Section 115. the Ear (sheet Vii.) Consists of an Essential Organ of

hearing, and of certain superadded parts. The essential part is called the internal ear, and is represented in all the true vertebrata (i.e., excluding the lancelet and its allies). In the lower forms it is a hollow membranous structure, embedded in a mass of cartilage, the otic capsule; in the mammal the latter is entirely ossified, to form the periotic bone. The internal ear consists of a central sac, from which three semicircular canals spring. The planes of the three canals are mutually at right angles; two are vertical, the anterior and posterior (p.v.c.) vertical canals, and one is horizontal, the horizontal canal (h.c.). There are dilatations, called ampullae, at the anterior base of the anterior, and at the posterior base of the posterior and horizontal canals. Indirectly connected with the main sac is a spirally-twisted portion, resembling a snail shell in form, the cochlea. This last part is distinctive of the mammalia, but the rest of the internal ear is represented in all vertebrata, with one or two exceptions. The whole of the labyrinth is membranous, and contains a fluid, the endolymph; between the membranous wall of the labyrinth and the enclosing bone is a space containing the perilymph. Strange as it may appear at first, the entire lining of the internal ear is, at an early stage, continuous with the general epidermis of the animal. It grows in just as a gland might grow in, and is finally cut off from the exterior; but a considerable relic of this former communication remains as a thin, vertical blind tube (not shown in the figure), the ductus endolymphaticus.