on, there would be no need of having the blood circulate. In some animals these changes are stopped in cold weather, and the blood stops moving. Great multitudes of such animals as frogs, bats, flies, spiders, &c., go into retirement in the autumn to sleep through the winter. Their bodies remain without change all this time. They are alive, but as still as death. The machinery of life is still, and so there is no wear and tear. There is therefore no repairing to be done, and so there is no need of having the blood circulate. And as there is no circulation there is no need of respiration.

24. When the warm weather comes, these animals wake up from this state of torpor. The blood begins to move again, and they breathe, so that the air can go into the lungs and change the blood. They begin to eat, too, so that some new blood can be made. A gentleman who was curious on this subject, kept some frogs and snakes in this torpid state in an ice-house for three years and a half, and then revived them by bringing them out into the warm a'r.

25. I have said that the useless particles that come from the wear and tear of the organs in their daily use are taken up to be carried away in the dark blood. I will now show you how this waste matter is disposed of. There are various ways of carrying it off. Some of it is discharged by the lungs. At every breath some part of the waste matter of our bodies is thrown off from the lungs in the form of carbonic acid gas, as

Why is the blood constantly in motion everywhere? What is the situation of some animals in winter? What effect does the warm weather of spring produce in them?

you learned in the chapter on Respiration. Other portions of the waste matter are removed by the skin, others by the kidneys, others by the liver, &c.

26. The waste matter is all sent off by these different organs, each doing its share, and in its own way. Observe how it is done. The waste matter is in the blood. Now as the blood goes to these organs, the lungs, the skin, the liver, the kidneys, each does its duty in cleansing the blood of the waste matter. And in doing this each organ seems to have the power of choosing just that part of the waste that it is its business to throw off. And it never makes a mistake, and throws off good particles instead of bad ones. But we know that these organs have no thought nor knowledge, and therefore cannot choose. And the perfect way in which they are made by the Builder of our bodies to do their duty is a mystery which we cannot understand. Each organ, I have said, has its own way of getting rid of the waste matter. For example, it is thrown off in the lungs in the air that we breathe out, while by the skin it is thrown off in a very different form-in the perspiration.

27. The skin is so important an organ in throwing off the waste of the body, that I will describe its structure. There are really two skins-a thick inner skin, and a thin outer one, called the scarf-skin. The coloring matter of the skin is not in the inner skin, but in the inner layers of the scarf-skin. When the skin is rubbed off, as it is expressed, it is only this

In what various ways is the waste matter of the body disposed of? Describe the manner in which it is done. What is said of the selecting power of the different organs in getting rid of the waste? Is it thrown off in the same form from the different organs?

scarf-skin that is removed, and the inner skin, called the true-skin, is laid bare. It is the scarf-skin that is raised when a blister is applied.

28. The true-skin is very sensitive, for it has a great number of nervous fibres in it. This is for the purpose of having the skin act as a sentinel for the inner organs. If the skin did not, by its nerves, warn of the approach of danger, these organs would be much oftener injured than they now are. The scarf-skin, which is not sensitive at all, makes a soft delicate covering for the inner sensitive skin. The true-skin, without this covering, would look badly, and would feel too keenly. I shall say something more about the skin as a sensitive organ in the chapter on the Nervous System.

29. The perspiration of the skin is in two different forms. When the skin feels soft, but does not look moist, there is a moisture going from it all the time in the form of vapor. This is called insensible perspiration, because we cannot see or feel it. If you place your hand, when it appears dry, upon a glass, and hold it there for some time, the glass will become moist from the insensible perspiration. The skin is sometimes covered with moisture, perhaps in drops, as, for example, after brisk exercise on a warm day. This is called sensible perspiration.

30. The perspiration is separated from the blood in the skin by innumerable little glands. The pores through which it is discharged are the outlets of the

Describe the structure of the skin. Where is the coloring matter of the skin? What is said of the sensitiveness of the skin? What is the use of the scarf-skin? What is the difference between sensible and insensible perspiration? By what is the perspiration formed? What are the pores of the skin?

tubes from these glands. There is another set of glands in the skin that separate an oily fluid from the blood. These oil-factories are most abundant wherever the skin particularly needs oiling, as at the joints, where one part of the skin rubs against another, and where the skin is exposed to the air, as in the face.

31. The skin then serves several different purposes. 1. It is a firm, but soft and beautiful covering for the body. 2. By its nerves it warns of the approach of danger. 3. It also, by its nerves, as the organ of touch, gives knowledge to the mind of objects around us. 4. It discharges, in its perspiration, much of the waste matter of the body.

32. The change that I have described in this chapter, as going on in all parts of the body, produces to a great extent the heat of the body. It does this by a kind of combustion or burning. You remember, that in the chapter on Respiration I told you that the oxygen of the air enters the blood in the lungs, and goes with it to the heart, and then is sent, with it, all over the body. Now, this oxygen, when it comes to the capillaries, is united to carbon, that is, charcoal, and makes carbonic acid gas. This gas goes to the heart with the dark blood, then is sent with it to the lungs, and there is thrown off, as I have already told you. At the same time, the oxygen also unites in the capillaries with another substance, a gas called hydrogen. It is this union of the oxygen with the carbon and the hydrogen that makes the heat of the body.

33. Now this is very much like what takes place in

What other glands are there in the skin, and where are they most numerous? Mention the four different purposes which the skin serves. Describe the process by which the heat of the body is chiefly produced?

combustion. When we burn charcoal, that is, carbon, in the air, it unites with the oxygen of the air; and it is this union that makes the heat that is given out. So too, when the chemist burns oxygen and hydrogen gas together, it is their union that produces the heat. So it is in the capillaries of the body. The union of the oxygen with the carbon and the hydrogen makes the heat of the body. There is a fire, then, we may say, in every part of the body in the fine blood-vessels, although there is no flame.

34. Carbonic acid gas is produced by the burning of charcoal in the air. So also it is produced by the fire in the capillaries. It is carried from them with the blood to the lungs, and there it is discharged through the windpipe. The windpipe may then be called the smokepipe or chimney, through which the smoke of this general combustion in the body is let off into the air. It will be interesting to notice here that the windpipe answers three different purposes. 1. It conducts fresh air to the lungs. 2. It carries off the bad air, or, as we may express it, the smoke of the fire that is in the body. 3. While carrying off the bad air it acts, when we speak or sing, as the conducting pipe of the organ of the voice in the throat from the bellows at work below, the chest.

35. Observe that in this combustion of the capillaries a part of the fuel is furnished from the waste of the body. The carbon and hydrogen that unite with the oxygen are, to a great extent, produced by this waste. As the oxygen comes in the red blood to the

Trace the resemblance between this and the process of combustion. What three purposes does the windpipe answer?