Copyright Goodheart-Willcox Co., Inc. Chapter 5 The Skeletal System 155 SELF CHECK 1. What is the anatomical word for joints that permit only slight motion? 2. List two examples of ball-and-socket joints. 3. Explain the differences between a condylar joint and a saddle joint. 4. What is the purpose of a tendon sheath? Articular Tissues The joints of any mechanical device must be properly lubricated if the movable parts are to move freely and not wear against each other. In the human body, articular cartilage covers the ends of bones at diar- throdial joints and provides a protective lubrication. Articular cartilage cushions the joint and reduces friction and wear. At some joints, articular fibrocartilage, shaped like a disc or a partial disc called a meniscus, is also present between the articulating bones. The intervertebral discs and the menisci of the knee are examples. These discs and menisci help to distribute forces evenly over the joint surfaces and absorb shock at the joint. Tendons, which connect muscles to bones, and ligaments, which connect bones to other bones, are also present at the diarthrodial joints. Composed of collagen and elastic fibers, these tissues are slightly elastic and return to their original length after being stretched, unless overstretched to the point of injury. The tendons and ligaments crossing a joint play an important role in joint stability. The hip joints, in particular, are crossed by a number of large, strong tendons and ligaments. One example of a condylar joint is the joint at the base of the index finger. Saddle joints are so named because their artic- ulating bone surfaces are both shaped like the seat of a riding saddle. Movement capability is the same as that of the condylar joint, but saddle joints have a greater range of movement. The joint at the base of the thumb is an example of a saddle joint. Ball-and-socket joints are the most freely movable joints in the body. In these joints, the surfaces of the articulating bones are reciprocally convex and concave, with one bone end shaped like a “ball” and the other like a “socket.” Rotation in all three planes of movement is permitted. In areas where the joint socket is relatively shallow, such as the shoulder, a large range of motion is permitted, but joint stability is sacrificed. Alter- natively, the deep socket of the hip joint maximizes stability but allows much less range of motion than the shoulder. Two structures often associated with diarthrodial joints are bursae and tendon sheaths. Bursae are small capsules lined with synovial membranes and filled with synovial fluid. Their purpose is to cushion the structures they separate. Most bursae separate tendons from bone, reducing the friction on the tendons during joint motion. Tendon sheaths are double-layered synovial structures surrounding tendons that are subject to friction because of their proximity to bones. These sheaths secrete synovial fluid to promote free motion of the tendon during joint movement. Many long muscle tendons crossing the wrist and finger joints are protected by tendon sheaths. SECTION 5.5 REVIEW Mini-Glossary amphiarthrosis a type of joint that permits only slight motion articular fibrocartilage tissue shaped like a disc or a partial disc called a meniscus that provides cushioning at a joint ball-and-socket joint a synovial joint formed between one bone end shaped roughly like a ball and a receiving bone reciprocally shaped like a socket bursae small capsules lined with synovial membranes and filled with synovial fluid that cushion the structures they separate condylar joint a type of diarthrosis in which one articulating bone surface is an oval, convex shape, and the other is a reciprocally shaped concave surface diarthrosis a freely movable joint also known as a synovial joint gliding joint a type of diarthrosis that allows only sliding motion of the articulating bones (continued)