The Skeleton and Joints (Anatomy cont.)

The Skeleton

Our bones form the framework of our body which we know as the skeleton.  These bones perform seven basic functions:

1. Support: Bones provide a framework that supports the body and cradles its soft organs.
2. Protection: The fused bones of the skull protect the brain.  The vertebrae surround the spinal cord, and the rib cage helps protect the vital organs in the chest.
3. Movement: Skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts.  As a result, we can walk, grasp objects, and breathe.  The design of a particular joint determines what type of movement is possible.
4. Mineral and Growth Factor Storage: Bone is a reservoir for minerals, most importantly calcium and phosphate.
5. Blood Cell Formation: Most blood cell formation occurs in the red marrow cavities of certain bones.
6. Triglyceride (fat) Storage: Fat, a source of energy for the body is stored in bone cavities.
7. Hormone Production: Bones produce osteocalcin, a hormone which not only helps regulate bone formation, but also protects against obesity, glucose intolerance, and diabetes mellitus.

In order for all of these bones to move, thereby imparting movement to our body, they all have to be connected in some way and have the ability for articulation.  The mechanisms that allow the muscles of the body to move the bones are the joints.

Joints

The majority of joints in the body, and those we have the most interest in, are known as Synovial Joints.  Synovial joints are those in which the articulating bones are separated by a fluid-filled cavity.  This structure permits substantial freedom of movement, and all synovial joints are freely moveable and limited only by the connective tissues, e.g. ligaments and tendons.

Structurally, synovial joints have six distinguishing features:

1. Articular Cartilage: Glassy smooth cartilage covers the opposing bone surfaces.  This cartilage acts as a cushion that absorbs compression in the joint.
2. Joint (articular) Cavity: The articular cavity is a potential space that contains a small amount of synovial fluid.
3. Articular Capsule: The joint cavity is enclosed within a two-layer capsule.  The outer layer is fibrous and its purpose is to strengthen the joint so that the bones are not pulled apart.  The inner layer is much softer and covers all of the joint surfaces not covered by cartilage.  The inner layer produces synovial fluid.
4. Synovial Fluid: Synovial fluid is viscous and has the consistency of egg whites.  Without this lubricant lining the surfaces of the joint, the joint would very rapidly wear out.
5. Reinforcing Ligaments: Synovial joints are reinforced and strengthened by a number of band-like ligaments.  The purpose of these ligaments is to hold the bones of the joint together and prevent the joint moving beyond its normal range.
6. Nerves and Blood Vessels: Synovial joints are rich with sensory nerve fibres which detect pain and monitor joint position and stretch.  The joints also carry a rich supply of blood vessels which supply the synovial membrane.

In addition to the basic components described above, certain joints have other structural features.  Some, such as the hip and knee joints have cushioning fatty pads between the synovial membrane and bone.  Others have discs or wedges are known as menisci. 

In the knee, for example, the menisci are located at the sides of the joint to prevent the knee joint from bending sideways.

These are the components the make up our basic structure, provide protection and allow movement to occur.  The challenge now is to effect that movement and that is where our muscles come in to play.  Next time, we begin looking at the major muscles of the body, their names and locations.