THE ORGAN OF VOICE.
The Voice-box or larynx forms the upper part of the windpipe. It is constructed of vari ous curiously-shaped pieces of cartilage (gristle), connected together by bands of ligament, and is clothed outside by muscles, and inside by a mucous membrane continuous with that of the rest of the air-passages. Fig. 154 shows the larynx and wind pipe stripped of muscles. The larynx (L) is seen to be formed of two pieces of cartilage (7'h and Cr), one placed above the other. The upper of the two is the thyroid cartilage, and the lower the cricoid cartilage.
The thyroid car tilage (Greek, thy reos, a shield) is formed of two ex tended wings meet ing at the middle line in front in a ridge; above and from the sides two horns project up wards (a, a), which are connected by bands to the hyoid bone (a), from which the larynx is suspended. The hyoid bone itself is attached by muscle and ligament to the skull. It lies at the root of the tongue, and the finger can feel it at the angle of junction of the chin and neck. From the under surface of the thyroid two horns (b, b) project down wards to become jointed to the cricoid. The thyroid cartilage thus rests and is movable on the cricoid, movable forwards or backwards, but not from side to side.
The cricoid cartilage is shaped like a sig net-ring (Greek, krikos, a ring), the narrow part of the ring being in front. Owing to this narrowness a small space (o) is left in front between the two cartilages. The space is closed by membrane. It is through this space that an opening is often made in cases of threatened suffocation owing to some obstruction higher up. The operation is known as laryngotomy (cutting the larynx), and is to be distin guished from trache otomy, in which the windpipe is opened.
The appearance of the larynx is very ' different when viewed from behind, as in Fig. 155. The thyroid cartilage is not com plete behind, while the cricoid is broader than in front. The cri coid carries, perched on its upper edge behind, two other cartilages, of great importance in the production of the voice.
The arytenoid cartilages (Ar, Fig. 155) are triangular in form, as shown in the figure. On their summits are perched small pieces of car tilage, and, when in their natural position, the two form a shape resembling the lip of a ewer, hence their name (Greek, arutaina, a pitcher).
Now these various cartilages form a frame work on which muscles and mucous membrane are disposed. Thus, towards the front, one
muscle passes from the cricoid up to the thyroid on each side, and when the muscle on each side contracts, the thyroid cartilage is pulled down wards and forwards on the pivot its lower horns form with the cricoid. Another muscle passes from each arytenoid cartilage behind to the thyroid cartilage, and when it contracts it pulls up the thyroid to its original position. Other muscles will be noted immediately.
The Vocal mucous membrane, which lines the inside of the box formed of the cartilages described, is thrown into various folds. In particular one fold passes horizon tally outwards from each side towards the middle line at the level of the base of the arytenoid cartilages. The free edge of each fold is formed by a band of elastic fibres that passes from each arytenoid cartilage straight forwards to become attached to the thyroid cartilage.
These folds are called the true vocal cords, by whose movements voice is produced. They are called true vocal cords be cause above them are folds of mucous membrane called the false vocal cords, which take no part in the pro duction of voice. The true vocal cords, projecting towards the middle line, encroach upon the space, and re duce the communi cation between the part of the larynx above them and the part below to a mere chink. This chink is called the glottis (from Greek, glotta, the tongue) or the chink of the glottis. Fig. 156 will perhaps ren der this more easily understood. It represents the larynx, &c., viewed from behind, with all the soft parts in connection with it. On looking down, the folds forming the true cords (c) are seen, iuclosing a V-shaped aperture, the glottis, the narrow part of the space being towards the front. Now by the contraction of various muscles the form of the aperture may be changed. The vocal cords may be brought so closely together that the space becomes a mere slit. Air forced through the slit will throw the edges of the folds into vibration, and a hound will be produced. Variations in the form of the opening will determine variations in the sound. This, briefly, is the mechanism of the production of voice. If all the muscles of the larynx be relaxed the folds do not pro ject nearly so far towards the middle line, the aperture of the glottis is wide, and air may enter and leave the windpipe during the acts of breathing without throwing the cords into vibration so as to produce any sound.