e centers, lower and higher. It is
made up of nerve cells and their dendrites, of the beginnings of axons
issuing from these cells and of the terminations of incoming axons.
The white matter, as was said before, consists of axons. An axon
issues from the {36} gray matter at one point, traverses the white
matter for a longer or shorter distance, and finally turns into the
gray matter at another point, and thus nerve connection is maintained
between these two points.

There are lots of nerve cells, billions of them. That ought to be
plenty, and yet--well, perhaps sometimes they are not well developed,
or their synapses are not close enough to make good connections.

[Illustration: Fig. 8.--A two-neurone reflex arc. (Figure text:
stimulus, skin, sensory axon, bit of the spinal cord, motor axon,
muscle)]

Examined under the microscope, the nerve cell is seen to contain,
besides the "nucleus" which is present in every living cell and is
essential for maintaining its vitality and special characteristics,
certain peculiar granules which appear to be stores of fuel to be
consumed in the activity of the cell, and numerous very fine fibrils
coursing through the cell and out into the axon and dendrites.

The _reflex arc can now be described_ more precisely than before.
Beginning in a sense organ, it extends along a sensory axon (really
along a team of axons acting side by side) to its end-brush in a lower
center, where it crosses a synapse and enters the dendrites of a motor
neurone and so {37} reaches the cell body and axon of this neurone,
which last extends out to the muscle (or gland). The simplest reflex
arc consists then of a sensory neurone and a motor neurone, meeting at
a synapse in a lower or reflex center. This would be a two-neurone
arc.

[Illustration: Fig. 9.--A three-neurone arc, concerned in respiration.
This also illustrates how one nerve center influences another.
(Figure text: white matter, gray matter, lung, respiratory center in
the brain stem, diaphragm, motor center in cord for the diaphragm)]

Very often, and possibly always, the reflex arc really consists of
three neurones, a "central" neurone intervening between the sensory
and motor neurones and being connected through synapses with each. The
central neurone plays an important role in cooerdination.

COOeRDINATION

The internal structure of nerve centers helps us see how cooerdinated
movement is produced. The question is, how {38} se