The cricoid cartilage, so called from its fancied resemblance to a signet-ring, is smaller but thicker and stronger than the thyroid, and forms the lower and back part of the cavity of the larynx. This cartilage is quite sensitive to pressure from the fingers, and is the cause of the sharp pain felt when we try to swallow a large and hard piece of food not properly chewed. Illustration: Fig. 149.—Diagram of a Sectional of Nasal and Throat Passages. C, nasal cavities; T, tongue; L, lower jaw; M, mouth; U, uvula; E, epiglottis; G, larynx; O, œsophagus. On the upper edge of the cricoid cartilage are perched a pair of very singular cartilages, pyramidal in shape, called the arytenoid, which are of great importance in the production of the voice. These cartilages are capped with little horn-like projections, and give attachment at their anterior angles to the true vocal cords, and at their posterior angles to the muscles which open and close the glottis, or upper opening of the windpipe. When in their natural position the arytenoid cartilages resemble somewhat the mouth of a pitcher, hence their name. 351. The Vocal Cords. The mucous membrane which lines the various cartilages of the larynx is thrown into several folds. Thus, one fold, the free edge of which is formed of a band of elastic fibers, passes horizontally outwards from each side towards the middle line, at the level of the base of the arytenoid cartilages. These folds are called the true vocal cords, by the movements of which the voice is produced. Above them are other folds of mucous membrane called the false vocal cords, which take no part in the production of the voice. The arrangement of the true vocal cords, projecting as they do towards the middle line, reduces to a mere chink the space between the part of the larynx above them and the part below them. This constriction of the larynx is called the glottis. Illustration: Fig. 150.—View of the Cartilages and Ligaments of Larynx. (Posterior view.) A, epiglottis; B, thyroid cartilage; C, arytenoid cartilage; D, ligament connecting lower cornu of the thyroid with the back of the cricoid cartilage; E, cricoid cartilage; F, upper ring of the trachea. 352. The Mechanism of the Voice. The mechanism of the voice may be more easily understood by a study of Fig. 150. We have here the larynx, viewed from behind, with all the soft parts in connection with it. On looking down, the folds forming the true vocal cords are seen enclosing a V-shaped aperture (the glottis), the narrow part being in front. The form of this aperture may be changed by the delicately coordinate activities of the muscles of the larynx. For instance, 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 sound will be produced.