it as much as is possible; that is, it must be kept from thinking. And what is true of particular parts of the machinery is true of it as a whole. When the whole body is disordered, as in fever, all the machinery must be kept as quiet as possible.

17. There is some of the machinery that never stops, either when we are sick or when we are asleep; it is the breathing and the circulating machinery. The heart is always beating, and the chest is always heaving; they never rest from their work, and they are stopped only by death.

18. In this chapter I have given you some general views of the machinery of the body. In the following chapters I shall describe particular parts of it, and shall explain to you how they operate. I shall speak of the machinery of digestion, of circulation, of respiration, the nervous machinery, &c., each of them separately.

CHAPTER II.

THE DIFFERENT STRUCTURES OF THE BODY.

1. BEFORE considering each subject particularly let us look in this chapter at some of the various things or structures that make up the machinery of the body. By doing this, these subjects will be more clear to you. For, as I shall mention different parts of the body, as I proceed, you will understand me better, if you have some knowledge of these parts at the beginning.

What parts of the machinery of the body never rest?

2. Notice first, the hard bones which are the framework of the body. These are very different in their shapes in different parts of the frame. For example, in the leg and arm they are long and slender, while in the head they make a box to hold the brain. They vary much in size also.

3. The bones are composed partly of mineral and partly of animal substance. When you see a pile of bones near a slaughter-house, which have been a long time exposed to the air, you see only the mineral part of them. The animal or soft part has been taken away by the heat of the sun and the washing of the rain. The same thing can be done, very quickly, by exposing a bone to a very hot fire. deprived of its animal part is very brittle, and breaks easily.

4. The animal part of a bone can be obtained also separate from the mineral part. This can be done by putting it into a mixture of an acid, called muriatic acid, and water. The acid takes the mineral part away, and leaves the animal part in perfect shape. While the mineral part is brittle, this soft animal part can be bent so as

A bone thus

FIG. 1.

What is said of the shapes of the bones, and of their size? Of what two parts is bone composed? separate from the other part? by itself?

How can the mineral part be obtained

How can the animal part be obtained

to be tied into a knot, if the bone be one of the long ones. Fig. 1 represents a thigh-bone thus tied, after being deprived of its mineral part.

5. In the child, when the bones are growing, they do not have as much of the mineral part as the bones of old persons do. It is well that they do not; for if they did, the frequent falls of the child would often give him a broken bone. If an old person should have as many falls as children commonly do, his brittle bones would very often snap asunder. A fall down stairs, which in the child is generally followed only by a momentary fright, a short crying-spell, and perhaps a bruise, is apt to break some bone in the old, and may even destroy life.

6. The bones are bound together by firm ligaments, so that while they move on each other at the joints, they are held in their places. The bones are moved by muscles. The muscles make up the bulk of the fleshy part of the body. Their color is red. The tendons are white and shining cords, by which the muscles pull the bones, in moving them.

7. As I must refer occasionally to the action of muscles before I come to the chapter on the muscles, I will explain to you now the manner in which they act. A muscle is composed of a great number of very small fibres or threads. When it acts, each one of these fibres shortens itself.

What is the difference between the young in regard to these two parts? if there were not this difference? together? By what are they moved? is a muscle composed of?

bones of the old and those of the
What would happen to the child
By what are the bones bound
What are the tendons? What

a

8. I will show how this shortening of the fibres moves the bones, by means of some figures. Suppose a, in Fig. 2, is a bone that is fixed so that it cannot be moved, and that b can be moved. Let с be a fibre that extends from the one bone to the other. If the fibre c shorten itself, it will draw the bone b towards a, as represented in the lower figure. The same thing is true of a number of fibres, as represented in Fig. 3. You see, then, how it is true of a multitude of fibres, as they are bound together in a muscle.

9. Let d, in Fig. 4, represent a bone that is fixed, and e a bone. that moves on d, with a hinge-like joint. If the fibres are relaxed, the bone e will be as in Fig. 4; but if the fibres contract, the bone will be as in Fig. 5. These two

figures show the action of the lower jaw, as it is moved up and down by the muscles, in eating. In Fig. 4, e is like the lower jaw when it is down; and in Fig. 5 it is like it when it is up, so that its teeth press against those of the fixed upper jaw.

Explain by figures 2 and 3, how the fibres of a muscle act. Illus trate the manner in which the muscles move the lower jaw in eating.

FIGS. 6, 7.

α

10. Figs. 6 and 7 show you how the muscle that bends the elbow acts. In Fig. 6 the bones a and b are represented as they are when the arm is extended out straight. The muscle which is represented by the line c, is relaxed. When it contracts or shortens itself, the bone 6 will be bent upon a, as seen in Fig. 7.

a

11. These two examples of muscular action will be sufficient for the present. In some of the succeeding chapters you will see examples of other ways in which the muscles operate; and, in the chapter on the muscles, the many various ways in which they act will be fully illustrated.

12. I have thus spoken of the bones with their liguments, and the muscles with their tendons. The limbs of the body are made up of these four structures. They compose also all the outer parts or walls of the trunk of the body and of the head. Within these walls are the three great cavities of the body, containing its most important organs.

13. In the cavity of the head is the brain. This delicate and soft organ is shut in very securely by that round box of bones, called the cranium or skull. The cavity of the chest contains the heart and the lungs. The walls of this cavity are the spinal column, or back-bone, as it called, the ribs, and the breast

Illustrate the manner in which the muscles bend the arm at the elbow. Of what four structures are the limbs of the body composed! What other parts d› they compose?