In both the arms and the legs, the half closest to the body has one strong, thick, long bone. The other half of the arm or leg, the part farthest from the body, has two thinner bones. There are 206 bones in the entire human body, so the skeletal structure of the legs and arms are actually quite simple. The hands and feet, though, have many little bones in them.
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The upper arm has one long bone. This is a strong bone called the humerus. At the shoulder joint, the head of the humerus is shaped like a hemisphere. This shape and the shape of the shoulder joint allows you a large range of movement of the arm. At the elbow joint, it has two protruding bumps called epicondyles, one on the inside of the elbow and one on the outside. When you "hit your funny bone," you are compressing a nerve against the end of the humerus, which gives you the jolt of pain up the arm.
The forearm has two bones in it. These are both long bones, called the radius and the ulna, which both stretch from the elbow to the hand. The radius is shorter and thicker than the ulna though. The radius is on the same side as the thumb and the ulna is on the other side. The range of movement the forearm has is due to the ability of the radius to rotate in the joint it shares with the ulna at the elbow, such as the movement you can see when you lay your forearm flat and flip your hand over and back.
The thigh, like the upper arm, only has one thick, strong long bone. It is called the femur. It has a rounded head sticking out at an angle from the majority of the long bone. This head fits into the hip and allows you a large range of movement of the leg. The end of the femur that fits into the knee joint has two epicondyles, one on each side, just like the humerus in the arm.
Compared to the arm, the leg has an extra bone. This is the patella, more commonly known as the kneecap. This small, flat bone is deep in the tendon of a big muscle covering the front of the thigh. It does not interact with the other bones of the leg, but instead helps to arrange the thigh muscle so it works most efficiently.
The calf has two long bones in it. The tibia is thicker and longer than the fibula. They are roughly side by side, with the tibia on the big toe side and the fibula on the little toe side. The thin fibula is not part of the knee joint, but rather attaches to the top of the tibia and, at the other end, to the ankle joint.