Although bones vary greatly in size and shape, they have certain structural similarities. Spongy bone forms the inner layer. Image from W.H. Freeman and Sinauer Associates, used by permission. New bone is formed by the osteocytes. The Haversian canals form a network of blood vessels and nerves that nourish and monitor the osteocytes.
Spongy bone occurs at the ends of long bones and is less dense than compact bone.
The spongy bone of the femur, humerus, and sternum contains red marrow, in which stem cells reproduce and form the cellular components of the blood and immune system . Yellow marrow, at the center of these bones, is used to store fats. The outer layer of the bones is known as the periosteum . The inner layer of the periosteum forms new bone or modifies existing bone to meet new conditions. It is rich in nerve endings and blood and lymphatic vessels.