Speech. The voice is a sound resulting from the vibrations which the air su&rs during its passage through the glottis, when expelled from the lungs. Speech or articulated voice is produced by this sound, modified by the motions of the tongue, lips, and other parts of the month. It is obvious, therefore, that no animals can have a voice, unless they possess lungs.
The larynx is the instrument of the voice, of which the rims glottidis is the immediate organ. Hence, if the trachea be opened below, so as to prevent the air from passing through, the voice is destroyed ; while, if the opening be made above, the speech only is destroyed.
It is universally agreed among physio logists, that the air, expelled from the lungs in expiration, striking against the sides of the rima glottidis (chordx vo cales) constitutes the voice. But it is necessary that the opening should be placed in some condition produced by an exertion of the will ; for although air is constantly passing to and fro, the voice is not flirmed unless by an express effort for that purpose; neither is it formed during sleep; nor after the muscles of the arytenoid cartilages have been para lysed by dividing their nerves.
The manner in which the voice is changed from acute to grave, and vice versu, has been much disputed: whether it arise from dilatation and contraction of the aperture, or from tension and relaxa tion of the chords: vocales. On the for. mer supposition the human larnyx may he compared to a wind instrument, in which the enlargement of the aperture renders the sound grave, and its diminu tion acute. By the latter explanation it resembles a stringed instrument. After considering the arguments on both sides, we should be inclined to admit the ope ration of both causes. The change of the voice from acute to grave at the time of puberty, when the larynx undergoes a remarkable developcment, as well as its acuteness in females, whose glottis is less by one-third than that of man, shew that the size of the aperture has a great in fluence Observing on the other hand that the vocal chords admit of considera ble tension and relaxation, we must allow that these variations will render them susceptible of executing, intt given time, vibrations more or less extensive and ra pid. And although they are neither dry, stretched, nor which are neces. sary conditions to the production of sound in those stringed instruments to which the larynx has been compared, yet they are analogous to vibrating ob. dies placed at the top of wind instru ments, as the reed in hautboys, the mouth-piece in flutes, Esc. and equally contribute to the formation and varied inflexions of vocal sound. That all the
changes and conditions of the vocal or gans, of whatever description, necessary to the production and modification of sounds, are produced by the muscles of the part, is rendered obvious by the ele gant experiment, in which the ligature or section of one or both recurrent nerves, or paria vaga, either signally im pairs, or entirely destroys, the vocal powers of the animal.
The modifications of the voice are also affected by the length of the trachea ; hence the larynx is manifestly drawn up in the neck, in the utterance of acute sounds, and as plainly descends when a grave sound is produced. In singing, where these effects take place in a greater degree, the head is thrown back upon the neck in the former case, and brought forwards on the chest in the lat ter.
The voice is stronger in proportion to the capacity of the thorax ; hence it is weaker after meals, when the stomach, distended by fiord, prevents the descent of the diaphragm, and in consumptive persons, where the capacity of the lungs is diminished by disease. It acquires more force and intensity, and becomes more sonorous, by its reflections in the mouth and nasal canals. Hence it is disagreeably altered when its passage in this direction is stopped by disease, as by polypus, and it is then commonly, but quite erroneously, said, that persons speak through the nose.
Whistling, which is common to man with singing-birds, is produced in the lat ter by their double larynx ; but in the for mer it is effected by a contraction and corrugation of the lips, in imitation of the effect produced by birds.
In singing, the voice runs through the different degrees of the harmonic scale with more or less rapidity, changing from acute to grave, and vice versa, with an expression of the intermediate notes. It requires much more exertion than speech. The glottis enlarges and contracts, the larynx is elevated or depressed, the neck elongated or shortened, inspirations are accelerated, prolonged or retarded ; ex pirations are long, or short and abrupt. The power of singing is peculiar to man, and forms the great prerogative of his vocal organs. Whistling is common al so to birds ; which are often taught to pronounce words without any great dif,fi culty. On the other hand. parrots are said, in two or ee instances, to have be. taught, by vast labour, to produce a kind of imitation of singing ; but no barbarous tribe has been hitherto met with, which has not been accustomed to employ singing as the natural expression of their feelings and passions.