Copyright Goodheart-Willcox Co., Inc. Chapter 5 The Skeletal System 153 by fibrous tissues. The fibers begin to ossify (turn to bone) in early adulthood and are eventually replaced completely by bone. The only sutures in the human body are the sutures of the skull. Syndesmoses, meaning “held by bands,” are joints in which dense, fibrous tissue binds the bones together, permitting extremely limited movement. Examples include the coracoacromial (kor-a-koh-a-KROH- mee-al) joint and the distal (lower) tibiofibular joints. Amphiarthroses The amphiarthroses are joints that permit only slight motion. These cartilaginous joints allow more motion of the articulating bones than the synarthrodial joints. This means they are somewhat better able to absorb shock. The two types of amphiarthroses are synchondroses and symphyses. Synchondroses, meaning “held by cartilage,” are joints in which the articulating bones are held together by a thin layer of hyaline cartilage. Examples include the sternocostal joints (between the sternum and the ribs) and the epiphyseal plates (growth plates). Symphyses are joints in which thin plates of hyaline cartilage separate a disc of fibrocartilage from the bones. Examples include the vertebral joints and the pubic symphysis. Diarthroses Freely movable joints are called diarthroses. They are also referred to as synovial joints because each joint is surrounded by an articular capsule with a synovial membrane lining that secretes a lubricant known as synovial fluid (Figure 5.34). There are six different types of diarthrosis each is structured to permit different types of motion (Figure 5.35). T he joints of the human body, also known as articulations, govern the extent and directions of movement of the bones that articulate (come together) at the joint. Although the range of motion at a given joint is affected by the tightness of the soft tissues crossing that joint, it is the structure of the bony articulation that determines the directions of motion permitted. Anatomists classify joints in different ways based on joint complexity, the number of axes present, joint structure, and joint function. Joint function determines movement capability, and it is the most easily remembered classification, so it will be used in this section. Types of Joints There are three main categories of joints with regard to function: the immovable joints, the slightly movable joints, and the freely movable joints. Moving is the function of the arms, hands, legs, and feet, so most of the joints in the body appendages are freely movable. The axial skeleton’s functions include stability and protection, so its joints are primarily immovable or slightly movable. Synarthroses The immovable joints are called synarthroses. The prefix syn- means “together” and the root word arthron means “joint.” Synarthroses are fibrous joints that can absorb shock but permit little or no movement of the articulating bones. There are two types of immovable joints: sutures and syndesmoses. Sutures, which you read about earlier in this chapter, are joints in which irregularly grooved, articulating bone sheets join closely and are tightly connected SECTION 5.5 Joints Objectives • Describe the general structures and functions of the three major categories of joints. • Explain the purpose of the articular tissues, including tendons, ligaments, bursae, and articular cartilage. Key Terms amphiarthrosis articular fibrocartilage ball-and-socket joint bursae condylar joint diarthrosis gliding joint hinge joint ligament pivot joint saddle joint symphysis synarthrosis synchondrosis syndesmosis synovial joint tendon tendon sheaths