, makes me angry; the
thought is then the stimulus arousing this emotional response.

If hearing you speak of Calcutta makes me think of India, your words
are the stimulus and my thought the response. Well, then, if I _think_
of Calcutta in the course of a train of thought, and next think of
India, what else can we say than that the thought of Calcutta acts as
a stimulus to arouse the thought of India as the response? In a long
train of thought, where A reminds you of B and B of C and C of D, each
of these items is, first, a response to the preceding, and, second, a
stimulus to the one following.

There is no special difficulty with the notion of "central stimuli"
from the physiological side. We have simply to think of one nerve
center arousing another by means of the tract of axons connecting the
two. Say the auditory center is aroused by hearing some one mention
your friend's name, {49} and this promptly calls up a mental picture
of your friend; here the auditory center has aroused the visual. What
happens in a train of thought is that first one group of neurones is
aroused to activity, and then this activity, spreading along the axons
that extend from this group of neurones to another, arouses the second
group to activity; and so on. The brain process may often be
exceedingly complex, but this simple scheme gives the gist of it.

The way nerve currents must go shooting around the brain from one
center or group of neurones to another, keeping it up for a long time
without requiring any fresh peripheral stimulus, is remarkable. We
have evidence of this sort of thing in a dream or fit of abstraction.
Likely enough, the series of brain responses would peter out after
awhile, in the absence of any fresh peripheral stimulus, and total
inactivity ensue. But response of one brain center to nerve currents
coming from another brain center, and not directly from any sense
organ, must be the rule rather than the exception, since most of the
brain neurones are not directly connected with any sense organ, but
only with other parts of the brain itself. All the evidence we have
would indicate that the brain is not "self-active", but only
responsive; but, once thrown into activity at one point, it may
successively become active at many other points, so that a long series
of mental operations may follow upon a single sensory stimulus.

The Motor Centers, Lower and Higher

A "center" is a collection of nerve cells, located somew