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.

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

vocal cords are nine in number, and take their names from their points of origin and insertion, viz.: the two crico-thyroid, two thyro-arytenoid, two posterior crico-arytenoid, two lateral crico-arytenoid, and one arytenoid muscles.

The crico-thyroid muscles, by their contraction, render the vocal cords more tense by drawing down the anterior portion of the thyroid cartilage and approximating it to the cricoid, and at the same time tilting the posterior portion of the cricoid and arytenoid cartilages backward.

The thyro-arytenoid, by their contraction, relax the vocal cords by drawing the arytenoid cartilage forward and the thyroid backward.

The posterior crico-arytenoid muscles, by their contraction, rotate the arytenoid cartilages outward and thus separate the vocal cords and enlarge the aperture of the glottis. They principally aid the respiratory movements during inspiration.

The lateral crico-arytenoid muscles are antagonistic to the former, and by their contraction rotate the arytenoid cartilages so as to approximate the vocal cords and constrict the glottis.

The arytenoid muscle assists in the closure of the aperture of the glottis. The inferior laryngeal nerve animates all the muscles of the larynx, with the exception of the crico-thyroid.

Movements of the Vocal Cords.-During respiration the movements of the vocal cords differ from those occurring during the production of voice.

At each inspiration, the true vocal cords are widely separated, and the aperture of the glottis is enlarged by the action of the crico-arytenoid muscles, which rotate outward the anterior angle of the base of the arytenoid cartilages; at each expiration the larynx becomes passive; the elasticity of the vocal cords returns them to their original position, and the air is forced out by the elasticity of the lungs and the walls of the thorax.

Phonation. As soon as phonation is about to be accomplished a marked change in the glottis is noticed with the aid of the laryngoscope. The true vocal cords suddenly become approximated and are made parallel, giving to the glottis the appearance of a narrow slit, the edges of which are capable of vibrating accurately and rapidly; at the same time their tension is much increased.

With the vocal cords thus prepared, the expiratory muscles force the column of air into the lungs and trachea through the glottis, throwing the edges of the cords into vibration.

The pitch of sounds depends upon the extent to which the vocal cords are made tense and the length of the aperture through which the air passes.

In the production of sounds of a high pitch the tension of the vocal cords becomes very marked, and the glottis diminished in length. When grave sounds having a low pitch are omitted from the larynx, the vocal cords are less tense and their vibrations are large and loose.

The quality of voice depends upon the length, size, and thickness of the cords, and the size, form, and construction of the trachea, larynx, and the resonant cavities of the pharynx, nose, and mouth.

The compass of the voice comprehends from two to three octaves. The range is different in the two sexes, the lowest note of the male being about one octave lower than the lowest note of the female; while the highest note of the male is an octave less than the highest note of the female.

The varieties of voices, e. g., bass, baritone, tenor, contralto, mezzosoprano, and soprano, are due to the length of the vocal cords, being longer when the voice has a low pitch, and shorter when it has a high pitch. Speech is the faculty of expressing ideas by means of combinations of sounds, in obedience to the dictates of the cerebrum.

Articulate sounds may be divided into vowels and consonants. The vowel sounds, a, e, i, o, u, are produced in the larynx by the vocal cords. The consonantal sounds are produced in the air passages above the larynx by an interruption of the current of air by the lips, tongue, and teeth; the consonants may be divided into: (1) mutes, b, d, k, p, t, c, g; (2) dentals, d, j, s, t, z; (3) nasals, m, n, ng; (4) labials, b, p, f, v, m; (5) gutturals, k, g, c, and g hard; (6) liquids, l, m, n, r.