CHAPTER XXII
THE SENSES, OR DOORS OF KNOWLEDGE
The Organs of Sense.—In order that our body may keep out of the way of other persons and find food and drink and do its work, the brain must have some way of receiving news about what is near us, how it looks, and of what it is made. Special organs for receiving knowledge of people and things about us are scattered over the surface of the body. They are called sense organs. The chief ones are the two eyes, the two ears, the nose, and many organs of taste in the mouth, and the thousands of tiny organs of feeling in the skin.
The Eye.—The eye consists of a globe called the eyeball and parts which move this and protect it from injury. Each eyeball is attached at its back part to the large nerve of sight ([Fig. 90]). This carries messages to the brain, telling it what the eye sees.
The eyeball is held in a socket in the front of the skull. A layer of fat lines the socket and keeps the eye from being injured by jars. The eyebrows at the lower edge of the forehead prevent the sweat from running into the eyeball.
Fig. 90 —Side of the face cut away to show the eyeball in its socket. n is the nerve of sight; the other letters show the muscles which move the eyeball.
The eyelids can close over the front of the eyeball to shut out dirt or anything else likely to hurt it. The lids have learned to do their work so well that we do not need to think to close them when anything flies toward the eye, for they are shut before we can think.
A salty fluid called tears flows from the tear gland at the upper and outer side of the eyeball. The tears keep the front of the eyeball clean.