What is the discussion mainly about? [br] The professor categorizes joints by ho

Listen to part of a class discussion on the different types of joints. (F= Professor, T = Tony, L = Louise)
P: Now, I know that most of you are healthy individuals who participate in some sort of physical activities pretty regularly. Some of you jog, others bicycle. You might throw around the Frisbee with your friends, or rollerblade. Our movements are so easy and fluid that you may not have stopped to think that the system of joints that makes them possible is highly complex. A joint is the place where two or more bones connect, and because that’s the technical definition, it actually includes some places that don’t move. Since it’s movement that we’re interested in, I won’t spend much time on the immovable joints. They’re called fibrous joints. The reason is that… uh, well, they’re joined together by bone fibers. It’s kind of like welding or soldering two pieces of metal together. There are fibrous joints, for example, between the different bones that make up the top part of your skull. The bones don’t move, right? I’m talking about the top of your head—forget your jaw for a second. If your jaw had a fibrous joint, you couldn’t talk! The round dome of the skull is made of several bone plates with fibers holding them together. Okay, so those are the fibrous joints. The other joints, the ones we’re interested in, are movable. In a movable joint, the bones aren’t fused to each other. They’re held together with ligaments. Ligaments are long and flexible; they’re kind of like ropes or cables. But because the bones have to slide or rub over each other when the joint moves, it also needs to be cushioned against abrasion. These joints are categorized by the types of tissues used to keep them working smoothly. Cartilaginous joints have a tough, smooth lining over the parts of the bone. It’s like a pad or cushion; it protects the bone from friction that could wear it down or cause it to splinter. And, in fact, if this cartilage—the protective tissue is called cartilage—if it becomes worn or damaged, joint movement may be painful or severely restricted because now you have bone hitting bone. That’s because cartilaginous joints are tight; the bones fit very closely together—for example, the bones in your spine. Some joints require more movement. They’re called synovial joints. There has to be a space between the bones, a joint cavity. Those more mobile joints, in addition to being padded with cartilage, are lubricated with liquid that flows through the cavity. The knuckles of your fingers are examples of synovial joints. Of course, the fluid has to be held in place, otherwise it’ll seep into the surrounding tissues and be absorbed back into the body, and what holds it in place is a little membrane-lined sac called a bursa. So the joint is encased in this membrane, and the membrane is a pocket for that lubricating fluid. By the way, the fluid also keeps the joint nourished—it doesn’t have an independent blood supply in adults, so the fluid absorbs nutrients from the blood through the joint’s outer membrane. That’s a synovial joint. Tony, do you have a question?
T: Yeah, I’m kind of confused. When we learned about the joints in high school, I don’t remember talking about them this way. I thought we talked about them more like machines… uh, like, the directions in which they moved.
P: Uh-huh, we’re getting to that. Okay, now that we have joints categorized by how their surfaces are put together—fibrous, synovial, and cartilaginous—we can talk about how the movable joints actually move. I mentioned your knuckles a few minutes ago, and they’re good examples of hinge joints. They work kind of like the hinge on a door, so the joint can flex and extend in one direction only. But hinge joints aren’t the only movable joints in the body. The hip, as well as the shoulder, is an example of ball-and-socket joints. Ball-and-socket joints allow the greatest range of movement, and therefore, they have to be the most anatomically complex. In a ball-and-socket joint, one bone has a rounded knob at the end, and it fits snugly into the socket, a round cavity, of another bone. Ball-and-socket joints are a type characteristically found in more evolved creatures, such as apes and us humans. They allow you to move through 360 degrees of motion, the way you can wind your arm back before you throw a ball. While we’re at it, can anyone think of another type of joint?
L: Well, I know that when I rollerblade, I have to turn my head constantly to make sure I don’t crash into anyone. But the neck doesn’t seem to be an example of either a hinge joint or a ball-and-socket joint.
P: Good example, Louise. The neck is actually a type of pivot joints. Pivot joints permit rotation, the way you can twist your head around. Well, it doesn’t go all the way around, but it moves around your neck. It pivots on an axis.