Final answer:
The main part or shaft of a long bone is called the diaphysis, which is a tubular shaft that contains the medullary cavity.
Step-by-step explanation:
The main part or the shaft of a long bone is referred to as the diaphysis. The diaphysis is a tubular shaft that extends between the proximal and distal ends of the bone. Encasing the medullary cavity filled with yellow marrow, the diaphysis's walls are constituted by dense, hard, and compact bone. In contrast, the ends of the long bone are termed the epiphysis (plural epiphyses), containing spongy bone and red marrow. The joining region between the diaphysis and epiphysis is known as the metaphysis, which includes the epiphyseal plate, a hallmark of growing bones. Covering the outer surface of the bone, except for areas lined with articular cartilage, is the periosteum, a fibrous membrane.