THE INTERNAL EAR OR LABYRINTH 777 



cells are several tall columnar cells which rapidly decrease in height 

 until they have attained the simple character of the general lining 

 of this tubule. Practically the entire surface of the organ of Corti 

 is covered by a thick fibrillated membrane, the tectorial membrane, 

 which takes its origin upon the upper surface of the limbus and sweeps 

 almost transversely through the lumen of this canal. 



The Function of the Organ of Corti. These different rows of cells 

 are continued spirally into the tip of the scala media. It has been 

 estimated that there are more than 2500 inner and 13,000 outer hair 

 cells. Their total number is generally given as at least 16,000. We 

 have every reason to believe that these hair cells are the elements which 

 receive the sound waves, this assumption being based principally 

 upon their general appearance and position. In the second place, it 

 is noted that the cochlear branch of the auditory nerve ascends through 

 the modiolus and directs its fibers radially through the spiral lamina 

 into the organ of Corti. Near the base of the lamina these fibers tra- 

 verse a ganglion, known as the ganglion spirale. The cells of this 

 structure are bipolar, their peripheral branches being continued onward 

 into the basilar membrane where they lose their medullary sheath 

 and enter the epithelium in the region of the inner hair cells. Some 

 of these fibers terminate here, while others continue onward and cross 

 the tunnel of Corti to enter the region of the outer hair cells. In this 

 region they terminate as fine filaments which invest the lower poles of 

 the corresponding cells of Deiters. 



The fact that the rods of Corti are not present in birds, which 

 doubtlessly possess a very keen sense of hearing, shows that these 

 elements are not essential to hearing. The same conclusion may be 

 drawn from the fact that their number is altogether too small to be 

 able to receive the large number of sound waves to which we may be 

 subjected. Retzius, for example, estimates their total number at less 

 than 10,000, of which 5600 are inner rod cells. This exclusion of the 

 rods as direct factors in the reception of the sound waves, leaves us free 

 to localize this function in the hair cells. In accordance with Helm- 

 holtz, it may then be held that the latter play the part of sympathetic 

 resonators which are capable of reducing musical sounds into their 

 components. 



The Activation of the Organ of Corti. In accordance with a sug- 

 gestion of Hensen, it has been advocated by Helmholtz that the constitu- 

 ents of the organ of Corti are activated from below by the sympa- 

 thetic vibrations of the radial fibers imbedded in the basilar membrane. 

 It is believed in this case that the vibrations of the perilymph in the 

 scala tympani are transmitted to these fibers and that the latter in 

 turn stimulate the hair cells above them. This contention harmonizes 

 with the fact that the basilar membrane contains about 24,000 of 

 these fibers, and that their length gradually increases from the base to 

 the tip of the cochlea (135/x to 234/i). Thus, the fibers in the base 

 of the cochlea would be adapted to high notes, and those near the heli- 



