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Limbs of the horse
The limbs of the horse are structures made of dozens of bones, joints, muscles, tendons and ligaments that support the weight of the equine body. They include two apparatuses: the suspensory apparatus, which carries much of the weight, prevents overextension of the joint and absorbs shock, and the stay apparatus, which locks major joints in the limbs, allowing horses to remain standing while relaxed or asleep. The limbs play a major part in the movement of the horse, with the legs performing the functions of absorbing impact, bearing weight, and providing thrust. In general, the majority of the weight is borne by the front legs, while the rear legs provide propulsion. The hooves are also important structures, providing support, traction and shock absorption, and containing structures which provide blood flow through the lower leg. As the horse developed as a cursorial animal, with a primary defense mechanism of running over hard ground, its legs evolved to the long, sturdy, light-weight, one-toed form seen today.
Good conformation in the limbs leads to improved movement and decreased likelihood of injuries. Large differences in bone structure and size can be found in horses used for different activities, but correct conformation remains relatively similar across the spectrum. Structural defects, as well as other problems such as injuries and infections, can cause lameness, or movement at an abnormal gait. Injuries to and problems with horse legs can be relatively minor, such as stocking up, which causes swelling without lameness, or quite serious. Even non-fatal leg injuries can be fatal to horses, as their bodies are designed to bear weight on all four legs and serious problems can result if this is not possible.
==Limb anatomy==
Horses are odd-toed ungulates, or members of the order Perissodactyla. This order also includes the extant species of rhinos and tapirs, and many extinct families and species. Members of this order walk on either one toe (like horses) or three toes (like rhinos and tapirs).〔(【引用サイトリンク】title=Introduction to the Perissodactyla )〕 This is in contrast to even-toed ungulates, members of the order Artiodactyla, which walk on cloven hooves, or two toes. This order includes many species associated with livestock, such as sheep, goats, pigs, cows and camels, as well as species of giraffes, antelopes and deer.〔(【引用サイトリンク】title=Introduction to the Artiodactyla )〕
Equine hooves and legs have evolved over millions of years to the form in which they are found today. The original ancestors of horses had shorter legs, terminating in five-toed feet. Over millennia, a single hard hoof evolved from the middle toe, while the other toes gradually disappeared into the tiny vestigial remnants that are found today on the lower leg bones. Prairie-dwelling equine species developed hooves and longer legs that were both sturdy and light weight to help them evade predators and cover longer distances in search of food. Forest-dwelling species retained shorter legs and three toes, which helped them on softer ground. Approximately 35 million years ago, a global drop in temperature created a major habitat change, leading to the transition of many forests to grasslands. This led to a die-out among forest-dwelling equine species, eventually leaving the long-legged, one-toed ''Equus'' of today, which includes the horse, as the sole surviving genus of the Equidae family.〔(【引用サイトリンク】title=On Your Toes )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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