AUDITORY ORGANS 203 



derm, on the inner surface with entoderm, represents the imperforate 

 wall of the cleft, while the Eustachian tube is the narrowed internal 

 end of the spiracle. The chain of ear bones has already been de- 

 scribed. It is to be noted that the chain consists of columella and 

 stapes in anura and sauropsida, while in the mammals columella is re- 

 placed by incus and malleus. In the urodeles and gymnophiones, 

 where no tympanic cavity is .developed, the quadrate articulates 

 with the stapes. 



The External Ear. — In the anura and in many reptiles the tym- 

 panic membrane is flush with the surface of the head, but in other 

 reptiles and in birds it is at the bottom of a canal, the external audi- 

 tory meatus, the simplest expression of an external ear. In the 

 mammals (whales, sirenians and some seals are exceptions) an exter- 

 nal conch is developed behind the meatus to collect the sound waves 

 and to direct them to the inner parts. In some birds the feathers 

 are arranged around the meatus so as to play the .same part. The 

 conch is strengthened by cartilage and is moved by muscles (fig. 

 149). There is evidence which points to the conch being homologous 

 with either the operculum of fishes or with the first external gill of 

 amphibians. 



Functions. — The vertebrate ear is primarily an organ of equili- 

 bration by which the animal recognizes all changes of position. 

 Though the purposes of the various parts are not accurately known, 

 the following conclusions seem warranted. Every movement of the 

 head affects the endolymph and the contained otoUths, causing them 

 to move (by gravity or by momentum, or by both) over the cristas 

 acusticae in the ampullae and the macula utricuU and thus to stimulate 

 the sense cells and nerves. The position of the semicircular canals in 

 approximately the three dimensions of space would seem to afford a 

 means for the recognition of the directions and amounts of the com- 

 ponents of any motion. The maculae, and especially that of the la- 

 gena, are probably concerned in the recognition of sound. In the 

 fishes the lagena is poorly developed, and while some fishes have been 

 proved to hear, others have given negative results. With the terres- 

 trial vertebrates the sound percipient functions of the ear are beyond a 

 doubt, while they still retain their equilibrational use. The sound 

 waves strike the tympanic membrane, are carried across the middle 

 ear by the auditory ossicles, and set the perilymph in motion and thus 

 affect the parts of the membranous labyrinth. 



