The Larynx and the Ear : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 23 of 27)

External Ear. - The external ear consists of the part on the outside of the head called the pinna, or auricle, and the tube leading into the middle ear, called the auditory canal. The pinna by its peculiar shape aids to some extent the entrance of sound waves into the auditory canal. It consists chiefly of cartilage. The auditory canal is a little more than an inch in length and one fourth of an inch in diameter, and is closed at its inner end by a thin, but important membrane, called

The Membrane Tympani. - This membrane consists of three thin layers. The outer layer is continuous with the lining of the auditory canal; the inner is a part of the lining of the middle ear; and the middle is a fine layer of connective tissue. Being thin and delicately poised, the membrane tympani is easily made to vibrate by the sound waves that enter the auditory canal. In this way it serves as a receiver of sound waves from the air. It also protects

The Middle Ear. - The middle ear, or tympanum, consists of an irregular cavity in the temporal bone which is lined with mucous membrane and filled with air. It is connected with the pharynx by a slender canal called the Eustachian tube. Extending across the middle ear and connecting with the membrane tympani on one side, and with a membrane closing a small passage to the internal ear on the other, is a tiny bridge formed of three small bones. These bones, named in their order from the membrane tympani, are the malleus, the incus, and the stapes. Where the malleus joins the membrane is a small muscle whose contraction has the effect of tightening the membrane. The Eustachian tube admits air freely to the middle ear, providing in this way for an equality of atmospheric pressure on the two sides of the drum membrane. The bridge of bones and the air in the middle ear receive vibrations from the membrane tympani and communicate them to the membrane of the internal ear.

Purposes of the Middle Ear. - The middle ear serves two important purposes. In the first place, it makes it possible for sound waves to set the membrane tympani in vibration. This membrane could not be made to vibrate by the more delicate of the sound waves if it were stretched over a bone, or over some of the softer tissues, or over a liquid. Its vibration is made possible by the presence of air on both sides, and this condition is supplied, on the inner side, by the middle ear. The Eustachian tube, by providing for an equality of pressure on the two sides of the membrane, also aids in this purpose.

In the second place, the middle ear provides a means for concentrating the force of the sound waves as they pass from the membrane tympani to the internal ear. This concentration is effected in the following manner:

1. The bridge of bones, being pivoted at one point to the walls of the middle ear, forms a lever in which the malleus is the long arm, and the incus and stapes the short arm, their ratio being about that of three to two. This causes the incus to move through a shorter distance, but with greater force than the end of the malleus.

2. The area of the membrane tympani is about twenty times as great as the membrane of the internal ear which is acted upon by the stapes. The force from the larger surface is, therefore, concentrated by the bridge of bones upon the smaller surface. By the combination of these two devices, the waves striking upon the membrane of the internal ear are rendered some thirty times more effective than are the same waves entering the auditory canal.

The Internal Ear, or labyrinth, occupies a series of irregular channels in the petrous process of the temporal bone. It is very complicated in structure, and at the same time is very small. Its greatest length is not more than three fourths of an inch and its greatest diameter not more than one half of an inch. It is filled with a liquid which at one place is called the perilymph, and at another place the endolymph. It is a double organ, being made up of an outer portion which lies next to the bone, and which surrounds an inner portion of the same general form. The outer portion is surrounded by a membrane which serves as periosteum to the bone and, at the same time, holds the liquid belonging to this part, called the perilymph. The inner portion, called the membranous labyrinth, consists essentially of a closed membranous sac, which is filled with the endolymph. The auditory nerve terminates in this portion of the internal ear. Three distinct divisions of the labyrinth have been made out, known as the vestibule, the semicircular canals, and the cochlea.

The Vestibule forms the central portion of the internal ear and is somewhat oval in shape. It is in communication with the middle ear through a small opening in the bone, called the fenestra ovalis, at which place it is separated from the middle ear only by a thin membrane. Sound waves enter the liquids of the internal ear at this point, the foot of the stapes being attached to the membrane. Six other openings lead off from the vestibule at different places. One of these enters the cochlea. The other five open into

The Semicircular Canals. - These canals, three in number, pass through the bone in three different planes. One extends in a horizontal direction and the other two vertically, but each plane is at right angles to the other two. Both ends of each canal connect with the vestibule, though two of them join by a common opening. The inner membranous labyrinth is continuous through each canal, and is held in position by small strips of connective tissue.

The purpose of the semicircular canals is not understood. It is known, however, that they are not used in hearing. On the other hand, there is evidence to the effect that they act as equilibrium sense organs, exciting sensations necessary for balancing the body. Their removal or injury, while having no effect upon the hearing, does interfere with the ability to keep the body in an upright position.

The Cochlea is the part of the internal ear directly concerned in hearing. It consists of a coiled tube which makes two and one half turns around a central axis and bears a close resemblance to a snail shell. It differs in plan from a snail shell, however, in that its interior space is divided into three distinct channels, or canals. These lie side by side and are named, from their relations to other parts, the scala vestibula, the scala tympani, and the scala media. Any vertical section of the cochlea shows all three of these channels.

D.C. Heath and Co. - Publishers
Original copyright 1909