396 THE SPECIAL SENSES. 



certain structures within the membranous cochlea are set into sym- 

 pathetic vibrations corresponding to the simple waves of which the 

 compound wave is constituted. Helmholtz first suggested that 

 the peculiar rods of Corti form the resonating apparatus, and 

 by sympathetic vibrations are capable of analyzing the compound 



Fenestra ovcd/'s-^^,^ „ , v? 7- 



Scalcc Ves/idaii \ i. ,■ ■, 



nelteoi re/no. 



fenestra roTunda-^ Scala tynipani 



Fig. 177. — Schematic figure from Auerbach to show the relative positions of the mem- 

 branes of the oval and round fenestras and the connection through the cochlea from one to the 

 other. 



movement. Later, however, this suggestion was abandoned, 

 since the number of the rods is not sufficiently great perhaps to 

 answer the requirements of this theory. According to Retzius, 

 the inner rods number 5600 and the outer ones 3850. Moreover, 

 these structures are absent from the bird's cochlea, and we must 

 assume that these animals are capable of appreciating musical 

 sounds. Helmholtz then adopted a suggestion of Hensen's, that 

 the basilar membrane constitutes the resonating apparatus. This 

 membrane forms the floor of the membranous cochlea, stretching 

 from the limbus to the opposite side of the bony cochlea (Fig. 

 172). Its middle layer consists of fibers, running radially, which, 

 though united to one another, are sufficiently independent to be 

 regarded as separate strings. In the portion covered by the rods 

 of Corti, the arcuate zone or zona tecta, the membrane is not dis- 

 tinctly fibrillated, but in the portion exterior to the outer rods, the 

 outer zone or zona pectinata, the membrane is composed of fibers 

 stretching from the foot-plate of the outer rods to the external 

 spiral ligament. This portion of the basilar membrane is supposed 

 in Helmholtz's theory to constitute the resonating apparatus. 

 From the base to the apex of the cochlea the membrane increases 

 in width, the length of the strings in the outer zone varying, ac- 

 cording to Retzius, from 135 /z in the basal portion to 220 \x in the 

 middle spiral and to 234 y. at the apex. According to Keith the 

 range is from 120 to 300 ix. The whole structure is estimated to 

 contain about 24,000 fibers varying gradually in length, as stated, 

 and resembling in general arrangement the strings of the piano. 

 Assuming that each of these fibers has its own period of vibration, 

 we may imagine that the entire collection forms an apparatus for 

 sympathetic vibration which is capable of analyzing each com- 

 pound wave motion into its constituent simple waves, each string 

 being set into strongest vibrations by the wave of the corresponding 

 period. Moreover, it is imphed or assumed in this theory that the 

 vibrations of each string are communicated to a corresponding 

 nerve fiber of the cochlear nerve, through which the stimulus is 



