By what means attention--perhaps the most wonderful of all the wondrous
powers of the mind--is effected, is an extremely obscure subject.
According to Muller,[44] the process by which the sensory cells of the brain
are rendered, through the will, susceptible of receiving more intense
and distinct impressions, is closely analogous to that by which the motor
cells are excited to send nerve-force to the voluntary muscles.
There are many points of analogy in the action of the sensory
and motor nerve-cells; for instance, the familiar fact that close
attention to any one sense causes fatigue, like the prolonged exertion
of any one muscle.[45] When therefore we voluntarily concentrate
our attention on any part of the body, the cells of the brain
which receive impressions or sensations from that part are,
it is probable, in some unknown manner stimulated into activity.
This may account, without any local change in the part to which our
attention is earnestly directed, for pain or odd sensations being
there felt or increased.
[43] `Lectures on Surgical Pathology,' 3rd edit.
revised by Prof. Turner, 1870, pp. 28, 31.
[44] `Elements of Physiology,' Eng.
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