here in the
brain or cord, which gives off axons running to some other center or
out to muscles or glands, while it also receives axons coming from
other centers, or from sense organs. These incoming axons terminate in
end-brushes and so form synapses with the dendrites of the local {50}
nerve cells. The axons entering any center and terminating there
arouse that center to activity, and this activity, when aroused, is
transmitted out along the axons issuing from that center, and produces
results where those axons terminate in their turn.

[Illustration: Fig. 12.--Side view of the left hemisphere of the
brain, showing the motor and sensory areas (for the olfactory area,
see Fig. 18). The visual area proper, or "visuo-sensory area," lies
just around the corner from the spot marked "Visual," on the middle
surface of the hemisphere, where it adjoins the other hemisphere.
(Figure text: frontal lobe, parietal lobe, central fissure, occipital
lobe, motor area, somesthetic area, auditory area, fissure of Sylvius,
temporal lobe, brain stem, cerebellum)]

The _lower_ motor centers, called also reflex centers, are located in
the cord or brain stem, and their nerve cells give rise to the axons
that form the motor nerves and connect with the muscles and glands. A
muscle is thrown into action by nerve currents from its lower motor
center.

The principal _higher_ motor center is the "motor area" of the brain,
located in the cortex or external layer of gray matter, in the
cerebrum. More precisely, the motor area is a long, narrow strip of
cortex, lying just forward of what is called the "central fissure" or
"fissure of Rolando".

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If you run your finger over the top of the head from one side to the
other, about halfway back from the forehead, the motor areas of the
two cerebral hemispheres will lie close under the path traced by your
finger.

[Illustration: Fig. 13.--(After Cajal.) Type of the brain cells that
most directly control muscular movement. (Figure text: Axon. Giant
pyramid cell from the motor area of the cerebral cortex, magnified 35
diameters. Cell body of same farther magnified)]

The motor area in the right hemisphere is connected with the left half
of the cord and so with the muscles of the left half of the body; the
motor area of the left hemisphere similarly affects {52} the right
half of the body. Within the motor area are centers for the several
limbs and other motor organs. Thus, at the top, n