The drawing shows the folds of mucous membrane, the vocal cords, which stretch from the tips of the arytenoid cartilages to the recess behind the median portion of the thyroid cartilage. To the outer side of each vocal cord is seen the thyro-arytenoid muscle (cut across), consisting of a broad outer portion, chiefly concerned in closing the glottis during the act of swallowing, and a smaller internal portion, which regulates the length and the thickness of the segment of the cord allowed to vibrate.
Fig. 45.—The Aperture of the Glottis seen from Above.
The leaf-like structure in front of it is the epiglottis; the two triangular structures at the back, the arytenoid cartilages; the white bands on either side, the vocal cords. A, The glottis is widely open during inspiration. Arrows show the lines of action of the muscles which rotate, and approximate, the cartilages. Attached to their outer angles, and pulling these angles forwards, the lateral crico-arytenoid muscles; pulling them backwards and inwards, the posterior crico-arytenoid muscles. Drawing the cartilages together, the arytenoid muscles. B, The glottis during speaking in a deep chest-voice, or when a low note of the lower register is being sung. C, During the production of a high note of the lower register. D, During the production of a note of the head-register. E, During the act of swallowing; the arytenoid cartilages are drawn towards the epiglottis the aperture is folded into a T; the pharynx (the tube behind the glottis) is distended.
The first cartilage below the thyroid—it may be felt with the finger—is termed “cricoid” (κρίκος, a ring), from its resemblance to a signet-ring. Narrow in front, its large signet projects upwards, within the V of the thyroid, behind, and on the top of the signet rest the two arytenoids. Each arytenoid is a triangular pyramid, its anterior, external, and upper angles prolonged into processes. It is united with the cricoid by a swivel joint, which allows its anterior process to swing inwards or outwards under the influence of two antagonistic muscles attached to its outer angle—the lateral and posterior crico-arytenoids. Another muscle attached only to the arytenoids draws them together. Still another muscle—or two muscles, for it is in two separate bands—unites the anterior process of the arytenoid with the back surface of the thyroid just on the outer side of the attachment into that cartilage of the vocal cord. The internal thyro-arytenoid muscle is a comparatively narrow band; the external thyro-arytenoid muscle is thick and broad.[3] By the simultaneous contraction of the encircling muscles the larynx is closely squeezed together, the anterior portion of the slit forming a T, with the transverse limb in front. This occurs only in swallowing. Under the co-operating contractions of the several muscles, the glottis assumes a variety of shapes. The external crico-arytenoids rotate the anterior angles of the arytenoid cartilages inwards ([Fig. 45, A]). If at the same time the arytenoid muscle draws the cartilages together, the glottis is reduced to a slit ([Fig. 45, C]). The posterior crico-arytenoid muscles rotate the cartilages outwards. If the arytenoid muscle is at the same time relaxed, the glottis gapes to its fullest extent ([Fig. 45, A]). The freer the opening, the less is the resistance to the blast of air, the gentler the vibrations of the cords, the lower the voice. The closer the slit, the greater is the resistance which the air in the windpipe has to overcome in passing through it, and consequently the more ample the vibrations into which it throws the vocal cords.
The vocal cords are the tongues of a reed-pipe, which, commencing in the chest at the point where the great bronchi join to form the windpipe, comprises the larynx, and, above the larynx, the complicated chambers of the throat, mouth, and nasal cavities, including the spaces within the bones of the head which open out of them. The pitch of the voice depends upon (1) the length of the vocal cords, and (2) their tension. The first factor is fixed for every individual. The voice is base, baritone, tenor, in a man; contralto, mezzo-soprano, soprano, in a woman—in proportion as the cords are long, of medium length, or short. A man’s vocal cords measure, on the average, 15 millimetres, a woman’s 11 millimetres. When a boy is from twelve to fifteen years of age his vocal cords double in length, and the “breaking” of the voice occurs as he gives up trying to get high notes out of his longer cords, and allows them to produce manly tones of an octave lower.
The lower posterior angles of the thyroid cartilages articulate with the cricoid. If the four cartilages are freed from all soft tissues without disturbing the thyro-cricoid, or crico-arytenoid joints, and if, while the thyroid is held in one hand, a finger of the other is placed on the front of the cricoid, it will be found that as this is depressed the arytenoid cartilages which rest upon its signet are tilted upwards and forwards within the thyroid; as it is raised, they are tilted away from it. In life this movement is effected by a muscle—the crico-thyroid ([Fig. 46])—attached to the front of the cricoid cartilage and to the under border of the lateral plate of the thyroid. This is the muscle of supreme importance in the production of the voice. The thyroid cartilage is slung in a fixed position by the hyoid bone (to be felt in the neck above it). The crico-thyroid muscle, being unable to depress the thyroid, raises the front of the cricoid cartilage, tilts back the arytenoids, tightens the vocal cords. As the voice ascends the scale, the tension of the cords is progressively increased, and their vibrations rendered proportionately more rapid. The range of the human voice is about three and a half octaves; of individual voices about two octaves; if the shrill cry of a baby, which may reach the third G above the middle C, or even higher (E⁗ or F⁗), be excluded. Exceptional voices have a range far greater than two octaves. Falsetto voice is produced by throwing half of the vocal cord out of vibration (the way in which this is accomplished is not clear), and at the same time raising the back of the tongue to the wall of the throat in such a manner as to cut off all the lower part of the upper resonating chamber, leaving it only the mouth and the cavities of the nose.