THE MECHANISM OF THE LARYNX
The larynx originally developed as a device to protect the lower respiratory passages against inundation. Protection remains its primary role, although phonation— the production of voice—is the function that most often comes to mind.
Protection of the lower passages against the entrance of food and drink is achieved in two ways. On swallowing, the larynx is drawn forward, and the epiglottis, tilted somewhat backward by coming against the root of the tongue, forms a partial cover to the laryngeal entrance. The resemblance between the outlines of the epiglottis and the aditus suggests a much closer fit than actually occurs. Solid foods are swiftly carried over the laryngeal entrance by the pharyngeal muscles, whereas fluids are deflected by the epiglottis through the piriform recesses of the pharyngeal floor. It is known that removal of the larger part of the human epiglottis does not interfere with normal swallowing. A second, active protection is provided at a deeper level by the glottis, which is closed by the adduction of the vocal folds. Inhibition of inspiration at this time further reduces the risk of food being drawn into the larynx. In fact, food comparatively rarely “goes down the wrong way,” but, when it does, contact with the vestibular mucosa initiates reflex coughing.
On inspiration, abduction of the vocal folds may widen the rima glottidis, but the effect is pronounced only when breathing is unusually vigorous. Abduction is the task of the dorsal cricoarytenoideus, and subsequent adduction is the task of the lateral cricoarytenoideus muscle (Figure 4—15/5,6 and arrows). It should be noticed that these antagonistic muscles are supplied by the same nerve, which is contrary to the common arrangement.
Closure of the glottis also occurs in a number of other functional contexts in which free passage of air to or from the lungs must be prevented.
A build-up of
Figure 4-15 Schematic transverse section of the larynx. Arrows on the left: action of cricoarytenoideus lateralis (6) on arytenoid cartilage; arrows on the right: action of cricoarytenoideus dorsalis (5) on arytenoid cartilage (10). 1, Location of the cricoarytenoid joint; 2, glottic cleft; 3, vocal ligament in vocal fold; 4, thyroarytenoideus; 5, cricoarytenoideus dorsalis; 6, cricoarytenoideus lateralis; 7, arytenoideus transverse; 8, thyroid cartilage; 9, cricoid cartilage; 10, arytenoid cartilage.
expiratory forces against a closed glottis allows for a forceful expulsion when the air is eventually released; this is the mechanism used when coughing to clear the lower passages of mucus accumulations or foreign matter. Sustained closure with elevation of the intratho- racic pressure is also used in activities involving straining: defecation, micturition, and parturition. The blockage of the escape route for air helps maintain the intrathoracic pressure and by so stabilizing the diaphragm aids the action of the muscles of the abdominal wall.
The skeleton of the thorax can also be more effectively fixed to provide a firm base for muscles attaching to the ribs when the glottis is closed. This combination of activities is well illustrated in ourselves when we attempt to lift a heavy weight or to draw the trunk toward a handhold above the head.
The production of voice is a further important function of the larynx. The sounds of human speech are more complex than those produced by other species, although no greater complexity of laryngeal structure is present. Indeed the complex laryngeal mechanism is not indispensable to this task; after the surgical removal of the larynx, an operation sometimes required by malignant disease, the human patient can learn to use the expulsion of air from the esophagus to produce voice, although it may be sadly unnatural.
Even in normal circumstances the voice does not issue from the larynx in its final form but is much modified and “colored” by the resonance chambers provided by other cavities of the head. Some controversy exists over the manner in which the basic sound is produced in the larynx. The airstream is made to vibrate as it passes through the glottis. The pitch is controlled by the thickness, the length, and the tension of the vocal folds and is thus to some extent variable and to some extent determined by permanent (or semipermanent, since a boy’s voice breaks with growth) and individual features of laryngeal anatomy. The tension of the folds, or of part of them, is varied by the cricothyroideus muscle acting as the coarse adjustment and the vocalis muscle as the fine adjustment. Most believe the folds are made to vibrate passively by the flow of air passing between them. An alternative theory suggests that the muscles contract and relax at the appropriate rate; however, as some tones of the human voice exceed 200 cycles per second and tonic contraction of the vocalis muscle occurs with stimuli repeated 67 times per second, this theory is untenable.Electromyographic studies show that purring in cats is produced by fast twitching of the laryngeal muscles and the diaphragm. The laryngeal muscles rapidly narrow and widen the glottis, which causes the respiratory air to vibrate and make the sound.