604 MANUAL OF PHYSIOLOGY. 



perilymph the waves pass up the fluid in the vestibular spiral 

 of the cochlea, and arriving at its summit, they descend by the 

 tympanic spiral to the round opening. In this course they pass 

 at first over and then under the fluid contained in the membran- 

 ous canal of the cochlea endolymph in which the special nerve 

 terminations of the cochlea are placed. 



For the construction of the labyrinth the student is referred 

 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 ap- 

 preciation of tones are those situated in the cochlea. 



The endings of the nerves which are found in the membranous 

 sacs 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 ampulla 

 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, partiajly 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 



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

 p. 639. 



