44 BRAIN AND BODY OF FISH 



plate of bone projecting downwards from the vertebral column to 

 which the anterior sac or air vesicle is not only attached by its outer 

 wall, but also by a very important muscle rising from an ossicle, 

 the first of the chain of small bones leading from the anterior sac 

 to the internal ear. These are known as the Weberian ossicles. 

 (•Plate 7.) 



In the carps the posterior air-sac, which acts as a buoyancy tank, 

 has a tube connecting it with the gullet, and this duct, known as the 

 pneumatic, allows swallowed air to be not only introduced into the 

 bladder by means of the pneumatic bulb or pump, but also to be 

 discharged when necessary in accord with the hydrostatic require- 

 ments. Between the two sacs there is a communicating duct which 

 is kept closed by a sphincter muscle, controlled by a nervous gang- 

 lion ; this enables the gaseous pressure in the anterior vesicle to be 

 kept at a level most suitable for the reception of vibrations ; the 

 function of the anterior sac is that of a drum, which acts as a hydro- 

 phone. It may be said that this drum combines the functions of 

 a vibrating membrane and that of a middle ear, while the " ductus 

 communicans " has the same physiological use as the Eustachian 

 tube in the human ear. Vibrations received by the body wall of 

 the fish are communicated to the anterior sac directly and not by 

 any external ear, which does not exist in fish. There is another 

 important fact to be noted, namely, that the orifice of the pneumatic 

 duct as it enters the gullet is surrounded by a ring of taste-buds, 

 which act as sentinels protecting the orifice, and only allow bubbles 

 of swallowed air to pass into the posterior sac. 



If the two diagrams (Plate 6 and 7) of the coimections of the 

 lung in man, and the swim-bladder in fish, with the auditory 

 organ are studied, a remarkable similarity in general arrangement 

 can be traced. In the human ear there is an external ear which 

 leads to a drum which forms an outer wall to the middle ear. To 

 this drum is attached a chain of ossicles or small bones which trans- 

 mit vibrations to the internal ear, and the gaseous pressure in the 

 middle ear is controlled by means of a tube, the Eustachian tube, 

 that leads into the upper part of the pharynx which may be looked 

 upon as an extension of the air-passages upwards. 



In the carp, an external ear being non-existent, the anterior sac 

 or air-vesicle receives the sound vibrations directly through the 

 body walls and from its anterior end there arises a chain of ossicles 

 that transmit these vibrations to the internal ear. Weber, who first 

 described these ossicles, named them after the small bones of the 

 human ear, malleus, stapes and incus ; but those comparative 

 anatomists, who followed him, did not consider that these bones 



