EAR AS AN ORGAN FOR SOUND SENSATIONS. 367 



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. 

 162). 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 fJ. in 

 the basal portion to 220 ft in the middle spiral and to 234 /j. 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 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 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 

 peculiar quality or timbre of the tone of each instrument is refer- 

 able, therefore, immediately to the number and relative intensities 

 of the simple tone sensations that it arouses. The fusion of these 

 elementary tone sensations into compound ones of different qual- 

 ities is comparable, in a general way, to the fusion of simple color 

 sensations, with this exception, however, that in the compound 

 tone sensations we are capable of distinguishing more clearly the 

 fact that they are composed of simpler elements ; the constituent 

 tones may be recognized by the trained ear at least. The mechan- 

 ism by which the vibrations of the strings of the basilar mem- 



