Halliday & Matthiessen (1999: 607):
These are the physiological systems and processes of the production and reception of speech: motor systems of articulation (air stream mechanisms, constrictions and oscillations of the larynx and other organs, movements of tongue and lips, shaping of the buccal cavity) and receptor systems of auditory perception in the various regions of the ear. When language comes to be written, analogous systems come into play for the production and reception of visual expressions.