Ferrier distinguishes just adjacent to this centre the motor centres for the tongue and mouth and upper extremities, showing an anatomical association of the processes of articulate and written speech.
But purely motor impulses comprise only a portion of the phenomena of speech. The external world must be brought into relation with the mind, and this is done through the perceptions. We may say that perceptions are apprehended sensations, and this apprehension demands a localized field of cerebral activity, as well as the motor energies. It seems natural enough that experiment should have located (Ferrier) perceptive, visual, and acoustic centres in the posterior cortical areas and temporo-sphenoidal lobes—that the motor and perceptive areas should be contiguous and sharing a common blood-supply.
The perceptive visual centre is found to occupy the occipital lobes, while the acoustic centre occupies the whole length of the first temporo-sphenoidal convolution.
As motor impulses found a path to the motor ganglia, and finally to the cord, so the course of sensory perceptive impressions can be traced back through the posterior internal capsule and through the optic thalami to the sensory columns of the medulla and cord.
Intuitive and sense perception, even when reinforced with motor power, cannot result in articulation. The mechanism of speech requires a co-ordinating centre, and this basal phonic centre of Kussmaul is located in the medulla near the origin of the hypoglossal and facial nerves. From the medulla proceed the nerves supplying the machinery of phonation, the superior laryngeal nerve to the mucous membrane of the larynx and to the crico-thyroid muscle, the most important muscle of phonation. The remaining laryngeal muscles are supplied by the recurrent laryngeal; the motor processes of articulation are guided by the hypoglossal, facial, and fibres of the glosso-pharyngeal.
The larynx is a reed, with the addition of numerous resonance-cavities producing abundant overtones. It may be considered as a box composed of two segments, the lower of which has vertical motion upon the joint at the posterior junction of the superior and inferior segments. By this motion the tongue of the reed (vocal cords, stretching antero-posteriorly from lower segment to junction with upper segment) is tightened or relaxed, the vibratory blast of air coming through the trachea from the lungs. The superior aperture of the tube is guarded by the epiglottis and false vocal cords. As auxiliaries are the pharyngeal, oral, and nasal cavities, with the associated bony cavities of the skull, the soft palate acting as a movable partition or switch, the hard palate as a sounding-board or resonance-surface. The reed is applicable to the production of musical sounds; the tongue, lips, and teeth are required for the checks in those sounds, constituting the consonants or division utterances.
The curious phenomenon of the falsetto voice is thought by Helmholtz to be produced by the attenuation of the true cords and the vibration of their thinned edges.
DEVELOPMENT OF LANGUAGE.—The study of language demonstrates its origin to have been largely in exclamations and imitative sounds, from which our vowels can easily have arisen. The growth of all synthetic language illustrates the aggregation of accessory sounds about the primitive root-sound, while the common tendency to the insertion of consonants shows their addition to the primitive vowels. That the long vowels should have undergone countless modifications from the physical peculiarities and environment of those speaking them is but natural, for the number of vowels remaining in actual use in any language is not large. Consonants serve to make more clear by their separation of vowel-sounds the meaning to be conveyed; their development resulted from vowel changes, and their number is small.
The written characters of language represent only the usage of the majority. Individual speech and pronunciation vary as greatly as do languages themselves, and it is evident that the speech of any individual is as truly peculiar as his physical conformation.
To recapitulate, we find ideas, the material of speech, formed in the cerebral cortex. Speech-volition becomes motor impulse at Broca's convolution; such impulse passes along the internal capsule to the corpus striatum, where it is co-ordinated probably in the formation of syllables, thence to the medulla, whence the mechanism of the larynx receives its co-ordinated stimulation.