TI1E INTERNAL EAR. or LARTRINT11. This con sists of three parts: the vestibule. the cochlea, and the semicircular canals. The vestibule is a small central cat ity communicating by openings ill it- posterior stall x‘itli the semicircular vaunts: anteriorly, the cochlea it by a single opening, the beginning of the seata resti bull described below, ttit its miter wall is the hiicstra oratis, and on its inner there are a num ber of minute orifices for the entrance of the filaments of the auditory nerve. The cochlea (Lat.. snail slic11) is situated in front of the vestibule. it is a spiral eanal wound around a bony axis, termed the modiolus. which is per forated at its base for the entrance of the filaments of the cochlear portion of the auditory nerve. The interior of the cochlea is subdivided by an os-co-membranons partition. the lamina spi 1.alis, into two passages: the sea/a open ing into the vestibule, and the stabs tympani, the communication of which with the tympanic eav ity—the fcncstra rotunda—is covered by a mem brane a- described above: the two seahe com municate at the apex of the eoehlea by an open ing called the hclicotnma. iletween the two scabs there is a third space, termed the scab, intermedia, which contains the terminations of the filaments of the auditory nerve in a com plicated arrangement of peculiarly formed neuro epithelial cells termed collectively the organ of (See Aunrrour NEuvE.) The semicircu lar canals are three in number. and open at both ends into the vestibule. They vary in length, and, notwithstanding their name, each is con siderably more than a semicircle, the superior vertical canal being the longest. Their aver
ape diameter is about a twentieth of an inch, the extremities of each canal exhibiting a slight dilatation called an ampulla. Each canal lies in a different plane, very nearly at right angles to the planes of the other two, hence their names of the superior vertical, the inferior vertical, and the horizontal canals. We now turn to the membranous labyrinth. The membranous and osseous labyrinths have the same shape, but the former is considerably smaller than the latter, a fluid, termed the perilymph, intervening in some quantity them. At certain points the membranous is firmly adherent to the osseous labyrinth. The vestibular portion consists of two sacs. an upper and larger ono, of an oval shape. tel'IllPfl the utrieulus, and a lower and smaller one of a more globular shape, called the saeenbas.
The membranous semicirenlar canals resemble in form and arrangement the osseous 1/1(.111.e them, but are only one-third of the diameter of the latter. The membranous laby rinth is tilled by a fluid which is termed the endolyniph; and in certain spots, especially at the terminations of the vestibular nerves, there are, suspended in the somewhat gelatinous endo lymph, crystals of calcium carbonate known as otoliths, i.e. ear-stones. which are supposed to intensif? the impression of the sound-waves upon the Ilene endings.
We now proceed to consider the different functions of the various parts of the ear.