The two bones below the humerus are called radius-ulna. Of these, the outer bone is called the radius and the inner bone is called the ulna. The radius and ulna are connected by a membrane called the interosseous membrane. The upper part of both bones is covered by the annular membrane and the lower part by the antibrachial sheath. The radius consists of the head, neck and lobes at the top and the styloid process at the bottom. Again, the upper part of the ulna consists of the coronoid process, olecranon process, trochlear notch and tuberosity and the lower part the styloid process.