The Axial Skeleton

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.

Spine

The Spinal Column

By Pereru (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

The human spine provides much of the support for the upper body. It is also the primary protection mechanism to prevent injury to the spinal cord. The spine consists of 24 vertebrae bones plus the larger fused bone called the sacrum, and the tail bone or coccyx. The spine is commonly considered to be divided into four separate sections, the cervical spine, thoracic spine, lumbar spine, and the sacral region. The diagram at right below provides a pictorial view of these various sections.

Spinal Sections

The cervical spine is composed of the seven vertebrae that are found in the neck. This includes the set of vertebrae beginning below the skull and reaching down to just above the ribs. The vertebrae are quite flexible offering the head a broad range of movement.

The thoracic spine consists of all vertebrae to which a rib is attached. This section of the spine consists of the twelve vertebrae positioned roughly from the collar bone down to the kidneys. Of all the vertebrae (with the exception of the sacrum) these are the least flexible and most stable.

The lumbar spine comprises the five vertebrae that we commonly think of as our lower back. These are usually the most heavily stressed of the vertebrae since they typically carry the heaviest loads. The lumbar vertebrae offer a broad range of motion for the hips and torso.

The curve shown in the side view of the above diagram helps the body absorb stresses, provide proper balance, and support twisting and leaning motions. Forward bends in the curve (anteriorly curved) are referred to as Lordosis curves. Bends in the curve posteriorly are called Kyphosis curves. The cervical and lumbar spines generally have a lordosis curve. The thoracic spine and sacrum have a kyphosis curve.

The back view shows the normal relatively straight alignment of the spine when viewed from behind. Any pronounced curve in the spine from this perspective indicates a possible abnormal condition that should be reviewed by a physician.

The vertebrae in the sacrum are fused together to form one large bone that functions as the base of the spine and the structural connection to the hips. The sacrum is tightly connected to the ilium (hips) on either side of the body via a strong interlocking bone connection and extensive ligaments. This joint is called the sacroiliac joint and is one of the largest joints in the human body. This joint provides shock absorption for stresses transferred from the legs and hips, and provides strong structural support for the upper body. The joint can and does move slightly, primarily to absorb and distribute stresses. The diagram below shows the numerous ligaments associated with tightly binding the ilia and sacrum together.

Sacroilliac

By Dr. Johannes Sobotta (Sobotta’s Atlas and Text-book of Human Anatomy) [Public domain], via Wikimedia Commons

The coccyx is the final section of the spinal column and consists of from three to five small vertebrae that may be fused together as a single structure. Often called the tailbone because it is the vestige of a primate tail, it maintains significant functionality in humans. It is an attachment point for various ligaments and tendons and plays a vital role in balance. The coccyx is also quite important for structural support and weight bearing while sitting, particularly when leaning back while sitting (i.e. when the shoulders rest posterior to the hips).

Between any two vertebrae in the spine is a fibrocartilage disc. These are referred to as Intervertebral Discs. Each disc in the spine is named for the vertebrae it separates. For example, the intervertebral disc between the T1 and T2 vertebrae is referred to as the T1-T2 disc. The discs function to absorb stress. This is due, in part to the high water content found in each disc. Discs also function to ensure the vertebrae do not rub against one another, which would eventually result in pain and deformity of the joint.

As a person ages the normally high water content in discs often decreases. This results in a decreased ability for the back to sustain stresses and also may result in a decrease in height for the individual. This is largely why people begin to get shorter as they age.

At each level in the spine a nerve bundle exits the spinal column near the intervertebral disc. These nerves provide support for adjacent areas of the body. A more detailed diagram of the relationship between vertebrae, intervertebral discs, the spinal column, and nerves is provided below.

Vertebrae Depiction

By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

Rib Cage

We previously studied the rib cage when discussing Hontaibu No Atemi Waza. We will now study the rib cage from a purely anatomical perspective.

There are twelve pairs of ribs in most human bodies. A small percentage of individuals have a thirteenth pair or ribs, either at the C7 or L1 vertebrae. We will discuss the most common arrangement of ribs.

Ribs are classified in several different ways. Some classifications address how ribs attach to the sternum. Other classifications address how the ribs attach to the vertebrae.

Ribs may be classified as either true ribs or false ribs. Ribs that attach directly to the manubrium or sternum (via intervening hyaline cartilage) are known as true ribs. Ribs that either do not attach to the sternum at all, or that attach to the cartilage from another rib rather than to the sternum are called false ribs. Ribs 1 through 7 are true ribs because they, via cartilage, connect directly to the sternum. Ribs 8, 9, and 10 are false ribs because they attach to the cartilage at rib 7, rather than directly to the sternum. Ribs 11 and 12 are false ribs because they do not attach to the sternum at all.

Ribs that connect directly or indirectly to the manubrium or sternum are called fixed ribs. Ribs 11 and 12, which do not have such a connection, are called floating ribs.

The shape of a rib is another way in which ribs are classified. Most ribs have a similar shape. The superior portion of the rib has a broad curved and rounded surface. The rib then narrows toward the inferior edge which is thinner and with a sharper edge than the superior portion of the rib. All of these attributes combine to determine whether a rib is classified as either typical or atypical.

A typical rib has the following characteristics:

  • The inferior interior face (aspect) of the rib contains a subcostal grove in which the intercostal blood vessels and nerves reside.
  • The end of the rib nearest the spine (posterior end) contains a head, a neck, and a tubercle. The head contains two facets or flat spots that form part of a joint. The inferior facet articulates with the rib’s same level thoracic vertebrae. The superior facet connects to the vertebrae immediately above (superior to) the rib. The tubercle articulates with the transverse process of the vertebrae associated with the rib.

For example, rib 4 has two articulations with vertebrae T4 and one articulation with T3. The superior facet of the head articulates with vertebrae T3. The inferior facet of the head articulates with vertebrae T4. The facet of the tubercle articulates with the transverse process of vertebrae T4.

The typical ribs are ribs 3 through 9.  Atypical ribs include ribs 1 and 2, which have a different shape than most ribs, and ribs 10 through 12 which do not connect to the next lower rib.

The first rib does not connect with the prior vertebrae, so it is an atypical rib. It is also atypical because it has a much flatter and wider shape than other ribs. Rib number two has a wider and more flattened shape than other ribs so it is considered to be atypical.

Rib number ten is atypical because it does not articulate with the superior vertebrae. Some medical references do not claim that rib 10 is atypical, but others do. Your choice. Ribs eleven and twelve are atypical because they have neither a neck nor tubercle and only articulate with their same numbered thoracic vertebrae.

By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

As you can see in the above diagram the front of each rib does not directly connect with the sternum. Instead a segment of hyaline cartilage lines between these two bones for all non-floating ribs. These cartilage sections are referred to as costal cartilage. The costal cartilage provides flexibility to the rib cage allowing the chest to expand and contract during respiration. Ribs 8, 9, and 10 do not connect directly to the sternum, but rather are attached to the cartilage associated with the seventh rib. As a result ribs 8, 9, and 10 (as well as ribs 11 and 12) are referred to as false ribs.

The Skull

At birth the human skull is composed of 44 different bony plates that are knitted together with collagen borders called fontanels. The fontanels are the “soft spots” that can be belt in an infant’s skull. The fontanels allow the skull to deform substantially during birth. As a child ages the fontanels slowly evolve, through a process called ossification, into bone. At adulthood all of the fontanels have been converted to bone, leaving the skull, with the exception of the mandible, as one single bone structure.

The skull consists of all of the bone structures of the face and head, including the independently movable mandible.

Here is a list of the bones that comprise the skull:

Count Bone
1 Mandible
1 Ethmoid Bone
1 Frontal Bone
1 Occipital Bone
1 Sphenoid Bone
1 Vomer Bone
2 Parietal Bones
2 Temporal Bones
2 Inferior Nasal Conchae Bones
2 Maxilla(e) Bones
2 Nasal Bones
2 Zygomatic Bones
2 Palatine Bones
2 Lacrimal Bones

There are several bones listed that do not appear in the pictured diagram. The Vomer and Inferior Nasal Conchae bones are located inside and at the base of the nasal cavity. The Vomer bone forms the interior portion of the nose septum and serves to divide the nasal passages. The Inferior Nasal Conchae rest at the back of the nasal cavity and provide a large surface area over which air is warmed and humidified before being delivered to the lungs. The Palatine bones form the base of the eye socket and the hard palette.

The Hyoid bone (not listed nor depicted) is a U-shaped bone that sits roughly level with the bottom of the mandible in the throat. The hyoid serves primarily as the anchoring point for the tongue, but some muscles of the larynx, pharynx, and epiglottis all attach to the hyoid bone to facilitate their movements as well. The hyoid bone is nearly always considered to be part of the skull, though it has no physical connection with any other bones in the skull.

The cranium is comprised of the eight bones that enclose and protect the brain. These include:

Count Bone
1 Ethmoid Bone
1 Frontal Bone
1 Occipital Bone
1 Sphenoid Bone
2 Parietal Bone
2 Temporal Bone

As you can see, the cranium is a small subset of the skull. When one speaks of the cranium one is referring only to that portion of the skull that encircles and protects the brain.

In addition to protecting the brain the skull provides a home for various sensory functions including vision, hearing, taste, smell, and feel (not unique to the skull). The skull also provides innumerable attachment points for muscles that control everything from movement of the head to facial expressions.

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