Function of the Tensor Tympani Muscle.-The function of this muscle is, as its name indicates, to increase the tension of the membrane in accordance with the pitch of the sound wave. The tendon of this muscle playing over the processus cochleariformis and attached at almost a right angle to the handle of the malleus will, when the muscle contracts, pull the handle inward, increase the convexity of the membrane, and at the same time increase its tension; with the relaxation of this muscle, the handle of the malleus passes outward and the tension is diminished. The contractions of the tensor muscle are reflex in character and excited by nerve impulses reaching it through the small petrosal nerve and otic ganglion. The number of nerve stimuli passing to the muscle and determining the degree of contraction will depend upon the pitch of the sound wave and the subsequent excitation of the auditory nerve. The tensor tympani muscle may be regarded as an accommodative apparatus by which the tympanic membrane is adjusted to enable it to receive vibrations of varying degrees of pitch.

Function of the Ossicles.-The function of the chain of bones is to transmit the sound waves across the tympanic cavity to the internal ear. The first of these bones, the malleus, being attached to the tympanic membrane, will take up the vibrations much more readily than if no membrane intervened. Owing to the character of the articulations, when the handle of the malleus is drawn inward, the position of the bones is so changed that they form practically a solid rod, and are therefore much better adapted for the transmission of molecular vibrations than if the articulations remained loose. As the stapes bone is somewhat shorter than the malleus, its vibrations are smaller than those of the tympanic membrane, and by this arrangement the amplitude of the vibrations is diminished, but their force increased.

The Function of the Stapedius Muscle is, according to Henle, to fix the stapes bone so as to prevent too great a movement from being communicated to it from the incus and transmitted to the perilymph. It may be looked upon, therefore, as a protective muscle.

The Function of the Eustachian Tube is to maintain a free communication between the cavity of the middle ear and naso-pharynx. The pressure of air within and without the ear is thus equalized, and the vibrations of the tympanic membrane permitted to attain their maximum, one of the conditions essential for the reception of sound waves. The impairment in the acuteness of hearing which is caused by an unequal pressure of the air in the middle ear can be shown: 1. By closing the mouth and nose

and forcing air from the lungs through the Eustachian tube into the ear, producing an increase in pressure. 2. By closing the nose and mouth, and making efforts at deglutition, which withdraws the air from the ear and diminishes its pressure. In both instances the free vibrations of the tympanic membrane are interfered with. The pharyngeal orifice of the Eustachian tube is opened by the action of certain of the muscles of deglutition, viz.: the levator palati, tensor palati, and the palato-pharyngei muscles.

The Internal Ear, or Labyrinth, is located in the petrous portion of the temporal bone, and consists of an osseous and membranous portion.

The Osseous Labyrinth is divisible into three parts, viz.: the vestibule, the semicircular canals, and the cochlea.

The vestibule is a small triangular cavity, which communicates with the middle ear by the foramen ovale; in the natural condition it is closed by the base of the stapes bone. The filaments of the auditory nerve enter the vestibule through small foramina in the inner wall, at the fovea hemispherica.

The semicircular canals are three in number, the superior vertical, the inferior vertical, and the horizontal, each of which opens into the cavity of the vestibule by two openings, with the exception of the two vertical, which at one extremity open by a common orifice.

The cochlea forms the anterior part of the internal ear. It is a gradually tapering canal, about one and a half inches in length, which winds spirally around a central axis, the modiolus, two and a half times. The interior of the cochlea is partly divided into two passages by a thin plate of bone, the lamina osseous spiralis, which projects from the central axis two thirds across the canal. These passages are termed the scala vestibuli and the scala tympani, from their communication with the vestibule and tympanum. The scala tympani communicates with the middle ear through the foramen rotundum, which, in the natural condition, is closed by the second membrana tympani; superiorly they are united by an opening, the helicotrema.

The whole anterior of the labyrinth, the vestibule, the semicircular canals, and the scala of the cochlea, contains a clear, limpid fluid, the perilymph, secreted by the periosteum lining the osseous walls.

The Membranous Labyrinth corresponds to the osseous labyrinth with respect to form, though somewhat smaller in size.

The vestibular portion consists of two small sacs, the utricle and saccule. The semicircular canals communicate with the utricle in the same manner as the bony canals communicate with the vestibule. The saccule communicates with the membranous cochlea by the canalis reuniens. In

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