The first modification from it is produced by the bending ventrally of the middle segments of both limbs upon the proximal segments, while the distal segment is bent in the opposite direction on the middle segment. Then the ventral surfaces of the antibrachium and crus come to look inwards, and their dorsal surfaces to look outwards. The brachium and manus, thigh and pes still have their dorsal surfaces facing upwards and their ventral surfaces facing downwards as before, and the relations of their pre-and postaxial borders remain as they were. Many Amphibians and Reptiles, such as tortoises, carry their limbs in this position. In all higher vertebrates, however, a further change takes place, each limb is rotated as a whole from its proximal end, the rotation taking place in opposite directions in the fore and hind limbs respectively. The anterior limb is rotated backwards from the shoulder, so that the brachium lies nearly parallel to the body, and the elbow points backwards, the antibrachium downwards, and the manus backwards; the pre-axial surface of the whole limb with the radius and pollex now faces outwards, and the postaxial surface with the ulna and fifth finger now faces inwards. In the Walrus and, to a certain extent, in the Sea lions the anterior limb remains throughout life in this position. The posterior limb is also rotated, but the rotation in this case takes place forwards, so that the thigh lies nearly parallel to the body, the knee-joint pointing forwards; the crus downwards and the pes forwards. The pre-axial surface of the whole limb with the tibia and hallux looks towards the middle of the body, the postaxial surface with the fibula and fifth toe looks outwards. This is the position in which the hind limb is carried in nearly all mammals. In nearly all mammals a further change takes place in the position of the anterior limb. The radius and ulna have hitherto been parallel to one another, but now the lower end of the radius, carrying with it the manus, comes to be rotated forwards round the ulna, so that the manus, as well as the pes, comes to be forwardly-directed, and its pre-axial surface faces inwards. In the majority of mammals the radius and ulna are permanently fixed in this, which is known as the =prone= position, but in man and some other mammals the manus can be pronated or turned into this position at will. When the radius and ulna are parallel throughout their whole length the manus is said to be in the =supine= position. The =extensor= side of a limb is that to which the muscles which straighten it are attached, the =flexor= side is that to which the muscles which bend it are attached. FOOTNOTES: