BOOK II. DISEASES OF THE EAR. CHAPTER XXX. ANATOMY AND PHYSIOLOGY OF THE EAR. 271. General Anatomy and Development.-The auditory organ is divided into the interior, middle, and external ear. The interior ear consists of the terminations of the auditory nerve in the interior of chambers hollowed out in the depth of the petrous bone. This, the apparatus for the perception of sound, is the first part of the ear developed in the embryo and the only part found in the lower vertebrates, especially the fishes. It develops at the side of the hind-brain vesicle in the form of a pit in the epiblast, which, as it sinks in deeper, changes into a vesicle detached from the surface. The auditory nerve, growing out from the brain, enters this vesicle. The latter elongates and divides into two portions-the upper becoming the utriculus, the lower, the sacculus. From the upper extrude hollow ridges which change into the semicircular canals. The lower cavity sends forth a tubular extension, the cochlea. This tube coils in a spiral manner in higher animals, reaching 2 turns in man. The semicircular canals, of which it is doubtful whether they serve for hearing, exist in all vertebrates, although the lowest fishes have but two. The cochlea, the organ of sound perception, is developed progressively in the animal scale beyond fishes. Semicircular canals and cochlea constitute the labyrinth contained in tunneled-out spaces in the petrous bone. |