walls around it. Then, too, the eye has a cushion of fat, and does not lie against the hard bone of the socket. Now, if the eye sees a blow coming, the muscle that makes the motion of winking shuts the lids, and pushes the eye back against this soft cushion. This, of course, not only covers it up, but sinks it deeper between the parapets of bone, and so puts it more out of the reach of the blow.

25. The eyelashes serve to keep light things, flying in the air, from entering the eye. The muscle that so quickly shuts the eye-lids, however, does more at this business of keeping out intruders. The eyebrows, besides being an ornament, are of some use as a protection. If they were not there, the perspiration on the forehead would continually run down into the eye, and would irritate and inflame it. The eyebrows are the eaves of the roof of the eyes' habitation, and the perspiration drops from them upon the cheek below.

J

a

Fig. 68.

Does the eye lie directly against the bone of its socket? What is done when the eye sees a blow coming? How is the eye protected by the eye-lashes? How by the muscle of the eye-lids? How by the eye-brows?

sented at a, in Fig. 68. The tears are carried from this factory by little ducts, as seen at b, and are poured over the surface of the eye. They serve to keep the eye moist, so that it can be moved about in its socket easily by the muscles. They also serve to wash out substances that get into the eye, and when they are needed for this purpose the tear-gland makes them abundantly. Fishes have no tear-gland, for the water in which they live answers the purpose of tears in their case. Neither have they any eye-lids, as they are not exposed to dust, or motes, or flying insects, as animals that live in the air are. For the purpose of moistening the eye, the tears come from the gland in a small amount all the time. Of course, there must be some contrivance for the passing off of the tears, or they would continually run over the edges of the lids. The contrivance is this. If you will look at the edges of the eye-lids, you will see on each, near the end toward the nose, a very little opening. Into these openings, seen at c c, in the Figure, the tears go, and pass through two ducts which unite in one, de. This ends in the nose. This sink-drain of the eye, as we may call it, is continually emptying its contents

there.

27. Sometimes this drain gets stopped up, and then the tears overflow their banks, the lids, and run down the cheeks. When one weeps, the tear factory makes

Describe the tear apparatus? What two purposes do the tears serve? Why have fishes no tear-glands and no eye-lids? Describe the drain by which the tears are carried off. From what causes may the tears overflow the lids?

FIG. 69.

b

tears so fast that the drain cannot take them all away, and there is an overflow. There is a curious contrivance for carrying off the tears when the eyes are closed in sleep. The lids close in such a way as to leave a three-cornered canal between them and the surface of the eyeball, as represented in Fig. 69. In this diagram the line b is the surface of the front of the eye, and a points to the edges of the two lids. The little open space which you see shows you the form of the canal. It is through this canal that the tears flow, all the time that we are asleep, to the openings that lead into the sink-drain.

28. There is still another contrivance, in regard to the tears, which I will notice. Along on the edge of each lid, among the roots of the eye-lashes, are some little glands that secrete an oily substance. This, besides oiling the eyelashes, serves to keep the tears in the eye. It makes an oily line all along the edge of the lid; and, as water does not mix with oil, the tears will not pass over this line unless they are more abundant than usual. If it were not for this simple but effectual contrivance, the tears would be constantly diffused over the edges of the lids, and the lids would therefore be all the time wet. This would certainly be the case with the lower ones.

29. Such is the wonderful apparatus of the human eye. It would be interesting now to show you how

What contrivance is there for carrying off the tears during sleep? Mention the contrivance for keeping the tears from moistening the outside of the eyelids.

the eyes of different kinds of animals vary from the human eye in their arrangements. I will speak, however, only of the compound eyes found in insects. They are made up of many eyes. Thus in the two eyes of the common fly there are eight thousand little eyes, as the microscope shows us. In some insects they amount to twenty thousand. Each of these is a tube, at the bottom of which an image can be formed, just as you have seen that there is on the retina of the human eye. Each of these eight thousand eyes in the fly sees perfectly of itself, having its own nerve of sight. The fly therefore can see in various directions, without turning its head, and it sometimes uses one part of this extensive optical apparatus and sometimes another, according to the direction in which it wishes to look or the number of things it wishes

to see.

30. It has been found by the microscope that the little eye-tubes, of which the eyes of insects are made, are not always of the same shape. In some they are hexagonal or six-sided. This is the case with the eye of the yellow beetle, or May-bug. A magnified view of a small portion of the surface of this insect's eye is given in Fig. 70. In some butter

flies the little eyes are of a square shape, as represented in Fig. 71. Why there should be this difference in shape we know not. These compound eyes of insects are

FIG. 70.

What is said of the eyes of insects? How many eyes has the common fly? How many have some other insects? Are the eyes of insects all shaped alike?

us.

FIG. 71.

among the most wonderful things that the microscope has revealed to We admire the skill and power of the Creator as we look at the construction of the human eye; but His skill and power appear vastly more wonderful, when we think of the eye of a mere common insect, as made up of thousands of optical instruments, each, though so minute, being more perfect than any instruments that man can make.

CHAPTER XI.

THE EAR.

1. THE mind acquires the knowledge of sounds by the apparatus of hearing. This apparatus is very complicated, and some of it is exceedingly delicate. Before describing it I will say something of sound, in order that you may better understand the operation of this apparatus.

2. Sound is caused by a vibration or shaking of some substance. You can perceive this vibration in a bell if you touch it after it has been struck. If the bell is quite large you can see as well as feel the vibration. You can see it in the string of a piano or a violin. It is the vibration of the cords in the larynx that produces the sound of the voice. It is not solid bodies alone that produce sound by their vibration.

Why are the compound eyes of insects more wonderful than the human eye? How is sound produced? Give examples of sound made by the vibration of air?