Speech is defined as the ability to communicate thoughts, ideas, or other information by means of sounds that have clear meaning to others.
Many animals make sounds that might seem to be a form of speech. For example, one may sound an alarm that a predator is in the area. The sound warns others of the same species that an enemy is in their territory. Or an animal may make soothing sounds to let offspring know that a parent is present. Most scientists regard these sounds as something other than true speech.
Some animals can copy human speech to a certain extent also. Many birds, for example, can repeat words that they have been taught. This form of mimicry also does not qualify as true speech.
True speech consists of two essential elements. First, an organism has to be able to develop and phrase thoughts to be expressed. Second, the organism has to have the anatomical equipment with which to utter clear words that convey those thoughts. Most scientists believe that humans are the only species capable of speech.
Speech has been a critical element in the evolution of the human species. It is a means by which a people's history can be handed down from one generation to the next. It enables one person to convey knowledge to a roomful of other people. It can be used to amuse, to rouse, to anger, to express sadness, to communicate needs that arise between two or more humans.
Spoken words are produced when air expelled from the lungs passes through a series of structures within the chest and throat and passes out through the mouth. The structures involved in that process are as follows: air that leaves the lungs travels up the trachea (windpipe) into the larynx. (The larynx is a longish tube that joins the trachea to the lower part of the mouth.) Two sections of the larynx consist of two thick, muscular folds of tissue known as the vocal cords. When a person is simply breathing, the vocal cords are relaxed. Air passes through them easily without producing a sound.
When a person wishes to say a word, muscles in the vocal cords tighten up. Air that passes through the tightened vocal cords begins to vibrate, producing a sound. The nature of that sound depends on factors such as how much air is pushed through the vocal cords and how tightly the vocal cords are stretched.
Anatomical structure: A part of the body.
Aphasia: The inability to express or understand speech or the written word.
Broca's area: The part of the brain that controls the anatomical structures that make speech possible.
Epiglottis: The flap at the top of the larynx that regulates air movement and prevents food from entering the trachea.
Larynx: A tube that joins the trachea to the lower part of the mouth.
Palate: The roof of the mouth.
Trachea: The windpipe; a tube that joins the larynx to the lungs.
Vocal cords: Muscular folds of tissue located in the larynx involved in the production of sounds.
The moving air—now a form of sound—passes upward and out of the larynx. A flap at the top of the larynx, the epiglottis, opens and closes to allow air to enter and leave the larynx. The epiglottis is closed when a person is eating—preventing food from passing into the larynx and trachea—but is open when a person breathes or speaks.
Once a sound leaves the vocal cords, it is altered by other structures in the mouth, such as the tongue and lips. A person can form these structures into various shapes to make different sounds. Saying the letters "d," "m," and "p" exemplifies how your lips and tongues are involved in this process.
Other parts of the mouth also contribute to the sound that is finally produced. These parts include the soft palate (roof) at the back of the mouth, the hard or bony palate in the front, and the teeth. The nose also provides an alternate means of issuing sound and is part of the production of speech. Movement of the entire lower jaw can alter the size of the mouth cavern and influence the tone and volume of the speech.
The tongue is the most agile body part in forming sounds. It is a powerful muscle that can take many shapes—flat, convex, curled—and can move front and back to contact the palate, teeth, or gums. The front of the tongue may move upward to contact the hard palate while the back of the tongue is depressed. Essentially these movements open or obstruct the passage of air through the mouth. During speech, the tongue moves rapidly and changes shapes constantly to form partial or complete closure of the vocal tract necessary to manufacture words.
Other animals have anatomical structures similar to those described above. Yet, they do not speak. The reason that they lack speech is that they lack the brain development needed to form ideas that can be expressed in words.
In humans, the part of the brain that controls the anatomical structures that make speech possible is known as Broca's area. It is located in the left hemisphere (half) of the brain for right-handed and most left-handed people. Nerves from Broca's area lead to the neck and face and control movements of the tongue, lips, and jaws.
The portion of the brain in which language is recognized is situated in the right hemisphere. This separation leads to an interesting phenomenon: a person who loses the capacity for speech still may be able to understand what is spoken to him or her and vice versa. The loss of the power of speech or the ability to understand speech or the written word is called aphasia.
Three speech disorders result from damage to the speech center, dysarthria, dysphonia, and aphasia. Dysarthria is an inability to speak clearly because of weakness in the muscles that form words. Dysphonia is a hoarseness of the voice that can be caused by a brain tumor or any number of other factors. And aphasia can be either the inability to express thoughts in speech or writing or the inability to read or to understand speech.
[ See also Brain ]