Hearing, and the Ear. 467 



The Lamprey ( Petromyzon ), a near, but somewhat advanced relative 

 of the Hag, has two semicircular canals connecting with the vestibular 

 sac, which no doubt is the remains of the original simple sac. In the 

 Rays, cartilaginous fishes, there are two canals quite circular and con- 

 nected by ducts with the original sac or vestibule. This seems to be an 

 unusual arrangement. The shark, a related animal, has the two canals 

 in the usual form. ( Huxley. ) 



In the Teleosteans, or ordinary bony fish, there are always three semi- 

 circular canals, offshoots from the vestibular sac. The sac contains a 

 prominent crista acustica, and also otoliths, usually two in number, large 

 and solid. Names have been given to these stones, the larger front one 

 being called the sagitta, and that in the rear, asteriscus. This fish ear 

 is within the cranium. 



FIG. 207. Ear of Salmon, 

 s. Membranous Sac 011 the inside of which the Acoustic 



Nerve is spread out. 

 o. Otolith also contained in Sac. 

 c c c. Semicircular Canals, 

 a a a. Ampullae. 



None of the fishes nor any of the animals be- 

 low them in the scale of being, possess any tym- 

 panum, or tympanic membrane, or auditory canal 

 FIG. 207. ( meat us ), or ear flap, or tympanic bones. The 



membrane which covers their ear cavity bears the same relation to the 

 inner ear that our fenestra ovalis does to ours. But this membrane is 

 only a continuation of the outside skin of the fish and comes in direct 

 contact with the water. In the course of the further development of the 

 ear in the animals above the fishes, this sac, now called the labyrinth, 

 together with the cavity containing it and the membrane covering the 

 cavity, sinks deeper into the head ; bones are built around it and another 

 membrane built across the mouth of the additional cavity so formed. 

 This second cavity becomes the tympanum or drum, and a sounding 

 post of cartilage or bone extends through it from the tympanic mem- 

 brane to the membrane of the inner cavity, which membrane is now 

 called the fenestra ovalis. The sounding post which is in one rod-like 

 piece is called the columella. Where the tympanum is thus enclosed on 

 the outside, there is a eustachian tube leading from it to the throat. 

 Such an ear as this, with the tympanum level with the head, is possessed 

 by most of the Amphibians. The eustachian tube sometimes is single 

 and median at its entrance into the throat, and further back it forks, 

 sending a prong to each ear drum. In some of the frog tribes, the high- 

 est of the Amphibia, there is also a fenestra rotunda, though it is not 

 settled that they possess a cochlea. The ears of some toads are like 

 those of the fishes, destitute of tympanum and columella. The ear of 

 the Salamander ( a batrachian ) is concealed by the flesh and is destitute 

 of a tympanum and columella, but has a little cartilaginous plate over 

 the fenestra ovalis. 



