races of man, that the effects are inherited. Turning to ordinary
sensations, it is well known that pain is increased by attending to it;
and Sir B. Brodie goes so far as to believe that pain may be felt in
any part of the body to which attention is closely drawn.[1341] Sir H.
Holland also remarks that we become not only conscious of the existence
of a part subjected to concentrated attention, but we experience in it
various odd sensations as of weight, heat, cold, tingling, or
itching.[1342]
Lastly, some physiologists maintain that the mind can influence the
nutrition of parts. Sir J. Paget has given a curious instance of the
power, not indeed of the mind, but of the nervous system, on the hair.
A lady who is subject to attacks of what is called nervous headache,
always finds in the morning after such an one, that some patches of her
hair are white, as if powdered with starch. The change is effected in a
night, and in a few days after, the hairs gradually regain their dark
brownish colour. [1343]
We thus see that close attention certainly affects various parts and
organs, which are not properly under the control of the will. 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,[1344] 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.[1345] 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.
If, however, the part is furnished with muscles, we cannot feel sure,
as Mr. Michael Foster has remarked to me, that some slight impulse may
not be unconsciously sent to such muscles; and this would probably
cause an obscure sensation in the part.
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