Development of Speech.
The study of speech, a complex function, enlisting at once the activities of mind and body, invites the physician to enter alike the domains of the psychologist and physiologist.
Distinguishing man from beast, articulate expression has its foundation in purely mental phenomena; its successful accomplishment requires the reflex mechanism common to man and beast.
Let us consider as concisely as possible the physiology of speech.
DEVELOPMENT OF SPEECH IN THE INDIVIDUAL.—The earliest observation has noted in the common name of the new-born child its speechless condition; it is the infant, or not-speaking being. Born with a nervous system in a highly unstable condition, the babe is a most favorable recipient of the many impressions which stream upon it from all sides. With sight and hearing undeveloped, the field of early infantile impressions is limited to hunger, need of sleep, and cutaneous impressions. Speech under such conditions is inarticulate, an animal cry, unconscious and without intelligence. But with the growth of the cerebrum the child's environment suggests to the embryonic intelligence its primal impression. The parental relation is dimly apprehended, and designations of the father and mother are uttered in scarcely articulate sounds.
Sight and hearing open extensive fields of sensation, and with their development begin the primal, physical impressions from which proceed the emotions. The acoustic and optic centres of perception become established, and the mental formation of symbols and signs, an imitative process, marks the stage of childish cerebration, beyond which the savage often does not proceed. The symbolic function is the basis of language and of pictorial representation; as man requires the trade medium of the symbol for the interchange of his ideas, so his legal tender appears with the first emergence of mental enterprise. The hieroglyphic and the onomatopoetic word are as old as humanity.
The co-ordinating mechanism of speech is of equal development. With the growth of the child the varied impressions of education, of surroundings, of heredity, all are influencing speech. With mental growth, stimulated by these numerous impressions, comes the gradual mental habit of forming ideas after certain models—of trading, so to speak, in the coin of the country, of making and using a vocabulary. Intuition, induction, and deduction are established, the general nervous function of memory develops, observations are made, and mind and body, master and instrument, enter upon full activity.
PHYSIOLOGY OF NORMAL SPEECH IN THE ADULT.—Let us consider normal speech in the adult. From a purely mental aspect speech is not included in the nervous reflexes. The conception before the individual may arise without sight or sound, it may be the object of the mental processes only; but if it is to be communicated, or transferred from the subjective to the objective, it calls into play the denotative faculty or facultas signatrix.
As we are cognizant through the senses of phenomena only, so we communicate only phenomena, or more particularly symbols; the spoken or written word, the gesture, are necessary to make ideas tangible to another mind. The symbolic faculty, then, is the mental faculty most concerned in speech.
Cerebral localization has not included this faculty in its areas; it is assigned to the anterior cortex, which as yet is the indiscriminate site of the formation of ideas. The idea conceived, the symbol formed, the motor area whose integrity is essential, is found in the region commonly known as Broca's convolution, the posterior third of the left third frontal convolution. This region is especially connected with the corona radiata coming from the corpus striatum, with the corpus striatum and the anterior portion of the internal capsule: like the anterior, or motor, cornua of the spinal cord, it possesses giant, or branching, nerve-cells; its anatomy would assign to it a motor function. Its blood-supply is derived through the inferior frontal branch of the Sylvian vessel, whose occlusion in a case cited by Charcot was followed by complete aphasia.