Lesson Explainer: The Axial Skeleton Biology • Third Year of Secondary School

In this explainer, we will learn how to describe the structure of the human axial skeleton.

An adult human skeleton contains 206 individual bones. These bones are essential to several functions. Our bones are the site of blood cell production and play a role in our immune systems. They are a method our bodies use to store certain ions and minerals. And, maybe most importantly, our skeletons are the internal support and framework of our bodies. The skeletal system consists of all of the human body’s bones, joints, ligaments, tendons, and cartilage. Our bones and joints allow motion, and parts of our skeletons protect our more delicate and vital internal organs. You can see an overview of the skeletal system in Figure 1.

The skeletal system is divided into two sections, the axial skeleton and the appendicular skeleton as shown in Figure 2. The axial skeleton consists of the bones of the head, chest, and back. These bones are found around the vertical axis of the body. The appendicular skeleton includes the bones of the arms and legs as well as the bones that attach them to the trunk of the body. Your arms and legs are also called appendages.

Let’s take a closer look at the structure and function of the axial skeleton.

Starting from the top of the axial skeleton, we first encounter the skull as shown in Figure 3. The skull can be divided into an anterior and a posterior section. Anterior is a word that means “front” or “forward,” and posterior means “back,” “rear,” or “behind.”

The posterior skull is composed of 8 bones that form the cranium, which protects the brain. At the base of the posterior of the skull is a large hole called the foramen magnum that plays a role in connecting the spinal cord to the brain. The anterior skull consists of the facial bones, such as those of the nose and the eye sockets, as well as the mandible, or jawbone.

The upper and lower jaws are lined with teeth. The teeth are considered to be a part of the skeletal system, but teeth are not bones. Teeth are harder and less flexible than bones. And, unlike our bones, they cannot grow back or mend themselves when they are damaged or broken.

The skull serves many important purposes. The bones of the skull anchor our facial muscles that allow us to speak, blink, and make facial expressions. The hinged mandible moves up and down to allow us to chew our food and help us speak. And the bones of the cranium protect the brain, one of our most important internal organs.

The base of the skull is directly attached to the top of the vertebral column, also called the spine or the spinal column. The vertebral column is made of a stack of irregularly shaped bones called vertebrae. The vertebrae near the top of the vertebral column are smaller and more delicate than those near the bottom because the bottom of the vertebral column supports significantly more of your body weight than the top.

The vertebral column is further divided into 5 sections as shown in Figure 4. The top 7 vertebrae are called cervical vertebrae. Cervical is a word that means “related to the neck.” The next 12 are called thoracic vertebrae. Thoracic is a word that means “related to the chest.” Each thoracic vertebra is attached to one pair of rib bones. Beneath the thoracic vertebrae are the 5 lumbar vertebrae. These thick, sturdy vertebrae make up the curve of the lower back. The cervical, thoracic, and lumbar vertebrae are separated by cartilage disks and are articulating vertebrae, meaning they are joined by a joint and are movable.

Next, there is a plate of 5 fused sacral vertebrae referred to as the sacrum. The sacrum connects the bones of the pelvis in the back of the body. Last, dangling beneath the sacrum, is a small group of 4 tiny fused coccygeal vertebrae called the coccyx. The coccyx is also commonly referred to as the tailbone.

The stacked bones of the vertebral column serve several functions. They are the site of attachment of many of the muscles of the back, chest, abdomen, arms, and legs. They protect the spinal cord, support the body, and allow movement of the head and torso.

Each vertebra is made up of a thick portion called the centrum or the “body” of the vertebrae that are connected laterally with two transverse processes. You can see the structure of the vertebrae in Figure 5. The bones of the vertebral column also support and protect the delicate spinal cord. Each vertebra has a hole called the vertebral foramen (or neural canal) that, when stacked with the others, creates a passage that the spinal cord is located within.

Example 3: Recognizing the Structures of a Vertebra

The diagram provided shows the basic structure of a single vertebra. Which letter indicates where the spinal cord passes through?

Answer

The vertebrae are the bones of the vertebral column, also called the spinal column. These bones serve two purposes. They support the body and maintain posture. The wide, flat parts of each bone are stacked together, joined by cushioning disks of cartilage, to form the structure of the vertebral column. The second purpose of the vertebral column is to protect the spinal cord. The spinal cord is an organ of the central nervous system. It relays information between the brain and the rest of the body. The spinal cord is delicate and is easily damaged. It traverses through openings in each vertebra, called a foramen. These openings, when stacked one above the other, form a protective channel for the spinal cord.

Using this information, we can conclude that the letter Y indicates where the spinal cord passes through the vertebra.

The axial skeleton also includes the thoracic cage, or rib cage. The thoracic cage is made of the 12 pairs of flat, curved rib bones. These bones are attached at their posterior end to the 12 thoracic vertebrae. And they are attached at their anterior end to the sternum, or breastbone.

Every rib bone is attached to a vertebra, but not every rib meets the sternum in the front. The two lowest pairs of ribs are attached in the back and not in the front. They are called floating ribs and you may be able to feel the ends of them if you trace them from your spine.

The thoracic cage is illustrated in Figure 6. It has several functions. The thoracic cage is the site of attachment for the bones and muscles of the arms. It anchors the sheet of muscle called the diaphragm, and the muscles between the ribs, called intercostal muscles, which move the ribs when we breathe. The thoracic cage is connected posteriorly to the thoracic vertebrae, while anteriorly, 10 pairs of ribs are attached to the sternum. Each of the upper 10 pairs of ribs is attached to the sternum through a length of specialized cartilage, which gives the rib cage some added flexibility. The lower two pairs of ribs are sometimes called floating ribs since they are not attached to the sternum at all.

When we breathe in, also called inhale or inspire, the ribs move up and outward to increase the volume of the thoracic cavity. When we breathe out, also called exhale or expire, the ribs move down and inward to decrease the volume of the thoracic cavity. The bones of the thoracic cage also protect vital organs including the lungs and the heart.

The skeletal system consists not only of the bones of the skeleton, but also the joints that connect them. There are three main categories of joints: fibrous, cartilaginous, and synovial joints.

The most movable of the three types is the synovial joint. Synovial joints are cushioned by a sac of fluid called synovial fluid. Some of the synovial joints in the axial skeleton are where the skull and the top of the neck meet. These allow us to move our heads in a range of directions. The hinges of the jaw are also synovial joints. A diagram of this joint is shown in Figure 7.

Cartilaginous joints are joints where bones are connected to each other directly through cartilage. Cartilage is a smooth, strong, and dense type of connective tissue. Cartilaginous joints are less movable than synovial joints and only allow a little bit of motion. The joints between the vertebrae are cartilaginous joints. A diagram of these joints is shown in Figure 8.

The least movable type of joint is a fibrous joint. Fibrous joints are also called fixed or immovable joints because they do not move. The joints that hold the platelike bones of the cranium together are a type of fibrous joint.

Let’s summarize the main points of this explainer.