HEARING. 611 
corresponding vibrations of the perilymph, which again is sent 
into oscillation by the movements of the stapes against the 
membrane covering the fenestra ovalis; so that the vibrations 
thus set up without the membranous labyrinth are trans- 
formed into similar ones within the vestibule and the scala 
vestibuli, and end, after passing over the scala tympani, against 
the membrane of the fenestra rotunda. The cochlear canal 
may be regarded as the seat of the most important part of the 
organ of hearing, and answers to the macula lutea of the eye 
in many respects. 
Fig. 451.—I. Transverse section of a turn of cochlea, II. Ampulla of a semicircular canal 
and its crista acoustica ; oP: auditory cells, one of which is a hair-cell. ILI. Diagram of 
labyrinth of man. IV. Of bird. V. Of fish: (After Landois.) 
The organ of Corti has given rise to certain speculations 
which require a brief notice. It has been supposed that, as the 
key-board of a piano may be said to cause certain tones by 
being associated with stretched wires of varying lengths, so 
the vibrations of the rods of Corti originate in certain nerve- 
fibers the sensations answering to the different tones we hear. 
It was found, however, (1) that these rods, though very nu- 
merous (6,000 to 10,000), are insufficient to account for the 
actual range of our hearing; (2) that they are absent in certain 
classes of animals that discriminate sounds very well, as birds; 
and (3) that the nerve-fibers do not terminate in these rods at 
all, but in the hair-cells of the organ of Corti. It is now pro- 
posed that the basilar membrane (present in birds) may, like a 
series of tense strings of different lengths, be the required 
organ. The failure of certain theories of vision should have 
