Writers are not in accord in explaining the pronunciation of letters, that is, the mechanism of articulate sounds; but, whether grammarians or physiologists, they all class the letters according to the parts of the vocal apparatus which co-operate in their pronunciation, as labials, dentals, gutterals, etc. The division of the signs of the alphabet into vowels and consonants expresses the universal idea that a vowel is a voice, a sound perfect in itself, while a consonant cannot be sounded without the help of a vowel associated with it. The consonants, indeed, do not make even a noise, a murmur; but they give a peculiar character to a vowel sound. We find something in the playing of an instrument analogous to this function of a consonant If we pinch the string of a violin, or strike a bell with a hammer, a sound is produced which we imitate with the voice by prefixing a t or d, as dinn, tinn; if the string or the bell be made to vibrate with the bow, the sound as reproduced by the vocal organ is preceded by the letters cr, whence the imitative French word crin-crin. The hammer and bow are the consonants, the note of the bell or the violin is the vowel.
Helmholtz has demonstrated, as we have already observed in speaking of hearing, that the timbre of sounds is determined by the harmonics. He was able by means of ingenious instruments to decompose the sounds which only produce a single sensation, and which seem to us simple, though they are really composed of elementary sounds more or less numerous. This analysis enabled him to discover the laws under which the quality of the sounds is constituted which are emitted by the glottis, and resound in the vocal tube under the form of vowels. Among the elementary sounds composing the sound emitted by the glottis, the vocal tube exalts a particular one by preference, and it is this one which gives to the vowel its characteristic timbre. The vocal tube disposes itself in a special form for each vowel. It lengthens or shortens, dilates or contracts; it places itself, in a word, in conditions essential to the strengthening of the sound which determines the timbre. Each vowel is therefore characterized by a note, but each one has a particular affinity for certain notes; it is sometimes difficult, or even impossible, to give such a note on another vowel than that with which it corresponds, and thus singers are sometimes forced to substitute one vowel for another.
In seeking in the different qualities of the voice, and especially in the vowels, for the seat of the resonance of sounds in the buccal tube, and the parts which co-operate in this resonance, Fournie' has made a classification of the letters, which he claims to have rendered more exact and more anatomical than any of his predecessors. The tongue, the teeth, the lips, and the throat, are the parts by which most of the letters are formed. To these Fournie adds the palate for some of them, and the glottis for the h, which, until recently, was classed among the gutturals. It is unnecessary to remark that in the study of the vowels in their relation to the mechanism of articulate sounds, the laryngoscope has given invaluable aid.
The manner of forming the vowels differs from that of the consonants in this respect: the parts which co-operate in the formation of the vowels must be fixed during the utterance of the vowel, while the articulation of the consonants is effected by a movement of the parts essential to their formation. Thus is enunciated by suddenly opening the lips which have been previously closed; and in the same way the other consonants are pronounced by some movement; and this movement is in accordance with the disposition of the parts necessary to the utterance of the vowel which precedes or follows the consonant.
Of all the parts which serve for the articulation of sounds, the tongue is the one which plays the principal part, and therefore it gives its name to the whole group of modulations of the voice which constitute language, or as we sometimes say, a tongue. And yet observation teaches us that the volume of the tongue may be greatly diminished, or may even exist only in a rudimentary state, without its being impossible to speak.
De Jussieu relates that he saw a girl fifteen years old, in Lisbon, who was born without a tongue, and yet she spoke so distinctly as not to excite the slightest suspicion of the absence of that organ.
The Transactions of the Royal Society of London (1742) contain a report of the commission which was appointed to investigate a case of a similar nature. It was a woman who had not the slightest vestige of a tongue, but who could, notwithstanding, drink, eat, and speak as well and as distinctly as any one, and even articulate the words in singing. Other instances have been known where individuals, after losing a portion of the tongue by accident or disease, have again been able to speak after a longer or shorter time.