Compact and cancellous — or spongy — bone are the two types of tissue found within most bones. Due to its function, compact bone is also referred to as strong bone; due to its structure, it is referred to as cortical bone. The two tissues serve different purposes in bones, with the main function of compact bone being to provide strength and protection to the bone. Cancellous bone also provides strength, but due to its structure, it has a much lower mass than compact bone.
When compact bone is studied, it is found to be made up of concentric circles called lamellae. Within each lamella, collagen is mixed with inorganic minerals like magnesium, calcium and phosphorus and layered around a Haversian canal. An artery, a vein, lymph vessels and nerve fibers are found within each Haversian canal. Osteocytes, or bone cells, are found in lacunae, which are spaces within the lamellae. The components of the lamellae and the Haversian canal are referred to as a Haversian system.
Most think that bone is a dead tissue, but this is not the case. It provides protection and support to the rest of the body, so must be able to grow, as well as repair and replace any damage. The very outer areas of bone are composed of compact bone. When stress is placed on a bone, a function of the compact bone is to provide inorganic components so that new bone tissue can be formed.
Although compact bone is made up of Haversian systems, it is almost solid. This makes it very dense, so it has a high mass. Even though the function of compact bone is extremely important to protecting a bone from damage, the mass of this type of bone is what limits it. If bones were made completely of compact bone, they would become too heavy for efficient use by the body.
As well as providing strength and support to the bone where it is found, there is another function of compact bone. The lamellae act as reserves for the inorganic molecules stored there. Not only are these molecules used to produce new bone cells, but they act as a reservoir for calcium and phosphorus for the body.
The function of compact bone is to help to keep the levels of these minerals at constant within the blood. The arteries and veins that run through the Haversian canals provide a means to transport the minerals to and from the compact bone. The minerals are deposited for storage within the lamellae when blood levels are high. When levels are low, minerals are absorbed out of the lamellae and transported throughout the body.