282 ORGANS OF THE SENSES. 



temporal bone, which envelopes the three semi-circular canals 

 and leaves the vestibule still free within the cavity of the 

 cranium. In the higher forms of cartilaginous fishes, how- 

 ever, in the rays and sharks, the whole labyrinth is at length 

 found to be completely surrounded by the firm elastic car- 

 tilaginous substance of the skull, excepting at the internal 

 entrances of the vessels and nerves and at the fenestra ovalis 

 which now communicates with an external meatus. The 

 external meatus is already seen in some of the osseous fishes, 

 as in the large ears of the lepidoleprus ; in the sharks it is 

 more distinct though closed externally by the skin, and in the 

 rays it is even double on each side as if to form a rudiment 

 of the Eustachian tube, the spiracula of these animals 

 having no relation to the organs of hearing. This imbedded 

 condition of the organ in the cartilaginous fishes better 

 enables them to perceive all the sonorous vibrations com- 

 municated to the soft substance of their skull, and we find 

 the whole internal ear preserve this position, so favourable 

 for receiving vibration, even in the densest forms of the 

 skull, from the cartilaginous fishes up to man. The cre- 

 taceous substances, generally three in number, found within 

 the vestibular succulum and its communicating smaller 

 sacculi, are soft and pulpy in the cartilaginous fishes and 

 of a stony density in those which have an osseous skeleton. 

 They vary in shape according to the species ; they are 

 normal parts of the auditory organ in most animals from 

 the cephalopods to man; they are composed of minute 

 rhomboidal crystals of carbonate of lime and are desti- 

 tute of internal organization like the excreted shells of 

 molluscous animals. The succuli of the vestibule in these 

 lowest forms of the ear appear to form the first rudiment 

 of a cochlea, and a rudiment of the tympanum is seen in 

 the subcutaneous passage destitute of air, leading from 

 the fenestra ovalis to the surface of the head in the highest 

 cartilaginous fishes. The acoustic nerve comes off sepa- 

 rately in fishes from the inferior part of their lobed medulla 

 oblongata beneath the cerebellum, and sends branches to be 

 distributed on the ampullae of the semi-circular canals, on 

 the vestibule, and on the sacculus or the yet undivided and 

 unconvoluted rudiment of the cochlea, so that its branches 

 are most affected by vibrations in these two last cavities by 



