The horse is a large, herbivorous ungulate mammal that evolved to travel over long distances in search of food. Its flattened table teeth allow it to chew and grind tough fibrous grass into a form that can be digested by the slow process of fermentation. The hind limbs are designed to carry its heavy body while providing balance and support for walking and galloping over rough terrain. The horse is one of the few mammals that are able to walk on its toes (plantigrade) rather than on its heel like humans or other mammals that are able to walk only on their heels or on all four feet.
The horse’s muscular system is adapted to the rapid movements it must make in order to keep up with its prey, which it is constantly chasing and eating. The horse has more muscle fibers in its forelimb than in its hind limb, which enables it to use its powerful muscles more efficiently. In addition, it has a special joint, the hyoid bone, in its neck and back that allows its long, flexible neck to be moved forward and backward by the action of muscles pulling on the bone at its base.
Compared to that of the human, the horse’s skeletal system has more bones, longer limbs and larger digits. These features are important in enabling it to move faster, leap higher and run further than any other land animal.
The structure of the hoof is similar to that of the human hand. Each digit has five metacarpals and phalanges. The phalanges are long, curved bones that form the ends of the toes. The joints in the hoof connect the phalanges to the digital cushion, a large mass of soft tissue that absorbs impact and helps to maintain the shape of the foot. The navicular bone, which is located between the second and third phalanges, reduces friction between the digital cushion and the tendon of the deep flexor muscle.
Fig. 16.8 Superficial Muscles of the Horse
Although it has been suggested that the drawing at lower right is an anatomical fantasy blending the bones of a man and those of a horse, Clayton and Philo  believe that its proportions of antebrachium to manus suggest a dog or wolf rather than a bear. They also believe that the oblique line crossing the medial face of the radius might represent the Vena cephalica, but this would run dorsomedially across the carpus and not dorsomedially along the antebrachium.
There are two drawings on the same sheet of paper in ‘Manuscrit K’, folio 109 verso and 110 recto (see Figure 9). They illustrate the comparative anatomy of the left hind limb of the horse with that of the human leg. It is possible that Leonardo intended to compare the lateral view of the horse’s pelvic limb with that of the human leg, but he did not keep his original proportions when making these sketches. He drew the femur in both drawings much too far distally, and his depiction of the rectus femoris muscle (represented here by different threads) fixes laterally at the tuber coxae, rather than at the lesser trochanter as it should be in the horse.