Final answer:
The inside of the bone is mainly composed of spongy (cancellous) bone tissue, which is lighter and less dense due to its porous structure of trabeculae with red bone marrow that produces blood cells.
Step-by-step explanation:
The inner part of the bone is primarily composed of spongy (cancellous) bone, which is much less dense than the outer layer of compact bone. This is due to its porous nature, containing a network of spaces within a lattice-like structure of trabeculae. The trabeculae are lamellae arranged as rods or plates, and red bone marrow is found in the spaces between them. Blood cells are formed within this red bone marrow, a process known as hematopoiesis. Additionally, these trabeculae are arranged along lines of stress, providing strength to the bone while allowing it to remain lighter for easier movement by muscles.
The periosteum, a tough fibrous membrane, covers and protects the outer surface of the bone, while spongy bone is found on the inside, particularly at the ends of long bones and in the interior of other bones. The intricate structure of spongy bone supports and protects the red marrow, where blood cells are produced. Spongy bone makes up about 20 percent of the bone mass, with compact bone accounting for approximately 80 percent.