ORGAN OF HEARING. 117 



sions and grooves, in which nerves from the N. acusticus are 



lodged ; they vary much in size and form, 



but in both respects show a remarkable 



constancy in the same kind of fishes. 



The vestibule is outwards in contact with 



the osseous side wall of the skull, inwards 



with the metencephalon and meduUa ^'^- 51.— otolith of 



11 . •. , • 1 r. Haddock (Gadns 



Oblongata; it contams another firm con- agiefinus). i. Outer, 

 cretion, and opens by five foramina ^^- ii™er aspect, 

 into the three semicircular canals. The terminations of 

 the acustic nerve are distributed over the vestibular con- 

 cretion and the ampulliform ends (Fig. 52 jp) of the semi- 

 circular canals, without being continued into the latter, which 

 are filled with fluid. The semicircular canals (Fig. 52 g), are 

 sometimes lodged in the cranial bones, sometimes partly 

 free in the cranial cavity. Many Teleostei have fontaneUes 

 in the roof of the skull, closed by skin or very thin bone only 

 at the place where the auditory organ approaches the surface, 

 by which means sonorous undulations must be conducted with 

 greater ease to the ear. ^ 



In many Teleostei a most remarkable relation obtains 

 between the organ of hearing and the air-bladder. In the 

 most simple form this connection is established in Percoids 

 and the allied families, in which the two anterior horns of the 

 air-bladder are attached to fontaneUes of the occipital region 

 of the skull, the vestibulum occupying the opposite side of 

 the membrane by which the fontanelle is closed. The con- 

 dition is similar, but more complicated in many Clupeoids. 

 The anterior narrow end of the air-bladder is produced into a 

 canal at the base of the skull, and divided into two very 

 narrow branches, which again bifurcate and terminate in a 

 globular swelling. An appendage of the vestibulum meets 

 the anterior of these swellings, and comes into close contact 

 with it. Besides, the two vestibules communicate with each 



