The outer layer of the bones is a dense tissue and does not contain large cavities and blood vessels. This layer is called compact bone. It contains a great number of tiny cavities that can only be seen with a microscope. This kind of bone is highly resistant to tension, torsion, and mechanical shocks.
The surfaces of the long bones that participate in mobile joints is covered by a layer of hyaline cartilage placed over a layer of compact bone. It is called articular cartilage.
The extremities of most long bones are dilated and called epiphysis. The midsection of long bones, between the two epiphysis is a hollow cilinder – the diaphysis .It is also called medullary canal because it contains the bone marrow besides besides other tissues and structures such as blood vessels and adipose tissue.
The interior of the epiphyses is made up of milseveral structures and tisssues such as millions of tiny interconnected plates of bone tissue called bone trabeculae. These three-dimensionally organized plates create a structure with countless small cavities visible macroscopically. This arrangement is called trabecular bone or spongy bone). The trabeculae of the bone are disposed according to the lines of force that act in this region, turning the spongy bone a quite strong yet lightweight structure.
The lower image shows a section of the interior of a bone at much a higher magnification than that in the upper figuro.
A peripheral region of compact bone is highlighted in red.
Most of the image contains spongy bone. The light pink regions are the surfaces of the small bone trabeculae, and the dark red areas are cavities between the laminae.
When sections of bone are viewed under a microscope, the trabeculae are usually seen sectioned and several of them are indicated by arrows.
