FISHES. 343 



The membranous labyrinth in the rays is entirely enclosed in a 

 larger osseous labyrinth, hollowed on the sides behind the cranium, 

 in the inner part of which it is supported by vessels and cellular mem- 

 brane, and it adheres by a sort of ligament to a portion of the supe- 

 rior surface of the cranium Avhich is pierced by a small foramen, and 

 closed by a membrane. On this membrane is a small membranaceous 

 excavation covered by the skin ; this is the whole of the communi- 

 cation of the labyrinth with the outside, and it has no communication 

 with the inside of the cranium except by foramina.^which serves for 

 the transmission of the nerves. 



The sturgeon and the moon-fish have only their semicircular 

 canals enveloped in the canals which are traced in the cartilage of the 

 cranium, but the remainder of their labyrinth is in the cranium itself. 

 In these [fishes there is also something Avhich approximates them 

 to the pikes. 



In a very great number of the osseous fishes, the whole of the 

 membranous labyrinth is suspended in the cavity of the cranium 

 which is merely a lateral depression of the large cavity in which the 

 brain is placed. Of the vestiges of the osseous labyrinth some osseous 

 or membranous traces remain, and aroinid them turn the semicir- 

 cular canals; there is also near them a cavity more or less deep, formed 

 in the base of the cranium above the basilar bone, where the sac is 

 merged, of which we shall presently speak. 



We should, however, remark a principal ligament which sus- 

 pends the tv\'o vertical semicircidar canals to the arch of the 

 cranium, near tlie posterior border of the parietal, and which has a 

 close resemblance to that which communicates with the fenestra 

 oralis in the ray. 



The oily or mucilaginous liquor which usually envelopes the 

 brain, penetrates also into the cavities, and surrounds the membra- 

 nous labyrinth. 



The semi-circular membranous canals, three in number each 

 swelled to an ampulla, which receives the filaments of the acoustic 

 nerve, differs from those of the superior classes only in their greater 

 extent. One of them is situated inferiorly, it is nearly horizontal, 

 having a direction towards the side of the brain, and two almost 

 vertical, one anterior, the other posterior. These two are united by 

 one of their extremities, so that all three together terminate by five 

 orifices only in the common cavity which represents the membranous 

 vestibule. 



The form of this cavity varies greatly ; sometimes it represents a 

 long canal, sometimes an oval sac, or triangular pyramid, &c. That 

 which is called the sac is an appendix of the vestibule, from which it 

 is distinguished by a constriction. This constriction is said to be so 

 impervious, that injections will not pass from one cavity to the 

 other; but this is not the case, at least, in the chondropterygians. 

 The membrane which forms the vestibule and the sac, appears 

 imiform, and is much thinner and finer than that of the semi-circular 

 canals. The sac is in general under, and most frequently behind the 

 vertibule; it is lodged in a concavity of the floor of the cranium, and 

 sometimes, this concavity is so closed by an osseous lamina, as only 



