Introduction to the Human Skeletal System

The human skeleton consists of all of the bones within the human body. Some of these bones are quite specialized. Many bones provide anatomical support while others provide more generalized functionality. For many purposes the human skeleton can be thought of as being comprised of two different skeletal components. The Axial Skeleton consists of the central core bones including the spine, ribs, and head. The Appendicular Skeletal system is comprised primarily of the hips, arms, hands, legs, and feet.

Below is a diagram showing the major bones of the human anatomy. Additional diagrams below will provide more detail about specific areas of the body. More detailed discussions follow the skeletal image.

There are at least 206 bones in the normal adult human body. These bones continue to grow and regenerate throughout a person’s lifetime. Bones are living tissue, not static mineralized structures.

The number of bones in an adult will normally exceed 206, but the additional bones are not consistent from person to person. These additional bones are sesamoid bones (discussed later) and grow as a result of activities and stresses on tendons. Since no two people have exactly the same stresses the number and location of sesamoid bones in two people are likely to be different. So the 206 bone count number refers to the common bones that appear in every individual, not the number of bones that appear in any single individual.

The diagram above provides colored references to specific portions of the skeleton. If you are viewing this diagram on non-color media you may wish to obtain a copy of this diagram in color. The color helps to identify the various skeleton regions as well as the types of cartilage found in various locations in the body. Pay attention to the location of the various forms of cartilage. We will discuss the various functions and types of cartilage in a separate article.

The skeletal diagram above only lists less than thirty of the bones in the skeleton. Some of these bones are found on both sides of the body (for example, the femur) so the chart addresses less than sixty of the various bones in the body. These are the primary bones with which you should become familiar. It is not necessary (though not discouraged either) that you know the name and location of each of the 206 bones. But we will cover nearly every bone in detail in the remainder of curriculum material. The more you can remember the better you will become at protecting yourself, maintaining a healthy skeletal structure, and using the skeleton of your opponent to your advantage.

Appendicular Skeleton

The Appendicular Skeleton

By LadyofHats Mariana Ruiz Villarreal (i did it myself) [Public domain], via Wikimedia Commons

The term Appendicular Skeleton refers to the set of bones associated with the limbs of the body. This is primarily the legs and arms and the support structures associated with these appendages. The name Appendicular Skeleton derives from the notion that legs and arms are appendages to the body.

The diagram at right provides an overview of the bones associated with the Appendicular Skeleton. It also provides common names for various regions of the skeleton. You can refer to the previous chart for the names of specific bones found in this part of the skeleton.

The Appendicular Skeleton is primarily responsible for movement. This includes locomotion capabilities largely provided by the legs and feet, and the grasping and manipulative capabilities provided by the arms and hands.

Most of the bones in this system function as levers. A muscle pulls against two bones to cause at least one of them to move. For example, your bicep muscles attached to the scapula at one end and the radius bone in the forearm on the other. When the muscle contracts the forearm moves, pulling toward the scapula.

We will cover the muscular system in a future belt curriculum, but you should appreciate that directed bone movement is caused by muscles pulling against bones, which function as simple levers. As we will discuss in a future section on this topic, various tendons, ligaments, and cartilages are involved in this process to ensure proper joint movement and to facilitate this movement in a nearly frictionless and painless manner.

For now, understand that the Appendicular Skeleton consists of the body’s appendages and the support structures to which major muscles attach to enable the bones to function in their role as levers.

Axial Skeleton

The Axial Skeletal System

By LadyofHats Mariana Ruiz Villarreal (i did it myself) [Public domain], via Wikimedia Commons

The Axial Skeleton is comprised of all of the bones of the body that are not part of the Appendicular Skeleton. Many of these bones protect vital organs. Many other bones serve ancillary purposes not related to support or organ protection.

The Ossicles are the smallest bones in the human body. They are located in the middle ear and serve to transmit vibrations from the eardrum to the inner ear. Without these bones our hearing would be seriously impaired. There are three bones in the middle ear called the malleus (hammer), incus (anvil), and stapes (stirrup). Sound vibrations travel through each bone on their way to the inner ear, where the sound vibrations are turned into nerve impulses that the brain can interpret as sounds.

The Hyoid bone (discussed later) is another small bone that has special purpose. It is not directly involved in supporting the body’s structure nor with the protection of internal organs.

The primary sections of the Axial Skeleton that are involved in organ protection and structural support are the spine, the ribs, and the skull. We will discuss each of these areas in more detail in the following sections.

Skeletal System Functions

Support

One of the primary functions of the skeletal system is support for the body. We would not be unable to stand or even sit without the structural support provided by the skeletal system.

This support is due, of course, to the shape and rigidity of the bones. But it is also largely due to the function and placement of ligaments that tightly bind joints together and limit the ways in which each joint can move. You would have great difficult standing if your knee could move in any direction at any moment. What would life be like if one knee could move forward while the opposite knee moved directly backward? It might be difficult to run a marathon or deadlift 300 lbs.

The feet, legs, pelvis, spinal column and shoulder girdle are all essential parts of our ability to stand erect. Bones and ligaments are the underpinnings that allow us to establish and maintain a standing, sitting, and even prone posture.

In addition to structural support the skeletal system provides muscle support. Most (but not all) muscles attach to one or more bones. Most organs are held in place through the direct or indirect support of bones and ligaments. Some obvious examples are the brain, teeth, eyes, and heart. If not for the skeletal system gravity would eventually allow all of these organs to fall to your feet. That would indeed make running that marathon quite problematic.

Movement

Our ability to move is fundamentally rooted in the skeletal system. The skeletal system provides basic support for our structure, but the interaction between muscles and the bones, ligaments, and tendons enables us to move. The skeletal muscles provide the ability for us to move one bone relative to another. The bones act as levers which the muscles pull to cause movement. Simply raising your arm involves numerous muscle contractions against several bones. Walking involves large numbers of muscles working on bones in your feet, legs, pelvis, back, chest, shoulders, and arms. It is amazing how we can move so many bones and muscles in such a coordinate effort without any conscious thought.

But movement may involve more than locomotion or placing your hands in guard position. Even smiling or grimacing uses your skeleton system to help produce the desired facial expression. Many facial muscles attach to the skull and when flexed or relaxed form the complex interplay of muscles we call facial expression.

Protection

If you look at our vital organs you will notice that most of them are protected to some degree by substantial elements of the skeletal system. The brain is protected by the cranium. The heart, and lungs are protected by the ribs. The liver, kidneys, stomach and pancreas are at least partially protected by the ribs as well. If you look at a skeleton and organ diagram you may notice that the pancreas does not appear to be covered by the ribs. However, the pancreas is located deep in the thoracic cavity and is protected by the spine and the posterior ribs. Only the skin and intestines are not offered substantial protection by the skeletal system.

Other organs are protected by the skeletal system as well. The shape of the skull affords protection to the eyes as well as the teeth and other organs of the mouth. The ears are protected by the skull. Let’s not forget that one of the primary purposes of the spine is to protect the spinal cord. Other non-vital organs such as the spleen and gallbladder receive significant protection via the rib cage.

You should come to understand and appreciate where your vital organs are in relation to various skeletal components. It will help you know whether a potential or previous strike may represent a significant threat to your health. It can also help you decide where best to place your guard.

Blood Cell Production

A critical and very active function of bones is the production of platelets as well as red and white blood cells. These blood components are created in the red marrow of large bones. The adult body produces about two million new red blood cells every second. That’s right, every second. About this same number of red blood cells die each second, so this process leaves the blood cell count in the human body at relatively stable levels in a healthy adult. Red blood cells make up about 45% of the volume of blood.

White blood cells only represent about 1% of the content of blood. Platelets represent roughly 4% of blood by volume. The remaining content of blood is largely plasma, which comprises about 55% of blood volume. These percentages are not exact and these ratios can vary in an individual at various times.

Each of these blood components is produced in red bone marrow from non-specific stem cells. These hematopoietic stem cells, which reproduce by cell division, can evolve to become any of the blood cell types. Since red bone marrow has a substantial blood supply these new blood cells are quickly absorbed into the blood stream and transported out of the bones.

In an infant nearly all bone marrow throughout the body is red bone marrow. As a person ages the marrow in smaller bones slowly evolves to become yellow bone marrow. In an adult only the larger bones in the body such as the ilium, scapula, ribs, vertebrae, skull, and sternum will contain red marrow. The marrow in most other bones, with a few minor exceptions, will be yellow marrow. This process continues into old age, at which point the vast majority of bone marrow is yellow marrow. Yellow marrow can convert back to red marrow if the body requires additional blood cell production, perhaps as the result of injury or disease. This conversion can happen very quickly, in the matter of only an hour or two.

Mineral Storage

The hard surfaces of bones contain large quantities of minerals. Ninety-nine percent of the calcium in the human body is stored in bones. Eighty-five percent of the phosphorus in the body is stored in bones. The body closely regulates the amount of minerals in the blood and will deposit or withdraw minerals from bone as necessary to maintain proper mineral percentages in the body. Minerals are released to or acquired from the blood stream as necessary.

Energy Storage

The yellow marrow in bones stores fat that can be quickly converted into energy. This is normally the last fat reserve to be depleted during periods of starvation.

Endocrine Regulation

It has recently been discovered that bones function as part of the body’s endocrine (hormonal) system. Bones secrete osteocalcin, a protein that is used by the body to help regulate insulin production. This protein stimulates beta cells in the pancreas to produce insulin and prompts the pancreas to create new beta cells when required. Osteocalcin has also been shown to interact with fat cells to control the body’s sensitivity to insulin. This recent discovery is expected to lead to a better understanding of diseases such as diabetes.

Additional articles within the Green Belt curriculum discuss various structures such as the spine, skull, hand, and other significant portions of the skeleton. We will also provide somewhat detailed information about bone structure, bone function, skeletal joints, cartilage, and ligaments.

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