Section 116. the Eighth Nerve Runs From the Brain Case (cr.), Into the

periotic bone, and is distributed to the several portions of this labyrinth. In an ordinary fish this internal ear is the sole auditory organ we should find; the sound-waves would travel through the water to the elastic cranium and so reach and affect the nerves. But in all air-frequenting animals this original plan of an ear has to be added to, to fit it to the much fainter sound vibrations of the compressible and far less elastic air. A "receiving apparatus" is needed, and is supplied by the ear-drum, middle ear, or tympanic cavity (T.). In the mammal there is also a collecting ear trumpet (the ear commonly so-called), the external ear, and external auditory meatus (e.a.m.). A tightly stretched membrane, the tympanic membrane, separates this from the drum. A chain of small bones, the malleus (m.), the incus (i.), the os orbiculare (o.or.), a very small bone, and a stirrup-shaped stapes, swing across the tympanum, from the tympanic membrane to the internal ear. At two points the bony investment of this last is incomplete-- at the fenestra rotunda (f.r.), and at the fenestra ovalis, (f.o.), into which latter the end of the stapes fits, and so communicates the sound vibrations of the tympanic membrane to the endolymph. A passage, the Eustachian tube, communicates between the tympanic cavity and the pharynx (Ph.), and serves to equalize the pressure on either side of the drum-head. A comparative study of the ears of the vertebrata brings to light the fact that, as we descend in the animal scale, the four ear ossicles are replaced by large bones and cartilages connected with the jaw, and the drum and Eustachian tube by a gill slit. We have, in fact, in the ear, as the student will perceive in the sequel, an essentially aquatic auditory organ, added to and patched up to fit the new needs of a life out of water.