Elements of the philosophy of the human mind. By Dugald Stewart. Rev. and abridged, with critical and explanatory notes, for the use of colleges and schools. By Francis Bowen ...

66 ATTENTION.
EBow the attention is distributed among simultaneous sensations.
— When two persons are speaking to us at once, we can attend
control which the will exercises, according to the best physiologists, is
often partial or imperfect. "There are many involuntary movements,"
says Miller, an eminent physiologist, "performed by muscles which are
[generally] subject to the will, -movements in some cases following as
regular a rhythm as do the motions of the heart. Certain muscles also,
which are quite independent of the influence of the will, are nevertheless
influenced by particular states of the mind."
The associate or consensual movements are those "which, contrary to our
uwill, accompany other, voluntary, motions... Very few persons indeed
can cause the different muscles of the face to act singly; they cannot, in
most instances, make the individual muscles act, except in groups with
other muscles." We find a difficulty in calling into action separately the
different fingers of the same hand; in extending, for instance, the third or
fourth finger without extending at the same time the first and second.
The muscles of the eyes have this tendency to association; it is impossible, for example, to turn one eye downwards and the other outwards, or
both outwards at the same time. When one eye is turned outwards, the
other is always rotated involuntarily inwards. During violent bodily exertion, many muscles act by association, although their action shows no
apparent purpose; a man using great muscular exertion moves the muscles of his face, as if they were aiding him in raising his load; during labored respiration, and in persons in a state of debility, the muscles of the
face act simultaneously, but involuntarily; although, except by raising the
"wings of the nostrils," they can in no way assist respiration.
" The less perfect the action of the nervous system," continues Mfiiller,
"' the more frequently do associate motions occur. It is only by education
that we acquire the power of confining the influence of volition in the production of voluntary motions to a certain number of nervous fibres issuing
from the brain. An awkward person, in performing one voluntary movement, makes many others, which are produced involuntarily by consensual nervous action. In the piano-forte player, we have an example, on
the other hand, of the faculty of insulation of the nervous influence in its
highest perfection."
Now, if education and habit, as is here stated, can insulate movements
which are by nature consensual, if they can enable us to perform separately
motions which were originally associated, it would seem that education and
habit might also associate acts which were at first independent of each
other, or, in other words, might teach us to perform by a single effort of
the will several movements each of which originally required a distinct volition. To adopt Stewart's illustration, the equilibrist or the rope-dancer
may need but one volition to put in motion several distinct muscles whose