There are certain articulate sounds called vowel or vocal, from the fact that they are produced by the vocal cords, and are but slightly modified as they pass out of the mouth.  The true vowels, a, e, i, o, u, can all be sounded alone, and may be prolonged in expiration.  These are the sounds chiefly used in singing.  The differences in their characters are produced by changes in the position of the tongue, mouth, and lips.

Consonants are sounds produced by interruptions of the outgoing current of air, but in some cases have no sound in themselves, and serve merely to modify vowel sounds.  Thus, when the interruption to the outgoing current takes place by movements of the lips, we have the labial consonants, p, b, f, and v.  When the tongue, in relation with the teeth or hard palate, obstructs the air, the dental consonants, d, t, l, and s are produced. Gutturals, such as k, g, ch, gh, and r, are due to the movements of the root of the tongue in connection with the soft palate or pharynx.

To secure an easy and proper production of articulate sounds, the mouth, teeth, lips, tongue, and palate should be in perfect order.  The modifications in articulation occasioned by a defect in the palate, or in the uvula, by the loss of teeth, from disease, and from congenital defects, are sufficiently familiar.  We have seen that speech consists essentially in a modification of the vocal sounds by the accessory organs, or by parts above the larynx, the latter being the essential vocal instrument.

Many animals have the power of making articulated sounds; a few have risen, like man, to the dignity of sentences, but these are only by imitation of the human voice.  Both vowels and consonants can be distinguished in the notes of birds, the vocal powers of which are generally higher than those of mammals.  The latter, as a rule, produce only vowels, though some are also able to form consonants.

Persons idiotic from birth are incapable of producing any other vocal sounds than inarticulate cries, although supplied with all the internal means of articulation.  Persons deaf and dumb are in the same situation, though from a different cause; the one being incapable of imitating, and the other being deprived of hearing the sounds to be imitated.

[Illustration:  Fig. 153.—­Direction of Pull of the Lateral Crico-Arytenoids, which adduct the Vocal Cords. (Dotted lines show position in adduction.)]

In whispering, the larynx takes scarcely any part in the production of the sounds; the vocal cords remain apart and comparatively slack, and the expiratory blast rushes through without setting them in vibration.

In stammering, spasmodic contraction of the diaphragm interrupts the effort of expiration.  The stammerer has full control of the mechanism of articulation, but not of the expiratory blast.  His larynx and his lips are at his command, but not his diaphragm.  To conquer this defect he must train his muscles of respiration to calm and steady action during speech.  The stutterer, on the other hand, has full control of the muscles of expiration.  His diaphragm is well drilled, but his lips and tongue are insubordinate.