Limb (anatomy)

A limb is a special part of the body that helps humans and many animals move and interact with the world. Limbs include arms and legs. They are made of bones, muscles, and joints that work together. The far end of a limb, called its extremity, is where hands or feet are located.

All animals with four limbs, like mammals, birds, and reptiles, have two pairs of limbs — one pair near the head (forelimbs or front legs) and one pair near the tail (hindlimbs or back legs). In humans, the forelimbs are called arms and the hindlimbs are called legs. Arms are good at reaching and grasping things, while legs are special for walking and running on two feet.

Human limbs are amazing. Our arms can move in many ways and are great for holding and using tools. The hands at the end of our arms have five fingers that can grip and manipulate small objects very well. Our legs are built for balanced walking and running, making us adaptable and clever toolmakers.

Anatomy

Limbs are parts of the body that connect to the middle section, or torso, through strong bands called girdles. There are two types of girdles: the pectoral girdle for the upper limbs, like arms, and the pelvic girdle for the lower limbs, like legs. These girdles help attach the limbs to the body.

All four-legged animals with a backbone, called tetrapods, have similar limb structures. Each limb has three main parts. The top part near the body is called the stylopodium—like the upper arm or thigh. The middle part has two bones and is called the zeugopodium—like the forearm or shin. The farthest part from the body is the autopodium, which includes the hand or foot. Hands and feet have smaller bones, and the ends of fingers or toes are often covered with hard skins like nails or claws.

Development

Main article: Limb development

Limb development is guided by special genes called Hox genes. In animals with jaws, limbs grow in a similar way. Growth starts from the top and moves toward the bottom. At the end of a growing limb, a layer of cells helps decide where bones will form. Another area helps decide how fingers and toes will develop.