188 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. 
Fic. 188.—Diagram of mammalian ear. a, ampulla of semicircular canals; an, acustic 
nerve; cn, cochlear nerve; em, external auditory meatus; ev, Eustachian tube; ft, fenestra 
tympani; 7, incus; m, malleus; p, perilymph space (black); pd, perilymph duct; ph, pharynx; 
5, Stapes; sc, sacculus; sm, st, sv, scala media, tympani et vestibuli; sg, spiral ganglion; ¢, 
tympanic cavity; ¢m, tympanic membrane; x, 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 criste 
acustice in the ampullz 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 
macule, and especially that of the lagena, are probably concerned in the 
recognition of sound. In the fishes the lagena is poorly developed, 
