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

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

The human skeletal system consists of 206 individual bones as well as joints, cartilage, tendons, and ligaments. The bones of the skeletal system serve many functions, from blood cell production to mineral storage. Perhaps most importantly, these bones and their joints allow and support the movement of our bodies. Actions like walking, climbing the stairs, or riding a bicycle would be impossible without the particular arrangement of our appendicular skeleton.

The skeletal system is divided into two sections: the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones along the central axis of the body. This includes the skull, the vertebral column, and the thoracic cage or rib cage. The appendicular skeleton consists of the limbs, also sometimes called appendages, and the bones that attach them to the trunk of the body. A diagram illustrating the axial and appendicular skeleton is shown in Figure 1.

Example 1: Identifying the Appendicular Skeleton in a Diagram

Which of the diagrams provided outlines the structure of the appendicular skeleton?

Answer

The adult human skeleton is made of 206 individual bones and the joints that hold them together. The skeleton is divided into two sections: the axial skeleton and the appendicular skeleton.

The axial skeleton refers to the parts of the body that align with the central, vertical axis of the body. This includes the skull, the vertebral column, and the thoracic cage.

The appendicular skeleton refers to the bones of the appendages, also known as the limbs. This includes the bones in the arms and legs and the bones that attach them to the axial skeleton. The upper appendages, or arms, are attached to the trunk of the body through the shoulder girdle, or pectoral girdle. The lower appendages, or legs, are attached to the trunk of the body through the pelvic girdle.

Therefore, the diagram that shows the appendicular skeleton is diagram X.

Let’s start our investigation of the appendicular skeleton by looking at the bones of our upper appendages, the arms.

The proximal, or closer, bone of the arm is called the humerus. Proximal is a term that means “closer to the point of attachment.” The humerus connects your shoulder to your elbow. The more distal, or farther, portion of the arm consists of two bones: the radius and the ulna. The radius is a slightly thicker and shorter bone that attaches from the elbow to the wrist on the side of the thumb. The ulna is a slightly thinner and longer bone that attaches from the elbow to the wrist on the side of the smallest finger. If you place your fingers on the bones of your arm, directly above the wrist, you can feel the distinct radius and ulna lying parallel to each other. The radius can rotate around the fixed ulna. The bones of the arm are illustrated in the diagram in Figure 2.

The distal end of the arm is attached to the hand by the bones of the wrist. There are 8 small, irregularly shaped bones within the wrist joint. These bones are tightly bound together and are called carpals as a group.

The bones of the palm of the hand are called metacarpals, which means “after” the wrist or carpals.

The bones of the fingers are each called a phalanx (plural: phalanges). Each of your fingers is made of three phalanges called the proximal, middle, and distal phalanges. The thumb is made of only two phalanges and does not have a middle phalanx.

You may be interested in knowing that these same bones can be found in the appendages of many vertebrates. From the wings of bats and birds to the hooves of horses, the legs of lizards, and the flippers of whales, the same arrangement of bones is present, with variations adapted for trotting, flying, climbing, or swimming.

The proximal end of the arm is attached to the trunk of the body by the bones of the shoulder. The top, or head, of the humerus is attached to the socket, or glenoid cavity, of the scapula. The scapula is a triangular, plate-like bone that is also commonly called the shoulder blade.

The scapula may appear to be attached to the back of the body, but in fact it floats above the thoracic cage there. In fact, the scapula is attached to the thoracic cage at the front of the body through the collarbone, or clavicle.

The clavicle is an S-shaped bone that attaches to the scapula through a process extending from the back of the scapula. The other end of the clavicle attaches to the top of the sternum. Both the clavicle and the scapula, together with their joints and attachments, are called the pectoral girdle, referring to the pectoral muscles of the chest.

Almost every one of the bones in the upper appendages, or arms, matches with a corresponding bone in the lower appendages, or legs. Their shapes and arrangements differ, making them suited to their different functions. The bones of the arms and legs can be seen in the illustration in Figure 3.

The proximal bone of the leg is called the femur. The femur is the largest and hardest bone in the human body. This bone connects the hip to the knee. The more distal portion of the leg, similar to the arm, consists of two bones. The much larger tibia is the bone of the shin. The much smaller fibula runs along the outer edge of the tibia. These two bones are very closely connected and bound together by connective tissues, so they are difficult to distinguish by touch. The distal end of the femur has two rounded portions called condyles, which articulate with the tibia.

The front of the knee joint includes a small, round bone called the patella, which is also commonly called the kneecap. The patella does not have a corresponding bone in the arm. The bones of the leg are shown in Figure 4.

Example 3: Identifying the Tibia in a Diagram

The diagram provided shows a basic outline of the appendicular skeleton. Which number indicates the tibia?

Answer

This question provides us with a diagram of the appendicular skeleton and asks us to identify the number of a specific bone. In order to answer this question, we need to correctly recall where in the body the tibia is located and then choose the number that points to it.

The appendicular skeleton consists of the bones of the appendages (arms and legs) and the bones that attach them to the trunk of the body. The tibia is a bone in the leg. The three major bones of the leg are the femur, the tibia, and the fibula. The femur is the bone of the thigh, or upper leg. In the lower leg, the tibia is the main bone of the shin and the fibula is a smaller bone that is located along the side of the tibia.

Using this information, we can conclude that the number that points to the tibia is the number 7.

The distal end of the leg is attached to the foot by the bones of the ankle. In the ankle, there are 7 irregular bones bound together, which as a group are called tarsals. These are similar to the carpals of the wrist. There are two tarsals worth noting in particular. The tarsal bone called talus directly connects the foot to the lower leg and is the main bone of the ankle joint. The tarsal bone called calcaneus juts out toward the rear of the foot and is the bone found in your heel. In Figure 4, the talus is shown in orange and the calcaneus is shown in green.

The bones of the arch of the foot are called metatarsals. These are similar to the metacarpals of the hand. Just like the fingers, each of the toes is made of three bones each called a phalanx (plural: phalanges). They are the proximal, middle, and distal phalanx, with the exception of the large toe, which has only two phalanges.

The proximal end of the leg is attached to the trunk of the body through what we call the pelvic girdle. The bones of the pelvis can be felt through the hips and buttocks. A detailed diagram of the pelvic girdle can be seen in Figure 5.

There are two large, flat, ear-shaped bones attached to either side of the sacrum. We call these bones the pelvic bones, and they attach to each other at the front of the body at a joint known as the pubis symphysis. The pelvic bones are fused and are made up of a large bone called the ilium, which is attached above the pubis and the ischium. The pubis is located above the ischium and is fused anteriorly (from the front) to the ilium, while the ischium extends below and behind the pubis and is fused posteriorly (from the back) to the ilium. These three pelvic bones form a socket called the acetabulum, where the head of the femur fits, forming the hip joint. If you press your hands on the front of your hips, you can feel the upper ridge of the pelvic bone, which is called the iliac crest. The structure of the pelvis can be seen in Figure 5.

The skeletal system consists not only of bones, but also of the joints that connect them. There are three major types of joints in the human body: synovial, cartilaginous, and fibrous joints. Synovial joints are highly mobile joints that allow a large range of motion. Cartilaginous and fibrous joints allow little motion or none at all.

The axial skeleton includes the skull, spine, and thoracic cage, while the appendicular skeleton includes the arms and legs. The appendicular skeleton has more articulation, or points and ranges of motion, than the axial skeleton, so it possesses more movable synovial joints and fewer cartilaginous and fibrous joints than the axial skeleton. A diagram of a synovial joint is shown in Figure 6.

Synovial joints consist of at least two bones cushioned in between by a sac of fluid called the synovial fluid. Examples of synovial joints include the joints between the phalanges of the fingers, at the elbow and knee, and in the shoulder and hip.

The ends of the bones in synovial joints are coated with a layer of articular cartilage, which helps them move against each other smoothly. The bones of a synovial joint are held in place by fibrous connective tissues called tendons and ligaments. Tendons connect muscles to bones, and ligaments connect bones to other bones.

Tendons and ligaments consist primarily of strong connective tissues and allow the movement of joints.

An example of a tendon is the Achilles tendon, which connects the gastrocnemius muscle of your calf with the bones of your foot. This tendon allows the extension of the ankle joint, which is essential in the pointing motion of the foot and a main feature of human gait. Damage to the Achilles tendon can render a person unable to walk. Treatments include anti-inflammatory drugs, using a medical splint, and surgery in extreme cases.

An example of a ligament is the ACL, or anterior cruciate ligament, a ligament that supports the knee joint. The cruciate ligaments cross from left to right within the knee joint, between the femur and tibia, preventing this joint from engaging in sideways motion. Ligaments have limited elasticity to allow some movement, but this keeps the flexing motion of the knee restrained to just one plane, forward and back. In sports that cause a lot of sideways strain on the knee, for example, basketball, damage and tear to the ACL are common.

Cartilage is a type of connective tissue found in the skeletal system as well as other parts of the body. Outside of the skeletal system, cartilage is found in the flexible structures of the ears and nose, the trachea and smaller air passages, and the larynx, or voice box. Cartilage connects bones in cartilaginous joints, and articular cartilage covers the ends of bones in synovial joints.

The bones of the skeleton develop from cartilage. Fetal bones are completely made of flexible cartilage. By the time a baby is born, they have about 300 individual bones with many cartilaginous parts. This is why babies are more flexible and cannot support themselves. The bones eventually fuse together into the 206 bones of the adult skeleton by converting most of the soft cartilage into tough bone.

Interestingly, cartilage does not possess blood vessels. The cells in cartilage tissues only receive nutrients slowly, through diffusion instead of directly from a blood supply. This is one of the reasons why cartilage heals extremely slowly once injured.

Let’s summarize what we have learned about the appendicular skeleton in this explainer.