<P> 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 . </P> <P> Each forelimb of the horse runs from the scapula or shoulder blade to the navicular bone . In between are the humerus (arm), radius (forearm), elbow joint, ulna (elbow), carpus (knee) bones and joint, large metacarpal (cannon), small metacarpal (splint), sesamoid, fetlock joint, first phalanx (long pastern), pastern joint, second phalanx (short pastern), coffin joint, outwardly evidenced by the coronary band, and the third phalanx (coffin or pedal) bones . Each hind limb of the horse runs from the pelvis to the navicular bone . After the pelvis come the femur (thigh), patella, stifle joint, tibia, fibula, tarsal (hock) bone and joint, large metatarsal (cannon) and small metatarsal (splint) bones . Below these, the arrangement of sesamoid and phalanx bones and joints is the same as in the forelimbs . When the horse is moving, the distal interphalangeal joint (coffin joint) has the highest amount of stresses applied to it of any joint in the body, and it can be significantly affected by trimming and shoeing techniques . Although having a small range of movement, the proximal interphalangeal joint (pastern joint) is also influential to the movement of the horse, and can change the way that various shoeing techniques affect tendons and ligaments in the legs . Due to the horse's development as a cursorial animal (one whose main form of defense is running), its bones evolved to facilitate speed in a forward direction over hard ground, without the need for grasping, lifting or swinging . The ulna shrank in size and its top portion became the point of the elbow, while the bottom fused with the radius above the radiocarpal (knee) joint, which corresponds to the wrist in humans . A similar change occurred in the fibula bone of the hind limbs . These changes were first seen in the genus Merychippus, approximately 17 million years ago . </P> <P> The anatomy of the forelegs begin at the scapula . This is the shoulder in which provides the ease of movement as it is connected to various bones surrounding it such as the cervical vertebra (a section of the spine). The next bone is the humerus which leads onto the radius below . The radius is then connected to the bones of the knee . The carpus is located at front of the knee and the pisiform is the back of the knee . Below the knee is the cannon bone which is also known as the 3rd metacarpal. 55 million years ago when the Eohippus existed, the cannon bone used to be the 3rd toe of the foot . It's fusion took place in order to increase height and power of the limb . Behind the cannon bone are the splint bones . The splint bones are also known as the 2nd and 4th metacarpal and fused 25 - 35 million years ago during the time of the Miohippus . They provide extra strength and support of the cannon bone and used to be the 2nd and 4th toes of the foot . Below the cannon bone is the fetlock joint in which lays a structure if many bones . Firstly are the sesamoid bones that act as part of the system that allows the leg to drop as pressure is applied and spring back up as pressure is released . Below this is the proximal phalanx also known as the long pastern which is followed by the middle phalanx (short pastern). Below these bones are the navicular bone and the distal phalanx . the distal phalanx can be known as the coffin bone or the pedal bone . Below the navicular bone is the navicular bursa . There are three main muscle groups of the forelimb . The triceps muscle straightens the elbow and foreleg, running from the elbow to the bottom of the shoulder blade . The muscles which extend the lower leg are called extensor muscles, while the flexion of the lower leg joints is achieved through movement of the flexor muscles . There are five main muscles and muscle groups in the hind legs . The vastus muscle flexes the hind leg and runs from stifle to hip, while the gluteal muscles, the large muscles in the hip, extend the femur . Forward motion and flexion of the hind legs is achieved through the movement of the quadriceps group of muscles on the front of the femur, while the muscles at the back of the hindquarters, called the hamstring group, provide forward motion of the body and rearward extension of the hind limbs . Extension of the hock is achieved by the Achilles tendon, located above the hock . </P> <P> There are two apparatus in the limbs of the horse - the suspensory apparatus and the stay apparatus . The fetlock joint is supported by group of lower leg ligaments, tendons and bones known as the suspensory apparatus . This apparatus carries much of the weight of the horse, both when standing and while moving, and prevents the fetlock joint from hyperextending, especially when the joint is bearing weight . During movement, the apparatus stores and releases energy in the manner of a spring: stretching while the joint is extended and contracting (and thus releasing energy) when the joint flexes . This provides a rebound effect, assisting the foot in leaving the ground . This ability to use stored energy makes horses' gaits more efficient than other large animals, including cattle . The suspensory apparatus consists of the suspensory ligament, the check ligament, the deep digital flexor tendon, the superficial flexor tendon, the common digital extensor tendon and the sesamoid bones . </P>

Where is the cannon bone located on a horse