CRYSTALLINE LENS.

189

most transparent substance, called the crystaliine lens, is placed, to answer the same purpose. It converges the rays of light which pass through it from external objects, and transmits them, so as to form an image on something extended behind it, at a proper distance to receive the impression. That something in the camera is the mirror I have shown you: that something in the eye is the retina, which I am now about to describe.

I dare say you have often heard persons speak of their nerves, and of other people being nervous. Indeed, you have very probably heard these words employed in so vague a manner that you have no distinct meaning attached to them. This confusion may very naturally arise from the office which the nerves fulfil in the human frame: they are, while life continues, the bond of union between the body and the mind; and are spoken of, in conversation, as sometimes belonging to the body, and forming part of its structure-sometimes as if they had

Blumenbach, ib. 195.

190

DISTRIBUTION OF THE NERVES.

reference only to the feelings of the mind. Perhaps the best way for you to have a distinct idea of the nerves, will be to compare them with another part of our frame, of which you all have some notion. I mean the blood-vessels, which proceed from the heart, like branches issuing from the trunk of a tree: these branches divide into slender sprays, or twigs; and, after distributing themselves over the body, return to the heart again. As the blood-vessels issue from the heart, so the nerves spring from the brain and spinal marrow, and spread, in minute ramifications, or branches, through the whole body. These are called nerves of motion, and correspond to the arteries issuing from the heart. If you desire to lift a book from the table, the impulse is conveyed from the brain, by the nerves of motion, to your hand. If you cut your finger, the nerves of sensation carry notice of the injury to the brain, and there excite a painful feeling. The nerves of sensation may be compared to the veins, which carry back the blood to the heart. So it goes on continually.

As there is a perpetual circulation of the blood, so there is a perpetual circulation of nervous influence. It is by means of these feelers, called the nerves, that we perceive what is going on in the world around us. But it is not my intention to speak further of them at present, than is necessary to give you some notion of the structure of the eye.

It is sufficient for that purpose if you think of the nerves as cords, more or less white and soft, through which impulses are conveyed to the different parts of the body, and sensations are brought back to the brain. One pair of these cords are called the optic nerves, and the globes of the eyes are fixed to them as an apple to its stalk; but with this difference, when the optic nerve has entered the dark chamber of the eye, it does not terminate like a stalk, but spreads into a membrane, or net-work, of most exquisite delicacy, which lines the back part of the eye, extending itself over the inner surface of the choroid, and forming the third coat of the eye.*

* Blumenbach, 192, 195, 247.

It is represented in the section by the innermost of these three lines, and spreads, you see, from the optic nerve at O.

It is on the fine net-work of the retina that the image of the object, to which the eye happens to be directed, is impressed: it corresponds with the mirror in the camera-obscura. Some have thought that the choroid, with its coat of black paint, is useful in rendering the image more distinct, as the silvering on the back of a mirror increases its power of reflection.* Others are of opinion that the function of the dark pigment is merely to absorb the useless light, leaving only those rays which are required to produce a distinct image.

For the image to be distinct, it is also necessary that the focus of the rays, which are refracted by passing through the crystalline lens, should fall exactly on the retina. When the lens becomes flattened by age, the focus is thrown farther back, beyond the retina; and wearing spectacles, with convex lenses, will as

Chambers.

FORMATION OF THE IMAGE.

193

sist the eye, and bring the focus, where it ought to be, on the retina.*

I think you will now be able to understand, on looking at this section of the eye, how the image of the cross, placed before it, is reflected on the

retina.

The dotted lines represent rays proceeding from the top and the bottom of the cross to the crystalline lens marked D: (of course, other rays proceed from every intermediate part of the object; but we notice only the upper and lower rays, to avoid confusion.) You see they converge in the centre of the lens, and crossing one another, they pass out of the lens diverging, and continue to diverge, till they reach the retina, where the image is formed. You perceive that, owing to the crossing of the rays, that which proceeds from the lower part of the object falls on the upper part of the image, and there producing the reflection of the part from which it came, the image on the retina appears inverted. You may very naturally suppose

*Joyce, v. 163.

K