sensations in 

 consciousness takes place in the cochlea. The impacts trans- 

 mitted from the tympanic apparatus and the cranial bones act 

 upon the organ of Corti, and excite the endings of the count- 

 less branches of the cochlear nerve by means of the hair -cells. 

 The great development of the organ of Corti in man and the 

 higher mammals, as well as clinical and experimental evidence 

 that lesions, such as partial or total destruction of the cochlea, 

 produce incomplete or complete deafness, directly prove this 

 view. 



Different theories have been advanced to account for the way 

 in which the vibrations of the plate of the stapes (or vibrations of 

 the bones in general) are capable of affecting the organ of Corti, 



Fin. 87. Diagram of normal organ of Corti. (After Kishi.) The membrana tectoria is extended 

 in contact with the ends of the hair-cells, and applied to the membrana reticularis. 



and exciting the endings of the cochlear nerve by means of the 

 labyrinthine lymph. Helmholtz was inclined in his earlier work 

 to regard the arches of Corti formed by the rods as the cochlear 

 elements designed essentially to vibrate in unison with the 

 tympanic apparatus. Certain structural conditions seem to 

 favour this hypothesis ; the arches of Corti, and particularly the 

 outer rods, are able to vibrate, freely, because they are surrounded 

 by endolymph, both on the posterior surface facing the tunnel of 

 Corti and the superior surface where a large space separates them 

 from the first series of hair-cells. But as the researches of Hasse 

 show that the organ of Corti in birds and amphibians, while 

 undoubtedly capable of transmitting auditory excitations to the 

 centres, is completely destitute of arches, Helmholtz and Hensen 

 hold it necessary to assume that the basilar membrane, on which 

 the organ of Corti is built up, must necessarily take part in the 

 vibrations that give rise to the auditory excitations. To explain 

 these excitations they assume that the vibrations of the basilar 

 membrane cause the hairlets of the hair-cells to impinge on the 



