316 A Manual of Veterinary Physiology. 



bridge, with a membranous and rigid apparatus containing 

 fluid. The cavities are the external and middle ear ; the 

 apparatus containing fluid is the internal ear ; the partition 

 between the external and middle chamber is the tympanum ; 

 and the flexible bridge connecting one air chamber with the 

 internal ear is composed of the bones of the ear or auditory 

 ossicles. 



The vibrations of sound are collected by the external ear, 

 which from its funnel shape is eminently calculated to con- 

 centrate them. From here they are directed on to the 

 tympanum ; this being set in vibration, the handle of the 

 malleus, then the incus, and lastly the stapes, are also 

 moved ; and the latter being attached to a membrane in 

 contact with fluid, the vibrations of the stapes are trans- 

 ferred to the fluid or perilymph, by which means a special 

 impression is made on the nerve endings in the cochlea 

 and so conveyed to the brain. 



It is therefore in the internal ear where the proper organ 

 of hearing is contained, and to which the eighth pair of 

 nerves is distributed ; and the arrangement is so peculiar 

 that we must momentarily glance at it. 



The internal ear consists of a cavity termed ' the laby- 

 rinth,' composed of the vestibule, cochlea, and semicircular 

 canals. All these, with the exception of two small surfaces, 

 are entirely covered with bone, yet within it we find a 

 membranous labyrinth containing a fluid termed ' the endo- 

 lymph,' and in the space between the membranous and 

 osseous labyrinth is a second fluid termed ' the perilymph." 

 Within the coils of the cochlea are spread out on a special 

 membrane some remarkable cells or organs, termed ' the 

 organs of Corti '; their general arrangement resembles the 

 keyboard of a pianoforte, and in connection with them, 

 through the medium of the auditory epithelium in which 

 they terminate, are fibres of the special nerve of hearing. 



The eighth pair of cranial nerves supplies the special sense 

 of hearing. The nerve is remarkably soft, and its fibres 

 very fine. It gives off branches to the vestibule, cochlea, 

 and semicircular canals. 



