VOICE AND SPEECH.—The human voice is produced by the combined action of all the muscles of the respiratory apparatus. With more or less force, the air expelled from the lungs acts upon the true vocal cords situated in the larynx, and these cords are kept in a certain state of tension by the action of the laryngeal muscles. The glottis, or space between the vocal cords, is contracted or dilated by the muscles of the larynx, thus determining the pitch of the sounds produced by the vibrations of the cords. The sounds are finally modified by the tongue, palate, teeth, lips, etc., whereby the several variations necessary for the formation of intelligible speech are brought about. The strength of the voice varies according to the force with which the air is expelled from the lungs ; and the pitch, as already mentioned, depends on the degree to which the glottis has been contracted, as well as on the rapidity of the vibrations of the vocal cords.
The accompanying illustration (Fig. 434) represents the right half of the human head, cut longitudinally through the middle. It shows the oral cavity with lips, teeth, and tongue, closed above by the hard palate, to the back of which are attached the soft palate and the uvula. When the mouth is opened wide, and the sound " ah " is produced, the soft palate will be seen to shoot upward ; on ceasing, it again recedes. The illustration shows a side view of this. The soft palate must be raised in the production of any sound except the nasal ones (m, n, rig) which, as the name indicates, are produced through the nose. II the soft palate were raised when these sounds are spoken, it would obstruct the posterior naves. A person who speaks without the soft palate being raised, is said to speak with a twang.
The lips, teeth, palate, etc., change the laryngeal sounds into vocal sounds. If the mouth be opened so that the laryngeal sound issues almost unchanged, the vowel " a " (as in " arm ") is heard ; if the lips be brought closer together, and rounded, the sound changes to " o." If the lips be closed still more, the sound of " oo " (as in " fool ") results (see Plates XXI. and XXII.). Consonants are formed in three ways : (r) with the lips and teeth (b, p, etc.) ; (2) with the teeth and the point of the tongue (d, t, etc.) ; and (3) between the back of the tongue and the palate (g, k, etc.).
From the foregoing it may be seen that the process of speech is extremely complicated. Innumerable muscles are- connected with it ; and these must be properly controlled so as not to create disturbances. This control is exercised by the so-called word-forming centre in the brain, in which, from a certain given point, the entire external speaking-apparatus and all its functions are cared for. As one learns to speak essentially by the ear, so there is another brain-centre of speech, called the auditory speech-centre, or the centre of word-understanding. Speech-disturbances may result from injury to either the external organs of speech or to the brain-centres. If the centre of word-formation be injured, as after a paralytic stroke, the person loses the ability to form words, and becomes dumb. If the centre of word understanding be injured, the person, though able to hear, does not understand what is said (word-deafness), See also the article on SPEECH DISTURBANCES.