Limbs

Limb refers to a paired appendage that extends from the body and is used to interact with the environment. In tetrapods, limbs are grouped as upper (forelimbs) and lower (hindlimbs). In humans, the upper limb includes the arm, forearm, wrist, and hand, while the lower limb includes the thigh, leg, ankle, and foot.

Anatomically, limbs consist of bones, joints, muscles, nerves, and blood vessels. The upper limb includes the shoulder girdle (clavicle and scapula) and the arm, forearm, wrist, and hand. The lower limb includes the pelvic girdle and the thigh, leg, ankle, and foot. Key joints are the shoulder, elbow, wrist, hip, knee, and ankle.

Functions include manipulation and dexterous action, locomotion, and weight bearing for balance and posture. The upper limb is optimized for precision and grasping, while the lower limb provides propulsion and stability. Limb form and function vary widely across species according to ecology and behavior.

Development and evolution: limbs arise from embryonic limb buds and share a conserved architectural plan across tetrapods. A typical five-digit pattern is common, but variation exists. Genetic signaling networks regulate growth, patterning, and differentiation, linking limb structure to function across lineages.

Clinical relevance includes injuries such as fractures, sprains, and dislocations, congenital differences like polydactyly or syndactyly, and the use of prosthetics or rehabilitation to restore function after loss or impairment.