PISCES. 



1003 



turgidly filled with a transparent fluid ; the 

 membranous vestibule, however, does not ad- 

 here to the walls of the capsule except at the 

 orifices leading into the cranium. The mem- 

 branous vestibule has its cavity divided into 

 several compartments by folds projecting into 

 its interior, and receives the auditory nerve, 

 which being changed into a pulpy mass spreads 

 out over its walls. 



In the Petromyzonidse therefore three 

 important parts of the auditory appa- 

 ratus, which are met with in the ear 

 of all other Fishes, are wanting, viz. 

 the sac of the otolithe, the utolitlie 

 itself, and the semicircular canals, ex- 

 cept indeed rudiments of the latter may 

 be represented by two curved folds of 

 the membrane of the vestibule, which 

 are joined superiorly to a similar fold, 

 an arrangement which is met with both 

 in the river and sea-lamprey. The 

 auditory nerve is derived immediately 

 from the brain. 



From the above description it would 

 appear that in the Lampreys there are two modes 

 whereby sonorous vibrations may be commu- 

 nicated to the vestibule, one through the car- 

 tilaginous capsule of the ear, the other through 

 the cranium, which communicating tremors 

 impressed upon it from without to the fluid 

 which is contained in its cavity, the vibration 

 reaches the tense membrane that closes the 

 large fenestra leading to the vestibule, and thus 

 affects the membranous vestibular sac itself. 



In a second group Weber includes those forms 

 of the ear which have no cartilaginous or osse- 

 ous vestibule separate from the cranial cavity. 

 This kind of ear exists in by far the greater 

 number of Fishes, being met with in all the 

 truly osseous and branchiostegous races as 

 well as in some Chondropterygians ; in none 

 of which is the membranous labyrinth en- 

 closed in a bony or cartilaginous envelope, the 

 internal ear being contained in the cavity of 

 the skull itself near the posterior part of the 

 cerebrum, with which, in fact, it is for the most 

 part in apposition ; for in these Fishes the 

 cranium being very large and having only a 

 small part of its cavity occupied by the brain 

 itself, performs the office of an osseous laby- 

 rinth, not only by furnishing a receptacle to 

 the internal ear in which every part necessary 

 to the performance of its functions may be fitly 

 suspended, but is filled with fluid with which 

 the membranous labyrinth is every where sur- 

 rounded, a provision not less necessary to the 

 sense of hearing than is the fluid contained in 

 the interior of the vestibule and semicircular 

 canals. In all such Fishes, therefore, the 

 auditory apparatus, consisting of a membra- 

 nous vestibule and semicircular canals, is lodged 

 on each side in cavities excavated in the base 

 of the cranium and bounded by the temporal 

 and lateral parts of the occipital bones. 



The internal ear itself (fig- 528) is composed 

 of the following parts: 1st. The membranous 

 vestibule (jig. 525, i). 2d. The sac of the 

 otolithe. 3d. The membranous semicircular 

 canals. 



The membranous vestibule is an elongated 

 smooth sacculus of very various form in diffe- 

 rent Fishes. Its parietes consist of a pellucid 

 membrane, and its outer surface is connected 

 by loose cellular tissue to the sides of the 

 cavity in which it is lodged. Its anterior 

 extremity is somewhat dilated and contains 

 a little otolithe; moreover into it open the 

 ampullae of the anterior and external semi- 



Fig. 533. 



Internal ear of Perch. (After Cuvier.) 



circular canals. The posterior extremity of the 

 vestibule is narrower, and into this part opens 

 the ampulla of the posterior semicircular canal 

 and the hinder termination of the external one. 

 Near the middle of the vestibular sac enters the 

 wide duct formed by the conjunction of the 

 terminations of the anterior and posterior semi- 

 circular canals; but whether this wide duct 

 ought rather to be looked upon as forming part 

 of the vestibule or of the semicircular canals 

 may be a matter of doubt, although the latter 

 supposition is the most probable. 



Thus the six extremities of the three semi- 

 circular canals communicate with the cavity of 

 the membranous vestibule, not by six, but by 

 five orifices. 



The membrane of which the vestibule con- 

 sists is considerably thinner than that which 

 forms the semicircular canals ; indeed it is so 

 delicate that if torn it at once collapses and is 

 scarcely distinguishable from the surrounding 

 parts. 



In the Pike (Esox Indus) there is a re- 

 markable appendage to the vestibule which is 

 not met with in other Fishes. This consists of 

 a pyriform membranous sacculus lodged in the 

 commencement of the spinal canal, which opens 

 into the vestibular cavity by a narrow orifice 

 near the entrance of the posterior semicircular 

 canal. The thickness of the walls of this 

 sacculus is much greater than that of the pa- 

 rietes of the vestibule, resembling rather in 

 this respect the ampullae of the semicircular 

 canals. Some of the upper spinal nerves are 

 distributed to this organ, but they give off no 

 branches, nor does it appear to receive any 

 filament from the auditory nerve. 



The sac of the otolithe in most Fishes is 

 immediately beneath and in close contact with 

 the membranous vestibule, but in some it is 

 hidden in the base of the occipital bone more 

 remote from the vestibular cavity, with which it 

 is joined by a narrower duct. The saccus is most 

 generally divided into two portions by a median 

 septum, in such a way, however, that the ante- 



