EAR AS AN ORGAN FOR SOUND SENSATIONS. 399 



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 



Fertesfra ^__^ _^ 



\helicotrefna. 



FenesTra rofancla,~m. Jcalci 



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

 branes of the oval and round fenestras and the course of the wave movement 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. These fibers in the portion covered 

 by the rods of Corti, the inner zone or zona tecta, are finer and 

 more difficult to separate than in the portion exterior to the outer 

 rods, the outer zone or zona pectinata. From the base to the apex 

 of the cochlea the membrane increases in width, the length of the 

 strings in the outer zone varying, according to Retzius, from 135 p. in 

 the basal portion to 220 // in the middle spiral and to 234 fJ. at the 

 apex. The whole structure is estimated to contain about 24,000 

 strings 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 compound wave motion into 

 its constituent simple waves, each string being set into strong- 

 est vibrations by the wave of the corresponding period. More- 

 over, it is implied or assumed hi this theory that the vibrations of 

 each string are communicated to a corresponding nerve fiber of 

 the cochlear nerve, through which the stimulus is conveyed to 

 the brain as a nerve impulse. We should be capable of perceiv- 

 ing, theoretically, as many distinct musical tones as there are 

 fibers in the basilar membrane, while a compound wave, by setting 

 a number of these mechanisms into action, gives a series 

 of sensations which are more or less fused in consciousness. The 



