l8o PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



from a bony ledge on the pillar to the outer wall of the 

 stairway, and dividing it into two semicylindrical spiral 

 hollows, one above and one below, but not quite reach- 

 ing the extreme end ; so that vibration might run spirally 

 around in the upper way, over at the top, and down 

 spirally by the lower way. These two ways are called 

 the one scala vestibuli, because it opens into the cavity of 

 the bony vestibule, and the other the scala tympani, be- 

 cause it abuts against the tympanum, being separated 

 from it only by the membrane of the foramen rotundum. 

 The whole bony cavity of the cochlea is of course filled 

 with perilymph. 



We have spoken as if there were but one membrane 

 separating the scala vestibuli from the scala tympani, 

 but really there are two, and these are separated by a 

 little space which is filled with endolymph. This space 

 is called the scala media and connects through the saccu- 



lus with the vestibular 

 sac. Now in the scala 

 media are found a great 

 number of stiffish rods 

 running from the cen- 

 tral pillar to the outer 

 wall, like stair rods, and 

 of diminishing length to 

 the very end. These are 

 the rods of Corti. There 

 are several thousands 

 of them. A branch of the auditory nerve runs up the 

 central pillar and sends its fibers into the scala media, 

 and the rods of Corti are supposed to be the percipient 

 terminals of these fibers, as are the rods and cones of 

 the retina terminals of the fibers of the optic nerve. 

 Here again, therefore, we have a most delicate arrange- 

 ment for perceiving the slightest vibratory movement 



Fig. 114. — Section through an ampulla, 

 showing a branch of the auditory 

 nerve, «, and the sensitive hairs, a a. 



