19 Chapter 19: Skeletal System
Bones make good fossils. While the soft tissue of a once living organism will decay and fall away over time, bone tissue will, under the right conditions, undergo a process of mineralization, effectively turning the bone to stone. A well-preserved fossil skeleton can give us a good sense of the size and shape of an organism, just as your skeleton helps to define your size and shape. Unlike a fossil skeleton, however, your skeleton is a structure of living tissue that grows, repairs, and renews itself. The bones within it are dynamic and complex organs that serve a number of important functions, including some necessary to maintain homeostasis.
Learning Objectives
After studying this chapter you should be able to:
- List and describe the functions of bones.
- Describe the difference between compact bone and spongy bone.
- Identify the major bones of the axial and appendicular skeletons as listed in the table below.
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Axial Skeleton Bone (s) Body region Skull Head Mandible Lower jaw Ribs Thoracic cage (chest) Sternum Chest Vertebral column Back Appendicular Skeleton Scapula Shoulder Clavicle Collarbone Humerus Upper arm Radius Forearm Ulna Forearm Carpals Wrist Metacarpals Hand Phalanges Fingers and thumb Pelvis Hip Femur Thigh Patella Knee cap Tibia Lower leg Fibula Lower leg Tarsals Ankle Metatarsals Foot Phalanges Toes
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- Define joints, ligaments, cartilage, and tendons, and understand how they work together to connect bones to each other and muscles to bones.
19.1 Functions of the Skeletal System
Bone, or osseous tissue, is a hard, dense connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints), cartilage, a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the body system composed of bones, cartilage, and ligaments and performs the following critical functions for the human body:
- supports the body
- facilitates movement
- protects internal organs
- produces blood cells
- stores and releases minerals and fat
Support, Movement, and Protection
The most apparent functions of the skeletal system are the gross functions—those visible by observation. Simply by looking at a person, you can see how the bones support, facilitate movement, and protect the human body.
Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin.
Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of view, bones act as levers and joints serve as fulcrums (Figure 19.2). Unless a muscle spans a joint and contracts, a bone is not going to move.
Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (Figure 19.3).
Mineral Storage, Energy Storage, and Hematopoiesis
On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix stores a number of minerals important to the functioning of the body, especially calcium and potassium. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses.
Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most bone is referred to as bone marrow (Figure 19.4). There are two types of bone marrow: yellow marrow and red marrow. Yellow marrow contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy. Red marrow is where hematopoiesis—the production of blood cells—takes place. Red blood cells, white blood cells, and platelets are all produced in the red marrow.
19.2 Bone Structure
Bone tissue (osseous tissue) differs greatly from other tissues in the body. Bone is hard and many of its functions depend on that characteristic hardness. Bone is also dynamic in that its shape adjusts to accommodate stresses.
Gross Anatomy of Bone
The structure of a long bone allows for the best visualization of all of the parts of a bone (Figure 19.5). A long bone has two parts: the diaphysis and the epiphysis. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow. The walls of the diaphysis are composed of dense and hard compact bone.
The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone. Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate), a layer of cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18–21 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line.
Bone Cells and Tissue
Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a surface for calcium crystals to adhere. The calcium crystals crystals give bones their hardness and strength, while the collagen fibers give them flexibility so that they are not brittle.
Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. The most common type of bone cell is called an osteocyte, and it is important for maintaining healthy bone.
The dynamic nature of bone means that new bone is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. Bones can modify their strength and thickness in response to changes in muscle strength or body weight. Thus, muscle attachment sites on bones will thicken if you begin a workout program that increases muscle strength. Similarly, the walls of weight-bearing bones will thicken if you gain body weight or begin pounding the pavement as part of a new running regimen. In contrast, a reduction in muscle strength or body weight will cause bones to become thinner. This may happen during a prolonged hospital stay, following limb immobilization in a cast, or going into the weightlessness of outer space.
Compact and Spongy Bone
Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone’s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution.
Compact Bone
Compact bone is the denser, stronger of the two types of bone tissue (Figure 19.6) and it provides support and protection. The microscopic structural unit of compact bone is called an osteon.
Spongy (Cancellous) Bone
Spongy bone, also known as cancellous bone consists of a lattice-like network of matrix spikes called trabeculae (singular = trabecula) (Figure 19.7). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where blood cells are made.
19.3 Bone Growth
How Bones Grow in Length
The epiphyseal plate (growth plate) is the area of growth in a long bone. It is a layer of cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified (turns into bone), and the diaphysis grows in length.
Bones continue to grow in length until early adulthood when the growth plate closes. All that remains of the epiphyseal plate is the epiphyseal line (Figure 19.8).
19.4 Divisions of the Skeletal System
The skeleton consists of the bones of the body. For adults, there are 206 bones in the skeleton. Younger individuals have higher numbers of bones because some bones fuse together during childhood and adolescence to form an adult bone. The lower portion of the skeleton is specialized for stability during walking or running. In contrast, the upper skeleton has greater mobility and ranges of motion, features that allow you to lift and carry objects or turn your head and trunk. The skeleton is subdivided into two major divisions—the axial and appendicular.
The Axial Skeleton
The axial skeleton forms the vertical, central axis of the body and includes all bones of the head, neck, chest, and back (Figure 19.9). It serves to protect the brain, spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back, and for muscles that act across the shoulder and hip joints to move their corresponding limbs.
The axial skeleton of the adult consists of 80 bones, including the skull, the vertebral column, and the rib (thoracic) cage. The skull is formed by 22 bones. Also associated with the head are an additional seven bones, including the hyoid bone and the ear ossicles (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a vertebra, plus the sacrum and coccyx. The thoracic cage includes the 12 pairs of ribs, and the sternum, the flattened bone of the anterior chest.
The Appendicular Skeleton
The appendicular skeleton includes all bones of the upper and lower limbs, plus the girdle bones that attach the limbs to the axial skeleton. The bones of the shoulder region form the pectoral girdle, which anchors the upper limb to the thoracic cage of the axial skeleton. The lower limb is attached to the vertebral column by the pelvic girdle. There are 126 bones in the appendicular skeleton of an adult.
19.5 Axial Skeleton
Skull
The skull (cranium) is the skeletal structure of the head that supports the face and protects the brain. It is subdivided into the facial bones and the brain case, or cranial vault (Figure 19.10). The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. The rounded brain case surrounds and protects the brain and houses the middle and inner ear structures.
In the adult, the skull consists of 22 individual bones (Figures 19.11-13), 21 of which are immobile and united into a single unit. The 22nd bone is the mandible (lower jaw), which is the only moveable bone of the skull.
contain muscles that act on the mandible during chewing.
Interactive Link
Watch this video to view a rotating and exploded skull, with color-coded bones.
Vertebral Column
The vertebral column is also known as the spinal column or spine (Figure 19.14). It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc. Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae.
Rib Cage
The rib cage (thoracic cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal cartilages and the sternum (Figure 19.16). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1–T12). The thoracic cage protects the heart and lungs.
19.6 Appendicular Skeleton
The appendicular skeleton includes all of the limb bones, plus the bones (pectoral girdle and pelvic girdle) that unite each limb with the axial skeleton. Because of our upright stance, different functional demands are placed upon the upper and lower limbs. Thus, the bones of the lower limbs are adapted for weight-bearing support and stability, as well as for body locomotion via walking or running. In contrast, our upper limbs are not required for these functions. Instead, our upper limbs are highly mobile and can be utilized for a wide variety of activities. The large range of upper limb movements, coupled with the ability to easily manipulate objects with our hands and opposable thumbs, has allowed humans to construct the modern world in which we live.
Pectoral Girdle
The bones that attach each upper limb to the axial skeleton form the pectoral girdle (shoulder girdle). This consists of two bones, the scapula and clavicle (Figure 19.17). The clavicle (collarbone) is an S-shaped bone located on the anterior side of the shoulder. It is attached on its medial end to the sternum of the thoracic cage, which is part of the axial skeleton. The lateral end of the clavicle articulates (joins) with the scapula just above the shoulder joint. You can easily palpate, or feel with your fingers, the entire length of your clavicle.
The scapula (shoulder blade) lies on the posterior aspect of the shoulder. It is supported by the clavicle and articulates with the humerus (arm bone) to form the shoulder joint. The scapula is a flat, triangular-shaped bone with a prominent ridge running across its posterior surface. This ridge extends out laterally, where it forms the bony tip of the shoulder and joins with the lateral end of the clavicle. By following along the clavicle, you can palpate out to the bony tip of the shoulder, and from there, you can move back across your posterior shoulder to follow the ridge of the scapula. Move your shoulder around and feel how the clavicle and scapula move together as a unit. Both of these bones serve as important attachment sites for muscles that aid with movements of the shoulder and arm.
The right and left pectoral girdles are not joined to each other, allowing each to operate independently. In addition, the clavicle of each pectoral girdle is anchored to the axial skeleton by a single, highly mobile joint. This allows for the extensive mobility of the entire pectoral girdle, which in turn enhances movements of the shoulder and upper limb.
Bones of the Upper Limb
The upper limb is divided into three regions. These consist of the arm, located between the shoulder and elbow joints; the forearm, which is between the elbow and wrist joints; and the hand, which is located distal to the wrist. There are 30 bones in each upper limb (see Figure 19.17). The humerus (Figure 19.19) is the single bone of the upper arm, and the ulna (medially) and the radius (laterally) are the paired bones of the forearm (Figure 19.20). The wrist of the hand contains eight bones, each called a carpal bone, and the palm of the hand is formed by five bones, each called a metacarpal bone. The fingers and thumb contain a total of 14 bones, each of which is a phalanx bone (plural = phalanges) of the hand (Figure 19.21).
The carpal bones form a U-shaped grouping, and a strong ligament called the flexor retinaculum spans the top of this U-shaped area. Together, the carpal bones and the flexor retinaculum form a passageway called the carpal tunnel, with the carpal bones forming the walls and floor, and the flexor retinaculum forming the roof of this space (Figure 19.22). The tendons of nine muscles of the anterior forearm and an important nerve pass through this narrow tunnel to enter the hand. Overuse of the muscle tendons or wrist injury can produce inflammation and swelling within this space. This produces compression of the nerve, resulting in carpal tunnel syndrome, which is characterized by pain or numbness, and muscle weakness in those areas of the hand supplied by this nerve.
Pelvic Girdle and Pelvis
The pelvic girdle (hip girdle) is formed by a single bone, the hip bone or coxal bone (coxal = “hip”), which serves as the attachment point for each lower limb. Each hip bone, in turn, is firmly joined to the axial skeleton via its attachment to the sacrum of the vertebral column. The right and left hip bones also converge anteriorly to attach to each other. The pelvis is the entire structure formed by the two hip bones, the sacrum, and the coccyx (Figure 19.23).
Unlike the bones of the pectoral girdle, which are highly mobile to enhance the range of upper limb movements, the bones of the pelvis are strongly united to each other to form a largely immobile, weight-bearing structure. This is important for stability because it enables the weight of the body to be easily transferred laterally from the vertebral column, through the pelvic girdle and hip joints, and into either lower limb whenever the other limb is not bearing weight. Thus, the immobility of the pelvis provides a strong foundation for the upper body as it rests on top of the mobile lower limbs.
Because the female pelvis is adapted for childbirth, it is wider than the male pelvis and has a wider and more shallow pelvic cavity. (see Figure 19.24).
Bones of the Lower Limb
Like the upper limb, the lower limb is divided into three regions. The thigh is that portion of the lower limb located between the hip joint and knee joint. The leg is specifically the region between the knee joint and the ankle joint. Distal to the ankle is the foot. The lower limb contains 30 bones. These are the femur, patella, tibia, fibula, tarsal bones, metatarsal bones, and phalanges (see Figure 19.17). The femur (Figure 19.25) is the single bone of the thigh, and it is also the longest and strongest bone of the body. The patella (Figure 19.25) is the kneecap and articulates with the distal femur. The tibia (shin bone) is the larger, weight-bearing bone located on the medial side of the leg, and the fibula is the thin bone of the lateral leg (Figure 19.26). The bones of the foot are divided into three groups. The posterior portion of the foot is formed by a group of seven bones, each of which is known as a tarsal bone (Figure 19.27), whereas the mid-foot contains five elongated bones, each of which is a metatarsal bone (Figure 19.27). The toes contain 14 small bones, each of which is a phalanx bone (plural = phalanges) of the foot (Figure 19.27).
19.7 Ligaments and Tendons
Ligaments connect bone to bone and help stabilize and hold joints together. Tendons connect skeletal muscles to bones. When the muscle contracts and shorten, the muscle pulls on the tendon, which pulls on the bone and moves it. Figure 19.28 shows the tendons and ligaments of the elbow joint.
19.8 Joints
The adult human body has 206 bones, and with the exception of the hyoid bone in the neck, each bone is connected to at least one other bone. Joints are the location where bones come together. Many joints allow for movement between the bones. At these joints, the articulating surfaces of the adjacent bones can move smoothly against each other. However, the bones of other joints may be joined to each other by connective tissue or cartilage. These joints are designed for stability and provide for little or no movement between the adjacent bones.
At joints that allow for wide ranges of motion (synovial joints) (see Figure 19.29), the articulating surfaces of the bones are not directly united to each other. Instead, these surfaces are enclosed within a space filled with lubricating fluid (synovial fluid), which allows the bones to move smoothly against each other. The articulating surface of each bone is covered by a thin layer of articular cartilage that acts like a Teflon® coating over the bone surface, allowing the articulating bones to move smoothly against each other without damaging the underlying bone tissue. These joints provide greater mobility, but since the bones are free to move in relation to each other, the joint is less stable. Most of the joints between the bones of the appendicular skeleton are this freely moveable type of joint. These joints allow the muscles of the body to pull on a bone and thereby produce movement of that body region. Your ability to kick a soccer ball, pick up a fork, and dance the tango depend on mobility at these types of joints.
Knee Joint
Let’s take a look at one of the synovial joints in the body, the knee joint, which is the largest joint of the body (Figure 19.30). The knee functions as a hinge joint, allowing flexion and extension of the leg. The knee is well constructed for weight bearing in its extended position, but is vulnerable to injuries associated with hyperextension, twisting, or blows to the medial or lateral side of the joint, particularly while weight bearing.
Located between the articulating surfaces of the femur and tibia are two cartilage discs, the medial meniscus and lateral meniscus (see Figure 19.30b). The menisci provide padding between the bones. Some areas of each meniscus lack an arterial blood supply, and thus these areas heal poorly if damaged.
The knee joint has multiple ligaments that provide support, particularly in the extended position (see Figure 19.30c). Outside of the articular capsule, located at the sides of the knee, are two extrinsic ligaments. The fibular collateral ligament (lateral collateral ligament) is on the lateral side and the tibial collateral ligament (medial collateral ligament) is on the medial side of the knee. In the fully extended knee position, both collateral ligaments are taut (tight), thus serving to stabilize and support the extended knee and preventing side-to-side or rotational motions between the femur and tibia.
Inside the knee are two ligaments, the anterior cruciate ligament (ACL) and posterior cruciate ligament, which help to resist knee hyperextension. The cruciate ligaments are named for the X-shape formed as they pass each other (cruciate means “cross”). The posterior cruciate ligament is the stronger ligament. It serves to support the knee when it is flexed and weight bearing, as when walking downhill. In this position, the posterior cruciate ligament prevents the femur from sliding anteriorly off the top of the tibia. The anterior cruciate ligament becomes tight when the knee is extended, and thus resists hyperextension.
Injuries to the knee are common. Since this joint is primarily supported by muscles and ligaments, injuries to any of these structures will result in pain or knee instability. Most commonly, injuries occur when forces are applied to the extended knee, particularly when the foot is planted and unable to move. Anterior cruciate ligament (ACL) injuries can result with a forceful blow to the front of the knee, producing hyperextension, or when a runner makes a quick change of direction that produces both twisting and hyperextension of the knee.
A worse combination of injuries can occur with a hit to the lateral side of the extended knee (Figure 19.31). A moderate blow to the lateral knee will cause the medial side of the joint to open, resulting in stretching or damage to the tibial collateral ligament. Because the medial meniscus is attached to the tibial collateral ligament, a stronger blow can tear the ligament and also damage the medial meniscus. This is one reason that the medial meniscus is 20 times more likely to be injured than the lateral meniscus. A powerful blow to the lateral knee produces a “terrible triad” injury, in which there is a sequential injury to the tibial collateral ligament, medial meniscus, and anterior cruciate ligament.
Arthroscopic surgery has greatly improved the surgical treatment of knee injuries and reduced subsequent recovery times. This procedure involves a small incision and the insertion into the joint of an arthroscope, a pencil-thin instrument that allows for visualization of the joint interior. Small surgical instruments are also inserted via additional incisions. These tools allow a surgeon to remove or repair a torn meniscus or to reconstruct a ruptured cruciate ligament. The current method for anterior cruciate ligament replacement involves using a portion of the patellar ligament. Holes are drilled into the cruciate ligament attachment points on the tibia and femur, and the patellar ligament graft, with small areas of attached bone still intact at each end, is inserted into these holes. The bone-to-bone sites at each end of the graft heal rapidly and strongly, thus enabling a rapid recovery.
Interactive Link
Watch this video to learn more about different knee injuries and diagnostic testing of the knee.
Adapted from Openstax Human Biology and Anatomy and Physiology
