The first step is to release breath from the inflated lobes of the lungs into the branched tubing of the respiratory system. The main channel, the trachea, is about eight inches long, rigid and segmented, a conduit for the breath toward the vocal cords, those twin infoldings of mucous membrane stretched horizontally across the larynx. They are thin muscular flaps, reminiscent of labia, that block the top of the trachea; when we breath, these flaps are loose; when we want to speak, they form a barrier by pressing together and sealing off the throat from the breath, which accumulates until the pressure becomes intense enough to release the cords - not all at once but fluidly, periodically, in a rippling motion. As the breath makes its dash upwards, the pressure below the vocal cords decreases, they seal again. More breath creates more pressure, another release point is reached, the pressure drops, the cords seal and so on in a rapid alternating dance of advance and retreat . . . in addition to the vocal cords, the respiratory system has a series of muscles, the depressor anguli oris, the posterior cricoarytenoid, the sternocleidomastoid, whose purpose is to manipulate the flow of breath as it passes out of the body. And then of course there are the tongue and the teeth and the lips. After crossing the lips, the breath collides with the air outside, its energy transforming into molecular movement. The air molecules in front of the mouth compress and open in pulses tuned to our words. These pulses travel forwards and outwards . . . if there is an ear handy, its pinna - those folds of cartilage that jut out from the sides of the head - will capture these pulses, this voice, and direct it into the auditory canal, where it will gain speed until it collides with the taut sheath of flesh called the tympanic membrane, or ear drum. The membrane booms, it vibrates and echoes, mimicking the complex vibrations of the air. Beyond lie three ossicles, little bones that rest lightly against each other so that the movement of one causes movement in the next. The hammer (malleus) strikes the anvil (incus) that strikes the stirrup (stapes) that strikes the cochlea - like a pin striking a bell. Very little energy is lost. The cochlea is snail-shell-shaped, hair-lined, fluid-filled. A spiralled, hollow, conical chamber of bone. Here molecular movement is converted to electricity, as the movement of fluid within the shell causes the hairs along its twisting length to vibrate. This vibration in turn stimulates the slender cells of the auditory nerve, the pathway to the brain, which transports the electrical signal of speech across the axons of its component neurons into that upper section of the brain known as the primary auditory cortex. This absorbs the incoming electrical impulses, somehow converting them into the comprehension of a voice. The exchange, from the lungs of the speaker to the brain of the listener, takes about one second to complete; but it is only a prologue. A voice does not begin its true existence until after the brain of the hearer has absorbed and converted the electrical signals of the neurons. We copy the external signal, discard the original and then remake the copy in our own image, based on the meaning the voice has for us. We mould and remould the voice over and over again, the copy deviating from the original that produced it and at the same time becoming inseparable from what it is stimulating. A voice has spoken and we have heard.

(Adapted from Daniel E Smith : Muses, Madmen and Prophets - Hearing Voices and the Borders of Sanity. Smith's own account depends upon Brian Moore's An Introduction to the Psychology of Hearing and a website: www.voice-center.com which, he laments, no longer exists in the form in which it was when he accessed it.)

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