HEARING. 1 59 



Describe the organ of Corti. 



Upon the basilar membrane is arranged a series of rafter-like 

 bodies which roof in a small canal (Fig. 46) : upon this are spread 



FIG. 46. 



Diagramatic Section of the Organ of Corti: 1, membrana basilaris; 2, 3, internal and 

 external fibres of the arch ; 4, epithelium cells near its inner and outer borders ; 

 5, Hair-cells lying in contact with the arch (magnified 500 diameters). 



the functional nerve-endings of the auditory nerve. They are 

 large nucleated cells, the rods of Corti, having hair-like processes 

 which project into the canalis cochlearis or scala media. When 

 looked at from above the cells have an appearance similar to the 

 keyboard of a piano. Fibres of the auditory nerve spread to these 

 cells from the bony lamina spiralis. 



What peculiarity of structure of the auditory nerve is noteworthy ? 

 Its fibres are non-medullated, and it contains numerous ganglion- 

 cells. In the cochlea there are many of these cells, and they form 

 plexuses of nerve-fibres to supply the hair-cells. The absence of 

 neurilemma in the auditory nerve gives it a soft feel which has 

 caused the name " portio mollis" to be given to it when it and the 

 facial are considered as a single nerve. 



What is the physiological action of the organ of Corti ? 



It is probable that each of the functional cells in the organ of 

 Corti responds to a particular shade of sound. How this occurs is 

 not understood, but the vibrations of the tympanum are communi- 

 cated to the stapes by the other ossicles ; and these cells seem to be 

 able to respond each to a particular tone by its sensitiveness in 

 selecting its particular rate of vibration. 



What is the musical range of human hearing ? 



About seven octaves. There are about three thousand hair-cells 

 in the organ of Corti, and it will be easily seen that this would 

 allow an enormous capability to differentiate sounds and musical 

 tones. 



