The other main morphological characteristic of artiodactyls is that the astragalus, one of the bones in the ankle, has upper and lower rounded articulations (areas of contact of bones) and no constricted neck, instead of simply one rounded articulation above a neck, as in other mammals.
The artiodactyl astragalus also has an articulation on its rear surface for the calcaneum (heel bone).
The three articulations are in nearly parallel planes, allowing the astragalus to rotate vertically.
Centred on the desert steppes of Central Asia and Mongolia, this floristic zone consists of 200 or more endemic genera and extends from the Caucasus to the Plateau of Tibet, with arid zone plants of the family Chenopodiaceae (goosefoot) and genera such as Salix (willow), Astragalus (milk vetch), and Picea (spruce) (Figure 1).
The articulation of one of the tarsal bones, the ankle bone (talus, or astragalus), with the fibula and tibia of the lower leg forms the actual ankle joint, although the general region is often called the ankle.
At heights of more than 6,500 feet (1,980 metres), the mountain is covered with ashes, sand, and fragments of lava and slag; there are a few scattered plants such as Astragalus aetnensis (local name: spino santo), which forms typical bushes almost 1 yard (about 0.9 metre) high, while some alpine plants manage to survive even near the top.
The navicular abuts on the flattened astragalus (or talus), the intermediate bone of the proximal row.
The articulation between the upper surface of the astragalus and the tibia is pulley-like and permits only fore-and-aft movement of the limb.
The talus (astragalus) articulates above with the bones of the lower leg to form the ankle joint.
Notable are the tubers of Gastrodia elata, the bark of the elmlike Eucommia ulmoides, the roots of Coptis chinensis (Chinese goldthread) and Astragalus membranaceus (milk vetch), and the fruit of Evodia rutaecarpa (Evodia fruit).
The lengthening of the hind limbs has been attained in part by the elongation of two bones (astragalus and calcaneum) in the foot.
