PR OB ABLE A CTION OF THE CO CHLEA. 1 1 8 1 



fenestra rotunda would be adapted for the higher, while those near the 

 vertex of the cochlea would be suitable for deep tones. Corti's arches 

 are therefore of secondary importance, serving either as supporting 

 structures, or for transmitting vibrations of parts of the basilar mem- 

 brane to the rows of hair cells placed on their backs. When we take 

 into account the fact that the filaments of the cochlear nerve do not 

 terminate in the rods, nor in the fibres of the basilar membrane, nor 

 even in the hair cells, but, as shown by Eetzius, 1 in fine filaments lying 

 against the hair cells, we must consider how vibrations of the fibres of 

 the basilar membrane could affect these filaments. 



The short hairs of the hair cells project through apertures in the 

 membrana reticularis, as if it were necessary to cause any oscillation 

 communicated to the hair cells to be always in the same direction. 

 Eesting upon the upper surface of the membrana reticularis is the 

 membrana tectoria ; in sections the latter appears often to be separated 

 from the former by a considerable interval ; but this is not always the 

 case, and in some sections the two may be seen to be almost in apposi- 

 tion. The question whether the two are in actual contact at any 

 time has not been settled by histologists. Suppose that, in accordance 

 with the view of Helmholtz, a segment of the basilar membrane were 

 thrown into sympathetic vibration, it would move in a direction at right 

 angles to the direction of its fibres. These movements would be com- 

 municated to the structures lying on its upper surface ; and if we sup- 

 pose the arches of Corti to be elastic, such movements would be trans- 

 mitted to the hair cells. These would move in the line of their long 

 axis, — in other words, their hairs would move up and down in the 

 meshes of the membrana reticularis, and strike against the under surface 

 of the membrana tectoria. A reaction would take place from the latter, 

 and thus the delicate nerve-endings between the hair cells would receive 

 pressures corresponding in frequency to the oscillations of the membrana 

 basilaris. In the cochlea of birds and amphibia, the mechanism is prac- 

 tically the same, but in consequence of the membrana basilaris not being 

 highly differentiated, there cannot be the nice appreciation of pitch of 

 tone which exists in the higher animals. There is every reason to 

 suppose that a bird has a power of discriminating pitch only through a 

 narrow range. The number of notes birds use in song is very limited, 

 and even those which can be taught to whistle tunes, like piping bull- 

 finches, can only use a very few tones. Parrots, mocking-birds, and 

 minas no doubt can imitate many tones, and it would be interesting to 

 examine the structure of the cochlea in these birds, an investigation not 

 yet carried out. 



The theory of the action of the cochlea above described is supported 

 by the results of certain experiments and by clinical observations ; but 

 the parts involved are so minute as to make it difficult to be accurate 

 as to the portion actually destroyed or modified. All such results, 

 therefore, must be received with great caution. The experiments of 

 Baginsky 2 on dogs show that, when the upper parts of the cochlea are 

 destroyed, deafness for low tones results, while destruction of the basal 

 portions causes deafness for high tones. Stepanofi', 3 however, could not 



1 Biol. Untcrsuch., Stockholm, YA. v. S. 35, Tafel xvii. fig. 1. 



2 Sitzungsb. d. k. Akad. d. Wisscnsch., Wien, 1863, S. 685 ; Vvrchow's Archir, Bd. xciv. 

 S. 65 ; Centralbl. /'. Physiol., Leipzig u. Wien, 1888, S. 299. 



3 Monatschr. f. Ohrenh., Berlin, 1888, S. 85. 



