nes.

[Illustration: Fig. 6.--The synapse between the two neurones lies just
above the arrow.]

The junction is good enough so that one of the two neurones, if itself
active, can arouse the other to activity. The end-brush, when a nerve
current reaches it from its own nerve cell, arouses the dendrites of
the other neurone, and thus starts a nerve current running along those
dendrites to their nerve cell and thence out along its axon.

Now here is a curious and significant fact: the dendrites are
receiving organs, not transmitting; they pick up messages from the
end-brushes across the synapse, but send out no messages to those
end-brushes. Communication across a synapse is always in one
direction, from end-brush to dendrites.

This, then, is the way in which a reflex is carried out, the pupillary
reflex, for example. Light entering the eye starts a nerve current in
the axons of the optic nerve; these axons terminate in the brain stem,
where their end-brushes arouse the dendrites of motor nerve cells, and
the axons of these {35} cells, extending out to the muscle of the
pupil, cause it to contract, and narrow the pupil.

Or again, this is the way in which one nerve center arouses another to
activity. The axons of the cells in the first center (or some of them)
extend out of this center and through the white matter to the second
center, where they terminate, their end-brushes forming synapses with
the cells of the second center. Let the first center be thrown into
activity, and immediately, through this connection, it arouses the
second.

[Illustration: Fig. 7.--Different forms of synapse found in the
cerebellum, "a" is one of the large motor cells of the cerebellum (a
"Purkinje cell"), with its dendrites above and its axon below; and
"b," "c" and "d" show three forms of synapse made by other neurones
with this Purkinje cell. In "b," the arrow indicates a "climbing
axon," winding about the main limbs of the Purkinje cell. In "c," the
arrow points to a "basket"--an end-brush enveloping the cell body;
while "d" shows what might be called a "telegraph-wire synapse."
Imagine "d" superimposed upon "a": the axon of "d" rises among the
fine dendrites of "a," and then runs horizontally through them; and
there are many, many such axons strung among the dendrites. Thus the
Purkinje cell is stimulated at three points: cell body, trunks of the
dendrites, and twigs of the dendrites.]

The "gray matter" comprises the nerv