My Body — Class 4 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Science, Unit 1. Internal organs, bones and muscles, teeth and oral health, and personal safety. Taught in June.
1. Organs you cannot see
Some body parts are easy to point at — eyes, nose, ears, hands, legs. These are external organs.
But most of the work of staying alive happens out of sight. The organs that lie inside the body are called internal organs, and each one has a job.
2. The brain
The brain is the commanding centre of the body. It is protected inside the skull, and it controls thinking, memory, movement and the senses.
An adult human brain weighs about 1.36 kg.
The brain has three main parts: the forebrain, the midbrain and the hindbrain. Nerves carry messages between the brain and the rest of the body, which is why nerves are not themselves a part of the brain.
3. The lungs
The lungs are a pair of spongy, sac-like organs in the chest. They take in the oxygen your body needs and let out the carbon dioxide it must get rid of.
Air travels in through the nose, down the windpipe, and into the two lungs. Put your hands on your ribs and breathe deeply — you can feel them expand.
4. The heart
The heart is the pumping organ. It pushes blood to every part of the body, carrying oxygen and food to each one.
It lies between the lungs, almost in the centre of the chest, and it is made up of muscle.
A healthy heart beats about 72 times in a minute. You can hear it with a stethoscope, or feel it as a pulse in your wrist.
5. The stomach and the kidneys
The stomach receives the food you swallow and begins breaking it down so the body can use it. Food is turned into energy through digestion.
You have two kidneys, and they are bean shaped. They clean the blood by filtering out excess water and harmful substances, which leave the body as urine.
Kidneys have a remarkable reserve: half of a single kidney can do the work that two kidneys normally share.
6. Bones and muscles
Press your upper arm. The part that feels hard is bone. The part that feels soft is muscle.
Bones give the body its shape. Together they form a frame that lets you jump, run, sit or lie down. Muscles cover the bones and pull on them to produce movement.
Some numbers worth remembering:
| Fact | Answer |
|---|---|
| Number of muscles in the body | More than 600 |
| Longest bone | The thigh bone |
| Smallest bone | The stapes, in the ear |
| Longest muscle | In the thigh |
7. Looking after each organ
| Organ | How to care for it |
|---|---|
| Brain | Sleep for eight hours |
| Heart | Avoid fatty food |
| Stomach | Eat healthy food on time |
| Kidneys | Drink more water |
| Bones and muscles | Exercise regularly |
8. Two sets of teeth
You grow teeth twice in your life.
Milk teeth are the first set. They begin to appear at about six months of age and there are 20 of them. At the age of 6 or 7 they start to fall out.
Permanent teeth replace them. This second set is meant to last the rest of your life, so there is no third chance. An adult has 32 permanent teeth, including the four wisdom teeth that come last.
9. The four types of teeth
Teeth are not all the same shape, because they do not all do the same job.
| Type | Number | Shape | Job |
|---|---|---|---|
| Incisors | 8 | Sharp, chisel-edged, at the front | Cutting, like scissors |
| Canines | 4 | Pointed | Tearing, like a fork |
| Premolars | 8 | Flat with ridges | Crushing and tearing |
| Molars | 12 | Broad and flat, at the back | Grinding |
10. Caring for your mouth and teeth
- Brush twice a day — after getting up and before going to bed.
- Change your toothbrush once every three months.
- Rinse your mouth with water after you eat.
- Avoid sticky food, which clings to teeth and feeds the bacteria that cause decay.
Night brushing matters even more than morning brushing. While you sleep, saliva flow drops, so bacteria multiply freely on any food left behind.
It is not only how much sugar you eat but how often. Every time you eat, bacteria in the mouth make acid for a while afterwards, so frequent snacking keeps the teeth under attack all day.
In many Indian villages people traditionally chew neem twigs as a toothbrush, and researchers believe this is part of the reason for their strong, healthy teeth.
11. Good touch and bad touch
Your body belongs to you. Learning the difference between a safe touch and an unsafe one is as important as any other lesson in this book.
Good touch is a touch that feels safe and caring:
- a hug or kiss from your parents
- your father patting you on the head
- a friendly hug from family members
- shaking hands
Bad touch is any touch that is not allowed or that makes you uncomfortable:
- touching the private parts of the body
- hitting, slapping, pushing, pinching
- anything that makes you feel scared, nervous or ashamed
- dirty talk or being shown dirty pictures
My body is my own. If someone touches you in a way you do not like, three things help: say no loudly, get away, and tell a trusted adult — a parent, a teacher, or another grown-up you trust. It is never your fault, and you should keep telling until someone listens.
Be alert to tricks people may use, such as saying your mother is in hospital and they will take you to her, asking you to guide them to an address, offering gifts, sweets or money, or suggesting private games. A safe adult does not ask a child to keep secrets from their parents.
The childline helpline 1098 is free and works day and night for anyone under 18.
12. Worked examples
Example 1. Pick the odd one out: heart, legs, brain, kidney.
Solution: Legs. The other three are internal organs; legs are external.
Example 2. Which organ is the commanding centre of the body?
Solution: The brain.
Example 3. Where is the heart located?
Solution: Between the lungs, almost in the centre of the chest.
Example 4. True or false: muscles are the soft parts that cover our bones.
Solution: True.
Example 5. How many milk teeth does a child have, and when do they start to fall?
Solution: 20 milk teeth, starting to fall at the age of 6 or 7.
Example 6. Which teeth tear food, and how many are there?
Solution: The canines, and there are 4.
13. Practice
- Name any four internal organs.
- About how much does the human brain weigh?
- How many times does the heart beat in a minute?
- What shape are the kidneys, and what do they do?
- Name the longest and the smallest bone in the body.
- How many muscles does the human body have?
- How many permanent teeth does an adult have?
- Which type of tooth grinds food?
- Give two examples of a good touch and two of a bad touch.
- How often should a toothbrush be changed?
14. Answers
- Brain, lungs, heart, stomach, kidneys (any four).
- About 1.36 kg.
- About 72 times.
- Bean shaped. They purify blood by filtering out excess water and toxins.
- Longest is the thigh bone; smallest is the stapes in the ear.
- More than 600.
- 32, including four wisdom teeth.
- The molars.
- Good touch: a parent's hug, a pat on the head. Bad touch: hitting or slapping, touching private parts.
- Once every three months.
15. Summary
- External organs can be seen; internal organs work out of sight.
- The brain is the commanding centre and weighs about 1.36 kg, with forebrain, midbrain and hindbrain.
- The lungs are a pair of spongy, sac-like organs that take in oxygen.
- The heart is a muscular pump lying between the lungs and beats about 72 times a minute.
- The two bean-shaped kidneys filter excess water and toxins from the blood.
- Bones give shape and muscles move them; the body has over 600 muscles, the thigh bone is longest and the stapes smallest.
- Milk teeth number 20 and appear at six months; 32 permanent teeth replace them from age 6 or 7.
- Incisors cut, canines tear, premolars crush and molars grind.
- Brush twice daily, change the brush every three months, and rinse after eating.
- Your body is your own: say no, get away, and tell a trusted adult. Childline is 1098.
