mesaxonic condition

The term "mesaxonic" describes a condition in zoology where the axis of the foot is formed by the middle digit. This is a characteristic feature of the order Perissodactyla, which includes animals like horses, rhinoceroses, and tapirs. In contrast, the paraxonic condition, found in Artiodactyla (even-toed ungulates), has the axis of symmetry passing between the third and fourth toes.123

The mesaxonic condition is associated with evolutionary changes in limb structure, particularly in the Equidae family (horses), where limbs became longer and more slender with a reduction in the number of digits. The middle digit (the third toe) became the most dominant, bearing most of the animal's weight. This specialization allows for efficient, swift running.3