THE MEMBRANOUS VESTIBULE AND THE CANALS 477 



(6) Not all the hair or sensory cells are affected by external 

 vibrations. The region inclosed by the rods, and the membrane 

 upon which they rest, does not respond as a whole to the vibra- 

 tions of the perilymph, that is to say, when a particular vibration 

 is set up in the perilymph only certain regions of the rod in- 

 closed portion vibrate with it. As a result, only certain ones of 

 the sensory cells are stimulated, and it is supposed that ability of 

 the brain to distinguish different somids depends upon the fibers 

 which are thus stimulated. 



(c) The character of the vibrations produced by a vibrating 

 body, and hence set up in the perilymph may vary in three ways, 

 in pitch or tone, in loudness, and in quality. These variations 

 are distinguishable by the brain through variations in the impulses 

 which the sensory cells transmit to it. 



(d) The final interpretation of sound impulses has been located 

 in certain convolutions of the temporal lobe of the cerebrum, 

 which is connected by association fibers with the medullary origin 

 of the auditory nerves. The exact area is much more difficult to 

 localize than the sight area. 



(e) The two organs of hearing produce in the brain not two 

 sensations but one. Their position on each side of the head aids, 

 however, in locating the source of vibrations. 



(J) The sensory cells become fatigued by continuous stimula- 

 tion in much the same manner as do the retinal cells of the eye. 



Functions of the membranous vestibule and the canals. — 



Divisions of the auditory nerve supply the utriculus, 

 sacculus, and canals with fibers. The endolymph of these 

 parts is also continuous with that of the cochlear part, and 

 they are surrounded by the perilymph of the bony laby- 

 rinth. Further, the fibers of these divisions of the auditory 

 nerve end in the ampulla, utriculus, and sacculus in 

 sensory hair cells similar to those of the cochlea. In spite, 

 however, of these similarities of equipment, these hair 

 cells do not appear to transmit ordinary sound impulses. 

 The present theory of the action of these parts may be 



