up, because they use the swollen root; and of those that come from the brain as slipping down, because they use the smooth root.

Since the two sets of fibers carrying messages in opposite directions are so close together, the ignorant per

THE SMOOTH AND THE
SWOLLEN ROOTS AS
THEY JOIN THE
CORD

son might wonder whether or not any mistakes are ever made in the work they do.

The answer is, that this

never happens. From birth to death every fiber that passes through the swollen root carries messages upward, while, at the same time, every fiber of the smooth root hurries messages downward. These countless messages, moving in opposite directions, cannot intermix, because each nerve fiber is separated from the others by its own particular outside wrapping. The two roots join each other and are wrapped together as a single spinal nerve; and thus are the sixtytwo spinal nerves formed. Each passes out through its own opening between the vertebræ, yet never in a single instance does any fiber in any bundle carry a message the wrong way or exchange its message for that which a neighbor fiber is carrying. Thus once again do nerve fibers remind us of telegraph wires.

And now we are ready for the explanation of the gray and white substance of the brain. The gray layer is a mass of millions of cell bodies packed together and joined to each other by dendrites and axons.

The white stuff is a compact mass of axons, each one of which stretches away with its silvery sheath from its

GANGLION OF A SPINAL ROOT CUT OPEN

Here we see cells and axons

individual cell in the gray layer. Millions of these axons join one part of the brain with another part of the same brain. Still other millions go downward towards the spinal cord, and there, within the firm protection of the backbone, impulses of every sort fly upward to the brain, while, at the same instant, on separate roads, countless commands go from the brain to the muscles of the body.

FROM SMALLER BRAIN TO LARGER

A, frog; B, pigeon; C, dog; D, chimpanzee; E, man. Each is three eighths its normal size. Notice the convolutions

that increase as intelligence advances

As a rule, white color anywhere in the nervous system stands for nerve fibers, and gray color shows cell bodies; but the arrangement of the two colors is rather interesting. In the brain it is gray outside, white inside; in the spinal cord it is white outside, gray inside.

Close examination proves that it is from the white part of the cord that the sixty-two bundles of axons, called spinal nerves, stretch away from the backbone, and that the spinal cord is therefore the highway of travel between brain and body.

It is easy to talk about neuron, about cell body, axon, and dendrite, yet each separate one is so small that without a microscope no human eye will ever find a neuron anywhere in the nervous system. In fact, even to the keenest eye, if it is unaided, the contents of the skull and backbone will always look like nothing but a mass of marrow.

In shape, however, the brains of living creatures may be arranged as a ladder. At the foot stands amphioxus, with its backbone, pointed at both ends, its nerve tube, and its simple nerve branches. At the top stands man, with his masterful brain and his superb nervous system.

CHAPTER XI

NEURONS AT WORK

When a baby sees a flame, laughs with joy, thrusts his fingers into it, and pulls them out again with a scream, several sets of fibers have been at work.

1. One set, from the eyes, compelled the brain to see a lovely color.

2. Another set brought word from brain to hand muscles," Feel of it."

3. A third set carried a stimulus to the brain, which seemed to say, "Something dreadful is happening to the fingers."

4. A fourth set brought the prompt command, "Pull the fingers out of the color as fast as possible." In the meantime other sets of fibers set other muscles to work, so that at one point the baby opened its mouth to laugh with joy, and a moment later opened it again to scream with pain. Still other fibers commanded the heart to pump faster and send more blood to the excited head. They commanded the tear glands to manufacture salt water with lightning speed and in great abundance. They set lungs and vocal cords to