CH. LIY.] 



THE ORGAN OF CORTI 



745 



above, the organ of Corti shows a remarkable resemblance to the 

 keyboard of a piano. The top of the organ is roofed by the 

 membrana tectoria (fig. 547, t) that extends from the end of the 

 limbus (Us, fig. 547), a connective tissue structure on the spiral lamina. 

 The spiral ganglion from which the cochlear nerve-fibres originate is 

 situated in the spiral lamina* The peripheral axons of its bipolar 

 cells arborise around the hair-cells of the organ of Corti : the central 

 axons pass down the modiolus, and thence to the pons (see p. 642). 



Physiology of Hearing. 



Sounds are caused by vibrations ; when a piano-string is struck, 

 it is thrown into a series of rapid regular vibrations; the more 



Fio. 548. Vertical section of the organ of Corti from the dog. 1 to 2, Homogeneous layer of the 

 membrana basilarls ; u, vestibular layer ; v, tympanal layer, with nuclei and protoplasm ; a, pro- 

 longation of tympanal periosteum of lamina spiralis ossea ; c, thickened commencement of the 

 membrana basilaris near the point of perforation of the nerves h ; d, blood-vessel (vas spirale) ; e, 

 blood-vessel ; /, nerves ; g, the epithelium of the sulcus spiralis interims; i, internal hair-cell, with 

 basal process k, surrounded with nuclei and protoplasm (of the granular layer), into which the 

 nerve-fibres radiate ; I, hairs of the internal hair-cell ; n. base or foot of inner pillar of organ of Corti ; 

 m, head of the same uniting with the corresponding part of an external pillar, whose under half is 

 missing, while the next pillar beyond, o, presents both middle portion and base; r, s, d, three 

 external hair-cells ; t, bases of two neighbouring hair or tufted cells ; x, supporting cell of Deiters ; 

 w, nerve-fibre arborising round the first of the external hair-cells ; I I to I, lamina reticularis. 

 x 800. (Waldeyer.) 



rapidly the vibrations occur the higher is the pitch of the musical 

 note; the greater the amplitude of the vibration, the louder or 

 more intense is the tone; if the vibrations are regular and simple 

 (pendular), the tone is pure ; if they are regular but compound, the 

 tone is impure, and its quality or timbre is dependent on the rate 

 and amplitude of the simple vibrations of which the compound 

 vibrations are composed. The vibrations are transmitted as waves, 

 and ultimately affect the hair-cells at the extremities of the 

 auditory nerve in the cochlea. The semicircular canals are not 

 concerned in the sense of hearing ; their function in connection with 

 equilibration is described in Chapter XLIX. The external and 



