54 BRAIN AND BODY OF FISH 



tympanic membrane, as found in the liigher vertebrates ; this 

 allows the incompressible fluid in the cavity to vibrate. It must be 

 understood that the membranous wall of the saccule lies in the 

 central cavity of the bony labyrinth of the ear, known as the vesti- 

 bule ; in the herring an appendix of the vestibule meets the 

 anterior spherical air vesicle, which we shall shortly describe, and 

 comes in close contact with it. The long silvery looking tube that 

 we see in the upper part of the abdomen of a fresh herring is the 

 swim-bladder. Hillier gives a very concise description of this 

 organ as follows : 



"It is a simple long sac, opening posteriorly by a fine canal 

 through a dense sphincter muscle, and anteriorly opening through 





Fig. v.— a dissection of the lateral wall of the Herring (enlarged) 

 from within to show the accessory auditory apparatus. 



The front wall of the anterior spherical air-vesicle has been chipped away so that 

 india-ink could be injected into the posterior air-vesicle through the canal 

 leading into the pear-shaped vesicle. The ink has also passed into the canal 

 leading into the swim-bladder. Below the pear-shaped vesicle is the outer 

 wall of the saccule closed by the auditory fenestra. 



a cartilaginous arch, into the fine duct that leads into the interior 

 of the labyrinth. A canal from the stomach, the pneumatic duct, 

 enters the swim-bladder about its middle. So that the swim- 

 bladder is unique among fishes in having three openings into its 

 cavity. It is curious to note that the communication from the 

 stomach is actually in line with the pharynx, and that the passage 

 to the lab3T:'inth is through a ring of cartilage ; for in this respect 

 it suggests the similar condition of things in mammals, with the 

 Eustachian tube running from the naso -pharynx to the cavity of 

 the middle ear." 



The accessory portion of the auditory organ commences by the 

 fine duct given off from the anterior end of the swim-bladder ; 

 this almost at once divides into two finer ducts, which pass forwards 



