This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. The voice is a sound produced in the larynx, or windpipe, by the passage of air, either to or from the lungs. It is generally caused in the act of respiration, and its seat is the larynx, which forms the very top of the wind pipe, and externally makes that prominence at the fore part of the neck, called Adam's apple. It is connected above with the bone of the throat, by means of small muscles, and behind with the esophagus, or passage to the stomach. | 2. The larynx is composed of four cartilages, closely connected by membranes ; these are .called the thyroid, the cricoid, and the two arytenoid cartilages. The thyroid cartilage is shaped somewhat like a shield, and hence its name. It forms the front and lateral part of the larynx, being broader in front than behind, and made up of two parts which join in the front of the neck at an acute angle, thus making the prominence spoken of above. It has two projections above, and two below; the former connect it with the bone of the tongue, and are called the upper horns; by the latter which are called the lower horns, it is united with the cricoid cartilage, by means of ligaments.
3. The cricoid cartilage is so called from its resemblance to a ring. It lies immediately below the thyroid cartilage, being broad at its sides, narrow in front, and connected with the thyroid cartilage behind by a still broader surface. In front it is not connected with the thyroid cartilage; but the space is occupied by the lining membrane of the larynx, covered with the common integuments.
4. The arytenoid cartilages are much smaller than the others, and are situated at the back part of the larynx, in connection with the cricoid cartilage. They have small muscles attached to them, by which they are moved sideways, and it is by these motions that the opening in the larynx, called the glottis, is enlarged or contracted. Two fibrous ligaments connect these cartilages with the thyroid ; these two are called vocal chords, as they are supposed to be particularly concerned in the production of voice. They pass from the artenoid cartilages to the angle formed by the two side pieces of the thyroid, and are about half an inch in length. The opening between them is called glottis, rima glottidis, or chink of the glottis.
5. The epiglottis is the little moveable cartilage ; which lies over the top of the wind pipe, at the root of the tongue, and has been compared to a trap door. It prevents the food from passing into the wind pipe when we swallow. From its elasticity, its position is usually perpendicular, except in the very act of swallowing, when the tongue is carried backwards, so as to bring it exactly over the opening ; this prevents the passage of foreign bodies into the lungs.
6. The larynx is abundantly supplied with nerves which are given off from the eighth pair. The thyroid gland is a body consisting of two lobes, lying one on each side, and somewhat below the larynx ; the use of which is unknown; it is the seat of the disease called goitre, or swelled neck, so common in Switzerland, and some other countries.
7. Such is the description of the organs immediately concerned in the voice ; the whole vocal apparatus, however, comprises the muscles concerned in breathing, the mouth, and nasal cavities, as well as the parts above mentioned. It is essential to the production of voice that the air should pass from the lungs through the larynx ; for if an opening be made in the wind pipe, so that the air escapes, the voice is lost. I lately witnessed this in a man who undertook to destroy his life by cutting his throat. He succeeded in cutting the wind pipe completely off, but as no large blood vessels were divided, he got well. This man could not utter a syllable, not even in a whisper, until the wind pipe healed up ; his voice Was then restored to its former condition.
8. Volition is necessary to the production of voice. If the nerves going to the larynx are cut, the voice will be lost. Palsy of the muscles of the larynx, also causes dumbness ; and fear effects the voice by paralyzing muscular effort. The epiglottis is found not to be concerned in the production of the voice, as it may be removed without affecting it; its sole office being to guard the wind pipe against the introduction of foreign substances.
9. It has been proved by experiments, that nearly all the larynx except the chorda vocales, or vocal chords, may be removed without destroying the voice. These are the small ligaments that pass from the arytenoid cartilages to the thyroid cartilages, and are usually called the inferior ligaments, or the lower vocal chords.
10. It is now ascertained that these chords perform the principal part in the production of the voice. In what way this is done, is not precisely known; but, we may suppose that the air in passing from the lungs in expiration, is forced out of small cavities, as the air cells and minute branches of the wind pipe, into the main channel; it it is thence sent through a narrow passage, on each side of which is a vibratory chord ; the vibrations of which, by the action of the air produces voice or sound.
11. It has been said that if this theory of the voice be correct, there ought to be sound produced by forcing air through the wind pipe of a dead animal. But this takes for granted what is not true, namely, that the voice is the result of a purely mechanical process ; instead of which, it is a vital function, performed by living agents, and therefore under the control of voluntary action.
12. In producing sound, there can be no doubt that numerous voluntary muscles are put in action, bringing the arytenoid cartilages in contact with each other, thus putting upon the stretch the lower vocal chords which are attached to them. It is therefore by no means strange that vocal sound cannot be made by forcing air through the larynx of a dead animal.
13. It has often been disputed whether the larynx was a wind or stringed instrument. Most physiologists at the present time, regard it as a wind instrument of the reed kind, such as the clarionet, hautboy, etc, and they differ chiefly in explaining the various modifications of the tone and quality of the voice. There is, however, great reason to believe that it partakes of the character of both.