One specimen should be cut open lengthwise in the middle line in front, and the other cut in the same way from behind.
354. Speech. Speech is to be distinguished from voice. It may exist without voice, as in a whisper. Speech consists of articulated sounds, produced by the action of various parts of the mouth, throat, and nose. Voice is common to most animals, but speech is the peculiar privilege of man.
Fig. 152.—Diagramatic Horizontal Section of Larynx to show the Direction of Pull of the Posterior Crico-Arytenoid Muscles, which abduct the Vocal Cords. (Dotted lines show position in abduction.)]
The organ of speech is perhaps the most delicate and perfect motor apparatus in the whole body. It has been calculated that upwards of 900 movements per minute can be made by the movable organs of speech during reading, speaking, and singing. It is said that no less than a hundred different muscles are called into action in talking. Each part of this delicate apparatus is so admirably adjusted to every other that all parts of this most complex machinery act in perfect harmony.
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.