ORGAN'S OF THE SENSES. 281 



concretions, and even without distinct semi-circular canals, 

 these canals being only represented by three perceptible folds 

 of the vestibular membrane. These two vestibular cavities 

 lodged in the lower and lateral parts of the skull are still 

 separated from the cranial cavity only by the dura mater, 

 and are pierced only by the acoustic nerves and blood vessels 

 which spread on the vestibular membranes, between the 

 peri- and ento-lymph. But in the higher osseous fishes, we 

 find in addition to a highly developed vestibule containing solid 

 calcareous bodies and even sometimes perforated externally by 

 a fenestra ovalis, distinct, large, free, semi-circular canals with 

 considerable ampullae at their terminations, as in other ver- 

 tebrated classes. These parts, however, of the acoustic 

 apparatus are not yet imbedded in or surrounded by the 

 solid bones of the head, but are simply lodged in a de- 

 pression on each side within the general cavity of the skull. 

 There is generally no external fenestra or vestibular opening 

 in osseous fishes, and the sonorous impulses communicated 

 to the body of the animal through the dense medium of the 

 water, are commonly conveyed to the air-sac of the trunk, 

 which is often bifurcated or even ramified, and from that 

 forwards to the base of the skull or into its interior near the 

 ear, and sometimes distinct small costal bones detached 

 from the transverse processes of the anterior vertebrae assist 

 in communicating these vibrations from the air-sac to the 

 organ of hearing. These bones, however, are ordinary ele- 

 ments of the vertebrae, and the air-sac is the rudiment of 

 the lungs and not analogous to the tympanic cavity of air- 

 breathing animals. The great size and length of the semi- 

 circular canals within the cranial cavity of osseous fishes, 

 where they are not yet enclosed within the substance of the 

 temporal bone, corresponds with the great development exter- 

 nally of the tympanic ossicula composing the opercular bones 

 which are likewise unrestrained by any tympanic membrane 

 or osseous walls in their extension outwards. Although in the 

 osseous fishes the whole labyrinth lies in an open groove on 

 each side, entirely within the general cavity of the skull, 

 separated from the brain only by the dura mater, and con- 

 nected only by ligaments with the temporal bone, we find in 

 the sturgeon, an operculated cartilaginous fish, that the 

 labyrinth is already partly buried in the substance of the 



