VOICE. The parts employed in the production of the voice are, the trachea, or wind-pipe, by which the air passes to and from the lungs : the larynx, which is a short and cylindrical canal at the head of the trachea ; and the glottis, which is a small oval chink between two semicircu lar membranes, extended horizontally from the entering side of the larynx.
The trachea so much resembles a flute, that the ancients attributed the for mation of the voice to the trachea, as much as the formation of the sound to the body of the flute ; and till the commence ment of the last century, it was generally imagined that the trachea had at least a considerable part in the production of the voice. M. Doilart has established the contrary. He observed, that we nei ther speak nor sing in drawing in our breath, but only when we expel that which we have inhaled ; and that the air thus expelled from -the lungs passes through vessels, which increase in size as their distance from the lungs increases ; and finally, through the trachea, which in, the most capacious of any : so that the air, instead of being there confined and increasing in velocity, loses it. But the opening, denominated the glottis, being very narrow in comparison with the size of the trachea, the air can never pass through it without acquiring a consider able degree of velocity : so that the air this compressed and forced on, commu nicates, as it passes, a vibratory motion to the particles of the two lips of the glottis, which produces that effect on the air which we call sound. The sound thus formed, passes into the cavity of the month and nostrils, where it reverbe rates; and Dodart proves that this rever beration is what principally gives the ef fect to the voice. The different parts of the mouth, each in its turn, contributes to these reverberations, and modifies them; and it is this mixture of different reverbe rations, well proportioned to one another, which produces in the human voice a har mony which no instrument can equal. When the parts are defective, much of this pleasure is lost. It is, then, the cavi ty of the mouth, &c. that more properly
answers to the body of the flute ; the tra chea only furnishes the air, like the sound-board of the organ.
The glottis, by means of different mus cles, can be extended or shortened, can be dilated or contracted ; and it is these changes which produce all the variety of tone. The narrower the opening the greater the rapidity with which the air passes, and the more acute the sound : hence those who wished to give -their voice a very high tone, would suffocate themselves ifthey continued it sufficiently long; for, as they almost entirely close the glottis, very little air can issue ; and they are in a similar situation with those whose respiration is stopped by hanging, drowning, &c. But if the opening of the glottis be too much dilated, the air will pass too easily to produce any vi bration : hence those who wish to give their voice too deep a tone, cannot pro duce any sound.
This power of contraction and dilatation is, perhaps, the most wonderful part of the mechanism of the voice. The dia meter of the glottis never exceeds Atlt of an inch : now, suppose a person capa ble of sounding twelve notes (to which the voice easily reaches), there must be the difference of part of an inch for each note.' But if we consider the subdivision of notes of which the voice is capable, the motion of the sides of the glottis appears still more minute ; for if of two chords, so stretched as to be ex. actly in unison, one be shortened the part of its length, a correct ear will perceive the difference of the two sounds ; and a good voice will sound the difkrence, which is only part °fa note. But suppose that a voice can divide a note into 100 parts, it will follow that the different openings of the glottis will be 1200 in the of an inch, each of which will produce a sound perceptible to a good ear. But the movement of each side of the glottis' being equal, it is necessary to double this number, and the side of the glottis actually qvides the Nth of an inch into 2400 equal parts; tat each vibration part of an inch.