CHAPTER VII.

THE NERVOUS SYSTEM.

1. I HAVE thus far told you only about building the body and keeping it in repair. You have seen in the previous chapters that the blood is the common building material of the body—that it is the office of the digestive machinery to separate from the food the nourishing part of it, and fit it to become blood—that the circulating machinery circulates the blood everywhere, that it may be used as building material-that the formative vessels, the real builders of the body, everywhere use it to build and repair-and that the machinery of the respiration keeps the blood fresh and in good condition, by continually changing it after it has been used by the builders.

2. Digestion, circulation, respiration, and formation, are then all engaged in building and repairing machinery. In the remaining chapters of this book I intend to explain to you the uses for which this machinery is built.

3. The mind uses this machinery. First, it uses some of it to learn what is going on around it. It uses the apparatus or machinery of the eye to see with; that of the ear to hear with; that of the nose to smell with; that of the mouth to taste with; and that of the skin to feel with. In all these cases the

Give the summary of subjects treated of in the previous chapters. What are to be the subjects of the following chapters as stated in § 2? What uses the machinery of the body? For what purpose does the mind use the machinery of the senses? Illustrate.

mind is acted upon. Light acts upon it by means of the eye; sounds act upon it by means of the ear; odors act upon it by means of the nose, &c. The mind is in these cases passive. It receives impressions.

4. But secondly, the mind is in an active state, in using some of the machinery of the body. That is, it makes impressions upon the world around. It does this by moving the muscles. For example, it moves the muscles of the voice, and thus produces sounds. It works the muscles of the face, to give it expression. It sets in motion the muscles of the arm and other parts of the body, when we work.

5. You see, then, that the purpose of all the machinery is to have a communication between the mind and all that is around it. The mind, by some parts of the machinery, makes impressions, and by other parts receives impressions. And you see, also, that all that goes into the mind gets there by means of the machinery of the senses, and all that comes out from the mind comes by the machinery of the muscles. That is, all the knowledge that enters the mind enters by the senses; and the mind uses this knowledge, in acting upon things and beings around it, by means of the muscles.

6. Now the mind makes all this use of the machinery of the body by means of the nervous system. The brain is the great centre of this system, and here the mind has its seat. And the mind is connected with the organs of the senses by the nerves that

In what state is the mind in using this machinery? In using what machinery is it in an active state? Illustrate. What is said of the communication between the mind and the world around it? By means of what does the mind make use of the machinery of the body?

branch out from the brain, and go to these organs. It receives impressions by the nerves that connect the brain with the organs of the senses, and it makes impressions by the nerves that go to the muscles.

7. The brain then may be considered the great central workshop of the mind. There it sits and operates by the nerves upon all the machinery of the body. It receives messages by one set of nerves, and sends out messages by another set. The nerves by which it receives messages are called nerves of sensation. The nerves by which it sends out messages are called nerves of motion. You can see how this is by a single example. If you put your finger too near the fire, you feel pain in it, and instantly draw it away. Now see what takes place in this case. The sensation made by the fire goes by the nerves of sensation to the brain, so that the mind feels it; and then the mind sends a message or order by the nerves of motion to the muscles of the arm, and they draw the finger away.

8. We know that the nerves are the means of this connection between the mind and the various parts of the body in this way. If the nerves of any part are divided, the part cannot move, and there is no feeling in it. For example, if the nerves of the hand are divided, you can pinch or prick it without producing any feeling, because the mind has lost its connection with it. And the muscles of the hand will not act if the mind sends a message to them, because

What is said of the brain? What of the nerves of sensation and of motion? Give the illustration in §7. How do we know that the nerves are the means of communication between the mind and the different parts of the body?

the message can go no further than where the nerves are divided, just as when a telegraph wire is broken, the electricity can go only to the point where it is broken.

9. The nerves are white cords. Each nerve is made up of a great number of tubes. These tubes are so small that they can be seen only by the aid of a very powerful microscope. Each tube is altogether by itself. It is never seen to communicate with any of the other tubes that are bound up with it in the same nerve. Each of these tubes then goes by itself from the brain, where it begins, to the place where it ends in the body. To every separate fibre of any muscle there is probably one of these tubes. This is the telegraphic wire by which the fibre is told by the mind in the brain to act. As each fibre receives a message by itself, whenever the muscle acts, what a multitude of messages are sent to the whole muscle! 10. Some of the tubes in the nerves are for sensation, and others are for transmitting the messages or impressions to the muscles. These two kinds of tubes are very commonly bound up together in the same nerve to go to any part. And yet they are entirely separate in their office. For example, in the great nerve that goes to the arm, the nervous tubes for the muscles and the tubes for sensation are bound up together. But as the nerve branches out to be distributed, the two kinds of tubes are separated. And the same tube never transmits sensation to the brain and brings back a message to a fibre or a muscle.

Of what are the nerves composed? Do the tubes in them communicate together? What two kinds of tubes are there in the nerves?

The sensation goes by one set of tubes, and the messages to the fibres of the muscles come by another set.

11. In the body and in the limbs the two kinds of tubes are bound together in the same nerves. But in the face the two kinds of tubes are in two separate sets of nerves. There we have nerves of sensation and nerves of motion separate from each other, while everywhere else they are mingled together. But where they are thus mingled, they are just as separate in their office as where they are in separate

nerves.

12. Here, in Fig. 36, is a representation of the brain and spinal marrow, with the nerves branching out from them in all directions. At a is the cerebrum, the upper large brain filling up a large part of the skull, and at b is the cerebellum, the smaller brain lying underneath the cerebrum. You see the spinal marrow extending from the brain down the back. It is very much like the brain in its structure, and should really be considered as an extension of the brain itself. You see that nerves branch out from the brain and the spinal marrow all over the body.

13. You observe that the whole of this nervous system has two exactly similar halves, just as it is with the system of bones, and the system of muscles. The cerebrum has two parts just alike, called the two hemispheres of the brain. So it is with the cerebellum. There are two sets of nerves also for each half of the body that are just alike.

In what parts of the body are these tubes mingled together in the same nerves? Where are they in separate nerves? Describe the arrangement of the nervous system represented in Fig. 36. Is the pervous system single? What are the hemispheres of the brain?