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the interior of the utricle and saccule, at the entrance of the auditory nerve; are small masses of carbonate of lime crystals, constituting the otoliths. Their function is unknown.

The membranous cochlea is a closed tube, commencing by a blind extremity at the first turn of the cochlea, and terminating at its apex by a blind extremity also. It is situated between the edge of the osseous lamina spiralis and the outer wall of the bony cochlea, and follows it in its turns around the modiolus.

A transverse section of the cochlea shows that it is divided into two portions by the osseous lamina and the basilar membrane: 1. The scala vestibuli, bounded by the periosteum and membrane of Reissner. 2. The scala tympani, occupying the inferior portion, and bounded above by the septum, composed of the osseous lamina and the membrana basilaris.

The true membranous canal is situated between the membrane of Reissner and the basilar membrane. It is triangular in shape, but is partly divided into a triangular portion and a quadrilateral portion by the tectorial membrane.

The organ of Corti is situated in the quadrilateral portion of the canal, and consists of pillars of rods, of the consistence of cartilage. They are arranged in two rows—the one internal, the other external;. these rods rest upon the basilar membrane; their bases are separated from each other, but their upper extremities are united, forming an arcade. In the internal row it is estimated there are about 3500, and in the external row about 5200 of these rods.

On the inner side of the internal row is a single layer of elongated hair cells; on the outer surface of the external row are three such layers of hair cells. Nothing definite is known as to their function.

The endolymph occupies the interior of the utricle, saccule, membranous canals, and bathes the strictures in the interior of the membranous cochlea throughout its entire extent.

The Auditory Nerve at the bottom of the internal auditory meatus divides into (1) a vestibular branch, which is distributed to the utricle and semicircular canals; (2) a cochlear branch, which passes into the central axis at its base, and ascends to its apex; as it ascends, fibers are given off, which pass between the plates of the osseous lamina, to be ultimately connected with the organ of Corti.

The function of the semicircular canals appears to be to assist in maintaining the equilibrium of the body; destruction of the vertical canal is followed by an oscillation of the head upward and downward ; destruction of the horizontal canal is followed by oscillations from left to right. When the canals are injured on both sides, the animal loses the power of maintaining equilibrium upon making muscular movements.

Function of the cochlea. It is regarded as possessing the power of appreciating the quality of pitch and the shades of different musical tones. The elements of the organ of Corti are analogous, in some respects, to a musical instrument, and are supposed, by Helmholtz, to be tuned so as to vibrate in unison with the different tones conveyed to the internal ear.

Summary.—The waves of sound are gathered together by the pinna and external auditory meatus, and conveyed to the membrana tympani. This membrane, made tense or lax by the action of the tensor tympani and laxator tympani muscles, is enabled to receive sound waves of either a high or low pitch. The vibrations are conducted across the middle ear by a chain of bones to the foramen ovale, and by the column of air of the tympanum to the foramen rotundum, which is closed by the second membrana tympani, the pressure of the air in the tympanum being regulated by the Eustachian tube.

The internal ear finally receives the vibrations, which excite vibrations successively in the perilymph, the walls of the membranous labyrinth, the endolymph, and, lastly, the terminal filaments of the auditory nerve, by which they are conveyed to the brain.

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VOICE AND SPEECH. The Larynx is the organ of voice. Speech is a modification of voice, and is produced by the teeth and the muscles of the lips and tongue, coordinated in their action by stimuli derived from the cerebrum.

The Structures entering into the formation of the larynx are mainly the thyroid, cricoid, and arytenoid cartilages; they are so situated and united by means of ligaments and muscles as to form a firm cartilaginous box. The larynx is covered externally by fibrous tissue, and lined internally with mucous membrane.

The Vocal Cords are four ligamentous bands, running antero-posteriorly across the upper portion of the larynx, and are divided into the two superior or false vocal cords, and the two inferior or true vocal cords ; they are attached anteriorly to the receding angle of the thyroid cartilages and posteriorly to the anterior part of the base of the arytenoid cartilages. The space between the true vocal cords is the rima glottidis.

The Muscles which have a direct action upon the movements of the

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