i88 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



(whales, sirenians and some seals are exceptions) an external 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 strength- 

 ened by cartilage and is moved by muscles (fig. 142). There is evi- 

 dence which points to the conch being homologous with either the 

 operculum of fishes or with the first external gill of amphibians. 



FIG. 188. Diagram of mammalian ear. a, ampullae of semicircular canals ; an, acustic 

 nerve; en, cochlear nerve; em, external auditory meatus; eu, Eustachian tube;//, fenestra 

 tympani; i, incus; m, malleus; p, perilymph space (black) ; pd, perilymph duct; ph, pharynx; 

 s, stapes; sc, sacculus; sm, st, sv, scalae media, tympani et vestibuli; sg, spiral ganglion; t, 

 tympanic cavity; tm, tympanic membrane; u, utriculus; v, vestibular nerve. 



Functions. The vertebrate ear is primarily an organ of equi- 

 libration 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 otoliths, causing them 

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

 acusticae in the ampullae 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 components of any motion. The 

 maculae, and especially that of the lagena, are probably concerned in the 

 recognition of sound. In the fishes the lagena is poorly developed, 



