asthma, the mind directs the use of the muscles of the chest, so as to get as much air as possible into the langs. But when the asthma is gone, the mind ceases to superintend these muscles, and the breathing goes on again by their involuntary action.

39. While the muscles of respiration are both voluntary and involuntary in their action, the heart is wholly an involuntary muscle or set of muscles. No one can make his heart beat more quickly or more slowly by determining that it shall. He may do it by exercise, or by thinking of exciting subjects, but he cannot do it by any direct action of the will.

CHAPTER X.

THE EYE.

1. THE eye is one of the principal instruments that the mind uses in getting a knowledge of the world around it. Though it is a small organ, it is a very complicated set of machinery. It has many structures in it differing much from each other, as you will see as I proceed. It has six nerves, as I have before told you, and four of these are the nerves by which the mind works the muscular machinery of this organ.

2. You will see, in this chapter, that the eye is an optical instrument. It is, therefore, constructed somewhat like the optical instruments that man makes,

Is the heart in any measure a voluntary muscle? What is said of the structure of the eye? What kind of instrument is it? What instruments is it like?

but is much more perfect. For example, it is like a telescope, and it is also like a camera-obscura. And I shall show you, that in some respects it is like the instrument used in taking daguerreotypes.

3. The eye is contained in a deep bony socket, which you see in Fig. 41, on page 111. As you look at the eye you see only its front, with a portion of its sides, as it rolls in its socket. It is in shape like a globe, but it is not perfectly round. Its front part, where the clear window is, stands out a little. The eye has a complete case enclosing all its soft and delicate parts. But this case is not all alike. It has two parts which are very different from each other. There is a thick white part called the sclerotic coat, from a Greek word, meaning hard. It is what is commonly called the white of the eye. It is this coat that gives the firm feeling to the eye when you press your finger upon it. But this coat does not extend over the front of the eye. There is there, as you see, a clear transparent part of altogether a different structure. This fits into the white coat very much as a watch-glass fits into the case. It is called the cornea.

4. Looking through this window of the eye, you see a little behind it a delicate colored curtain, called the iris, with a round opening in it called the pupil. The iris is differently colored in different persons. And we call the eye blue or brown or gray, &c., according to the color of this curtain. Through the pupil we look into the very inner chamber of the eye.

What is the situation of the eye? sclerotic coat? What is the cornea? coat? What is the iris? What is the we look through the pupil?

This

What is its shape? What is the How does it fit into the sclerotic pupil of the eye? Into what do

always has a dark appearance, because, as you will soon see, this chamber has a lining of a dark color.

5. The iris makes the eye very beautiful. But it is not a mere ornament. It is an important part of the machinery of the eye. Its chief office is to regulate the quantity of light that goes into the eye. You can see how this is done. If you look into the eye of any person while you hold a light at some distance, you see that the pupil is quite wide open. Now bring the light near the eye, and you will see the pupil instantly grow smaller. This is because the iris or curtain has contracted its opening in order to keep too much light from going into the eye. In the glare of a bright sun this opening that lets in the light becomes very small; but in the dark it is very wide open, because the eye then needs all the light that it can get. 6. I will now explain the way in which the iris acts in regulating the quantity of light that enters the eye. The iris has two sets of fibres, straight and circular. These are represented in Fig. 63. In a the pupil is wide open. Here the circular fibres are relaxed, and the straight ones are contracted. In b the pupil is contracted. And here the fibres are

a

FIG. 63.

just the reverse of what they are in a-the straight ones are relaxed and the circular ones are contracted.

7. The fibres of this round curtain must be very nicely arranged, and be made to act with great exactness; for the iris is always perfectly even as it en

What is the chief use of the iris? How would you show its usel Explain the manner in which the iris acts.

larges or contracts its opening, and there are never any wrinkles in it. If any person should attempt to construct a curtain after this form, with a round opening in it, he could not in any way fix the strings by which the opening would be made smaller or larger, so as to keep the curtain always smooth and its edge always regular.

8. I will now give you a more full and particular description of the parts of the eye. In Fig. 64 you have a map of the eye. It has three coats, as

they are called.

coat, the sclerotica.

α

At a is the thick, strong, white Into it the cornea, e, the clear window of the eye, fits, as I have before told you, like a crystal of a watch in the case. These two parts, the cornea and the sclerotica, really together make one coat. Inside of the sclerotic coat is the This is of a dark color. Why

choroid coat, b.

What is said of the regularity of action of the iris? Describe the parts of the eye as represented in Fig. 64.

this is so I will tell you in another place. At c is the retina. This is very thin, and is chiefly composed of the fibres of the optic nerve, d, that enters the eye at its back part.

9. The eye has three humors, as they are called. There is the aqueous, or watery humor, f, in the front part of the eye behind the cornea. The iris, gg, is in

the midst of this humor. It divides the chamber that contains the humor into two parts. The part of the chamber which is in front of the iris is, as you see, much larger than that which is behind it. At h is the crystalline humor, or lens, as it is more often called. This lens is a substance like hard clear jelly. Then behind this, filling up all the space, i, is the vitreous, or glassy humor, which is like soft jelly. You see that it is this vitreous humor that fills up a great portion of the ball of the eye.

10. The object of all this apparatus that I have described is to have images of objects formed upon the retina, c. That these images are formed there can be proved by an experiment. Take an ox's eye, and peel off carefully the thick rind at the back part, that is, the sclerotic coat, so that very little but the retina is left. If now you hold a candle before it, you can see the image of the candle on the retina at the back part of the eye. So, if you place it in a hole in the window-shutter, the images of objects, such as houses, trees, &c., can be seen pictured on the retina.

11. This picturing upon the retina takes place with every object that we see. If we look at a single ob

What is the object of the apparatus of the eye? How can you prove that images are formed on the retina!