The speech center lies in the parietal lobe of the left hemisphere of the brain for right-handed persons and most left-handed. The area of the brain responsible for motor control of the anatomic structures is called Broca's motor speech area. It is named for Pierre Paul Broca (1824-80) a French anatomist and surgeon who carried out extensive studies on the brain. The motor nerves leading to the neck and face control movements of the tongue, lips, and jaws.
The language recognition center usually is situated in the right hemisphere. Thus 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-dysarthria, dysphonia, and aphasia-result from damage to the speech center. Dysarthria is a defect in the articulation and rhythm of speech because of weakness in the muscles that form words. Amyotrophic lateral sclerosis (Lou Gehrig's disease) and myasthenia gravis are two diseases with which such muscle weakness can be associated. Dysphonia is a hoarseness of the voice that can be caused by a brain tumor or any number of nonneurologic factors. Aphasia can be either motor aphasia, which is the inability to express thoughts in speech or writing, or sensory aphasia, the inability to read or to understand speech.
The ability to speak is inherent in the human species. An infant is born with the ability to learn language but not to speak. Language is passed from one generation to the next. Children learn basic language easily and at a young age. From that time they add to their vocabulary as they accrue education and experience. A child will learn a language with the regional inflections inherent in his parents' and peers' speech.