The intensity, or loudness, of the voice is governed by the force with which the air is expelled from the lungs. The vibrations of the cords, however, are greatly reënforced by the peculiar structure of the upper air passages, as stated above.
Production of Speech.—The sounds that form our speech or language are produced by modifying the vibrations from the vocal cords. This is accomplished by "mouthing" the sounds from the larynx. The distinct sounds, or words, are usually complex in nature, being made up of two or more elementary sounds. These are classed either as vowels or consonants and are represented by the different letters of the alphabet. The vowel sounds are made with the mouth open and are more nearly the pure vibrations of the vocal cords. The consonants are modifications of the vocal cord vibrations produced by the tongue, teeth, lips, and throat.
Words and their Significance.—In the development of language certain ideas have become associated with certain sounds so that the hearing of these sounds suggests the ideas. Our words, therefore, consist of so many sound signals, each capable of arousing a definite idea in the mind. To talk is to express ideas through these signals, and to listen is to assume an attitude of mind such that the signals may be interpreted. In learning a language, both the sounds of the words and their associated ideas are[pg 358] mastered, this being necessary to their practical use in exchanging ideas. From spoken language man has advanced to written language, so that the sight of the written or printed word also arouses in the mind the associated idea.
THE EAR
The Ear is the sense organ which enables sound waves to so act upon afferent neurons as to excite impulses in them. The effect upon the mind which these impulses produce is known as the sensation of hearing. In the performance of its function the ear receives and transmits sound waves and also concentrates them upon a suitable exposure of nerve cells. It includes three parts—the external ear, the middle ear, and the internal ear.
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 (Fig. 151). The pinna by its peculiar shape aids to some extent the entrance of sound waves into the auditory canal.[119] 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 Membrana 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 membrana tympani is easily made to vibrate by the sound[pg 359] waves that enter the auditory canal. In this way it serves as a receiver of sound waves from the air. It also protects
Fig. 151—Diagram of section through the ear, showing relations of its various parts. (See text.)
The Middle Ear.—The middle ear, or tympanum,[120] 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 membrana 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 membrana tympani, are the malleus, the incus, and the stapes (Fig. 151). Where the malleus joins the membrane is a small muscle whose contraction has the effect of tightening[pg 360] 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 membrana tympani and communicate them to the membrane of the internal ear.