CHAPTER XLVII.

PHYSIOLOGY OF THE ORGANS OF HEARING.

963. HEARING is that function by which we obtain a knowledge of the vibratory motions of bodies, which constitute sounds. The precise function of all the different parts of the ear is not known.

964. The function of that part of the external ear which projects from the head is to collect sounds and reflect them into the meatus.

965. The membrana tympani serves to facilitate the transmission of sounds, and also to moderate their intensity. It is so arranged that it can be relaxed or tightened.

Observation. This membrane, when healthy, has no opening; and it must be apparent that the apprehension which is often expressed, that insects will penetrate further, is groundless. The pain is owing to the extreme sensibility of the membrana tympani.

966. The supposed office of the tympanum is to transmit the vibrations made on the membrana tympani to the internal This is effected by the air which it contains, and by the chain of small bones that are enclosed in this cavity.

ear.

967. The use of the Eustachian tube is to admit air into the tympanum, which renders the pressure on both sides equal, and thus its membrane is kept in a proper state of tension.

963-971. Give the physiology of the organs of hearing. 963. What is hearing? Are the precise functions of the different parts of the ear known? 964. What is the function of the external ear? 965. Of the membrana tympani? What observation in reference to this membrane ? 966. What is the supposed office of the middle ear? 967. What is the use of the Eu- stachian tube?

Observation. When near a cannon, or a field-piece, about being discharged, by opening the mouth the impression upon the auditory nerve will be diminished, and the unpleasant sensation lessened. This is the result of the air in the middle ear escaping through the Eustuchian tube, when the vibrations of the membrana tympani are violent.

F.g. 148. A view of all the parts of the ear. 1, The tube that leads to the internal ear. 2, The membrana tympani. 3, 4, 5, The bones of the ear. 7, The central part of the labyrinth, (vestibule.) 8, 9, 10, The semicircular canals. 11, 12, The channels of the cochlea. 13, The auditory nerve. 14, The channel from the middle ear to the throat, (Eustachian tube.)

968. But little is known of the functions of the internal ear; its parts are filled with a watery fluid, in which the filaments of the auditory nerve terminate.

969. Many of the parts just enumerated aid in hearing, but are not absolutely essential to this sense But if the vestibule and auditory nerve are diseased or destroyed, no sound is then perceived. If this sense is destroyed in early life, the person also loses the power of articulating words. Hence a man born deaf is always dumb.

970. The transinission of sound through the different parts of the ear will now be explained by aid of fig. 148. The vibrations of air are collected by the external ear, and conducted through the tube (1) to the membrana tympani, (2.) From the membrane vibrations pass along the chain of bones, (3, 4, 5.) The bone 5 communicates with the internal ear, (7, 8, 9, 10, 11, 11, 11, 12, 12, 12.) From the internal ear the impression is transmitted to the brain by the nerve, (13.)

971. The auditory nerve, like the optic, has but one function, that of special sensibility. The nerves which furnish the ear with ordinary sensibility, proceed from the fifth pair.

HYGIENE OF THE ORGANS OF HEARING.

972. Hearing, like the other senses, is capable of very great improvement. By cultivation, the blind are enabled to judge with great accuracy, the distance of bodies in motion, and even the height of buildings. It is also capable of improvement when all the other senses are perfect. Thus the Indian will distinguish sounds that are inaudible to the white

man.

969. What parts of the ear are essential in order to hear sounds? What follows loss of hearing? 971. What is the office of the auditory nerve? What nerves convey ordinary sensibility to the ear? 972978. Give the hygiene of the organs of hearing. 972. Is this sense capable of improvement? How does this sense aid the blind? Is it also capable of improvement when all the other senses are perfect? In whom is this illustrated?

Note. Let the anatomy and physiology of the organs of hearing be reviewed, from fig. 148, or from anatomical outline plate No. 10.

973. Acute hearing requires perfection in the structure and functions of the different parts of the ear, and that portion of the brain from which the auditory nerve proceeds. Deafness is by no means unfrequent. We will now advert to some of the common causes of imperfect hearing.

974. The structure or functional action of the brain may be deranged by inflammation, by compression, or by debility, and produce deafness. The first is seen during inflammatory affections of the brain, and in fevers; the second is seen in accidental injuries of the head; the third is seen in old age, and after severe diseases of the head, and fevers. In these cases, applications to, and operations upon, the ear do no good. The only remedy is to remove, if possible, the diseased condition of the brain.

975. Imperfect hearing may be produced by the destruction of the membrana tympani, or removal of the bones of the ear, or the parts within the labyrinth. In these instances, medical treatment is of no avail, as the destroyed parts cannot be restored.

976. Hearing may be rendered defective by a diminution of the vibratory character of the membrana tympani. This may result from a thickening of this membrane, or from an accumulation of wax upon its outer surface. The increased thickness is usually the result of inflammation, either acute or chronic. The proper treatment is such as is efficient to remove inflammatory action.

Observations. 1st. The introduction of heads of pins into the ear is a frequent cause of chronic inflammation of the membrana tympani. Hence this practice should never be

adopted, and if acquired, should be abandoned.

'973. On what does acute hearing depend? 974. State effects on the hearing in some conditions of the brain. How relieved? 975. Of the effect on hearing when the bones of the ear or the labyrinth are destroyed? Is medical treatment of any avail? 976. What conditions of the drum of the ear may impair hearing? How relieved? What is said of the introduction of pins to cleanse the ear?

2d. The accumulations of viscid wax may be softened by dropping some animal oil into the ear, and then removing it by ejecting warm soap suds a few hours subsequent to the use of the oil. This may be repeated for several successive days.

977. Hearing may be impaired by obstruction of the Eustachian tube. The closure of this canal diminishes the vibratory character of the air within the tympanum, in the same manner as closing the opening in the side of a drum. For the same reason, enlarged tonsils, inflammation and ulceration of the fauces and nasal passages during and subsequent to an attack of scarlet fever, and the inflammation attending the 36 sore throat" in colds, are common causes of this obstruction.

978. The treatment of such cases of defective hearing, is to have the tonsils, if enlarged, removed by a surgeon; for the inflammation and thickening of the parts remedial means should be applied, directed by a skilful physician. The nostrums for the cure of deafness are generally of an oleaginous character, and may be beneficial in cases of defective hearing caused by an accumulation of wax upon the drum of the ear, but in this respect they are no better than the ordinary animal oils.

What is the remedy where there is an accumulation of wax? 977. What is the effect on hearing if the Eustachian tube is obstructed? 978. What is the treatment when deafness is caused by inflammation or ulceration e the fauces? What is said of the nostrums used for deafness?