CHAPTER XXV.

HOW WE HEAR, SEE, SMELL, TASTE, AND FEEL.

1.  The Senses.—­We have five senses—­hearing, seeing, smelling, tasting, and feeling.  These are called special senses because they are very different from each other.  They also differ from the general sense of feeling by means of which we feel pain when any part is hurt.

2.  Organs of the Special Senses.—­Each of the special senses has a special set of nerves and also special cells in the brain which have charge of them.  We say that we see with our eyes, hear with our ears, feel with our fingers, etc.; but, really, we see, hear, taste, and smell in the brain just as we feel in the brain.  The eyes, ears, nose, and other organs of the special senses are the instruments by means of which the brain sees, hears, smells, etc.

3.  Sound and the Vibrations which it Causes.—­All sounds are made by jars or vibrations of objects.  Sounds cause objects to vibrate or tremble.  A loud sound sometimes jars a whole house, while other sounds are so gentle and soft that we cannot feel them in the same way that we feel loud sounds.  But Nature has made for us an ingenious organ by means of which we can feel these very fine vibrations as well as loud ones.  We call this organ the ear.

4.  The Ear.—­The part of the ear which we can see is shaped somewhat like a trumpet.  The small opening near the middle of the ear leads into a canal or tube which extends into the head about an inch.  At the inner end there is a curious little chamber.  This is called the drum of the ear, because between it and the canal of the ear there is stretched a thin membrane like the head of a drum.  The ear-drum is also called the middle ear.

[Illustration:  THE EAR.]

5.  Bones of the Ear.—­Within the drum of the ear there are three curious little bones which are joined together so as to make a complete chain, reaching from the drum-head to the other side of the drum.  The last bone fits into a little hole which leads into another curious chamber.  This chamber, which is called the inner ear, is filled with fluid, and in this fluid the nerve of hearing is spread out.  A part of the inner ear looks very much like a snail shell.

[Illustration:  THE INSIDE OF THE EAR.]

6.  How we Hear.—­Scratch with a pin upon one end of a long wooden pole.  Have some one listen with the ear placed close against the other end of the pole.  He will tell you that he hears the scratching of the pin very plainly.  This is because the scratching jars the ear and especially the drum-head, which vibrates just as the head of a drum does when it is beaten with a drum-stick.  When the drum-head vibrates it moves the bones of the ear, and these carry the vibration to the nerves of hearing in the inner chamber.  We hear all sounds in the same way, only most sounds come to the ear through the air.