The bone shown in the figures is a dry bone (from which all soft tissue has been removed) and then cut longitudinally.
The outer layer of the bone 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 . Its central cavity the medullary canal It contains the bone marrow besides other tissues and structures such as blood vessels and adipose tissue.
The interior of the epiphyses and of several flat bones is made up by millions of tiny interconnected plates of bone tissue called bone trabeculae (singular: trabecula). These three-dimensionally organized plates create a structure with countless small cavities that can be seen withou a microscope. Because of the presence of so many trabeculae these regions of bones are called trabecular bone or spongy bone).
The presecne of the numerous cavities of the spongy bone and the disposition of the trabeculae result in a quite strong yet lightweight structure.
The lower image shows higher magnification of the interior of spongy bone.
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 trabeculae.
When sections of bone are viewed under a microscope, the trabeculae are seen as sectioned plates. Several of them are indicated by arrows.
