THE COCHLEA. 605 



to the text books of anatomy, as space only admits of a brief 

 account of the special arrangements of the nerve ending being 

 given. 



The nervous mechanisms which are most important for the 

 appreciation of tones are those situated in the cochlea. 



The endings of the nerves which are found in the membranous 

 sacks in the vestibule are connected with peculiar epitheloid cells, 

 to which are attached fine, bristle-like processes. These processes 

 lie in the endolymph, and are related to calcareous masses called 

 otoliths. Waves in this endolymph possibly bring the otoliths 

 into collision with the hairs, and thus give a stimulus to the 

 nerve endings. Thus, noises may be heard, but no fine impres- 

 sions of tones can be explained. The exact use of the nerves 

 going to the other parts of the labyrinth, namely, the ampullae 

 of the semicircular canals, is somewhat doubtful, and possibly not 

 immediately connected with hearing.* The coils of the snail- 

 shell-like cochlea are, throughout their entire length, even in the 

 dried state, partially divided into two by a kind of shelf project- 

 ing from its central axis into the spiral cavity. This is called 

 the osseous spiral lamina. In the fresh state the separation of the 

 spiral canal into an upper (vestibular) and a lower (tympanic) 

 coil is completed by a membranous partition, which stretches 

 from the bony spiral lamina to the opposed side of the spiral 

 canal. This is called the membranous spiral lamina, and forms 

 the base upon which the special nerve endings of the organ of 

 hearing are spread out. An extremely delicate membrane called 

 the membrane of Reissner stretches from the upper side of the 

 spiral partition obliquely upward to the outer wall of the spiral 

 cavity, so as to cover the special organ, and shut off that part of 

 the vestibular coil which lies over the membranous spiral lamina. 

 This canal of the cochlea is triangular in section, and is separate 

 from the rest of the spiral cavity. Its floor is made up chiefly 

 of the membranous spiral lamina, particularly the part called 

 the basilar membrane, while the oblique roof is composed of only 



* Compare equilibration, in connection with which they will be described} 

 p. 638. 



