Hence more blood flows
into these glands, and they secrete a copious supply of saliva.
Now it does not seem an improbable assumption, that, when we
reflect intently on a sensation, the same part of the sensorium,
or a closely connected part of it, is brought into a state of activity,
in the same manner as when we actually perceive the sensation.
If so, the same cells in the brain will be excited, though, perhaps,
in a less degree, by vividly thinking about a sour taste, as by
perceiving it; and they will transmit in the one case, as in the other,
nerve-force to the vaso-motor centre with the same results.
To give another, and, in some respects, more appropriate illustration.
If a man stands before a hot fire, his face reddens. This appears
to be due, as Mr. Michael Foster informs me, in part to the local
action of the heat, and in part to a reflex action from the vaso-motor
centres.[46] In this latter case, the heat affects the nerves of the face;
these transmit an impression to the sensory cells of the brain,
which act on the vaso-motor centre, and this reacts on the small arteries
of the face, relaxing them and allowing them to become filled with blood.
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