Which layer of the periosteum contains blood vessels?

The outer layer of the periosteum is the correct choice because it is rich in blood vessels and serves essential functions in supplying nutrients to the bone and facilitating healing. This vascular network plays a crucial role in bone health and metabolism. The outer layer also contains sensory nerve endings, which contributes to the pain sensation associated with bone injuries or inflammation.

In contrast, the inner layer of the periosteum, while also important, mainly consists of osteogenic cells responsible for bone growth and healing. The middle layer is not typically recognized as a distinct component of the periosteum, and the fibrous layer—though it sounds relevant—primarily refers to the dense connective tissue that contributes to this outer layer's structural integrity. Hence, the rich presence of blood vessels distinguishes the outer layer as the one specifically involved in vascular supply to the bone.