Читать книгу The Skull of Quadruped and Bipedal Vertebrates - Djillali Hadjouis - Страница 16

The Family Equidae reached during its evolution the maximum of its specialization by the development of its third digit (also in the Rhinoceros and Tapirs), the second and fourth being vestigial. The last phalanx ending in a powerful hoof supporting a body with symmetrical postural balance in each leg (Figure 2.4). The cephalo-caudal axis shows a template whose withers and rump illustrate a masterful structure reminiscent of the suspension bridge (Figure 2.3). In fact, the fore and rear-end are in a front-to-rear axis at equidistant height. The spine, from the first four thoracic vertebrae to the caudal vertebrae, shows a curvature with a lower concavity. The cantilevered head is positioned high, due to a verticalized cervical spine, exceeding in height the withers by five cervical vertebrae. Because of this balance, the gravitational forces have developed the same robustness in the fore and hind limbs, including both sexes. In spite of a little apparent sexual dimorphism on the bones, only the absence of canines in the mare constitutes a difference.

Figure 2.3. Posture of the horse’s head and quadruped body. The Equidae are often used as an example to represent perfectly balanced posture and locomotion. The withers and the rump are located at the same height with a center of gravity positioned in the middle; the head is held high up by a verticalized neck that passes the withers by five cervical vertebrae (© Hadjouis and Le Bihan)

Figure 2.4. Probable hybrid calvarium of a horse (Equus sp.) and mandible from above showing the symmetry of the paired dental and cranial parts (© Hadjouis)

The wild and domestic forms, free of transport, draught and pack work, do not present any particular post-cranial asymmetries. Similarly, occlusion is perfectly balanced and only animals in captivity develop abnormalities or particular dental pathologies due to tooth wear against the bars of menageries.