Limb (anatomy) facts for kids

A limb is a body part that sticks out from the main body of an animal. Think of your arms and legs – those are limbs! Limbs are usually jointed, meaning they have parts that can bend, like your elbows and knees. They also have muscles that help them move.

Animals use their limbs for many things. They help them move around, like walking, running, or climbing. Limbs also help animals interact with the world, such as picking up objects or holding onto things.

In humans, our upper limbs are our arms, forearms, and hands. Our lower limbs are our thighs, legs, and feet. Our legs and feet are great for walking and standing on two feet. Our arms are very flexible and can grasp things and do detailed tasks, like writing or drawing. Most animals have limbs attached to their main body by strong bone structures called girdles.

Different Kinds of Limbs

Animals have many different types of limbs, depending on how they live and what they need to do.

Tetrapod Limbs

Tetrapod limbs are found in animals with four limbs, like humans, dogs, and horses. These animals are called "tetrapods" because "tetra" means four. They have two pairs of limbs:

• The front pair, closer to the head, are called forelimbs or front legs.
• The back pair, closer to the tail, are called hindlimbs or back legs.

These limbs are usually arranged in a balanced way on both sides of the body.

Arthropod Legs

Arthropod legs belong to creatures like insects, spiders, and crabs. These legs are different because they have a hard outer shell, like armor. They are also jointed, allowing arthropods to walk, run, climb, or jump.

Bird Limbs

Birds have two lower limbs, which are their legs. These legs help them walk and perch. Instead of upper limbs like arms, birds have two wings. Their wings are specially designed for flying. Birds also have a strong, keeled chest bone (called a sternum) that helps support their powerful flight muscles.

Prehensile Limbs

Some animals, like monkeys and certain reptiles, have prehensile limbs. "Prehensile" means they can grasp and hold onto objects. These special limbs can be either front or back limbs. They are very useful for climbing trees or picking up food.

Fins

Some water animals, like certain fish, have fins instead of limbs. Fins are flat body parts that help these animals swim through water. They are a key part of how these creatures move.

Tail Limbs

A few animals, like kangaroos (which are called macropods), use their tails as an extra limb! Their strong tails help them balance and even push off the ground when they hop.

How Limbs Develop

The way limbs grow and form in an animal is a fascinating process. It's controlled by special instructions in their genes, called Hox genes. All animals with jaws, from fish to humans, develop their limbs in a similar way.

Limbs grow from the part closest to the body (proximal) outwards to the tips (distal). At the very end of the growing limb, there's a special area called the apical ectodermal ridge (AER). This area helps the bones and other parts of the limb form. Another important spot, called the Zone of Polarizing Activity (ZPA), helps make sure that fingers and toes develop correctly.