Forelimbs

Forelimbs are the paired front limbs of most tetrapods. They connect the trunk to the appendages that interact with the environment and are used for locomotion, grasping, manipulation, and, in some lineages, flight. In fishes, the homologous structure is the pectoral fin; in land-dwelling vertebrates these fins have evolved into rigid limbs with specialized regions.

Anatomy typically includes a proximal segment (the humerus), two parallel forearm bones (radius and ulna), a carpal set (wrist), metacarpals, and digits (phalanges). The shoulder girdle anchors the limb to the trunk, enabling a wide range of movements. Across tetrapods the exact proportions and bone counts vary, but the general plan remains recognizable as a limb built for bending and rotating at several joints.

Development and evolution: Forelimb formation begins as limb buds on the body axis, with proximal-distal patterning guided by the apical ectodermal ridge and signaling centers such as SHH and FGFs. The pentadactyl pattern is common in many lineages, though evolution has modified or reduced digits in various ways (for example, horses with a single dominant toe, birds with fused bones in the wing).

Diversity and function: Forelimbs support terrestrial locomotion in many animals and are highly specialized in others. They form wings in bats and birds, flippers in whales and seals, and dexterous grippers in primates. Muscular and skeletal adaptations reflect the environmental demands placed on each lineage, producing a wide range of shapes from robust, weight-bearing limbs to light, streamlined wings.