98 COMPARATIVE ANATOMY. 



VERTEBRATA. Organ divided for convenience into inter- 

 nal, middle, and external ears. 



The internal ear (labyrinth) is the true otic capsule, and 

 corresponds to the ear of Invertebrata. It is constantly di- 

 vided into vestibule (sacculus) and semicircular canals; and 

 in all Vertebrate above cartilaginous fishes is enclosed in an 

 osseous case composed of three bones, named pro-otic, epi- 

 otic, and opisthotic. It may be entirely closed, as in fishes 

 and perennibranchiate batrachians, or present a single orifice 

 anteriorly (fenestra oral is), which may be occupied by end of 

 eolumnella, or stapes. When the cochlea (which is recog- 

 nized as a modified semicircular canal) is added, a second 

 opening (fenestra rotundum) is seen, which is covered by mem- 

 brane. Otolithic contents commonly present. The middle 

 >ear (cavity of tympanum) is a chamber placed in advance of 

 otic capsule, and defined within by its anterior portion, with- 

 out by the tympanic membrane. It is lined with mucous 

 membrane, communicates with pharynx through Eustachian 

 tube, and is traversed by a single bone (Batrachia, Reptilia, 

 .and Aves), or a chain of bones (Mammalia). The external 

 ear is all that portion beyond the middle ear. It is divided 

 anto an external auditory canal (meatus) and an auricle 

 i(3onch). 



Pisces. In Amphioxus and marsipobranchiate fis"hes, ear 

 enclosed within cranial walls; semicircular canals two in 

 .number, not distinct; otolithes absent. In Myxine the cap- 

 usule is a single circular canal. In Raidae, auditory sac, em- 

 bedded in walls of cranium, communicates with the exterior 

 by several openings, and contains a quantity of gritty matter; 

 otolithes less dense than in osseous fishes. In osseous fishes 

 the ear is enclosed in a bony envelope. The semicircular 

 canals are always three in number. The main body of cap- 

 sule (vestibule) contains a large otolithe nearly filling the 

 chamber: no communication seen with either pharynx or 

 integument. With some fishes the labyrinth is extended by 

 a portion which unites with that of the opposite side, and 

 .afterward, on the outside of the occiput, to terminate in a 

 ^chamber on the first vertebra. Here, by means of a small 



