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The femur, or thigh bone, is the most proximal (closest to the body) bone of the leg in vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in the rear legs.
In human anatomy, the femur is the longest and largest bone. Along with the temporal bone of the skull, it is one of the two strongest bones in the body. The average adult male femur is 48 centimeters (18.9 in) in length and 2.34 cm (0.92 in) in diameter and can support up to 30 times the weight of an adult. It forms part of the hip joint (at the acetabulum) and part of the knee joint, which is located above. There are four eminences, or protuberances, in the human femur: the head, the greater trochanter, the lesser trochanter, and the lower extremity. They appear at various times from just before birth to about age 14. Initially, they are joined to the main body of the femur with cartilage, which gradually becomes ossified until the protuberances become an integral part of the femur bone, usually in early adulthood.
The shaft of femur is cylindrical with a rough line on its posterior surface (linea aspera).
The intercondylar fossa is present between the condyles at the distal end of the femur. In addition to the intercondylar eminence on the tibial plateau, there is both an anterior and posterior intercondylar fossa (area), the sites of anterior cruciate and posterior cruciate ligament attachment, respectively.
In primitive tetrapods, the main points of muscle attachment along the femur are the internal trochanter and fourth trochanter, and a ridge along the ventral surface of the femoral shaft referred to as the adductor crest. The neck of the femur is generally minimal or absent in the most primitive forms, reflecting a simple attachment to the acetabulum. The greater trochanter was present in the extinct archosaurs, as well as in modern birds and mammals, being associated with the loss of the primitive sprawling gait. The lesser trochanter is a unique development of mammals, which lack both the internal and fourth trochanters. The adductor crest is often also absent in mammals, or reduced to a series of creases along the surface of the bone.
Some species of whales, snakes, and other non-walking vertebrates have vestigial femurs. One of the earliest known vertebrates to have a femur is the Eusthenopteron, a prehistoric lobe-finned fish from the Late Devonian period. In invertebrates, the name femur tite is also given to the most proximal full-length jointed segment of the legs of some arthropods such as spiders.
The word femur is Latin for thigh. Theoretically in strict usage, femur bone is more proper than femur, as in classical Latin femur means "thigh", and os femoris means "the thigh's bone".
In medical Latin its genitive is always femoris, but in classical Latin its genitive is often feminis, and should not be confused with case forms of femina, which means "woman".