Part 62 (1/2)

What is the supposed office of the middle ear? 967. What is the use of the Eustachian 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 Eustachian tube, when the vibrations of the membrana tympani are violent.

[Ill.u.s.tration: Fig. 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 ca.n.a.ls. 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.

What observation in this connection? 968. What is the function of the internal ear?

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 transmission 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 pa.s.s 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? 972-978.

_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 ill.u.s.trated?

_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 acc.u.mulation 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.