Limb (anatomy)

Limbs are body parts like arms and legs. They have bones and muscles. Most animals have four limbs. They use them to walk, run, or swim.

Front limbs are near the head. Back limbs are near the tail. Some animals use them to climb trees. Birds and bats use them to fly.

Humans use arms to grab things. Our hands have five fingers. We use our feet to walk. Limbs help us do many jobs.

A limb is a part of the body with muscles and joints. Most four-legged animals have four limbs. These limbs come in two pairs. One pair is near the head. We call these forelimbs. The other pair is near the tail. We call these hindlimbs.

Animals use limbs in many ways. Some use them to run or jump. Others use them to swim or climb. Some animals have flippers to move in water. Birds and bats have limbs that help them fly.

Inside the limb is a bone frame. This is called the appendicular skeleton. The limb has three main parts. The first part is near the body. It has one long bone. The next part has two bones. The last part is the extremity. This is the hand or the foot.

In humans, we call these arms and legs. Our hands have five digits called fingers. We use them to grab and move small things. Our feet help us walk on two legs. Even though our limbs are not very strong, they are very useful.

A limb is a jointed part of the body. It has muscles and is used to move. Many animals use limbs to carry weight. They also use them to walk or run on land. Limbs help animals touch or grab objects. The very end of a limb is called the extremity.

Limbs work in a very organized way. They attach to the main body at a point called a girdle. The front limbs use a pectoral girdle. The back limbs use a pelvic girdle. These girdles connect to the limbs using ball-and-socket joints. This type of joint allows for movement. The bones grow from the body outward toward the tip.

Most four-legged animals share a similar bone pattern. This pattern comes from ancestors that lived a long time ago. These ancestors were fish-like creatures called sarcopterygians. They lived during a time called the Late Devonian. These animals survived a major event called the Late Devonian extinction. This shared history explains why many different animals have similar limbs.

Each limb has three main bone sections. The first part is the stylopodium. It has one long bone like the humerus or femur. The next part is the zeugopodium. It has two bones like the radius and ulna. The last part is the autopodium, which is the hand or foot. This part includes small bones called the mesopodium.

Humans use their limbs in special ways. Our upper limbs are our arms. Our lower limbs are our legs. We walk on two legs, which is called bipedalism. Our hands have five digits called fingers. These fingers help us do fine motor skills. This means we can move small things with great care. We can even make tools with our hands.

A limb is a jointed, muscled appendage found in humans and tetrapod vertebrate animals. These structures are essential for life on land or in water. They allow animals to bear weight and perform terrestrial locomotion, which means moving across land. Limbs also enable physical interaction with the environment, such as grasping, climbing, or striking. The very end of a limb is called the extremity. All tetrapods possess an appendicular skeleton, which is the bony endoskeleton of the limbs. This skeletal structure is homologous among all tetrapods. This means they share a common evolutionary origin.

Limbs attach to the main body via specialized structures called girdles. The cranial pair of limbs, located closer to the head, are called forelimbs. These connect to the torso through the pectoral girdle, also known as the shoulder girdle. In terrestrial animals, the pectoral girdle is quite mobile. It floats over the rib cage and connects via the clavicles and muscles. The caudal pair of limbs, located closer to the tail, are called hindlimbs. These connect to the pelvis through the pelvic girdle. Unlike the shoulder, the pelvic girdle is typically an immobile ring. It is often fused to the vertebral column at the sacrum. Both types of girdles connect to the limbs using ball-and-socket synovial joints.

Each limb follows a consistent structural pattern divided into three main segments. The first segment is the proximal half, known as the stylopodium. This section contains one long bone. In the forelimb, this is the humerus. In the hindlimb, it is the femur. The middle section is the zeugopodium, which contains two long bones. In the forearm, these are the radius and ulna. In the lower leg, they are the tibia and fibula. The distalmost portion is the autopodium, which includes the hand or foot. The hand is technically called the manus, while the foot is called the pes.

The autopodium is further divided into several complex parts. The proximal region of the autopodium is the mesopodium. This area contains small, nodular bones. In the wrist, these are the carpals. In the ankle, they are the tarsals. The middle part of the autopodium is the metapodium. This consists of slender bones called metapodials. In the hand, these are the metacarpals. In the foot, they are the metatarsals. Finally, the distalmost part consists of the digits, such as fingers or toes. These digits contain multi-jointed phalanges. Many animals cover the ends of these digits with keratin, forming nails or claws.

All tetrapod limbs share a similar body plan due to their evolutionary history. They branched from a single lineage of stegocephalians. These ancestors survived the Late Devonian extinction. This shared lineage is why limbs show pentadactyly, or a five-digit pattern. This pattern is seen across many different species. Even though animals look different, their limb bones remain homologous. This means they are built from the same ancestral blueprint. This connection links fish-like sarcopterygian ancestors to modern mammals and birds.

Different environments have shaped limbs in unique ways. Arboreal species, which live in trees, often have prehensile forelimbs for grasping. Some primates can even use their hindlimbs to grasp. Birds and bats have expanded forelimbs to achieve flight. They use specialized feathers or membranes to create lift. Aquatic animals often have webbing to help with propulsion. Marine mammals and sea turtles have evolved flattened, paddle-like limbs called flippers. These are examples of convergent evolution, where different species develop similar traits for the same purpose.

Human limbs show remarkable specialization for a bipedal lifestyle. Humans walk on two legs, which is called bipedalism. Our upper limbs are called arms, and our lower limbs are called legs. In academic anatomy, "arm" specifically refers to the upper arm. The lower part is the forearm. Similarly, "leg" refers to the lower leg, while the upper part is the thigh. Human arms have a great range of motion. The hands are highly adapted for fine motor skills. This dexterity allows humans to manipulate objects with precision. While our limbs may be proportionally weaker than some mammals, our ability to make tools compensates for this.

Limb development is a highly regulated biological process. It is controlled by specific groups of genes called Hox genes. Growth occurs in a specific direction, from the proximal part toward the distal part. At the distal end, a structure called the apical ectodermal ridge, or AER, helps differentiate skeletal elements. This ridge expands in rays to guide growth. Additionally, a Zone of Polarizing Activity, or ZPA, is located at the rear of the AER. The ZPA coordinates the differentiation of the digits. This ensures that fingers and toes form in the correct positions. All jawed vertebrates surveyed so far organize their limb buds in a similar way.