Thus both in voice and speech the production and modification of vocal sound depend on the laws of acoustics, while the adjustments of the various parts of the apparatus which pitoduce and modify the voice depend on voluntary muscular movement. The one is mechanical, the other physiological.
It is familiarly known that the movement of every organ is effected by muscular contraction ; that both voluntary and involuntary con traction of the muscles depend on the nerves ; that all voluntary contraction is regulated by the brain ; and also that the voluntary contraction of one muscle is accompanied by an adjusted voluntary relaxation of ita antagonist muscle.
The speech apparatus may be considered as consisting of the lungs or bellows, which can send a current of air through the trachea or windpipe to the larynx, which is situated on its summit. In the larynx this current of air can be vocalised at will into song, or into speech-sounds, which, on passing through the variable cavity, consist ing of the mouth, pharynx, and nose, can be further modified into speech. Thus the speech-apparatus, as a whole machine, consists of the respiratory, the vocal, and the enunciative organs.
The respiratory apparatus consists of the chest, the lungs, and the air-passages. The respiratory movements are involuntary and periodic ; the inspiration of breath alternates with its expiration ; and in both acts the breath flows in a continuous stream. There is a periodic action of the inspiratory muscles, but whether their action alternates with an action of the expiratory muscles, or simply with the spon taneous return of the parts by their elasticity and gravity, is yet un determined. The muscular actions during the tranquil respiration of repose appear to be limited to periodic inspiratory movements.
The movements of respiration include the motions of the diaphragm, the abdominal and thoracic muscles, and those of the larynx, which dilate and contract the aperture of the glottis. The nerves engaged in these movements are the phrenic, the spinal accessory, the vague, many of the spinal nerves, and the portio dura of the seventh. The will can influence and somewhat modify the movements of respiration; thug we can prolong or shorten the duration of an inspiration, and we can delay or;hasten to begin one. We have similar control over the expiration of the breath ; and we have power also to limit, or nearly so, the movements of respiration to sub-groups of muscles : thus we can breathe by the diaphragm alone, or by the ribs alone.
Indeed in the act of voealising, whether for speech or song, the involuntary is almost superseded by the voluntary act of respiration.
The will gives a different play to the chest. We breathe less by the diaphragm and more by the ribs ; we shorten the duration of the inspiration and completely change the character of the expiration.
There are two modes of involuntary respiration : in the one, the breath issues in one continuous unbroken stream, as in the ordinary breathing of unruffled tranquillity, which by some emotions is hurried and involuntarily vocalised, producing sighs, groans, &c. ; in the other mode the stream of breath is interrupted so that it issues In detached portions, which during; some emotions is also involuntarily vocalised, producing laughter, crying, &c.
The will has power to produce voluntary expirations in both modes. The unbroken stream is termed the exhausting breath, which is often required for a long-drawn note in song. The broken stream Is termed the holding breadth, which is constantly required in lengths suitably adjusted to the demands of the syllables as they occur in speech. The general conditions of respiration, vocalisation, enunciation, and articu under which stammer occurs, are subjoined.
I. Respiration.—l. Most stammerers manage their respiration badly, although nearly all can speak freely in a whisper. 2. They feel that they have insufficient breath to speak. This sensation, however, arises less from an insufficiency than from attempting to speak on an involuntary inspiration. The breath is expired to be vocalised by the voluntary action of the ribs, which mechanically contract the cheat's cavity. The ribs, however, cannot accomplish this when they are in the position in which an involuntary inspiration leaves them ; they must be raised to that position to which a voluntary inspiration carries them, before they can act with mechanical effect on the cheat to expire a holding breath for the purpose of conversation. 3. With the sensa tion of insufficiency of breath, some feel also a pain at the pit of the stomach. This pain is connected with attempting to speak on an involuntary inspiration, and its severity is commonly increased by struggling to speak.