The human skeleton, and the skeletons of most of the animals we have examined, are built upon one general plan as regards the limbs. There are two pairs of limbs. In birds the upper limbs are modified to form wings, but the structure even here is essentially the same.
Let us examine the human limbs now. The arm has thirty-two distinct bones, and consists of three parts-an upper arm, a fore-arm, and a hand; the leg also consists of three corresponding parts-an upper leg (thigh), a lower leg, and a foot. It contains thirty separate bones.
The upper arm and the thigh are each formed by a single long bone. The fore-arm consists of two bones (one larger than the other), jointed at the elbow to the bone of the upper arm. The lower leg also consists of two bones (one larger than the other), jointed at the knee tothe thigh-bone. The knee-joint, however, differs from the elbow-joint in having a small bone-the kneecap-placed over the joint. There is no corresponding bone in the arm.
The hand and the foot are built on very much the same plan. The wrist of the one corresponds to the ankle of the other. The wrist has eight small bones; the ankle seven.
