humming the tune, viz. with closed lips, so that the sound waves can only
escape through the nose; under such circumstances the only definite
conscious muscular sensation I have is from the effect of closure of the
lips; the sensations from the larynx are either non-existent or quite
ill-defined, although I hear mentally the tonal sensations of the melody.
No doubt by closing the lips in silent humming I am in some way
concentrating attention to the sensori-motor sphere of phonation and
articulation, and by reactive association with the auditory sphere
reinforcing the tonal sensations in the mind. The vocal cords (ligaments)
themselves contain very few nerve fibres; those that are seen in the deeper
structures of the cords and adjacent parts mainly proceed to the mucous
glands. This fact, which I have ascertained by numerous careful
examinations, is in accordance with the fact that there are no conscious
kinæsthetic impressions of alterations of position and tension of the vocal
cords. A comparative microscopic examination of the tip of the tongue and
the lips shows a remarkable difference, for these structures are beset with
innumerable sensory nerves, whereby every slightest alteration of tension
and minute variations in degrees of pressure of the covering skin is
associated with messages thereon to the brain. The sense of movement in
articulate speech is therefore explained by this fact. There is every
reason then to believe that auditory tonal images are the sole primary and
essential guides to the minute alterations of tension in the muscles of the
larynx necessary for the production of corresponding vocal sounds. By
humming a tune we concentrate our attention and thereby limit the activity
of neural processes to systems and communities of neurones employed for the
perception of tonal images and their activation in motor processes; and
this helps to fix the tune in the memory.