SENSE OF HEARING. 905 



knowledge of the laws according to which sonorous vibrations are transmitted. 

 Regarding the special function of the Cochlea, there is precisely the same 

 uncertainty. This part of the organ is peculiar in one respect, that the expan- 

 sion of the auditory nerve is here spread out (within the lamina spiralis) in 

 closer proximity with the bone itself, than it is in any other part of the laby- 

 rinth ; and moreover the peri -lymph is here deficient, so that the membranous 

 lining of the cochlea is in absolute contact with its osseous wall. It is not 

 easy to see, however, what can be the peculiar object of this disposition, in 

 regard to the function of hearing. It has been surmised by M. Duges that, by 

 the cochlea, we are especially enabled to estimate the pitch of sounds, particu- 

 larly of the voice ; and he adduces, in support of this idea, the fact that the 

 development of the cochlea follows a very similar proportion with the compass 

 of the voice. This is much the greatest in the Mammalia ; less in Birds; and 

 in Reptiles, which have little true vocal power, the cochlea is reduced to its 

 lowest form, disappearing entirely in the Amphibia. That there should be an 



Fig. 228. 



The Cochlea divided parallel with its axis, through the centre of the Modiolus ; after Breschet : 1, the 

 modiolus ; 2, the infundibulum in which the modiolus terminates ; 3, 3, the cochlear nerve, sending its fila- 

 ments through the centre of the modiolus; 4, 4, the scala tympani of the first turn of the cochlea; 5, 5, the 

 scala vestibula of the first turn ; 6, section of the lamina spiralis, its zonula ossea ; one of the filaments of 

 the cochlear nerve is seen passing hetween the two layers of the lamina spiralis to be distributed upon the 

 membrane which invests the lamina ; 7, the membranous portion of the lamina spiralis ; 8, loops formed by 

 the filaments of the cochlear nerve ; 9, 9, the scala tympani of the second turn of the cochlea; 10, 10, scala 

 vestibula of the second turn; the septum between the two is the lamina spiralis; 11, the scala tympani of 

 the remaining half turn ; 12, the remaining half turn of the scala vestibula the dome placed over this half 

 turn is the cupola ; 13, the lamina of bone which forms the floor of the scala vestibula curving spirally 

 round to constitute the infundibulum (2) ; 14, the helicotrema through which a bristle is passed ; its lower 

 extremity issues from the scala tympani of the middle turn of the cochlea. 



acoustic relation between the voice and ear of each species of animal, cannot be 

 regarded as improbable ; but the speculation of M. Duges can at present only 

 be received as a stimulus to further inquiry. 



906. We have now to consider the functions of the accessory parts the 

 External Ear, and the Meatus. The Cartilage of the external ear may propa- 

 gate sonorous vibrations in two ways ; by reflection and by conduction. In 

 reflection, the concha is the most important part, since it directs the reflected 

 undulations towards the tragus, whence they are thrown into the auditory pas- 

 sage. The other inequalities of the external ear cannot promote hearing by 

 reflection ; and the purpose of the extension of its cartilage is evidently to 

 receive the sonorous vibrations from the air, and to conduct them to its point of 

 attachment. In this point of view, the inequalities become of importance ; 

 for those elevations and depressions upon which the undulations fall perpen- 

 dicularly, will be affected by them in the most intense degree ; and in conse- 

 quence of the varied form and position of these inequalities, sonorous undula- 

 tions, in whatever direction they may come, must fall advantageously upon 



