The eye; that wonderful organ whose healthful function enables us to see surrounding objects, and thus avoid many injuries, as well as experience many joys. The eye is of a globular form, composed of a number of humors, which are covered by membranes, and inclosed in several coats. On the front surface there is a slight depression, and in this is situated the crystalline lens. This is a body of considerable thickness and strength, and has the form of a double-convex lens. It is placed in a perpendicular direction immediately behind the pupil, and is kept in its situation by a membrane which is called its capsule. ^{1The flattening of this lens by old age or other causes, produces the defect in vision known as long-sightedness. When more than ordinarily convex, it causes near-sightedness.} In front of the crystalline lens, and occupying the whole of the front part of the eye, is the aqueous humor. It is composed principally of water, with a few saline particles, and a very small portion of albumen. A curtain with an opening in its center floats in the aqueous humor, but is attached to one of the coats of the eye at its circumference. This curtain is called the iris, and the opening in it is the pupil. It derives its name from the various colors it has in different individuals, and it is the color of the iris that determines the color of the eye. All the light admitted into the eye passes through the pupil, which is dilated or contracted according to the intensity of the light and power of the eye. The eye has three coats. The outer or sclerotic coat-the white of the eye-is that to which the muscles that move the eye in various ways are attached. Within the sclerotic coat is the choroid coat, composed mostly of blood-vessels and nerves. The inner coat is called the retina. It is either an expansion of the optic nerve, or composed of nervous filaments attached to it.HBH 126.4

288. How is the eye and its various parts moistened?HBH 127.1

There are what are called lachrymal glands, which constantly supply the eyes with moisture, not only when they are open and in action, but also when closed and quiet in sleep. There are two small openings from the eyes into the nose, through which the fluid secreted by the lachrymal glands is conveyed. When these glands are much excited by irritations of the eyes or nose, or by strong emotion of the mind, they pour their fluid into the eyes faster than the small nasal ducts can convey it into the nose, and it flows down the cheeks in tears.HBH 127.2

289. What is the medium of sight to the eye, and how is sight effected?HBH 128.1

Light is the medium of vision, and the light conveys the impression of the object to the retina. A good illustration of the action of the eye may be made by cutting a hole in a window shutter large enough to receive a spectacle glass, excluding all light from the room except what comes through the hole. If the sun is shining brightly upon the shutter the rays of light will be seen in the room drawing together till they come to a focal point, and then the rays pass on diverging from one another, but the angle will be alike both sides of the focal point. At this focal point all the rays coming through the glass cross each other, so that the top rays at the glass are the bottom ones beyond the point. If a sheet of white paper be placed a little beyond the focal point, a beautiful miniature image will appear upon it of whatever the rays of light may come from, but this image will be upside down, and turned side for side, caused by the crossing of the rays of light. If instead of a spectacle glass, a small glass globe filled with water be placed in the hole in the window shutter, the rays will cross and diverge before they get through it, and the image will be thrown upon the back part of the globe. The interior of the eye is represented by the darkened room; the cornea by the transparent window glass; the iris by the shutter; the pupil by the hole through which the rays of light enter; the aqueous, crystalline, and vitreous humors, constitute a lens of so great a convexity, that the rays cross and diverge before they get through the globe, and throw their inverted image upon the retina. Here the mind perceives it, and by usage, views the object as though it were right side up.HBH 128.2

290. What care is necessary in relation to the eye?HBH 129.1

Reading early in the morning, before the just-wakened eyes are accustomed to the light, reading by twilight or lamp-light, are all injurious to the eyes. Never read facing the light of the lamp or window, but let the light shine over your shoulder upon your book. Those who read much should spend considerable time out-of-doors, looking at distant objects. Close application to reading tends to shorten the sight and weaken the eye. It is supposed one reason city people lose their eye-sight sooner than those in the country, is because their sight is confined to objects near at hand. Another reason, however, is the impurity of the air, it being infected with smoke, coal-dust, etc. Smoke of any kind, especially tobacco smoke, is very injurious to the eyes. But it must ever be remembered that all organs of the body sympathize with each other and are affected by diseases of each other; so to cure diseases of the eyes there must be care given to the vital interests of the whole domain of organic life.HBH 129.2

ORGAN OF SMELL

THE ORGAN OF HEARING: THE EAR