Chapter One:

Sight

The eyes allow you to see what is around you. The eyes and the brain have to work together to be able to see. The eye is made up of many parts, including the cornea, iris, pupil, lens, and retina. These parts have to work together to focus on images and light. The eyes then use special nerves to send what the eye sees to the brain, so the brain can recognize and process what is being seen.

Diseases with Sight

Glaucoma is a group of eye conditions that can lead to total blindness. Myopia is when things close to you are clear, but things that are far away may be blurry. This is also called nearsightedness. Astigmatism is when your eye isn't curved like it should be. It causes images to be unclear because the light rays can't meet at a common focus. Presbyopia is when the eyes can't focus on nearby objects anymore because of age. The lens can't stretch like it used to. Usually the lens is clear. If it gets cloudly, it is called a cataract. Like looking through a cloudy window, you can't see well. Conjunctivitis is when the conjunctiva, which covers your eye, gets infected. Your eye will be pink and itchy.

Chapter Two:

Hearing

The ear is split into three parts - the outer ear, the middle ear, and the inner ear. These parts have to work together so you can hear and process the sounds around you. The outer ear, also called the pinna, picks up sound waves and they travel through the canal of the outer ear. The sound waves then move into the inner ear and hit the eardrum, which starts to vibrate. When the eardrum vibrates, it moves three small bones in the ear. These tiny bones are called the hammer (malleus), anvil (incus), and stirrup (stapes). They help sound move into the inner ear. The vibrations then move to the cochlea, which is a spiral shaped part that is filled with liquid and has tiny hairs on its surface.

The sound vibrations make the tiny hairs move. The hair sends the sound to your hearing nerve, which then sends it to your brain. This is what allows you to hear.

Diseases with Hearing

Otitis is simply an ear infection. Germs can get into your ear, causing fluid to build up. This can make your ear very sore. Have you ever felt like you were spinning? People with Meniere's disease may feel like that a lot. It makes them have episodes of vertigo (spinning) and may make them not hear as well. Otosclerosis is a disorder that makes you lose your hearing because the ear can't amplify sound, or increase the sound. Do you often have to yell for your grandparents to hear you? They may have presbycusis, which is hearing loss related to age.

Chapter Three:

Taste

Your tongue is a hard working muscle that helps you talk and eat. The front part of your tongue can move around a lot. It helps you eat by moving food around while you chew. Your tongue pushes the food to the back teeth so they can grind it up. The back of the tongue is important for eating as well. Once the food is ground up and mixed with saliva (spit), the back muscles of your tongue start to work. They move and push food down into your esophagus, which is a food tube that leads to your stomach. Look at the top of your tongue in a mirror. Notice how it's rough and bumpy, not smooth. Those bumps are called papillae. They help grip food and move it around while you chew.

They also contain your taste buds, so you can taste everything you put in your mouth. Your taste buds can understand sweet, bitter, sour, and salty flavors. Each taste bud is made up of taste cells, which have sensitive, tiny hairs called microvilli. These tiny hairs send messages to the brain, which identifies the taste for you. The nose also can help you taste things by smelling them before putting them in your mouth. The tongue and the nose work together to allow your taste buds to taste what they do.

Diseases with Taste

Imagine eating your favorite food. It tastes amazing. Now imagine that you could never taste it again. Ageusia is the loss of taste functions. People who have this can never taste their favorite food. They can't taste sweetness, sourness, saltiness, or bitterness. Hypogeusia is similar, but you can't just taste as well as you should be able to, unlike ageusia where you can't taste at all.

Chapter Four:

Smell