and thus approximate the cords. (4) The arytenoids pass from the
one arytenoid cartilage to the other, and in action these cartilages
will be approximated and slightly depressed. (5) The aryteno-epiglottidean
muscles arise near the outer angles of the arytenoid; their
fibres pass obliquely upwards, decussate and are inserted partly into
the outer and upper border of the opposite cartilage, partly into
the aryteno-epiglottis fold, and partly join the fibres of the
thyro-arytenoids. In action they assist in bringing the arytenoids together,
whilst they also draw down the epiglottis, and constrict the upper
aperture of the larynx. The vocal cords will be also relaxed by the
elasticity of the parts.
Fig. 6.—Diagrams explaining the action of the muscles of the larynx. The dotted lines show the positions taken by the cartilages and the true vocal cords by the action of the muscle, and the arrows show the general direction in which the muscular fibres act. A, Action of crico-thyroid: 1, cricoid cartilage; 2, arytenoid cartilage; 3, thyroid cartilage; 4, true vocal cord; 5, thyroid cartilage, new position; 6, true vocal cord, new position. B, Action of arytenoid: 1, section of thyroid; 2, arytenoid; 3, posterior border of epiglottis; 4, true vocal cord; 5, direction of muscular fibres; 6, arytenoid, new position; 7, true vocal cord, new position. C, Action of lateral crico-arytenoid; same description as for A and B; 8, posterior border of epiglottis, new position; 9, arytenoid in new position. D, Action of posterior crico-arytenoid; same description. (From Beaunis and Bouchard.)
2. Physiology of Voice Production.—The vocal cords are tightened by the action of the crico-thyroid, or, as it might Muscular mechanisms. be more appropriately termed, the thyro-cricoid muscle. It stretches the thyro-arytenoid ligaments, the free edges of which, covered by mucous membrane, form the vocal cords. The adductors of the cords are the lateral crico-arytenoids, while the posterior crico-arytenoids are the abductors. The arytenoid muscle brings the cords together. Many of the fibres of the thyro-arytenoid are inserted obliquely into the sides of the cord, and in contraction they tighten the cord by pulling on the edge and making it curved instead of straight. Some such action is indicated by the elliptical shape of the rima glottidis in passing from the chest register to the middle register. Other fibres, however, running parallel with the cord may tend to relax it in certain circumstances. All the muscles except the thyro-cricoid (which is innervated by the superior laryngeal) receive nerve filaments from the inferior laryngeal branch of the vagus, the fibres being derived from the accessory roots. Both the abductor and adductor nerves come therefore from the inferior laryngeal. When an animal is deeply anesthetized stimulation of the inferior laryngeal nerve causes abduction of the cord, but if the anaesthesia is slight, then we have adduction. The tonic contraction of the abductors is stronger than that of the adductors, so in a state of rest the glottis is slightly open. The centre of innervation is in the medulla oblongata, and this is dominated by a centre in the Rolandic region of the cerebral cortex.
The intensity or loudness of voice depends on the amplitude of the movement of the vocal cords. Pitch depends on the number of vibrations per second; and the length, size and degree of tension of the cords will determine the number of vibrations. The more tense the cords the higher the pitch, and the greater the length of the cords the lower will be the pitch. The range of the human voice is about three octaves—that is, from fa1 (87 vibrations per second) to sol4 (768 vibrations). In men, by the development of the larynx, the cords become more elongated than in women, in the ratio of General physiological characters. 3 to 2, so that the male voice is of lower pitch and is usually stronger. At the age of puberty the larynx grows rapidly, and the voice of a boy “breaks” in consequence of the lengthening of the cords, generally falling an octave in pitch. A similar change, but very much less in amount, occurs at the same period in the female. At puberty in the female there is an increase of about one-third in the size of the glottis, but it is nearly doubled in the male, and the adult male larynx is about one-third greater than that of the female. In advanced life the upper notes of the register are gradually weakened and ultimately disappear, whilst the character of the voice also changes, owing to loss of elasticity caused by ossification, which first begins about middle life in the thyroid cartilage, then appears in the cricoid, and much later in the arytenoid. Eunuchs retain the voices of childhood; and by careful training it is possible in normal persons to arrest the development of the larynx so that an adult male can still sing the soprano parts sometimes used in cathedral choirs. The ranges of the different varieties of voice are shown in the following diagram, where the dotted lines give the range of certain remarkable voices, and the figures represent vibrations per second, taking the middle C of the piano as 256 vibrations per second.
A basso named Gaspard Forster passed from fa−1 to la3; the younger of the sisters Sessi had a contralto voice from do2 to fa5; the voice of Catalani ranged three and a half octaves; a eunuch singer, Farinelli, passed from la1 to re5; Nilsson, in Il Flauto Magico, could take fa5; and Mozart states that he heard in Parma in 1770 a singer, Lucrezia Ajugari, range from sol2 to do6, which she gave purely, whilst she could execute trills on re5. The latter is the most highly pitched voice referred to in musical literature, an octave and a half above the highest ordinary soprano. It will be observed that the lowest note of Gaspard Forster's voice is not much above the pitch at which
