FIRST BOOK IN PHYSIOLOGY.

CHAPTER I.

THE MACHINERY OF THE BODY.

1. WHEN you look at any machine made by man, you inquire what it is intended to do. You find, commonly, that it is for some one purpose. Thus, a nailmachine makes nails, and does nothing else; a papermachine makes paper; a locomotive draws cars on a track; and so of other machines.

2. But the human body is not a single machine for a single purpose. It is a complicated machine, and serves many purposes. It differs very much in this respect from the machines that man makes. While for example, it is a machine that walks, walking is not the only thing that it does. It is not like the locomotive, that does nothing but draw cars. It can perform a great variety of motions besides walking. It can run, jump, leap, climb, &c.

3. You see the same difference, if instead of looking at the body as a whole, you look at any particular part of it. Look, for example, at the hand, and com

What is said of machines made by man? How does the machinery of the body differ from these

pare it with the most ingenious machinery that man has ever made. The variety of things that it can do is almost endless. So, too, if you open your mouth before a looking-glass, and move about that busy little machine, the tongue, you will get some idea of the great variety of motion that it can perform.

4. But besides being a locomotive machine, capable of all this variety of motion, the human body is also a machine in which many things are made. Blood is made in it. This red fluid is made out of the food which this machine puts into its mouth and eats. And then from the blood are made all the various parts of the body.

5. In order that the blood may be used to construct all parts of the body, it must be carried everywhere. There is a wonderful set of machinery to do this. The heart is pumping night and day, sending out the blood through the pipes that branch out from it all over the body.

6. Then the blood, when it has been used, is not fit to be used again until it is changed. There is, therefore, a set of machinery in the chest for the purpose of changing the blood. The blood is carried to the lungs, and there it is exposed to the air that we breathe into the lungs every time that we draw a breath. By being aired in this way, it is fitted to be used again, and it goes back to the heart that it may be pumped out again all over the body.

7. But the most wonderful machinery in the body ntion some particular parts of the body in which this difference is What is made by the machinery of the body? By what ma

the blood circulated! How is the blood changed after it has

is that which we find in the nervous system. The brain is the great central organ of this system. From it branch out white cords, called nerves, which are found in every part of the body. This nervous system is somewhat like a telegraph, though it is much more perfect and mysterious. The brain may be considered as the central office, where the mind has its seat. The nerves may be called the wires, by which messages are sent forth and received by the mind.

8. Messages are sent by means of the nerves to the muscles, whenever the mind wills that any part of the body move. Thus, when you wish to move your hand, messages are sent from the brain to the muscles that move this part. When the mind wills that the whole body shall move, a great number of these messages are sent in all directions at once.

9. The mind too receives messages through the nerves. It receives them from the senses. When we see, something is sent by means of the nerves of the eyes to the brain, and thus reaches the mind, just as electricity goes along the wires of a telegraph. And the same may be said of the other senses.

10. Observe now how great a variety of machinery there is in the body. The digestive machinery grinds up the food with its teeth and mixes it with juices in such a way that blood is made out of it. Then the machinery of the circulation moves the blood about everywhere in the body, so that all the parts may be made out of it and be kept in repair. The breathing

What is the most wonderful machinery in the body? What is it like? And how? Describe what is done when the muscles act. How and from what does the mind receive messages? Give what is stated in 10 about the variety of the machinery in the body

machinery continually purifies the blood after it has been used, and so fits it again for use. Then by means of the nervous machinery the mind uses the parts that are thus constructed from the blood-viz., the muscles, the bones, and the organs of the senses.

11. You see that some of the machinery of the body is for the purpose of making other machinery. This is the business of the machinery of the digestion, the circulation, and the respiration. This machinery makes nerves, and muscles, and bones, and the brain, and the eye, and the other organs of the senses. The object then of eating and drinking and breathing and having the blood circulate, is to make machinery for the mind to use.

12. There is one difference between the machinery of the body and the machines constructed by man, that I have not yet mentioned. When man makes a machine he cannot use it till it is completed. If he wishes to alter it or repair it, he cannot use it at all while he is doing this. But the machinery of the body is constantly altered while it is in use. I will illustrate this difference.

13. The machinery of the child's body is small ma chinery, but every part of it gradually becomes larger, and in manhood it is of its full size. But no machine made by man can grow to be a larger one. Now, the machinery of the body not only grows, but it is kept in use while it is growing. A small telescope never grows to be a large one, but the little eye of the infant

Can the machinery that man makes be kept in use while he is altering or repairing it? How is it with the machinery of the body? What is said of the growing of the machinery of the body'

grows to be the large eye of a man, and is used every day while this is done. A cord does not grow to be a rope, but the muscles grow as we use them.

14. The machines that man constructs cannot be repaired while they are in use; they have to lie by for repair, as it is expressed. It is not so with the machinery of the body; repairing is going on while it is in use. In the machinery made by man it is done only now and then, but in the machinery of the body it is done all the time, every day, every hour, every

moment.

15. One thing is to be noticed, however, about this repairing of the body. Some of its machinery must have seasons of rest, in order that the repairing may be thoroughly done. This is the case with the brain, the nerves, and the muscles. When the mind has worked these parts of the machinery during the day, the rest of night is needed to repair fully the wear and tear. Though the business of repairing them is going on all the time, more of it is done while they are at rest in the hours of sleep than when we are awake.

16. Another thing to be remarked is, that when the machinery is much deranged by disease, more rest than is commonly taken at night is needed. There must be some lying by for repair now. Thus, if a limb be inflamed, it must be kept still. An inflamed eye needs to have the light shut out from it. If the brain be diseased, the mind must be kept from using

Does the machinery of the body lie by for repair? What parts of the machinery of the body must have seasons of rest to have the repairing well done? What is said of the need of rest for repairing in disease?