Yet again, even as the amœba has several arms, so, too, have most nerve cells. In the case of the nerve cell, however, one arm is apt to be entirely different in appearance from all the others. It stretches off to a greater

A and C, from the cerebellum; B, from the spinal cord; D, from the cerebrum; a, the axon. The cells A and D are stained so that the main body and the dendrites are black; B and C show the nucleus

distance and has branches that extend from it at right angles. This long arm has its own name. It is called an axon. A nerve cell never has more than one axon. All the nerves that we see stretching everywhere on the surface of the body and within it are nothing more nor

less than larger or smaller bundles of these axons, and each axon belongs to its own particular cell somewhere at the center of things.

When a nerve cell has several arms it is always easy to pick out the axon, because the other arms are shorter; they also branch in forked and crooked fashion like the twigs of a tree. Indeed, they are called dendrites, because "dendron" is the Greek word for tree.

Since the nerve cell as a whole is a complete individual, it is natural that it should have its own distinct name. Scientists call it neuron, and we shall adopt the same name, for that one word covers the entire structure of cell body, axon, and dendrites. We might express it thus:

neuron =

cell body

axon

dendrites

CHAPTER X

STRUCTURE OF NERVE MACHINE (CONTINUED)

The amoeba with its nucleus moves from place to place; and, as we know, it keeps up an ever-changing life.

But the neuron, which also has a nucleus, never moves from its place. Axons do certainly stretch out from the cells while the child grows from two feet to six feet in height; dendrites may also increase in number on the different cells, but the cell body of each neuron is forever stationary. It is not entirely separated, however, from other cells of its kind. On the contrary, the cell bodies of the neurons are clustered together in bunches. Multitudes of them are in the brain, protected by the skull; still other multitudes are in the spinal cord, protected by the backbone; and yet others are placed in groups that are unprotected by any special bony covering. Each of these latter bunches is called a ganglion.

Wherever nerve cells are clustered, whether in brain, spinal cord, or ganglion, there we have that interesting place, a nerve telegraph station. It resembles a city telegraph station in two ways:

OF BRAIN

1. It has axons that do the work of wires, and stretch away from the central station to different points here and there. These carry messages hither and thither.

2. If an axon is separated from its own particular cell in that central cluster, it is as useless as is a telegraph wire after it has been separated from its telegraph station. We see, then, that the vital part of each neuron is the gray cell body, and we realize why it is that a cluster of hundreds and thousands of these cells becomes one of the most fascinating centers of activity in the world. Especially so, as it appears that each axon wire

SPINAL CORD AND UNDERSIDE that enters the central station is responsible for one sort of message alone, and that it can never carry a message of any

the spinal nerves

other sort. See how this is in the spinal cord, within the backbone. A list of statements will make the case plain.

1. Thirty-one bones called vertebræ are joined together to make up the human backbone.

2. By the way the bones are joined, one above the other, a round opening is left on both sides of each bone, making sixty-two holes in all.

3. Through each of these sixty-two holes goes a nerve about as large as a quill toothpick. It is called a spinal nerve, because it comes from the spinal cord.

4. Each of the sixty-two spinal nerves is made up of two roots which join each other before they come out of the backbone through the hole.

5. On one root of each spinal nerve there is always a spot that looks swollen. It is a ganglion, that is, a bunch of cell bodies.

6. The other root of each spinal nerve is smooth. It has no ganglion.

7. Every message of every kind that goes to the brain from the body travels upward through the swollen root; it goes by the road of the ganglion.

8. Every message of every kind that goes to any part of the body from the brain goes down through the smooth root.

To remember which messages use which root we might think of those that go to the brain as climbing