Each eye has six muscles that turn it in whatever direction you want to look. These muscles are very delicate, and for true sight they must be exactly adjusted. If the muscles on one side of the eye are stronger than those on the other side, you will be cross-eyed or wall-eyed. If one of the muscles in one eye is stronger than the corresponding muscle in the other eye, it pulls the eyeball out of place and you "see double." [Sidenote: The use of the outer ear] The portion of the ear that you see on the side of your head has as much to do with hearing as the outer rim of the horn into which you speak has to do with making a record for the phonograph. You know that the record is really made at the little end of the horn, while the big end simply collects the sound. The outside portion of the ear simply collects sounds, and the real hearing is done with the portion of the ear that is not seen. [Illustration: FIG. 87. The ear, showing the outer, middle, and the inner part.] The outer ear connects with the short tube that leads to the _drum_, which is a thin membrane that separates the middle ear from the outer ear. This drum does not have so much to do with hearing as is supposed. To have a hole in the drum does not mean that you cannot hear. [Sidenote: The middle ear] The middle ear is that portion which is just inside the drum. In this we find a chain of little bones. The one attached to the drum is shaped much like a hammer, and is called the _malleus_, which means mallet or hammer. The next one is shaped like an anvil and is called _incus_, which means anvil, and the third is called _stapes_, which means stirrup. The flat part of the stapes fits into a small opening that leads to the internal ear. [Sidenote: The inner ear] The internal ear is shaped like a snail shell. It makes several turns, which are hollow like the ends of a conch shell. In these channels the nerve that receives the impressions made by the sounds is distributed. This nerve carries the impressions to the brain. [Illustration: FIG. 88. The nervous system.] THE NERVOUS SYSTEM [Sidenote: Importance of the nervous system] This system is so important in its use and so difficult to understand in its details, that a description of it should have more space than can be given in this summary. Can you imagine having no feeling and being unable to move? This would be your condition if you had no nerves. [Sidenote: Nerves compared to telegraph system] Briefly, the nerves might be called the telegraph system of the body. There is a great central station called the brain where messages are received and sent out; there are many sub-stations that make up the spinal cord. Twelve great nerves pass directly from the brain to the body; all others pass from the spinal cord. [Sidenote: Extent of nervous system]