According to Arizona State University (ASU), "spongy" bone is not dense because it contains holes or "pores." These open areas contain blood marrow, nerves, and blood vessels that transport cells and nutrients. Although the appearance of these bones resembles a sponge, they are actually very hard.
ASU explains that most spongy bone is located at joints and at the ends of bones. These regions account for about 20 percent of a person's bones. The rest of the bones are "compact" and are extremely dense and hard. Nerves and blood vessels go through these bones too, but the openings, referred to as "canals," are much smaller than those found in spongy bones.