tion and for motion, as you learned in § 10 and § 11, but there are also different nerves for different kinds of sensation. Thus, the nerve that informs the mind of a tickling or a pain in the nose, is not the same nerve that informs the mind of the odors that you smell. The snuff-taker feels the tingling of the snuff with one nerve, and smells it with another. So, too, in the eye, the nerve with which the mind feels a pain there is not the same nerve with which it sees.

23. There are also about the face different nerves for different kinds of motions. Thus, the nerve through which the lower jaw is moved, in eating, is not the same nerve by which the mind works the muscles of the jaw in laughing and in speaking.

24. There is no organ that has so many different nerves as the eye. It has two different nerves for sensation and four for its various motions. Its machinery of nerves and muscles is therefore very complicated.

25. All parts of the body are not equally supplied with nerves. Some parts are very scantily supplied, and therefore have but little feeling, as it is expressed. There are few nerves in the bones, and so when a limb is cut off, the sawing of the bone occasions no pain. There is not much feeling in the muscles, for although they are well supplied with nerves, the tubes in these nerves are the tubes for motion mostly, and few of them are for sensation. The skin has a full

What is said in §22 of the different kinds of nerves? What is said of the nerves of motion in the face? What organ has more nerves than any other? What is said of different parts of the body in regard to their supply of nerves?

supply of the nerves of sensation. In cutting off a limb, therefore, the chief suffering is in dividing the skin.

26. The skin is fully supplied with nerves for two purposes: 1, that it may act as the organ of the sense of touch, and 2, that it may warn of danger. It is, as I have said in another place, a sentinel to guard the organs inside against injury. It feels the least touch. Its nerves at once send their warning of danger to the mind, and the mind sends its orders for action to the muscles, so that the danger may be retreated from. And as the skin stands guard so faithfully, there is no need that the muscles and bones and other internal parts should be very sensitive.

27. There is one office of the nerves that I have not mentioned. The different organs of the body sympathize with each other, and it is through the nerves that they do this. Thus, when you have a headache from a disordered stomach, it is because the brain sympathizes with the stomach. When tears flow in grief, it is because the tear-glands are excited to unusual action through the nerves. The sympathy in this case is with the brain. The sorrowful mind, by its thoughts, affects the brain. Then the tear-glands, by means of the nerves which go from the brain to them, sympathize with it, and so they make and pour forth a flood of tears.

28. There are many actions in the body that result from the connection of different parts by the nerves.

For what two purposes is the skin fully supplied with nerves? Why is there no need that the muscles and bones should be very sensitive? By what is the sympathy of the different organs of the body maintained?

Thus, when there is something in some of the pipes of the lungs, causing an irritation there, we cough in order to throw it off through the windpipe. But the muscles that perform the coughing motion are not in the pipes. They are outside of the lungs in the framework of the chest, and remove the irritating substance by forcing the air out against it. Now the reason that these muscles are excited to this action is that they are connected by nerves with the pipes where the irritation is felt. So it is in sneezing. If something irritate the lining membrane of the nose, the muscles of the chest throw the air from the lungs with great force up through the nose to expel the offending substance. They could not act in this way if they were not connected by nerves with the lining membrane of the nostrils. A message, as we may say, is sent from the irritated spot down to the muscles of the chest through the nerves, telling them to send up a blast of air to expel the intruder.

29. From what I have told you in this chapter in regard to the nerves, you can see that the nervous system is a very complicated system. I have spoken only of those things in it which you can easily understand. In my larger work on Physiology I go into this subject much more extensively, and what you have learned in this book will prepare you to understand fully what is contained in that.

Upon what do many actions in the body depend? Illustrate by the acts of coughing and sneezing.

CHAPTER VIII.

THE BONES.

1. THE bones of the body serve many different purposes. The three principal of these I will notice. 1. They form the solid framework of the body. In this respect they are to the body what timbers are to a building. 2. Some of the principal bones also form cavities in which important organs are securely inclosed. Thus, the soft delicate brain is made very secure by being inclosed in that round box of bones called the skull or cranium. So, also, the lungs and the heart, as you saw in the chapter on Respiration, are very carefully shut up in a barrel-shaped frame of bones, united firmly by ligaments and muscles. 3. The bones serve another important purpose in being moved by muscles. When they move upon each other at the joints it is the muscles that make them move. The bones are, therefore, a part of the machinery that the mind moves by means of the nervous system. This use of the bones shows you why I introduce this subject here.

I have told you in Chapter II, §3, of what two substances the bones are composed, and shall, therefore, say nothing here on that subject.

2. The bones in our bodies are covered up from view by the ligaments, muscles, tendons, and the skin. But this is not so with the skeletons of all animals. Some have their skeletons on the outside of the body.

What are the three principal purposes that the bones answer? Of what two kinds of substance are the bones composed?

This is the case, for example, with turtles, crabs and lobsters. With them the skeleton is a coat of mail to defend the soft parts from injury.

3. The bones, although they are so hard, grow together with all the soft parts that surround them. Thus, when the arm of a child grows to be the stout arm of a man, the bones enlarge equally with the muscles, tendons, &c. They enlarge, just as these other parts do, from particles added by the formative vessels from the blood; for, although they are so hard, the blood circulates in them.

4. The teeth, which are so much like the bones, differ from them in regard to growth. When a tooth first shoots up out of the gum, its body is as large as it ever will be. It cannot grow larger, as the bones do, because the hard enamel can have no circulation in it. If the teeth could grow larger there would be no need of having a second set to take the place of the first. There would then need to be simply an addition of more teeth as the jaw enlarged. But as it is now, all the first set are removed, because they would be too small for the large jaw of the adult, and thirty-two large teeth take the place of the twenty small ones of the first set.

5. The bones are not perfectly solid. They would be too heavy if they were so. Some parts of them are made up of cells, as the large ends of the long bones. The shafts of these bones are hollow. This is for the purpose of making them strong, and at

By what are the bones in our bodies covered? Are they covered up in all animals? What is said of the growth of bones? How do teeth differ from bones? Why are there two sets of teeth?